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V/ (200

Date

0-7639

 

LIBRARY
Michigan State
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This is to certify that the

thesis entitled

Kinetic Studies for Cationic Photopolymerization
of Epoxides

presented by

Nguyen, Khanh Phuong

has been accepted towards fulfillment
of the requirements for

Masters degree in Chemical Engineering

mm,

Major professor

6/12/00

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11m mm.“

KINETIC STUDIES FOR CATIONIC
PHOTOPOLYMERIZATION OF EPOXIDE MONOMERS

By

Khanh P. Nguyen

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Chemical Engineering

2000

ABSTRACT
KINETIC STUDIES FOR CATIONIC PHOTOPOLYMERIZATION OF EPOXIDE
MONOMERS
By

Khanh P. Nguyen

The general objective of this research is to provide a more thorough
understanding of the reaction kinetics of cationic photopolymerizations. Photo-
differential scanning calorimetry (PDSC) experiments will be performed to
systematically characterize the effects on the rate of polymerization and the ultimate
limiting conversion by a host of variables in reaction mixtures containing an epoxide
monomer, which may have different number of epoxide rings in the monomer molecule.
The variables are temperature, type and concentration of photoinitiator; type and
concentration of photosensitizer, initiating light intensity, and finally monomer structure
and functionality.

The initiating light intensity above 52 mW/cm2 produces a large increase in the
observed rate of polymerization, but only about 2 % increase in final conversion. Among
the three photoinitiators studied, tolycumyl iodonium tetrakis pentafluorophenyl borate
and diaryliodonium hexafluoroantimonate have the comparable effects; whereas the
triarylsulfonium hexafluorophosphate is the least effective. The optimum photoinitiator
concentration is 8.0 x 10’3 molal. When the reaction is photosensitized by 8.0 x 104
molal, the reaction rate increases dramatically, but the final conversion improves by only

a few percent. CPTX is determined to be a better photosensitizer than anthracene. The

effects of temperature are investigated over a range of 30°C to 40°C and finally to 50°C.
Both reaction rate and final conversion increase accordingly; however, the effect is much
greater in reaction rate than in conversion.

In short, the rate of polymerization and the ultimate limiting conversion increase
when the magnitude of the testing variable is increased; however, the relatively small
increase in conversion may be attributed to the monomer structure and functionality. The
reactivity of the three mono-epoxides investigated is as follows: phenyl glycidyl ether >
butyl glycidyl ether > octyl glycidyl ether; but for conversion: PGE > OGE > BGE. With
respect to monomer functionality, the reactivity is determined in this order: di-epoxides
2 tri-epoxides > mono-epoxides; and for conversion: mono-epoxides > di-epoxides > tri-

epoxides.

Copyright by
KHANH PHUONG NGUYEN

2000

ACKNOWLEDGMENTS

First and foremost, I would like to thank my major advisor, Dr. Alec B. Scranton
for giving me the opportunity to work in the area of cationic photopolymerization. His
knowledge, guidance, and support have been instrumental to the successful completion of
my thesis project. I feel very fortunate to have the opportunity to work with and learn
from him. Next, I would like to especially thank Dr. Hawley and Dr. Worden for serving
as committee members. The assistance and suggestions they offer have contributed to the
enhancement of my knowledge base.

I would like to thank Mr. Long Le and Mr. Ruiping Huang for their training and
assisting me in the usage of the NMR and FI‘IR spectrometers respectively. Their
expertise and experience help me learn and operate those instruments more efficiently. I
would like to thank the two student assistants, Elena Tran and Steve Shubuski, for their
contribution in performing some of the experiments. The results obtained are significant
to the progress and timely completion of this research project.

I would like to thank my colleagues in our research group: Kiran Baikerikar,
Bernhard Drescher, Julie Jessop, and Katy Padon. Our friendship has grown strongly
over the last three years by the support we have for and the anecdotes we share with each
other. These special times will remain in my fondest memories. My special appreciation
goes to the staff members of the chemical engineering department, Julie Caywood and
Candy Master, for their enthusiasm and assistance.

Last but not least, I would like to thank my parents who have instilled in me the
values of hard work. Their love and words of encouragement have always given me

comfort and strength.

TABLE OF CONTENTS

 

 

LIST OF TABLES ...... viii
LIST OF FIGURES. ....... xi
1. Introduction ............. . ........................ . ........................................... 1
1.1. Overview and Motivation for Photopolymerization l

1.2. UV-Initiated Photopolymerization.... ........... 2

1.3. Motivation for Research on Cationic Photopolymenzatron 5

1.4. References .............................................................................. 8

2. Background ............... . ................................................................... 11
2.1. Photopolymerization: Characteristics and Applications 11

2.2. Cationic Polymerizations ............................................................ 12

2.3. Cationic Photopolymerizations of Epoxides.... 13

2.4. UV- initiated Cationic Photopolymerizations.... .... 15
2.4.1. Diaryliodonium and Triarylsulfonium Salt Photo1n1t1ators ........ 15

2. 4. 2. Development and Applications of Onium Salt Photoinitiators 19

2.5. ExperimentalTechniquesw .. 22
2.5.1. Photo-Differential Scanning Calonmetry ................................ . 22

2.5.2. Fourier Transform Infrared W(F1‘IR) Spectroscopy ...................... 24

2.5.3. H-NMR Spectroscopy... . 25

2.6. References .............................................................................. 28

3. Research Objectives .......................................................................... 32
4. Characterization of Photoinitiating Systems ........ . ................................... 33
4.1. Selection of the Components in the Photoinitiating Systems... 33

4.2. Absorption of the Photoinitiating System Constituents... .. . .. 37

4.3. Verification of Monomer Molecular Structures by NMR Spectroscopy 51
4.3.1 Introduction.. 51

4.3.2. Analysis of H—NMR Spectra 58

4.4. References .............................................................................. 65

5. Kinetic Studies of Cationic Photopolymerization of Epoxide Monomers ........ 66
5.1. Photo—differential Scanning Calorimetry (PDSC)”. Studies... 66
51.1 Introduction... ... 66

5.1.2. Experimental... 67

5.1.3. Data Analysis 70

5.14 Results and Discussion... 78

5.1.5. Conclusions. .... 138

5.2. Conversions by Fourier Transform Infrared Spectroscopy (FTIR) ............... 140

5. 2. 1. Introduction.. ... 140

5.2.2. Experimental... 140

5.2.3. Results and Discussion................................. 141

vi

5.2.4 Conclusions ................................................................................. 145

5.3. References .............................................................................. 146
Conclusions and Recommendations ...................................................... 147
6.1. Summary of Results ................................................................... 147
6.1.1. Effect of Incident Light Intensity on Polymerization Kinetics 148
6.1.2. Effect of Photoinitiator Concentration on Cationic
Photopolymerization Kinetics ..................................................... 149
6.1.3. Effect of Photosensitizer and Photosensitizer Concentration
on Polymerization Kinetics... 150
6.1.4. Effect of the Monomer Structure and Functronahty on
Polymerization Kinetics" .150
6.1.5. Effect of Temperature on Polymerization Kinetics ..................... 151
6.2. Recommendations for Future Works ............................................................ 152

vii

Table 4.1

Table 4.2

Table 4.3

Table 4.4

Table 5.1

Table 5.2

Table 5.3

Table 5.4

Table 5.5

Table 5.6

Table 5.7

LIST OF TABLES

Chapter 4
Molecular structures of epoxide monomers. .......................................... 35
Molecular structures of photoinitiators and photosensitizers. ................ 36

Absorption properties of epoxide monomers, photoinitiators and
photosensitizers. ..................................................................................... 38

Weight per mole of epoxide groups and density of epoxide
monomers. .............................................................................................. 64

Chapter 5

Summary of the effects of incident light intensity on the rates of
polymerization and conversions for two representative reaction

systems. The photoinitiator was diaryliodonium
hexafluoroantimonate at 8.0 x 10'3 molal ............................................... 83
Effect of different photoinitiators on reaction rates (RP) of cationic
photopolymerization for various reaction systems at 50°C. ................... 87
Concentration effects of photoinitiator diaryliodonium
hexafluoroantimonate on reaction rates and conversions of
monoepoxy butyl glycidyl ether at 50°C ................................................. 93

Concentration effects of photoinitiator tolycumyl iodonium tetrakis
(pentafluorophenyl) borate on reaction rates and conversions of

mono—epoxy butyl glycidyl ether at 50°C. .............................................. 94
Concentration effects of photoinitiator diaryliodonium
hexafluoroantimonate on reaction rates and conversions of di-epoxy
1,4-butanediol diglycidyl ether at 50°C. ................................................ 94

Concentration effects of photoinitiator tolycumyl iodonium tetrakis
(pentafluorophenyl) borate on reaction rates and conversions of di-
epoxy 1,4-butanediol diglycidyl ether at 50°C. ..................................... 95

Effects on the polymerization rate, R,, for a reaction system
containing 8.0 x 10‘3 molal of photoinitiator tolycumyl iodonium
tetrakis (pentafluorophenyl) borate, B-[F5]4' and 8.0 x 10'4 molal of
photosensitizer and a mono-epoxide monomer ...................................... 119

viii

Table 5.8

Table 5.9

Table 5.10

Table 5.11

Table 5.12

Table 5.13

Table 5.14

Table 5.15

Effects on the polymerization rate, R,, for a reaction system
containing 8.0 x 10'3 molal of photoinitiator diaryliodonium
hexafluoroantimonate, [Sng]' and 8.0 x 104 molal of
photosensitizer and a mono-epoxide monomer at various

temperatures. ..........................................................................................

Photosensitizer effects on the limiting conversion for a reaction
system containing 8.0 x 10'3 molal of photoinitiator tolycumyl
iodonium tetrakis (pentafluorophenyl) borate, B-[F5]4' and 8.0 x 10'4
molal of photosensitizer and a mono-epoxide monomer at various

temperatures. ..........................................................................................

Photosensitizer effects on the limiting conversion for a reaction
system containing 8.0 x 10'3 molal of photoinitiator diaryliodonium
hexafluoroantimonate, [Sde' and 8.0 x 10“ molal of
photosensitizer and a mono-epoxide monomer at various

temperatures. ..........................................................................................

Effects on the polymerization rate, R,, for a reaction system
containing 8.0 x 10'3 molal of photoinitiator Tolycumyl iodonium

tetrakis (pentafluorophenyl) borate, B-[F5]4' and 8.0 x 10'4 molal of

photosensitizer and a di-epoxide monomer at various temperatures. .....

Effects on the polymerization rate, R,, for a reaction system
containing 8.0 x 10'3 molal of photoinitiator diaryliodonium
hexafluoroantimonate, [SbFJ and 8.0 x 104 molal of

photosensitizer and a di-epoxide monomer at various temperatures. .....

Photosensitizer effects on the limiting conversion for a reaction
system containing 8.0 x 10'3 molal of photoinitiator tolycumyl
iodonium tetrakis (pentafluorophenyl) borate, B-[F5]4' and 8.0 x 10'4

molal of photosensitizer and a di-epoxide monomer ..............................

Photosensitizer effects on the limiting conversion for a reaction
system containing 8.0 x 10'3 molal of photoinitiator diaryliodonium
hexafluoroantimonate, [Sde' and 8.0 x 10" molal of

120

121

122

123

124

125

photosensitizer and a di-epoxide monomer at various temperatures. ..... 126

Photosensitizer effects on pol erization rate, R,, for a reaction
system containing 8.0 x 10' molal of photoinitiator tolycumyl
iodonium tetrakis (pentafluorophenyl) borate, B-[F5]4' and 8.0 x 10'4

molal of photosensitizer and a tri-epoxide monomer. ............................ 127

ix-

Table 5.16

Table 5.17

Table 5.18

Table 5.19

Table 5.20

Table 5.21

Table 6.1

Photosensitizer effects on the polymerization rate, Rp, for a reaction
system containing 8.0 x 10’3 molal of photoinitiator diaryliodonium
hexafluoroantimonate, [soar and 8.0 x 10“ molal of

photosensitizer and a tri-epoxide monomer at various temperatures.

Photosensitizer effects on conversion for a reaction system
containing 8.0 x 10’3 molal of photoinitiator tolycumyl iodonium
tetrakis (pentafluorophenyl) borate, B-[F5]4' and 8.0 x 10‘4 molal of

photosensitizer and a tri-epoxide monomer at various temperatures.

Photosensitizer effects on conversion for a reaction system
containing 8.0 x 10'3 molal of photoinitiator diaryliodonium
hexafluoroantimonate, [SbF5]' and 8.0 x 104 molal of

photosensitizer and a tri-epoxide monomer at various temperatures.

Concentration effects of photosensitizer CPTX on reaction systems
containing 8.0 x 10'3 molal of photoinitiator diaryliodonium

hexafluoroantimonate and mono-epoxy butyl glycidyl ether. ................

Effect of monomer structure and functionali on the conversion of
reaction mixtures comprising of 8.0 x 10' molal photoinitiator
tolycumyl iodonium tetrakis (pentafluorophenyl) borate, B-[Fsla' and

different epoxide monomer at 50°C. ......................................................

Percent conversion by cationic photopolymerization for various

reaction systems using the method of FTIR. ..........................................

Chapter 6

Summary of distinct reaction formulations in the kinetic studies of

128

129

130

131

137

144

cationic photopolymerization of epoxide monomers. ............................ 148

Figure 2.1

Figure 4.1

Figure 4.2

. Figure 4.3

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7

Figure 4.8

Figure 4.9

Figure 4.10

Figure 4.11

Figure 4.12

Figure 4.13

LIST OF FIGURES

Chapter 2

Experimental set-up for the Photodifferential scanning calorimetry
PDSC)experiments... . .

Chapter 4
Emission spectrum of the PDSC lamp, 200 Watt Hg/Xe arc lamp .....

Absorption spectrum of the mono-epoxide monomer octyl glycidyl
ether .....................................................................................................

Absorption spectrum of the mono-epoxide monomer butyl glycidyl
ether .....................................................................................................

Absorption spectrum of the mono-epoxide monomer phenyl
glycidyl ether .......................................................................................

Absorption spectrum of the (ii-epoxide monomer 1,4-butanediol
diglycidyl ether ....................................................................................

Absorption spectrum of the di-epoxide monomer cyclohexane
dimethanol diglycidyl ether .................

 

Absorption spectrum of the tri-epoxide monomer tn'methyol
propne triglycidyl ether .......................................................................

Absorption spectrum of the photoinitiator tolycumyl iodonium
tetrakis (pentafluorophenyl) borate .....................................................

Absorption spectrum of the photoinitiator diaryliodonium
hexafluoroantimonate ..........................................................................

Absorption spectrum of the photoinitiator tn'arylsulfonium
hexafluorophosphate ...........................................................................

Absorption spectrum of the photosensitizer 1-chloro-4-propoxy -
9H-thioxanthen-9-one (CPTX) ............................................................

Absorption Spectrum of the photosensitizer anthracene ....................
1H-NMR spectrum of the mono-epoxide monomer octyl glycidyl
ether .....................................................................................................

xi

23

39

40

41

42

43

45

46

47

48

49

50

52

Figure 4.14

Figure 4.15

Figure 4.16

Figure 4.17

Figure 4.18

Figure 4.19

Figure 4.20

Figure 4.21

Figure 4.22

Figure 4.23

Figure 4.24

Figure 5.1

Figure 5.2

1H-NMR spectrum of the mono-epoxide monomer butyl glycidyl
ether .....................................................................................................

lI‘I-NMR spectrum of the mono-epoxide monomer phenyl glycidyl
ether .....................................................................................................

1H-NMR spectrum of the (ii-epoxide monomer 1,4-butanediol
diglycidyl ether ....................................................................................

1H-NMR spectrum of the (Ii-epoxide monomer cyclohexane
dimethanol diglycidyl ether .................................................................

1H-NMR spectrum of the tri-epoxide monomer trimethyol propane
triglycidyl ether ...................................................................................

Molecular structure of the mono-epoxide monomer octyl glycidyl
ether .....................................................................................................

Molecular structure of the mono-epoxide monomer butyl glycidyl
ether .....................................................................................................

Molecular structure of the mono-epoxide monomer phenyl
glycidyl ether .......................................................................................

Molecular structure of the di-epoxide monomer 1,4-butanediol
diglycidyl ether ....................................................................................

Molecular structure of the di-epoxide monomer cyclahexane
dimethanol diglycidyl ether

Molecular structure of the tri-epoxide monomer trimethyol
propane triglycidyl ether .

Chapter 5

PDSC reaction exotherm for a reaction system containing di-
epoxide monomer 1,4-butanediol diglycidyl ether and 8.0 x 10'3
molal of photoinitiator tolycumyl iodonium tetrakis
(pentafluorophenyl) borate, (B'[F5]4' ) at 50°C ...................................

A profile of the rate of polymerization, R,, for a reaction system
containing (ii-epoxide monomer 1,4-butanediol diglycidyl ether
and 8.0 x 10'3 molal of photoinitiator tolycumyl iodonium tetrakis
(pentafluorophenyl) borate, (B-[F5]4') at 50°C ...................................

xii

53

54

55

56

57

58

59

60

60

61

62

71

72

Figure 5.3 Integrated released heat of reaction for a reaction system
containing di—epoxy 1.4-butanediol diglycidyl ether and 8. 0 x 10 3
molal of photoinitiator tolycumyl iodonium tetrakis
(pentafluorophenyl) borate, (B-[F5]4' ) at 50°C. ................................. 74

Figure 5.4 Conversion profile for the reaction system containi3ng di-epoxy
1, 4- butanediol diglycidyl ether and 8. 0 x 10'3 molal of
photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl)
borate, (B‘[F5]4. ) at 50°C. .................................................................. 75

Figure 5.5 Profile of 191M“) for a reaction system containing (ii-epoxide
monomer 1,4 butanediol diglycidyl ether and 8 .o x 10 3 molal of
photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl)
borate, (B- [F5]4 ) and 8. o x 10“ molal photosenstizer CPTX at
50°C. ................................................................................................... 77

Figure 5.6 Reaction rates for cationic photopolymerization of monoepoxy
octyl glycidyl ether and 8. 0 x 10'3 molal of photoinitiator
tolycumyl iodoniumetrakis (pentafluorophenyl) borate (B [F5]4 )
at various temperatures... .. .. . 79

Figure 5.7 Temperature effect on conversion for cationic
photopolymerization of octyl glycidyl ether in the same reaction
systems as Figure 56 79

Figure 5.8 Reaction rates for cationic photopolymerization of 1.4-butanediol
diglycidyl ether and 8. o x 10 3 molal of photoinitiator tolycumyl
iodonium tetrakis (pentafluorophenyl) borate (B-[F5]4 )at various
temperatures ........................................................................................ 80

Figure 5.9 Effect of temperature on total conversion for cationic
photopolymerization of 1,4-butanediol diglycidyl ether and 8. 0 x
10’3 molal of photoinitiator tolycumyl iodonium tetrakis
(pentafluorophenyl) borate (B-[F5]4')at various temperatures ............ 80

Figure 5.10 Rates of polymerization for cationic photopolymerization of tri-
epoxide monomer trimethyol propane triglycidyl ether with 8. 0 x
10 3 molal of tolycumyl iodonium tetrakis (pentafluorophenyl)
borate, ---B [F5]4 at various temperatures... . 81

Figure 5.11 Effect of temperature on total conversion for cationic
photopolymerization of tri-epoxide monomer trimethyol propane
triglycidyl ether with 8. 0 x 10 3 molal of tolycumyl iodonium
tetrakis (pentafluorophenyl) borate, ---B [F5]4 at various
temperatures... . 81

xiii

Figure 5.12

Figure 5.13

Figure 5.14

Figure 5.15

Figure 5.16

Figure 5.17

Figure 5.18

Figure 5.19

Figure 5.20

Figure 5.21

Profile of reaction rate vs. conversion for the reaction system of
mono-epoxy octyl glycidyl ether and 8.0 x 10'3 molal of
photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl)
borate at various temperatures ............................................................

Profiles of reaction rate vs. conversion for the reaction systems of
di-epoxy 1,4-butanediol glycidyl ether and 8.0 x 10‘3 molal of
photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl)
borate at various temperatures ............................................................

Effects of light intensity for cationic polymerization of octyl
glycidyl ether photoinitiated by 8.0 x 10'3 molal of diaryliodonium
hexafluoroantimonate at 50°C .............................................................

Effects of light intensity on both reaction rates and conversions for
cationic photopolymerization of octyl glycidyl ether photoinitiated
by 8.0 x 10'3 molal of diaryliodonium hexafluoroantimonate at
50°C .....................................................................................................

Rates of polymerization for cationic photopolymerization of 1,4-
butanediol diglycidyl ether at different initiation light intensities
by 8.0 x 10'3 molal of photoinitiator diaryliodonium
hexafluoroantimonate at 50°C .............................................................

Effects of light intensity on conversion profiles for cationic
photopolymerization of (Ii-epoxide monomer 1.4-butanediol
diglycidyl ether by 8.0 x 10‘3 molal of photoinitiator
diaryliodonium hexafluoroantimonate at 50°C ...................................

Effect of photoinitiators tolylcumyl iodonium tetrakis
(pentafluorophenyl) borate, B-[F5]4-; and diaryliodonium

hexafluoroantimonate, [SbF5]- for mono-epoxy octyl glycidyl
ether at 50°C ........................................................................................

Effect of photoinitiators on conversion profiles in the same
reaction systems as in Figure 5.18. .....................................................

Effect of photoinitiators tolylcumyl iodonium tetrakis
(pentafluorophenyl) borate, B-[F5]4.; and diaryliodonium

hexafluoroantimonate, [Sng]- for mono-epoxy butyl glycidyl
ether at 50°C. .......................................................................................

Effect of photoinitiators on conversion profiles in the same
reaction systems as in Figure 5.20. .....................................................

xiv

82

82

84

84

85

85

88

88

89

89

Figure 5.22 Effect of photoinitiators tolylcumyl iodonium tetrakis
(pentafluorophenyl) borate, B-[Fslfl or diaryliodonium
hexafluoroantimonate, [Sng]’ for mono—epoxy phenyl glycidyl
ether at 50°C. ....................................................................................... 90

Figure 5.23 Effects of photoinitiators on conversion profiles for reaction
systems containing tolylcumyl iodonium tetrakis
(pentafluorophenyl) borate, B-[F5]4'; or diaryliodonium
hexafluoroantimonate, [Sde' and mono-epoxy phenyl glycidyl
ether at 50°C ........................................................................................ 90

Figure 5.24 Effects of photoinitiators tolylcumyl iodonium tetrakis
((pentafluorophenyl)) borate, B-[F5]4-; or diaryliodonium

hexafluoroantimonate, [SbFol- for di—epoxide monomer 1,4-
butanediol diglycidyl ether at 50°C. ................................................... 91

Figure 5.25 Effect of photoinitiators on conversion profiles in the same
reaction systems as in Figure 5.24. ..................................................... 91

Figure 5.26 Effect of photoinitiators tolylcumyl iodonium tetrakis
((pentafluorophenyl)) borate, B-[F5]4-; and diaryliodonium

hexafluoroantimonate, [SbF6]- for di-epoxide monomer
cyclohexane dimethanol diglycidyl ether at 50°C ................................ 92

Figure 5.27 Effect of photoinitiators on conversion profiles in the same
reaction systems as in Figure 5.26. ..................................................... 92

Figure 5.28 Concentration effect of photoinitiator tolycumyl iodonium tetrakis
((pentafluorophenyl» borate, B-[F5]4' on mono-epoxy butyl
glycidyl ether at 50°C. ......................................................................... 95

Figure 5.29 Effects of photoinitiator concentration on conversion in systems
comprising of mono—epoxy butyl glycidyl ether and photoinitiator
tolycumyl iodonium tetrakis (pentafluorophenyl) borate, B-[F5]4'
at 50°C. ................................................................................................ 95

Figure 5.30 Profiles of the rate of polymerization, Rp, versus conversion for
mono-epoxy butyl glycidyl ether with various concentrations of
photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl)
borate, B-[F5]4' at 50°C. ...................................................................... 96

Figure 5.31 Concentration effects of photoinitiator diaryliodonium

hexafluoroantimonate, [SbFJ on mono-epoxy butyl glycidyl ether
at 50°C. ................................................................................................ 97

XV

Figure 5.32 Effects of photoinitiator concentration on conversion in systems
comprising of mono-epoxy butyl glycidyl ether and photoinitiator
diaryliodonium hexafluoro-antimonate, [SbFsl'at 50°C ...................... 97

Figure 5.33 Profiles of the rate of polymerization versus conversion for mono-
epoxy butyl glycidyl ether with various concentrations of
photoinitiator diaryliodonium hexafluoroantimonate, [SbF5]'at
50°C. .................................................................................................... 98

Figure 5.34 Concentration effects of photoinitiator tolycumyl iodonium tetrakis
(pentafluorophenyl) borate, B-[F5]4' on di-epoxy 1.4-butanediol
diglycidyl ether at 50°C. ...................................................................... 99

Figure 5.35 Effects of photoinitiator concentration on conversion in systems
comprising of di-epoxy 1,4-butanediol diglycidyl ether and
photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl)
borate, B-[F5]4' at 50°C. ...................................................................... 99

Figure 5.36 Profiles of the rate of polymerization versus conversion for di-
epoxy 1,4-butanediol diglycidyl ether with various concentrations
of photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl))
borate, B-[F5]4' at 50°C. ...................................................................... 100

Figure 5.37 Concentration effects of photoinitiator diaryliodonium hexaflouro-
antimonate, [Sde' on di-epoxy 1.4-butanediol diglycidyl ether at
50°C. .................................................................................................... 101

Figure 5.38 Effects of photoinitiator concentrations on conversion in reaction
systems comprising of di-epoxy 1,4-butanediol diglycidyl ether
and photoinitiator diaryliodonium hexafluoroantimonate, [SbF5]'at
50°C. .................................................................................................... 101

Figure 5.39 Profiles of the rate of polymerization versus conversion for di-
epoxy 1,4-butanediol diglycidyl ether with various concentrations
of photoinitiator diaryliodonium hexafluoroantimonate, [SbFJat
50°C. .................................................................................................... 102

Figure 5.40 Temperature effects for reaction systems of mono—epoxy octyl
glycidyl ether with 8.0 x 10‘3 molal photoinitiator diaryliodonium
hexafluoro-antimonate, [SbFJ and 8.0 x 10“ molal
photosensitizer CPTX .......................................................................... 105

Figure 5.41 Effect of photosensitization on final conversion for the same
reaction systems as in Figure 5.40. .................................................... 105

xvi

Figure 5.42

Figure 5.43

Figure 5.44

Figure 5.45

Figure 5.46

Figure 5.47

Figure 5.48

Figure 5.49

Figure 5.50

Figure 551

Figure 5.52

Temperature effects for reaction systems of monoepoxy octyl
glycidyl ether with 8.0 x 10'3 molal photoinitiator tolycumyl
iodonium tetrakis (pentafluorophenyl) borate, B-[F5]4’ and 8.0 x
104 molal photosensitizer CPTX. ....................................................... 106

Effect of photosensitization on final conversion for the same
reaction systems as in Figure 5.42. ..................................................... 106

Photosensitizer effect for reaction s stems of mono-epoxy octyl
glycidyl ether with 8.0 x 10' molal of photoinitiator
diaryliodonium hexafluoroantimonate and 8.0 x 10‘4 molal of
photosensitizer at 50°C. ....................................................................... 107

Photosensitizer effect on final conversion for the same reaction
systems as in Figure 5.44. .................................................................. 107

Photosensitizer effect for reaction system of mono-epoxy octyl
glycidyl ether with 8.0 x 10‘3 molal photoinitiator tolycumyl
iodonium tetrakis ((pentafluorophenyl)) borate, B‘[F5]4° and 8.0 x
104 molal photosensitizer at 50°C ....................................................... 108

Photosensitizer effect on final conversion for the same reaction
systems as in Figure 5.46. ................................................................... 108

Photosensitizer effect on the conversion for reaction systems
comprising of octyl glycidyl ether and 8.0 x 10'3 molal
photoinitiator tolycumyl iodonium tetrakis ((pentafluorophenyl))
borate, B-[F5]4'. ................................................................................... 109

Temperature effects for reaction systems of (ii-epoxide monomer
1,4-butanediol diglycidyl ether with 8.0 x 10'3 molal of
photoinitiator diaryliodonium hexa-fluoroantimonate, [SbFJand
8.0 x 10*4 molal of photosensitizer CPTX. .......................................... 110

Photosensitizer effect on final conversion for the same reaction
systems as in Figure 5.49. ................................................................... 110

Temperature effects for reaction systems of di-epoxy 1,4-
butanediol diglycidyl ether with 8.0 x 10'3 molal of photoinitiator
Tolycumyl iodonium tetrakis ((pentafluorophenyl)) borate, B-[F5]4’
and 8.0 x 10“ molal of photosensitizer CPTX. ................................... 111

Photosensitizer effect on final conversion for the same reaction
systems as in Figure 5.51. ................................................................... 111

xvii

Figure 5.53

Figure 5.54

Figure 5.55

Figure 5.56

Figure 5.57

Figure 5.58

Figure 5.59

Figure 5.60

Figure 5.61

Figure 5.62

Figure 5.63

Photosensitizer effect for reaction system of di-epoxy 1,4-
butanediol diglycidyl ether with 8.0 x 10'3 molal of photoinitiator
diaryliodonium hexafluoroantimonate and 8.0 x 104 molal of

photosensitizer at 50°C. .......................................................................

Photosensitizer effect on final conversion for the same reaction

systems as in Figure 5.53 ....................................................................

Photosensitizer effect for reaction system of di—epoxy 1,4-
butandiol diglycidyl ether with 8.0 x 10’3 molal of photoinitiator

tolycumyl iodonium tetrakis (pentafluorophenyl) borate, B’[F5]4-

and 8.0 x 10'4 molal of photosensitizer at 50°C ...................................

Photosensitizer effect on final conversion for the same reaction

systems as in Figure 5.55. ...................................................................

Temperature effects for reaction systems of tri-epoxy trimethyol
propane triglycidyl ether with 8.0 x 10‘3 molal of photoinitiator

diaryliodonium hexafluoroantimonate, [SbFJand 8.0 x 104 molal

of photosensitizer CPTX. ....................................................................

Photosensitizer effect on final conversion for the same reaction

systems as in Figure 5.57. ...................................................................

Temperature effects for reaction systems of tri-epoxide monomer
trimethyol propane triglycidyl ether with 8.0 x 10'3 molal of

tolycumyl iodonium tetrakis (pentafluorophenyl) borate, B-[Fs];

and 8.0 x 10‘4 molal of photosensitizer CPI'X. ...................................

Photosensitizer effect on final conversion for the same reaction

systems as in Figure 5.59. ...................................................................

Photosensitizer effect for reaction system of tri-epoxy trimethyol
propane triglycidyl ether with 8.0 x 10'3 molal of photoinitiator
diaryliodonium hexafluoroantimonate and 8.0 x 104 molal of

photosensitizer at 50°C. .......................................................................

Photosensitizer effect on final conversion for reaction system of
tri-epoxy trimethyol propane triglycidyl ether with 8.0 x 10'3 molal
of photoinitiator diaryliodonium hexafluoroantimonate and

8.0 x 10'4 molal of photosensitizer at 50°C. ........................................

Photosensitizer effect for reaction system of tri-epoxy trimethyol
propane triglycidyl ether with 8.0 x 10'3 molal of photoinitiator

xviii

112

112

113

113

114

114

115

115

116

116

Figure 5.64

Figure 5.65

Figure 5.66

Figure 5.67

Figure 5.68

Figure 5.69

Figure 5.70

Figure 5.71

Figure 5.72

tolycumyl iodonium tetrakis ((pentafluorophenyl)) borate, B-[Fs]; 1 17
and 8.0;t10‘3 molal of photosensitizer at 50°C .....................................
Photosensitizer effect on final conversion for the same reaction
systems as in Figure 5.63. ................................................................... 117

Effects of photosensitizer concentration for reaction 8 stems
containing mono-epoxy butyl glycidyl ether and 8.0 x 10' molal

of photoinitiator diaryliodonium hexafluoroantimonate, [SbF5]- at
O
50 C. .................................................................................................... 132
Effects of photosensitizer concentrations on final conversion of the
same reaction systems in Figure 5.65. ................................................ 132

Rates of polmerization for various mono-epoxide monomers
photoinitiated by 8.0 x 10’3 molal of tolycumyl iodonium tetrakis

(pentafluorophenyl) borate, B-[Fs]; at 50°C. ..................................... 135

Conversion profiles of various mono-epoxide monomers
photoinitiated by 8.0 x 10'3 molal of tolycumyl iodonium tetrakis

(pentafluorophenyl) borate, B-[Fs]; at 50°C. ..................................... 135

Effect of monomer’s functionality on reaction rates for reaction
systems with 8.0 x10'3 molal of photoinitiator tolycumyl iodonium

tetrakis (pentafluorophenyl) borate, B-[F5]4' at temperature of
50°C. ....................................................................................................

136
Effect of monomer’s functionality on conversion for reaction
systems with 8.0 x 10'3 molal photoinitiator tolycumyl iodonium

tetrakis (pentafluorophenyl) borate, B-[Fs]; at temperature of
50°C. ....................................................................................................

136
EUR spectra for cationic photopolymerization of mono-epoxy
octyl glycidyl ether initiated by 8.0 x 10'3 molal of tolycumyl
iodonium tetrakis pentafluorophenyl borate and 8.0 x 10 molal
CPTX. .................................................................................................. 142

FITR spectra for cationic photopolymerization of di-epoxy 1,4-
butanediol diglycidyl ether initiated by 8.0 x 10'3 molal of
tolycumyl iodonium tetrakis pentafluorophenyl borate and

8.0 x 10‘4 molal CPTX ......................................................................... 143

xix

Chapter 1

Introduction

1.1. Overview and Motivation for Photopolymerizations

The emission of volatile organic components (VOCS) from curing inks, films and
coatings can contribute to atmospheric pollution. Typically, an ink or coating
formulation must be sufficiently fluid to be easily applied (usually onto a rapidly moving
substrate through a series of transfer rollers), but must cure rapidly to a hard film. These
processing demands have been traditionally met through the use of a rapidly evaporating
organic solvent in the ink and coating formulations. However, when these volatile
organic solvents enter the atmosphere, they result in the formation of smog and air
pollution. Therefore, there is an urgent need for the development of hi gh-performance
inks and coatings that will not emit VOCs to the atmosphere.

The curing of an ink or coating formulation to form a highly crosslinked polymer
film could be initiated by a variety of energy sources, including heat, electromagnetic
radiation, and electron beam irradiation. Unfortunately, heat-initiated thermal systems
typically require high temperatures to achieve reasonable cure rates. The elevated
temperatures not only lead to high energy costs, but can also result in significant
distortions in substrate dimensions in many cases.1 High-energy irradiation such as
gamma radiation and electron beam radiation can lead to rapid cure rates in a variety of
systems; however, these techniques will likely have very limited applications because

they would typically result in degradation of the substrate”. Light-induced

photopolymerizations have many potential advantages for ink and coating formulations
including very high reaction rates at room temperature, low energy requirements, and
versatility since a wide variety of monomers may be initiated photochemically. In
addition, degradation of the substrate can be avoided when use with appropriate choice of

the wavelength of the light source.

1.2. UV-Initiated Photopolymerizations

Photopolymerizations initiated by ultraviolet (UV) light have gained prominence
in recent years for the rapid, pollution-free curing of polymer films.” These solvent-free
polymerizations proceed very rapidly with a fraction of the energy requirements of
thermally cured systems, and create films with excellent properties. Ultraviolet light is a
convenient energy source for photopolymerization because a variety of readily available
compounds will initiate chain polymerizations upon absorption of UV light.°’12 UV-
sensitive photoinitiators are currently available for free-radical or cationic
polymerizations. These photoinitiators are typically effective for a variety of incident

wavelengths.”

This feature is useful for UV-curable inks and coatings because the
commonly used pigments may be strong absorbers of light in the visible and ultraviolet
wavelengths, and an initiator which will be effective at a wavelength outside this window
must be chosen.

Free-radical photopolymerizations were first reported in the literature nearly fifty

68.13

years ago,13 and are currently receiving considerable attention. By far the most widely

used classes of monomers for UV-initiated free-radical photopolymerizations are

multifunctional acrylates and methacrylates. These monomers polymerize very rapidly,
and are easily modified on the ester group, allowing materials with a variety of properties
to be obtained.6 However, the acrylates are relatively volatile and have an unpleasant
odor.‘5 Moreover, recently there has been growing concern over potential health hazards

associated with the acrylates."‘“

Several recent investigations have demonstrated that
free-radical polymerizations of multifunctional acrylates and methacrylates exhibit
unusual kinetic behavior, including immediate onset of autoacceleration with the

6.15.20

formation of heterogeneous polymers, and the attainment of a maximum conversion

631-24

significantly less than unity. Finally, the free radical photopolymerizations are

”"3 such as

inhibited by oxygen and must be canied out under an inert atmosphere,
nitrogen.

UV-initiated cationic photopolymerizations display several advantages when
compared with the free-radical photopolymerization discussed above. First of all the
cationic photopolymerizations are not inhibited by oxygen.7"3'“” This feature provides an
important practical advantage for industrial processes since it is not necessary to blanket
the system with nitrogen to achieve rapid cure rates. Secondly, in contrast to the free-
radical polymerizations which experience a rapid decrease in polymerization rate when
the light source is removed (due to radical-radical termination reactions), the cationic
polymerizations will proceed long after the irradiation has stopped, consuming nearly all
of the monomer)” Finally, cationic photopolymerization is a very versatile technique,
and may be used to polymerize important classes of monomers, including epoxides and

vinyl ethersfw” Although these classes of monomers cannot cure by free-radical

polymerizations, they exhibit many desirable properties, including low volatility, good

rheological properties and negligible toxicity.7 Furthermore, the cured polymer films
associated with these monomers exhibit excellent clarity, adhesion, abrasion resistance,
and chemical resistance.”““”'30

Despite the advantages of UV-initiated photopolymerizations discussed above,
cationic photopolymerizations have received considerably less attention than the
analogous free-radical reactions. This fact may be attributed to the lack of suitable UV-
sensitive cationic photoinitiators until decades after radical photoinitiators were
available.” Crivello and Lam reported two classes of thermally stable photoinitiators
for cationic polymerizations: diaryliodonium and triarylsulfonium salts.’o Upon
photolysis, these compounds undergo irreversible fragmentation in which the carbon-
iodine or carbon-sulfur bond is cleaved to produce an aryliodonium or an arylsulfonium
cation-radical capable of initiating cationic polymerization.n While the diaryliodonium
and triarylsulfonium salts actively initiate cationic polymerization in the presence of UV
light, they are remarkably latent in the absence of light. In fact, fully formulated
solutions of these salts in epoxide monomers have shelf lives of several years.12

The wavelength at which the diaryliodonium and triarylsulfonium salts indicate
their maximum UV absorbance depends somewhat upon substituents attached to the ring,
but typically falls between 225 and 275 nm.”8 Although the initiators absorb strongly at
wavelengths near 250 nm, their absorption diminishes at longer wavelengths.” This fact
could limit the efficiency of the initiators for photopolymerizations driven by mercury
lamps which provide most of their emission at wavelengths above 300 um?” However,
the spectral region over which the initiators are effective may be expanded by the

addition of a variety of photosensitizers, including hydrocarbons, ketones and

heterocyclic compounds.9 These compounds actively sensitize the iodonium and
sulfonium initiators by an electron transfer process, rendering them effective in the long

UV and visible wavelengths of lightfm'

1.3. Motivation for Research on Cationic Photopolymerizations

UV-initiated cationic photopolymerizations have considerable potential for the
development of improved coatings and inks. These reactions may be used to rapidly
form highly crosslinked polymer films exhibiting excellent adhesion, abrasion resistance,
and chemical resistance without emitting volatile organic components and using a
fraction of the energy requirements of thermal systems. Unlike free radical
photopolymerizations, the cationic reactions are not inhibited by atmospheric oxygen,
and may be used to polymerize epoxides and vinyl ethers. These monomers exhibit many
desirable properties, including low volatility, good rheological characteristics and
negligible toxicity?” Fully formulated systems containing monomer, pigment and initia-
tor exhibit shelf lives of more than a year,‘2 and initiators may be chosen to be effective
for wavelengths at which the pigments have low extinction coefficients. Finally, the UV
irradiation will not significantly degrade the substrate (unlike electron beam curing) and
the cationic polymerization can withstand moderate amounts of water without a
significant decrease in reaction rate.'”’

Despite the promise of UV-initiated cationic photopolymerizations, these
reactions have received only limited attention. This fact may be attributed primarily to

the lack of suitable initiators until recent years. In fact, most of the work on cationic

photopolymerizations reported in the literature focuses on the initiation step of the

842.2549

reaction. The development of thermally stable UV-sensitive cationic photoinitiators

has led to increased interest in the cationic photopolymerizations of epoxides and vinyl

ethers in the last ten years; 2"”

however, the field is significantly less developed than that
of free-radical photopolymerizations. For example, the reaction kinetics of free radical
photopolymerizations of multifunctional acrylates and methacrylates have been

extensively investigated,°"'”

revealing anomalous kinetic behavior, including immediate
onset of autoacceleration, the formation of heterogeneous networks, and the attainment of
a maximum conversion significantly less than unity. However, similar experiments have
not been reported for cationic photopolymerizations. The reaction kinetics of cationic
photopolymerizations of diepoxides and bisvinyl ethers typically have been characterized
only macroscopically in terms of the tack-free time of the polymer film.

The broad objective of this thesis is to provide a more detailed characterization of
the kinetics of cationic photopolymerizations. The specific aims of this research effort
will be outlined in Chapter 3. A number of parameters will be investigated for their
effects on the rate of polymerization and the final conversion. The parameters are
temperature, type and concentration of photoinitiator, type and concentration of
photosensitizer, initiating light intensity, and finally the structure and functionality of the
epoxide monomer. There are three different classes of epoxide monomers that will be
investigated: 1) each of the three mono-epoxide monomers has only one epoxide ring in
the molecule, but they differ from each other by the length and type of pedant group

attached to the epoxide ring; 2) each of the two di-epoxide monomers has two groups of

epoxide rings in the molecule, and differs from each other by the middle group

connecting the two epoxide rings; and 3) one tri-epoxide monomer which has three
epoxide rings in the molecule. The reactive rnixuture must contain a photoinitiator and
may contain a photosensitizer. There are three different photoinitiators and two distinct
photosensitizers that will be investigated. All these components will be thoroughly
characterized and the specific reasons for their selection in this research can be found in
chapter 4. The primary experimental technique is called Photo-differential scanning
calorimetry (PDSC). Based on the analysis and interpretation of the raw data provided by
the PDSC experiments, substantial information can be obtained with regard to the
kinetics of cationic photopolymerizations (chapter 5). In addition to PDSC, other
techniques, such as ‘H-NMR spectroscopy is used to verify the monomer molecular
structure, and the technique of Fourier Transform Infrared (FI'IR) spectroscopy will be
utilized to obtain degree of monomer conversion. All experimental techniques are
described in chapter 2. Finally, chapter 6 presents valuable results and findings with

respect to the kinetics of cationic photopolymerizations of epoxide monomers.

10.

ll.

12.

1.4. References

E. L. Grarninski, “Fundamental Chemical and Engineering Issues in Paper
Currency Production,” National Science Foundation, Washington D. C., 1
(1991).

P. E. Sundell, S. Jonsson, and A. Hult, “Radiation Curing of Polymeric
Materials,” edited by C. E. Hoyle and J. F. Kinstle, ACS Symposium Series,
Vol. 417, American Chemical Society, Washington D. C., 459 (1989).

H. C. Kim, A. M. El-Naggar, G. L. Wilkes, Y. Y00 and J. E. McGrath,
“Radiation Curing of Polymeric Materials,” edited by C. E. Hoyle and J. F.
Kinstle, ACS Symposium Series, Vol. 417, American Chemical Society,
Washington D. C., 474 (1989).

V. Stannett and A. Deffieux, “Cationic Polymerization and Related Processes,”
edited by E. J. Goethals, International Union of Pure and Applied Chemistry,
Oxford, UK, 307 (1984).

J. Pacansky and R. J. Waltrnan, “Radiation Curing of Polymeric Materials,” °
edited by C. E. Hoyle and J. F. Kinstle, ACS Symposium Series, Vol. 417,
American Chemical Society, Washington D. C., 498 (1989).

J. G. Kloosterboer, Adv. Polym. Sci., 84, 1 (1988).

C. G. Roffey, “Photopolymerization of Surface Coatings,” Wiley, New York,
(1981).

S. P. Pappas, “UV Curing, Science and Technology,” Vol. 2, Technology
Marketing Corporation, Norwalk, CT, (1985).

J. V. Crivello, Adv. Polym. Sci., 62, 1 (1984).

J. V. Crivello and J. L. Lee, J. Polym. Sci., Polym. Chem. Ed, 27, 3951 (1989).
A. Ledwith, S. Al-Kass, and A. Hulm-Lowe, “Cationic Polymerization and
Related Processes,” edited by E. J. Goethals, International Union of Pure and
Applied Chemistry, Oxford, UK, 275 (1984).

J. V. Crivello, “Cationic Polymerization and Related Processes,” edited by E. J.

Goethals, International Union of Pure and Applied Chemistry, Oxford, UK, 289
(1984).

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

25.

26.

27.

28.

A. Reiser, “Photoreactive Polymers,” Wiley, New York, NY (1989).

S. P. Lapin, “Radiation Curing of Polymeric Materials,” edited by C. E. Hoyle
and J. F. Kinstle, ACS Symposium Series, Vol. 417, American Chemical
Society, Washington D. C., 363 (1989).

H. M. J. Boots, J. G. Kloosterboer, and G. van de Hei, Brit. Polym. J., 17, 219
(1985).

H. Galina, B. Kolarz, P. Wieczorek, and M. Wojczynska, Brit. Polym. J., 17,
215 (1985).

W. Funke, Brit. Polym. J., 21, 107 (1989).

J. Bastide and L. Leibler, Macromolecules, 21, 2649 (1988).

A. Matsumoto, H. Matsuo, H. Ando and M. Oiwa, Eur. Polym. J., 25, 237
99:81:19, M. Llorente, I. Hernandez-Ruentes, and I. Pierola, Eur. Polym. J.,
25, 471 (1989).

D. Turner, Z. Haque, S. Kalachandra and T. Wilson, Polym. Mater. Sci. Eng.
Proceed, 56, 769 (1989).

G. P. Simon, P. E. M. Allen, D. J. Bennett, D. R. G. Williams and E. H.
Williams, Macromolecules, 22, 3555 (1989).

J. G. Kloosterboer, G. F. C. M. Lijten and C. P. G. Zegers, Polym. Mater. Sci.
Eng. Proceed, 60, 122 (1989).

P. E. M. Allen, D. J. Bennett, S. Hagias, A. Hounslow, G. Ross, G. P. Simon,
D. R. G. Williams and E. H. Williams, Eur. Polym. J., 25, 785 (1989).

J. G. Kloosterboer, G. F. C. M. Lijten and H. M. J. Boots, Makromol. Chem,
Macromol. Symp., 24, 223 (1989).

J. V. Crivello and J. H. W. Lam, “Epoxy Resin Chemistry,” edited by R. S.
Bauer, ACS Symposium Series, Vol. 114, American Chemical Society,
Washington DC, 1 (1979).

S. P. Pappas, Prog. Org. Coat., 13, 35 (1985).

J. V. Crivello, “Organic Coatings, Science and Technology,” Vol. 5 edited by
G. D. Parfitt and A. V. Patsis, Marcel Dekker, Inc., New York, NY, 35 (1983).

29.

30.

31.

32.

W. R. Watt, “Epoxy Resin Chemistry,” edited by R. S. Bauer, ACS Symposium
Series, Vol. 114, American Chemical Society, Washington DC, 17 (1979).

F. Loshe and H. Zweifel, Adv. Polym. Sci., 78, 61 (1986).

J. V. Crivello, J. L. Lee and D. A. Conlon, J. Polym. Sci., Polym. Chem. Ed.,
21, 1785 (1983).

J. V. Crivello, J. L. Lee and D. A. Conlon, J. Polym. Sci., Polym. Chem. Ed.,
28, 479 (1990).

10

Chapter 2

Background

2.1. Photopolymerizations: Characteristics and Applications

Photopolymerizations are reactions initiated by light. Photoinitiation is a
photochemical event in which the initiator produces active centers (usually free radicals
or cations) upon absorption of a photon of the appropriate wavelength. This wavelength
typically lies in the ultraviolet (UV) or visible region of the light spectrum.1 Once the
active centers are produced, the polymerization reaction proceeds rapidly by propagation
of the active center with successive monomer units in the same manner as traditional
thermal polymerizations.

Photopolymerizations offer many advantages that may be suitable for a variety of
applications. First, these polymerizations can rapidly form polymer without the use of
diluting solvents at a fraction of the energy of traditional thermal systems. Second,
photopolymerizations provide significant spatial control over the reaction since the
initiating light may be directed to locations of interest in the reactive system, and
temporal control since the light may be appropriately shuttered on or off during the
reaction. These advantages have been exploited in many applications, including films,
inks and coatings; a host of emerging high-tech systems such as fabrication of printed
circuit boards, coatings for optical fibers, and replication of optical disks; and recently in

stereolithography for the production of three dimensional parts.

11

2.2. Cationic Polymerizations

Cationic polymerizations are chain reactions in which a propagating cationic
center successively reacts with many monomer units to form long polymer chains.
Classes of monomers which will undergo cationic polymerizations include a-olefrns, 1,3-
dienes, vinyl ethers, and epoxides. Cationic polymerizations are not as well characterized
as radical polymerizations partially because they proceed at very rapid rates, and may be

sensitive to small concentrations of impurities.2

Cationic initiators are typically protonic
or Lewis acids, and the resulting reactive cations are sufficiently stable to have a
reasonable lifetime for chain growth by propagation. The polymer chain growth may be
terminated by chain transfer of the reactive center, or occasionally by counterion
combination. Compounds such as water, alcohols, and esters are particularly effective
chain transfer agents. Although chain transfer results in a decrease in the average
primary polymer chain length, it typically will not affect the polymerization rate because
the resulting cation will continue to propagate.2

Kinetic data for cationic polymerizations are difficult to interpret for several
reasons.2'4 First of all, the reactions often proceed so rapidly that the steady-state
assumption is not valid for the reactive centers.2 The analysis is further complicated by
the fact that the nature of the propagating center and the counterion are often unclear.2'4
For kinetic modeling of these reactions, it is common to assume several types of
propagating centers ranging from completely free (unpaired) ions to covalent species.
The unpaired species typically exhibit much higher kinetic constants for propagation than

pair species. Cationic polymerization rates are often higher than those for free radical

polymerizations for a variety of reasons. For cationic polymerizations of unsaturated

12

monomers the kinetic constants for propagation are usually higher than those for free
radical reactions, while the rate constants for termination are lower. Both of these trends
contribute to higher reaction rates for the cationic reactions. Furthermore, the
concentration of propagating species is usually two orders of magnitude higher for
cationic polymerizations than for free radical polymerizations.

While cationic polymerizations are not used extensively for the commercial
synthesis of long polymer chains, they show considerable promise for
photopolymerizations of highly crosslinked polymer films. Due to the predominance of
transfer processes during the course of the reaction, linear cationic polymerizations tend
to yield polymers of moderate molecular weight and relatively broad molecular weight
distribution. These facts tend to limit the usefulness of the reactions for the synthesis of
linear high polymers. However for photopolymerizations of multifunctional monomers, a
high linear primary chain length is not necessary because the resulting highly crosslinked
polymers derive their excellent properties from the network structure, not from long
linear chains. For these systems the insensitivity to oxygen and high reaction rates

exhibited by cationic polymerizations represent significant advantages.

2.3. Cationic Photopolymerizations of Epoxides

The development of thermally stable cationic photoinitiators in the late 1970s has
lead to increased interest in cationic photopolymerizations. Most of the work reported in
the literature has focused on the synthesis and development of appropriate monomers and
initiators for cationic photopolymerizations of high performance polymer films."13

Several classes of monomers exhibiting desirable properties and rapid polymerization

13

10.11-13

rates have been reported, including epoxides, novel silicon-containing epoxy

15.1720 and urethane bisvinyl ethers.‘°'15’1° The selection of appropriate monomers

resins
and initiators is now reasonably broad, however the characteristics of the cationic
photopolymerization reactions have received only limited attention. There is a need for
fundamental studies of the kinetics of the reactions, as well as the structure and physical
properties of the resulting highly crosslinked polymer networks. Such studies would
allow the optimum formulations and curing procedures to be established.

Experimental studies of the cationic photopolymerizations of epoxides and vinyl
ethers have provided information about the salient features of the reactions. The
observed polymerization rate depends upon several variables including the type and
concentration of the initiator, the intensity and wavelength of the UV light source,
temperature, and the structure of the monomer. Until now, most investigators have used

mercury lamps for the UV light source.‘°’13

Many monomers exhibit a substantial
increase in cure rate as temperature is increased.'°'""° This fact presents potential
complications for kinetic studies of extremely rapid cationic photopolymerizations.
Polymerization reaction of epoxides and vinyl ethers are highly exothermic and can
liberate significant amounts of heat in a very short period of time. The cure rate of
cationic photopolymerizations is typically characterized by the tack-free time of the
polymer film. Reported tack-free times range from less than one second to several
minutes;""3 however the meaning of the tack-free time in terms of the degree of cure is
unclear. Differential scanning calorimetry experiments13 indicate that reaction proceeds

long after the tack-free time. The studies also indicate that UV cured epoxides typically

exhibited a cure rate which passed through a maximum, then rapidly decreased to smaller

14

values. This behavior was attributed to diffusion limited propagation upon vitrification in
the highly crosslinked films.1°’21 This interpretation is supported by the fact that tack free
time may be decreased by the addition of chain transfer agents which delay vitrification.1°
Unfortunately the chain transfer agents cause a reduction in crosslink density,

undermining the mechanical properties and the chemical resistance of the cured film.'°

2.4. UV-initiated Cationic Photopolymerizations

In recent years, cationic photopolymerizations have been given special attention.
This technique has made tremendous progress due to its many advantages: high
reactivity, efficiency, and non-polluting. Therefore, cationic photopolymerization is a
technique applied in many fields, such as release paper, photodoping, the graphic arts,
microlithography, and holography. A host of highly efficient cationic photoinitiators is
commercially available, especially the onium salts: triarylsulfonium and diaryliodonium

salts.

2.4.1. Diaryliodonium and Triarylsulfonium Salt Photoinitiators

2.4.1.1. General Characteristics and Synthesis Methods
Diaryliodonium salts are generally colorless to yellowish white crystalline com-
pounds, which are stable in the absence of light at temperatures up to their melting points.

They are often soluble in many common organic solvents. Compounds containing

weakly nucleophilic anions such as SbF6-, BF; are ionic salts, while those with simple

halide anions exhibit some covalent character.22

15

Ar—Y+-—Ar’
x- Y+ =1, Br, Cl ; x' = SbF6', BF;
The major reactions of diaryliodonium salts include the attack at the positively charged

iodine atom by nucleophiles X-, resulting in the displacement of an aryl group.

Triarylsulfonium salts are generally colorless crystalline compounds, which are
soluble in common organic solvents and slightly soluble in water. The triphenylsulfonium
cation adopts a pyramidal configuration with all the carbon-sulfur bonds lying nearly in
the same plane.23 There is considerable positive charge dispersal throughout the molecule
because of the significant pit-d7: bonding between the sulfur atom and the aromatic rings.
For the same reason, these compounds are thermally stable and the cations are not

Ar

Ar—IJI+ x' M+= S, Se, Te; x' = PFe', SbF,’
.1.

readily susceptible to nucleophilic displacement reactions at the sulfur atom. The
synthesis methods for both types of onium salts have been well characterized. In
addition, the preparative methods for triarylsulfonium salts are more easily done than
those for the diaryliodonium salts?”

2.4.1.2. Photochemistry : Photolysis of Photoinitiators

Upon absorption of a photon of the appropriate wavelength, the photoinitator

undergoes an irreversible fragmentation to generate the active centers, the radical-cations.
Crivello and Lam” proposed the following mechanistic pathway to account for the

photoproducts which had been observed.

16

Scheme 1

Ar21+x' —h‘-’—> [Arzfx'ri (2.1)
[Atzr+x']* ———> ArI-i-X- + Ar (2.2)

In this mechanism, light is absorbed first to produce an electronically excited
diaryliodonium salt. Rapid decay of the photoexcited species then takes place with the
resultant cleavage of a carbon-iodine bond to give an aryliodonium cation-radical and an
aryl radical.

A similar mechanistic pathway has been proposed for the photolysis of

triarylsulfonium salts as follows.

Scheme2
Ar3s+x' £> [Ar3S +x']* (2.3)
[Ar3S +x']* .... Ans-ix + M (2.4)

The evidence for the formation of these active species in both reaction schemes
had been shown by the works of Pappas28 and coworkers and Crivello and Lee.29
2.4.1.3. Mechanisms of Photoinitiation and Propagation

The cation-radicals produced by photolysis are capable of directly initiating
polymerization. Crivello and Lam3° have suggested that the cation-radicals interact with

the solvent or monomer, R—H, by hydrogen abstraction to generate the strong protonic

acid, Hx' (Eq. 2.5).

17

ArI-i-X- + An fl HX- + products (2.5)

Initiation then occurs by the addition of the acid to the monomer (M) with the
formation of a carbenium species (Eq. 2.6). Propagation then follows suit (Eq. 2.7).

Scheme 3

Hx' + M —> HM*X’ (2.6)

HM“x' + nM —> H(M),.-1M*x' (2.7)
2.4.1.4. Photosensitization
The majority of the diaryliodonium and triarylsulfonium salts absorb strongly the
wavelength range of 230 - 250 nm. Even the introduction of simple substituents on the
aryl rings does not markedly alter their spectral sensitivity. The poor absorptivity of
these onium salts in the 300-450 nm region is of great concern since it severely limits
their efficiency or light utilization in the region in which the commonly available medium
and high pressure mercury arc lamps provide a substantial portion of their emission. In
addition, the products of photolysis also display absorption bands at or near those of the
photoinitiators. Thus, as they are formed, these products suppress further photolysis of
the photoinitiators due to screening effects. One solution to these difficulties is to
synthesize photoinitiators which incorporate chromophores that allow absorption at
longer wavelengths. Another method is through the use of photosensitization. In the
photosensitization process, the function of the photosensitizer is to absorb energy of
specific wavelengths and to mediate its transfer to the initiator.
Based on the work of several investigators,3 1'33 The two possible mechanisms for

photosensitization have been investigated and characterized: l) electron transfer, and 2)

18

free-radical induced decomposition. The electron transfer mechanism was investigated by
Pappas and co-workers33 using diaryliodonium and triarylsulfonium salts. They have
showed that diaryliodonium salts, which have lower oxidation potentials, are more easily
photosensitized than triarylsulfonium salts. Working with divinyl ethers, Nelson et. al
have also demonstrated that the electron transfer occurs from the triplet state of the

sensitizer anthracene to the initiator.34

2.4.2. Development and Applications of Onium Salt Photoinitiators

A typical photopolymerizable formula contains monomer resin, a photbinitiator,
and may contain an additive (photosensitizer, dye, filler, diluent, etc.). The first two
components are the basic requirements for the reaction to occur. The overall reaction
kinetics of cationic photopolymerization may be characterized in terms of the reactivity
of these components.
2.4.2.1. Effect of photoinitiator

The aryldiazonium salts formed the first class of photoinitiators that could
efficiently induce cationic polymerization. However, they were thermally unstable, and
the resultant short shelf lives had precluded their use in practical UV-curing applications.

During the 1970’s, three new classes of cationic photoinitiators were developed, namely

35-37 38-40 41.42

diaryliodonium, triarylsulfonium, and ferrocenium salts. Subsequent
advances have focused on modifying these onium salts to tailor their absorption
characteristics, to enhance solubility, and to reduce toxicity. With respect to absorptivity,
Crivello has shown that for triarylsulfonium salts, their spectral responses to the initiating

light depend directly on the structure of the aromatic groups attached to the positively

19

charged sulfur atom. One strategy was to incorporate long-wavelength chromophores;
however, the synthetic complexities of this method make it impractical in real-life
applications. A much better alternative is through photosensitization, as discussed in
section 2.4.1.4.‘3

The reactivity of the initiating species generated from the photolysis of the onium

salt with a monomer in (Eq. 2.6) depends to a great extent on the character of the anion

Xi These anions or counterions can participate in the nucleophilic attack on the reactive

species, thus terminating the polymerization. In order to avoid this complication, the

least nucleophilic anions such as Sng', ASF6-, PF.{, and BF; have been used. However,

these essentially non-nucleophilic anions could still show considerable variation in the
reactivity of their corresponding onium salt photoinitiators. For the same reason, water
or many bases can effectively terminate polymerization.

Since the majority of the onium salts are ionic, solubility could become a problem
for non-polar monomer resins. The solutions could include clever substitutions on the
aromatic groups without compromising the absorption properties of the photoinitiators.
For example, the long alkyl groups attached to the phenyl rings can greatly enhance
solubility.

Toxicity is also a concern for some onium salts, specifically those carrying the
heavy metals such as antimony (Sb) or arsenic (As) in their counterions. For example,
diphenyliodonium hexafluoroantimonate has an oral LDso of 40 mg/Kg (rats).44 In
addition, the hexafluoroantimonate anion is sensitive to humidity,45 which poses a
problem for long-term storage. Those problems dealing with solubility and toxicity could

be eliminated with the relatively recent invention4° of a new kind of iodonium salt,

20

namely diaryliodonium tetrakis (pentafluorophenyl) borate salts. These borate-based
salts also display high reactivity, low toxicity and relative insensitivity to humidity.
2.4.2.2. Effect of Monomer Structure and Functionality

The structure of an epoxide molecule can affect its rate of ring-opening in cationic
photopolymerization. Therefore, the monomer’s reactivity depends directly on the degree
of strain brought about by the three-member ring. For example, it is a fact that epoxy
cyclohexanes possess greater ring strain than their open-chain counterparts.”48 In
addition, Crivello and coworkers have recently shown that the presence of ester and ether
group in the monomer tend to depress their reactivity.49 Their works have proven without
a doubt that the monomer’s structure and its degree of ring strain significantly contribute
to the overall polymerization reactivity.”52 They successfully engineered a series of
novel epoxies, including a novel hybrid monomer having both oxirane and vinyl
moieties.
2.4.2.3. Other factors affecting reaction rates in cationic photopolymerization

In addition to the reactivity of specific monomer and photoinitiator used, other

factors like temperature, initiating light intensity, and even level of impurities present in

the reactive mixture can affect the overall rate of polymerization.

21

2.5. Experimental Techniques

2.5.1. Photo-Differential Scanning Calorimetry

Photo- differential scanning calorimetry (PDSC) is the primary technique used
throughout this research effort. Along with the technique of real-time infrared (RTIR)
Spectroscopy, photo-differential scanning calorimetry (PDSC) has been applied widely
for in situ characterization of reaction kinetics and polymerization rate constants
involving photopolymerizations.53'59 The response time of PDSC is relatively fast, about

2-3 seconds.58

Therefore, it can be used to monitor the kinetics of cationic
photopolymerizations for divinyl ethers57 and epoxiesss'58 The DSC experiments can be
run at isothermal conditions or with temperature ramped with time as in the
determination of glass transition temperature, Tg. Photo-DSC requires the use of light to
initiate the reaction.

The basic operating principle for DSC is relatively straightforward. It measures
the difference between sample and reference cells during a thermal event, such as a
chemical reaction or a controlled temperature change.59 Changes in the heating rates of
the sample relative to the reference can be converted into heat capacity and enthalpy
change. The DSC typically uses temperature sensors placed in heat sinks, which are
located just below the sample and reference cells to monitor changes in temperature.
Because the cells are maintained independently, heat flow can be controlled for each cell.
Heat can either be removed from or supplied to either or both cells depending on the

system being tested. In other words, DSC can characterize both exothermic and

endothermic reactions. Therefore, DSC is well suited for kinetics characterization of

22

cationic photopolymerizations, which are highly exothermic.

The heat of reaction profile obtained from DSC can be calculated to obtain
several kinetics features, such as the rate of polymerization, the product of
polymerization rate constant and concentration of active centers. In addition, the
conversion profile as a function of time and the final conversion of a given formulation
can be calculated. Another useful application of photo—DSC is the determination of the
termination rate constant in the “dark” reaction. The sample is illuminated to initiate the
reaction, and then the light is shuttered off in order to monitor the dark reaction.

In Short, the versatility of DSC will be utilized in this research to study the effects
of temperature, type and concentration of photoinitiator, type and concentration of

photosensitizer, and light intensity for various systems of epoxy resin, initiator, and

sensitizer. The experimental set-up for PDSC is shown in Figure 2.1 below.

:-_*ull spectrum mirror

H20 filter
removes IR

 

 

$ Fused silica
window
Perkin-Elmer DSC 7

 

 

       

 

I Ni purge I

Figure 2.1. Experimental set-up for the Photo-differential scanning calorimetry (PDSC)
experiments.

23

2.5.2. Fourier Transform Infrared (FTIR) Spectroscopy

A general introduction to the basic principles of infrared spectroscopy is
presented here. Detailed discussion on the physics, development, and applications of
FI‘IR can be found in many textbooks and published articles.°°'62 The infrared (IR)
region of the light spectrum covers the wavenumbers between 10,000 cm'1 and 10 cm".
The region of interest for this research is the mid-infrared, spanning 4000 to 400 cm“.
The basic operating principle behind IR spectroscopy is that when infrared light is passed
through a sample, certain frequencies are absorbed while others are transmitted, resulting
in an absorption (or transmittance) spectrum that is dependent upon the molecular
vibrational frequencies of that sample.

In polymers, molecular bonds have different vibrational frequencies, which
correspond to characteristic IR absorption bands. For example, for the (ii-epoxide
monomer 3,4—epoxycyclohexylmethyl 3’ ,4’-epoxycyclohexane carboxylate
(cycloaliphatic di-epoxy ERL-4221), the characteristic absorption band is at 790 cm’1
corresponding to the C—H bond stretching vibration of the epoxide group.°3'°5 By
monitoring the change of this absorption band, the degree of conversion can be obtained.
In addition, the C—H absorption band should lie in the region of 740 - 850 cm",
depending on the molecular structure of a particular epoxide molecule.66

The technique of FTIR will be utilized in this research effort. The results in terms

of the degree of conversion will be compared to those obtained from PDSC studies.

24

2.5.3. lH-NMR Spectroscopy

A brief introduction to the basic principles of NMR is presented here. Detailed
discussion about the physics, development and applications of this technique can be
found in many textbooks and published articles.“70 Nuclear magnetic resonance is a
powerful technique that enables the user to deduce the molecular structure of a compound
of interest based on the information given by a NMR spectrum.

NMR is based on the interactions between the nucleus of a particular atom with
the applied magnetic field. The interacting atom is either a hydrogen (proton) or a carbon
atom, thus giving the name pmr (proton magnetic resonance or 1H-NMR) spectrum or
cmr spectrum (based on 13C nuclei). The following discussion is based on proton, but the
same principles operate on the l3C-NMR. The nucleus of a hydrogen atom, 1H, has a net
charge and can spin. The spinning generates a magnetic moment along the axis of spin.
When a proton is placed in an external magnetic field, its magnetic moment can be
aligned with or against the external field according to quantum mechanics. Alignment
with the field is more stable, and aligning against the field is less stable because energy is
absorbed to “flip” the proton magnet over. The exact amount of energy required to cause
the flipping depends on the strength of the external magnetic field: the stronger the field,
the greater the tendency to remain lined up with it, and the higher the frequency of the

radiation needed to do the job because AB = hv.

4%
2n

where, v = frequency in Hz; H0 = magnetic field strength in gauss

y = the gyromagnetic ratio, a constant of 26,750 for proton

25

In principle, a substance can be placed in a magnetic field of constant strength,
and its NMR spectrum is obtained by passing radiation of steadily changing frequency
through the substance while observing the frequency at which radiation is absorbed. In
practice, however, it is more convenient to keep the radiation frequency constant and to
vary the strength of the magnetic field. At some value of the field strength, the energy
required to flip the proton matches the energy of the radiation; absorption occurs and a
Signal is observed. Such a spectrum is called a nuclear magnetic resonance (NMR)
spectrum.

The actual environment, that the proton is in a substance, is far more complex.
Due to the electron density at the proton as well as presence of other protons, the
frequency at which a proton absorbs depends on the magnetic field at which that proton
feels, and this effective field strength is not exactly the same as the applied field strength.
For example, at a given radiofrequency, all protons absorb at the same effective field
strength, but they absorb at different applied field strength. It is this applied field
strength that is measured, and against which absorption is plotted to produce a NMR
spectrum.

The position of an absorption signal for a particular proton is plotted on the 6
(delta) scale. The reference signal is from tetramethylsilane (TMS) and is taken to be 0.0
ppm. The position of an absorption signal can be shifted with respect to the TMS signal
depending on the particular electronic environment the proton is in. Such shifts in the
position of NMR absorptions are called chemical shifts. For a given molecule, protons
with different electronic environments - non—equivalent protons - have different chemical

Shift; and protons with the same electronic environment - equivalent protons - have the

26

same chemical shift. Most chemical shifts have 8 values between 0 and 10 ppm. A small
6 value represents a small downfield shift, i.e., going to the left on the nmr spectrum chart

and toward lower applied magnetic field strength. For example“, a spectrometer

operating at a frequency of 300 MHz or 300 x 10° Hz,

_ observed shift (Hz) x10°
300x10° (Hz)

 

6

For analysis of NMR spectra there are four basic features of the spectrum that can
be used to characterize the molecular structure and composition of the sample: 1)the
number of signals (or peaks), corresponds to the number different “kinds” of protons (as
determined by electronic environment) there are in a molecule; 2) the positions of the
signals, characterizes the electronic environment of each kind of proton; 3) the intensities
of the signals (integral of the area beneath the peak), determines tell how many protons of
each kind there are; and 4) the splitting of a signal into smaller peaks characterized the

number of protons covalently bonded by a neighboring carbon.

27

 

10.

11.
12.

13.

14.

15.

16.
17.
18.

 

 

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G. Odian, Principles of Polymerization, 2nd Edition, Wiley, New York, NY
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P. H. Plesch, Cationic Polymerization and Related Processes, edited by El.
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D. J. Dunn, Developments in Polymerization, edited by R.N. Haward, Applied
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A. Ledwith, S. Al-Kass, A. Hulme-Lowe, Cationic Polymerization and Related
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A. Reiser, Photoreactive Polymers, Wiley, New York, NY (1989).
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IF. Kinstle, ACS Symposium Series, Vol. 417, American Chemical Society,
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S. P. Pappas, Prog. Org. Coat, 13, 35 (1985).
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F. Loshe and H. Zweifel, Adv. Polym. Sci., 78, 61 (1986).
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M. Simonetta and A. Garezotti, The Chemistry of the Sulfonium Group, Vol. 1, 13,
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A. Ledwith et. al., Polymer, 22, 143 (1981).

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B. M. Trost and L. S. Melvin Jr., Sulfitr Ylides — Emerging Synthetic
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J. P. Marino, Topics in Sulfitr Chemistry, Vol. 1, 1, A. Sennig (Ed), George T.
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J. V. Crivello and J. H. W. Lam, Macromolecules, 10, 1307 (1977).

S. P. Pappas et. al., Proc. Soc. Photogr. Sci. and Eng., 22'“1 Fall Symp.,
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G. H. Smith, US patent 4,069,054 Jan. 17, 1978.
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J. V. Crivello and J. H. W. Lam, J. Polym Sci., Polym Chem. Ed.,l6, 2441,
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S. P. Pappas, L. R. Gatechair, and J. H. Jilek, J. Polym Sci.: Polym Chem. Ed,
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30

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N. B. Colthup, L. H. Daly, and S. E. Wiberley, Introduction to Infrared and
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Boston, Massachusetts, pp. 420 - 422 (1973).

31

Chapter 3

Research Objectives

Based on the discussion above it is clear that there is a need to provide a more
thorough understanding of the kinetics of cationic photopolymerizations. Therefore, the
broad objective of this research effort is to perform a systematic series of experiments
that will investigate the effects of a host of variables on the resulting polymerization
kinetics and observed limiting conversion of epoxide monomers.

The specific objectives are as follows:

i. to obtain complete, time-resolved profiles of the rate of polymerization as a
function of time using photo-differential scanning calorimetry to characterize the
effects of the following variables:

0 temperature;

0 type of photoinitiator (and for a representative system, the effect of

photoinitiator concentration);

0 type of photosensitizer (and for a representative system, the effect of

photosensitizer concentration);

0 initiating light intensity;

ii. to obtain complete, time-resolved profiles of conversion as a function of time with
respect to the same variables above;

iii. to investigate the effect of the monomer structure and functionality on

the photopolymerization kinetics and limiting conversion.

32

Chapter 4

Characterization of the Photoinitiating Systems

4.1. Selection of the Components in the Photoinitiating Systems

A typical photopolymerizable formulation contains a monomer resin, an initiator,
and may contain other additives such as a sensitizer, dye, filler, diluent, etc. It is obvious
that the monomer resin and initiator are the basic requirements for the reaction to occur
upon light irradiation. In order to use most efficiently the initiating power of the
irradiating light, all these components except the initiator and/or the sensitizer should to
be transparent to the initiating wavelength.

Three classes of monomers were investigated in this study as listed in Table 4.1:
l) epoxides containing a single epoxide group, 2) epoxides containing two epoxide
groups, and 3) epoxides containing three polymerizable epoxide groups. These classes
will henceforth be denoted mono-epoxide, di-epoxide and tri-epoxide monomers,
respectively. Three different mono-epoxide monomers were investigated: 1) octyl
glycidyl ether, 2) butyl glycidyl ether, and 3) phenyl glycidyl ether. These mono-epoxide
monomers were chosen to investigate the effect of the length and type of the pendant
group attached to the epoxide ring. Two different di-epoxide monomers were used: 1)
1,4-butanediol diglycidyl ether, 2) cyclohexane dimenthanol diglycidyl. The comparison
of the results from the mono-epoxide studies and the di-epoxide studies will allow the
effect of crosslinking to be investigated, while comparison among the two di-epoxide

monomers will allow the effects of the length and flexibility of the chain between the two

33

reactive groups to be investigated. Finally, the tri-epoxide monomer trimethyol propane
triglycidyl ether was investigated. All epoxide monomers were obtained from Shell
Chemical Company.

As illustrated in Table 4.2, three different photoinitiators were used, including two

iodonium salts and one sulfonium salt. The two iodonium salts were: 1) tolylcumyl
iodonium tetrakis pentaflourophenyl borate, (B-[Fsh' counterion) from Rhodia Inc; 2)

diaryliodonium hexafluoro-antimonate, ([SbFJ counterion) from Sartomer Company.
These diaryliodonium salts differ from each other by the counterion. One is the
(tolycumyl) iodonium with the bulky counter ion tetrakis (penta-fluorophenyl) borate.

The other diaryliodonium salt has a relatively smaller counterion hexafluoroantimonate

(Table 4.2). The third photoinitiator was triarylsulfonium hexafluorophosphate ([PF6]'
counterion) from Sartomer Company. The triarylsulfonium salt has the counterion
hexafluorophosphate. Two photosensitizers from Aldrich Chemical Company, Inc. were
used: 1) anthracene, and 2) 1—chloro-4-propoxy-9H-thioxanthen-9-one (CPTX).

As indicated by their absorption spectra (Figures 4.8 - 4.10), the photoinitiators
absorb strongly in the deep UV region of 230 - 254 nm. In order to increase their spectral
sensitivities, specific photosensitizers will be added to the initiating systems. The two
photosensitizers are anthracene and 1-chloro—4~propoxy-9H-thioxanthen-9—one (CPTX)
and their molecular structures are shown in Table 4.2. Figures 4.11 - 4.12 show that both
anthracene and CPTX have absorption maxima in the 310 - 380 nm region, which
coincides with the strong emission peaks of the emission spectrum of the initiating light
(source, a 200 Watt Hg-Xe arc lamp (Figure 4.1). Thus, these photosensitizers were

selected due to their absorbance characteristics.

34

Table 4.1. Epoxide rmnorrers used in the cationic photopolymerizations

0
CH2 — CH—Cflz—O—(Cflzb—Cl-Ig Butyl glycidyl ether
0
/ \
CH2 — CH—Cfla—O—(Cflzh—cfls Octyl glycidyl ether
0
CH2 — cu—cnz—o Phenyl glyc1dyl ether
0 ' 0

cu;— cu—cuz—o—cuz—(cum—cnz— o— Cflz—C/H —\ctt2

1,4- butanediol diglycidyl ether
0
/ \ /_
Cl-lz—CH—CHz—O—Cflz Hf—O—CHf—CH H2

Cyclohexane dirretlnnol diglycidyl ether

0
o—cuz—cn — CH2

0 Trirrethyol propane
CHa—Cflz—C—CHz— o— CHz—CH — cit2 uiglycidyl ether

 

CH:

0
CH2— 0— CHr—CH — CH2

 

35

Table 4.2. Photoinitiators and photosensitizers for cationic photopolymerizations
Photoinitiators

H, (tolylcumyl) iodonium tetrakis
¢>_©_ .2_©_ CH3 3" F (pentafluorophenyl) borate
CH3

MW = 1015.7 g/mole

 

 

 
  

F r
4
55"; Diaryliodonium Hexafluoroantimonate
MW = 746.23 g/mole
l + Mixed Triarylsulfonium Hexaflourophosphate
‘ P“ Average“ MW = 1693.44 g/mole

3" a 50 wt% solution of propylene carbonate

Photosensitizers

OCO Anthracene, MW = 178.23 g/mole

0| I
O \ Q CPTX (1-chloro-4-prop0xy-9H-thioxanthen-9-one)
‘ / MW = 304.80 g/mole

OCHzCHzCI-Ia

36

4.2. Absorption Spectra of the Photoinitiating System Constituents

One of the important parameters that can affect the overall kinetics of the
polymerization reaction is the absorption of the incident light by the constituents in the
reactive mixture. The knowledge of these absorption spectra is critical in the optimal
selection of monomers, initiators and/or sensitizers. From the emission spectrum,
obtained by Ocean Optics, of the light source, a 200 Watt Hg-Xe arc lamp, (Figure 4.1),
the location of the strongest emission peaks and their associated wavelengths can be
determined. The absorption spectra were obtained using the UV—vis spectrophotometer
(HP8452A Diode Array). For example, an amount of initiator was weighed in the Metler
200 scale, with accuracy to 0.1 milligrams. The number of moles was calculated and
then dissolved in the appropriate volume of the solvent methylene dichloride (CH2C12) to
obtain the desired concentration. The absorbance was then taken. Serial dilutions were
done to obtain an absorbance value of 2 or less. The extinction coefficient was calculated
using Beer’s Law,

Absorbance = ecl (4.1)

Where, 8 a extinction coefficient in L.mole'1.cm'l
c 5 concentration of monomer resin in mole / Liter
l a path length in cm

The absorption spectra of all of the system constituents are shown in Figures 4.2-4.12.
From these absorption spectra, the wavelength of maximum absorbance and the
corresponding extinction coefficient were calculated. Table 4.3 provides a summary of
the absorption properties for all monomers, photoinitiators, and photosensitizers used in

this research effort.

37

Table 4.3. Absorption properties of epoxide monomers, photoinitiators, and

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

photosensitizers
Peak Concentration Calculated
Monomers Absorbance Wavelength (moi/L) in Extinction Coef.
(nm) CHzClz (L’mole'1‘3cm'1)
n-octyl glycidyl 0.463 232 1.000 0.463
ether
n-butyl glycidyl 1.621 240 0.100 16.214
ether
Phenyl glycidyl 1.313 228 0.001 1313.324
ether 1.668 270 0.001 1668.045
1,4-butanediol 0.492 240 0.100 4.916
diglycidyl ether
Cyclohexane 0.652 228 0.100 6.520
dimethanol 0.615 240 0.100 6.153
diglycidyl ether
Cycloalphatic
diepoxies ERL 0.502 228 0.010 50.166
4221
Photoinitiators
(tolycumyl)
iodonium tetrakis 1.590 230 1.00E-3 1.59E3
(pentafluorophen
yl) borate 1.539 252 1.00E-3 1.54E3
Diaryliodonium 1.131 230 5.00E-5 2.26134
Hexafluoro- 1.294 260 5.00E—5 2.59E4
antimonate
Triarylsulfonium 1.542 232 5.00E—5 3.08134
Hexafluoro- 0.674 298 5.00E-5 1.35E4
phosphate
Photosensitizers
1-chloro-4- 1.968 258 5.00E-5 3.94E4
propoxy-9H 0.530 316 5.00E-5 1.06E4
thioxathen-9-one 0.333 386 5.00E-5 6.66E3
(CPTX)
3.217 248 5.00E-5 6.43E4
Anthracene 0.412 358 5.00E-5 8.25E3
0.379 378 5.00E-5 7.58E3

 

 

 

 

 

 

38

 

 

 

 

 

 

 

 

 

l l l l

100 200 300 400 500 600 700 800 900

 

 

wavelength (nm)

 

Figure 4.1. Emission spectrum of the 200 Watt Hg-Xe arc lamp for the photo-
differential scanning calorimetry (PDSC) studies.

As shown in Figure 4.1, the emission spectrum has strong emission peaks at wave-
lengths of 310 nm and 365 nm, which are in the region where the photosensitizers strongly

absorb. Figures 4.1 through 4.12 are presented in color.

39

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50

4.2. Verification of Monomer Molecular Structures by NMR
Spectroscopy

4.2.1. Introduction

It is imperative to know the exact weight per mole of epoxide groups present in
each of the monomer molecules investigated in this research. The verification of the
monomer molecular structures is performed using the technique of lH-NMR
spectroscopy. NMR is based on the interactions between the nucleus of a particular atom
with the applied magnetic field. The interacting atom is either a hydrogen (proton) or a
carbon atom, thus giving the name PMR (proton magnetic resonance or 1H-NMR)
spectrum or CMR spectrum (based on 13C nuclei). Further description of this technique is
found in Chapter 2, section 2.5.3. The 1H—NMR spectra were obtained for all epoxide

monomers and are shown in Figures 4.13 through 4.18 in the following section.

51

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57

4.2.2. Analysis of 111-NMR Spectra

NMR spectroscopy was used to characterize and verify the molecular structures
(including the molecular weights) of the epoxide molecules that are used in this research
effort. All 1H-NMR spectra were obtained using the NMR instrument VXR 300 by
Varian using standard methods and standard values for the instrumental parameters such
as the delay and the pulse width. For each spectrum, 32 free induction decays (FIDs)
were signal averaged without apodization before Fourier transformation, and the resulting
spectra exhibited signal-to-noise ratios of at least 200 to 1. All samples were dissolved in
deuterated chloroform.
4.2.2.1 1H-NMR spectrum of mono-epoxide monomer octyl glycidyl ether

The NMR spectrum for the mono-epoxide monomer octyl glycidyl ether is shown
in Figure 4.13. The proposed molecular structure with assignments for different or non-

equivalent protons corresponding to the signals on the spectrum is as follows:

Fa Eb Hb Hb b Tb Tic Ta Tue 0
H3 12— 11 ——Ll:o — "5 "4 C fi—Ié—L —Hh
HG Ld Hg Li

Figure 4.19. Molecular structure for the mono-epoxide monomer octyl glycidyl ether.

 

ӣ11bebebe

Based upon this structure, there are 24 total protons per molecule, and nine
distinct electronic environments or nine different kinds of protons as assigned a, b, c, d, e,
f, g,h and i for the molecule octyl glycidyl ether. They correspond to the nine different
signals as seen on the NMR spectrum (Figure 5.13). The numbers beneath the ppm scale

are values for the integrated area of the signals or signal intensity, which is directly

58

proportional to the number of protons giving rise to the signal. For example, the number
three at about 0.8 ppm corresoponds to three protons H, which form the methyl group.
Likewise, the signal at about 1.25 ppm is for the twelve protons Hg, Each signal at about
3.1 ppm, 2.7 ppm and 2.5 ppm correspond to proton Hg, Hg, and Hi. The remaining four
protons: 2 Ha, I H. and l Hf are the peaks found in the region of roughly 3.3 ppm to 3.7
ppm. Thus, the formula for octyl glycidyl ether is C9H1902C3Hs and the formular weight
is 186.294 g/mole.
4.2.2.2. 1HuNMR spectrum of mono-epoxide monomer butyl glycidyl ether

The analysis for this monomer is very similar to that for octyl glycidyl ether
because the only difference here is the butyl group. The molecular structure is illustrated
in Figure 4.20 below. As shown in Figure 4.14, the peak at about 0.85 ppm corresponds
to the three protons H.. The two signals at 1.3 ppm and 1.5 ppm are due the two protons
Hb and two protons Hc respectively. Next, the three signals at 2.5 ppm, 2.7 ppm and 3.1
ppm belongs to each single proton of Hh, Hi and Hg. The two protons Hd, l proton He and
l proton Hf are located in the region of 3.3 ppm to 3.7 ppm. Therefore, the formula of

butyl glycidyl ether is C4H902C3H5 with the formular weight of 130.186 g/mole.

Hal—i: —T:—I: —L—O—I1e—C/2— —-Hh
lLa J'lb ll‘ld 1‘ld J"If lllg J'li

Figure 4.20. Molecular structure for the mono-epoxide monomer butyl glycidyl ether

4.2.2.3. 1H-NMR spectrum for the mono-epoxide monomer phenyl glycidyl ether

Figure 4.21 shows that there are a total of 10 protons which can be determined

59

from seven different signals in the 1H—NMR spectrum of phenyl glycidyl ether (Figure

4.15). The five aromatic protons H. ande are shifted downfield as the two signals at

Ha Hb
Hd O
l / \
Ha O—Cr —Cz—Ca—Hh
ll
a HD 9 9

Figure 4.21. 1H-NMR spectrum for the mono-epoxide monomer phenyl glycidyl ether.

about 7 ppm and 7.15 ppm respectively. Each of the proton Hd and I-L is due to a signal

at about 4 ppm and 4.1 ppm. Finally, the three signals at approximately 2.85 ppm , 2.95

ppm and 3.2 ppm correspond to each proton of Hg, Hh and Hf. So the formula for

phenyl glycidyl ether is C5H502C3Hs with its formular weight of 150.177 g/mole.

4.2.2.4. 1H-NMR spectrum for (ii-epoxide monomer 1,4-butanediol diglycidyl ether.
This molecule is symmetric with a perpendicular plane bisecting the C5-- C5 bond.

There are seven different kinds of protons, namely a, c, d, e, f, g, and h which correspond

Hd Hc Ha Ha Ho Td

SIT—l” l | | | __
m —°W‘_VTTTFTTE:T’W

He He Ha Ha C

Figure 4.22. 1H-NMR spectrum for di-epoxide monomer 1,4-butanediol diglycidyl ether

to the seven signals as observed on the spectrum (Figure 4.16). Each half of the molecule
has a total of 9 protons. The signal at 1.5 ppm is due to two protons 11.. Each proton H...

Hg, and Hf corresponds to a signal at 2.5 ppm, 2.7 ppm and 3.0 ppm. The remaining four

60

protons (2 Hc, 1 Ha and l H.) are found in the region of 3.2 ppm to 3.7 ppm. The formula
of phenyl glycidyl ether is determined to consist of two groups of C2H402C3H5, each of
which contains an epoxide ring; thus the weight per mole of epoxide groups (or rings) for
1,4-butanediol diglycidyl ether is 101.23 g/mole.

4.2.2.5. 1H-NMR spectrum for the (ii-epoxide monomer cyclohexane dimethanol
diglycidyl ether.

 

Figure 4.23. Molecular structure of di—epoxide cyclohexane dimethanol diglycidyl ether
This molecule is symmetric with respect to the vertical and perpendicular plane
bisecting the cyclohexane ring. There are a total of 12 protons, which derive from 8
different electronic environments, corresponding to 8 different kinds of protons. They
are: a, b, c, d, e, f, g, and h (Figure 4.23). Looking at the 1H-NMR spectrum in Figure
4.17, each of the signal at roughly 2.5 ppm, 2.7 ppm and 3.05 ppm is due to each of the
proton H... Hg, and Hf accordingly. So each proton has an integrated value of about 0.60.
Based on this assignment, a peak at 1.75 ppm, a broad and split signal centered at about
1.4 ppm, and a signal 0.9 ppm correspond to the five protons H. and Hg, attached to the
cyclohexane. Thus a single molecule of cyclohexane dimethanol diglycidyl ether is made
up of two groups, each of which contains an epoxide ring. The weight per mole of
epoxide groups for the di-epoxide monomer cyclohexane dimethanol diglycidyl ether is

determined to be 128.17 g/mole.

61

4.2.2.6. lH-NIVIR spectrum of monomer trimethyol propane triglycidyl ether

As shown in Figure 4.24, a molecule of the monomer trimethyol propane
triglycidyl ether has three epoxide groups. There are a total of 26 protons, which can be
determined by 8 different signals from the 1H-NMR spectrum (Figure 4.18).
Specifically, the signal at about 0.75 ppm is due to three protons 11,. So each proton has

an integrated value of about 0.33. Based on this designation, the signal at 1.3 ppm is due

 

 

 

 

 

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triglycidyl ether.

62

two protons Hb. Each signal at 2.5 ppm, 2.7 ppm and 3.0 ppm correspond to 3 protons
H.., 3 protons Hg, and 3 protons Hf of the three epoxide rings. The remaining 12 protons
(6 IL, 3 IL, and 3 IL) are located in the region of 3.2 ppm to 3.7 ppm. 80 the average
weight per mole of epoxide groups for the tri-epoxide monomer trimethyol propane
triglycidyl ether is determined to be 100.79 g/mole. The results of lH-NMR analysis of
molecular structure of the monomers and their weight per mole of epoxide groups in a
molecule are presented in Table 4.4. In addition, Table 4.4 also presents the calculated
density for each epoxide monomer. The method to obtain the calculated densities is
described in the next chapter, section 5.1.2.1.

In summary, the molecular structures of all the epoxide monomers used in this
research effort have been verified by analyzing their 1H—NMR spectra. The results of this
analysis provide an accurate account of the weight per mole of epoxide groups present in
a single monomer moleclue for each monomer. The weight per mole of epoxide as well
as the density are two important constants used in the calculation of the observed rates of
polymerization and the degree of conversion based on the primary data obtained from the
photodifferential scanning calorimetry (PDSC) studies. In addition, the NMR spectra for
some of these epoxide monomers have already been published by Aldrich.7 They were

used for comparative purposes.

63

Table 4.4. Weight per mole of epoxide groups and densities of all epoxide monomers
used in this research effort.

 

 

 

 

 

 

Wt. per mole
Monomer epoxide groups Cale. Density (g/ml)
(g/mole)
Octyl glycidyl ether 186.2936 0.9061 :1: 0.0004
Butyl glycidyl ether 130.1864 0.9365 i 0.0001
Phenyl glycidyl ether 150.1768 1.1074 1: 0.0001
1,4-butanediol diglycidyl ether 101.1249 1.1012 :1: 0.0001
Cyclohexane dimethanol diglycidyl ether 128.1076 1.0928 1 0.0001
Trimethyol propane triglycidyl ether 100.7889 1.1603 :1: 0.0002

 

 

4.3. References

J. K. M. Sanders and B. K. Hunter, Modern NMR Spectroscopy — A Guide for
Chemists, Oxford University Press, New York, 1987.

R. T. Mom'son and R. N. Boyd, Organic Chemistry, 3rd ed., Allyn and Bacon, Inc.,
Boston, Massachusetts, 1973.

E. Breitrnaier, Structure Elucidation by NMR in Organic Chemistry - A Practical
Guide, John Wiley & Sons, New York, 1993.

R. Freeman and G. A. Morris, Bull. Mag. Reson., 1, pp. 1 - 28, 1979.

R. T. Morrison and R. N. Boyd, Organic Chemistry, 3ml ed., Allyn and Bacon, Inc.,
Boston, Massachusetts, pp. 420 — 422 (1973).

E. Breitrnaier, Structure Elucidation by NMR in Organic Chemistry - A Practical
Guide, John Wiley & Sons, New York, pp. 178, (1993).

C. J. Pouchert, The Aldrich Library of NMR Spectra, ed. 11, Vol. 1, Aldrich
Chemical Company, Inc., Milwaukee, Wisconsin, pp. 173 -— 214 (1983).

65

Chapter 5

Kinetic Studies for Cationic Photopolymerization of Epoxide
Monomers

5.1. Photo-differential Scanning Calorimetry (PDSC) Studies
5.1.1. Introduction

Photo-differential scanning calorimetry (PDSC) is a convenient and reliable
method that can be used to characterize the kinetics of photopolymerization reactions.
Because the polymerizations are highly exothermic, the reaction rate may be measured by
monitoring the rate at which heat is released from the polymerizing sample. The profiles
of the heat of reaction versus time are obtained from PDSC and may be used to
characterize the reaction kinetics, such as the rate of polymerization RP, and to elucidate
the behavior of the propagation rate constant kp. Several authors have used PDSC to
characterize free radical“5 and cationic“9 photopolymerizations. However, cationic
polymerizations are highly exothermic, care must be taken in order to ensure the released
heat of reaction does not exceed the time resolution of the instrument, which is in the
order of 2-3 seconds. Therefore, appropriate incident light intensities and small sample
sizes (about 15 mg) are used to allow the removal of heat evolved on the time scale of the
equipment in order to maintain isothermal conditions.

In this thesis, PDSC experiments were performed to determine effects of the
following parameters on reaction rate and limiting conversion of cationic
photopolymerization of epoxides: temperature, type and concentration of photoinitiator,

type and concentration of photosensitizer, monomer structure, and light intensity.

66

5.1.2. Experimental

5.1.2.1 Materials

Three classes of monomers were investigated in this study as listed in Table 4.1:
1) epoxides containing a single epoxide group, 2) epoxides containing two epoxide
groups, and 3) epoxides containing three polymerizable epoxide groups. These classes
will henceforth be denoted mono-epoxide, (ii-epoxide and tri-epoxide monomers,
respectively. Three different mono-epoxide monomers were investigated: 1) octyl
glycidyl ether, 2) butyl glycidyl ether, and 3) phenyl glycidyl ether. These mono-epoxide
monomers were chosen to investigate the effect of the length and type of the pendant
group attached to the epoxide ring. Two different di-epoxide monomers were used: 1)
1.4-butanediol diglycidyl ether, 2) cyclohexane dimenthanol diglycidyl. The comparison
of the results from the mono-epoxide studies and the di-epoxide studies will allow the
effect of crosslinking to be investigated, while comparison among the three di-epoxide
monomers will allow the effects of the length and flexibility of the chain between the two
reactive groups to be investigated. Finally, the tri—epoxide monomer trimethyol propane
triglycidyl ether was investigated. All epoxide monomers were obtained from Shell
Chemical Company.

The molecular weight and density are important constants used in the calculation
of the rate of polymerization and the degree of conversion based on the primary data
obtained from PDSC. The density was readily computed as a ratio of the sample’s mass
over its volume. For example, a 25-ml volumetric flask was filled up to the mark with
the resin of butyl glycidyl ether. The mass of the resin was obtained via the 200 Metler

scale, and the density was then calculated. The molecular weight was determined from

67

the molecular structures that were verified in the previous chapter. Table 4.4 provides a
summary of the molecular weight and density at room temperature for all monomers.
As illustrated in Table 4.2, three different photoinitiators were used, including two

iodonium salts and one sulfonium salt. The two iodonium salts were: 1) tolylcumyl
iodonium tetrakis pentaflourophenyl borate, (B-[Fs]; counterion) from Rhodia Inc; 2)
diaryliodonium hexafluoroantimonate, ([SbF6]' counterion) from Sartomer Company.

The third photoinitiator was triarylsulfonium hexafluorophosphate ([PF6]' counterion)
from Sartomer Company. Two photosensitizers from Aldrich Chemical Company, Inc.
were used: 1) anthracene, and 2) CPTX (l-chloro-4-propoxy-9H-thioxanthen-9-one).

As detailed in the previous chapter, the structures of all the monomers were
characterized using lH--NMR spectroscopy (section 4.2.2). Prior to their usage in the
kinetic experiments, the monomers were dried over molecular sieves (4-8 mesh beads,
from Aldrich Chemical Company, Inc.) to remove any dissolved water. This dehydration
step was necessary because water molecules can participate in the nucleophilic attack of
the propagating active centers, thereby terminating the polymerization reactions
prematurely. The photoinitiators and photosensitizers were used as received. A typical
reactive mixture’s weight was about 15 milligrams, comprising of the monomer resin and
other constituents. The concentrations of the photoinitiator and photosensitizer were
calculated on molal basis (numbers of moles of photoinitiator or photosensitizer per
kilogram resin), in order to maintain consistent concentrations among different initiating
systems and to allow for their quantitative comparison. A representative formulation
contained 8.0 x 10‘3 moles / kg resin in photoinitiator with or without 8.0 x 10" moles / kg

resin in photosensitizer. In all, 42 different formulations of monomer, photoinitator,

68

and photosensitizer were studied at a number of different temperatures and light
intensities.
5.1.2.2. Methods for Photo-DSC Experiments

The PDSC experiments were conducted using a Perkin Elmer model DSC-DPA 7
differential scanning calorimeter. As shown in Figure 2.1, this instrument is equipped
with a photocalorimetric accessory, including the transfer optics to produce full-beam
ultraviolet (UV) light. The DSC chamber contains two cells, one for the reference and
the other for the test sample. During the reaction, the test sample was held in an
uncovered DSC aluminum pan and the reference pan was kept empty and also uncovered.
However, the entire reaction chamber was covered with a fused silica window that
allowed for full transmission of the light source and contained the heat generated from
the reaction. A recirculating water bath, maintained at 10°C, was installed to filter out the
IR emissions of the initiation light source, which was a 200-watt Hg/Xe arc lamp. The
intensity of light striking the reference and the sample was controlled by varying the
distance between the light source and sample and/or by placing appropriate neutral
density filter (from Oriel Corp) in the beam path. In addition, the incident light intensity
was measured by placing black graphite discs in the sample and reference cells and
measuring the heat generated in the discs due to absorption of light. With the reference
disc kept dark, light was shuttered on and illumination was allowed to proceed for about
2 minutes until a constant value of heat flow (mW) had been reached. All polymerization
reactions were run isotherrnally at specified reaction temperatures and light intensities.
All experiments were performed in triplicate and average results are reported. All images

and figures are presented in color.

69

5.1.3. Data Analysis

5.1.3.1. Evaluation of Kinetics Parameters
5.1.3.1a. Rate of Photopolymerization

A profile for the rate of polymerization, R1,, can be calculated based on the PDSC
profiles of the heat released by the reaction versus time. The rate of polymerization, RP,
is directly proportional to the rate of heat released from the polymerizing sample, and is
thereby proportional to the height of the PDSC exotherm measured in ng. The
instantaneous rate of polymerization in units of moles (epoxide groups)/Losec is

calculated by the following relationship:

 

_d[1W]_heightOfexotherm(W/g),.l p (g/cm3)

R” dt AHP

(5.1)

where,
Rp in moles (epoxide groups)lL-sec,
p, density of the reaction mixture is taken as the density of the monomer
resin because the amounts of photoinitiator and photosensitizer are so

small and insignificant.

.AI-Ip (J/moles) is the amount of heat evolved from one mole of epoxide
groups and is a constant of 92,000 1! mole (please see reference 10).

An example of the data analysis to obtain the rate of polymerization, RP, is shown
in Figures 5.1 and 5.2 below. As illustrated in Figure 5.1, immediately after illumination
(which occurs at time zero) the PDSC exotherm curve increases sharply as active centers
are photochemically produced and begin to propagate with the epoxide groups of the

monomer. During this initial portion of the curve, rate of polymerization increases

70

largely because the concentration of active cationic centers is increasing. The plot of heat
released soon reaches a maximum, and then declines as the monomer concentration is
reduced by polymerization. Eventually the reaction rate goes to zero and the curve
flattens out. It is important to note that the reaction rate may become zero even though
both active centers and unreacted epoxide groups remain in the sample. The vitrification
phenomenon occurs because the reactive groups do not have enough mobility to find and
react with one another. The PDSC exotherm curve was mathematically converted to the

rate of polymerization, Rp curve using Eq. 5.1 (Figure 5.2).

2.0 -

 
 
   

1.5-

Released heat
of reaction

1.0-

Average heat flow (W / g)

0.5 -I

 

 

0.0.) oooooooooooooooooooooooooooooo

 

Time (min)

Figure 5.1. PDSC reaction exotherm for a reaction system containing (ii-epoxide
monomer 1,4-butanediol diglycidyl ether and 8.0 x 10‘3 molal of photoinitiator tolycumyl

iodonium tetrakis (pentafluorophenyl) borate, (B-[F5]4' ) at 50°C.

71

0.02 -

0.01 -

Rp (moles/L‘sec)

 

 

0.00 -

 

 

O-l
0|:
.5
O
u.
0|
3
N
U!
8

Time (min)

Figure 5.2. A profile of the rate of polymerization, RP, for a reaction system containing
(ii-epoxide monomer 1,4-butanediol diglycidyl ether and 8.0 x 10‘3 molal of

photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl) borate, (B-[F5]4' ) at
50°C.

72

5.1.3.1b. Conversion by Photopolymerization

The concentration of epoxide groups at a given time, [M], and the epoxide
conversion can be computed from the integral of the reaction rate as a function of time.
Therefore, the instantaneous concentration of epoxide groups may be obtained from the

PDSC profiles using the following equations:

 

 

 

[M]=[M0]_(exotherm area(J/g)—p] (5.2)
Mb
Conversion = exotherm area (J/g) 'p (5.3)
AH.)— [Mo]
where,
AHpandparethe sameasinEq. 5.1
Conversion =(integrated exotherm area . Wt.) (54)
AH)
where,

Wt. is weight per mole of epoxide groups, (Table 4.4)

A plot of the integrated heat of reaction is shown in Figure 5.3. A complete
profile of the concentration of the epoxide groups or conversion may be calculated by
determining the area under the PDSC exotherm as a function of time and applying Eq.
(5.3) or Eq. (5 .4). The baseline was drawn as in Figure.5.l in order to obtain the accurate
value of the total heat released from the reaction. The baseline was taken as the
horizontal line that passes through the PDSC exotherm when it flattens out. Taking the
integral of the exotherm curve in Figure 5.1, then dividing by the heat that would be

released if all of the epoxide groups had reacted, provides a profile of the epoxide

73

conversion, as shown in Figure 5.4. In this figure note that the conversion reaches a flat
plateau value of the ultimate limiting conversion, and that the sample had achieved less

than 100% conversion when the reaction had stopped.

 

 

 

A 12-

or

E

g o.

c I

.9 8-

‘6

m J _ Integrated released

2 6 heat of reaction

.._ '1

o

B

.c 4‘

E 21

c:

2 «l

E 0..
l V l f l ' I r
O 5 10 15 20

Time (min)

Figure 5.3. Integrated released heat of reaction for a reaction system containing di-
epoxy 1,4-butanediol diglycidyl ether and 8.0 x 10'3 molal of photoinitiator tolycumyl

iodonium tetrakis (pentafluorophenyl) borate, (B-[F5]4' ) at 50°C.

74

 

 

 

so-
60-
"o‘
21 i
C
.9
9 4o-
‘3
g .l
20-
OJ
1 l ' l j ' I
o 5 10 15 20

Time (min)

Figure 5.4. Conversion profile for the reaction system containing di-epoxy 1,4-
butanediol diglycidyl ether and 8.0 x 10'3 molal of photoinitiator tolycumyl iodonium

tetrakis pentafluorophenyl borate, (B-[F5]4' ) at 50°C.

75

5.1.3.1c. Propagation Rate Constant
According to standard kinetic analysis, the rate of polymerization RP is equal to
the product of the propagation rate constant, kp, multiplied by the concentration of the

active centers, [M+], and concentration of the epoxide groups, [M], as shown below:

=M=kpwuwl (5.5)
dt
kle*l=3"— (5.6)

[M ]

Therefore, the quantity kpM] can be calculated for each PDSC exotherm as
shown in Figure 5.5 below. In this figure, [rpm initially increases sharply and reaches a
plateau value. It remains relatively constant and then begins to level off and finally
decreases rapidly as the limiting conversion is reached. The shape of the profile of
kpM’] is determined by two factors that may both be changing as the reaction proceeds:
the concentration of active centers [MT] and the propagation rate constant kp. One
possible reason for a change in the effective propagation constant, kp, for cationic
photopolymerizations is a change in the reactivity of the active centers due to the
proximity of the counterion. In the early stage of the reaction, polymerization leads to a
large local increase in viscosity that causes reduction diffusional mobility. The active
centers retains considerable mobility through propagation because they can effectively
moves through the reaction mixture by reacting with new monomers. This type of
mobility has been named “reaction diffusion” and may become the dominant mechanism
for mobility of the active center in highly crosslinked polymerizations.”5 However, the
bulky pentafluorophenyl borate counterion experiences a decrease in mobility as the

viscosity increases. As a result, the counterion and the active center become separated,

76

and since separated ions are orders of magnitude more reactive than ion pairs,” this
would lead to a large increase in the propagation rate constant, kp. This type “reaction

diffusion” is a prominent feature in the reaction kinetics of highly crosslinked free radical

 

 

 

 

 

systems. “5
1E4 1
'70 1
3.3; l
E 1E-5 1
1E-6
i
d I l ' l T l j
o 20 4o 60 so
Conversion (96)

Figure 5.5. Profile of kp[M+] for a reaction system containing di-epoxide monomer 1,4-
butanediol diglycidyl ether and 8.0 x 10'3 molal of photoinitiator tolycumyl iodonium

tetrakis pentafluorophenyl borate, (B-[F5]4- ) and 8.0 x 104 molal photosenstizer CPTX
at 50°C.

77

5.1.4. Results and Discussion
5.1.4.1. Effect of Temperature on the Cationic Photopolymerization Kinetics for
Systems with no Photosensitizers.
Reactions were carried out at a series of temperatures from 30, 40 to 50°C. The
PDSC profiles of the rate of polymerization for cationic polymerization of mono-epoxide

monomer octyl glycidyl ether, photoinitiated by 8.0 x 10'3 molal of tolycumyl iodonium

tetrakis pentafluorophenyl borate, B-[F5]4', are shown in Figure 5.6. The figure
illustrates that, as expected, the rate of polymerization, Rp, and total conversion (Figures
5.6 and 5.7) increase with increasing temperature. This is evident from the fact that the
exotherms reach the maximum rate in a shorter time and also exhibit a larger integrated
heat value as the temperature is increased (recall that the integrated heat released by the
reaction is directly proportional to the total conversion). Figures 5.8 and 5.9 illustrate the
temperature effects of the same photoinitiator for di-epoxide monomer 1,4«butanediol
diglycidyl ether, while the corresponding plots for this initiator in the tri-epoxide
monomer trimethyol propane triglycidyl ether are shown in Figure 5.10 and 5.11. In each
of these monomers, the same general trend for the effect of temperature is observed. As
illustrated in Figures 5.12 and 5.13, the conversion value at the peak maximum of the rate
of polymerization, R , is shifted to the right as the temperature is increased. This
observation further confirms the significant effect of reaction temperature on the rate of
polymerization, RP, and the conversion of monomer. The temperature effects were also
observed in reaction mixtures containing a photosensitizer as discussed in the section

5.1.4.5.

78

0.010 -

 

.'"\.
i R
0.008- ; ‘-._

3, Temperature
§ 0.006- 5 \._ 30°C
: s‘

o -
g 0.004-
D.
I
0.002-
0.000-

 

 

 

Time (min)

Figure 5.6. Reaction rates for cationic photopolymerization of monoepoxy octyl glycidyl
erer and 8.0 x 10'3 molal of photoinitiator tolycumyl iodoniumetrakis pentafluorophenyl

borate,([F5]4- ) at various temperatures.

  
 

 

 

v 50-
5 Temperature
E
2 — 30°C
5:) - ----- 40°C
——- 50°C
0..
l ' l ' I ' l ' l
0 10 20 30 40

Time (min)

Figure 5.7. Effect of temperature on conversion for cationic photopolymerization of
octyl glycidyl ether in the same reaction systems as Figure 5.6.

79

0.035 -

 

 

 

 

0.030 d ;' ‘,
- a.
0.025 - ‘;
A ‘.
§ - -.
O 0.020 - 'l
g . ,' ‘l‘ '3. Temperatu re
E : \i
v 0.015 - 1
e p : \l, 30°C
\ ' ' ' ' 40°C
0.010 q ‘. 50°C
0.005 -
0.000 - . '-
l l l ' l
0 10 20 30 40
Time (min)

Figure 5.8. Reaction rates for cationic photopolymerization of 1,4-butanediol diglycidyl
ether and 8.0 x 10'3 molal of photoinitiator tolycumyl

iodonium tetrakis
pentafluorophenyl borate,---B‘[F5]4-at various temperatures.

'_-..-._.;—‘.-.— — _ _ _ —— — —

50— .- I

  
 

Temperature

Conversion (96)

 

0.1
. . , . . . .
O 10 20

Time (min)

Figure 5.9. Effect of temperature on total conversion for cationic photopolymerization of
1,4-butanediol diglycidyl ether in the same reaction systems as in Figure 5.8.

80

0.01 -

Fip (moi/L‘sec)

0.00 -

 

‘.
."‘~'\. Temperature
5] $3..
a. l... — 30°C
5: \.‘~ 40°C

    

 

Figure 5.10. Rates
monomer trimethyol

 

Time (min)

of polymerization for cationic photopolymerization of ui-epoxide
propane triglycidyl ether with 8.0 x 10'3 molal of tolycumyl

iodonium tetrakis pentafluorophenyl borate,---B-[F5]4'at various temperatures.

........
o

50 - ,- ,r

Conversion (%)

 
   
 

Temperature

_. 30°C
---- 40°C
- ----- - so°c

 

 

- - I - I v I ' I ' I
0 10 20 30 4o 50 60
Time (min)

Figure 5.11. Effect of temperature on total conversion for cationic photopolymerization
of the same reaction systems as in Figure 5.10.

81

0.010

Ftp (moi/L'sec)

 

 

Temperature
30°C

Conversion (%)

Figure 5.12. Profile of reaction rate vs. conversion for the reaction .. of mono-epoxy
octyl glycidyl ether and 8.0 x 10'3 molal of photoinitiator tolycumyl iodonium tetrakis
pentafluorophenyl borate at various temperatures.

0.020

0.01 5

0.010

Rp (moles/L‘sec)

0.005

 

0.000

Temperature

 

 

Conversion (%)

Figure 5.13. Profiles of reaction rate vs. conversion for the reaction systems of di-epoxy
1,4-butanediol glycidyl ether and 8.0 x 10'3 molal of photoinitiator tolycumyl iodonium
tetrakis pentafluorophenyl borate at various temperatures.

82

5.1.4.2. Effect of Light Intensity on the Cationic Photopolymerization Kinetics
PDSC experiments were also performed to elucidate the effect of the initiating
light intensity on the photopolymerization kinetics. Figures 5.14—5.15 show the reaction
rates and conversion profiles of octyl glycidyl ether photoinitiated by the photoinitiator
diaryliodonium hexafluoroantimonate in an isothermal polymerization at temperature of
50°C with the light intensity varying from 35 mW/cm2 to 90 mW/cmz. A similar set of
studies for the di-epoxide monomer 1,4-butanediol diglycidyl ether are shown in Figures
5.14-5.15. Based on the results observed from the two reaction systems, it is evident that
as the light intensity is increased, the rate of polymerization is observed to increase. In
addition, the total conversion was found to exhibit a small increase with increasing light
intensity. It appeared that with the photoinitiator concentration at 8.0 x 10'3 molal and
light intensity at 52 mW/cmz, the total conversion of both mono-epoxide and di-epoxide
monomers had closely reached its limiting point. Table 5.1 gives a result summary for
these two reaction systems.
Table 5.1. Summary of the effects of incident light intensity on the rates of

polymerization and conversions for two representative reaction systems. The
photoinitiator was diaryliodonium hexafluoroantimonate at 8.0 x 10'3 molal.

 

 

 

 

 

Epoxide Light intensity Peak time Peak rate of Total
monomer (mW/ cmz) (min) polymerization conversion (%)
(mol/L*sec)
Octyl glycidyl 35 2.26 i 0.10 0.009 1: 0.001 80.01 :i: 1.07
ether 52 1.37 i 0.03 0.015 :t 0.001 82.60 :i: 1.05
90 0.78 :t 0.04 0.023 i 0.001 82.94 i 1.12
1,4-butanediol
diglycidyl 50 1.31 :i: 0.01 0.028 t 0.002 74.95 :i: 1.58
ether 74 0.82 :i: 0.03 0.052 i 0.001 76.60 :i: 0.20

 

 

 

 

83

 

 

 

l
0.02 -
8
i” q
:
o O I
E. Light IntenSIty
g- 0.01 -
------ 35 mW lcm2
—— 52 mW lcm2
- - - - 90 mW lcm2
0.00- ........
l I
15 20

Time (min)

Figure 5.14. Effects of 1i ght intensity for cationic polymerization of octyl glycidyl ether
photoinitiated by 8.0 x 10' molal of diaryliodonium hexafluoroantimonate at 50°C.

  

 

 

0.02 - f '\‘
A :’ ‘\.
§ .!
. ' .\ C O
g . , _ ... ‘, Light Intensity
! I ’ ”b ‘ s \
v i ’ \ \ .'\
o. g I \\ 4‘ ——35 mW/cm2
I oat-g} \ "~. ----52mW/cm2
i,’ ‘ - ----- - 90 mW lcm2
(K.
\O
\‘t
O 00 V I ' I ' I ' rll fl I
0 20 40 so so 100

Conversion (96)

Figure 5.15. Effects of light intensity on both reaction rates and conversions for cationic
photopolymerization of octyl glycidyl ether photoinitiated by 8.0 x 10'3 molal of
diaryliodonium hexafluoroantimonate at 50°C.

 

0.05 -
0.04 -
’6
0)
J” 0.03 -
—‘ r . .
>0 i ‘- Light IntenSIty
E i
a 0-02‘ 5' . - ----- 50 mW/cm2
II . \
_l \ — 74 mW / cm2
l
0.01 —
.i
0.00 -

 

 

 

Time (min)

Figure 5.16. Rates of polymerization for cationic photopolymerization of 1,4-butanediol
diglycidyl ether at different initiation light intensities by 8.0 x 10'3 molal of photoinitiator
diaryliodonium hexafluoroantimonate at 50°C.

 

 

 

 

E
C
2
it
E Light Intensrty
o ------ 50 mW lcm2
74 mW lcm2
T 1
15 20

Time (min)

Figure 5.17. Effects of light intensity on conversion profiles for cationic
photopolymerization of (ii-epoxide monomer 1,4-buranediol diglycidyl ether by 8.0 x 10'3
molal of photoinitiator diaryliodonium hexafluoroantimonate at 50°C.

85

5.1.4.3. Effect of Different Photoinitiators on Cationic Photopolymerization Kinetics
To investigate the effect of the photoinitator on the photopolymerization kinetics,
PDSC experiments were performed using three different photoinitiators. The two

iodonium salts with different counterions were: 1) tolycumyl iodonium tetrakis
pentafiuorophenyl borate, B-[F5]4'; 2) diaryliodonium hexafluoroantimonate, [Sng]’.

The third photoinitiator was triarylsulfonium hexafluorophosphate, [PF6]’. Figures 5.18
through 5.23 show the effects that the two iodonium salts have on three different mono-
epoxide monomers: octyl glycidyl ether, butyl glycidyl ether and phenyl glycidyl ether.
Similar curves for the di-epoxide monomers are shown in Figures 5.24 through 5.27.
These reactions were run isotherrnally at 50°C and constant light intensity of about 52
mW/cmz. The effect of the photoinitiator on the polymerization kinetics were relatively
subtle and small, however, in all three cases, the rate of polymerzation, RP, with the
diaryliodonium hexafluoroantimonate was a little faster than that with the tolycumyl
iodonium tetrakis pentafluorophenyl borate.

Compared to the iodonium salts, the triarylsulfonium salt exhibited a considerable
slower rate of polymerization and lower conversion. As illustrated by the data in Table
5.2, the triarylsulfonium hexafluorophosphate initiator exhibited low conversions and
comparable polymerization rates even with relatively high initiator concentrations (16.0 x

10'3 molal) and high light intensity (90 mW/cm’).

86

Table 5.2.

photopolymerization for various reaction systems at 50°C.

Effect of different photoinitiators on reaction rates (RP) of cationic

 

 

 

 

Phenyl glycidyl 1,4-butanediol Trimethyol
ether diglycidyl ether propane
triglycid I ether
Photoinitiator & Light % % %
concentration intensity Peak Rp Conv. Peak Rp Conv. Peak R, Conv.
0.008 molal 0.024 94.0 0.022 76.3 0.012 59.6
Initiator A 52 :i: 0.2 :i: 0.001 :1: 0.8 i: 0.001 i 1.1 :i: 0.001 :1: 1.2
0.008 molal 0.018 98.4 0.028 74.6 0.012 59.4
Initiator B 53 i 0.1 d: 0.001 :1: 0.9 :i: 0.002 :1: 2.7 i 0.001 :I: 0.5
0.016 molal 0.041 94.4 0.137 72.2 0.040 49.5
Initiator C 90 :i: 0.1 :i: 0.002 :i: 1.9 :i: 0.006 :1: 0.8 i 0.001 :1: 0.6

 

 

 

 

 

 

Note: A = tolycumyl iodonium tetrakis pentafluorophenyl borate, B-[F5]4'

B = diaryliodonium hexafluoroantimonate, [Sde-

C = triarylsulfonium hexafluorophosphate, [P136]-
Rp in moles/L*sec
Light intensity in mW/cm2

87

 

    
 

 

 

 

% 0.01 J .\
O '\ -
“g ’x. --------- 8 mmolal B-[F5]4
V \O
a? 8 mmolal [Sde-
0.00 1 '\ ............
5 E in

Time (min)

Figure 5.18. Effect of photoinitiators tolylcumyl iodonium tetrakis pentafluorophenyl

borate, B-[F5]4-; and diaryliodonium hexafluoroantimonate, [SbFa]- for mono-epoxy
octyl glycidyl ether at 50°C.

  
 

 

 

 

100 -

E

C

3

2 50 r -
E --------- 8 mmolal B'[F5]4
O

o -

8 mmolal [Sde
0 o g ‘ 1'0

Time (min)

Figure 5.19. Effect of photoinitiators on conversion profiles in the same reaction
systems as in Figure 5.18.

88

0.02 -

 

  
 

 

 

 

 

Ii . ‘-
§> 0.01 - . """"" 3 "”0101 B-[F514-
0% i 8 mmolal [Sde
0.00- '--—
6 i» 4 it 5 1'0 1'2

Time (min)

Figure 5.20. Effect of photoinitiators tolylcumyl iodonium tetrakis pentafluorophenyl
borate, B-[F5]4'; and diaryliodonium hexafluoroantimonate, [Sde- for mono-epoxy

butyl glycidyl ether at 50°C.

80-

60-4

Conversion (%)
6
1

 

o-

  
 

N-

’.-.-.-.-.-.-.-.-..
.

 

--------- 8 mmolal B-[Fs];

 

s mmolal [355.]’

i l f I ‘I
6 8 10 12
Time (min)

#1

Figure 5.21. Effect of photoinitiators on conversion profiles in the same reaction

systems as in Figure 5.20.

89

   
  

----- 8 mmolal B—[FS];

Rp (moles/L‘sec)
O
8
I

s mmolal [35126]”
0.01 -

0.00 -

 

 

 

 

-
d
d

Time (min)

Figure 5.22. Effect of photoinitiators tolylcumyl iodonium tetrakis pentafluorophenyl

borate, B-[F5]4-; and diaryliodonium hexafluoroantimonate, [SbF6]' for mono-epoxy
phenyl glycidyl ether at 50°C.

  
 

 

 

 

100 -
g
g 50 . _
é -- - - -- 8 mmolal B‘[F5]4
o -
. 8 mmolal [Sde
0 - I I I I v I V I ' I
0 2 4 5 8 10 12
Time (min)

Figure 5.23. Effects of photoinitiators on conversion profiles in the same reaction
systems as in Figure 5.22.

0.03 -

  

 

 

 

 

 

 

E?
g 0.02 -
.2
E
E ----- 8 mmolal B-[Fslr
a: 0.01- -
8 mmolal [SbFfi]
0.00-
I l j T r I
5 10 15 20

Time (min)

Figure 5.24. Effects of photoinitiators tolylcumyl iodonium tetrakis (pentafluorophenyl)

borate, B-[F5]4-; and diaryliodonium hexafluoroantimonate, [SbF6]- for di-epoxide
monomer 1,4-butanediol diglycidyl ether at 50°C.

50‘

Conversion (%)

 

  
 

 

------... 3 mmolal B-[Fslr'

 

8 mmolal [sorrl’

 

I r I

10 20

Time (min)

Figure 5.25. Effect of photoinitiators on conversion profiles in the same reaction

systems as in Figure 5.24.

91

 

 

 

 

 

0.015 -
e
0
:0 0 010 -
a .
In
2
O
E.
6:1 0.005 P ------- 8 mmolal B-[Fs];
—- s mmolal [Sng]-
0.000 -
0 10 20

Time (min)

Figure 5.26. Effect of photoinitiators tolylcumyl iodonium tetrakis (pentafluorophenyl)

borate, B-[F5]4-; and diaryliodonium hexafluoroantimonate, [SbF6]- for di-epoxide
monomer cyclohexane dimethanol diglycidyl ether at 50°C.

 

 

E

8

E 50 b

O

E -

8 ------- 8 mmolal B-[F514
__ 8 mmolal [Sde-

o . ' ‘ I j
0 1 o 20

Time (min)

Figure 5.27. Effect of photoinitiators on conversion profiles in the same reaction
systems as in Figure 5.26.

92

5.1.4.3. Effect of Photoinitiator Concentration on the Photopolymerization Kinetics
In addition to temperature, the effect of the photoinitiator concentration on the
rate of polymerization was investigated. All the reactions were run under isothermal
conditions at 50°C with various concentrations of the photoinitiator. Figures 5.28
through 5.32 illustrate the effects of the concentration of two different iodonium
photoinitiators (tolycumyl iodonium tetrakis pentafluorophenyl borate and
diaryliodonium hexafluoroantimonate) on the cationic polymerization kinetics for the
mono-epoxide monomer butyl glycidyl ether. For both photoinitiators, the
polymerization kinetics exhibit a large increase in reaction rate and conversion as the
initiator concentration is increased from 2.0 x 10’3 to 4.0 x 10'3 and to 8.0 x 10'3 molal.
Further increase of initiator concentration to 10.0 x 103, 12.0 x 10'3 and 16.0 x 10'3 molal
only produced a small increase in reaction rates and improved very little with respect to
conversions. Similar effects were observed in the case with the di-epoxide monomer 1,4-
butanediol diglycidyl ether using the same reaction conditions (Figures 5.34 to 5.39).
Therefore, the photoinitiator concentration of 8.0 x 10‘3 molal was chosen as the
concentration to be used in other reaction systems. This choice would allow for the
quantitative comparison among different reaction systems and would also prevent
potential solubility problems. Tables 5.4 through 5.7 provide the result summary for the
effects of photoinitiator concentration on the cationic photopolymerization kinetics.
Table 5.4. Concentration effects of photoinitiator diaryliodonium hexafluoroantimonate

on reaction rates and conversions of monoepoxy butyl glycidyl ether at 50°C

Concentration Peak time (min) Peak height (mollLrsec) % conversion

 

2.0 x 10'3 2.05 :i: 0.05 0.0074 i 0.0003 48.2 1 1.0

 

 

 

 

4.0 x 10'3 1.72 i 0.02 0.0144 i 0.0004 62.3 :i: 1.4

 

93

 

 

 

 

 

8.0 x 10'3 1.63 1 0.09 0.0224 1 0.0005 71.1 1 4.0
12.0 x 10'3 1.62 1 0.07 0.0235 1 0.0007 70.7 1 0.9
16.0 x 10'3 1.62 1 0.04 0.0253 1 0.0006 72.0 1 2.0

 

Table 5.5. Concentration effects of photoinitiator tolycumyl iodonium tetrakis
pentafluorophenyl borate on reaction rates and conversions of mono-epoxy butyl

 

 

 

 

 

 

 

 

glycidyl ether at 50°C.
Concentration Peak time (min) Peak height (mol/Lcsec) % conversion
2.0 x 10'3 2.34 1 0.13 0.0065 1 0.0003 57.2 1 3.4
4.0 x 10'3 2.31 1 0.14 0.0106 1 0.0007 64.5 1 1.9
8.0 x 10'3 2.11 1 0.01 0.0161 1 0.0002 69.0 1 0.2
12.0 x 10'3 2.07 1 0.08 0.0183 1 0.0011 67.5 1 3.1
16.0 x 10‘3 1.93 1 0.07 0.0213 1 0.0005 69.7 1 1.1

Table 5.6. Concentration effects of photoinitiator diaryliodonium hexafluoro-antimonate
on reaction rates and conversions of di-epoxy 1,4-butanediol diglycidyl ether at 50°C.

 

 

 

 

 

 

 

 

Concentration Peak time (min) Peak height (moi/Lesa) % conversion
2.0 x 10'3 1.04 1 0.08 0.026448 1 0.0014 71.2 1 2.9
4.0 x 10'3 1.06 1 0.06 0.033048 1 0.0014 73.9 1 1.3
8.0 x 10'3 0.91 1 0.04 0.040756 1 0.0003 74.1 1 1.2
12.0 x 10'3 0.98 1 0.14 0.038819 1 0.0002 74.9 1 0.1
16.0 x 10'3 1.08 1 0.02 0.039257 1 0.0010 74.9 1 1.1

Table 5.7. Concentration effects of photoinitiator tolycumyl iodonium tetrakis
pentafluorophenyl borate on reaction rates and conversions of di-epoxy 1,4-butanediol
diglycidyl ether at 50°C.

 

 

 

 

 

 

 

 

Concentration Peak time (min) Peak height (mollLasec) % conversion
2.0 x 10r 1.45 1 0.05 0.0186 1 0.0003 73.2 1 0.4
4.0 x 10'3 1.37 1 0.02 0.0256 1 0.0005 73.9 1 0.7
8.0 x 10'3 1.10 1 0.02 0.0330 1 0.0002 75.5 1 0.3
12.0 x 10'3 1.07 1 0.04 0.0353 1 0.0003 74.5 1 0.3
16.0 x 10'3 1.10 1 0.02 0.0383 1 0.0007 74.6 1 0.3

94

0.02

 

 

 

 

3 Photoinitiator concentration
:0
g - - - 2 mmolal
3 ----- 4mmmm
o _
E 0.01 8 mmolal
Z - ----- 12 mmolal
r: ------ 16 mmolal
-— ~ _ ~ ‘
\
...-o-o-O>--- -----
0.00 I I
20 30
Time (min)

Figure 5.28. Concentration effect of photoinitiator tolycumyl iodonium tetrakis
(pentafluorophenyl) borate, B-[Fs]; on mono-epoxy butyl glycidyl ether at 50°C.

50'1

Photoinitiator Concentration

- - — 2 mmolal
-.---4 mmolal
__.8 mmolal
_ ..... 12 mmolal
...........- 16 mmolal

Conversion (%)

 

 

01

I 1 I

O:
J
O
N
O

30
Time (min)

Figure 5.29. Effects of photoinitiator concentration on conversion in systems comprising

of mono—epoxy butyl glycidyl ether and photoinitiator tolycumyl iodonium tetrakis
pentafluorophenyl borate, B-[Fs]; at 50°C.

95

------ .-- ._
..a -.~“
-__
-
.‘~_.
I.-.
“‘n.‘
-._
...
'-

0.01 -—

 

0.02 '-
’6‘
O
(n
O
—| -i
\
U)
2
O
E.
D.
I
0.00

Photoinitiator Concentration

..... 2mmmm
......... 4mmmm
—— 8 mmolal
..--.12 mmolal
-..-...... 16 mmolal

 

 

Conversion (%)

Figure 5.30. Profiles of the rate of polymerization, RP, versus conversion for mono-
epoxy butyl glycidyl ether with various concentrations of photoinitiator tolycumyl

iodonium tetrakis pentafluorophenyl borate, B-[Fs]; at 50°C.

96

   

0.02 - Photoinitiator Concentration, moll kg
..... 2mmo|al

......... 4 mmolal

_ 8 mmolal

..... 12 mmolal

_ ........ - 16 mmolal

Ftp (mol/L'sec)

0.01 -

   

I
....

I
.---—r"
- — — 1 — ’.

..........-......-. -“ ----------

 

 

0.00 '1
I

 

Time (min)

Figure 5.31. Concentration effects of photoinitiator diaryliodonium
hexafluoroantimonate, [SbF6]- on mono-epoxy butyl glycidyl ether at 50°C.

It)! Inns——-

 

 

 

2\°, ' ' ’ —————————
5 . , ’
.E ’
g
5 Photoinitiator Concentration, mol/ kg
0
- - - 2 mmolal
----- 4 mmolal
—8 mmolal
- ----- 12 mmolal
------- 16 mmolal
l ' I ' l ' I
0 10 20 30

Time (min)

Figure 5.32. Effects of photoinitiator concentration on conversion in systems comprising
of mono-epoxy butyl glycidyl ether and photoinitiator diaryliodonium hexafluoro-

antimonate, [SbF61'at 50°C.

97

’3 0.02-
O
m
.
_.I
E
O 1
E
E
5‘; 0.01-1

 

  
 
 
 
 

Photoinitiator Cone. (mol/ kg)

- - - ~2mmolal
------ 4mmmm
.‘ — 8 mmolal

\ .‘ - ----- 12 mmolal

------- 16 mmolal

I

O
n“'

00‘”.
......

‘poo-C"
iguooofl.

40 60 80

Conversion (%)

Figure 5.33. Profiles of the rate of polymerization versus conversion for mono-epoxy
butyl glycidyl ether with various concentrations of photoinitiator diaryliodonium

hexafluoroantimonate, [SbF6]-at 50°C.

98

0.04 -

 
  
 

 

 

 

 

P3"
0.03- Photoinitiator concentration, mole/kg
3’7
:0 ------ 2 mmolal
: ......... 4 mmolal
g 0.0% — 8 mmolal
E . ---..-“ 12 mmolal
cc .‘ ......... 16 mmolal
0.01- '
0.00 ‘ “N - ------
I ' I I I if I
0 5 10 15 20 25
Thne(nfin)

Figure 5.34. Concentration effects of photoinitiator tolycumyl iodonium tetrakis
pentafluorophenyl borate, B-[F5]4- on di-epoxy 1,4-butanediol diglycidyl ether at 50°C.

 

 

’3
21 50-
S
T": Photoinitiator concentration, mole/kg
0
I
E ,- I ----2mmolal
o ------ 4 mmolal
— 8 mmolal
- ----- 12 mmolal
------- 16 mmolal
OJ
. . . . . , . . . .
o 5 10 15 20 25
Time (min)

Figure 5.35. Effects of photoinitiator concentration on conversion in systems comprising
of di-epoxy 1,4-butanediol diglycidyl ether and photoinitiator tolycumyl iodonium

tetrakis pentafluorophenyl borate, B-[Fs]; at 50°C.

99

Photoinitiator concentration, mole/kg
o.o4 -

    
  

’ ----- s.‘ - - - .2 mmOIaI
J I!’ """" ~ .‘."\ ...... 4 mmOIal
.I’O, '0‘ .\.‘. —— 8 mm0|a|
0.031 - N — ----- 12 mmolal
A - ------- 16 mmolal
0
0
a:
C
i 0.02 .
O
E
a.
r:
0.01-
0.00 . ' T

 

 

Conversion (%)

Figure 5.36. Profiles of the rate of polymerization versus conversion for di-epoxy 1,4-
butanediol diglycidyl ether with various concentrations of photoinitiator tolycumyl

iodonium tetrakis (pentafluorophenyl) borate, B-[Fs]; at 50°C.

As illustrated in Figures 5.30, 5.33, 5.36 and 5.39, the rate of polymerization, R ,
and conversion increase with increasing concentration of the photoinitiator. Furthermore,
the conversion value at the peak maximum is shifted to the right, which further confirms
the significant effect of photoinitiator concentration on the rate of polymerization, R , and
conversion. Since the photoinitiator is by the far the most expensive component in
reactive system (typical cost is ~$40lpound), it is important to identify the optimum
initiator concentration. Our studies indicate that concentrations above 8.0 x 10'3 molal

provide little added benefit in terms of reaction rate and final conversion.

100

0.04 '-

0.03 '

0.02 -

Rp (moles/L'sec)

 

 

0.00

Photoinitiator concentration, mole/kg

...... 2mmmm
-..“..- 4 mmolal
___8mmmm
...... 12 mmolal
_ .......... 16 mmolal

 

 

Figure 5.37.

Time (min)

Concentration effects of photoinitiator diaryliodonium hexaflouro-

antimonate, [Sde- on di-epoxy 1,4-butanediol diglycidyl ether at 50°C.

50‘

Conversion (%)

 

o-

 

  
 

Photoinitiator concentration, mole/kg

...... 2mmmm
......-.-4 mmolal
__—8mmmm
--------- 12 mmolal
- ...... - 16 mmolal

I

10
Time (min)

20

Figure 5.38. Effects of photoinitiator concentrations on conversion in reaction systems
comprising of di-epoxy 1,4-butanediol diglycidyl ether and photoinitiator diaryliodonium

hexafluoroantimonat

e, [Sbm'at 50°C.

101

0.04
A 0.03
O
0
m
C
—l
\
m
2 0.02
O
E
a.
r:
0.01
0.00

 

  
  

Photoinitiator concentration, mole/kg

/' .--‘.;\ - - - 2 mmolal

‘~:\ ----- 4 mmolal

“‘,\ —8 mmolal
‘ —-—-- 12 mmolal

------- 16 mmolal

 

Conversion (%)

Figure 5.39. Profiles of the rate of polymerization versus conversion for di-epoxy 1,4-
butanediol diglycidyl ether with various concentrations of photoinitiator diaryliodonium

hexafluoroantimonate, [SbFJat 50°C.

102

5.1.4.5. Effect of Photosensitizer on the Photopolymerization Kinetics

The absorption spectra of the photoinitiators (Figures 4.8-4.10) indicate their
strong absorption in region below 300 nm. In order to increase their spectral sensitivity
to the emission spectrum of the ZOO-Watt Hg/Xe arc lamp, two different photosensitizers
were investigated: 1-chloro—4-propoxy-9H—thioxanthen-9-one (CPTX) and anthracene.
These photosensitizers absorb strongly in the region of 310-380 nm, (Figures 4.11-4.12)
and therefore may be used to shift the effective initiation wavelength to this near-UV
region of the spectrum. The photosensitizers generally operate through an electron
transfer mechanism in which an excited state photosensitizer molecule transfer an
electron to the iodonium photoinitiator.""19 In this study two different photosensitizers
were investigated: l-chloro-4-propoxy-9H—thioxanthen-9-one (CPTX) and anthracene.

The addition of either photosensitizer to the reaction mixture significantly
affected the rate of polymerization for the epoxide monomers, as shown in Figures 5.40
through 5.46. Specifically, Figures 5.40 and 5.41 show the temperature effect in reaction
systems containing mono—epoxide octyl glycidyl ether with 8.0 x 10" molal of
photosensitizer CPTX and 8.0 x 10'3 of photoinitiator diaryliodonium
hexafluoroantimonate. In Figure 5.42 and 5.43, the photoinitiator was switched to
tolycumyl iodonium tetrakis pentafluorophenyl borate. Then in Figures 5.44 and 5.47,
The effects of different photosensitizers (CPTX and anthracene) were observed. In each
case, the concentration of photosensitizer was 8.0 x 104 molal. The figures showed that
the mono-epoxide octyl glycidyl ether was more reactive in the presence of CPTX than it
was with anthracene, but that the addition of anthracene resulted in a higher

polymerizaton rate than the photosensitizer-free system. The relatively large effect of

103

CPTX can be attributed to the stronger molecular absorption by the CPTX as compared
to that by the anthracene (Figures 4-11 and 4-12). As a result, at the initial stage of the
reaction, more CPTX molecules were photolyzed and became active to undergo electron
transfer to the photoinitiator molecules, thereby setting off the initiation of the chain
polymerization reactions. Although the reaction rate increases remarkably in the
presence of a photosensitizer, the final limiting conversion does not increase by much
(Figure 5.48). This phenomenon can be attributed to the fact that photosensitizer acts
through the electron transfer process to increase the active centers available for
polymerization while the final limiting conversion depends primarily on the glass
transition temperature of the reacting mixture, which is determined by the molecular
structure of the monomer. Tables 5.9, 5.10, 5.11 and 5.12 summarize the effects of the
two photosensitizers CPTX and anthracene on polymerization rate and conversion for the
three mono-epoxide monomers: butyl glycidyl ether, octyl glycidyl ether, and phenyl
glycidyl ether with each photoinitiator (diaryliodonium hexafluoroantimonate or
tolycumyl iodonium tetrakis pentafluorophenyl borate).

The same set of reaction conditions were then applied in the cases with the two
di-epoxide monomers 1,4-butanediol diglycidyl ether and cyclohexane dimethanol
diglycidyl ether, and the tri-epoxide monomer trimethyol propane. Similar results on the
rate of polymerization and limiting conversion were observed. For the (ii-epoxide
monomers, the effects on polymerization rate are illustrated in Figures 5.48 through 5.56
and the result summary for polymeization rate and conversion are presented in Tables
5.13 through 5.16. The same information for the tri-epoxide monomer trimethyol propane

triglycidyl ether are found in Figures 5.57 through 5.64, and Tables 5.17 to 5.20.

104

0.03 -

’o‘ .i

0 ::

(I) II

a :t

.1 I:

B °-°2 "it Temperature
% 53.

E ~51 ---- 30°C
v ' ‘ 0
a 1 —-— 40C
II 0.01

 

 

0.00 ’

 

Time (min)

Figure 5.40. Temperature effects for reaction systems of mono-epoxy octyl glycidyl

ether with 8.0 x 10’3 molal photoinitiator diaryliodonium hexafluoro—antimonate, [SbFfil-
and 8.0 x 104 molal photosensitizer CPTX.

 

 

 

g;
C
.9
g Tem peratu re
E ---- 30°C
o __ 40°C
- ....... 50°C
0 4 1 1 l
0 20 40

Time (min)

Figure 5.41. Effect of photosensitization on final conversion for the same reaction
systems as in Figure 5.40.

105

0.03 -"

’6‘

0

3° . e

d 0.02 -g g

U) I -

.g 3 Temperature
.5, 'x, ---- 30°C
E 0.01 “ 40°C

0.00

 

 

 

 

Time (min)

Figure 5.42. Temperature effects for reaction systems of monoepoxy octyl glycidyl ether
with 8.0 x 10'3 molal photoinitiator tolycumyl iodonium tetrakis pentafluorophenyl

borate, B-[F5]4-and 8.0 x 10‘4 molal photosensitizer CPTX.

50-

  
 

Conversion (%)

Temperature
---- 30°C
— 40°C
- ----- -- 50°C

 

A l A l a l

o 10 20 so
Time (min)

Figure 5.43. Effect of photosensitization on final conversion for the same reaction
systems as in Figure 5.42.

106

 

 

 

 

 

’6 0.02 —
:3 Photosensitizer
.1
3 - — — Control
1.5. — CPTX
a. — ----- Anthracene
II 0.01 —
~\‘_s ‘
.\ \
0.00 . l 4—4 J
0 1o 20
Time (min)

Figure 5.44. Photosensitizer effect for reaction systems of mono-epoxy octyl glycidyl
ether with 8.0 x 10' molal of photoinitiator diaryliodonium hexafluoroantimonate and
8.0 x 104 molal of photosensitizer at 50°C.

 

 

 

.3
S
-- 50>-
g I: I
> ,- I Photosensitizer
C I I
o .
0 ','I -- - Control
,-, — CPTX
:1 — ----- Anthracene
,9
,r
o - l . I . I
o 5 10 15

Time (min)

Figure 5.45. Photosensitizer effect on final conversion for the same reaction systems as
in Figure 5.44.

107

 

7; Photosensitizer
3 0.02 -
.d - — - Control
'6 —— CPTX
E ..1- .\ — ----- Anthracene
n. I .'
r: 0.01 ~ -'
f I
,- /
:I
[I
i
0.00 —

 

 

 

Time (min)

Figure 5.46. Photosensitizer effect for reaction system of mono-epoxy octyl glycidyl
ether with 8.0 x 10'3 molal photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl)

borate, B-[F5]4_and 8.0 x 104 molal photosensitizer at 50°C.

  
 

 

 

23
C
.9
9 so -
2 Photosensitizer
o I
o f ,’ --- Control
.' I — CPTX
,-’ ’l — ----- Anthracene
: l
o I I I
o 5 10 15

Time (min)

Figure 5.47. Photosensitizer effect on final conversion for the same reaction systems as
in Figure 5.46.

108

IConttol
ICPTX
lAnlhreeene

 

 

3000 40°C 50°C
Temperature (‘0)

Figure 5.48. Photosensitizer effect on the conversion for reaction systems comprising of
octyl glycidyl ether and 8.0 x 10'3 molal photoinitiator tolycumyl iodonium tetrakis

(pentafluorophenyl) borate, B-[Fs];

109

0.15-

 

 

 

 

 

’5 .'
S 0.10- :=
;! '1
3 J .
'3' Temperature
g- o.os - " " ' " 23$
_ ..... -. 50°C
0.00 3 ---------- F __________ I
20 so
Time (min)

Figure 5.49. Temperature effects for reaction systems of di-epoxide monomer 1,4-
butanediol diglycidyl ether with 8.0 x 10'3 molal of photoinitiator diaryliodonium hexa-

fluoroantimonate, [Sbm’and 8.0 x 10‘4 molal of photosensitizer CPTX.

 

 

”
23 ;" ,I’
8 5°‘ .5 ,’
E i ,’
“g g Temperature
0 : I
o g ,I 30°C
;; — 40°C
- ----- 50°C
V
o I I ' I 1 I
0 10 20 30
Time (min)

Figure 5.50. Photosensitizer effect on final conversion for the same reaction systems as

in Figure 5.49.

110

 

 

0.15 1
3!
5;
... ii
8 ii
CD I]
C I:
-\J 0.10 fig
CD ii
2 =:
o 2:
.5.. i". Temperature
.1
a ; ---- 30°C
a: 0.05 j __ 40°C
_ ...... 50°C
0.00 W
20 30

 

Time (min)
Figure 5.51. Temperature effects for reaction systems of di-epoxy 1,4-butanediol

diglycidyl ether with 8.0 x 10'3 molal of photoinitiator Tolycumyl iodonium tetrakis
(pentafluorophenyl) borate, B-[F5]4-and 3.0 x 10'4 molal of photosensitizer CPTX.

..........
o"
a

 

 

 

35
C
.9
U3
3
E Temperature
0 an 30°C
— 40°C
- ----- - 50°C
0 n l n l
O 10 20

Time (min)

Figure 5.52. Photosensitizer effect on final conversion for the same reaction systems as
in Figure 5.51.

111

0.15 - n

 

23‘

(D

0)

1’ Photosensitizer

B 0.10 "

g .. - .. Control

E — CPTX

v _ ----- Anthracene
0.

cc

0.05 '-

0.00 '-

 

 

 

 

 

Time (min)

Figure 5.53. Photosensitizer effect for reaction system of di-epoxy 1,4-butanediol
diglycidyl ether with 8.0 x 10'3 molal of photoinitiator diaryliodonium
hexafluoroantimonate and 8.0 x 10“ molal of photosensitizer at 50°C.

  
 

 

 

 

ix; so -
C
.2
g Photosensitizer
>
C
8 - - - Control
----- CPTX
Anthracene
0 . r t r . I
o s 10 15
Time (min)

Figure 5.54. Photosensitizer effect on final conversion for the same reaction systems as
in Figure 5.53.

112

 

 

0.15 -

a i

o D

.0,

5 0.10 - ..

93 Photosensrtlzer

é - — - Control

a. —— CPTX

a: 0.05 - - ----- Anthracene
o.oo

 

 

 

Time (min)

Figure 5.55. Photosensitizer effect for reaction system of di-epoxy 1,4-butandiol
diglycidyl ether with 8.0 x 10’3 molal of photoinitiator tolycumyl iodonium tetrakis

pentafluorophenyl borate, B—[Fsh'and 3.0 x 10“ molal of photosensitizer at 50°C.

 

 

 

 

.13
C
.9
9
m
>
E Photosensitizer
.. .. - Control
__ CPTX
- ----- Anthracene
L l I
10 15

Time (min)

Figure 5.56. Photosensitizer effect on final conversion for the same reaction systems as
in Figure 5.55.

113

0.04 1

0.03 -

0.02 .

Fip (moles/L'sec)

0.01 -1

 

0.00 -

3 Temperature

---- 30°C

 

 

 

Time (min)

Figure 5.57. Temperature effects for reaction systems of tri-epoxy trimethyol propane
triglycidyl ether with 8.0 x 10'3 molal of photoinitiator diaryliodonium

hexafluoroantimonate, [SbF5]-and 8.0 x 10‘4 molal of photosensitizer CPTX.

Conversion (%)

 

 

 

Temperature
---- 30°C
—— 40°C
’ """ ‘ 50°C

 

I f I ' I ' I

O 20 40 60

Time (min)

Figure 5.58. Photosensitizer effect on final conversion for the same reaction systems as

in Figure 5.57.

114

o.oe- ;

0.05- ii,
55.

8 E!
g 0.04- i,
2 . gt Temperature
m ! i
g 033‘!“ _-_- 300C
g g 1 — 40°C
Q 0.02“: '-
a:

0.01 '1

 

0.00 -

 

 

 

Time (min)

Figure 5.59. Temperature effects for reaction systems of tri-epoxide monomer
trimethyol propane triglycidyl ether with 8.0 x 10'3 molal of tolycumyl iodonium tetrakis

pentafluorophenyl borate, B-[F5]4-and 8.0 x 104 molal of photosensitizer CPTX.

 

 

 

 

so- _______
e ........
T; "
.2
e
2
g Temperature
---- 30°C
— 40°C
- ----- - 50°C
0 I ' I ' I
0 20 40
Time (min)

Figure 5.60. Photosensitizer effect on final conversion for the same reaction systems as
in Figure 5.59.

115

0.04 '-

2?
O
.03
'3’ Photosensitizer
2 0.02 - ..
g : \. --- Control
v .-' 't — CPTX
g g, 3 - ----- Anthracene
5 .I’ ‘
7
I

 

 

F
0.00

.— _o--1

40

 

Time (min)

Figure 5.61. Photosensitizer effect for reaction system of tri-epoxide trimethyol propane
triglycidyl ether with 3.0 x 10'3 molal of photoinitiator diaryliodonium
hexafluoroantimonate and 8.0 x 10'4 molal of photosensitizer at 50°C.

 

 

 

E

r:

g

8

g Photosensitizer

O

0 - - - Control
— CPTX
- ----- Anthracene
20 . 40

Time (min)

Figure 5.62. Photosensitizer effect on final conversion for the same reaction systems as
in Figure 5.61.

116

0.06 '-

’8

3

1' 0.04 r- . .

g Photosensitizer
é - - - Control

C? ,v \ —— CPTX

0.02 - -' a - ----- Anthracene

 

 

0.00

--P— -1

25

 

 

Time (min)

Figure 5.63. Photosensitizer effect for reaction system of tri-epoxy trimethyol propane
triglycidyl ether with 8.0 x 10'3 molal of photoinitiator tolycumyl iodonium tetrakis

(pentafluorophenyl) borate, B-[Fs]; and 8.0x10'4 molal of photosensitizer at 50°C.

 

 

 

......... :.;-. -..-Fl — — -- -
I ’ ’ ’
' I
A so - or I
’ I
a ,
s , ,
g .-’ ,’ Photosensitizer
s ,
° I I
o " I - - - Control
" ’ —— CPTX
; ’ - ----- Anthracene
-' I
.' I
I' l
o " ‘ L
° 10 20
Time (min)

Figure 5.64. Photosensitizer effect on final conversion for the same reaction systems as
in Figure 5.63.

117

As illustrated in Figure 5.57, the rate of polymerization increases sharply to
exhibit the effect of both temperature and photosensitization. With just the photoinitiator
alone, the reaction still occurs. Recall that the photosensitizer CPTX can absorb more
strongly of the initiating light in the wavelength region above 300 nm than the sensitizer
anthracene can. As a result, more energy is being transferred to the initiator by the
electron transfer process, which in turn causes in an increase in the generation of the
active cationic centers, which is manifested by the sharp increase in the rate of
polymerization. The tri-epoxide monomer trimethyol propane triglycidyl ether has three
epoxide rings in the molecule, which consequently gives rise to the highest local
concentration of epoxide rings available for reaction. Theoretically, the rate of
polymerization should be faster than that of the (ii-epoxide and mono-epoxide monomers.
The actual observed rate of polymerization is higher than that of the mono-epoxide but is
lower than that of the (Ii-epoxide monomer. One possible explanation lies in the time
when vitrification sets in. Because vitrification occurs much faster than that in the case
with tri-epoxide, the mobility of the active centers is severely handicapped. As a result,
the rate of polymerizztion is reduced. Figure 5.58 shows that the ultimate limiting
conversion is more affected by increasing reaction temperature than that observed in the
di- and mono-epoxide monomers. The difference can be explained by the same principle,
conversion depends strongly on the glass transition temperature of the polymer, which is
primarily dependent on the structure of the monomer. Higher temperature translates into
higher kinetic energy of the active polymer chains, and may result in higher rate of
effective collisions or reaction has just occurred. The same trends are observed in the

case with the photoinitiator tolycumyl iodonium tetrakis pentafluorphenyl borate.

118

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120

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126

Table 5.17. Photosensitizer effects on the polymerization rate, RP, for a reaction system
containing 8.0 x 10'3 molal of photoinitiator tolycumyl iodonium tetrakis

(pentafluorophenyl) borate, B-[F5]4' and 8.0 x 10'4 molal of photosensitizer and a tri-
epoxide monomer at various temperatures.

 

 

 

 

 

 

 

 

 

 

Trimethyol propane triglycidyl ether
Photosensitizer Temp. (°C)
Peak time (min) Peak height (mole/L*sec)
30°C 7.24 :1: 1.15 - 0.0037 i 0.0004
No sensitizer 40°C 5.02 :1: 0.11 0.0072 1 0.0005
50°C 3.28 :t 0.10 0.0123 :1: 0.0006
30°C 0.74 :I: 0.06 0.0124 :1: 0.0003
CPTX 40°C 0.68 :1: 0.01 0.0251 :1: 0.0013
50°C 0.54 i 0.01 0.0605 1: 0.0012
30°C 3.64 :I: 0.15 0.0055 :1: 0.0001
Anthracene 40°C 2.34 :1: 0.06 0.0120 1 0.0001
50°C 1.48 :t 0.06 0.0259 :1: 0.0003

 

 

 

127

Table 5.18. Photosensitizer effects on the polymerization rate, RP, for a reaction system
containing 8 .0 x 10'3 molal of photoinitiator diaryliodonium hexafluoroantimonate,

[Sde and 8.0 x 104 molal of photosensitizer and a tri-epoxide monomer at various
temperatures.

 

 

 

 

 

 

 

 

 

 

Trimethyol propane triglycidyl ether
Photosensitizer Temp. (° C)
Peak time (min) Peak height (mole/L*sec)
30°C 7.38 i 0.24 0.0035 i 0.0003
No sensitizer 40°C 4.96 i 0.17 0.0065 :1: 0.0003
50°C 3.48 :1: 0.11 0.0116 :t 0.0002
30°C 1.41 :1: 0.06 0.0084 1: 0.0004
CPTX 40°C 1.32 :1: 0.03 0.0171 :1: 0.0004
50°C 0.93 i 0.01 0.0363 :1: 0.0007
30°C 4.74 d: 0.34 0.0050 i 0.0001
Anthracene 40°C 3.17 :1: 0.07 0.0107 i 0.0001
50°C 2.97 :1: 0.05 0.0196 :1: 0.0002

 

 

 

128

Table 5.19. Photosensitizer effects on conversion for a reaction system containing 8.0 x
10'3 molal of photoinitiator tolycumyl iodonium tetrakis pentafluorophenyl borate,

B-[Fsh' and 8.0 x 104 molal of photosensitizer and a tri-epoxide monomer at various
temperatures.

 

 

 

 

 

 

 

 

 

 

 

 

Trimethyol propane triglycidyl ether
Photosensitizer Temp. (°C)
% conversion

30°C 47.9 :1: 1.0

No sensitizer 40 C 56-4 :1: 2.2
50°C 59.6 11.2

30°C 43.5 :1: 1.2

CPTX 4° C 58.6 :1: 0.8
50°C 60.2 a; 0.3

30°C 50.7 i 0.3

Anthracene 40 C 59.1 i .02
50°C 59.2 :1: 0.6

Table 5.20. Photosensitizer effects on conversion for a reaction system containing 8.0 x

10'3 molal of photoinitiator diaryliodonium hexafluoroantimonate, [SbFJ and 8.0 x 10"
molal of photosensitizer and a tri-epoxide monomer at various temperatures.

 

 

 

 

 

 

 

 

Trimethyol propane triglycidyl ether
Photosensitizer Temp. (°C)
% conversion
30°C 50.1 :1: 0.7
0

No sensitizer 40 C 55-2 i 1.6
50°C 59.3 :1: 0.5

30°C 52.1 i 0.8

CPTX 40°C 55.9 i 0.6
50°C 60.7 :1: 0.8

30°C 50.1 :1: 0.9

Anthracene 40°C 54.0 1: 0.7
50°C 61.2 i 0.9

 

 

 

 

129

5.1.4.6. Effect of Photosensitizer Concentration on Photopolymerization Kinetics
Recall that the addition of photosensitizer to the reaction mixtures increased the
spectral sensitivity of the photoinitiator, and the photosensitizer CPTX was shown in
Section 5.1.3.5 to be more efficient than anthracene. In this section, the efficiency of the
photosensitizer CPTX was further studied by varying its concentration in a series of

reaction systems containing 8.0 x 10'3 molal photoinitiator diaryliodonium

hexafluoroantimonate, [Sde' and mono-epoxide butyl glycidyl ether. Figures 5.65-5.66
show the significant effects of photosensitizer on the overall reaction rate, RP, and the
total conversion. The figures illustrate that the most dramatic enhancement in the rate of
photopolymerization is observed as the photosensitizer concentration is increased from 0
to 0.8 millimolal. As the concentration is increased another order of magnitude to 8.0
millimolal, the reaction rate continues to increase, but in a much less dramatic manner.
These trends are illustrated by the values for the peak polymerization rate. As the
photosensitizer concentration is increased from 0 to 0.8 mmolal, the peak polymerization
rate more than doubles (from 0.0249 mollL sec to 0.0527 mollL sec); however, the
further increase in initiator concentration to 8 mmolal (a ten fold increase in
concentration) the peak rate increases only another 22% to a value of 0.0646 mollL sec.
Increases in the photosensitizser concentration had an even smaller effect on the limiting
conversion. The final limiting conversion increases slightly from ~72% to ~75% as the
photosensitizer concentration was increased from 0.8 to 8 mmolal. This relatively
modest effect on the final limiting conversion is not surprising since the photosensitizer
increases the active center concentration, and the limiting conversion is not highly

dependent on the active center concentration. The limiting conversion is priman'ly

130

determined by the glass transition temperature of the polymer, and this is primarily
determined by the structure of the polymer chain (which is determined by the monomer
structure). This effect will be illustrated more clearly in section 5.1.4.7. The results on
the effect of the photosensitizer are presented in Table 5.21 below.

Table 5.21. Concentration effects of photosensitizer CPTX on reaction systems

containing 8.0 x 10'3 molal of photoinitiator diaryliodonium hexafluoroantimonate and
monomer mono-epoxy butyl glycidyl ether.

 

 

 

 

 

 

 

 

 

Concentration Peak time (min) Peak height (mollL4sec) % conversion
No sensitizer 1.23 :t 0.08 0.0249 :1: 0.0005 71.3 i 4.0
8.0 x 10“ 0.41 1: 0.05 0.0527 :1: 0.0039 72.5 :1: 5.2
2.0 x 10'3 0.40 1.0.01 0.0530 i 0.0006 73.7 :1: 0.6
4.0 x 10'3 0.34 :1: 0.03 0.0650 :1: 0.0035 75.1 5: 1.0
6.0 x 10'3 0.36 i 0.01 0.0566 :1: 0.0004 73.8 i 1.2
8.0 x 10'3 0.33 :1: 0.02 0.0646 5: 0.0021 74.5 :1: 0.3

131

0.004

0.04 ‘

Hp (moles/L'sec)

0.02 -

 

0.00 -

Photosensitizer concentration, mole/kg

----- CONTROL 0 500C
—— 0.8 mmolal CPTX
- ------- 2.0 mmolal CPTX
----- 6.0 mmolal CPTX
----- 4.0 mmolal CPTX

I ‘. --------- 8.0 mmolal CPTX

 

 

 

 

 

Time (min)

Figure 5.65. Effects of photosensitizer concentration for reaction systems containing
mono-epoxy butyl glycidyl ether and 8.0 x 10'3 molal of photoinitiator diaryliodonium

hexafluoroantimonate, [Sng]- at 50°C.

504

Conversion (%)

 

 

  
 

Photosensitizer concentration, mole/kg

----- Control 0 5000

— 0.8 mmolal CPTX
----- 2.0 mmolal CPTX
------- 6.0 mmolal CPTX
--— 4.0 mmolal CPTX
------ 8.0 mmolal CPTX

I T l

0 5 10

Time (min)

Figure 5.66. Effects of photosensitizer concentrations on final conversion of the same
reaction systems in Figure 5.65.

132

5.1.4.7. Effect of Monomer’s Structure and Functionality on the Cationic Photo-
polymerization Kinetics

The purpose of these studies was to determine the effect of the monomer structure
and functionality on the cationic photopolymerization kinetics and on the ultimate
limiting conversion. For the purposes of this thesis, the functionality of a monomer is
defined as to the number of epoxide groups in a single monomer molecule. In this
research, the three mono-epoxide monomers were: butyl glycidyl ether, octyl glycidyl
ether, and phenyl glycidyl ether. The two di-epoxide monomers were: 1,4-butanediol
diglycidyl ether, cyclohexane dimethanol diglycidyl ether. Finally, the tri-epoxide
monomer was trimethyol propane triglycidyl ether.

First we will examine the effect of the monomer structure on the
photopolymerization kinetics and limiting conversion of the three mono-epoxide
monomers. These results are illustrated in Figure 5.67 and 5.68. Figure 5.67 illustrates
that the phenyl glycidyl ether exhibits the highest polymerization rate, following by butyl
glycidyl ether, and that the octyl glycidal ether exhibits the lowest polymerization rate.
This result can be explained by considering the size and geometry of the monomer
molecules. The highest polymerization rates will be exhibited by monomers that have a
high probability of colliding with the active cationic centers in an orientation that will
allow reaction. The octyl glycidyl ether possesses a relatively long pendant hydrocarbon
chain that can preclude collisions of the proper orientation, and therefore exhibits the
slowest polymerization rate. Similarly, the phenyl glycidyl ether has the most compact
molecular structure and exhibits the highest polymerization rate. It is interesting that the
octyl glycidyl ether exhibits a lower polymerization rate than the butyl glycidyl ether, but

achieves a higher final limiting conversion. This may be attributed to the fact that the

133

smaller pendant butyl group allows the polymer to form a glass more readily, and
therefore the system vitrifies and loses mobility at a lower conversion for the butyl
glycidyl ether than for the octyl glycidyl ether.

The functionality of the monomer had a significant effect on the reaction rate and
conversion as shown in Figures 5.69-5.70. Figure 5.69 illustrates that the di-epoxide
monomer 1,4-butanediol diglycidyl ether exhibits a higher polymerization rate than the
mono-epoxide monomer phenyl glycidyl ether (which was the most reactive mono-
epoxide monomer). This higher polymerization rate can be attributed to the relative high
local epoxide concentration in the vicinity of the active center. Figure 5.70 illustrates that
the limiting conversion is significantly lower for the di-epoxide monomer. This is due
the fact that the crosslinking reaction that occurs in the di-epoxide case restricts the
mobility of the polymer chains and facilitates vitrification. In addition, the crosslinking
reaction can produce pendant epoxide groups that are buried within the crosslinked
structure and are not accessible to the cationic active centers. Not surprisingly, the tri-
epoxide monomer exhibit the lowest limiting conversion of all, as illustrated by the
values in Table 5.22. In this case there is a high probability of producing pendant

epoxide groups that are not accessible to the cationic active centers.

134

fi.‘ alnrulw—

 

 

 

 

 

0.03 « ._
g 3‘ .......... Phenyl glycidyl Ether
5 0.02 a i Butyl glycidyl Ether
a .3 2‘
é "" - Octyl glycidyl Ether
Z
“5 0.01 -

0.00 ~ '1‘1‘; -..- .

1'0 1'5

 

Time (min)

Figure 5.67. Rates of polmerization for various mono-epoxide monomers photoinitiated

by 8.0 x 10'3 molal of tolycumyl iodonium tetrakis pentafluorophenyl borate, B-[Fs]; at
50°C.

100 '-

------------

50-

  
 

Conversion (%)

- ------- Phenyl glycidyl ether
Octyl glycidyl ether
- — - - Butyl glycidyl ether

 

 

l a l

o 5 10
Time (min)

Figure 5.68. Conversion profiles of various mono-epoxide monomers photoinitiated by

8.0 x 10'3 molal of tolycumyl iodonium tetrakis pentafluorophenyl borate, B-[Fs]; at
50°C.

135

0.035 -

   

 

 

 

 

0.000 -
J Monomer type

A 0.025 ' - - - - Phenyl glycidyl ether
§ ,’ —— 1,4-butanediol diglycidyl ether
1% 0.020 - .1 - ------- Trimethyol propane triglycidyl ether
_ 1 i
g 0.015 - '1
E ' ,.
1: 0.010 - ,-"

0.005 3 j. ‘

i0 ' s Q . ‘ .....
0,000. i ....................... _
T F I
0 10 20 30

 

Time (min)

Figure 5.69. Effect of monomer’s functionality on reaction rates for reaction systems
with 8.0 x10'3 molal of photoinitiator tolycumyl iodonium tetrakis pentafluorophenyl

borate, B"[F5]4' at temperature of 50°C.

 

  
 

 

 

100-
l I ' '
I
00- ’I _
J
E 60- ......................... _
s .. -----
E
‘>’ 40.
g Monomer type
‘ .- - - - - Phenyl glycidyl ether
20- f —— 1,4-butanediol diglycidyl ether
.." - ------- Trimethyol propane triglycidyl ether
0' l . ' I ' l r 1
0 10 20 30
Time (min)

Figure 5.70. Effect of monomer’s functionality on conversion for reaction systems with
8.0 x 10'3 molal photoinitiator tolycumyl iodonium tetrakis pentafluorophenyl borate,

B-[F5]4' at temperature of 50°C.

136

Table 5.22. Effect of monomer structure and functionality on the conversion of reaction
mixtures comprising of 8.0 x 10'3 molal photoinitiator tolycumyl iodonium tetrakis

pentafluorophenyl borate, B‘[F5]4' and different epoxide monomer at 50°C.

 

 

 

 

 

 

Light %
Monomer functionality Monomer structure intensity Conversion
(mW/cmz)
butyl glycidyl ether ~ 52 79.2 :1: 0.7
Mono-epoxide octyl glycidyl ether ~ 52 82.0 :1: 0.8
phenyl glycidyl ether ~ 52 93.9 :1: 0.8
1,4-butanediol ~ 52 76.2 1: 1.1
diglycidyl ether
Di-epoxide
cyclohexane dimethanol ~ 52 76.8 :1: 0.3
diglycidyl ether
Tri-epoxide trimethyol propane ~ 36 59.6 i 1.2
triglycidyl ether

 

 

 

137

5.1.5. Conclusions

PDSC experiments were performed to study the reaction kinetics of cationic
photopolymerization of epoxides. This method was indeed useful in studying various
parameters that can affect the rate of polymerization, RP, and final conversion. The
chemical compounds used in this research effort were seven different epoxide monomers,
three photoinitiators, and two photosensitizers. This gave a total of 40 different possible
combinations of individual reaction systems.» After the data analysis was done, there was
undoubtedly some variability among the reaction systems. However, there were four
general trends that had been established. First of all, the reaction rate increased with
increasing temperature (as supported by the behavior of the propagation rate constant kp).

The conversion also increased by a small amount going from 30, 40 to 50°C. Second, the

photoinitiator tolycumyl iodonium tetrakis pentafluorophenyl borate, B-[Fs]; was proven

to be as effective (if not better in some cases) as the photoinitiator diaryliodonium
hexafluoroantimonate, [SbF6]'. The photoinitiator triarylsulfonium hexafluorophosphate,

[PFG]- was the least effective one of the three photoinitiators. The concentration of the
photoinitiator also influenced the reaction rate, RP and final conversion; however, this
effect became less significant above 8.0 x 10’3 molal. Third, with respect to
photosensitization to increase spectral sensitivity of the photoinitiators to the initiating
light source, the photosensitizer CPTX (l-chloro-4-propoxy-9H-thioxanthen-9-one) was
more efficient than anthracene was. This was mainly due to the ability of CPTX to
absorb more strongly in the region of 320-390 nm. The study of concentration effect of
photosensitizer indicated that concentration above the molality of 8.0 x 10‘4 still

enhanced the reaction rate RP by a small amount, but did not improve the final conversion

138

by much. Lastly, the structure and functionality of the monomer also had an effect. For
reactivity: (ii-epoxide > tri-epoxide > mono-epoxide; and for conversion: mono-epoxide

> (ii-epoxide > tri-epoxide.

139

5.2. Conversion by Fourier Transform Infrared Spectroscopy (FTIR)

5.2.1. Introduction

The method of Fourier Transform Infrared Spectroscopy (FTIR) has also been

applied by many researchers”15

to study the reaction kinetics of cationic
photopolymerization. The basic operating principle for FI‘IR is that when infrared red
light is passed through a sample, certain frequencies are absorbed while others are
transmitted, resulting in an absorption (or transmittance) spectrum that is dependent upon
the molecular vibrational frequencies of that sample. For the epoxide compounds used in
this research effort, the absorption band corresponding to the C—H bond should lie in the
region of 740 — 920 cm'l, depending on the molecular structure of the particular epoxide

molecule. By monitoring the change of this absorption band, the degree of conversion

can be obtained.

5.2.2. Experimental

The FI‘IR experiments were conducted using the Nicolet IR/42 spectrometer by
N icolet Instruments, Inc.. The formulation was applied onto the polyethylene membrane
of an IR card (# 61 by 3M). The absorption spectrum of the sample was taken before the
UV irradiation. The light source was a 12-watt UV lamp that can produce a light beam at
wavelength of 365 nm. The sample was irradiated for a specified amount of time to
allow the polymerization reaction to proceed at room temperature and in the open. Next,

the sample was scanned in the FTIR spectrometer to obtain the absorption spectrum, from

140

which the change of the absorption band in the region of 740-920 cm'1 would be used to
determine the degree of conversion as a result of the UV irradiation. Because of the

experimental set-up, this was not an in situ process.

5.2.3. Results and Discussion

Each epoxide monomer has a characteristic absorption band corresponding to the
C—H bond. As shown in Figure 5.71, the absorption peak is at 914 cm’1 for mono-epoxy
octyl glycidyl ether. ‘6 The conversion was calculated by the following relationship:

Ct-Cto.

Conversion =

 

100 (5.8)

where,
Cm is the absorption peak before UV irradiation
C, is the absorption peak for a time t of UV irradiation
Figure 5.72 shows the FTIR spectra for the cationic photopolymerization of di-
epoxide monomer 1,4—butanediol diglycidyl ether by 8.0 x 10‘3 molal of Tolycumyl
iodonium tetrakis pentafluorophenyl borate, B-[F5]4' and 8.0 x 10“ molal of
photosensitizer CPTX. The characteristic absorption peak“5 occurred at 912 cm".
Additional FI‘IR experiments were done by the same procedure for other epoxide
monomers with different photoinitiators and photosenstizers. The results in terms of

percent conversion were tabulated and reported in Table 5.23.

141

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144

5.2.4. Conclusions

The method of Transform Fourier Infrared (FTIR) spectroscopy was used in this
research effort primarily for confirming the conversion values obtained by the PDSC
experiments. As described in the experimental set-up, essentially two FI'IR absorption
spectra were used to calculate the percent conversion of a cationic photopolymerization.
The results in Table 5.23 show a great deal of similarity in terms of conversion and
general trends with respect to monomer structure and functionality as those obtained by

PDSC experiments.

145

 

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5.3. References

J. G. Kloosterboer, Adv. Polym. Sci., 84, l (1988).

K. S. Anseth, C. M. Wang and C. N. Bowman, Macromolecules, 27, 650 (1994).
T. Doomkamp and Y. Y. Tang, Polym Commun., 31, 362 (1990).

K. S. Anseth, C. M. Wang and C. N. Bowman, Polymer, 35, 3243 (1994).

C. Dekker and K. J. Moussa, Coatings Tech., 62, 55 (1990).

J. V. Crivello, J. L. Lee and D. Conlon, J. Rad. Curing, 10, 6 (1983).

E. W. Nelson, T. P. Carter, and A. B. Scranton, J. of Poly. Sci., Poly. Chem Ed., 33,
247 (1995).

E. W. Nelson, T. P. Carter, and A. B. Scranton, Macromolecules, 33, 247 (1995).
C. Priou, A. Soldat, J. Cavezzan, F. Castellanos and J. P. Fouassier, J. of Coatings
Tech., 67, 851:71, (1995).

J. M. Barton, Adv. Polym. Sci., 72, 111 (1985).

J. V. Crivello and S. S. Liu, J. Polym. Sci., Part A: Polym. Chem, 37, 1199 (1999).
Y. Toba, M. Saito, and Y. Usui, Macromolecules, 32, 3209 (1999).

J. V. Crivello, S. Liu, Chem. Mater., 10, 3724 (1998).

S. K. Rajaraman, W. A. Mowers, and J. V. Crivello, Macromolecules, 32, 36
(1999).

R. Narayan and J. V. Crivello, Macromolecules, 29, 439 (1996).

W. A. Patterson, Anal. Chem, 26, 823 (1954).

J. P. Kennedy, and E. Marechal, Carbocationic Polymerization, John Wiley & Son,
$3.133; L. R. Gatechair, and J. H. Jilek, J. Polym Sci. .- Polym. Chem. Ed., 22,
77 (1984).

E. W. Nelson, T. P. Carter and A. B. Scranton, J. Polym. Sci.: Polym. Chem. Ed.,
33, 247 (1995).

146

Chapter 6

Conclusions and Recommendations

This research effort has contributed to the fundamental understanding of the
reaction kinetics of cationic photopolymerization of epoxide monomers. The results from
the PDSC experiments of these UV-initiated cationic photopolymerizations presented in
this thesis will complement the more macroscopic studies currently in literature and are
imperative if the potential of these reactions is to be realized. In addition, the
experimental techniques used in this research can be readily applicable in high-speed
polymerization reactions. This chapter also provides some conclusions and

recommendations for future work.

6.1. Summary of Results

The technique of Photo-differential scanning calorimetry (PDSC) has proven to
be quite a useful tool to systematically characterize the reaction kinetics of cationic
photopolymerization for a series of epoxide monomers with three distinct photoinitiators,
and two different photosensitizers over a temperature range from 30°C to 40°C and
finally to 50°C. Table 6.1 provides a summary of all the constituents that make up
different combinations of reaction formulations. Basically, there are 42 total distinct
reaction mixtures, each of which contains an epoxide monomer and a photoinitiator and
may contain a photosensitizer. Several parameters that can affect the rate of

polymerization and the ultimate limiting monomer conversion have been studied. The

147

parameters are initiating light intensity, type and concentration of photoinitiator, type and
concentration of photosensitizer, and temperature. The kinetics results may be correlated
with monomer structure and functionality and reaction conditions to provide insight into
the underlying fundamental reaction processes.

Table 6.1. Summary of distinct reaction formulations in the kinetic studies of cationic
photopolymerizations of epoxide monomers.

 

 

 

 

Photoinitiator Photosensitizer Epoxide monomer
Butyl glydidyl ether
Tolycumyl iodonium l-chloro-4~propoxy - Octyl glycrdyl ether
tetrakis 9H-thioxanthen-9- Phenyl glycidyl ether
gntafluorophenyl) one (CPTX) 1,4-butanediol diglycidyl ether
ra e

 

Cyclohexane dimethanol diglycidyl ether

 

Trimethyol propane triglycidyl ether

 

 

Note: Two other photoinitiators are: diaryliodonium hexafluoroantimonate and
triarylsulfonium hexafluorophosphate, and another photosensitizer anthracene.

The two significant kinetics aspects of cationic photopolymerizations are the rate
of polymerization, RP, and the final monomer conversion, both of which can be affected

by the parameters with the results presented in the following sections.

6.1.1. Effect of the Incident Light Intensity on Polymerization Kinetics

Our studies show that the incident (initiating) light intensity can affect
tremendously the rate of polymerization. For example, when light intensity is raised
from 35 mW/cm2 to 90 mW/cmz, the reaction rate for mono-epoxy octyl glycidyl ether
with 8.0 x 10'3 molal diaryliodonium hexafluoroantimonate at 50°Ca reaction, the

reaction rate more than doubles and the final conversion increases from 81% to 83%

148

(Table 5.1). This general trend with huge increase in rate of polymerization and relatively
smaller increase in final conversion was also found in the case with di—epoxy 1,4-
butanediol diglycidyl ether under the same initiator concentration and reaction

temperature.

6.1.2. Effect of Photoinitiator and Photoinitiator Concentration on Cationic
Photopolymerization Kinetics
The effect on the rate of polymerization is for the most part comparable for both
photoinitiators diaryliodonium hexafluoroantimonate and tolycumyl iodonium tetrakis
(pentafluorophenyl) borate, although there is some variability when it comes to
monomers with a different number of epoxide rings in the monomer molecule. With the
antimonate salt, the effect is a little more pronounced with the mono-epoxide monomers
than with the di- and tri-epoxide monomers probably because of the crosslinking which '
severely limits the mobility of the active polymer chains from propagating with other
epoxide groups. For a given photoiniatator at the same reaction temperature and
initiating light intensity, the rate of polymerization is almost the same for tri and di-
epoxide monomers and slowest for the mono-epxide monomer. The final conversion is in
the reverse trend: mono-di-epoxide > di-epoxide > tri—epoxide (~ 85 %, ~ 76 % and ~ 60
% respectively). The effect of photoinitiator concentration was studied to show that the
concentration of 8.0 x 10‘3 molal would make the most sensible choice because the
reaction rate was high and the final conversion reached the limiting point in most cases.
The further increases in initiator concentration only result in small increase of the rate of

polymerization and improve very little in conversion.

149

6.1.3. Effect of Photosensitizer and Photosensitizer Concentration on
Polymerization Kinetics

The photosensitizer is added to the reaction mixture in order to increase the
spectral sensitivity of the photoinitiator to the initiating light, which has strong emission
peaks in the 300 - 380 nm region (near UV). The photosensitizer causes a remarkable
increase in the rate of polymerization by increasing the active centers at the initial phase
of the reaction. As a result, the final conversion also increases. Overall, CPT X was
determined to be a better photosensitizer than anthracene because CPTX absorbs more
strongly of the initiating light (Figure 4.11). The efficiency of the photosensitizer CPTX
was studied by changing its concentration in a series of reaction mixtures containing
butyl glycidyl ether and 8.0 x 10‘3 molal of diaryliodonium hexafluoroantimonate. Our
studies indicate that at sensitizer concentration of 8.0 x 1-4 molal, the reaction rate more
than doubles, but the conversion only increases from ~ 71 % to ~ 73%. When raised to
8.0 x 10'3 molal (a tenfold increase), the reaction rate only increased by about 22% and

the conversion changed from ~73 % to ~75 %.

6.1.4. Effect of the Monomer Structure and Functionality on Polymerization
Kinetics
Monomer molecular structure (as illustrated in Table 4.1) was determined to
affect the reaction kinetics of cationic photopolymerization. Among the three mono-
epoxide monomers in this research, the monomer phenyl glycidyl ether is most reactive
and consequently has the highest final conversion; the monomer butyl glycidyl ether was

a little more reactive than octyl glycidyl ether, but has lower conversion. This

150

observation may be attributed to butyl glycidyl ether’s low glass transition temperature.
Between the two di-epoxide monomers, 1,4-butanediol diglycidyl ether was observed to
generally have a slightly higher reaction rate and conversion as compared to those of the
cyclohexane dimethanol diglycidyl ether. With respect to functionality (number of
epoxide rings in a monomer molecule), monomer reactivity is ranked as follows: di-
epoxides > tri-epoxides > mono-epoxides; and for final conversion: mono-epoxides > di-

epoxides > tri-epoxides.

6.1.5. Effect of the Temperature on Polymerization Kinetics

As expected, reaction temperature was observed to affect the both the rate of
polymerization and final conversion. The effect of temperature by itself in general is
smaller than with the addition of a photosensitizer and a given photoinitiator. For
example, for a reaction mixture containing butyl glycidyl ether with 8.0 x 10’3 molal
photoinitiator tolycumyl iodonium tetrakis (pentafluorophenyl) borate, the reaction rate
increases from 0.0044 mol/L*sec at 30°C to 0.0316 mol/L*sec at 50°C for the control
sample , whereas the rate increases from 0.0138 mol/L*sec at 30°C to 0.0551 mol/L*sec
at 50°C with the photosensitizer CPTX at 8.0 x 10'4 molal (Table 5.9).

In general, as the reaction temperature increases, the rate of polymerization also
increases rather significantly, however the final conversion does not necessarily increase

with the same magnitude due to the difference in monomer structure and functionality.

151

‘ -4“,-

W. .,_.‘_

6.2. Recommendations for Future Works

The focus of this research effort is on the final outcomes with respect to the rate
of polymerization and final limiting conversion. Further investigation can be
implemented in the following areas:

0 The mechanism on the rate of the active centers generation.

0 Determination of the propagation rate constant and elucidate its behavior

throughout the course of the reaction.

0 Determination of the termination rate constant.

152

    
  

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