FACTORS AFFECTING THE PHOTODEGRADAnON RATES OF POLYMERS
by
BEVIN COLLEEN DAGLEN
A DISSERTATION
Presented to the Department of Chemistry
and the Graduate School of the University of Oregon
in partial fulfillment of the requirements
for the degree of
Doctor ofPhilosophy
September 2008
11
University of Oregon Graduate School
Confirmation of Approval and Acceptance of Dissertation prepared by:
Bevin Daglen
Title:
"FACTORS AFFECTING THE PHOTODEGRADATION RATES OF POLYMERS THAT
CONTAIN (-C5H4(CO)3MO-MO(CO)3C5H4-) IN THE BACKBONE"
This dissertation has been accepted and approved in partial fulfillment of the requirements for
the degree in the Department of Chemistry by:
Catherine Page, Chairperson, Chemistry
David Tyler, Advisor, Chemistry
David Johnson, Member, Chemistry
Darren Johnson, Member, Chemistry
Miriam Deutsch, Outside Member, Physics
and Richard Linton, Vice President for Research and Graduate Studies/Dean of the Graduate
School for the University of Oregon.
September 6, 2008
Original approval signatures are on file with the Graduate School and the University of Oregon
Libraries.
© 2008 Bevin Colleen Daglen
111
IV
An Abstract of the Dissertation of
Bevin Colleen Daglen for the degree of Doctor ofPhilosophy
in the Department of Chemistry to be taken September 2008
Title: FACTORS AFFECTING THE PHOTODEGRADAnON RATES OF
POLYMERS THAT CONTAIN (-CsHiCO)3Mo-Mo(CO)3CsHc) IN THE
BACKBONE
Approved:
Prof. David R. Tyler
There are compelling economic and environmental reasons for using degradable
plastics in selected applications and considerable research is now devoted to devising new
photodegradable polymers with improved performance. Controlling the degradation of
these materials in a prescribed fashion is still a difficult problem because the parameters
that influence degradation rates are not completely understood. In order to predict polymer
lifetimes, to manipulate when a polymer starts to degrade, and to control the rate of
degradation, it is necessary to identify the experimental parameters that affect polymer
degradation rates and to understand how these parameters affect degradation.
This dissertation describes the results ofexperiments designed to gain fundamental
insight into the factors that affect the rate of polymer photodegradation. The key
experimental strategy employed was the incorporation of organometallic dimers into the
vbackbone ofthe polymer chains, specifically, [CpRMo(COhh (CpR = a substituted
cyclopentadienyl (l'{CsI-4R)). Incorporating these organometallic units into a polymer
chain make the polymer photodegradable because the metal-metal bond can be cleaved
with visible light. The photogenerated metal radicals can then be trapped by O2 or
chlorine, resulting in degradation of the polymer. Another benefit from incorporating this
chromophore into the polymer backbone is that it provides the experimentalist with a
convenient spectroscopic handle to monitor degradation rates.
Using these model polymers, several experimental factors that can affect polymer
photodegradation rates have been explored. For example, experiments showed that radical
trap concentration affects degradation rates below, but not above, the polymer glass
transition temperature. In addition, degradation rates as a function of irradiation
temperature, tensile stress, and time-dependent morphology changes were explored for
various polymers. The results of these studies suggest that the ability of the photogenerated
radicals to escape the radical cage is the dominant factor in determining photodegradation
efficiencies.
This dissertation includes previously published and unpublished co-authored
material.
CURRICULUM VITAE
NAME OF AUTHOR: Bevin Colleen Daglen
PLACE OF BIRTH: Camp Lejeune, North Carolina
DATE OF BIRTH: 25 September 1977
GRADUATE AND UNDERGRADUATE SCHOOLS ATTENDED:
University of Oregon, Eugene, Oregon
Boise State University, Boise, Idaho
DEGREES AWARDED:
Doctor ofPhilosophy in Chemistry, 2008, University of Oregon
Master of Science in Chemistry, 2005, University of Oregon
Bachelor of Science in Chemistry, 2003, Boise State University
AREAS OF SPECIAL INTEREST:
Physical Polymer Chemistry
Photochemistry
Organometallic Chemistry
Science Education
PROFESSIONAL EXPERIENCE:
Polymer Program Volunteer, Materials Science Institute, University of Oregon,
2004-2007
Graduate GK-12 Fellow, Materials Science Institute, University of Oregon,
2004-2007
Graduate Teaching Fellow, Materials Science Institute, University of Oregon,
2003-2004&2007
VI
VB
Women in Graduate Science Past President and Founding Member, University of
Oregon, 2004-2008
Adjunct Faculty, Department of Chemistry, Boise State University, 2003
GRANTS, AWARDS AND HONORS:
NSF-IGERT Fellowship, University of Oregon, 2007
Women in Graduate Science Travel Award, University of Oregon, 2007
NSF Debating Science Program, University of Montana, 2007
NSF-GKI2 Fellowship, University of Oregon, 2004-2007
ACS Summer School on Green Chemistry, The Johnson Family Foundation, 2006
PUBLICATIONS:
Daglen, B. C.; Tyler, D. R. Application of a Perrin-like Model to Explain the
Biphasic Photochemical Degradation Rates of Polymers. Macromolecules
submitted.
Tyler, D. R.; Daglen, B. C.; Shultz, G. V. Polymers with Metal-metal Bonds as
Models in Mechanistic Studies of Polymer Photodegradation. In
Macromolecules Containing Metal and Metal-like Elements:
Photochemistry and Photophysics; Abd-EI-Aziz, A. S., Eds.; Wiley-
Interscience: Hoboken, NJ, 2008 under review.
Daglen, B. C.; Harris, J. D.; Dax, C. D.; Tyler, D. R. Measuring Solid-state
Quantum Yields: The Conversion of a Frequency-doubled Nd:YAG Diode
Laser Pointer Module into a Viable Light Source. Rev. Sci. Instr. 2007, 78,
07410411-074104/4.
Daglen, B. C.; Harris, J. D.; Tyler, D. R. Factors Controlling the Rate of
Photodegradation in Polymers; the Effect of Temperature on the
Photodegradation Quantum Yield in a PVC Polymer Containing Metal-
metal Bonds in the Polymer Chain. J. Inorg. Organomet. Polym. Mat.
2007,17,267-274.
Vlll
Daglen, B. C.; Tyler, D. R. Factors Controlling the Rate of Photodegradation in
Polymers. In Degradable Polymers and Materials; Khemani, K. c.;
Scholz, c., Eds.; ACS Symposium Series 939; American Chemical
Society: Washington, DC, 2006; pp 384-397.
Chen, R.; Meloy, J.; Daglen, B. c.; Tyler, D. R. Photochemically Reactive
Polymers. Identification of the Products Formed in the Photochemical
Degradation of Polyurethanes that Contain (CsH4R)(CO)3Mo-
Mo(CO)3(CsH4R) Units Along their Backbones. Organometallics 2005,
24, 1495-1500.
IX
ACKNOWLEDGMENTS
I wish to express my sincere appreciation to my research advisor, Prof. David R.
Tyler, for supporting my research endeavors and enduring my numerous extracurricular
activities. I am thankful to have studied in a laboratory that tackles a diversity of difficult
scientific questions. I am equally honored to have been a part of the team of talented
individuals who have shared in the frustration and elation experienced while attempting
to answer those challenging scientific questions (Team Tyler pushing back the forefront
of science). I would like to extend an honorable mention to those who have collaborated
on the polymer project during my tenure in the Tyler lab: Rui Chen, John Harris, Ginger
Shultz, Katherine Klug, and Jennifer Zemke. This is not the space to acknowledge all lab
members individually but there are two in particular I must mention for their inspiration,
their laughter, and their courage: Takiya 1. Ahmed and Ginger V. Shultz. Thank you.
My life is more consequential for having spent these years in your company.
More appreciation for those who enriched my Graduate School experience: David
Senkovich and Kris Johnson were instrumental in maintaining the Oz-free atmosphere in
our dryboxes. David and Kris along with John Bossinger and Jeffrey Garman of the
machine shop also counseled me in numerous trouble-shooting and repair projects. (They
are the cornucopia of knowledge and skill that keeps the University scientists'
experiments going.) Clifford Dax was critical in the improvements of the irradiation
source used in the degradation experiments. Clarisse Heinhorst and Jim Rasmussen were
useful for the timely acquisition of needed materials. Thanks to the assistance of the
xfaculty and staff of the university's Materials Science Institute, I was able to accomplish
my GK-12 and IGERT responsibilities. Lastly, critical to my education and career
development was my involvement in the UO Women in Graduate Science founded by my
dear friend Sara J. Staggs. I will never underestimate the power of a mentor.
As for mentors: Prof. Michael Haley recruited me to the University of Oregon for
graduate school and I am very grateful. My committee, Darren Johnson, Dave Johnson,
and Miriam Deutsch, was also a great resource over the years. To my committee chair,
Cathy Page, I appreciate you taking on the extra load of chair, I especially enjoyed the
opportunity to teach Inorganic Chemistry, and I learned much from you imparting your
teaching and research experiences. I also had a series of great instructors, David Tyler,
Mark Lonergan, Jim Hutchison, and Cathy Page, who all inspired me to become an even
more enthusiastic teacher.
To my family: I am thankful for your support. My mother has forever
encouraged me to continue my education which was the beginning really. My
grandmother showed me the usefulness of thrift and creativity, which is handy in the
research lab. My grandfather taught the importance of voice projection and a firm
handshake. My mother- and father-in-law are wonderful role models and have proved
very useful in solving calculus problems. And finally, I cannot express the extent to
which my husband, Michael Daglen, has contributed to the success of my graduate
career. Life is just more fun with you in it.
The investigation was supported by NSF Graduate K-12 program (DGE-0231997)
NSF IGERT program (DGE-0549503) and the Petroleum Research Fund of the ACS.
To Michael Todd Daglen, my Judo partner, my friend, my love.
"The harmonious development
and eventual perfection
of the human character."
- Jigoro Kano
Xl
XlI
TABLE OF CONTENTS
Chapter Page
1. PHOTODEGRADABLE POLYMERS AND SOLID-STATE
PHOTOCHENIISTRY 1
1.1 Introduction 1
1.2 Photodegradable Polymer Design 5
1.3 Factors Affecting Degradation Rates 12
1.3.1 Effect of Radical Trap Concentration 12
1.3.2 Effect of Temperature.................................................................. 14
1.3.3 Effect of Stress 16
Plotnikov Hypothesis 17
Decreased Radical Recombination Efficiency Hypothesis 18
Zhurkov Hypothesis 20
Evidence Supporting the DRRE Hypothesis 20
1.4 Kinetic Models of Polymer Photodegradation 21
1.5 Dissertation Outline 23
1.6 Acknowledgments 24
II. PHOTOCHEMICALLY REACTIVE POLYMERS. IDENTIFICATION OF
THE PRODUCTS FORMED IN THE PHOTOCHEMICAL
DEGRADATION OF POLYURETHANES THAT CONTAIN
(C5H4R)(CO)3Mo-Mo(COMC5~R)UNITS ALONG THEIR
BACKBONES 26
2.1 Introduction 26
2.2 Results and Discussion 28
2.2.1 Syntheses and Photochemical Reactions of Polymers Hypol-Mo
and Hypol-PEG-Mo.................................................................. 28
2.2.2 X-ray Photoelectron Spectroscopic Investigation 33
2.2.3 Photochemical Reactivity of the Polyurethanes in Air 35
2.2.4 Photodegradation ofPU-70 Under a Nitrogen Atmosphere 36
2.3 Key Insights and Conclusions 38
xiii
Chapter Page
2.4 Endnotes 39
III. FACTORS CONTROLLING THE RATE OF PHOTODEGRADAnON IN
POLYMERS 40
3.1 Introduction 40
3.2 Results and Discussion 41
3.2.1 Experimental Approach to the Problem 41
3.2.2 Polymer Synthetic Strategy......................................................... 43
3.2.3 Synthesis of the PU-XX Polymers 44
3.2.4 Photodegradation Occurs in the Absence of02 45
3.2.5 Effect ofTg on the Efficiency of Photodegradation 46
3.2.6 Effect ofRadical Trap Concentration 48
3.3 Summary and Conclusions 49
IV. THE EFFECT OF TEMPERATURE ON THE PHOTODEGRADATION
QUANTUM YIELDS OF POLYMERS CONTAINING METAL-METAL
BONDS IN THE POLYMER CHAIN 52
4.1 Introduction 52
4.2 Results and Discussion 53
4.2.1 Polymer Preparation and Characterization 53
4.2.2 Temperature Dependence on the Degradation ofPVC Polymer
and Model Systems 54
4.2.3 Secondary Thermal Transitions Hypothesis 61
4.2.4 Activation Energies of Polymer Secondary Thermal Transitions.. 64
4.3 Conclusions 65
V. THE EFFECT OF MORPHOLOGY CHANGES ON POLYMER
PHOTODEGRADAnON EFFICIENCIES. A STUDY OF TIME-
DEPENDENT MORPHOLOGY AND STRESS INDUCED
CRySTALLINITy.................................................................................... 67
5.1 Introduction 67
5.2 Results and Discussion 69
xiv
Chapter Page
5.2.1 Polymer Preparation and Characterization 69
5.2.2 Effect of Time-dependent Morphology Changes on the
Degradation Efficiency of a Segmented Polyurethane............... 69
Quantum Yield as a Function of Time for PU-35 69
Identification of Peaks in the Infrared Spectrum ofPU-35.............. 71
Method oflnfrared Peak Deconvolution 73
PU-35 Infrared Peak Areas as a Function of Time.......................... 75
5.2.3 Effect of Tensile Stress on the Degradation Efficiencies of
PVC-Mo.................................................................................. 76
Decreased Radical Recombination Efficiency Hypothesis 76
Previous Data Supporting the DRRE Hypothesis 77
Recent Data for Quantum Yield versus Tensile Stress for
PVC-Mo-2 79
5.3 Conclusions 81
5.4 Endnotes 83
VI. APPLICATION OF A PERRIN-LIKE KINETICS MODEL TO EXPLAIN
THE BIPHASIC PHOTOCHEMCIAL DEGRADATION RATES OF
POLyMERS.............................................................................................. 84
6.1 Introduction 84
6.2 Results and Discussion 86
6.2.1 Fitting the Biphasic Photodegradation Data 86
6.2.2 Derivation of a Perrin-like Photochemical Model 90
6.2.3 Analysis of a Literature Example Using the Perrin-like Model...... 97
Literature Data Capture Method 98
6.2.4 Fitting Parameters as a Function of Temperature 99
6.3 Conclusions 103
6.4 Endnotes 104
VII. CONCLUDING REMARKS 105
7.1 Chapter Summaries 105
7.2 Concluding Hypothesis 107
xv
Chapter Page
APPENDICES 110
A. MEASURING SOLID-STATE QUANTUM YIELDS: THE CONVERSION
OF A FREQUENCY-DOUBLED Nd:YAG DIODE LASER POINTER
MODULE INTO A VIABLE LIGHT SOURCE .. 110
A.1 Introduction........................................................................................ 110
A.2 Experimental Apparatus 112
A.2.1 The Merlin Detection Unit.......................................................... 112
A.2.2 Conversion of the Frequency-doubled Nd:YAG Laser Pointer
Module into a Viable Irradiation Source 114
A.2.3 The Nd:YAG Laser as a Light Source......................................... 115
A.2A The 200 W High-pressure Hg Arc Lamp as a Light Source......... 116
A.3 Experiments and Discussion................................................................ 117
A.3.1 Reagents..................................................................................... 117
A.3.2 Results ofthe Solid-state Experiments........................................ 118
A.4 Summary 120
A.5 Endnotes 121
B. SYNTHESIS AND PREPARATION OF PHOTOCHEMICALLY REACTIVE
SPECIES USED IN THE DEGRADATION STUDIES 122
B.1 Materials 122
B.2 Characterization Instruments 123
B.3 Synthesis of [(lls-CsH4CHzCHzOH)(CO)3Moh 124
B.3.1 Synthesis of Sodium Cyclopentadiene......................................... 124
B.3.2 Synthesis of Cyclopentadienylethanol......................................... 124
B.3.3 Synthesis of2-(1-Methoxy-1-methyl-ethoxy)-ethyl-
cyc10pentadiene 125
B.3A Synthesis of(lls-CsH4CHzCHzOHhMoz(CO)6............................ 125
BA Synthesis ofthe Polyurethane Copolymer of Hypol-2000 and
(CsH4CHzCHzOHhMoz(CO)6 (Hypol-Mo) 125
B.5 Synthesis of the Polyurethane Copolymer ofHypol-2000, PEG-1000
and (CsH4CHzCHzOHhMoz(CO)6 (Hypol-PEG-Mo) 126
B.6 Synthesis of Acyl-substituted Poly(vinyl chloride) (PVC-CDCl) 127
XVI
Chapter Page
B.7 Synthesis and Preparation ofPoly(vinyl chloride) Crosslinked with
(CSH4CHzCHzOH)zMoz(CO)6 (PVC-Mo) 127
B.8 Synthesis and Preparation of the Polyurethane Copolymers ofTDI,
TDI-PPG, l,4-Butanediol and (CSH4CHzCHzOH)zMoz(CO)6
(PU-XX) 128
B.9 Preparation of PVC Embedded with Cp'zMOZ(CO)6 (Mo in PVC) 129
B.10 Preparation ofCp'zMoz(CO)6 Solution (Mo in Soln) 130
REFERENCES 131
Page
XVll
LIST OF FIGURES
Figure
CHAPTER I
1. The Properties of Both a Typical Polymer and an Ideal Photochemically
Degradable Polymer Demonstrating Tunable Onset of Degradation and
Rapid Degradation 3
2. Example of an Inorganic Photosensitizer Attached to a Polymer Chain
Through the Pendant Groups..................................................................... 9
3. Examples of Polymers with Transition Metal Dimers in the Backbone of
the Polymer 11
4. Graphical Representation of the Three Hypotheses for the Effect of
Tensile Stress on the Quantum Efficiency of Polymer Photodegradation
as Postulated by the: a. Plotnikov, b. Decreased Radical Recombination
Efficiency, and c. Zhurkov Hypotheses 17
5. The Photophysical Description ofthe Plotnikov Hypothesis 18
6. An Illustration of the Decreased Radical Recombination Efficiency
Hypothesis Regarding the Effect of Tensile Stress on Polymer
Photodegradation 19
7. Photodegradable Polymer (PVC-Mo) Made by Crosslinking Poly(vinyl
chloride) with a Metal-metal Bond Chromophore 21
CHAPTER II
1. Photos ofthe Photooxidative Degradation ofHypol-PEG-Mo Under
Ambient Light and Atmospheric Temperature and Pressure. (a)
Unexposed Sample; (b) Sample Exposed to Light for 5 h; (c) Sample
Exposed for 3 Days 31
2. Infrared Spectra Showing the Changes that Occur When a Film of
Hypol-Mo on a NaCl Plate was Exposed to Ambient Light 33
CHAPTER IV
1. Plots ofthe Quantum Yields for the Disappearance ofthe Cp2Mo2(CO)6
Unit in PVC-Mo, Cp'2Mo2(CO)6 Dispersed in PVC, and Cp'2Mo2(CO)6
in Hexane/CC14 56
Page
XV111
Figure
CHAPTER IV
2. Illustrated are the Relationships Between: a) the Viscosity of a Solution
ofCp'zMoz(CO)6 in Hexane/CCl4 at Variable Temperature, b) the
Quantum Efficiency ofCp'zMoz(CO)6 Photolysis in Hexane/CCl4 at a
Constant Temperature and Variable Viscosity, and c) the Quantum
Efficiency of Cp'zMoz(CO)6 in a Hexane/CCl4 Solution at Variable
Temperature... 58
3. Plot of the In<l> Versus r 1 for PVC-Mo 63
4. Two Examples of Molecular Motion that Result in Secondary Thermal
Transitions. 65
CHAPTER V
1. Illustration of the Solid-state Morphology of a Polymer with Soft and
Hard Segments.......................................................................................... 68
2. Quantum Yield as a Function of Time for PU-35 70
3. N-H Stretching Region and C=O Stretching Region of the Infrared
Spectrum of a Solution ofPU-35 in Tetrahydrofuran and a Polymer Film
ofPU-35.................................................................................................... 73
4. Carbonyl Region of the Infrared Spectrum of a PU-35 Polymer Film and
the Calculated Contributions to the Observed Peaks Using a Linear
Combination ofLorentzian Curves 75
5. Ratio of the Areas of the Hydrogen-bonded C=O v Peaks to the Non-
hydrogen-bonded C=O v Peak over time for PU-35 76
6. Literature Data for the Quantum Yield Versus Tensile Stress for PVC-
Mo-l......................................................................................................... 78
7. Literature Data for the Ratio of the Infrared Absorbances, AI4ZS/AI436, for
the Bending Modes of CHz in the Amorphous and Crystalline Regions of
PVC-Mo-l as a Function of Tensile Stress 79
8. Quantum Yield Versus Tensile Stress for PVC-Mo-l and PVC-Mo-2 80
9. Ratio of the Infrared Absorbances, AI4ZS/AI436, for the Bending Modes of
CHz in the Amorphous and Crystalline Regions ofPVC-Mo as a
Function of Tensile Stress 81
Page
XIX
Figure
CHAPTER VI
1. A Plot Illustrating the Typical Biphasic Decay for the Photodegradation
of a Polymer 85
2. Relative -eH2CsH4(CO)3Mo-Mo(CO)3CsH4CH2- Concentration as
Function ofIrradiation Time at Room Temperature for Polymer 1 and 2
with Fits for the 1st Order and 2nd Order Equations 87
3. Relative -eH2CsH4(CO)3Mo-Mo(CO)3CsH4CH2- Concentration as a
Function ofIrradiation Time at Room Temperature for Polymer 1 and 2
with Fits for the Simple Diffusion, Diffusion-controlled, and Perrin-like ... 89
4. Perrin Model used to Describe the Observed Rates for the Fluorescence
Decay of a Chromophore in the Solid-state 92
5. Relative Radical Concentration as a Function of Time at 343.5 K
Following Irradiation of Polymer 3 96
6. Fitting Parameters Xo and Yo from Equation 7 as a Function of
Temperature for Polymers 1 and 2 100
7. Erying Plots for the Fitting Parameters ki and k2 from Equation 7 for
Polymers 1 and 2 102
APPENDIX A
1. The Experimental Setup for Both Liquid and Solid-state Quantum Yield
Measurements using Either a Hg Arc Lamp or Nd:YAG Laser as the
Source 114
2. A Schematic of the Nd:YAG Laser 115
3. Beam Profiles for Nd:YAG Laser and High-pressure Mercury Arc 117
Lamp .
4. The Structure of Polymer 1 Used in the Photochemical Degradation
Studies 118
5. Irradiation of a Polymer Containing the Chromophore Unit Cp(CO)3Mo-
MO(CO)3Cp using the Nd:YAG Laser and the Attenuated Hg Arc Lamp.. 119
xx
LIST OF TABLES
Table
CHAPTER II
1. Mo 3ds12 Binding Energies of Different Materials
CHAPTER III
Page
34
1. Quantum Yields for Various Polymers with Different Glass Transition
Temperatures and for a Model Complex 47
2. Quantum Yields for Polymer Degradation for Polymers with Different
Concentrations of Metal-radical Trap 49
CHAPTER IV
1. Thermal Transitions and Activation Parameters for PVC-Mo and PU-70 .. 64
CHAPTER V
1. Literature Values for the N-H and C=O Stretching Frequencies Found in
Polyurethanes 71
CHAPTER VI
1. Apparent Enthalpy and Entropy of Activation Values Obtained from the
Eyring Plot ofthe Rate Constants, k] and k2, Versus Temperature for the
Photodegradation of Polymers 1 and 2 102
APPENDIXB
1. The Starting Material Quantities for the Polyurethane Series PU-XX 129
XXI
LIST OF SCHEMES
Scheme Page
CHAPTER I
1. Mechanism of the Norrish Type I (a) and II (b) Reactions in a Ketone-
containing Polymer 6
2. The Autooxidation Process Where R is Some Carbon Species 7
3. Examples of Organic Peroxide Reactions 7
4. Synthesis of a Ferrocene-containing Polymer using Step Polymerization
Techniques 10
5. Example Synthesis of a Metal-metal Bond-containing Polymer using
Step Polymerization Techniques 10
6. Photochemical Degradation of a Generic Polymer with Metal-metal
Bonds in the Backbone 12
7. Homolysis of a Bond and Subsequent Trapping Reaction Depicting the
Radical Cage Intermediate 14
8. Mechanism for the Photochemical Norrish Type II Reaction Showing the
Formation of the 6-membered Ring Intermediate 16
9. Simplified Polymer Photodegradation Mechanism Showing Bond
Homolysis 17
10. Degradation Pathway ofPVC-Mo Used in Studying the Effect of Stress
on Photochemical Quantum Yields 21
CHAPTER II
1. Example of a Polyurethane Synthesis Used to Incorporate Molybdenum
Dimers into the Backbone of a Polymer Chain 27
2. Example Photodegradation Reaction of Polyurethane Containing
Molybdenum Dimers with a Halide Radical Trap in Solution 28
3. Synthesis of the Polyurethane Hypol-Mo 29
4. Synthesis ofthe Polyurethane Hypol-PEG-Mo 29
5. Photodegradation ofHypol-Mo under Ambient Conditions 36
6. Synthesis ofPolymer PU-70 37
Scheme
CHAPTER III
XXll
Page
1. Example of a Step Polymerization Technique Used to Incorporate an
Organometallic Complex into a Polymer Backbone 43
2. Example of a Polyurethane Synthesis Used to Incorporate Molybdenum
Dimers into the Backbone of a Polymer Chain 44
3. Syntheses ofPU-XX Polymers 45
4. Photochemical Reaction of the PU-XX Polymers 46
5. Photodegradation of a Model Complex, 1, in Solution with CC14 as the
Radical Trapping Agent 48
CHAPTER IV
1. Synthesis and Photodegradation of PVC-Mo 54
2. Reaction of a Photoreactive Species to Form a Caged Radical Pair
Followed by Escape of the Radicals from the Cage and Subsequent
Radical Trapping Reaction 60
3. Pathway for the Increased Separation Efficiency of the Radicals Formed
by Irradiation ofPVC-Mo 62
CHAPTER V
1. Simplified Polymer Photodegradation Mechanism Showing Bond
Homolysis................................................................................................. 77
CHAPTER VI
1. Illustration of the Reaction of Photogenerated Metal Radicals with
Trapping Atoms in a Solid-state Matrix 93
CHAPTER VII
1. Photodegradation Mechanism of a Metal Dimer Containing Polymer
Illustrating the 'Solvent' Cage 108
APPENDIX A
1. The Photochemical Reaction of Polymer 1 119
1CHAPTER I
PHOTODEGRADABLE POLYMERS AND SOLID-STATE PHOTOCHEMISTRY
1.1 Introduction
According to the Society of the Plastics Industry, the United States produces over
$379 billion worth of plastic annually, making plastics the third largest manufacturing
industry in the United States. Furthermore, the industry has grown at a rate of 3.2%
annually since the 1980's. The reason for the resilience of this industry is the numerous
benefits plastics have had and continue to have on our lives. A couple examples include
providing a protective barrier from pathogens in the medical and food industries and
making cars lighter by using plastic in lieu of metal parts, and consequently, making
vehicles more fuel efficient. Because plastics are so ubiquitous, scientists have made
great advances in the development and improvement of polymeric materials. This year
alone, over 38,000 publications on polymers have appeared in the scientific literature.
This particular piece of literature will address a niche in the plastics industry that deals
with the photodegradation of polymers.
Photochemically reactive polymers are of significant interest for two main
reasons: their use as degradable plastics for a variety of applications and for the study of
polymer degradation. In regards to the former, photodegradable polymers are extensively
used for photolithography and biomedical applications. 1-5 Another niche for
photodegradable polymers is in the 'green' plastics market. In the environmental area,
photodegradable plastics are finding increased use as packaging materials for items that
2have a high probability of becoming litter. Photodegradable consumer plastics have also
found their way into homes as degradable trash bags and plastic kitchenware. Another
area that is experiencing increasing utility in photodegradable polymers is plasticulture.
In this application, the ground is covered with plastic sheeting (typically a polyolefin);
this acts as a mulch to prevent the growth of weeds (thus requiring the use of fewer
herbicides), decreases water demand, and extends the growing season by keeping the
ground warmer. By making these agricultural films out of degradable plastics,
considerable labor and money can be saved in the plastics recovery phase of the
technique.6•9
The second reason photochemically reactive polymers are of significant interest is
for use in the study of polymer degradation. In order for photodegradable polymers to be
suitable in the applications mentioned above, the performance of these polymers must be
well understood before they are incorporated into materials. The ideal photodegradable
polymer has (at least) three ideal properties (illustrated in Figure 1).8,10 First, the onset of
degradation should be reliably predictable. Although it is obvious why this property is
desirable for practical applications, it is noted that it is difficult to predict polymer
lifetimes in practice because light intensities vary, as do temperatures and a host of other
mechanistic variables that control degradation rates and degradation onsets. Second, the
onset of degradation should be tunable. Photodegradable polymers have different
applications and each application will generally require different polymer lifetimes.
Methods must be found for manipulating polymer lifetimes. Third, the polymer should
degrade completely and quickly once degradation starts. This characteristic is important
3for practical reasons because most polymer mechanical properties are related to
molecular weight. Small amounts of degradation can drastically decrease the molecular
weight (and thus mechanical properties) of a plastic, yet to all appearances the plastic
piece is visually unchanged. In essence, the plastic is still present but it is not structurally
sound and hence useless and perhaps dangerous. Under such circumstances, it may as
well be completely degraded. Photochemically reactive polymers are studied in an effort
to understand and control the life cycle of materials. This includes not only polymers that
are engineered to degrade but also those that are meant to last indefinitely in the case of
structural plastics.
100
o
....
...................~.::;;~ .
............
.......... '- ideal
.'
..•.....
tunablility
irradiation time
Figure 1. The properties of both a typical polymer and an ideal photochemically
degradable polymer demonstrating tunable onset of degradation and rapid degradation.
Controlling the degradation of a material in a prescribed fashion, so that the rate
and onset of degradation is both tunable and predictable for the particular application, is
of immense interest. In order to achieve this goal, it is necessary to understand how
environmental parameters and specific polYmer properties affect photodegradation. For
4example, it is important to understand the effect of temperature because of the range of
temperatures plastic materials experience in their diverse applications. Specifically, it is
known that many polyolefins as well as polyethers, polyketones, and polysulphones have
accelerated degradation rates at elevated irradiation temperatures. 11 -16 Temperature
dependence also has extensive implications in service lifetime predictions where
accelerated rate testing is the industry standard. Temperatures employed in these types of
tests are typically greater than the intended application conditions because it is often
impractical to perform real time testing on plastics whose expected use is over several
decades. In these cases and in general, polymer thermal transitions must be taken into
consideration in elucidating the effects of temperature on degradation rates. Specifically,
the rates of polymer photodegradation above and below the polymer glass transition have
been shown to vary significantly. 12, 17-19
There are several challenging experimental problems that hinder the rigorous
mechanistic exploration of polymer photodegradation. One of the primary difficulties is
that polymer degradation is mechanistically complicated. This is not to say that the
mechanisms are not understood; in fact, they are understood in detail. Rather, the
mechanisms are intricate, often involving multiple steps, cross-linking, and side-
reactions; this makes pinpointing the effects of various experimental parameters difficult.
This chapter will discuss methods for designing photodegradable polymers and exploring
their corresponding degradation mechanisms. In addition, several factors that influence
photodegradation rates as well as specific models that describe the reaction kinetics will
also be explored.
51.2 Photodegradable Polymer Design
Photodegradable polymers are typically designed for commercial use in one of
two ways: 1) by incorporating a photosensitive chromophore into in the backbone of the
polymer chain or 2) by physically combining 'non-degradable' polymers with small
molecule photosensitizers that absorb light and initiate photochemical degradation
reactions with the polymer?O, 21 The first strategy is accomplished primarily by
incorporation of a carbonyl group into the backbone of the polymer?2 The most
commercially successful polymer of this type is E-CO, which was generated by the
copolymerization of ethylene and carbon monoxide in the presence of a catalyst to
produce a polyketone.23 This polymer has been used since the 1970's to make the Hi-
Cone™ six-pack rings following environmental concerns and is still used today. The
photochemistry of these types of polymers were first studied by Guillet and Hartley and
their findings made the advancement of these materials a commercial possibility?4,25
They discovered that the primary mode of photodegradation of polymers containing
carbonyls as the chromophore occurs by the Norrish type reactions upon irradiation with
UV light as illustrated in Scheme 1. The primary experimental evidence for support of
such a mechanism was the formation of terminal alkenes. It is also worth mentioning that
the Norrish type II reaction is also a significant contributor to the degradation of
polyesters and polyacrylates owing the presence of the carbonyl group. I, 25
6a.
o hv 0~~~.+.~
b.
Scheme 1. Mechanism of the Norrish type I (a) and II (b) reactions in a ketone-
containing polymer.
The second design for making photodegradable polymers is to add small molecule
radical initiators to otherwise light stable carbon-based polymers. Some common
initiators include peroxides, metal oxides (e.g. Ti~, CuO, ZnO), metal chlorides (e.g.
cobalt(II), lithium, iron(III), nickel(II), copper(II), M(acac)n and M(stearate)n complexes,
benzophenone, and quinines.23, 26-30 These systems degrade by a complex system of
reactions that starts when the photosensitive additive absorbs a photon and begins the
autooxidation process shown in Scheme 2. This results in the formation of peroxide and
organic radical species. (It is interesting to note that the autooxidation process is
autocatalytic because another carbon radical is generated during the hydrogen abstraction
step). Organic peroxide species generated in the autooxidation mechanism can lead to
polymer main chain cleavage via two routes: direct cleavage by ~-scissionor through a
more circuitous route by formation of carbonyl carbons in the backbone and subsequent
Norrish type reactions. Some photoreactions ofperoxides are shown in Scheme 3.
7Initiation
Initiator
Propogatioo
R· + O2 .. ROO·
ROO· + R-H .. ROOH + R·
Tennioatioo
Radical coupling or diproportionation reactions
Scheme 2. The autooxidation process, where R is some carbon species.
~+
hv
~
OH
~
I~
·OHO·
~
------.... OH~+·OH
o
~~ + .~ + ·OH
Scheme 3. Examples of organic peroxide reactions.
8Other examples of the two strategies discussed above are noteworthy for both
their relevance to inorganic and organometallic containing polymers genre and for their
innovative attributes. For example, a novel synthetic strategy is currently being explored
to covalently link commonly used inorganic photosensitizers to polymer pendent groups
using sol-gel synthesis techniques (Figure 2 polymers 1 and 2).31,32 Polymers containing
pendant radical initiators have the advantage of discouraging large inorganic aggregates
in the solid-state. Polysilanes (Figure 2 polymers 2 and 3) are another class of
photodegradable polymers which are primarily developed for photolithography
applications, given their transparent nature and photoconductivity.32,33 Recent progress
in this area has been made toward elucidating the mechanism of photodegradation. It
was shown, for the case of polyurea containing silane units, that the photodegradation
pathway involves a single-electron transfer between the silyl and carbonyl groups, silyl
.. d I I . 33group mIgratIOn, an so vo YS1S.
9r s> H H s> s> H H s>~~-R'-~-C-N-R-N-cM~CH2SiCH2~-C-N-R-N-C mPh Ph Ph Ph 0
3
Figure 2. Example of an inorganic photosensitizer attached to a polymer chain through
the pendant groups.
Another method for generating photodegradable polymers that utilizes the first
strategy of building in photosensitive groups into the backbone is the incorporation of
organometallic dimer complexes into the backbone of the polymer chain. lO A general
synthetic route for incorporating metal-metal bonds into polymer backbones is based on
the step polymerization techniques used to integrate ferrocene into polymer backbones?4-
36 This was accomplished by functionalization of the cyclopentadienylligands creating
the necessary difunctional monomer unit for step polymerization and reacting the
ferrocene monomer with an appropriate difunctional organic molecule (Scheme 4). An
analogous strategy for synthesizing metal-metal bond containing polymers also utilizes
the functionalization of cyclopentadienylligands. A sample polymerization reaction is
shown in Scheme 5, which illustrates the reaction of a metal-metal bonded 'diol' with
10
hexamethylene diisocyanate (HMDI) to form polyurethane. Other synthetic strategies
also have been utilized to incorporate metal dimers into polymer backbones including
(Figure 3): 1) the chain copolymerization of styrene and an iron dimer with vinyl
cyclopentadiene ligands, 5, 2) acyclic diene metathesis polymerization of a molybdenum
dimer with a,ffi-alkene substituents, 6, and 3) cross-linking of a functionalized
prepolymer with a disubstituted molybdenum dimer species, 7. 10,37-40
n
o 0
II II
C1-C-R-C-C1
•
~CH2CH20H
I
Fe
HOCH2CH2--.b
Scheme 4. Synthesis of a ferrocene-containing polymer using step polymerization
techniques.
cat.
Scheme 5. Example synthesis of a metal-metal bond-containing polymer using step
polymerization techniques.
511
6
~ C~O
O· cco
\ '-~OC-Mo-Mo-CO
oC~t~o
a CI CI
7
a CI
Figure 3. Examples of polymers with transition metal dimers in the backbone of the
polymer.
Polymers that contain metal-metal dimers in the backbone of the polymer chain,
for example polymers 4-7, will photodegrade in the presence of visible light (Scheme 6).
This occurs because of the photoreactivity of the metal-metal bond which homolyizes
upon irradiation resulting in the generation of two metal radical species.41 , 42 In the
absence of a radical trap, these two metal radicals will recombine for no net reaction.
However, in the presence of a radical trap, for example oxygen or chlorine atom, the
photogenerated metal radical can be trapped before recombination occurs. This results in
an overall reduction in polymer molecular weight.
metal oxides
~
CI-MLn""""LnM-IVILnuvvv
12
(MLn = CpMo(COh, CpW(COh, Mn(CO)5' Re(CO)5'
or CpFe(COh and Cp = 'll5-CsH4
Scheme 6. Photochemical degradation of a generic polymer with metal-metal bonds in
the backbone.
A more complete list of photodegradable polymers and polymer photodegradation
reactions can be found in the many monographs that exist in the literature.26, 43-47
1.3 Factors Affecting Degradation Rates
1.3.1 Effect ofRadical Trap Concentration
The radical trap concentration has been shown to influence the degradation rate of
polymers both in the solid-state and in solution. In the case of the autooxidative
degradation pathway of carbon-based polymers shown in Scheme 2, the carbon free-
radical species that is formed in the initiation step must be trapped by an oxygen
molecule for degradation to proceed. (Note, it is true that the carbon free radical may
abstract a hydrogen atom in lieu of reacting with oxygen but that would result in no net
reaction). In the solid-state, oxygen diffusion is often the rate limiting step in the
autooxidative degradation of polymers. This is contingent on the sample thickness,
morphology, and permeability of the polymer toward oxygen).48,49 For example, by
sampling the molecular weight of a photochemically degraded polystyrene sample as a
13
function of depth, it was found that there was variability in the degradation efficiency that
could only be explained by depletion in oxygen at greater sample depths.48, so For this
polymer, samples with thicknesses greater than 0.1 mm will experience rate limiting
oxygen diffusion.
Radical cage effects are of particular importance to the radical trapping reaction in
polymer photodegradation (see Scheme 7 for a simplified example of a cage reaction.) It
is known that when a bond is homolyzed in the solid or liquid phase, the two radicals that
are formed (termed geminate radicals) exist inside a solvent cage,SI-S3 It is the ability of
this geminate radical pair to escape the solvent cage that determines whether the two
radicals will either recombine for no net reaction or escape the radical cage and become
free radicals. When free radicals form, they will either react with another species (a
radical trapping molecule) or react with another non-geminate radical for no net reaction.
Because of the cage effect, the photodegradation efficiencies of photosensitive molecules
can be manipulated by changing the concentration of the radical trapping agent.S4 For
example, by adjusting the concentration ofCCl4 (radical trapping agent) in the photolysis
reaction of (CSHS)2M02(CO)6, the quantum yield can be varied from zero (no radical trap
present) to 0.6 (> 0.1 M CCl4 in hexanes). Radical trap concentration affects the
probability of non-geminate radical recombination; at high concentrations of trap, any
radical that escapes the cage is trapped.
14
R-R hv [R·,·R] kcJ 2 R· T = trap 2 R-T
-
~
t
kc cage pair free radicals I<t
kr
Scheme 7. Homolysis of a bond and subsequent trapping reaction depicting the radical
cage intermediate where kc is for the geminate recombination, kd is for the diffusion from
the radical cage, kr is for non-geminate recombination, and kt is for the radical trapping
step.
1.3.2 Effect ofTemperature
The effect of temperature on the thermal degradation rates of polymer has been
extensively explored because of the need to predict the service lifetime of consumer
plastics. However, only a few studies have probed the effect of temperature on the
photochemical degradation rates of polymers. These studies are rather inconclusive with
respect to a general model for predicting the dependence of photodegradation rates on
temperature. Several studies found that the results are consistent with an Arrhenius
relationship, but other studies have found non-Arrhenius behavior. II , 55-58 There is some
indication that the non-Arrhenius behavior is due to the complex degradation pathways
referred to in the proceeding sections. For this reason, interpreting Arrhenius plots of
photochemical reactions can be non-trivial. Balzani notes that the relationship between
the temperature and the activation parameters in a photochemical reaction is a complex
one, and the "apparent activation energies" obtained should be regarded with care.59
Another hypothesis regarding non-Arrhenius is that the behavior is caused by
changes in the molecular mobility of the polymer chains. Several groups have observed
non-linearity in Arrhenius plots that coincide with the polymer's thermal transitions, most
15
notably, the glass transition. Is, 17, 18 One stark example is the photodegradation of
poly(vinyl ketone) (PVK). In that study, Guillet found that the quantum yields of
degradation below the glass transition temperature, Tg, increased gradually with
increasing temperature (A<I> = 0.04 for T = 20-100 dc. To explain this behavior, Guillet
noted that PVK degrades by a Norrish type II mechanism, which proceeds via formation
of a six-membered ring intermediate (Scheme 8). He proposed that, because the
degradation pathway required substantial rearrangement of the polymer chain to form the
six-membered ring intermediate, the slight increase in quantum yields with increase
temperature below Tg was a reflection of the increase in free-volume of the solid-state
polymer. A larger free-volume allows for easier rearrangement of the chain to the six-
membered ring intermediate. It is interesting to note that at Tg, Guillet observed a
dramatic increase in the quantum yield to a value similar to that in solution (<I>PVK =
0.24). The sudden increase in <I> at the glass transition temperature was attributed to the
ability of the polymer chains to easily form the intermediate because of facile chain
movement above Tg. These findings can be summarized by stating that increasing the
photoreaction temperature can have several effects (both direct and indirect) on the rate
of polymer degradation, notably increases in kinetic energy, free-volume, and molecular
mobility.
H-b~
\
o CH2\\ I
C-CH/ 2
, ,
hv He
I • \
::::;r====~), 0 CH2\. IH-abstraction C-CH2/
Norrish
type II
»
16
Scheme 8. Mechanism for the photochemical Norrish type II reaction showing the
formation of the 6-membered ring intermediate.
1.3.3 Effect ofStress
An interesting outcome of polymer degradation studies is the discovery that
tensile and shear stress can accelerate the rate of photochemical degradation. For
example, recent studies of this phenomenon have shown that tensile stress will accelerate
the photodegradation of numerous polyolefins as well as polycarbonates, nylon, and
acrylic-melamine coatings.60-73 Currently, there are three prevailing theories that attempt
to explain this phenomenon: the Plotnikov, the Decreased Radical Recombination, and
the Zhurkov Hypotheses (Figure 4). Each of the hypotheses refer to the different rate
constants in Scheme 9 to explain the occurrence of increased degradation rates with
applied tensile stress. The overall rate, or quantum efficiency, of the photoreaction is
defined by eq 1,
ktrap[RX]
<I> =--~----
khOmOIYSiS + ktrap[RX]
where RX is the concentration of radical trapping species and kl and kr are the rate
constants for recombination and trapping steps, respectively.
(1)
a.
stress
b.
stress
c.
stress
17
Figure 4. Graphical representation of the three hypotheses for the effect of tensile stress
on the quantum efficiency of polymer photodegradation as postulated by the: a.
P10tnikov, b. Decreased Radical Recombination Efficiency, and c. Zhurkov hypotheses.
khomolysis
~==~...~ 2 oJ\fVV\.
k:combination
2 RX
---~~~ 2 .J\.IV\. X + 2 R •
ktrap
Scheme 9. Simplified polymer photodegradation mechanism showing bond homolysis.
P10tnikov Hypothesis
P10tnikov theorized that tensile stress affects the activation barrier of the excited state
of the individual bond involved in homolysis.74 The photophysical phenomenon can be
represented by a linear perturbation of the Lennard-Jones potential as shown in Figure 5,
which compares an unstressed bond to a stressed bond in a photochemical reaction. The
mathematical perturbation results in a lower activation barrier for bond homolysis in the
case of a stressed bond. The predicted consequence is that the lowered activation energy
for homolysis would increase the quantum yield for disappearance of the starting material
as shown in Figure 4a. There is a mathematical limitation on the extent of perturbation,
which causes the increasing quantum yield to level off at high stresses. Another
consequence of the perturbation is that it results in the potential well taking on a negative
18
slope (not shown in Figure 5) as the atom distance approaches infinity, which suggests
that the photogenerated radicals become more thermodynamically stable at infinite
separation. The negative slope is a mathematical artifact and is not meant to imply
thermodynamic stability at infinite distance.
E
Unstressed Bond
bond length
E
Stressed Bond
bond length
Figure 5. The photophysical description of the Plotnikov hypothesis.
Decreased Radical Recombination Efficiency Hypothesis
The Decreased Radical Recombination Efficiency hypothesis postulates that the
degradation efficiency is affected by the ability of the geminate radical to recombine?S-78
This hypothesis is shown in Figure 6 as a series of conditions under which radical
recombination can occur. The fIrst condition is at zero or minimal stress levels where an
amorphous polymer has an intrinsic quantum yield. When tensile stress is applied to a
polymer, the chains, which are primarily in a random coil state, become elongated and
19
straightened. In this case, the radicals generated during bond homolysis recoil to resume
the more entropically favored random coil. This results in an increase in radical distance,
and increase in the likelihood of radical trapping and consequently an increase in
degradation efficiency. An increase in applied tensile stress also has other consequences
that predict a deviation from a continued increase in degradation efficiency with
increased stress. It is well known, and often utilized in plastic production, that tensile
stress can impart a high degree of polymer strand orientation and induce the formation of
crystalline regions in some polymer systems. Bonds homolyzed in these regions of high
thermodynamic stability are unlikely to recoil into a random coil configuration. This
would result in an enhanced rate of radical recombination and a general decrease in
degradation efficiency. The qualitative illustration of the effect of stress on the quantum
yield is shown in Figure 4b.
no stress
(amorphous)
applied stress
(amorphous)
applied stress
(crystalline)
;;=: ::::> ..
:::::::::>'-'"'""c:::=::::: =
-------1~hV
;=: ::::> -------.~
:::::::::>~
-------
Figure 6. An illustration of the Decreased Radical Recombination Efficiency hypothesis
regarding the effect of tensile stress on polymer photodegradation.
20
Zhurkov Hypothesis
The Zhurkov hypothesis is an extension of the research performed to investigate the
thermal degradation of polymers under tensile stress and is empirical in nature.79 It was
observed that the degradation efficiency increased as a function of stress in an
exponential fashion. This can be quantitatively described by eq 2, where't is the lifetime
of a polymer under stress (0) at a given temperature (T), 'to is the period ofthermal
oscillation of the bond, y is a 'structural coefficient which defines the actual loads in a
stressed body', and Uois the 'apparent' activation energy for bond scission (the
'apparent' activation energy is a composite of khOmolysis, krecombination, and ~rapping)' It is
hypothesized that the same trend would hold true for photochemical degradation of a
polymer under stress and have the same general equation:74
(2)
The qualitative illustration of the effect of stress on the quantum yield is shown in Figure
4c.
Evidence Supporting the DRRE Hypothesis
There is one prevailing set of experiments that tested the hypotheses listed above.
The work was performed by evaluating the quantum efficiency of a model
photodegradable polymer (PVC·Mo is shown in Figure 7) as a function of stress and
comparing the results to the possible outcomes illustrated in Figure 4.80 The model
polymer system does not degrade by the typical photooxidative mechanism but rather by
a simplified mechanism shown in Scheme 10. Using this system, the degradation
21
efficiency was measured as a function of tensile stress and found to follow the Decreased
Radical Recombination Efficiency hypothesis.
CI ~ CI <51 CIa aa~ac,. ' I ~caac~MO-Mr"ca~a
a a
CI CI CI 't CI
Figure 7. Photodegradable polymer (PVC-Mo) made by crosslinking po1y(viny1
chloride) with a metal-metal bond chromophore.
~~
(COhMo-Mo(COh ~ 2
~~
~~
\(COhMo.
2~
Scheme 10. Degradation pathway of PVC-Mo used in studying the effect of stress on
photochemical quantum yields.
There are a number of factors not discussed in this section that can affect the
photodegradation efficiencies of polymers. A brief list includes light intensity,
chromophore concentration, irradiation wavelength, molecular weight, tacticity, moisture,
etc. Monographs on these subjects can be found in other references.45.81-83
1.4 Kinetic Models of Polymer Photodegradation
To devise a photodegradable polymer with a tunable onset of degradation and
with a specific degradation rate, it is necessary to identify the experimental parameters
22
that affect degradation rates and to detennine how those parameters affect the
degradation mechanism. A typical first step toward this goal is to detennine the kinetics
of the degradation reaction. An interesting observation from kinetic studies of the
photodegradation rates of solid-state polymers is that the plots of degradation versus
irradiation time are often biphasic, showing a relatively fast rate during the initial period
of irradiation but a slower rate at longer times.33 ,44 Several hypotheses have been
proposed to explain this observation but no definitive explanation has come forth.
One hypothesis is that the photodegradation reaction is a simple number-ordered
reaction: zeroth, first, or second order. Owing to the fact that the plots are not linear, a
simple zeroth-order reaction, which is typical for many photochemical reactions, is not
feasible in the case of a biphasic plot. First-order reactions however do yield biphasic
plots and have been appropriately fit to the data. For example, photooxidative
degradation in the presence of excess oxygen has been shown in several cases to exhibit
first-order kinetics.84-87 In all cases, which include polyethers, conjugated polymers, and
poly(vinyl chloride), the main chain scission results from fonnation of a carboxyl carbon,
usually an ester linkage, which is subsequently involved in ~-scission. (A more detailed
description of the fonnation of carbonyl groups in polymers is given in the preceding
section and in the associated references.) Second-order kinetics in polymer
photodegradation is less prevalent in the literature. To site one example, second-order
kinetics were observed for the photodegradation of neoprene in the presence of FeCl3 .88
However, no definitive explanation was offered as to the origin of this behavior.
23
Other hypotheses used to explain the biphasic behavior exhibited in polymer
photodegradation attribute the unique reactivity to solid-state diffusion kinetics. A
possible explanation is that the reaction rate is not solely determined by a specific
chemical transformation but rather by diffusion processes. One representation of
diffusion behavior is simple-diffusion.76 Simple-diffusion models are often used to
describe diffusion processes involving metal-metal interfaces but they have also been
employed in polymeric systems. For example, simple-diffusion kinetics were used to
describe the degradation of solid-state polyolefins where the decay of photogenerated
radicals were hypothesized to be controlled either by oxygen diffusion into the polymer
material or by radical diffusion via hydrogen abstraction to the crystalline surfaces where
radical-trapping oxygen was present.89 Another diffusion model to describe the biphasic
kinetic behavior of polymer photodegradation was developed by Waite. His hypothesis
was a modification of the simple-diffusion model called the diffusion-controlled mode1.90-
92 This model describes bimolecular reactions in the solid-state or in highly viscous
solutions where both diffusion and spatial distribution of the reactive species are taken
into account. The diffusion-controlled model has been used to fit the radical decay
kinetics of several photodegradation reactions of polyolefins and polyethers as monitored
by electron spin resonance spectroscopy.13,14, 76, 93
1.5 Dissertation Outline
This dissertation is composed of advances made on the front of understanding
polymer photodegradation using unique polymer models that contain the chromophore
(-CH2CsH4(CO)3Mo-Mo(CO)3CsH4CH2-)' Chapter II describes the reactivity of these
24
model polymers in the solid-state in the presence and absence of oxygen and identifies
the photochemical products. Chapter III discusses the reactivity of these polymers in the
presence of various radical trapping agents and provides insights into the relationship
between degradation efficiencies and both the radical trapping concentration and glass
transition temperature. Chapters IV and V describe the effect of temperature on two
different polymer systems and the effect that secondary thermal transitions have on
polymer degradation below the glass transition temperature. Further findings on the
relationship between tensile stress and photodegradation efficiencies are discussed in
Chapter VI. Finally, an investigation into the biphasic nature of the kinetic traces of
polymer degradation as a function of irradiation time is presented in Chapter VII.
Included in the appendix are the synthetic details for the polymer films and solutions
employed and the experimental instrumentation for the photochemical studies.
1.6 Acknowledgments
This dissertation contains my co-authored material. Chapter II contains co-
authored material with R. Chen, 1. Meloy, and D. R. Tyler. The manuscript was primarily
written by R. Chen with myself and J. Meloy as contributors. My specific contribution was
the oxygen free experimentation and corresponding write-up as well as editorial
responsibilities. The experiments were performed by myself, R. Chen, and J. Meloy. Tyler
provided editorial assistance. Chapter III contains co-authored material with D. R. Tyler.
The experiments were performed by myself. D. R. Tyler and B. C. Daglen being the co-
contributors to the writing. Chapter IV contains co-authored material with J. Harris and D.
R. Tyler. The solid-state experiments were performed by myself and the solution data was
collected with the assistance of Harris. The manuscript was prepared by myself with
25
editorial assistance provided by D. R. Tyler. Authorship of chapters V, VI, VII, and VIII are
shared with D. R. Tyler who acted as my advisor. The experiments were performed by
myself. Appendix I was co-authored with J. Harris, C. Dax and D. R. Tyler. The
experiments were performed by myself and J. Harris. C. Dax was critical in the design and
assembly of the instrumentation. D. R. Tyler provided editorial assistance.
26
CHAPTER II
PHOTOCHEMICALLY REACTIVE POLYMERS. IDENTIFICATION OF THE
PRODUCTS FORMED IN THE PHOTOCHEMICAL DEGRADATION OF
POLYURETHANES THAT CONTAIN (CsH4R)(CO)3Mo-Mo(CO)3(CsH4R) UNITS
ALONG THEIR BACKBONES
Chen, R.; Meloy, J.; Daglen, B. c.; Tyler, D. R. Organometallics 2005,24, 1495-1500.
2.1 Introduction
Our synthesis of metal-metal bond containing polymer uses metal-metal bonded
organometallic dimers with functionally substituted cyclopentadienylligands, a synthetic
method borrowed from earlier work on polyferrocenes. 1-s An example of a polyurethane
synthesis is shown in Scheme 1.6 Thus far, polyurethanes, polyureas, polyethers,
polyvinyls, polyamides, and several types of copolymers with metal-metal bonds along
the backbone have been synthesized.7 As predicted, all of these polymers are
photodegradable, both in solution and in the solid state, provided that a radical trap is
present to capture the metal radicals and prevent reformation of the metal-metal bond by
radical-radical recombination. In solution, organic halides such as CC14 are excellent
traps, while ambient oxygen serves that purpose in the solid-state reactions.
27
OCN(CH2)6NCO
•
DBTA
p-dioxane
n
Scheme 1. Example of a polyurethane synthesis used to incorporate molybdenum dimers
into the backbone ofa polymer chain. (DBTA = dibutyltin diacetate)
In the case of the CCl4 radical traps in solution, the organometallic portion of the
product that forms is (11 s-CsH4R)Mo(CO)3CI (Scheme 2).7 Previously, the product
formed when oxygen was the radical trap had not been identified. Discussed in this
chapter are the results of experiments designed to identify the metal product that forms
when oxygen is the radical trap. Also described is the synthesis of a new polymer that is
capable of degrading in the absence of oxygen (or any other exogenous trap) because it
has CI atoms "built-in" along the polymer backbone.
28
n
hv jeel,
el(eOhMO-~-eH2eH20~NHeH2NH~oeH2eH2-o-MO(eOhel
Scheme 2. Example photodegradation reaction of polyurethane containing molybdenum
dimers with a halide radical trap in solution.
This work is published in Volume 24 ofthejoumal Organometallics in 2005. The
work on the photodegradation in the presence of oxygen was performed by R. Chen and
1. Meloy, with the anaerobic work being performed by myself. D. R. Tyler and R. Chen
prepared the manuscript with myself providing editorial assistance as well as the written
experimental portion of the manuscript that coincided with my work.
2.2 Results and Discussion
2.2.1 Syntheses and Photochemical Reactions of Polymers Hypol-Mo and Hypol-
PEG-Mo
The polyurethane Hypol-Mo was prepared by reacting (115-
C5H4CH2CH20H)2Mo2(CO)6 with Hypol 2000 in THF solution at 50 DC (Scheme 3). As
mentioned in the Experimental Section, Hypol 2000 is a toluene diisocyanate endcapped,
lightly branched poly(ethylene glycol). The structure was analyzed and reported in a
prior paper.8 (Note the branching feature of the prepolymer structure is not shown in
Scheme 3.)
29
Scheme 3. Synthesis of the polyurethane Hypol-Mo.
Scheme 4. Synthesis of the polyurethane Hypol-PEG-Mo.
The polyurethane copolymer Hypol-PEG-Mo was prepared in a similar fashion,
except that PEG-I 000 was also present (Scheme 4). Both polymers retain the deep red
30
polymers form films suitable for irradiation in the solid-state.6 Samples of such films
were prepared on glass microscope slides, and then separate samples were exposed to one
of four different conditions: (1) ambient light in air, (2) ambient light under a nitrogen
atmosphere, (3) air but no light, and (4) neither light nor air. Only the samples exposed
to both light and air degraded, as monitored by UV-vis spectroscopy. ("Ambient light" in
this instance refers to the normal light present in the laboratory from the overhead
fluorescent lights and from the windows but not from direct sunlight.) These results are
consistent with the results found for other polymers with metal-metal bonds along their
backbones and are interpreted as showing that both light and a radical trap are necessary
in order for degradation to occur; i.e., a net photochemical reaction only occurs if a
radical trapping agent is present (in this case, O2).7 In the absence of a trapping agent, it
is proposed that the geminate (Mo,Mo) radical cage pair recombines, leading to no net
reaction: LnMo-MoLn t+ (LnMo-,-MoLn).7
Thin films of the samples exposed to light and air showed interesting changes in
their physical appearance. After 3-5 hours of light exposure, numerous small bubbles
appeared in the polyurethane films (Figure 1). On further exposure to light, the bubbles
continued to grow, eventually merging with each other to form larger bubbles.
Ultimately, polymers turned into a viscous liquid and dripped from the (vertical) glass
microscope slide. (Note that both Hypol2000 and PEG-1000 are viscous liquids; thus, as
the Mo-Mo bonds are photochemically cleaved, the prepolymer units are effectively
regenerated, hence the formation of a liquid product.) As shown in Figure 1, the
polyurethane films turned from red to brown-green during the photochemical reactions.a
(a) (b) (c)
31
Figure 1. Photos of the photooxidative degradation of Hypol-PEG-Mo under ambient
light and atmospheric temperature and pressure. (a) unexposed sample; (b) sample
exposed to light for 5 h; (c) sample exposed for 3 days.
To gain more insight into what is happening during the photodegradation reaction,
the photochemical reaction ofHypol-Mo in air was monitored by infrared spectroscopy
using a film prepared on a NaCI salt plate. During the reaction, two new bands appeared
at 2170 and 2110 cm- I and increased in intensity as the bubbles grew in (Figure 2). These
bands agree well with those reported for gaseous CO (see Figure 2, insert), and it is
logical to suggest that the bubbles are formed by CO released from the Mo center during
the degradation reaction.b Consistent with this interpretation, the spectra show the
decrease in intensity ofthev(C=O) bands of the (-OCH2CH2CsH4)(CO)3Mo-
MO(CO)3(CsH4CH2CH20-) unit at 1900, 1947, and 2004 cm'l as irradiation progressed
(see Figure 2).9 Accompanying the decrease of these bands, new bands appeared at 1771,
1865, and 2046 cm'], but continued irradiation eventually caused these bonds to
disappear (Figure 2). The v(C=O) bands at 1771, 1856, and 2046 cm- I are tentatively
assigned to disproportionation products of the (Y]s-CSH4CH2CH20-)M02(CO)6 unit. This
32
assignment is based on the fact that, in solution, the ('YJs-CsHs)M02(CO)6 species readily
photochemically disproportionates in the presence of virtually any ligand (L) to form ('YJs-
CsHS)(CO)3Mo- and ('YJs-CsHS)(CO)3MoL+ products. The anionic product has bands at
1771 and 1890 em-I, while the cationic species has a band located in the 2025-2060 cm-1
region, depending on the identity ofL. lO In the photochemical reaction of the polymer,
the ligand L may possibly be an oxygen donor from the polyether segment or an oxygen
of a C=O group in the urethane linkage. The ionic disproportionation products are air-
sensitive, and the disappearance of these bands is consistent with the eventual reaction of
the ions with oxygen to form metal oxide final products that do not contain CO ligands.
33
18002400 2200 2000
Wavenumbers (cm-1)
2600
o
Carbon Monoxide
30- \ (gas phase) 1
0.8 0 ('t)
......
,.....
\ .~ / ,..... ,.....N N0.4
, \)~ 20 \ !iC \JI
n:s 0.0~
E 2300 2200 2100 2000
C/) Wavenumber (em·l )
C
n:s
L..
I- 10~
'#.
Figure 2. Infrared spectra showing the changes that occur when a film ofHypol-Mo on a
NaCl plate was exposed to ambient light. The intensity of the two bands at 2170 and
2113 cm-1 (attributed to CO(g)) appeared and grew in intensity, the v(C:=O) bands of the
(-OCH2CH2CsH4)(CO)3Mo-Mo(CO)3(CsH4CH2CH20-) unit at 1900, 1947, and 2004
cm-1 disappeared, and bands at 1771,1856, and 2046 cm-1 transiently appeared and then
disappeared with prolonged exposure. The inset shows the spectrum of CO gas for
reference.
2.2.2 X-ray Photoelectron Spectroscopic Investigation
To identify the fate of the Mo in the photochemical degradation reactions of the
polyurethanes, samples of polymers were analyzed by X-ray photoelectron spectroscopy
(XPS). XPS spectra were also obtained for (ils-CsHs)M02(CO)6 and Mo03 for
comparison. The Mo 3d core level presents two peaks, 3d3/2 and 3ds/2, due to spin-orbit
34
splitting, and accordingly the spectra were fit to two peaks. The binding energy
difference between 3d3/2 and 3ds/2 peaks (3.1 eV) and peak intensity ratios
(I(3dsn)/I(3d3/2) = 3/2) were kept constant for all of the fits. The Mo 3ds12 binding
energies and the corresponding content in the sample surface are listed in Table 1. (The
3ds/2 peak is listed in the table because it is the more intense of the two peaks.) The
values have been corrected for the effects of charge neutralization.
Table 1. Mo 3ds12 binding energies of different materials.a
Mo 3ds12 binding energies (eV)b
Material Mo(O) Mo(I) Mo(IV) Mo(V) Mo(VI)
Pure Mo (Mo(O» 227.8
Mo02(Mo(IV) 229.7
MoO) (Mo(VI» 232.7
(11 5-Cs~(CH2)20H)2Mo2(CO)6 (Mo(I» 228.9
Hypol-Mo (tirradiation = 0 min) 229.1
Hypol-Mo (tirradiation = 10 min) 229.1 231.5(74%) (26%)
Hypol-Mo (tirradiation = 120 min) 231.7 232.9(20%) (80%)
a C Is hydrocarbon = 285.0 eV. b The percentages listed under the binding energies are
the percentages of the total 3ds/2 peak area accounted for by that peak.
Note that the unexposed polymer Hypol-Mo has essentially the same binding
MO(CO)3(Cs1L(CH2CH20-) units with Mo in the +1 oxidation state. (The slight shift in
35
binding energies between the two samples may be attributed to a slightly different
chemical environment ofMo in the polymer compared to that in the solid microcrystals.)
After exposure to ambient light for 10 minutes, a sample of the polYmer showed 3ds/2
peaks at 229.1 (Mo(I) in the undegraded polymer) and at 231.5 eY. On the basis of
literature data, the oxidation state ofMo in the latter species is likely +5. 11 • 12 The ratio of
Mo(I) to Mo(V) is 74% to 26%. After further exposure to ambient light for 2 hours, the
XPS spectrum showed a major peak at 232.9 eV (assigned to a Mo(VI) species) and a
smaller peak at 231.7 eY. The latter peak is again assigned to undegraded Mo(I). The
ratio of Mo(VI) to Mo(V) was 80% to 20%.
2.2.3 Photochemical Reactivity ofthe Polyurethanes in Air
The key results in the sections above are (1) the Mo-Mo bond disappears in the
photochemical reaction of the polymers (as demonstrated by the disappearance of the
v(C=O) bands for the (-OCH2CH2CsH4)(CO)3Mo-Mo(CO)3(CsH4CH2CH2Q-) unit and
by the color change of the films), (2) CO is released in the photochemical reactions (as
demonstrated by the appearance of bubbles in the films and the presence of free CO in
the infrared spectrum), (3) no other Mo-carbonyl species is formed as a final product, (4)
02 is necessary to obtain a net photochemical reaction, (5) the ultimate fate of the Mo is a
Mo(VI) species, and (6) the fate of the organic portion of the polymers is low-molecular-
weight liquid, likely PEG units. Taken together, these results suggest the photochemical
reaction shown in Scheme 5 (shown for Hypol-Mo, but an analogous reaction applies for
Hypol-PEG-Mo).
36
Scheme 5. Photodegradation of Hypol-Mo under ambient conditions.
2.2.4 Photodegradation ofPU-70 Under a Nitrogen Atmosphere
A long-range goal is to develop ways to control the rate of degradation in
photodegradable polymers. One approach is to control the efficiency with which any
photogenerated metal radicals are captured. (The capture efficiency will affect the rate of
degradation, because the non-trapped radicals can back-react to reform the metal-metal
bond.) To control the rate of the radical trapping reaction, we are designing polymers
with radical traps built into the backbone. It is suggested that it should be possible to
control more easily the degradation of polymer with built-in radical traps, because the
concentration of the trap can be manipulated. In a prior paper, we reported the
photochemical degradation reactions of a PVC polymer modified with Mo-Mo bonds that
has an abundance of chlorine atoms to act as radical trapping agents (Figure 7 of Chapter
1).13 To explore the effect of a lower concentration of Cl, polymer PU-70 was
synthesized (Scheme 6).
CH2CIqNCOI +~
NCO
37
+ HO~OH +
HO~
\(COhMo-Mo(COh
~OH
iOH HO tII I I IIC-N-R'-N-C-O-R"-O n
Scheme 6. Synthesis of polymer PU-70.
The photochemical reaction of polymer PU-70 (Airradiation 546 nm) under a
nitrogen atmosphere was monitored by infrared and electronic absorption spectroscopy.
The latter method showed the disappearance of the absorption band at 512 nm, and
infrared spectroscopy showed the disappearance of the v(C=:O) bands in the
cm-1 and the appearance of new bands at 2048 and 1972 em-I, attributable to the
38
(-OCHzCHzCsH4)(CO)3Mo-Cl component.c The reaction is the same as in solution and
an example of the photochemistry is illustrated in Scheme 2.
The quantum yield for the reaction ofpolymer PU-70 was determined to be 0.35.
(The quantum yield, <1>, is defined as the rate of a photoreaction divided by the absorbed
light intensity; i.e., <I> = reaction rate/absorbed intensity.) Perhaps surprisingly, this
quantum yield is higher than the quantum yield (0.15) for the similar reaction for the
modified PVC polymer PVC-Mo, which has a considerably high concentration of C-Cl
bonds for metal-radical trapping. 13 These values can also be compared to the quantum
yield (0.36) of plasticized PVC, PPVC_MO.1l4 One possible explanation is the difference
in the glass transition temperature, Tg, values of the polymers. The irradiation of the
polymers was performed at 25 DC, which is below the Tg volume of polymer PVC-Mo
(65 DC) and above the Tg value of polymer PU-70 (-44 DC) and plasticized polymer
PPVC-Mo (20 DC). It is suggested that chain movement to achieve proper orientation for
radical capture and C-Cl concentration are both important factors in determining the
efficiency of photochemical degradation.
2.3 Key Insights and Conclusions
Films of polymers containing the (-OCHzCHzCs~)(CO)3Mo­
MO(CO)3(CsH4CHZCHzo-) unit along their backbones react photochemically with
visible light in the presence of oxygen to form Mo(V) and Mo(VI) oxides, CO(g), and
lower molecular weight organic-backbone polymer species. In the absence of metal-
radical traps (e.g., Oz), no net photochemical reaction occurs, because the photogenerated
Mo radicals recombine. PU-70 contains built-in radical traps (C-Cl bonds), and this
39
polymer efficiently degrades in the absence of oxygen, because the Mo radicals abstract
the Cl atoms to form Mo-Cl bonds, thus preventing the recombination reactions of Mo
radicals. Despite the relatively low concentration of Cl in polymer PU-70, it degrades
more efficiently at room temperature than polymers containing considerably higher
concentrations of Cl. It is proposed that, because Tg of polymer PU-70 is lower than the
reaction temperature, chain movement to achieve the proper orientation for the
abstraction reaction is relatively facile, resulting in a relatively efficient photochemical
reaction. In Chapter III, data is presented that provides greater insight into the role of
radical trapping concentration and the glass transition temperature.
2.4 Endnotes
a. Mixtures of Mo(V) and Mo(VI) oxides are frequently highly colored. See for
example: Siokou, A.; Leftheriotis, G.; Papaefthimiou, S.; Yianoulis, P. Surf. Sci.
2001,482-485 (Part 1),294.
b. NIST Mass Spec Data Center, S. E. Stein, director, "Infrared Spectra" in NIST
Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P. 1.
Linstrom and W. G. Mallard, March 2003, National Institute of Standard and
Technology: Gaithersburg, MD, 20899 (http://webbook.nist.gov).
c. The band at 1972 cm-1 was merged into the strong absorption from the parent
dimer.
40
CHAPTER III
FACTORS CONTROLLING THE RATE OF PHOTODEGRADATION IN
POLYMERS
Daglen, B. c.; Tyler, D. R. In Degradable Polymers and Materials; Khemani, K. c.;
Scholz, C., Eds.; ACS Symposium Series 939; American Chemical Society: Washington,
DC, 2006; pp 384-397.
3.1 Introduction
In order to predict polymer lifetimes, control when a polymer starts to degrade,
and control the rate of degradation, it is necessary to identify the experimental parameters
that affect polymer degradation rates and to understand how these parameters affect
degradation. Among the parameters that have been identified as affecting polymer
lifetime are temperature, exposure to ultraviolet radiation, light intensity and wavelength,
oxygen diffusion rates in the polymer, tensile stress, compressive stress, chromophore
concentration, molecular weight, humidity, and polymer morphology. 1-4 In this chapter,
we report how the glass transition temperature and the radical trap concentration also
affect polymer photodegradation rates.
This work was published in Volume 939 of the American Chemical Society
Symposium Series on Degradable Polymers and Materials in 2006. All the experiments
in this chapter were performed by myself. D. R. Tyler and myself were co-authors of the
manuscript.
41
3.2 Results and Discussion
3.2.1 Experimental Approach to the Problem
Several challenging experimental problems hinder the rigorous experimental
mechanistic exploration of polymer photodegradation. One of the difficulties is that
polymer degradation is mechanistically complicated.5 This is not to say that the
mechanisms are not understood; in fact, they are understood in detail.5 Rather, the
mechanisms are intricate, often involving multiple steps, cross-linking, and side-
reactions; this makes pinpointing the effects of stress difficult. Another complication is
that oxygen diffusion is the rate-limiting step in photooxidative degradations, the primary
degradation mechanism in most polymers.6, 7 This can add to the intricacy of the kinetics
analysis because cracks and fissures develop in the polymer as degradation proceeds;
these fractures provide pathways for direct contact of the polymer with oxygen, which
will then no longer degrade at a rate controlled by oxygen diffusion. To circumvent these
experimental and mechanistic complexities and therefore make it less difficult to interpret
data and obtain fundamental insights, we use three key experimental strategies in our
investigations. First, we study the problem using special photodegradable polymers of
our own design that contain metal-metal bonds along the backbone.8-13 These polymers
are photodegradable because the metal-metal bonds can be cleaved with visible light and
the resulting metal radicals captured with an appropriate radical trap, typically and
organic halide or molecular oxygen (the reaction is discussed in more detail in Chapter
II).
42
By studying these "model" systems, we are able to extract information without
the mechanistic complications inherent in the degradation mechanisms of organic
radicals. (For example, metal radicals do not lead to cross-linking, so we can avoid this
complicating feature found with organic radicals.) The second key experimental strategy
is to use polymers that have built-in radical traps, namely C-CI bonds. 14, IS By
eliminating the need for external oxygen to act as a trap, we exclude the complicating
kinetic features of rate-limiting oxygen diffusion. The third experimental strategy is to
use the distinctive M-M bond chromophore to spectroscopically monitor the
photodegradation reactions of the polymers. This allows us to compare the efficiencies
of the photodegradations by measuring the quantum yields of the reactions. (The
quantum yield, <I>, is defined as the rate of a photoreaction divided by the absorbed light
intensity; i.e., <I> = reaction rate/absorbed intensity.) The use of quantum yields to
quantify and compare the various degradation rates is a crucial advance because polymer
degradation reactions have typically been monitored by stress testing, molecular weight
measurements, or attenuated total reflection (ATR) spectroscopy, all of which can be
laborious and time consuming. 16 Relative to these techniques, quantum yield
measurements are straightforward. (Note that quantum yields in regular carbon-chain
polymers cannot be measured conveniently by UV-vis spectroscopy because there are
generally no suitable chromophores.) To further expedite our quantum yield
measurements, we use a computerized apparatus that automatically measures the
quantum yields on thin film polymer samplesY' 18
43
3.2.2 Polymer Synthetic Strategy
Our general synthetic route for incorporating metal-metal bonds into polymer
backbones is based on the step polymerization techniques for incorporating ferrocene into
polymer backbones. 19-21 Step polymers of ferrocene can be made by substituting the
cyclopentadienyl (Cp) rings with appropriate functional groups followed by reaction with
appropriate difunctional organic monomers (Scheme 1).22-24
n
a a
II II
CI-C-R-C-CI
..
~CH2CH20H
I
Fe
HOCH2CH2~
Scheme 1. Example of a step polymerization technique used to incorporate an
organometallic complex into a polymer backbone.
The analogous strategy for synthesizing metal-metal bond-containing polymers
also used difunctional, cyclopentadienyl-substituted metal dimers. A sample
polymerization reaction is shown in Scheme 2, which illustrates the reaction of a metal-
metal bonded "diol" with a hexamethylene diisocyanate (HMDI) to form a polyurethane.
This step polymerization strategy is quite general, and a number of metal-metal bond-
containing polymers have been made from monomers containing functionalized Cp
ligands.
44
~NCO
Y
NCO
jDBTAp-dioxane
Scheme 2. Example of a polyurethane synthesis used to incorporate molybdenum dimers
into the backbone of a polymer chain.
3.2.3 Synthesis ofthe PU-XX Polymers
Using the synthetic strategy in Schemes 1 and 2, the polymers for this study were
synthesized by the route shown in Scheme 3. Note that the amount of Cl-containing
aromatic diisocyanate was varied, which gave polymers with different glass transition
temperatures as well as polymers that have different radical trap to metal atom ratios. For
example, PU-90 has a Tg of 35 °c and a 9: 1 [C-CI]: [Mo] ratio, and PU-70 has a Tg of
-44 °c and a 7:1 [C-el]:[Mo] ratio. (The XX number in the PU-XX nomenclature
indicates the mole fraction of aromatic diisocyanate in the overall amount of diisocyanate
used in the formulation.)
PU·35
PU-50
PU·60
PU-70
PU-90
65%
50%
40%
30%
10%
CH2CI~NCO
Y
NCO
35%
50%
60%
70%
90%
45
HO~
\
+ (COhMo-Mo(COh +
~OH
5%
J
i~J-R'-~J-O-R'-Ot
HO~OH
95%
Scheme 3. Syntheses ofPU-XX polymers.
3.2.4 Photodegradation Occurs in the Absence of0 1
As expected, polymers containing C-Cl bonds photochemically degraded in the
absence of oxygen. Spectroscopic monitoring of the reactions showed the disappearance
of the CpzMOZ(CO)6 chromophore ("-max = 390 and 510 nm; v(CsO) 2009, 1952, and
1913 em-I) and the appearance ofthe CpMo(CO)3Cl unit (v(CsO) 1967 and 2048 cm-\
46
In addition, the number average molecular weight decreased steadily during the course of
the reaction. The reaction in Scheme 4 is suggested.
hv
Scheme 4. Photochemical reaction of the PU-XX polymers.
3.2.5 Effect ofTg on the Efficiency ofPhotodegradation
The quantum yields for PU-90 and PU-70 at 26°C are shown in Table 1. In
addition, the quantum yields for PVC-Mo and PVC-Mo plasticized with DOP (PPVC-
Mo) (20%) are provided.
47
Table 1. Quantum yields for various polymers with different glass transition
temperatures and for a model complex.
Polymer TgeC) Quantum Yield (<I» at 26 DC
PVC-Mo 75 0.15 ± 0.03
PU-90 35 0.05 ± 0.02
PU-70 -44 0.35 ± 0.04
PPVC-Mo «RT 0.35 ± 0.07
model complex 1 solution phase 0.43 ± 0.03
The quantum yields were measured at 26 DC. Note in the table that the quantum
yields are 0.35 for the two polymers with Tg < 26 DC, while the two polymers with Tg >
26 DC have considerably lower quantum yields.
The quantum yields for the polymers with Tg < 26 DC are close to the quantum
yields for model complexes (e.g., <I> = 0.43 for model complex 1 at 0.38 cP; Scheme 5.)
This result suggests that when the irradiation takes place above Tg, chain mobility is
facile enough that a radical trap (i.e., a C-Cl bond) is encountered before metal-radical-
metal-radical recombination occurs. In contrast, when irradiation takes place below Tg,
chain mobility is limited and metal-radical-metal-radical recombination occurs in many
instances before a metal-radical encounters a C-CI bond. The quantum yields above Tg
are similar to those in solution because, in solution, chain mobility is quite facile and
radical traps are readily encountered.
48
..
hexane
<P = 0.43
Scheme 5. Photodegradation of a model complex, 1, in solution with CCl4 as the radical
trapping agent.
3.2.6 Effect ofRadical Trap Concentration
The effect of radical trap concentration on the quantum yields ofpolymer
degradation was studied using the polymers PU-35, PU-50, PU-60, PU-70, and PU-90.
The results are shown in Table 2 and can be summarized as follows: IfTg < Tirradiation
then the quantum yield (<1» is independent of trap concentration. Thus, <1> = 0.35 for PU-
35, PU-50, PU-60, and PU-70. In contrast, ifTg > Tirradiation then the quantum yield is
dependent on trap concentration. Thus, <1> = 0.05 for PU-90 (Tg = 35 DC) under an N2
atmosphere but <1> = 0.10 for PU-90 under an O2 atmosphere. (The O2 is a radical trap so
the experiment performed under an Ch atmosphere has a larger concentration of radical
trap.) The explanation for these observations again lies in chain mobility. When the
irradiation takes place above Tg, the radical chain ends are relatively mobile, and the
reaction of the metal radicals with the radical traps is kinetically saturated with trap at the
concentrations of trap used in these experiments. In contrast, when irradiation occurs
below the glass transition temperature, because oflimited chain mobility, the reaction
49
kinetics are not saturated in trap concentration and the quantum yields depend on the
concentration of radical trap.
Table 2. Quantum yields for polymer degradation for polymers with different
concentrations of metal-radical trap.
Polymer (atmosphere) Tg (OC) Quantum Yield (4)>) at 26°C
PU-35 (N2) -47 0.35 ± 0.04
PU-50 (N2) -46 0.35 ± 0.01
PU-60 (N2) -46 0.35 ± 0.02
PU-70 (N2) -44 0.35 ± 0.04
PU-90 (N2) 35 0.05 ± 0.02
PU-90 (02) 35 0.10 ± 0.03
3.3 Summary and Conclusions
The photochemical reactivity ofpolymers is of considerable interest because
photodegradable plastics have a number ofapplications. Additional interest in polymer
photoreactivity stems from the need to limit and control "weathering" in polymer
materials. Photodegradation is an important component of polymer weathering, and a
proper understanding of degradation processes and of the experimental factors that affect
degradation is necessary for the accurate estimation of polymer lifetime and for the
development of stabilizing systems.
Among the experimental parameters that have been shown to affect polymer
degradation rates are light intensity, temperature, oxygen diffusion, chromophore
concentration, polymer morphology and stress. This study added two more experimental
50
parameters to this list, namely the glass transition temperature, Tg, and the concentration
of radical trap. To investigate the effects ofTg and trap concentration on degradation
efficiencies, specially designed polymers with metal-metal bonds along their backbones
were synthesized. These polymers degrade by a straightforward mechanism that makes it
possible to extract meaningful information. Using these polymers, it was shown that
when polymers are irradiated above their glass transition temperature their quantum
yields of degradation are similar to their quantum yields in solution. When irradiated
below their glass transition temperatures, the quantum yields of photodegradation are
small, i.e., the degradation reactions are inefficient. The explanation suggested is that
when the irradiation takes place above Tg, chain mobility is facile enough that a metal-
radical trap is encountered before radical-radical recombination occurs. In contrast, when
irradiation takes place below Tg, chain mobility is limited and metal-radical-metal-radical
recombination occurs in many instances before a metal-radical encounters a trap. The
quantum yields above Tg are similar to those in solution because, in solution, chain
mobility is facile and radical traps are easily encountered. Chain mobility also explains
the affect of radical trap concentration on the efficiency of photodegradation. When the
irradiation takes place above Tg then the radical chain ends are relatively mobile, and the
reaction of the metal radicals with radical traps is kinetically saturated with trap at the
concentrations of trap used in these experiments. In contrast, when irradiation occurs
below the glass transition temperature then, because of limited chain mobility, the
reaction kinetics are not saturated in trap concentration and the quantum yields are
dependent on the concentration of radical trap. In Chapter IV, the photodegradation
51
efficiencies of a polymer as a function of temperature are evaluated and compared to two
model systems.
52
CHAPTER IV
THE EFFECT OF TEMPERATURE ON THE PHOTODEGRADATION QUANTUM
YIELDS OF POLYMERS CONTAINING METAL-METAL BONDS IN THE
POLYMER CHAIN
Daglen, B. C.; Harris, J. D.; Tyler, D. R. J. Inorg. Organomet. Polym. Mat. 2007, 17,
267-274.
4.1 Introduction
The effect of temperature on the thermal degradation rates of polymers has been
extensively explored because ofthe need to predict the service lifetime of consumer
plastics. However, only a few studies have probed the effect oftemperature on the
photochemical degradation rates ofpolymers. These studies are rather inconclusive with
respect to a general model for predicting the dependence of photodegradation rates on
temperature. Several studies found that the results are consistent with an Arrhenius
relationship, but other studies have observed non-Arrhenius behavior.1-s There is some
indication that the non-Arrhenius behavior is due to the complex degradation pathways
referred to above, and therefore the relatively straightforward degradation process in
polymers with metal-metal bonds might provide some fundamental insights that are not
obtainable with standard carbon-chain polymers. Accordingly, we began a study to
investigate the effect of temperature on the degradation efficiencies of metal-metal bond
containing polymers. In this chapter we report data showing the effect of temperature on
the degradation efficiencies of a PVC polymer that has metal-metal bonds incorporated in
its chain.
53
This chapter was published in volume 17 in the Journal of Inorganic and
Organometallic Polymers and Materials in 2007. (With the exception of the experiments
in section 4.2.4 which were performed by myself subsequent to the publication.) The
solution studies were performed with the assistance of 1. Harris and the solid-state
experiments were performed by myself. D. Tyler and I were the co-authors of the
manuscript.
4.2 Results and Discussion
4.2.1 Polymer Preparation and Characterization
PVC-Mo was prepared by reacting acyl chloride-substituted poly(vinyl chloride)
with diol ('115-C5H4CH2CH20H)2M02(CO)6, as shown in Scheme 1. The polyurethane,
PU-70 was prepared by reacting two diisocyanates: a poly(propylene glycol) prepolymer
and radical trap containing monomer, and two diols: ('11 5-C5H4CH2CH20H)2M02(CO)6
and 1,4-butanediol. Previous work showed that thin films ofPVC-Mo are
photochemically reactive (A. = 532 or 546 nm) in the absence of oxygen.6 Infrared
spectroscopic monitoring of the polymer chemical reaction showed the disappearance of
the v(CsO) bands of the [Cp2M02(CO)6] unit at 2005, 1950, and 1970 cm-I and the
appearance of bands attributed to the [CpMo(CO)3CI] unit at 2048 and 1976 em-I. The
reaction is thus similar to the photochemical radical trapping reactions of the
Cp2M02(CO)6 dimer that takes place in solution in the presence of an alkyl halide, and an
analogous mechanism is proposed (Scheme 1).7
54
hv
Scheme 1. Synthesis and photodegradation of PVC-Mo.
4.2.2 Temperature Dependence on the Degradation ofpvc Polymer and Model
Systems
The temperature dependence of the quantum yields for the degradation of
polymer PVC-Mo could depend on: (1) the inherent temperature dependence of the
photolysis and radical trapping reaction of the [Cp2Mo2(CO)6] unit: (2) the temperature
dependent behavior of the polymer morphology, or (3) a temperature-dependent
dynamical property of the photogenerated radicals in the polymer. To differentiate
between these possibilities, two control experiments were carried out, namely the
photolysis of Cp'2Mo2(CO)6 dispersed in a PVC polymer matrix and the photolysis of
55
Cp'2M02(CO)6 in hexane/CCl4 solution. The quantum yields for the disappearance of the
Cp2M02(CO)6 unit in PVC-Mo, for Cp'2M02(CO)6 dispersed in PVC, and for
Cp'2M02(CO)6 in hexane/CCl4 solution are plotted versus temperature in Figure 1. Note
that all of the solid-state data were collected below the glass transition temperature of the
polymer films (Tg = 65-72 °C). The plots show that there is a significant increase in the
quantum yields for the polymer PVC-Mo with increasing temperature. In contrast, for
Cp'2M02(CO)6 dispersed in PVC and for Cp'2M02(CO)6 in hexane/CCl4 solution (in
which the Mo-Mo chromophores are unattached to the polymer chains) there are only
slight increases in the quantum yields over this temperature range. An immediate
conclusion is that the large increase in the quantum yields with temperature for PVC-Mo
is not attributable to an inherent temperature dependence of the photolysis and
subsequent radical trapping reaction of the Cp2M02(COk (Otherwise, the quantum
yields for Cp'2M02(CO)6 in the hexane/CCl4 solution would also show a sizeable
temperature dependence.) Also, because the quantum yields for Cp'2M02(CO)6 dispersed
in PVC show only a slight temperature dependence, the temperature dependence
observed for PVC-Mo cannot be ascribed solely to changes in PVC morphology.
(Otherwise, the Cp'2M02(CO)6 dispersed in PVC and PVC-Mo would show a similar
temperature dependence because the morphologies of PVC and PVC-Mo are similar in
regard to crystallinity, modulus (1300 ± 100 vs. 1200 ± 50 MPa), and glass transition
temperature (65 ± 4 vs. 72 ± 3°C).
56
1.4
•
•
•
• •
o Cp'2Mo2(CO)6 dispersed in PVC
1.2 .• PVC-Mo
1.0 f::, Cp'2Mo2(CO)6 in hexane/CCl4
0.8
<I> I__--e--.-JL----t;;~·~-----------j~l0.6 ~ •
0.4
0.2 .
0.0
10 20 30 40 50
Temperature (DC)
60
Figure 1. Plots of the quantum yields for the disappearance of the Cp2Mo2(CO)6 unit in
PVC-Mo, Cp'2Mo2(CO)6 dispersed in PVC, and Cp'2Mo2(CO)6 in hexane/CCl4.
As an aside, it is noted that the small increase in the quantum yield for
Cp'2Mo2(CO)6 in the hexane/CCl4 solution is attributed to the slight decrease in the
viscosity of the solution with the increase in temperature. It is well established that the
quantum yields for photolysis and subsequent trapping reactions of metal-metal bonded
dimers are viscosity dependent because of the cage effect, and the temperature
dependence of the quantum yields observed in this study is likely just an example of this
phenomenon.8 To check the validity of this statement, two experiments were performed:
1) observe the change in viscosity ofthe solution over the temperature range evaluated
and 2) observe the effect of viscosity on the quantum efficiency of this reaction using a
viscogen rather than variable temperature to change the viscosity. The results of these
two experiments are shown in Figure 2. Figure 2a shows the change in viscosity for the
Cp'2Mo2(CO)6 in the hexane/CCl4 solution over the temperature range of the
57
photochemical experiments. The viscosity ranged from about 1.6 cP at 25°C to 0.8 cP at
50°C. Solutions of Cp'2M02(CO)6 in the hexane/CCl4 were prepared with varying
amounts of squalene to represent the range of viscosities observed in the variable
temperature experiment illustrated in Figure 2a. The photochemical quantum yields for
those solutions with variable viscosities obtained at room temperature are shown in
Figure 2b. A crude linear relationship can be fit to both of these plots and an equation
extracted. (The fit is crude because the overall relationship between temperature and
viscosity and viscosity and quantum yield is not linear but a linear relationship can be
approximated over this small range.) These two equations, with viscosity as the common
variable, can be combined to solve for an equation that represents the change in quantum
yield as a function of temperature as predicted from changes in viscosity with
temperature. (Note again that the resulting equation is derived only for this particular
temperature range and is not applicable outside of it.) Shown in Figure 2c is the plot for
the experimentally determined quantum yield versus temperature for the Cp'2M02(CO)6
in the hexane/CCl4 solution and the predicted values from the calculated relationship.
The experimental and calculated values are similar and it is therefore concluded that the
temperature affects the viscosity of the solution and subsequently the ability of the metal
radicals to escape the radical cage.
58
a. 2.0
.-.. 1.6 x
~ x
~ x
;: 1.2
.... x
·Vl
o 0.8 x~
CIJ
.•
.. 0.4
0.0
0 20 40 60 80
T (0C)
b. 1.0
0.9 I %
0.8 :I: ~
«I> 0.7
0.6 )I(
0.5
0.4
0.0 0.5 1.0 1.5
viscosity (cP)
C. 0.8
0.7
•
«I> 0.6 ~ ~
a !!!
0.5 B
o experimental
• calculated
0.4
0 20 40 60 80
T (0C)
Figure 2. Illustrated are the relationships between: a) the viscosity of a solution of
Cp'zMOZ(CO)6 in hexane/CCl4 at variable temperature, b) the quantum efficiency of
Cp'ZM02(CO)6 photolysis in hexane/CCl4 at a constant temperature and variable viscosity,
and c) the quantum efficiency of Cp'zMOZ(CO)6 in a hexane/CCl4 solution at variable
temperature.
For the Cp'zMOZ(CO)6 dispersed in PVC, the small increase in quantum yields
with increasing temperate is attributed to an increase in the free-volume. 9 This
59
explanation is based on a suggestion by Guillet to explain a similarly small temperature
dependence in the quantum yields for degradation of poly(vinyl ketone), PVK. IO In that
study, Guillet found that the quantum yields of degradation below the glass transition
temperature, Tg, increased gradually with increasing temperature (~<I> = 0.04 for T = 20-
100°C. To explain this behavior, Guillet noted that PVK degrades by a Norrish type II
mechanism, which proceeds via formation of a six-membered ring intermediate. He
proposed that, because the degradation pathway required substantial rearrangement of the
polymer chain to form the six-membered ring intermediate, the slight increase in
quantum yields with increased temperature below Tg was a reflection of the increase in
free-volume of the solid-state polymer, which allows for easier rearrangement ofthe
chain to the six-membered ring intermediate. It is interesting to note that, at Tg, Guillet
observed a dramatic increase in the quantum yield to a value similar to that in solution
(<I>PVK = 0.24). The sudden increase in <I> at the glass transition temperature was
attributed to the ability of the polymer chains to easily form the intermediate because of
facile chain movement above Tg. (The experimental apparatus used in the present study
was not able to attain the temperatures required to study this phenomenon with PVC-
Mo.)
In contrast to PVK, the degradation of polymer PVC-Mo does not require the
polymer chains to extensively reorganize, and consequently some parameter other than
free-volume is likely responsible for the observed temperature dependence of the
quantum yield. In order to get better insight into what parameter is controlling the
temperature dependence of <I> in PVC-Mo, it is necessary to look in more detail at the
60
mechanism of Cp'2M02(CO)6 photolysis and the subsequent radical capture reaction.
(Recall from the discussion above that the increase in <I> with increasing temperature in
PVC-Mo cannot be attributed to either an intrinsic temperature-dependent reactivity
property of the Cp'2M02(CO)6 molecule or to changes in PVC morphology.) At 25.4 °C,
the quantum yields for disappearance of the Mo-Mo chromophore are as follows: PVC-
Mo, <I> = 0.20; Cp'2M02(CO)6 dispersed in PVC, <I> = 0.07; and Cp'2M02(CO)6 in the
hexane/CCl4 solution, <I> = 0.35. As expected, the quantum yield in solution is
considerably higher than that for Cp'2M02(CO)6 dispersed in PVC polymer because the
solution state is considerably less viscous that the solid state. Perhaps surprising,
however, is the much larger quantum yield for PVC-Mo (0.20) compared to
Cp'2M02(CO)6 dispersed in PVC polymer (0.07). The substantial difference in the two
quantum yields is proposed to be attributable to a difference in the radical-radical
recombination efficiencies (the "cage effect") in the two polymers. I I The cage effect is
illustrated in Scheme 2, which shows the elementary steps involved in the photochemical
generation of metal radicals and their subsequent capture reaction with a trapping
molecule.
~-MO(COh~(~O.'.MO(CO)' )~2~d 2~<G;Ji + 2~~. kc \ ~ k, \ \~VVVV" ~ (COhMoe (COhMo-CI
polymer caged radicals free radicals degraded polymer
Scheme 2. Reaction of a photoreactive species to form a caged radical pair followed by
escape of the radicals from the cage and subsequent radical trapping reaction.
61
4.2.3 Secondary Thermal Transitions Hypothesis
It is proposed that the temperature dependence ofPVC-Mo arose from the
temperature dependence of the kd step. Specifically, it is suggested that the polymer
segments to which the radicals are attached are conformationally stressed. There are two
possible modes for the newly formed radicals to relax and become separated: they can
rotate or recoil away from each other (Scheme 3). These secondary motions of the
polymer arise from the relaxation of unfavorable bond conformations that are formed
during the polymer casting process. The increased thermal energy facilitates the rotation
and recoil relaxation processes, which effectively increases the rate constant for diffusion
of the radicals out of the cage, kd. This leads to decreased radical-radical recombination
and consequently an increase in photodegradation efficiency.
hv o~~(COhMo••Mo(COh
~O
62
° °
kd radical separation by
secondary chain movements
(recoil or rotation)
O~
° (COhMo·
Scheme 3. Pathway for the increased separation efficiency of the radicals formed by
irradiation of PVC-Mo. A rotation process is shown but radical recoil will also lead to
increased radical-radical separation. Relaxation of the polymer chains leads to an
increase in kd and a subsequent increase in the quantum yield for degradation.
The quantum yield data for PVC-Mo in Figure 3 has an exponential dependence
on the inverse temperature, and it is therefore tempting to extract activation parameters
from the natural log plots of quantum yield vs. inverse temperature. Balzani, however,
has cautioned that the relationship between the temperature and activation parameters in a
photochemical reaction is a complex one, and the "apparent activation energies" thus
obtained must be interpreted with care. 12 With that disclaimer in mind, the activation
energy obtained from the In <I> vs. r l plot (Figure 3) is 14.1 ± 0.3 kcal mor l . This value
is typical for secondary relaxation chain movements in polymers (which generally fall in
the range of 5-20 kcal mor l ) and is consistent with the proposal that the temperature
63
dependence of <I> results from chain movements involved in recoil and rotation
processes. 13
.......
............. .
. ....
....
•
.... It..
................
....
3.353.30
i
------------ --------.-----1
.......
.... .....
....
.......
....
....
.....
- --.---
•
3.20 3.25
T (lOOO/K)
•
..................... .
-------.----------...! -- - - ----
..................... -!
•
..... -
•
3.15
•
.....
•
0.0
-0.5 i
-2.0 i
I
-2.5 t
I
-3.0 !
3.10
-& -1.0
.s
-1.5
Figure 3. Plot of the ln<l> versus r l for for PVC-Mo (shown as .) and PU-70 (shown as
+).
To further investigate this hypothesis, a second polymer system, PU-70,
containing the CpzMOZ(CO)6 subunit was evaluated as a function of temperature. It was
found that the quantum efficiency ofPU-70 also exhibited exponential dependence on
temperature. The activation energy obtained from the In <I> vs. r l plot (Figure 3) is 10 ±
I kcal mor l for PU-70. This data is also in line with the hypothesis that the temperature
dependence of <I> results from chain movements involved in recoil and rotation processes.
To further support this hypothesis, it was also necessary to compare the activation
barriers of the photochemical reaction obtained from the Arrhenius plots with the
64
activation energy for the molecular motions that are suspected to facilitate radical cage
escape for both polymer systems
4.2.4 Activation Energies ofPolymer Secondary Thermal Transitions
The activation energies for polymer molecular motions that may facilitate escape
of the photogenerated metal radical species from the radical cage were determined by
dynamic mechanical analysis. First, however, the relevant molecular motions occurring
during the temperature range of the degradation experiments were determined. The glass
transition temperature, Tg, and the secondary thermal transition, T~, for PVC-Mo and
PU-70 are shown in Table 1. The temperature range of data acquisition for the quantum
efficiency versus temperature was obtained below the polymer Tg and above T~, so there
were no thermal transitions that occurred during the data acquisition of the experiment.
The values obtained also indicate that, during the experiments, there was no cooperative
segmental motion of the polymer backbone associated with the onset of the glass
transition. However, since the experiments were performed above the p-transition, there
were smaller segmental motions, i.e. polymer backbone corkscrewing or pendant chain
rotation (Figure 4).
Table 1. Thermal transitions and activation parameters for PVC-Mo and PU-70.
photochemical molecular
reaction motion
Polymer Tg (OC)8 Til (OC)8 (kcal/mol)b (kcal/mol)8
PVC-Mo 65 ± 4 -120 ± 10 14.1 ± 0.3 14.7 ± 0.4
PU-70 55 ± 3 -40 ± 10 10 ± 1 8±1
a - values were determmed by dynamiC mechanical analysIs. b - values were determmed from the Arrhenius plots
of the photochemical quantum yields versus temperature.
65
•
•
•
•
•
•
•
•
•
•
b. ~ -----,
•
•
••
• • ••• •
•. .~ .~ ~
• "0---6' • •
•
•
a.
Figure 4. Two examples of molecular motion that result in secondary thermal transitions.
a - corkscrew rotation in the polymer backbone. b - polymer pendent group rotation.
The activation energies of the photochemical reactions are shown in Table 1 along
side the activation energies for the secondary thermal transitions for PVC-Mo and PU-
70. It can be seen that the energies of activation for the photochemical reaction are
equivalent within error to the energy of activation of the ~-transition for both polymers.
This evidence supports the hypothesis that polymer molecular motions, to which the
photogenerated radical species are a part of, facilitate the escape of the radicals from the
'solvent' cage thus decreasing the radical-radical recombination efficiency and
consequently increasing the photodegradation efficiency.
4.3 Conclusions
Control experiments show that the quantum yields for the photolysis and radical
capture reactions of Cp'2M02(CO)6 units in solution or dispersed in a polymer are
normally not very sensitive to temperature. What little temperature dependence there is
can be attributed to changes in viscosity of the solution or to small changes in the free-
volume of the polymer. In contrast, when the Cp2M02(CO)6 unit is incorporated into a
polymer chain, the quantum yields of degradation are strongly temperature dependent. A
66
cage effect model is proposed to explain these results. When a polymer chain is cleaved,
the chains will relax by secondary chain movements. It is proposed that increased
thermal energy facilitates the rotation and recoil relaxation processes, which effectively
increases the rate constant for diffusion of the radicals out of the cage, kd• In effect, the
cage recombination efficiency is decreased and this leads to an increase in the efficiency
of degradation. In support of this proposal, the apparent activation energies obtained
from the temperature dependence of the quantum yield of polymers PVC-Mo (14.0 ± 0.3
kcal mor l ) and PU-70 (10 ± 1 kcal mor l ) are consistent with the secondary relaxation
chain movements in polymers. In addition, these values coincide with the activation
energies of the b-transitions of the polymers PVC-Mo (14.7 ± 0.4 kcal mor l ) and PU-70
(8 ± 1 kcal mor l ).
This chapter summarizes the effect of temperature on polymer photodegradation.
In the next chapter, Chapter V, two more factors that affect polymer photodegradation
efficiencies will be discussed: time-dependent morphology changes and tensile stress.
67
CHAPTER V
THE EFFECT OF MORPHOLOGY CHANGES ON POLYMER
PHOTODEGRADATION EFFICIENCIES. A STUDY OF TIME-DEPENDENT
MORPHOLOGY AND STRESS INDUCED CRYSTALLINITY
Daglen, B. C.; Tyler, D. T. J. Inorg. Organomet. Polym. Mat. 2008, submitted.
5.1 Introduction
The previous chapters have discussed several parameters that affect polymer
photodegradation rates including glass transition temperature, radical trap concentration,
and temperature. This chapter will focus on two additional parameters: time-dependent
morphology changes and tensile stress. First, the time-dependent morphology changes in
the segmented polyurethane, PU-35 , will be discussed as well as how these changes
affect degradation efficiencies. Segmented polyurethanes are defined as polyurethane
copolymers that possess hard and soft segments in the polymer backbone (Figure 1).1
Soft segments are typically oligomers of polyol and hard segments can be aromatic rings
or regions were extended hydrogen bonding can occur (i.e. urethane or urea groups).
These segmented polyurethanes have distinct environments in the solid-state where the
soft segments form amorphous regions and the hard segments form highly crystalline
regions. An interesting phenomenon in these polymers is not just the formation of these
regions, but also the expansion of these domains in the solid-state over days or even
weeks? The thermodynamic driving force for the change in morphology is relaxation of
residual conformational strain from either the film casting process or foam formation
68
which results in an increase in thennodynamically stable regions of randomly coiled
polymer segments (soft) and extended hydrogen-bonding networks (hard). The time-
dependent morphology changes that occur may affect the rates of polymer degradation.
Accordingly, the investigation of time-dependent morphology changes in our model
polymers was investigated.
Figure 1. Illustration of the solid-state morphology of a polymer with soft (A) and hard
(B) segments.
The second factor that affects polymer photodegradation efficiencies presented in
this chapter is tensile stress. An interesting outcome of polymer degradation studies is
the discovery that tensile stress can accelerate photochemical degradation.3 There are
several theories to explain this phenomenon: Plotnikov hypothesis, decreased radical
recombination efficiency hypothesis, and Zhurkov hypothesis (Chapter I section 1.3.3).
Previous experiments on PVC·Mo testing these hypotheses support the idea that stress
69
affects the ability of the photogenerated geminate radical to recombine.4-6 Additional
results will be presented that investigate degradation efficiency as a function of tensile
stress.
5.2 Results and Discussion
5.2.1 Polymer Preparation and Characterization
The crosslinked polymer, PVC-Mo and PU-35 were prepared as described in
Chapter IV. (Note PVC-Mo-1 and PVC-Mo-2 were prepared by different
experimentalist but are chemically equivalent). The glass transition temperatures (Tg)
and the secondary thermal transitions (T~) were determined by dynamic mechanical
analysis and the results are discussed below. The time-dependent morphology changes in
the polyurethane, PU-35 were monitored by infrared spectroscopy using 32 scans at 4
em-I. The deconvolution of overlapping peaks in the carbonyl region of the IR spectra
were performed using IGOR Carbon Pro software. The changes in crystallinity as a
function of tensile stress for PVC-Mo were monitored by infrared spectroscopy as well.
5.2.2 Effect ofTime-Dependent Morphology Changes on the Degradation Efficiency
ofa Segmented Polyurethane
Quantum Yield as a Function ofTime for PU-35
The photodegradation efficiency of a segmented polyurethane film was monitored
over the course of several weeks. The quantum yield as a function of time are shown in
Figure 2. The initial quantum yield was relatively high (<I> = 0.58 ± 0.08). Over the
course of two weeks at 30°C, the quantum efficiency decreased significantly and then
leveled off at <I> = 0.35 ± 0.06. It was hypothesized that the segmented structure ofPU-
70
35 caused the polymer to change morphology over time and consequently cause a change
in the quantum efficiency of the photoreaction. (Recall that PU-35 was synthesized using
a 'soft' poly(propylene glycol) prepolymer and can form the 'hard' extended hydrogen
bonding networks characteristic of polyurethanes). It was shown that increased
crystallinity in the solid-state decreases the degradation rates of model polymers
containing metal-metal bonds in the polymer backbone.6 (Note that photooxidative
degradation of polymers, in general, is retarded by increased crystallinity.) Infrared
spectroscopy was used to identify and quantify any changes in crystallinity as a function
of time.
50
•
•
40
•
20 30
time (days)
•
'." - . .
-... -.•..... _----_._-----------
•
10
I •
I
I
\
\
\
\
\ .
\
\
\
...
. \
\
\
\
\ ,
,
,
,
,
,
,
0.70
0.65
0.60
0.55
0.50
~ 0.45
0.40
0.35
0.30
0.25
0.20 ,-I---------------------
o
Figure 2. Quantum yield as a function of time for PU-35. The dashed line is meant to
guide the eye and is not a quantitative fit.
71
Identification of Peaks in the Infrared Spectrum ofPU-35
To probe the time-dependent morphology changes of PU-35, infrared spectra
were analyzed as a function of time. To detect the formation or extension ofa hydrogen
bonding network, peaks associated with the urethane linkage, specifically the carbonyl
stretch (C=O v) and the N-H stretch (N-H v), were identified using values found in the
literature (shown in Table 1).7-11
Table 1. Literature values for the N-H and C=O stretching frequencies found in
polyurethanes.7-11
Infrared transition H-bonded wavenumber (em-I)
N-Hv no 3350
N-Hv yes 3300
C=Ov no 1730-1740
C=Ov yes (loosely) 1710
C=Ov no (strongly) 1700
Figure 3 shows two example infrared spectra: a solution ofPU-35 in
tetrahydrofuran (THF) and a polymer film ofPU-35 (5 days after casting). (Note that a
solvent background spectrum was subtracted from the solution spectrum.) Figure 3a
focuses on the N-H v region, where peak a is the solution spectrum and peak b is the
polymer film spectrum. Figure 3b focuses on the C=O v region, where peak c is the
solution spectrum and peaks d-e are the polymer film spectrum. In the solution
spectrum, the N-H v peak (a) at 3278 cm- I is in the range ofH-bonded amines and is
assumed to be interacting with the hydrogen-bond accepting solvent, THF. The C=O v
72
peak (c) at 1734 cm- l was identified as non-hydrogen-bonding in solution. In the solid-
state, the N-H v peak (b) at 3308 cm- l was identified as bound to a hydrogen; this
hydrogen-bond is likely with a carbonyl oxygen ofa urethane linkage. Four peaks were
identified in the carbonyl region of the polymer film spectrum: d) non-hydrogen-bonded
c=o v peak at 1737 cm- l , e) residual THF peak at 1727 cm- l , f) hydrogen-bonded C=O v
peak at 1712 cm-\ and g) strongly hydrogen bonded C=O v peak as a shoulder at 1700
cm-
I
•
12 To determine the relative peak contributions to the observed spectrum in the
carbonyl region, it was necessary to perform a mathematical deconvolution.
a. 1.0
""'""' 0.8
=i
$~ 0.6
Col§
~ 0.4
iii
~
0.2
b
73
0.0
3500
--------- --~~,
3400 3300 3200 3100
wavenumber (em-I)
b. 2.5
e
2.0
~
=!
= 1.5'-'
~ g
=
= 1.0,Q
'"'0~
~ 0.5
0.0
-1800 1750 1700 1650
wavenumber (em-I)
Figure 3. a) N-H stretching region and b) C=O stretching region of the infrared spectrum
of a solution ofPU-35 in tetrahydrofuran (THF) and a polymer film ofPU-35 (5 days
after casting).
Method of Infrared Peak Deconvolution
To quantify the contribution of the individual peaks in the carbonyl region of the
spectrum, a deconvolution method was employed. First, it was assumed that the
spectrum was composed of only the four peaks identified in the previous section and the
observed spectrum was the linear combination of those four peaks (i.e.
74
Aobserved = Al + A2 + A3 + A4). It was further assumed that the vibrational modes
associated with those peaks would yield a Lorentzian shaped curve. The Lorentzian
equation was chosen because it accounts for line-broadening that occurs due to
interactions of the vibrating species with the surrounding matrix. 13 The shape of a
Lorentzian curve is defined in eq 1,
A _ A max
- 4(Vmax _V)2 + 1
~V2
where A is the absorbance at frequency, v, Amax is the maximum absorbance for the
(1)
particular vibrational mode, Vmax is the frequency at Amax, and ~v is the full width at half
maximum (FWHM). Using IGOR Carbon Pro, the infrared spectra were fit to a linear
combination of eq 1.a The original data, the fit, and the contributing peaks are shown in
Figure 4. It is interesting to note that the FWHM for the strongly hydrogen-bonded C=O
v peak is significantly greater than the other peaks. This is expected since the hydrogen-
bonded species is often broader and lower in energy than the non-hydrogen-bonded
. 14
specIes.
75
2.5 1-------------------------------------------------------
I
I
2.0 .~.-~.,- -~-.--
""":'
~
~ 1.5
~§j 1.0 i
C'lI 0.5 I__
I
0.0
1800 1780 1760 1740 1720 1700 1680 1660 1640
wavenumber (em·1)
Figure 4. Carbonyl region of the infrared spectrum of a PU-35 polymer film and the
calculated contributions to the observed peaks using a linear combination of Lorentzian
curves.
PU-35 Infrared Peak Areas as a Function of Time
To determine the changes in the spectra over time, the peak area was compared.
The peak area was found by integrating the Lorentzian equation (eq 1) to generate eq 2.
nA /::iv
area(peak) = max
2
The change in crystallinity was monitored by observing the ratio of the areas of the
(2)
hydrogen-bonded C=O v peaks at 1710 and 1700 cm- l to the non-hydrogen-bonded C=O
v peak at 1740 em-I. The ratio as a function of time is shown in Figure 5. According to
the infrared data, PU-35 increased in crystallinity as a function oftime, which is
characteristic of segmented polyurethanes. It is hypothesized that the corresponding
decrease in quantum efficiency is a result of the photogenerated radicals being formed in
76
more thermodynamically stable regions of the polymer. Radicals generated in stable
regions have no driving force for radical separation (relaxation of stressed conformations)
and therefore the probability of radical cage escape is decreased.
3.4
~ 3.2
"...
-< 3.0
--.
=>
...
" 2.8...
-<
+ 2.6~
... iii
-< 2.4
--
2.2
2 4 6
time (days)
8 10
Figure 5. Ratio of the areas of the hydrogen-bonded C=O v peaks (at 1710 and 1700
cm-
I ) to the non-hydrogen-bonded C=O v peak (at 1740 cm- I ) over time for PU-35.
5.2.3 Effect ofTensile Stress on the Degradation Efficiencies ofPVC-Mo
Decreased Radical Recombination Efficiency Hypothesis
The Decreased Radical Recombination Efficiency (DRRE) hypothesis postulates that
degradation efficiency is affected by the ability of the geminate radical to recombine
(krecombinacion in Scheme 1).15-18 This hypothesis, as discussed in Chapter I, outlines a series
of conditions under which radical recombination can occur. To reiterate, at zero or
minimal stress levels, the semi-crystalline polymer has an intrinsic quantum yield. When
tensile stress is applied to a polymer, the chains which are in a random coil state become
77
elongated and taut. Under these conditions, the radicals generated during bond homolysis
recoil to resume a more entropically favored random coil configuration. This results in
an increase in radical distance. Greater radical separation increases in the likelihood of
radical trapping and consequently increases the degradation efficiency. An increase in
applied tensile stress may affect polymer morphology cause a deviation from a continued
increase in degradation efficiency with increased stress. Tensile stress can impart a high
degree of polymer strand orientation and induce the formation of crystalline regions in
polymer systems. Bonds homolyzed in these regions of high thermodynamic stability are
unlikely to recoil into a random coil configuration and thus remain in close proximity.
This results in an enhanced rate of radical recombination and a decrease in degradation
efficiency. (For a pictoral representation of this hypothesis, refer to section 1.3.3 in
Chapter I.)
khomolysis
,..
krecombination
2 \fVVV".
2 RX
---~~... 2 UVV\. X + 2 R •
ktrap
Scheme 1. Simplified polymer photodegradation mechanism showing bond homolysis.
Previous Data Supporting the DRRE Hypothesis
The DRRE hypothesis predicts a distinct quantum yield response to applied
tensile stress: I) an intrinsic quantum yield at low stress, 2) an increase in quantum yield
due to polymer chain relaxation into a random coil following bond homolysis, then 3) a
decrease in quantum yield from stress-induced crystallization. Figure 6 shows an
example of this phenomenon from the literature (polymer PVC_Mo_l).4,6 The stress-
78
induced crystallization was monitored by observing the CH2 bending mode (CH2 b) in the
infrared spectra where the absorbance at 1425 cm-1 is the CH2 b mode in the crystalline
region and the absorbance at 1436 cm-1 is the CHz b mode in the amorphous regions. 19, 20
Figure 9 shows the ratio of the peak intensities as a function of tensile stress. The
crystallinity of the polymer increased with applied tensile stress and the quantum yield
versus tensile stress plot was characteristic of the DRRE hypothesis (Chapter 1, Figure
4b).
0.5
0.4
0.3
cI> -~0.2
0.1
0
0 2 4 6 8 10
Stress (MPa)
Figure 6. Literature data for the quantum yield versus tensile stress for PVC-Mo-1.4
0.90
0.88
\C
~ 0.86....
~
N
0.84.".....
<
0.82
0.80
0 2 4 6
Stress (MPa)
8 10
79
Figure 7. Literature data for the ratio of the infrared absorbances, A142S/A1436, for the
bending modes of CH2 in the amorphous (A1436) and crystalline (A142S) regions ofPVC-
Mo-l as a function of tensile stress.4,6
Recent Data for Quantum Yield versus Tensile Stress for PVC-Mo-2
To investigate the DRRE hypothesis and the role of crystallinity further, another
sample ofPVC-Mo (PVC-Mo-2) was investigated. Quantum yield data as a function of
tensile stress for PVC-Mo-l and PVC-Mo-2 are shown in Figure 8. The quantum yield
responses to applied tensile stress were noticeably different for these two polymers. Both
samples exhibited similar quantum efficiency at zero stress (<I>(PVC-Mo-2) = 0.15 and
<I>(PVC-Mo-l) = 0.17). However, the quantum yield ofPVC-Mo-2 did not change as a
function of increased stress, where as PVC-Mo-l quantum yield varied from 0.17 to 0.42
before decreasing to 0.21. The morphologies of these polymers were compared to
investigate the discrepancy in response to tensile stress.
80
1086
•
4
•
•
2
-,---1
____________________________________________ t
•
0.5
0.4
0.3
<I>
0.2
0.1
0
0
Stress (MPa)
Figure 8. Quantum yield versus tensile stress for PVC-Mo-l and PVC-Mo-2.
• - Literature data for PVC-Mo-l. • - Recent data for PVC-Mo-2.
Figure 9 shows the ratios of the infrared absorbances, A142S/A1436, for the CH2 6 in
the amorphous (A1436) and crystalline (A142S) regions ofPVC-Mo-l and PVC-Mo-2 as a
function of tensile stress. The data indicate that the initial percent crystallinity (polymer
morphology before the application of tensile stress) ofthese polymers was markedly
different: A142S/A1436(PVC-Mo-l) = 0.83 and A142S/A1436(PVC-Mo-2) = 1.03. PVC-
Mo-2 was therefore more crystalline than PVC-Mo-l before tensile stress was applied.
Also, unlike PVC-Mo-l, the relative crystallinity in PVC-Mo-2 did not change with
increased tensile stress. This result is consistent with the DRRE hypothesis. PVC-Mo-2
was a highly crystalline polymer that could be said to be in the third 'case' where stress-
induced crystallinity results in an increase in radical recombination and, consequently, a
low quantum yield. However, the crystallinity in PVC-Mo-2 was formed during the
solvent casting process and not stress-induced as in the case ofPVC-Mo-l. The infrared
81
data indicated that tensile stress did not affect the morphology of the polymer over the
load applied. Since the environment experienced by the photogenerated radical species
was equivalent throughout the experiment, the quantum yield was not predicted to
change.
1.10
1.05
~ 1.00
....
~ 0.95
....
-< 0.90
0.85
----------f----------'-------------J'--------j
0.80
o 2 4 6
stress (MPa)
8
Figure 9. Ratio of the infrared absorbances, A142S/A1436, for the bending modes ofCH2 in
the amorphous (A1436) and crystalline (A142S) regions of PVC-Mo as a function of tensile
stress. • - Literature data for PVC-Mo-l. (No error bars were provided in original
publication.) + - Recent data for PVC-Mo-2.
5.3 Conclusions
The effects of time-dependent morphology changes and tensile stress on
photodegradation efficiencies were investigated. The photodegradation efficiency of a
segmented polyurethane was observed to decrease over two weeks time. It was found
that changes in polymer morphology, which are characteristic of polyurethanes, are the
cause of the changes in quantum yield. It was also found that tensile stress mayor may
82
not have an affect on photodegradation efficiencies depending on the initial morphology
of the polymer. PVC-Mo-l exhibited variable quantum yield as a function of stress that
was consistent with the DRRE hypothesis. PVC-Mo-2, which had a greater degree of
crystallinity, was found to not change morphology with increased stress and consequently
the applied stress did not affect the quantum yield. The results of the stress-induced and
time-dependent morphology changes indicate that thermodynamic stability of the
polymer affects the photodegradation efficiency. In the case of the polyurethane, the
change in morphology decreased residual conformational strain from the initial casting
process and created thermodynamically stable soft (random coil) and hard (hydrogen-
bonded) regions which effectively increased the efficiency of the radical-radical
recombination. In the case of PVC, external stress had two potential consequences.
First, stress decreased the rate of radical-radical recombination by inducing recoil of the
taut photogenerated radical polymer segments. Second, stress induces crystallinity,
which effectively increased radical recombination efficiency. Restated, a
thermodynamically stable polymer will have a slower degradation rate than a polymer
that residual or applied stress.
Thus far, the following external parameters that affect polymer degradation have
been discussed: radical trap concentration, glass transition temperature, reaction
temperature, time-dependent morphology changes, and tensile stress. The next chapter,
Chapter VI, explores the reaction kinetics of the photodegradation ofPVC-Mo and PU-
70. Several models found in the literature were evaluated and a new model based on
Perrin kinetics is presented.
83
5.4 Endnotes
a. The starting values ofAmax were set to l, Vmax was approximated from the spectra,
and FWHM was approximated to be 20 em-I. Subsequent starting values were
approximated from the initial fit.
84
CHAPTER VI
APPLICATION OF A PERRIN-LIKE KINETICS MODEL TO EXPLAIN THE
BIPHASIC PHOTOCHEMICAL DEGRADATION RATES OF POLYMERS
Daglen, B. C.; Tyler, D. R. Macromolecules, 2008, accepted.
6.1 Introduction
A major goal in the field of polymer science is to devise photodegradable
polymers with a tunable onset of degradation and with a specific degradation rate. In
order to design polymers with these properties, it is necessary to identify the experimental
parameters that affect degradation rates and to determine how those parameters affect the
degradation mechanism. A necessary step toward this goal is to determine the kinetics of
the degradation reaction. An interesting observation from these kinetic studies is that the
photodegradation rates of solid-state polymers are often biphasic, showing a relatively
fast rate during the initial period of irradiation but a slower rate at longer times, Figure
1.1,2 Several hypotheses have been proposed to explain this observation but no definitive
explanation has come forth. As part of our ongoing study of polymer photodegradation,
we began an investigation into the origin of the biphasic behavior.
85
y
irradiation time
Figure 1. A plot illustrating the typical biphasic decay for the photodegradation of a
polymer. The data represented on the y-axis will vary with the degradation monitoring
technique; two examples are radical concentration in the case of electron spin resonance
(ESR) experiments or number average molecular weight obtained from gel permeation
chromatography (GPC).
In this chapter we present a new explanation for the biphasic photochemical
degradation rates of polymers. Specifically, a mechanistic-based 4-parameter equation is
presented and tested. The new model was assessed on two polymers that contain M-M
bonds in their backbones, one a cross-linked poly(vinyl chloride) and the other a
polyurethane with soft and hard segments. To investigate the range of application of this
new model, literature data for the photodegradation of a polyoxymethylene was also
tested.3 The coefficients extracted from the fits were evaluated as a function of reaction
temperature, which led to further mechanistic insights.
This work was recently accepted in the journal Macromolecules. All of the
experiments in this chapter were performed by myself. D. R. Tyler and myself were co-
authors of the manuscript.
86
6.2 Results and Discussion
6.2.1 Fitting the Biphasic Photodegradation Data
Concentration data for the photodegradation of polymers 1 and 2 as a function of
irradiation time are shown in Figure 2.3 (Note the data in Figure 2 were normalized by
dividing the concentration values by the initial concentration.) The traces in Figure 2
exhibit biphasic character, showing a relatively fast rate during the initial period of
irradiation but a slower rate at longer times. In an attempt to fit the data, several
mechanistic models were evaluated. Owing to the fact that the plots are not linear, the
reactions are not the usual simple zeroth-order photochemical reactions. (Note, however,
that the slopes at long reaction times do appear linear.) To test for the potential origin of
the biphasic kinetics, the data were fit to the expression CICo=Ae-kt, which represents
first-order kinetics in the disappearance of starting material. In this case, the reaction
kinetics would be dependent on the concentration of the molybdenum dimer
chromophore. The first-order fits to the degradation data for polymers 1 and 2 are shown
in Figures 2a and b. The fits clearly do not represent what is occurring at short reaction
times. The second-order equation, CICo = (I + Bt)"l, was also evaluated but the fits were
no better (Figures 2a and b).
a. 1.000
0.995
=U 0.990
--U
0.985
87
0.980 -r-------,;------,----------,;-------,
o 1000 2000 3000
time (sec)
4000
b. 1.000
0.998
0.996
=U 0.994
--U
0.992
0.990
0.988
.'..
\~,> ....
'~., "
,f;,'
" .,••-f.~i,.~~.I .
.' ....... """ ~
....
.....
o 500 1000 1500 2000 2500 3000
time (sec)
Figure 2. Relative -CH2CsHiCO)3Mo-Mo(CO)3CsH4CH2- concentration as function of
irradiation time at room temperature for polymer 1 (a) and 2 (b) with fits for the 1st order
(_._._) and 2nd order (- ..- ..- ) equations.
It was hypothesized that the biphasic behavior exhibited by these polymers may
be attributed to unique reactivity in the solid-state. One possible explanation is that the
reaction rate is not solely determined by a specific chemical transformation but rather by
diffusion processes. One representation of diffusion behavior, as observed for the
disappearance of starting material, is the simple-diffusion equation, C/Co =(1 + De /2r'.4
88
Simple-diffusion kinetics were used to describe the degradation of solid-state polyolefins
where the decay of the photogenerated radicals was hypothesized to be controlled by
either oxygen diffusion into the polymer material or by radical migration to the
crystalline surfaces where radical-trapping oxygen was present.5 The fits of the
degradation data for polymers 1 and 2 to the simple-diffusion equation are shown in
Figures 3a and c. Note that, although the simple-diffusion model is a more promising fit
than a simple first- or second-order equation, it clearly overestimates the reaction rates at
both short (see Figures 3b and d) and long reaction times and is consequently not a
suitable description of this system.
89
a. 1.000
0.995
~
~ :::j
0.980 1-,-----,-,-----,---,----",---~
o 1000 2000 3000
time (sec)
b. 1.000
0.998
BO.996
U
0.994
0.992
(~. .'
,:-".:'
, ....." ."
.. ~'~"
..
...
...
...
--:-" .
...
o 100 200 300 400
times (sec)
500
....
600
'\ ..
. '.
'\. '.
-~ ..
''':'.~...:~..~
0.999
0.998
~
~U 0.997
0.996
0.995:
0.994 1-,--'-,--'-,----,---,----,-,----,---,----"
o 100 200 300 400 500 600
time (sec)
d. 1.000
o 500 1000 1500 2000 2500 3000
time (sec)
c. 1.000
0.998
0.996
~
~
U 0.994
0.992
0.990
Figure 3. Relative -CH2C5HiCO)3Mo-Mo(CO)3C5H4CH2- concentration as a function
of irradiation time at room temperature for polymer 1 (a and b) and 2 (c and d) with fits
for the simple diffusion (---- ), diffusion-controlled (•••••), and Perrin-like (--)
equations. The plots in band d show the initial data in plots a and c, respectively, on
expanded axes.
Waite developed a modification of the simple-diffusion model called the
diffusion-controlled mode1.6-s This model describes bimolecular reactions in the solid-
state or in highly viscous solutions where both diffusion and spatial distribution of the
reactive species are taken into account. The model (described by the equation,
C/Co = (l + Bt + Dt1l2t 1) has been used to fit the radical decay kinetics of several
photodegradation reactions of polyolefins and polyethers as monitored by electron spin
90
resonance spectroscopy.3,4,9, 10 Figure 3 shows the fits for the diffusion-controlled model
developed by Waite. Like the simple-diffusion model, it can be seen (particularly in
Figures 3b and 3d) that the diffusion-controlled model overestimates the initial reaction
rate. However, in contrast to the simple-diffusion model, Waite's model does more
closely represent the data at longer reaction times. Be that as it may, none of these
conventionally used models could adequately describe the kinetic behavior of polymers 1
and 2; accordingly, a new model based on Perrin kinetics was developed to explain the
biphasic behavior.
6.2.2 Derivation ofa Perrin-like Photochemical Model
There are many instances in the literature where a chemical species is converted
to a single product in the same reaction vessel by two or more mechanistically different
routes that possess different observed rates. One example is heterogeneous catalysis,
where a reactant may experience different catalytic sites on the reactive surface.ll , 12
Another example, which is the focus of this paper, involves a photoreactive species
embedded in a solid polymer matrix. Polymers can be classified morphologically as
either amorphous or crystalline, but the majority of polymers are semi-crystalline
(possessing both crystalline and amorphous regions). These polymers may visually
appear to be homogeneous but, under the scrutiny of an electron microscope, are in fact
heterogeneous with both crystalline and amorphous domains .13 One consequence of this
microheterogeneity is that a reactive species in a polymer matrix can reside in different
molecular landscapes. For example, it is well known that oxidation reactions in highly
crystalline polymers have different reaction rates at the crystal grain boundaries than
91
within the crystalline regions. 14 These two different environments result in an observed
reaction rate that is a composite of two intrinsically different reaction pathways that result
in the same product. The rate equation for such a reaction is derived from the model
A - B, where reactant A can be described as two components, X and Y, as expressed in
equation 1.
[A]o = [x]o + [Y]o (1)
Another case found in the literature where the observed reaction rate was the
combination of two separate rates was described by Perrin in the early part of the
twentieth century.15 The Perrin model, shown in Figure 4, was established to explain the
observed non-exponential fluorescence decay of small molecules in solid po1ymers.15 ,16
Perrin considered the rnicroheterogeneity of solid-state polymers and the relative
immobility of atoms in the solid-state. For fluorescence decay, he proposed that when an
acceptor was in the quenching sphere of an electronically excited donor molecule, the
fluorescence would be quenched. Therefore, the observed rate of fluorescence decay was
the combination of the decay rate of excited molecules in the presence of a quencher and
the natural decay rate of molecules in the absence of a quencher. A mechanistic analogy
can be made for photogenerated radical species in solid-state polymers: the observed rate
of radical decay will be the combination of the rate where a radical trapping agent is in
the reactive sphere of the radical and where it is not. (The term "reactive sphere" is
equivalent to the term "quenching sphere" used in the case of the original Perrin model.)
This is represented pictorially in Schemes 1a and lb.
92
a. 0".0,0 0
, ,
GeCo@D
I, ,
0''0''0 0
b. 0".0,0 0
, ,
Ge~O
0''0/0 0
e = excited species
= fluorescent quencher
~ =) = quenching sphere
Figure 4. Perrin model used to describe the observed rates for the fluorescence decay of
a chromophore in the solid-state.
93
a.
b.
:' -', reactive sphere
'-'C!) radical trap
~~~-p~-,.~,'~. I, . .T \" """ ,
'- "tI:J'
Scheme 1. Illustration of the reaction of photogenerated metal radicals with trapping
atoms in a solid-state matrix. In one case (a), there is a trap in the reactive sphere of the
metal radical. In the other case (b), the trap is initially outside of the reactive sphere.
To generate the observed rate equations for the disappearance of the starting
material as illustrated in Scheme 1, one must consider the kinetics of the photoreaction
and how the coinciding individual cases affect the kinetics. For the simple case where a
photon causes a singular photoreaction, the rate equation for the conversion of A to B can
be written as
(2)
94
where lj> is the quantum yield of the reaction, 1mis the number of einsteins absorbed per
unit volume per unit time, Eis the chromophore molar absorptivity, cA is the
concentration of the chromophore, and l is the path length, S is the surface area, and V is
the volume of the irradiated sample.17 Equation 2 can be simplified by considering the
magnitude of photon absorption by the sample. For instance, if the absorbance value is
large (EcAl > 2) then l_lO-fcAl ,..., 1 and the rate can be written as
(3)
which is a zeroth-order reaction. Conversely, if the absorbance of the substance is small
(EcAl < 0.1) then 1_IO-fcAl,..., 1 and the rate can be written as
dA S
- - = 2.303ECA llj>lm- =constant· cAdt V
which is a first-order reaction. To determine the rate equation that corresponds to the
individual cases illustrated in Scheme 1, the relative absorbances of the chromophore
(4)
with and without a trap in the reactive sphere will be evaluated. An assumption is made
that the experimental path lengths and molar absorptivity of both cases are the same
because both species exist in the same sample and, in both cases the absorbing species is
the same. Therefore, the relative absorbances will be determined by the relative
concentrations of both species. Returning to Figure 3, it is observed that < 1% ofthe
absorbing species is consumed in the initial fast reaction rate. This species, termed Y, is
in very low concentration and therefore is predicted to exhibit first-order kinetics, eq 5,
where Yo is the relative concentration of species Y at time zero and kz. is a "rate constant"
for the photoreaction.18 (Note that kz. is equal to 2.303E llj>lmS/V. To make comparisons of
95
the "rate constants" possible, the parameters 1, 1m , S, and V were kept constant from
sample to sample and polymer to polymer.) Integrating eq 4 in terms of concentration of
Y as a function of time, t, yields eq 5, where Yo is the relative concentration of species Y
at time zero and k2 is the rate constant for the photoreaction.
[Y] =Y oe-k2f (5)
The remaining 99% of the photoreactive starting material that reacts at a slower rate,
species X, is in high concentration and is predicted to show the usual zeroth-order
kinetics.17 The resulting rate for the disappearance of X is shown in eq 8, where Xo is the
concentration of species X at time, t, zero and kj is the corresponding "rate constant." (k j
is equal to </>lmS/V.).
(6)
By combining equations 5 and 6, a new overall rate equation for the disappearance of A
is formed, eq 7. This equation represents the combined contributions to the overall rate
for the two cases derived from the Perrin-like model as illustrated in Figure 4.
[A] =Xo + kjt + Y oe-k2f (7)
The Perrin-like expression, eq 7, was used to fit the kinetic traces of polymers 1
and 2, and the results are illustrated as the solid line in Figures 3a and b. It is apparent
that the Perrin-like equation is the best fit for the data at both long and short reaction
times. At short reaction times, the dominant species contributing to the observed fast
reaction rate is the photogenerated radical species where there is a radical trap in the
reaction sphere. The Perrin-like model describes the fast reaction kinetics as a first-order
exponential. At longer reaction times, the reaction rate is dominated by the relatively
96
slow reaction that involves migration of a radical trapping species into the reactive sphere
of the photogenerated metal radicals. This is described in the Perrin-like model as simple
zeroth order kinetics. The next section will investigate the applicability of the Perrin-like
model in other polymer systems by analyzing degradation data found in the literature.
a.
= 0.8~
0.7
0.6
+
0 500 1()()() 1500 2000 2500
time (s)
b. C.
0.95 .
0.9 ~..
0.90 ,\,......
0.8 0.85
, ......
= = " ".I.:? I.:? ... .....
U U 0.80 ...0.7
0.75
0.6
0.70
500 1000 1500 2000 2500 200 400 600 800
time (sec) time (sec)
Figure 5. Relative radical concentration as a function of time at 343.5 K following
irradiation of polymer 3 (+). The fits are for the lSI order (_._._) and 2nd order (_ .._.._)
equations (a) and simple diffusion (----), diffusion-controlled (•••••), and Perrin-like (-
-) fit equations (b). The plot in c shows the initial data in plot b on expanded axes.
97
6.2.3 Analysis ofa Literature Example Using the Perrin-like Model
To analyze the versatility of the Perrin-like model to other polymer systems,
literature data were fit to eq 7. Figure S shows data for the photodegradation of
polyoxymethylene, 3, under gamma irradiation.3 (The literature data points were
obtained using a literature capture method described in the next section of this chapter.)
The degradation mechanism of polyoxymethylene is known; upon irradiation, three
. . .
radical species are formed: -0 - CH -, -0 - CH2 , and 0- CH2 - .18. 19 The data
presented are for the disappearance of the two carbon-centered radicals as monitored by
EPR spectroscopy. The kinetic trace for the photodegradation of polymer 3, like that of
polymers 1 and 2, is biphasic in that there is a fast reaction at short times followed by a
slower conversion at extended reaction times.
3
Figure Sa shows the fits of the data for the reaction of polymer 3 to first- and
second-order equations (i.e., C/Co =Ae-kt and C/Co =(l + Btrl , respectively). The fits are
poor, and therefore these two equations do not accurately represent the reaction
mechanism for the degradation of this polymer. Figure Sb shows the fits for the other
three models: simple-diffusion, diffusion-controlled, and the Perrin-like model. As was
the case with polymers 1 and 2, it is evident that these three equations more closely fit the
data than the first- and second-order equations. Taking a closer look at the initial reaction
98
rates in Figure 5c, it is apparent that both the simple-diffusion and the diffusion-
controlled models overestimate the initial reaction rate. This is similar to the findings
reported above for polymers 1 and 2. The Perrin-like model (eq 7), however, fits the data
well at both long and short reaction times and is therefore a suitable model for the
photodegradation of this polymer system. The interpretation is that the fast, initial rate is
due to reaction of the carbon-centered radicals with another photogenerated radical
species or an adjacent abstractable atom such as a hydrogen atom in the reaction sphere,
and the slower rate is for the reaction of the carbon-centered radicals involving a
diffusive reaction.
Literature Data Capture Method
The original data was 'captured' using pixel conversion?O The data image was
scanned from the original journal article and opened in previewing software. The cursor
was used to locate the pixel coordinates for the data points in the graph as well as the
origin and maximums of the x- and y-axes. The pixel values for the data were recorded
in an array and converted to the 'original' values,X and Y, using the following previously
derived eqs 8 and 9,20
X mall- X minX =X min+ (pX - pXmin)----
pX m,,- pX min
Y max- Y min
Y = Y min+ (pY min- pY)----
pY min- pY m"
(8)
(9)
where Xmin , Ymin , Xmax ' and Ymax are the axes origin and extreme values extracted from the
plot, pXmin , pYmin , pXmax ' and pYmax are the corresponding pixel coordinates for the axes,
99
and pX and pY are the pixel coordinates for the data points of interest. The generated
arrays of 'original' data were fit to model equations using IGOR Pro Carbon.
6.2.4 Fitting Parameters as a Function ofTemperature
The photodegradation rates of polymers 1 and 2 were evaluated as a function of
reaction temperature. As in the sections above, the data were fit to first-order, second-
order, simple-diffusion, diffusion controlled, and Perrin-like equations. All data were
found to be best fit by the Perrin-like model (eq 7). The parameters obtained from the
best fit of the data were plotted as a function of irradiation temperature (Figure 6). The
parameters Yo and Xoin eq 7 represent the relative concentrations of the photogenerated
radical species with and without a radical trapping agent in the reactive sphere,
respectively. As shown in Figure 6, Yo is small, ranging from 0.5% to 2.5% over the
temperature range 25-50 °C. Note also that Yo is larger in polymer 1 than in polymer 2.
100
a.
1.000
•
0.995 ... •I --- ..... _ •
-.-
---
•
0.990 .............. I ---!.
... ...... • --~
......
....... •. ...
0.985 ...... •......
Xo
......
...
........
...
...... ...
0.980 ...... ....... ...
......
......
·
...
0.975
... •
0.970 I
25 30 35 40 45 50
T ('C)
b.
0.030
0.025
... t
...
-0.020 -
-
...
-
... ...
0.015
-
~....... --t'
•Yo .'1:
_.J("'"
•0.010 ...
- • --.
-
I --; .
- ... ---~--
0.005 L_--r- .
•
•
0.000
25 30 35 40 45 50
T('C)
Figure 6. Fitting parameters Xoand Yo from equation 7 as a function of temperature for
polymers 1 (... ) and 2 (e).
The relative ratio of Xo to Yo is dependent on the relative concentration of the
radical trapping agent to the concentration of the photogenerated radical. The radical trap
(chlorine atom) to metal species (molybdenum) ratio is approximately 100: I for polymer
1 and 9: I for polymer 2. An increase in radical trap concentration with respect to
potential metal radical species will increase the probability of a radical trap being located
in the reactive sphere. Therefore, it is predicted that polymer 1 would have a larger
101
number of photogenerated radicals with a Cl in the reaction sphere than polymer 2, i.e.,
Yo(l) > Yo(2). As shown in Figure 6, this is indeed the case. For example, at room
temperature, Yo for polymer 1 is approximately 0.008 and Yo for polymer 2 is 0.005.
For both polymers 1 and 2, the value of Yo increased gradually with an increase in
temperature and there was a corresponding decrease in Xo. (Over the temperature range
of the experiment, the value of Yo for polymers 1 and 2 increased at a rate of 0.065% ±
0.009% K 1 and 0.027% ± 0.007% K 1, respectively.) This trend is likely due to an
effective increase in the reaction sphere caused by the enhanced kinetic motion of the
radicals and the trapping species. This results in a greater likelihood of a radical trapping
agent being in the reactive sphere of the photogenerated radical species.
The experimentally determined values for kl and k2 (eq 7) for polymers 1 and 2 as
a function of temperature were evaluated. Over this temperature range, the value of kl
(the rate constant for the zeroth-order (slower) reaction) ranges from 10-6 to 10-5 S-I, while
k2 (the rate constant for the first-order (faster) reaction) is on the order of 10-3 S-I.b
Restated, the rate constant for the non-diffusive reaction, Is, is greater than the rate
constant for the diffusive reaction, kl • This result shows that diffusion is the rate-limiting
step in the diffusive reaction, as expected.
To extract apparent activation enthalpies and entropies, the data were fit to an
Eyring plot (Figure 7) and the resulting values are shown in Table I.e The enthalpies of
activation for the non-diffusive process are much smaller than those for the diffusive
process for both polymers 1 and 2, again consistent with rate-limiting diffusion in the
diffusive reaction. The entropies of activation for both processes and polymers are all
102
negative. However, due to the size ofthe error in the activation entropies, no conclusions
can be drawn from the values in Table 1.
a.
-17.0 I
-17.4 &
~ :::.: &
-18.6
-19.0 -------------
3.1E-03 3.2E-03 3.2E-03 3.3E-03 3.3E-03 3.4E-03 3.4E-03
Iff (K-I)
c.
b.
-9.5 ,-------------
-10.0 •
E. -10.5
~
.5
-11.0
-11.5 '----- -----J
3.1E-03 3.2E-03 3.2E-03 3.3E-03 3.3E-03 3.4E-03 3.4E-03
Iff (K-I)
-11.4 ,----------------
-11.6
-11.8
~ -12.0
~
.5 -12.2
-12.4
-12.6
•
i •
~: .
-&~
& &
&
-12.8 '----- ----J
3.1E-03 3.2E-03 3.2E-03 3.3E-03 3.3E-03 3.4E-03 3.4E-03
Figure 7. Erying plots for the fitting parameters k l and k2 from equation 7 for polymers 1
and 2.
Table 1. Apparent enthalpy and entropy of activation values obtained from the Eyring
plot of the rate constants, kl and k2 , versus temperature for the photodegradation of
polymers 1 and 2.
k, (diffusive) k2 (non-diffusive)
Polymer AH* (kcal/mol) AS* (kcallK mol) AH* (kcal/mol) AS* (kcal/K mol)
1 7±1 -60 ± 40 2±1 -70 ± 40
2 6±2 -50 ± 40 2±1 -70 ± 40
103
6.3 Conclusions
The photodegradation rate data of two polymer films, a cross-linked po1y(viny1
chloride) and a segmented polyurethane, were compared to six kinetics models. It was
found that the degradation did not follow simple zeroth-, first-, or second-order kinetics.
Two other models, a simple-diffusion and a diffusion-controlled model, were analyzed
and found to more closely represent the data than the zeroth-, first-, and second-order
equations. However, these two models still overestimated the initial reaction rates.
Accordingly, a new kinetics model, the Perrin-like model, was introduced that utilized
basic photochemical rate equations and the heterogeneous nature of solid-state polymers.
This equation fit the data with excellent accuracy. The model was also compared to the
photodegradation rate of an example found in the literature, po1yoxymethy1ene. The
Perrin-like model equation was found to be an accurate fit of the literature data presented.
An investigation of the effect of temperature on the parameters extracted from the fits to
the Perrin-like model, showed that only a few percent of the radical species generated had
radical trapping agents in the reactive sphere (Perrin-like environment) and that the
majority of metal radicals did not have radical trapping agents in the reactive sphere
(diffusive environment). It was also found that apparent ~H:j: for the photoreaction in the
Perrin-like environment was lower than that of the diffusive reaction. It is hypothesized
that the rate-limiting step in the diffusive reaction is diffusion of the photogenerated
metal radical to the radical trapping agent.
104
6.4 Endnotes
a. Polymers 1 and 2 are polymers PVC-Mo and PU-70, respectively.
b. Rate coefficients for zeroth- and first-order reactions have units of M S-I and S-I,
respectively. However, because the data were presented and fit as the relative
concentration versus time, the values of k l and k2 both have units of S-I. For the
purpose of comparison, kl and k2 will retain the unit S-I and be referred to as rate
constants or coefficients.
c. The activation parameters obtained from photochemical reactions as a function of
temperature must be interpreted with caution. See ref. 18, page 12.
105
CHAPTER VII
CONCLUDING REMARKS
7.1 Chapter Summaries
Sometime during the life of a polymeric material, it is exposed to light and some
other variable, such as extreme temperature or mechanical stress. In order to predict the
service lifetime of polymeric materials and to design environmentally degradable
plastics, it is critical to understand the photochemical degradation reactions of polymers.
To control polymer degradation in a prescribed fashion, so that the rate and onset of
degradation is both tunable and predictable, it is also necessary to understand how
external parameters and specific polymer properties affect degradation rates. This
dissertation described the results of degradation experiments that probed several factors
that affect polymer photodegradation rates: radical trap concentration, glass transition
temperature, irradiation temperature, time-dependent morphology, and tensile stress.
Chapter I outlined what was understood to date about polymer photodegradation
mechanisms and how external parameters and polymer properties affect degradation
rates. Chapter II introduced a novel class of polymers that incorporated organometallic
molybdenum dimers into the polymer backbone. In solution, these polymers are
photodegradable in the presence of visible light and a suitable radical trap (i.e. 02 or
CCI4). In the solid-state, these polymers were shown to degrade in the presence of~ or
a radical trap built into the polymer chain (i.e. C-el units).
106
Chapters III, IV, and V discussed how parameters affect degradation rates. One
unifying theme throughout the experiments is the mobility of the photogenerated radical.
When a bond is homolyzed, two radicals are formed and exist initially in a solvent cage.
These radicals can either recombine for no net degradation, or they can escape the solvent
cage to potentially be trapped by a radical trapping agent. The findings suggest that
external influences and intrinsic polymer properties directly affect the mobility of the
photogenerated radicals and consequently the ability of the radicals to escape the solvent
cage. In Chapter III, for example, polymers with different the glass transition
temperatures (Tg) were shown to have significantly different quantum yields. Polymers
irradiated below Tg, where polymer mobility is limited, had a slower degradation rate
than polymers irradiated above Tg• In fact, polymers irradiated above Tg had degradation
efficiencies similar to those in solution. This suggests that photogenerated radicals have
the same mobility in solution as in the solid-state above Tg.
The effect of temperature on polymers below Tg was discussed in Chapter IV. It
was discovered that quantum efficiency had an exponential dependence on reaction
temperature. The activation energies for the photochemical reactions corresponded with
the activation energies of the secondary thermal transitions of the polymers. This
suggested that polymer molecular motions, to which the photogenerated radical species
are a part of, facilitate the escape of the radicals from the 'solvent' cage (increase in kd)
and consequently increasing the photodegradation efficiency.
The final two parameters, time-dependent morphology and tensile stress, were
presented in Chapter V. These two studies both involve the formation of crystalline
107
regions in the solid-state. The results indicate that thermodynamic stability of the
polymer affects the efficiency with which photogenerated radicals escape the radical
cage. In the case of time-dependent morphology changes, the change in morphology
decreased the residual strain induced in the casting process and created
thermodynamically stable soft and hard regions which effectively decreased the ability of
the photogenerated to escape the cage. In the case of tensile stress, there were two
interesting results. First, tensile stress increased the rate of radical escape from the recoil
of 'taut' photogenerated radical polymer segments. Second, stress induces crystallinity,
which effectively decreased the rate of radical diffusion out of the cage and decreased
degradation rates. Restated, thermodynamically stable polymers had slower degradation
rates than polymers with residual or applied stress.
Chapter VI evaluated the reaction kinetics of the photodegradation reaction of the
metal-metal bond containing polymers. Several model equations were used to fit the
experimental data including a variety of diffusion models, however, none of the models
properly fit the data. A new kinetic model based on Perrin-like kinetics was presented
and found to fit the data accurately.
7.2 Concluding Hypothesis
The experiments preformed using novel metal-metal bond containing polymers
suggest that polymer degradation efficiency is strongly influenced by the mobility ofthe
photogenerated radicals. In Chapter III, the effect of glass transition temperature on the
degradation of polymers was investigated and it was found that the quantum efficiency
was greater above Tg than below Tg. This suggested that polymer molecular was a key
108
factor. This was further supported in Chapter IV were irradiation temperature was
explored. It was hypothesized that the quantum yield was affected by the ability of the
photogenerated radicals to escape the radical cage, kcage escape (shown in Scheme 1). This
was supported by the similarity in the activation energies of the photoreaction and the
activation energies of the relevant molecular motions required for radical cage escape.
khomolysis
\IVVV"M- MJVVV'" "".
kbonding
[ ]
kcage escape
.rvvvoM• ,. M.rvvvo ~ ... 2.rvvvoM•
kcage reformation
CI
I
~
-----c..~ 2 .rvvvoM-CI
k trap
Scheme 1. Photodegradation mechanism of a metal dimer containing polymer illustrating
the 'solvent' cage.
Other evidence that molecular motion is key was presented in Chapter V. The
experiments on tensile stress and time-dependent morphology demonstrated that
degradation efficiencies vary with changes in polymer morphology. Tensile stress has
two effects on polymers. It can increase radical-radical separation from the recoil of
'taut' photogenerated radical polymer segments which increases kcage escape or it can
decrease kcage escape from stress-induced crystalline formation. In the later case, the
photogenerated radicals were essentially held in place in the crystalline matrix resulting
in a decrease in radical separation efficiency. This was also shown in the case of
polyurethane that, with time, there was an increase in crystallinity and a corresponding
decrease in quantum efficiency.
109
In summary, it has been shown that the mobility of the photogenerated radicals is
the primary contributing factor that influences polymer degradation efficiencies. A
decrease in the mobility of the photogenerated radicals decreases the radical separation
efficiency and, consequently, the degradation rates decline.
110
APPENDIX A
MEASURING SOLID-STATE QUANTUM YIELDS: THE CONVERSION OF A
FREQUENCY-DOUBLED Nd:YAG DIODE LASER POINTER MODULE INTO A
VIABLE LIGHT SOURCE
Daglen, B. C.; Harris, J. D.; Dax, C. D.; Tyler, D. R. Rev. Sci. Instr. 2007, 78, 07410411-
074104/4.
A.I Introduction
Accurate quantum yield measurements are difficult to obtain for reactions in the
solid-state. The difficulty arises from three problems. First, the surface area of
irradiation must be accurately known so that quantum yield values can be accurately
calculated. 1. 2 Second, the samples are not homogeneous following irradiation. As
explained below, this can lead to errors in the calculation of the quantum yield. Third,
light scattering artificially decreases the absorbance of a sample, which leads to errors in
rate measurements and hence the quantum yield.3-5 (This problem is particularly
prevalent in opaque or non-transparent samples.) Unfortunately, light scattering results
from the intrinsic properties of the sample, and it is difficult to correct by instrumental
methods. Nevertheless, the first two difficulties can be addressed by proper instrument
design, and in this section, we report an irradiation apparatus that circumvents the first
two problems.
With regard to the first problem, it is noted that the quantum yield is defined as
the rate of the reaction divided by the rate of photon absorption, Equation 1.1,6
(Equivalently, is the number of molecules reacted divided by the number of photons
111
absorbed by the reactant.) For solid-state studies, the reaction rates (numerator) are
determined by iterative irradiation and degradation monitoring, typically by molecular
weight determination, end-group analysis, mechanical testing, etc. However, for these
reactions, it is not necessarily trivial to obtain the rate of photon absorption (denominator)
because the volume of irradiation must be known. It is important therefore that sample
thickness be accurately known and that the surface area of irradiation be measurable and
consistent. Obtaining an accurate surface area of irradiation is made complicated when a
divergent source such as a high-pressure Hg arc lap is used as the irradiation source.
Small translational movement along the path of irradiation may occur during iterative
sample replacements. This will produce a change in surface area of irradiation that scales
with the square of the distance of translational motion.
~ = rate ofreactionlrate ofphoton absorption (1)
With regard to the second problem, it is noted that the sample can become
inhomogeneous if the light beam is smaller that the surface area of the sample. This
leads to inhomogeneity because only the irradiated parts of the sample will have reacted.
For example, if the sample is moved from the light beam to the spectrometer for
interrogation then upon further irradiation the sample must be returned to precisely the
same location as during the previous irradiation so that the exact same region of the
sample can be irradiated again. Because solid-state samples can rarely be repositioned
exactly, this leads to errors in the ~(absorbance) measurements and hence to errors in the
rate of reaction.
112
To overcome the problem of obtaining accurate quantum yields in the solid-state,
we built a computer-controlled device that simultaneously irradiates and
spectroscopically monitors samples in the solid-state. It measures and records the
absorbed light intensity as a function of time and also records a reference (unabsorbed)
light intensity. A key feature of the apparatus is that the same light beam is used to
irradiate and simultaneously monitor the reaction. The films can thus be left in one place,
and consequently there are no "positioning errors" associated with these measurements.
Another important feature is that a collimated light source (i.e., laser) is used to irradiate
the sample, thus reducing errors attributed to inconsistencies in the "area of irradiation"
of the sample.
This work is published in volume 78 of the journal Review of Scientific
Instruments in 2007. The experiments were performed by J. Harris and myself. C. Dax
was critical in the design and assembly of the instrumentation. 1. Harris and myself
prepared the manuscript with D. Tyler providing editorial assistance.
A.2 Experimental Apparatus
A.2.1 The Merlin Detection Unit
An Oriel Merlin radiometry system was used to measure quantum yields. The
details of the experimental setup for this system are shown in Figure 1. The major
components of this system are (1) an irradiation source, (2) an Oriel 100 mm2, NIST-
calibrated silicon photodiode (Model 70356) detector, (3) an Oriel Merlin radiometry
control unit, and (4) a personal computer. The light source could quickly be interchanged
between the Nd:YAG (yttrium aluminum garnet) laser and the high-pressure Hg arc lamp
113
by the addition or removal of a mirror. Light from the source was passed through a
sample holder, which rested on a temperature control unit (set to 25.0 ± 0.1 DC for the
data herein). Solid samples were mounted in an airtight aluminum casing with glass
plates for light transmittance. (A solution sample apparatus consisting of a 1 cm cell
holder and a stirring mechanism could also be employed.) Light passing through the
sample holder was focused through a lens and a five-blade chopper (Oriel Model 75163)
onto the working area of the signal detector head. The signal was chopped at a frequency
of 30.0 Hz, to eliminate light fluctuations due to electrical line noise. The photodiodes of
the detector have operator selectable transimpedance resistors, which were set to 104
gain. Housing the detector inside the insulated, temperature-controlled box minimized
errors attributed to the detector's sensitivity to fluctuations in temperature. The data were
processed using Merlin radiometer system (Model 70100). The Merlin system contained
all the hardware and protocols required for detection of low light levels, lock-in
amplification, and digital processing of the signal.
114
@ to computer9 .-w ter ..~in/out p
C e- 0 m~I~O f llJ ~ l-l
Figure 1. The experimental setup for both liquid and solid state quantum yield
measurements using either a Hg arc lamp or Nd: YAG laser as the source. The
components of this system are as follows: (a) Hg arc lamp, (b) monochromator, (c) iris,
(d) focusing lens, (e) light valve, (f) mirror, (g) Nd:YAG laser, (h) mask for the laser
beam, (i) variable temperature mounting plate, U) sample holder, (k) focusing lens, (1)
chopper, and (m) detector.
A.2.2 Conversion of the Frequency-doubled Nd:YAG Laser Pointer Module into a
Viable Irradiation Source
A 15 mW frequency-doubled Nd:YAG diode laser pointer module (DPSS-5) was
purchased from "Beam of Light Technologies."? Unmodified, the laser pointer module is
inappropriate for quantum yield measurements. (Both Amax and the intensity are
temperature sensitive and the system requires cooling for operating times of more than 20
seconds.) In addition, the beam size, profile, and shape of the Nd:YAG laser were
inappropriate for solid-state quantum yield measurements and were modified
accordingly. The beam size was expanded by replacing the collimating lens included in
the module with a 4 inch focal length plano-convex lens placed 2.25 inches away from
the nodule (Figure 2). The beam was given a round shape with a top-hat profile by
passing it through an 8 mm diameter hole drilled into the aluminum housing that encased
the laser. Although the size, shape, and profile of the beam were adjusted during the
o115
conversion process, the key to successfully converting a laser pointer module into a
usable irradiation source for quantum yield measurements is the ability to precisely
control the temperature of the laser. Precision temperature control of the Nd:YAG laser
was achieved in two ways. First, the laser quantum yield measurements were performed
inside an insulated box, whose internal temperature was maintained at 25 ± 1 °C (see the
schematic in Figure 1). Second, the aluminum housing was kept at a temperature of25.0
± 0.1 °C by combination of both water-cooling and the use of a conventional servo
feedback loop used to control a thermoelectric cooler module (see schematic in Figure 2).
Front View
~
c
Figure 2. A schematic of the Nd:YAG laser, which consists of (a) the laser module, (b)
temperature set, (c) thermoelectric cooler modules, (d) a water cooling module, (e), a
thermocouple, and (f) lens.
A.2.3 The Nd: YAG Laser as a Light Source
The collimated light source was the Nd:YAG laser discussed in the previous
section. Before irradiating the sample, the 8 mm beam was first passed through a mask to
116
adjust the beam dimensions to a 2 x 8 mm2 rectangle. The beam profile is shown in
Figure 3(a); Amax = 532.48 ± 0.01 nm with a full width at half maximum (FWHM) of 0.79
± 0.02 nm. A silvered mirror (New Focus 5103) was used to precisely align the masked
beam through the sample and onto the detector (see the schematic in Figure 1).
A.2.4 The 200 W High-pressure Hg Arc Lamp as a Light Source
The divergent light source was a 200 W high-pressure mercury arc lamp (Oriel
Model 6135). The beam profile is shown in Figure 3(b); Amax = 546.88 ± 0.00 nm with a
FWHM of 1.17 ± 0.01 nm. The arc lamp was fitted with a quartz-ended IR filter (Oriel
Model 77263), a collimating mirror, and a flexible light tube. The other end of the
flexible light tube was attached to the slit input ofa monochromator (Oriel Model 77250)
equipped with a ruled diffraction grating of 1200 lines/mm line density (Oriel Model
77298). The entrance and exit slits were set to 0.2 mm. The exiting light was focused
into the insulated box and on the samples using a focusing lens. To reduce fluctuations in
beam intensity, the light was attenuated by passing it through a light valve optimized for
546 nm light with an adjustable feedback loop (see the schematic in Figure 1).
a.
6000
Z. 5000
.~ 4000
Q)
-.S 2)000
2000
'-' (;.
o i)
'.:'
~~~-. '\~~~~":i
528 532 536
wavelength (nm)
b.
117
::,
)1 ':1 ttl .J ~.}U·
I I I
~44 ~4(; ~5~
wavelength (nm)
Figure 3. Beam profiles for (a) Nd:YAG laser with Amax = 532.48 ± 0.01 nm and FWHM
= 0.79 ± 0.02 nm, and (b) high-pressure mercury arc lamp with Amax = 546.88 ± 0.00 nm
and FWHM = 1.17 ± 0.01 nm. The beam intensities are (a) 1.6 x 10-3 ± 2 x 10-6 and (b)
2.4 x 10-4 ± 3 x 10-6 W. The beam profiles were obtained by reflecting the beam off a
white surface onto an Ocean Optics USB4000 spectrometer and analyzed using Igor
Carbon Pro fitting program.
A.3 Experiments and Discussion
A.3.1 Reagents
All manipulations were carried out in the absence of water and atmospheric
oxygen using standard glove box techniques. The light sensitive nature of the samples
required that their preparation and storage be under darkroom conditions. Films of
polymer 1 (Figure 4) were prepared using the method described in the literature.8
118
CI ~ CI <51 CIOO~
I
(COhMo-Mo(COh
~O~O
CI <;;1 CI r CI
1
Figure 4. The structure of polymer 1 used in the photochemical degradation studies.
A.3.2 Results ofthe Solid-state Experiments
The photochemical reaction of polymer 1 is shown in Scheme 1.9 The reaction
was monitored by the disappearance of the Mo-Mo chromophore at 546 urn (using the
Rg arc lamp) or at 532 nm (using the Nd:YAG laser). The reaction was allowed to
proceed to 1%-3% completion. The quantum yields were calculated using standard
methods:) <I> (Rg arc lamp) = 0.06 and <I> (Nd:YAG) = 0.07. Figure 5 compares data
acquired during the irradiation of polymer 1 using the Nd:YAG laser and the high-
pressure Hg lamp. Note that the intensity of the Nd:YAG laser (1.6 x 10-3 ± 2 x 10-6 W)
is greater than the intensity of the Hg arc lamp (2.4 x 10-4 ± 3 x 10-6 W) (Figure 3). The
benefit of using a source that produces higher intensity light is that less time is needed to
achieve the same percent conversion. For the experiments in Figure 5,86 minutes were
needed for 1% conversion of polymer 1 using the Hg arc lamp, while only 25 minutes
were needed for 1% conversion using the Nd:YAG laser. These values correspond to a
reaction rate of 5.3 Mlmin for the laser and 2.3 Mlmin for the arc lamp.a
~«;2)
\ hv(COhMo-Mo(COh ~ ... 2
~~
CI
I
vvvvvo
----).~ 2
119
Scheme 1. The photochemical reaction of polymer 1.
0.292
0.290
-~ 0.288
-.....
0 0.286~
I
0 0.284~
......
0.282
0.280
0 20 40 60 80 100 120
Time (min)
Figure 5. Irradiation of a polymer containing the chromophore unit Cp(CO)3Mo-
MO(CO)3Cp using the Nd:YAG laser (represented with circles) at A= 532 nm and the
attenuated Hg arc lamp (represented with squares) at A = 546 nm.
Figure 5 also illustrates an improvement in the signal-to-noise [Nd:YAG laser
source (SIN = 1150); Hg arc lamp (SIN = 190)]. The improvement is a result of the
increased stability of the irradiation source from proper temperature control. The
increased signal-to-noise ratio allows for a more accurate determination of the reaction
rate and can be directly compared by the significant improvement in the coefficient of
determination from R2(Hg arc lamp) = 0.42 to R2(Nd:YAG) = 0.98.
Other concomitant benefits of using the Nd: YAG laser as an irradiation source
should also be noted. (l) The Nd: YAG laser is more cost effective than a traditional
120
high-pressure Hg arc lamp. The cost of the frequency-doubled Nd:YAG diode laser
pointer module (DPSS-5; Beam ofLight Technologies) is $55, while the cost of a
replacement bulb (200 W Hg bulb from Oriel) for a high-pressure mercury arc lamp is
$193. (2) The useful lifetime of the Nd:YAG laser is greater than that of the high-
pressure Hg arc lamp for quantum yield measurements. Although an accurate lifetime of
the Nd:YAG crystal has yet to be determined, the laser thus far has lasted for 1500 hours
of irradiation. (Note, the only change over the 1500 hours has been a slow decrease in
beam intensity from 1.6 x 10-3 to 1.2 X 10-3 W.) In comparison, the useful lifetime of a
high-pressure Hg arc lamp bulb for solid-state quantum yield measurements is
approximately 200 hours. 10 (After about 200 hours, the arc path begins to move, which
results in intensity fluctuations and renders it useless for these types of experiments.) (3)
The replacement and disposal of the Nd:YAG laser module are easy and safe.
Replacement of the laser module requires the removal of the old module from the
circuitry, the removal of the stock lens from the new laser module, and the attachment of
the new laser module to the circuitry. The used laser module can be disposed of safely in
the trash. In contrast, replacement of Hg arc lamp bulb involves the handling of high-
pressure bulbs that contain toxic mercury. Thus before a mercury bulb can be properly
disposed of, it is necessary to discharge the high pressures and mercury form the bulb.
A.4 Summary
The design of our modified Nd:YAG diode laser/computer-controlled device
allows us to circumvent two ofthe main problems associated with traditional solid-state
quantum yield measurements. First, errors attributed to inconsistencies in the "area of
121
irradiation" of the ample were eliminated by the collimated nature of the laser beam.
Second, using the same beam of light to both irradiate and monitor the sample, this
allowing the sample to remain stationary during the experiment, eliminated "positioning
errors". The inclusion of the Nd:YAG laser in the computer-controlled device allows for
faster data acquisition times with an improved signal-to-noise ratio. Overall, the
improvements allow a significant reduction in errors associated with solid-state quantum
yield measurements.b
A.5 Endnotes
a. Note that the differences in the reaction times and reaction rates are also
dependent on the molar absorptivity ofthe chromophore (£546 = 1615 L mol/em
and £532 = 1995 L mol/em), so the values cannon be compared directly.
b. One disadvantage of the laser is that it provides only one wavelength for
irradiation. The mercury arc lamp is an indispensable part of the system if other
irradiation wavelengths are needed.
122
APPENDIXB
SYNTHESES AND PREPARAnON OF PHOTOCHEMICALLY REACTIVE
SPECIES USED IN THE DEGRADATION STUDIES
B.1 Materials
All manipulations were carried out using standard Schlenk techniques or in a
Vacuum Atmospheres Co. glove box under a nitrogen atmosphere. Unless otherwise
noted, HPLC grade solvents were deoxygenated by passage through columns of alumina
and copper oxide under an argon atmosphere. (YJs-CSH4CHzCHzOH)zMoz(CO)6 was
prepared as previously described. I, z Hypol 2000 was donated by the W. R. Grace Co. It
was stored at 5 °C and deoxygenated with a nitrogen purge before use. Hypol 2000 is a
toluene diisocyanate-endcapped, branched poly(ethylene glycol) (ca. 6.7 wt %
isocyantate) with an Mn ~ 2,000 and an equivalent weight of 625 g/equiv of isocyanate.
Poly(ethylene glycol) (PEG-l 000) (Mn ~ 1,000) was obtained from Aldrich and
deoxygenated with a nitrogen purge before use. Carboxylated poly(vinyl chloride) (Mn ~
220,000 and 1.8 wt % carboxyl content) was purchased from Aldrich. Thionyl chloride
(Fluka, 99%) was stored in a desiccator. 1-(Chloromethyl)-2,4-diisocyanatobenzene
(97%) was obtained from Aldrich, stored in the drybox, and used as received. 1,4-
Butanediol (98%; Aldrich) was distilled under reduced pressure and stored under
nitrogen in the drybox. Tolylene 2,4-diisocyanate terminated poly(propylene glycol) (3.6
wt % isocyanate; Mn ~ 2,300) was obtained from Aldrich. Dibutyltin diacetate (DBTA;
Aldrich) was stored in the refrigerator in the dark. Carbon tetrachloride was prepared in a
123
nitrogen atmosphere by distillation over phosphorus pentoxide and then filtered though a
silica plug.
B.2 Characterization Instruments
Infrared spectra were recorded on a Nicolet Magna 550 FT-IR spectrometer with
OMNIC software. Thermal transitions were determined using a TA Dynamic
Mechanical Analyzer Q800 with an oscillating frequency of 0.1 S-1 or a TA 2920 DSC
instrument with a heating rate of 10°C/min. The modulus measurements were carried
out at room temperature using an Instron 4444 mechanical tester and are reported as
stress (MPa)/% strain. Molecular weights were determined either by a Knauer vapor
pressure osmometer (VPO) (at 45°C with THF as the solvent) or A Waters 515 HPLC
pump with HR3 and HR4 stryragel columns and a Waters 410 Differential Refractometer
gel permeation chromatograph (GPC) unit (molecular weight are respect to polystyrene
standards in THF). Oxidation states of molybdenum were obtained by Dratos His X-ray
photoelectron spectroscopy (XPS) using the monochromatized Al source (200 W). The
vacuum in the main chamber was < 3 x 10-9 Torr during data acquisition. To achieve
optimal signal-to-noise ratios and peak widths, high-resolution spectra were acquired at
20 eV pass energy. The binding energies were referred to the C Is peak at 285.0 eV.
124
B.3 Synthesis of [(11 s-CSH4CH2CH20H)(CO)3Moh.l, 2
B.3.1 Synthesis ofSodium Cyclopentadiene.
Freshly cut sodium (2.105 g, 0.092 mol) was added to 80 mL of nitrogen-purged
dicyclopentadiene at room temperature in a 500 mL three-neck flask equipped with a
magnetic stir bar, reflux condenser, N2 inlet, and a stopper. On heating, the solution
turned blue and then sodium sand formed. The mixture was heated for 6 hrs at 160°C.
A white precipitate was observed as well as dihydrogen evolution. The remaining
cyclopentadiene was filtered off the white product. The product was washed with four
100 mL portions ofhexanes and dried under vacuum. Yield: 8.017 g (99%). IH NMR
(d6-DMSO, 300 MHz): 0 5.3 (s, 5H).
B.3.2 Synthesis ofCyclopentadienylethanol.
Sodium cyclopentadiene (4.5 g, 0.051 mol) was added to 30 mL of nitrogen-
purged THF in a 100 mL Schlenk flask equipped with a magnetic stir bar and rubber
stopper in the drybox; the solution was brown/red. The reaction flask was removed from
the drybox and immersed in an ice-water bath. Ethylene oxide (3 mL, 0.06 mol) was
measured out in a graduated cylinder that was cooled in an ice-water bath and 7 mL of
nitrogen-purged, 0 °C THF was added. The ethylene oxide/THF mixture was cannulated
into the reaction flask over a 1 hr period; the solution was a burgundy color. The solution
was stirred at 0 °C for 2 hrs and then allowed to warm to room temperature. 1.0 M HCI
was added until the pH was acidic; the solution would temporarily change to orange/red
upon addition of the acid. All subsequent steps were preformed in air. The suspension
125
was transferred to a 1000 mL separatory funnel. 50 mL of water and 50 mL of ether
were added to the funnel. The water layer was removed and the ether layer was washed
three times with 100 mL of water. The water extracts were combined and washed three
times with 100 mL of ether. The ether layers were combined and dried with Na2S04.
The ether was vacuumed off to leave a brown/red oil. The product was vacuum distilled.
IH NMR (CDCh) m 6.19 - 6.47 (m, 3H, alkenyl CsHs); 3.80 (q, 2H, -CfuOH); 3.01 (s,
1H, allylic CsHs); 2.95 (s, 1H, allylic CsHs); 2.68 (m, 2H, CsHsCfu).
B.3.3 Synthesis of2-(1-Methoxy-l-methyl-ethoxy)-ethyl-cyclopentadiene.
Was prepared according to the literature.
B.3.4 Synthesis of(1Js-CsH4CH2CH20H)2MolCO)6.
Was prepared according to the literature.
B.4 Synthesis of the polyurethane copolymer of Hypol-2000 and
(CSH4CHzCHzOH)zMoz(CO)6 (Hypol-Mo).
Hypol 2000 (00400 g, 0.640 mmol of -NCO) and (lls_
CS~CH2CH20H)2M02(CO)6 (0.185 g, 0.185 g, 0.0640 mmol of -oH) were added, under
nitrogen, to a 100 mL Schlenk flask containing 33 mL of THF and equipped with a
magnetic stirbar and condenser. DBTA (0.020 mL; 0.075 mol) was syringed into the
mixture, which was then refluxed for 8 h. The reaction was monitored by IR
spectroscopy. The reaction was stopped when the isocyanate band at 2273 cm-1 had
126
disappeared. Absolute ethanol (0.5 mL) was added to destroy any unreacted isocyanate
groups. The solution was stirred for 0.5 h while cooling. Hexane (50 mL) was
cannulated into the cooled solution to precipitate the polymer, which was isolated by
decanting the solution and then drying under vacuum. Mn (VPO, THF) = 7900 g/mol.
B.5 Synthesis of the polyurethane copolymer of Hypol-2000, PEG-IOOO and
(CSH4CH2CH20H)2Mo2(CO)6 (Hypol-PEG-Mo).
Hypol 2000 (0.422 g, 0.675 mmol of -NCO) was placed in a 100 mL Schlenk
flask equipped with a magnetic stir bar and condenser and then degassed, followed by a
nitrogen purge. ('Y]s-CSH4CHzCHzOHhMoz(CO)6 (0.087 g, 0.300 mmol of-OH) and
poly(ethylene glycol) (PEG-WOO; 0.188 g, 0.375 mmol of-oH) were deoxygenated
under nitrogen and then dissolved in 30 mL of freeze-pump-thawed, dry THF and
cannulated into the Schlenk flask containing Hypo12000. DBTA (0.020 mL, 0.075 mol)
was added to the reaction solution, and the solution was refluxed for 8 h, at which time
the isocyanate band at 2273 cm-! had disappeared in the infrared in the infrared spectrum.
Absolute ethanol (0.5 mL) was added to destroy any unreacted isocyanate groups, and the
solution was stirred for 0.5 h while cooling. Hexane (50 mL) was cannulated into the
cooled solution to precipitate the polymer, which was isolated by decanting the solution
and then drying under vacuum. Mn (VPO, THF) = 13500 g/mol.
127
B.6 Synthesis of Acyl-substituted Poly(vinyl chloride) (PVC-COCI).
Using Schlenk techniques, carboxylated poly(vinyl chloride) (PVC-COOH; 2.0 g,
0.8 mmol of-COOH) was dissolved in 150 mL THF in a 3-neck round bottom flask
fitted with a stir bar, Schlenk adapter, addition funnel, reflux condenser, and a drying
tube for scrubbing acid. Thionyl chloride (1.5 mL, 2.45 g, 20.6 mmol) dissolved in 50
mL THF was added to the addition funnel. The reactants were slowly combined over 1 h
and then brought to reflux for 36 h. The solution resulting solution was clear orange and
became more viscous when vacuum concentrated. Hexanes (50 mL) were added and a
white precipitant of the polymer formed. The hexanes were cannulated off and the
polymer was washed three times with 50 mL portions of hexanes. The residual solute
was removed by vacuum and the remaining white solid polymer was dried under vacuum
for 2 days. IR (KBr pellet): v(C=O) 1775 (s) and 1731 (m) em-I.
B.7 Synthesis and preparation of poly(vinyl chloride) crosslinked with
(CSH4CHzCHzOH)zMoz(CO)6 (PVC-Mo).
In a drybox under a red light, PVC-COCI (0.4 g, 0.11 mmol of-COCl) was
dissolved in THF in a round bottom flask containing a stir bar. (Y1s-
CSH4CHzCHzOH)zMoz(CO)6 (0.04 g, 0.14 mmol of-oH) were dissolved in 40 mL THF
and slowly added by pipette to the reaction flask, followed by addition of 1 mL of
triethylamine. After two days, the salt precipitate was removed by filtration. The
remaining clear, red solvent was concentrated by vacuum, poured into a Teflon-coated
mold, left for 24 h, then placed under a vacuum for two days. IR (polymer film):
128
v(C=O) 1770 (s) and 1731 (m) em-I; v(C=O) 2004 (m), 1951 (s), and 1907 (s) em-I.
Modulus = 1300 ± 100 MPa (using a grip separation speed of 5 mm/min). Tg = 65 ± 4
°c.
B.8 Synthesis and Preparation of the polyurethane copolymers ofTDI, TDI-PPG,
1,4-butanediol and (CSH4CH2CH20HhMo2(CO)6 (PU-XX).
1-(Chloromethyl)-2,4-diisocyanatobenzene, tolylene 2,4-diisocyantae terminated
poly(propylene glycol), (115-C5H4CH2CH20H)2Mo2(CO)6, and 1,4-butanediol were
added, under nitrogen, to a 150 mL Schlenk flask containing 50 mL of THF and equipped
with a magnetic stirbar and condenser (reagent quantities expressed in Table 1). DBTA
(0.020 mL, 0.075 mmol) was syringed into the reaction mixture and the solution was
refluxed for 8 h. Absolute ethanol (0.1 mL) was added, and the solution was stirred for
30 min while cooling. The solution was cast onto a Teflon surface and the solvent
allowed to evaporate under an inert atmosphere for 2 days, after which time the film was
thoroughly dried under vacuum. The polymer films were insoluble in all common
organic solvents and consequently, Mn could not be determined by VPO or GPC.
129
Table 1. The starting material quantities for the polyurethane series PU-XX.
TDI TDI-PPG Mo Dimer 1,4-butanediol
Polymer (g, mmol-NCO) (g, mmol-NCO) (g, mmol -OH) (g, mmol-OH)
PU-35 0.136,0.700 0.817,0.700 0.029, 0.100 0.086, 1.90
PU-50 0.089, 0.850 0.992, 0.850 0.024, 0.850 0.073, 0.850
PU-60 0.113, 1.08 0.840,0.720 0.026, 0.090 0.077, 1.71
PU-70 0.208,2.00 1.00,0.857 0.041,0.143 0.122,2.72
PU-90 0.376,3.60 0.467,00400 0.057,0.200 0.171,3.80
B.9 Preparation of PVC Embedded with Cp'2Mo2(CO)6 (Mo in PVC).
In a drybox, PVC-COOH (0.4 g) was dissolved in 20 mL THF. Cp'2M02(CO)6
(40 mg, 0.08 mmol) was dissolved in 5 mL THF and added to the polymer solution. The
excess solvent was concentrated by vacuum, poured into a Teflon-coated mold, left for
24 hr to evaporate slowly then placed under vacuum for two days. Tg = 72 ± 3°C.
Modulus = 1200 ± 50 MPa. The microcrystallinity of the Cp'2M02(CO)6 in the PVC
matrix could not be determined by DSC. Crystallization phases were observed at
approximately 170 and 215°C in the samples of raw PVC, Cp'2M02(CO)6, and a mixture
of the two. These peaks have not yet been identified, but there presence in all three
samples prevents drawing any conclusion as to whether the Cp'2M02(CO)6 dispersed in
PVC contains any microcrystalline regions of Cp'2M02(CO)6.
130
B.IO Preparation of Cp' zMOZ(CO)6 Solution (Mo in Soln).
In a drybox, Cp'2Mo2(CO)6 (30 mg, 0.06 mmol) and CC14 (10 mL, 15.9 g, 0.1
mol) were added to a 100 mL volumetric flask and diluted with hexane to the mark. The
solution was 6 mM in Cp'2Mo2(CO)6 and 1 M in radical trapping agent (CC14).
131
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