DEVELOPMENT OF NOVEL CYCLOALIPHATIC SILOXANES FOR THERMAL
AND UV-CURABLE APPLICATIONS
A Dissertation
Presented to
The Graduate Faculty of the University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Doctor of Philosophy
Ruby Chakraborty
May, 2008 DEVELOPMENT OF NOVEL CYCLOALIPHATIC SILOXANES FOR THERMAL
AND UV-CURABLE APPLICATIONS
Ruby Chakraborty
Dissertation
Approved: Accepted:
______Advisor Department Chair Dr. Mark D. Soucek Dr. Sadhan C. Jana
______Committee Member Dean of the College Dr. Sadhan C. Jana Dr. Stephen Cheng
______Committee Member Dean of the Graduate School Dr. Erol Sancaktar Dr. George R. Newkome
______Committee Member Date Dr. George G. Chase
______Committee Member Dr. Chrys Wesdemiotis
ii ABSTRACT
Siloxanes have been extensively used as additives to modulate surface properties
such as surface tension, hydrophobicity/hydrophobicity, and adhesion, etc. Although,
polydimethyl -siloxane and polydiphenylsiloxane are the most commonly used siloxanes,
the properties are at extremes in terms of glass transition temperature and flexibility. It is
proposed that the ability to control the properties in between the these extremes can be provided by cycloaliphatic substitutions at the siloxane backbone. It is expected that this substitution might work due to the intermediate backbone rigidity.
In order to achieve the above objectives, a synthetic route was developed to
prepare cycloaliphatic (cyclopentane and cyclohexane) silane monomers followed by subsequent polymerization and functionalizations to obtain glycidyl epoxy, aliphatic amine and methacrylate telechelic siloxanes. The siloxanes were either thermally or UV- cured depending on end functionalizations. Chemical characterization of monomers, oligomers and polymers were performed using 1H, 13C, 29Si-NMR, FT-IR and GPC. The
curing kinetics of photo-induced reactions were investigated through photo-differential
scanning calorimetry (PDSC). The oxygen permeability, mechanical, coatings, and
release properties of siloxanes were studied as a function of the backbone substitutions.
The mechanical, coatings and released properties of cycloaliphatic siloxanes improved with respect to polydimethylsiloxanes. The thermal analysis of the cured films were carried out using differential scanning calorimetry (DSC). Viscoelastic properties of the
iii cured siloxanes due to the variation of substitution at the siloxane backbone were measured using dynamic mechanical thermal analysis (DMTA). The cycloaliphatic substituted siloxanes showed an increased glass transition temperature and permeability but reduced crosslink density, conversion, and rate of curing with respect to polydimethylsiloxanes.
Hybrids of siloxanes were prepared with linseed oil based alkyds to study the effect of variation of alkyd oil lengths and cycloaliphatic substitutions on siloxane backbone. The oil length of an alkyd resin is defined as the number of grams of oil used to produce 100 grams of resin. Three linseed oil based alkyds representing long, medium, and short oil lengths were grafted with siloxanes substituted with methyl, cyclopentyl, and cyclohexyl groups. The reaction was monitored through FTIR and 1H-NMR. The hybrids were formulated with standard drier package and thermally cured for detailed film characterization. Improvement in crosslink density, flexibility, and reverse impact resistance were found as function of oil length. However, tensile modulus, elongation, glass transition temperature, drying time and fracture toughness decreased with increase in oil length. For hybrids, the cycloaliphatic substituents at the siloxane backbone showed enhanced mechanical and coating properties as compared to hybrids with polydimethylsiloxanes.
Random and block copolymer of polydimethylsiloxanes with polydicycloaliphatic- siloxanes were synthesized and compared with homopolymers of polydicycloaliphatic siloxanes. The chemical characterization of the copolymers and homopolymers were carried out through 1H, 13C, 29Si-NMR, and FT-IR. The glass transition temperatures (Tg) of the synthesized polymers were obtained through DSC and
iv advanced rheometric expansion system (ARES). The Tg of random copolymers were found to be higher than the corresponding block copolymers. There was very small difference in Tg between cycloaliphaticsiloxanes homopolymers and corresponding random copolymers. From the above results, it can be inferred that the cycloaliphatic substitutions at the siloxane backbone can be used as a means to obtain properties intermediate to polydimethyl- and polydiphenyl siloxanes.
v
DEDICATION
To my parents for their exceptional love and support
vi ACKNOWLEDGEMEMNTS
At this juncture of submitting my thesis, I would like to express my sincere
appreciation to people who have made this possible.
First of all, I would like to express my sincere gratitude towards my advisor Dr.
Mark Soucek, for his exceptional guidance and support through out my stay at Polymer
Engineering. My stay in this group has imparted in me the much needed multi tasking
abilities and networking skills. Sincere thanks are due to my dissertation committee
members who have taken time out of their hectic schedule to help me with this research work. I would like to express my special thanks to Dr. Jana, if it were not for him, I would not have landed in Akron.
I would like to thank Dr. Venkat Dudipala for his help in setting up NMR
experiments and Sham for helping with rheological measurement. Sincere thanks belong to Dr. Swain and Dr. Thatte for their valuable suggestions during this work. I would like
to thank my group mates and friends Dr. Dworak, Dr. Nebioglu, Dr. Uma, Mayela, Elif,
Serkan, Kosin, Jamie, Dr. Gua, Neel and Veronica for providing a healthy and happy
environment to do research and have fun.
Finally I would like to thank all faculty and staff of Polymer Engineering for
directly or indirectly helping me in achieving this goal.
vii TABLE OF CONTENTS
Page
LIST OF TABLES………………………………………………………………………xiii
LIST OF FIGURES ……………………………………………………………………...xv
CHAPTER
I. INTRODUCTION ……………………………………………………………………1
II. BACKGROUND………………………………………………………………………5
2.1 An overview on polysiloxanes …………………………………………………..5
2.2 Nomenclature of polysiloxanes …………………………………………………7
2.3 Properties………………………………………………………………………...8
2.4 Monomer synthesis …………………………………………………………….12
2.4.1 Hydrosilation…………………………………………………………...14
2.5 Synthesis of polyorganosiloxanes ……………………………………………...17
2.5.1 Mechanism for polysiloxane synthesis ………………………………...17
2.5.1.1 Equilibrium polymerization ………………………………….19
2.5.1.2 Anionic polymerization………………………………………21
2.5.1.3 Cationic polymerization ……………………………………...24
2.5.2 Structure of synthesized polysiloxanes ………………………………...27
viii 2.5.2.1 Linear polysiloxanes …………………………………………27
2.5.2.2 Block and graft polysiloxanes ………………………………29
2.5.2.3 Side group modified polysiloxanes …………………………33
2.5.2.4 Polyorganosilesquioxanes ……………………………………34
2.6 Backbone substitution and functionalization of polysiloxanes ………………...36
2.7 Hybrids of siloxanes with other polymers…...…………………………………39
2.8 Curing of siloxanes …………………………………………………………….41
2.8.1 Radiation curing ………………………………………………………42
2.8.1.1 UV initiated cross-linking ……………………………………42
2.8.1.1.1 Free radical initiated UV-curing …………………44
2.8.1.1.2 Cationic UV-curing ……………………………45
2.8.1.2 Electron beam (EB) curing…………………………………...47
2.8.2 Thermally induced cross-linking……………………………………….47
2.8.3 Moisture induced cross-linking………………………………………...50
2.8.4 Addition Curing………………………………………………………...51
III. EXPERIMENTAL …………………………………………………………………...53
3.1 Materials ………………………………………………………………………53
3.2 Synthesis ………………………………………………………………………54
3.2.1 Synthesis of dicycloaliphatic dichlorosilane …………………………...55
3.2.2 Synthesis of cyclic oligomer of polydicycloaliphaticsiloxane …………55
3.2.3 Activation of ion exchange resin……………………………………….56
3.2.4 Synthesis of hydride terminated polydimethylsiloxane. ……………….57
3.2.5 Synthesis of hydride terminated polydicycloaliphaticsiloxane ………..57
ix 3.2.6 Synthesis of glycidyl epoxide terminated PDMS, PDPS, and PDHS ………………………………………………………58
3.2.7 Epoxy equivalent weight determination (EEW) ……………………...59
3.2.8 Synthesis of t-butoxycarbonyl (BOC) protected allyamine ……………60
3.2.9 Synthesis of amine terminated PDMS, PDPS, and PDHS……………..60
3.2.10 Synthesis of methacrylate terminated PDMS, PDPS, and PDHS ……...62
3.2.11 Synthesis of linseed oil based short oil, medium oil and long oil alkyds…………………………………………………………………...63
3.2.12 Synthesis of Block Copolymer of PDMS with PDPS PDMS-block-PDPS) and PDHS(PDMS-block-PDHS) ………………64
3.2.13 Synthesis of homopolymers of PDPS and PDHS………………………65
3.2.14 Synthesis of random copolymer of PDMS with PDPS (PDMS-ran-PDPS) and PDHS (PDMS-ran-PDHS) …………………..66
3.3 Characterization………………………………………………………………...67
3.3.1 Proton, Carbon and Silicon Nuclear Magnetic Resonance (1H NMR, 13C-NMR, and 29Si NMR) ………………………………….67
3.3.2 Fourier Transform Infrared Spectroscopy (FT-IR) …………………….67
3.3.3 Gel Permeation Chromatography (GPC) ………………………………67
3.3.4 Differential Scanning Calorimetry (DSC)……………………………...68
3.3.5 Photo-Differential Scanning Calorimetry (PDSC)……………………..68
3.3.6 Dynamic Mechanical Thermal Analysis (DMTA) …………………….68
3.3.7 Oxygen permeation analysis …………………………………………..69
3.3.8 X-Ray diffraction ………………………………………………………70
3.3.9 Fracture toughness ……………………………………………………..70
3.3.10 Tensile testing …………………………………………………………72
x 3.3.11 Contact angle…………………………………………………………...73
3.3.12 Release Measurements …………………………………………………73
3.3.13 Drying Time ……………………………………………………………74
3.4 Film preparation ……………………………………………………………….74
3.4.1 Film preparation from glycidyl epoxy and amine terminated siloxanes………………………………………………………74
3.4.2 Film preparation from methacrylate terminated siloxanes …………………75
3.4.3 Film preparation from hydride functional siloxane and alkyds ……………76
3.5 Film Characterization …………………………………………………………….77
IV. SYNTHESIS OF AMINE AND EPOXIDE TELECHELIC SILOXANES …………80
4.1 Introduction …………………………………………………………………….80
4.2 Result and Discussion ………………………………………………………….82
4.3 Conclusion……………………………………………………………………...94
V. MECHANICAL AND FILM PROPERTIES OF THERMAL CURABLE POLYSILOXANES ……………………………………………………96
5.1 Introduction …………………………………………………………………….96
5.2 Result and Discussion …………………………………………………………97
5.3 Conclusion…………………………………………………………………….109
VI. SYNTHESIS OF TELECHELIC METHACRYLIC SILOXANES WITH CYCLOALIPHATIC SUBSTITUENTS FOR UV-CURABLE APPLICATIONS …………………………………………………………………..110
6.1 Introduction ...…………………………………………………………………110
6.2 Result and Discussion ...………………………………………………………111
6.3 Conclusion ……………………………………………………………………129
xi VII. NEW APPROACH TO GRAFT SILOXANES TO ALKYDS …………………..130
7.1 Introduction …………………………………………………………………130
7.2 Result and Discussion ………………………………………………………132
7.3 Conclusion ……………………………………………………………………146
VIII. SYNTHESIS AND CHARECTERIZATION OF HOMOPOLYMERS AND COPOLYMERS OF CYCLOALIPHATIC SILOXANES ………………...148
8.1 Introduction …………………………………………………………………148
8.2 Result and Discussion ………………………………………………………150
8.3 Conclusion ……………………………………………………………………162
IX. CONCLUSIONS …………………………………………………………… 163
REFERENCES………………………………………………………………………….166
xii LIST OF TABLES
Table Page
3-1 Reagent amounts for methacrylated siloxane synthesis ………………………….62
3-2 FTIR assignments of methacrylic functional PDMS, PDPS and PDHS…………63
3-3 1H & 13C-NMR Resonance- assignments of methacrylic functional PDMS, PDPS and PDHS ………………………………………………………63
3-4 Reagent amount for long, medium and short oil alkyd synthesis………………...64
3-5 Formulation for thermal cured film formation ………………………………….75
3-6 Formulation for UV-cured film formation ……………………………………….75
3-7 Formulation for film formation of long oil (LO), medium oil (MO) and short oil (SO) alkyds with PDMS, PDPS and PDHS ………………………..76
5-1 Heat of curing for polysiloxanes and reactive diluents …………………………101
5-2 The mechanical properties of UV-cured siloxanes ……………………………104
5-3 The mechanical properties of thermally cured siloxanes ……………………….105
5-4 Coating properties of thermally cured PDMS, PDPS and PDHS ………………106
6-1 Summary of PDSC results for methacrylic functional PDMS, PDPS and PDHS……………………………………………………………………….119
6-2 Storage modulus, crosslink density and glass transition temperature of methacrylic functionalized PDMS, PDPS and PDHS ………………………120
6-3 Oxygen permeability, contact angle and coating properties of methacrylic functionalized PDMS, PDPS and PDHS………………………….126
6-4 Release properties of UV-cured methacrylated siloxanes………………………127
xiii 7-1 The drytime measurement data for hybrids of long oil (LO), medium oil (MO) and short oil (SO) alkyds with PDMS, PDPS and PDHS ………………..137
7-2 The crosslink density of silicone alkyd hybrids ………………………………..143
7-3 Coating properties of alkyd siloxane hybrid …………………………………...145
8-1 Comparison of Tg values obtained experimentally and predicted by fox equation …………………………………………………………………….161
xiv LIST OF FIGURES
Figure Page
2-1 Shorthand notation for common siloxanes………………………………………...7
2-2 Schematic representation of siloxane backbone …………………………………8
2-3 Chalk-Harrod mechanism for hydrosilation using karstedt catalyst ……………..15
2-4 Cyclization in dilute, non-aqueous and non-polar solution………………………19
2-5 Mechanism for anionic polymerization of cyclic siloxanes……………………...23
2-6 Cleavage of cyclic siloxane oligomers facilitated by H-bonding (a) in absence of water (b) in presence of water …………………………………26
2-7 Sol-gel condensation catalyzed by DBTDL ……………………………………..29
2-8 General route to synthesize organo bifunctional siloxanes ………………………30
2-9 (a) Hydrosulfidation and (b) Hydrophosphination reactions with vinyl functional siloxanes……………………………………………………………...34
2-10 General synthesis of silesquioxanes ……………………………………………35
2-11 Deactivation pathways of the excited photoinitiator …………………………….43
2-12 Free radical generation from unimolecular photoinitiators………………………44
2-13 Free radical generation from bimolecular photoinitiators………………………..45
2-14 The reaction route for cationic UV curing ………………………………………45
2-15 Usage of onium salts for free radical photoinitiation …………………………….46
2-16 Excitation of resins by e-beam …………………………………………………47
xv 2-17 Peroxide initiated vinyl polymerization ………………………………………….48
2-18 Mechanism for epoxy amine curing……………………………………………...49
2-19 A general moisture curing mechanism of silicones ……………………………50
2-20 Complexation of hydrosilation catalyst…………………………………………..51
3-1 Structure of reactive diluents and photoinitiators used …………………………..54
3-2 Sample chamber cross-section ………………………………………………….70
3-3 (a) A thin film with single edge notch in tension (b) Stress distribution in the vicinity of a crack tip ……………………..….……70
4-1 Synthesis of glycidyl epoxy functional siloxane (a) Hydrosilation of cycloaliphatic alkene with Dichlorosilane gas (b) Hydrolytic Condensation to prepare Cyclic Oligomer (c) Hydride functional Polysiloxane synthesis (d) Hydrosilation to give Glycidyl functional Polysiloxane ……………………83
4-2 Synthesis of aliphatic amine functional siloxane (a) t-BOC protection of allylamine (b) Hydrosilation of t-BOC protected allylamine with hydride functional siloxane (c) Deprotection of hydrosilated product……………………84
4-3 FTIR of hydride terminated polydicyclopentylsiloxane …………………………87
4-4 FTIR of glycidyl epoxy functional polydicyclopentylsiloxane ….……………..87
4-5 NMR spectra of glycidyl epoxy functional PDPS a. 1H-NMR b. 13C-NMR c. 29Si-NMR ………………………………………………………89
4-6 FTIR of amine functional polydicyclopentylsiloxane…...……………………….90
4-7 NMR spectra of amine functional PDPS a.1H-NMR b. 13C-NMR c. 29Si-NMR………………………………………………………………………91
4-8 DSC of siloxane epoxy/ siloxane amine cured systems ………………………..92
5-1 Schematic representation of telechelic epoxide siloxane homopolymerization ……………………………………………………………..98
5-2 Schematic representation of reaction between telechelic epoxide and amine ……………………………………………………………...99
xvi 5-3 Curing exotherm of (a) telechelic epoxide and amine PDMS, reactive diluents, and cured siloxane with reactive diluents (b) telechelic epoxide and amine PDPS, reactive diluents, and cured siloxane with reactive diluents (c) telechelic epoxide and amine PDHS, reactive diluents, and cured siloxane with reactive diluents ……………100
5-4 Exotherm for cationic photopolymerization of glycidyl epoxide functionalized (a) PDHS (b) PDPS (c) PDMS at 60 °C for 10 s ……………..102
5-5 Tan δ plot of thermally cured PDMS, PDPS, and PDHS ………………………105
5-6 WAXD spectrum of thermally cured siloxanes ………………………………108
6-1 Schematic representation of methacrylic functionalized siloxane synthesis ………………………………………………………………………...112
6-2 NMR spectra of glycidyl epoxy functional PDPS a) 1H-NMR b) 13C-NMR……………………………………………………………………..114
6-3 NMR spectra of methacrylated PDPS a) 1H-NMR b) 13C-NMR ……………...115
6-4 FTIR spectra of A) epoxide and B) methacylate functionalizated PDPS ………116
6-5 Temperature and time effect for methacrylic functionalized PDHS (a) at 60 °C for 10 sec (b) at – 10 °C for 10 sec and (c) at -10 °C for 20 sec…………………………………………………………………………...117
6-6 Effect of pendant group variation on siloxane backbone on reaction rate measured at 60 °C at 10 sec (a) methacrylated PDMS (b) methacrylated PDPS (c) methacrylated PDHS ………………………………………………...118
6-7 WAXD spectrum of UV- cured siloxanes………………………………………121
6-8 Plain stress fracture toughness methacrylic functionalized PDMS, PDPS, and PDHS……………………………………………………………………….122
6-9 Energy release rate per unit crack area at fracture (GIC) of methacrylic functionalized PDMS, PDPS, and PDHS ………………………………………123
6-10 Tensile properties of UV- cured methacrylic functionalized PDMS, PDPS, and PDHS : (a) Tensile strength (b) Tensile modulus (c) elongation at break ………………………………………………..124
7-1 Reaction pathway for dehydrocoupling reaction ……………………………….133
xvii 7-2 FTIR spectras of (a) hydride functional PDHS (b) long Oil (LO) alkyd (c) hydride functional PDHS-LO alkyd hybrid …………………….134
1 7-3 P H-NMR spectras of (a) hydride functional PDHS (b) long oil (LO) alkyd (c) hydride functional PDHS-LO alkyd hybrid …………………...135
7-4 13C-NMR spectras of (a) hydride functional PDHS (b) long oil (LO)alkyd (c) hydride functional PDHS-LO alkyd hybrid ……………………136
7-5 Tensile properties of alkyd-siloxane hybrid (a) Tensile strength (b) Tensile modulus (c) Elongation at break……………………………………….139
7-6 The Tan δ of (a) PDMS (b) PDPS and (c) PDHS based alkyd- silicone hybrids ………………………………………………………………...141
7-7 Plane stress fracture toughness of alkyd-siloxane hybrids……………………...144
8-1 Route for synthesis of homo and copolymers (a) Cyclic oligomers synthesis by hydrolytic condensation of dichlorosilanes, (b) Block copolymer synthesis by ROP of cyclic oligomers (c) Homopolymer synthesis by condensation of di substituted dichlorosilane (d) Random copolymer synthesis by condensation of two different disubstituted dichlorosilane …………………………………………………………………..151
8-2 FT-IR of homopolymer and copolymer of polydicyclohexylsiloxane (PDHS) ………………………………………………………………………….152
8-3 1H-NMR of (a) polydicyclohexyl- siloxane (PDHS) homopolymers (b) block copolymer of PDHS and PDMS (c) random copolymer of PDHS and PDMS …………………………………………………………...154
8-4 13C-NMR of of (a) polydicyclohexylsiloxane (PDHS) homopolymers (b) block copolymer of PDHS and PDMS (c) random copolymer of PDHS and PDMS……………………………………156
8-5 29Si-NMR of (a) polydicyclohexylsiloxane (PDHS) homopolymers (b) block copolymer of PDHS and PDMS (c) random copolymer of PDHS and PDMS …………………………………………………………...158
8-6 DSC curves of homopolymers and copolymers of (a) PDHS (b) PDPS ………159
8-7 Glass transition temperature (α-transition) of homopolymers and copolymers of (a) PDHS (b) PDPS ………………………………………..…...160
xviii CHAPTER I
INTRODUCTION
Applications of siloxanes are diverse due to semi-organic form of silicon known
as siloxanes or organosiloxane polymer. The application of siloxane technology in
various applications involving synthetic materials such as sealants, lubricants, surfactants,
rubbers, coatings and adhesives have been made possible due to versatility of siloxanes.1
Due to the unique physico-chemical attributes compared to other materials, siloxanes are irreplaceable in certain applications. These are available as low molecular weight volatile liquids, high molecular weight fluid like gums, solid resins and as cured elastomers.
The siloxanes are one of the most important materials for design of release coatings and pressure sensitive adhesives (PSAs). This is due to the distinctive molecular structure of siloxanes. The siloxane polymer backbone can adapt different structures.
This creates opportunity to change physical, chemical and as a result performance characteristics of functionalized siloxanes from the most commonly obtained siloxane i.e. polydimethylsiloxane. The semi-inorganic siloxane backbone is highly flexible with large bond angles, long bond lengths and huge freedom of rotation. This rotational freedom allows the siloxane polymers to orient in a helical polymeric configuration consisting of an inorganic Si-O-Si backbone (high surface energy) with an organic pendant
1 group. This flexibility causes the siloxanes to have easier release than the similarly low
surface energy, but non-interacting organic release counterparts.
Some of the major requirements for a siloxane compound to be used in pressure
sensitive products are (a) a nonflowable crosslinked film should be obtained on curing,
(b) the siloxane compound should wet nearly any substrate surface and (c) the siloxane polymer should exhibit appropriate rheological and viscoelastic properties to adjust the properties of a pressure sensitive adhesive product. The very nature of siloxanes satisfy most of the requirements for low adhesion. When cured well, siloxane networks are fairly inert and provides low surface energy. The inert and immiscible behavior of siloxanes with most organic polymers is advantageous in pressure sensitive adhesive applications.
Inhibition of interdiffusion and entanglement results in easy release.
Siloxanes with modified organic substituents can provide altered reactivity,
adhesion, surface energy, thermal stability, hydrophilicity, etc. Functional groups can be
substituted into siloxanes as side groups, end groups or as grafts, enabling the incorporation of siloxanes into various systems. The organic modification of siloxanes can improve solubility, diffusivity, permeability and release properties when used as components of pharmaceutical product.2 The application of siloxanes acrylates in release
systems is a recent development 3, though UV-curable siloxanes were known since
1970s.4 Both in epoxide and acrylic siloxane systems, the ratio of siloxane backbone to
functional material governs the properties associated with release i.e. cure speed, transfer
etc.5 Some of the advantages of these systems includes no requirement of solvent and
thermal energy for curing. One of disadvantages of acrylics is inhibition by oxygen,
hence there is a need of an inert atmosphere in the cure chamber to avoid air inhibition.
2 One of the ways to improve the performance of siloxanes as a component of
pressure sensitive adhesives and other products is to increase the glass transition
temperature (Tg) of the siloxanes. In pressure sensitive adhesives, higher Tg corresponds
to tighter, more controlled peel at a given peel rate.6 Though there is an increase in both
storage and loss modulus, effect on loss modulus is more pronounced. Reduction in
flexibility of siloxane network, increases friction, and results in a rigid surface which
sustains higher stress levels. One of the ways to increase Tg of the siloxane systems is to
incorporate silicate resins by blending with siloxane formulations before coating.
Noticeable improvement in release forces was found only when silicones having high
crosslink density were used above 30 wt %.7 Since blends are not one part system, there
is an added requirement to compatiblize the blend components.
A way around this problem is to develop siloxane systems with organic
substituents in the same polymer to achieve desired combination of properties. The
polymerization chemistry of siloxanes can afford possible structural variation on the
siloxane moiety.8 Replacement of methyl group with bulky organic groups on the
siloxane backbone, increases the thermal and oxidative stability and organic solubility apart from increasing the glass transition temperature. The modification also makes siloxane backbone less regular unlike PDMS, thus inhibits crystallization. One of the most common available bulky substituent is the phenyl group. But the photochemical instability of aromatic compounds rules out phenyl group as a substituent on siloxane polymer backbone, in outdoor and UV-applications. Thus the present study is aimed at developing new siloxane systems with cycloaliphatic substitutions, which will provide intermediate Tg and rigidity between polydimethyl- and polydiphenyl siloxanes.
3 This dissertation is divided into eight chapters. Following the present introduction
(chapter I), the literature review of siloxanes are covered in chapter II. Chapter III encompasses the experimental details, materials, instrumentation, and characterization techniques. The studies related to synthesis of dicycloaliphatic silane monomers from gaseous dichlorosilane and cycloalkene (cyclopentene and cyclohexene), polymerization of the silane monomers to yield hydride, glycidyl epoxide and aliphatic amine telechelic methyl, cyclopentyl and cyclohexyl siloxanes are covered in chapter IV. The thermal curing of the synthesized glycidyl epoxide and aliphatic amine telechelic siloxanes for film formation and the detailed investigation of the film properties are described in chapter V. The study of synthesis of methacrylic functional siloxanes, chemical characterization, film formation by free radical UV curing mechanism and film charecterization are elaborated in chapter VI. In chapter VII, the hybrid formation between linseed oil based alkyds and siloxanes, the thermal curing, film formation and properties are described. Chapter VIII focuses on synthesis of homopolymers, block and random copolymers between methyl, cyclopentyl and cyclohexyl siloxanes and the chemical and thermal characterization.
4 CHAPTER II
BACKGROUND
2.1 An overview on polysiloxanes
Organosiloxane polymers are hybrid materials consisting of pendant organic groups along the inorganic siloxane backbone, belonging to class named “semi- inorganic” polymers. The modern siloxane industry came to being in 1940s. But the history of siloxanes dates backs to 1824, when Berzelius 9 prepared elemental silicon as
an amorphous brown solid by heating potassium flurosilicate with potassium. Then
crystalline silicon was prepared by Deville10 in 1854 by action of tetrachlorosilane over
molten aluminum. The first organosilicon compound, tetraethylsilane was synthesized by
Charles Friedel11 and James Mason Crafts, the two chemists whose started research with the goal to bring organosilicon chemistry along the lines of organometallic chemistry.
Friedel and Crafts were the first chemists to obtain the first siloxane polymer
“diethylsiloxane” in 1866 by oxidation of tetraethylsilane.12 Albert Ladenburg provided
a practical synthesis of polysiloxanes. Alfred Stock developed and systematized the
silicon hydride chemistry.13, 14 , 15 Researchers like Pape, Polis, Dilthey and Schlenk
became active in this area towards end of 19th and start of 20th century. In 1885, Polis
synthesized aromatic derivatives via Wurtz-Fittig reaction.16 Frederic Stanley Kipping,
an English chemist, established the groundwork for explosive growth of organosilicon
chemistry between 1899-1936, and lived to see its early important stages. He used
5 grignard reagents to good advantages, and prepared mono, di, tri silanes from silicon
tetrachloride17 and later compounds of the type RR’R’’R’’’Si.18 He studied the
hydrolysis, and condensation of those silanes.19, 20 Kipping being a pure academician, did
not think seriously about adhesive and film forming properties of siloxanes. He characterized siloxanes as macromolecules in 1927. The new polymers were first named
silicoketones or silicones having the empirical formula R2Si=O as an analogy to ketones
having a formula of R2C=O. 8 Structural studies elucidated the true structure as,
All the earlier siloxane synthesis, involved the reaction of silicon halides with an
organo- metallic compound. The synthesis of silicon halide from silicon were multistep
process having low monomer yield. Hyde from Corning Glass and Rochow from General
Electric Company started the industrial research in organosilicone chemistry in 1930s.
They made scale-up reactions of organosiloxane possible by using “direct process”, in
which elemental silicon is heated with alkyl halides at high temperatures.21, 22
Preparation of silicones by the hydrolysis of dialkyldimethoxysilane and a ring-opening
process that Rochow patented in 1945 remains the basis of modern polymerization
methods. Related derivatives of dimethylsiloxanes were developed and application range
of these materials were also broadened. The growth relates to many interesting properties
including low glass transition temperature, excellent thermal and oxidative stability, low surface energy, high gas permeability, stability to UV radiation, low temperature coefficient of viscosity, and hydrophobicity. The siloxanes can be incorporated with
6 different classes of polymers to form copolymers or blends. These are due to the ease
with which various functional moieties like epoxy, amine, carboxyl, anhydride, nitro,
hydroxyl, vinyl, acryl can be introduced along the main chain or as side groups to form
novel polymers.
2.2 Nomenclature of polysiloxanes
Certain structures and structural segments appears over and over again in the
siloxane area, hence the system of abbreviation was started to be used in specialized areas
of literature. A silicon atom in a polysiloxane backbone can be bonded to one, two or three organic groups, remaining valences being used by oxygen. By using tetramethylsilane Si(CH3)4 as reference, substituting one or more methyl groups by alkoxy groups (-OR), one can assemble siloxanes with four types of structural units. The monofunctional unit R3SiO1/2 is designated as “M”, the difunctional unit R2SiO2/2 as “D”,
trifunctional unit as RSiO3/2 as “T”, and quadrifunctional SiO4/2 as “Q”. This
nomenclature can be applied to dimer, cyclic, and linear molecules as given in Figure 2-
1.
CH H3C 3 CH CH 3 3 Si CH3 H3C O O
H3C Si O Si CH3 Si Si CH CH 3 3 O O H3C Si CH3 [MM] H3C CH3
[D4]
CH3 CH3 CH3
H3C Si OOSi Si CH3 n CH CH 3 CH3 3
[MDnM] 7 Figure 2-1. Shorthand notation for common siloxanes
Additionally, there is a well established terminology to identify various siloxanes
according to the organic substituents on the siloxane backbone. This kind of
nomenclature is used mostly in patent literatures. Dimethyl substituted structures
(CH3)2SiO are denoted as D, methyl-phenyl substituted structure is denoted as D’, diphenyl substituted structures (C6H5)2SiO are denoted as D’’, 1,1,1-trifluoropropyl substituted structures (CF3CH2CH2)CH3SiO are denoted as F.
2.3 Properties of polysiloxanes
Siloxane polymers have one of the most flexible backbone structures due to several structural features. Structure of a PDMS is given in Figure 2-2.
Si O O Si R θ R Si φ R R R
Figure 2-2. Schematic representation of Siloxane backbone23 (Reproduced by permission of John Wiley and Sons)
One of the key structural feature governing the properties of siloxane chains is the
larger size of silicon atom relative to small oxygen atom. Also, the silicon oxygen (Si- O)
bond length (1.64 Ao) is significantly larger relative to carbon carbon (C-C) bond length
(1.53 Ao) but smaller than the value (1.83 Ao ) calculated from additivity of Si (1.17 Ao ) and O (0.66 Ao) atomic radii.24 This results in much diminished steric interferences.23, 24,
25, 26 Also due to the large difference in the electronegativities of Si and O, the Si-O bond
8 can be viewed as being 50 % ionic in nature. Hence the dissociation energy for hemolytic
bond cleavage of Si-O bond (108 kcal mol-1) is much higher than that of C-C bond (83
kcal mol-1). Hence the Si-O polymer backbone is thermally stable but susceptible to electrophilic and nucleophilic attack. The side groups are unencumbered by oxygen atoms, due to its size and having di-valency needed for the chain structure to continue.
The siloxanes are typically atactic with Si-O-Si bond angle ((180-θ) ~ 143°) much larger
than usual tetrahedral bond angle occurring usually ~ 110° and can invert 27 through the
linear form with very little rotational barrier. Energy barrier for torsional rotations is also
very low. The inequality of Si-O-Si bond angle (143°) to O-Si-O (110°) angle causes the
all-trans form of the molecule to form a cyclical projection that comes back to itself after
about 11 repeat units and thus becomes helical for long chains.28 Hence dynamic
flexibility of the siloxane increases immensely due to these structural features.24, 26
Another factor that affects the chemistry of polysiloxanes is the electron
distribution of silicon atom.29 The silicon oxygen bond consists of σ bonding between the
two atoms. The empty d orbitals of silicon is utilized to form (d-p)π dative covalent
bond.30 The large angle at oxygen is usually attributed to O (pπ ) →Si(dπ )bonding, i.e.
bonding of π -symmetry electrons on an oxygen atom to one of the empty 3d orbitals of a
silicon atom. The dative covalent bond causes the oxygen to be less basic than expected.
31 Since all the d orbitals are empty, there is no restriction to rotation by the π bonding.
Since lone pair of electrons of the oxygen atom are back donated to the empty d orbitals of silicon atom, the d-π bonds exist concomitantly with the usual σ bonds.
In the siloxane chain, trans states are of lower energy than the gauche states. 23, 32
This preference in conformation is due to Vander Waals interactions between the pairs of
9 organic substituents separated by four bonds in trans states. This interaction is more
important than favourable Coulombic interactions between oppositely charged Si and O
atoms separated by three bonds. Due to reduced distance in gauche state, the Coulombic
interactions is larger in gauche states than in trans states.
Bonds of silicon atoms with electronegative atoms like oxygen and nitrogen have
a large ionic component, hence unlike carbon compounds, the sp3 hybridized bonds are
not directionally rigid. Silicon compounds also have larger molecular volume (75.5
cm3/mole) than analogous carbon compounds.33 The flexible Si-O bond results in large
intermolecular distance and low van der Waal’s forces results in exceptionally low glass
transition temperatures 34, which is -123 oC for PDMS and surface energy and high
speading coefficient. In any application that requires modification or control of interface,
surface properties of siloxanes can be utilized. Thus these polymers are used in
suppressing foams, coupling agents, surfactants, release coatings, pressure sensitive
adhesives, etc.
More significant properties of polysiloxanes are directly derived from the nature
of siloxane bond and low intermolecular forces between the polymer chains. Even when
vinyl, methyl or phenyl groups are attached to silicon atom, the siloxane molecules rotate
freely around the Si-O bond, since the molecule are highly flexible. Siloxanes can be
viewed as an inorganic core similar to a one dimensional silicate structure with high bond energy protected by sheath of organic substituents e.g. methyl groups in case of PDMS that hide the core, providing low surface energy and organic characteristics. This explains the unique set of properties like high resistance to harsh environments, namely high temperatures, radiation, oxygen and ozone. As a result, silicone rubbers are
10 remarkably stable, and have higher gas permeability than any other elastomer. Due to low
surface energy, the siloxane component of a phase separated siloxane containing
copolymer or polymer blend system migrates to air/polymer interface resulting in
siloxane rich surface. These features of siloxanes can be tailored and exploited in
applications requiring physiological inertness and atomic oxygen resistance. 35, 36, 37
Si-O-C linkage is susceptible to hydrolysis due to environmental moisture,
although Si-O and Si-C bonds are stable. This was used successfully for room
temperature vulcanization.The Si-O bond is susceptible to attack by acids and bases, and the rubber vulcanizates are relatively weak and readily swollen by hydrocarbon oils.
Non-vulcanized, low-molecular-weight polysiloxanes make excellent lubricants and hydraulic fluids and are known as silicone oils. The high permeability of polysiloxane matrix, inherently the liquid nature of the polymer in addition to facile displacement reaction of alkoxy and carboxy functional groups are the basis for application of siloxanes in many types of sealants and adhesives. Even though this technology is well established, none of the existing cure systems are ideal. The ideal cure system would be a one package system, that can be activated under mild conditions, and should not release volatile by-products.
In addition to weak intermolecular forces, mild temperature dependency of polysiloxanes can be explained by another point. The linear polysiloxanes have helical
structure, and approximately four repeating units form one spiral of rotation. This can be
explained through stereochemistry 38 and weak dipolar interaction.39, 40 Due to large Si-
O-Si bond angle and large radius of silicon atom, the molecule project outward from the
11 helix. This attributes to non-polar, hydrophobic nature of polysiloxanes. The helix uncoils
with increase in temperature, resulting in increased intermolecular interactions, which in
usual systems translates into increase in viscosity. But in the case of siloxanes, the
increased intermolecular interactions compensates for the increase in molecular movements. Hence, the viscosity of the polysiloxanes is only slightly affected by
temperature.
Ring-chain equilibrium is one of the common features of siloxane chemistry.
Polycondensation reaction unlike addition reaction, catalyzed by an acid or base, is
reversible in nature and leads to formation of cyclosiloxanes of different molecular
weights. A well defined distribution of these cyclic siloxanes are obtained at equilibrium.
This distribution is a function of size of substituents on silicon atom, system viscosity,
and the strain of the cyclic compounds.41 Due to the difference in properties between
large sized cyclosiloxanes and the linear homologues, the interest in cyclosiloxanes is
increasing. Some of these properties are hydrodynamic volume 42, bulk viscosity, critical
entanglement mass 43, 44, and glass transition temperatures.45 The large cyclics can also be used to synthesize macromolecules, with no free chain ends and no covalent bonds between the individual repeat units.46
2.4 Monomer synthesis
Siloxane polymers are widely synthesized industrially from chlorosilanes by
either direct hydrolysis and condensation reactions or by ring opening polymerizations.
The materials properties can be readily tailored by varying the identity of the alkyl side
12 chains and terminating groups, the molecular weight distributions, and the degrees of
crosslinking. A relatively small number of silane monomers generate vast multitude of
functional siloxanes, the properties of which can be better understood by considering the
chemistry of these monomers and the various combinations.47 The silicon element is
obtained from the reduction of sand at high temperature 48, 49 as shown in equation (2-1).
Reacting silicon with chlorine gas gave tetrachlorosilane, equation (2-2).
Tetrachlorosilane is reacted with Grignard reagent to form organosilanes, equation (2-3).
SiO +2C Si +2CO (2-1) 2
Si + 2Cl SiCl (2-2) 2 4
SiCl +2RMgX R SiHCl +2MgClX (2-3) 4 2 2
The above relatively complicated reaction, which was used initially was replaced
by “Direct Process” or “Rochow Process". 48, 50, 51 Rochow pocess involves an
exothermic reaction of silicon with organic halide in presence of copper/silver catalyst,
equation (2-4).
Si + 2RCl R SiCl (2-4) 2 2
The reaction yields byproducts such as RSiCl3, R3SiCl, R4Si, HSiCl3, RHSiCl2 which can be removed by distillation. Later metals like zinc, tin, aluminum, antimony, arsenic, bismuth and phosphorus were found to have beneficial effects.52 The main product, R2SiCl2, is the precursor for the preparation of large variety of compounds
13 having both organic and inorganic character 53, 54 , 55, hence this reaction is very significant. The course of reaction is a function of temperature, time of contact of the reactants, type of catalyst used, and the interaction of silicon and catalyst. 22 Though the reaction rate increases with temperature, but the chance of pyrolysis of free radicals also increases and the product mixtures becomes richer in halogen and lower in organic groups. Linear siloxanes with high molecular weight is obtained if the reaction condition involves basic catalysis and high temperature. Cyclic siloxanes of low molecular weight are obtained using acidic catalysts.56
The organochlorosilanes undergo hydrolysis followed by self condensation to produce a mixture of linear diols and cyclics of various sizes. The ratio of cyclic to linear products can be controlled by varying the reaction conditions.16 Yeild of cyclics are as high as 90 % when the hydrolysis is conducted in the presence of water-insoluble, non- polar solvents. It is due to the fact that halosilanes and organosilanes are solubilized by non-polar organic solvents, thus reducing the concentrations in the aqueous phase. Hence, intramolecular condensation dominates over intermolecular condensation and formation of cyclics are preferred.
2.4.1 Hydrosilation
Hydrosilation is one of the most versatile, fundamental and widely used approaches currently available for laboratory and industrial synthesis of customized silane monomers, organosilicon compounds and organic silyl derivatives. It is second in importance in silicon organic chemistry after Rochow reaction, but its reaction
14 mechanism is not yet totally undersood. This reaction is not limited to alkenes and
occurs in general for molecules having CCCCCOCS= ,,,≡==, and C≡ N groups.57
In this process a organosilane containing Si-H group is added to an unsaturated compound, equation (2-5).
R CH CH + HSi RCH CH Si (2-5) 2 2 2
Speier studied hexachloroplatinate (IV) as a catalyst for hydrosilation 58, 59 in
which Pt is reduced from IV to Pt(0) in the presence of silane or siloxane. The Chalk-
Harrod mechanism is the most widely accepted mechanism for platinum catalyzed hydrosilation reaction. Chalk-Harrod catalytic cycle for hydrosilation is represented in
Figure 2-3.
Si R' Me2Si HSi O Pt(II) Me Si 2 H
R' Si
Si Si Me Si 2 Me2Si Me2Si O Pt(0) O Pt(II) O Pt Me Si R' 2 Me2Si Me2Si H R'
Si
R' Me2Si Pt ' Si O R Me2Si H
Figure 2-3. Chalk-Harrod mechanism for hydrosilation using karstedt catalyst
15 This process may be mechanistically divided into three steps. In the first step, a π
bond of the unsaturated compound (C=C,C≡ C, C=O, C=S, or C≡ N) in the vicinity of a silanic group is cleaved. In the second step, silanic hydrogen shifts from silicon to one of the carbon atoms originally involved in the π bond, then in the next step the unsaturated
silicon and carbon atoms bind to one another. This process involves oxidative addition of
silicon hydrogen compound and reductive elimination of the hydrosilated alkene, and
regeneration of the catalyst. Actually, the regiochemistry of addition is reported to be
anti-Markovnikov, i.e, silicon goes onto the carbon with the most hydrogen atoms.
According to bonds formed and broken, hydrosilation reaction is found to be an
exothermic reaction. Si-H bond (94 kcal/mol) and the C=C bond (65 kcal/mol) are broken
and Si-C (92 kcal/mol) and C-H (100 kcal/mol) bonds are formed. The resulting enthalpy of reaction for a generic hydrosilation reaction is 94+65-92-100 = -33 kcal/mol.
Hydrosilation can be triggered by heat, peroxides, azonitriles, and high gamma or
UV-radiation. 60 The reaction is generally catalyzed by silicone soluble group VIII precious metal compounds, e.g. complexes of platinum, rhodium etc. It is the one of the best method for curing solventless coatings. The reagent (HSi ≡), solvent, and the source
of platinum (Speier’s catalyst or Karstedt’s) have a large effect on incubation times and
stability state of platinum. Electron-withdrawing alkenes, such as maleates, stabilize platinum(0). The mechanism shows that sterically hindered and internal alkenes would
have a difficult time undergoing hydrosilation. As a result, high temperature and
pressure are needed to provide the hindered alkene with enough kinetic energy to
overcome the activation energy barrier required for hydrosilation. 61 Complexes of this
16 type are not active at room temperature for hydrosilation to occur. This property is used to deactivate platinum present in the reaction mixtures, and to process the liquid while curing can be initiated by raising the temperature of the system.
2.5 Synthesis of polyorganosiloxanes
Synthesis of polysiloxanes can be classified broadly according to the mechanism used for synthesis and structure of siloxanes produced. Equilibrium, anionic and cationic polymerization are the three generally used mechanisms for polysiloxane synthesis.
2.5.1 Mechanism for polysiloxane synthesis
Chlorosilanes are the building blocks of all polysiloxanes, which are exclusively obtained from direct process. In industry, the general route for linear polysiloxane synthesis consist of two steps. 29 First step is hydrolytic polycondensation of distilled and purified chlorosilanes leading to a mixture of linear and cyclic oligosiloxanes. The second step is the transformation of oligomers into a high molecular weight polymer. The synthesis of the high molecular weight polymer can be accomplished either by polycondensation of the hydroxyl terminated, linear short chain polysiloxanes or by ring opening polymerization of the cyclic oligomers. The reaction can be schematically shown in equation (2-6). 29
(2-6) (m+n)Me2SiCl2 + (m+n+1)H2O HO(Me2SiO)mH+ (Me2SiO)n + 2(m+n)HCl
Polycondensation Ring opening polymerization
High MW Polymer
17 Heterofunctional condensation, equation (2-8), becomes important in hydrolytic
polycondensation when concentration of HCl is high in the system. On the other hand,
homofunctional silanol condensation, equation (2-10) is important when the reaction is
carried out in large excess of water. The condensation reaction equation (2-6), is
addition of two reversible reactions, equation (2-7) and (2-8).
SiCl + H2O SiOH + HCl (2-7)
SiCl +SiOH SiOSi + HCl (2-8)
2 SiCl + H2O SiOSi + 2HCl (2-9)
SiOH +SiOH SiOSi (2-10)
This process can be optimized to give either cyclics or linear hydroxy ended
polysiloxanes as the main products.16, 62 The hydrolytic polycondensation reactions are exothermic, and a considerable contribution, comes from the heat of hydration of HCl. 63
-1 The heat of hydration ( Δ H298) for Me2SiCl2 is -134.3 kJ mol . One of the products, HCl catalyzes the hydrolysis by protonating the substrate at the siloxane oxygen. The reaction is reversible leading to fast Cl-OH functional group exchange.
The yield of cyclics versus linear siloxane is controlled by competition of
kinectics of unimolecular intramolecular condensation and bimolecular intermolecular
condensation. Hence in dilute, non-aqueous and non-polar solution, cyclization, Figure 2-
4 is preferred. 64, 65 This is due to organohalosilanes being soluble in non-polar, organic
solvent, the concentration in aqueous phase decreases. Hence, organohalosilanes can
react with water, hydrolysis product is retained in organic solvent, and protected from
liberated aqueous acid. A pure linear or cyclic polysiloxane is converted to a mixture of
cyclic and linear polysiloxanes in presence of strong acid or alkali. 18 a a b Si O H Si O Si b H + O Si c H c Si O + Si O H H H
Figure 2-4. Cyclization in dilute, non-aqueous and non-polar solution
A high yield of linear polysiloxanes can be obtained by carrying out the condensation in aqueous acidic medium, since acid catalyzes the polymerization of the cyclic species. In dilute solution a high yield of cyclic oligomers can be obtained. There is no control of molecular weight by this polymerization process due to absence of chain terminating groups. Control of molecular weight can be obtained by varying the ratio dichlorosilane (chain extenders) to monochlorosilane (chain blockers). Molecular weight increases or decreases as the proportion of chain blockers is reduced or increased in the reaction mixture.
Ring opening polymerization (ROP) of cyclic siloxanes 16, 65 , 66 , 67 , 68 can yield a
high molecular weight polysiloxanes with higher precision than hydrolytic polycondensation methods. ROP of cyclic siloxanes can be performed either by equilibrium or non- equilibrium polymerization method. According to the structure of the active propagation center, ROP of cyclic siloxanes can be classified as anionic or cationic.
2.5.1.1 Equilibrium polymerization
Equilibrium polymerization of cyclic oligomers is referred to as
thermodynamically controlled polymerization and is carried at equilibrium state of the
19 process. The equilibrium does not depend on the initiator used or ring size of the
monomer used and can be reached by both cationic and anionic routes. The
polymerization results in complex equilibria of cyclic and linear polymers. There is a relation between equilibrium ring concentrations and the statistical conformations of the corresponding open chain molecules. This equilibrium can be represented as follows:
1/Kcn
Dx-n + Dn Dx
Dx + Dy Dx+n + Dy-n
Dn represents the n-meric cyclic in equilibrium with open chain species
containing n and x-n. The yield of polymerization is obtained from information about
equilibrium between cyclic and open chain populations. From the above equation, the
equilibrium constant (Kcn) for cyclic formation is:
⎡D cy ⎤ ⎣ n ⎦e K = cn pn
⎡⎤D cy where ⎣⎦n e = molar concentration of cyclic at equilibrium,
and p = extent of reaction
In ROP, since there is no change in the number of chemical bonds of any type,
Δ Hc is related exclusively to strain energy of cyclic species. Larger cycles consisting of
four or more repeating units are stain free, and at equilibrium the presence of strained
20 cyclic trimer can be neglected. The yield of polymer, is independent of temperature and
69 to a good approximation Δ Hc ≈ 0. 65, Ring opening leads to increase in entropy ( Δ Sc
is negative) due to higher conformational freedom of opened siloxane chain, compared
with cyclic siloxane. With the increase in size of the cyclic oligomers and the polarity of
the organic substituents on silicon atom, the entropy gain on polymerization decreases.
So, the yield of the polymer at equilibrium decreases, if the monomer with bulky or polar
substituents are used.
2.5.1.2 Anionic polymerization
Developments of anionic polymerization in 1950s provided versatile
methodologies for the synthesis of polymers with well defined structures and low degree
of compositional heterogeneity. The discovery of “living polymers” in 1956 by M.
Szwarc et. al. was of tremendous importance for developing fundamental concepts and
practical application of anionic polymerization. 70, 71, 72 Strong inorganic, organic or
organometallic bases for example hydroxides, alcoholates, phenolates, silanolates,
quarternary ammonium and phosphonium hydroxides etc are generally used as catalyst
for initiation of anionic ring opening polymerization of cyclic siloxanes. Due to lower
catalytic activity of lithium ions in siloxane redistribution reaction among the
organoalkali bases, initiators containing lithium ions are preferred compared to sodium
and potassium ions. Initiation leads to the ring opening of the cyclic siloxanes and
formation of the active propagation center, the silanolate anion, Figure 2-11. The
silanolate anion then attacks another cyclic oligomers, Figure 2-12, to form another
21 pentacovalent silicon species, which upon electron rearrangement, opens up to yield a
longer silanolate chain.
+ - M B + Si O Si B Si SiO- M+ (2-11)
Silanolate Anion
B Si SiO- M+ + Si O Si B Si SiOSi SiO- M+ (2-12)
The propagation step, equation (2-12) is reversible due to the back biting reaction of the propagation center with its own chain. Back biting process results in generation of series of monomer homologs of various ring sizes. In the early stages of polymerization, the rate is higher due to abundance of cyclic siloxanes. As the polymerization proceeds, concentration of cyclics reduces and linear species increases. Since the propagating active center is reactive towards both cyclic and linear siloxanes, randomization occurs. An equilibrium is reached and linear and cyclic siloxanes can be separated by distillation. In the absence of any chain transfer agents, the reaction proceeds without termination, hence has to be quenched to deactivate the silanolate anion. In both base and acid catalyzed reaction, termination reaction consists of condensation of intermediates and liberation of the acid or base catalyst. The mechanism for base induced anionic polymerization for cyclic oligomers is depicted in Figure 2-5.
22 OH R R R R R R + M+OH- Si M + Si O Si _ O Si - + R R R R HO Si SiO M R R
R R R R R R - + R R HO Si SiO M + Si O Si HO Si Si O Si SiO- M+ R R R R R R R R
R R R R R R back-biting R Si Si - + - + HO O O Si O Si O M HO Si O Si O M + Si O a b a-1 R R R R R R R b+2
Figure 2-5. Mechanism for anionic polymerization of cyclic siloxanes
The rate of anionic polymerization of cyclic siloxane is dependent on several parameters, for e.g., initiator, medium, monomer, ring size, substituents on silicon atom etc. Cyclic oligomers with small ring size show high reactivity for ring opening due to ring stain. The rate of anionic polymerization increases with the increase in the bulk of counterion in the silanolate73, 74 in the order SiOLi < SiONa < SiOK < SiORb < SiOCs
≈ SiONMe4 ≈ SiOPBr4. In case of bigger counterions, ion aggregation and ion pair
interaction is weaker, thus availability of anion is more and hence the reactivity increases.
With increase of solvent polarity also the ion aggregation loosens and reactivity
increases. 75 Substituents on silicon atom has counter balancing influence on the rate of
polymerization. The electron withdrawing substituents increases the reactivity by making
the silicon center more electrophilic. But the decreased electron density on silicon atom
lowers the basicity of silanolate ion. The interaction between siloxane oxygen and
silanolate ion in the propagation step also become less effective.76 Equilibrium anionic
polymerization is performed at elevated temperatures in industry. Thermo-labile
23 silanolates, like (Me4N+)(-OSi ≡), (Bu4P+)(-OSi ≡) etc, are used as initiators since
77 decomposition occurs on heating, producing Me3N and Bu3PO respectively. The non-
equilibrium anionic ROP is used for synthesis of end-functionalized polysiloxanes,
particularly macroinitiators78 and macromonomers.79
2.5.1.3 Cationic polymerization
Cationic polymerization of cyclic siloxanes is a very important and convenient
reaction and used extensively both in industry and research laboratories for synthesis of
linear polysiloxanes. In 1983, Higashimura and Sawamoto reported the first living
cationic polymerization. 80, 81, 82 , 83 Strong Bronsted-Lowry protic acids with unreactive
counterions (H2SO4, HClO4, HOSO2CF3), Lewis acids with (BF3, AlBr3, FeCl3, TiCl4), or
the mixtures of the two are generally used as initiators for cationic polymerization of
cyclosiloxanes. Living cationic polymerization have extensive application in the
synthesis of amphiphilic block copolymers, silesquioxanes and silica network.
Initiation of polymerization by protic acids initiates with protonation of oxygen in
cyclic siloxane resulting in a cyclic silyloxonium salt, equation (2-13). The
rearrangement and decomposition of the bonds in silyloxonium salt results in a siloxane
chain terminated with silylenium-anion pair and a silanol group. The concentration of
silylenium-anion and silanol is kept constant by fast reversible reactions, equation (2-14)-
(2-16).
H+A- + H + Si O Si A- Si O Si HOSi SiA (2-13)
24 ~SiOH + HA ~SiA + H2O (2-14)
~SiA + ~SiOH ~SiOSi~ + HA (2-15)
+ ~SiOSi~ + H O (2-16) ~SiOH ~SiOH 2
The linear siloxane chains formed from the decomposition of the cyclic silyloxonium complex can condense in two different manners. The first route is through the reaction of a silanol with a silylenium-anion to eliminate acid (acidolysis- condensation), equation (2-15). The second route is through the reaction of two silanol groups to yield water, equation (2-16). The liberated acid from acidolysis-condensation is then free to protonate another cyclic siloxane which opens to provide additional short siloxane chains. This route of polymerization is termed as acidolysis-condensation polymerization.84
The initiating power of the protic acid increases with increase in acid strength and
decrease in nucleophilicity of the anion. Favorable entropy factors favors ring opening of strained cyclic species, yielding macromolecular polysiloxane chains. Stationary concentrations for free acid, acid complex, and silanol are established much faster in case of stronger acid. The acid mediated cleavage of cyclic siloxanes for found to be higher in order with respect to acid 85, due to cooperative hydrogen bonding facilitating the transfer
of proton, Figure 2-6a. 29 Water released as byproduct from silanol condensation also
has activation effect, as can be explained by formation of hydrogen bond with acid,
Figure 2-6b. Since acid molecules are replaced by water in hydrogen bonded complex,
so released acid molecules can initiate polymerization in other cyclic siloxanes.
25 H (HA)n-1 (HA)n-1 O H H O H A H
HA HOH H O O Si O Si H
(a) (b)
Figure 2-6. Cleavage of cyclic siloxane oligomers facilated by H-bonding (a) in absence of water (b) in presence of water
Lewis acids complexed with proton releasing co-catalyst can induce cationic
polymerization, in virtually all categories of monomers. 86, 87, 88 Initiation is possible at
much low temperatures, and high molecular weight polymers are produced in high yield.
These catalyst accepts the lone pair of electron from oxygen of cyclic siloxane, forming an adduct. The cyclic oligomers is cleaved resulting in bond formation between the silicon atom and lewis acid. If a stable adduct cannot be formed between siloxane and
lewis acid, the lewis acid is regenerated and further reacts with other siloxanes. 89, 90
Cationic polymerization of cyclic oligomers in presence of chain stoppers91, 92, 93 is one of the important routes to introduce desired functionality at the chain ends and to control molecular weights, equation (2-17). This route can be used to synthesize telechelic polysiloxanes which can be further used to synthesize siloxane-siloxane and siloxane-organic block copolymers.94, 95
R R R' R R' R
X(SiO)nSiX + Si O X(SiO)n( SiO ) Si X (2-17) m m R R R R R'' R''
26 2.5.2 Structure of synthesized polysiloxanes
The synthesized polysiloxanes may have linear, block, graft, side chain modified,
or silsesquioxanes structure.
2.5.2.1 Linear polysiloxanes
Silanols and chlorosilanes are versatile precursors for synthesizing siloxanes since
they can condense with many functional silanes, including self condensations. 16, 96 , 97
The reactivity of silanols depends on the structures98 and the reactivity increases with the
increase in the number of hydroxy groups at the silicon carbon for e.g. R3SiOH <
R2Si(OH)2 < RSi(OH)3. Linear siloxane is obtained by condensation of silanediols which
is dependent on size of substituents on the silicon atom. 99 The size dependence of silanol
condensation can be explained by intramolecular catalysis by silanol groups. Figure 2-4 shows the acid catalyzed silanol condensation. With small alkyl groups, silanols and silanediols are unstable and condense spontaneously to form linear or cyclic siloxanes.
Bulky organic groups stabilize silanediols in a manner that condensation do not occur
even under the influence of mineral acids or heating in an autoclave.100 This is in
contrast to carbon chemistry, where compounds with H-acidic groups bonded to same
carbon atom are unstable. Disproportion of polysiloxanols, equation (2-18), usually accompanied by silanol self condensations. It becomes important in presence of strong bases and weaker acid.
R R R R R R R R a a b b acid or base a a b b Si O SiOH + HOSi O Si SiOH + HOSi O Si O Si (2-18) R R R R R R R R
Heterofunctional condensations of silanol with chlorosilanes, alkoxysilanes or
silicone hydrides can also yield siloxanes. Chlorosilane and silanol condensation process 27 is facilitated in presence of basic and nucleophilic catalysis. Acid catalysts promote self
condensation of silanols by preferentially protonating ≡ SiOH and not ≡ SiCl. Tertiary amines and aromatic nitrogen heterocycles, such as, pyridine promote heterofunctional condensations in two ways. First by forming hydrogen bonded complex with silanol ,
equation (2-19) i.e. operates as a bronsted base, and reacts with chlorosilanes, equation
(2-20) at the rate limiting step. Second, by acting as HCl acceptor, a byproduct of condensation thereby promoting the condensation to proceed forward.
δ − δ + SiOH + NR3 SiO H NR3 (2-19)
SiO H NR + SiCl . 3 SiOSi +R3N HCl (2-20)
Uncharged nucleophile can also effectively catalyze this heterofunctional condensation by formation of strong electrophilic complex with chlorosilanes, equation
(2-21), which then further reacts with silanol as in equation (2-19) and equation (2-20).
+ - SiCl SiOH . +Nu Si Nu Cl SiOSi +R3N HCl + Nu (2-21) NR3
Reaction of silanols with alkoxysilanes, equation (2-22) are used for synthesis of
linear polymers, block copolymers and also in sol-gel process for polysiloxane network
preparation.101
SiOR +SiOH SiOSi + ROH (2-22)
A diverse variety of catalysts promote this reaction, for e.g. stannous octoate,
dibutyltin dilaurate (DBTDL), CF3COOH, amines etc. The mechanism for sol-gel
condensation catalyzed by DBTDL is given in Figure 2-7. The proposed mechanism 28 involves the formation of intermediate complex of the catalyst with both the reactants.
The group (II) metal oxides, amines, tin carboxylates are effective in promoting selectively heterocondensation over silanol self-condensation.
O O R' = C4H9 R'2 O Sn O R''' = C11H23 R''' R'''
H2O
O
HO R''' O R'2 O Sn OH R''' SiOR
ROH
O R'2 O Sn O Si R''' SiOH
O R'2 R''' O Sn OH SiOSi
Figure 2-7. Sol-gel condensation catalyzed by DBTDL
Dehydrocoupling reaction is a heterofunctional condensation between silanol and
silicone hydrides. This reaction is very chemoselective and side reactions like
homocondensation of silanols and disproportionation reactions are not significant in this
system. Transition metal complexes for e.g. wilkinson catalyst, tin salts etc are used as
catalyst for this coupling. The hydrogen generated used as a by-products, can be used for
foaming.
2.5.2.2 Block and graft polysiloxanes
Polysiloxanes polymers containing multiphases, as blocks, segmented, or graft
polymers have been studied for more than 40 years. 8 Due to the high surface activities, 29 the siloxane copolymers have been used in various applications, such as surface
modifying agents in paints, coatings, fibers. Most commonly, synthesis of copolymer requires difunctional polymers. Block copolymers can be subdivided according to functionality of siloxane (one or two terminal groups), sequence of monomers in the
backbone (random and block) and according to the functionality directly attached to a
silicon atom or separated by atleast one organoalkyl group.
Difunctional α, ω-siloxanes may be obtained by cationic equilibration of cyclic
oligomers in presence of functional disiloxane. If functional disiloxane is 1,1,3,3-
disiloxane, a large number of organofunctional endgroups can be introduced through
hydrosilation. The disiloxane can be used as an endblocker in the equilibration of cyclic
siloxane. The separation of cyclic and linear polymers is difficult in this case. According
to the step chosen for hydrosilation and hydrolysis, different possibilities can be
considered, Figure 2-8. According to the nature of functional groups to be introduced,
the polymerization can be either cationic or anionic. For example, aminoalkyl group is
introduced through anionic polymerization initiated by silanolate or
tetraalkylammnonium hydroxide. 102
ClSiR (CH ) X X 2 2 2 H2O + - Cl SiR2 H X Dn ( H / HO ) HO SiR2 (CH2)2X HO SiR2 H Y Dn Y H2O HSiR O Dn OSiR2H Dn 2 X
Y= OSiR (CH ) X 2 2 2
Figure 2-8. General route to synthesize organo bifunctional siloxanes
30 Monofunctional siloxanes are usually synthesized by anionic polymerization of
cyclic siloxanes initiated with silanol in presence of an activator such as tetrahydrofuran.
Monofunctional chlorosilanes is then used to deactivate the silanolate catalyst to introduce the functional group. The groups that are reactive with silanolate ion can also
be introduced by this process, if excess of chlorosilanes to avoid side reactions. 103, 104
The siloxane-organic multiblock copolymers can be distinguished according to
the type of linkage joining the two blocks i.e Si-C or Si-O-C links. Each class of
copolymers can further be classified into two categories, true multiblocks and random
block copolymers. The basis of classification are the reactants used for the synthesis.
Copolymers synthesized by reacting two α,ω-bifunctional polymers gives true
multiblocks, while the copolymers prepared by reacting a α,ω-bifunctional polymer with
one or more α,ω-bifunctional monomers yields random blocks. The bifunctional
monomers and polymers may have same or different functional groups. Better
distribution of blocks and control of polydispersity can be achieved by separately
preparing and then reacting α,ω-bifunctional polymers.
The copolymers having Si-O-C functionality were obtained by reacting reactive
functional groups attached to silicon atom for example, chlorosilanes, aminosilane,
alkoxysilanes, silanol, etc. An example of random block copolymer having Si-O-C
linkage obtained by monomer polymer condensation is PDMS-polycarbonate multiblock
copolymer.105 It was obtained by reacting α,ω-dichlorooligosiloxane with bisphenol-A.
Examples of random block copolymer having Si-O-C linkage obtained by condensation
of two α,ω-bifunctional polymers are reaction of α,ω-dichloro-PDMS with α,ω-dihydroxy
aliphatic polyester 106, α,ω-diamino-PDMS with α,ω-dihydroxypolysulphone 107, 108,
31 α,ω-dihydroxypolycarbonate , α,ω-dihydroxypoly- arylester109. A disadvantage of this
type of copolymers is the sensitivity of Si-O-C bond. In this respect the blocks linked by
Si-C is preferred.
Various α,ω-difunctional PDMS have been reacted with polyesters ,
polycarbonates110, polyamide111, polyurethanes112, polyurea113 etc to yield random block
copolymers with Si-C linkage. A two step procedure is preferred with excess of one
reactant reacted with bifunctional siloxane in the first step and second reactant added in
next step to adjust stoichiometric balance. The most significant of all the block copolymers are the multiblock copolymers prepared by reacting two α,ω-bifunctional polymers coupled by Si-C linkage. This class of polymers show several interesting properties as phase separation, separate glass transition temperatures etc. A favorable route to synthesize this class of polymer is by hydrosilation. For e.g. multiblock PDMS- polystyrene and PDMS-poly(α-methylstyrene) copolymers were synthesized by reacting vinylsilane end-capped polystyrene and poly(α-methylstyrene) with hydride terminated
PDMS.114
Graft copolymers of siloxane with organic polymer can be classified depending on whether the siloxane is forming the polymer backbone or the branches. 29 To enable
grafting of organic polymers on polysiloxane backbone, presence of reactive groups for
e.g. hydroxy 115, hydride, amino, carboxy etc functionality is desired at polymer trunk, to react with the organic polymers. An alternate route is to introduce functional group into the polysiloxane backbone and then using these groups to graft organic polymers. For example, a siloxane backbone formed by hydrolytic condensation of dimethyldichlorosilane and di(cyanopropyl)dimethylsilane resulted in carboxylate
32 functionality at the backbone.116, 117 The carboxylates can then initiate polymerization of
ε-caprolactam, lactones etc which forms the side chains. Siloxane polymers having mono or di functional groups can be incorporated as side chains on organic polymer backbone.
For example PDMS having unsaturation on one end have been copolymerized with vinyl or acrylic monomers.118 Dihydroxy functional PDMS have been used to introduce siloxane functionality to polyurethane backbone.119
2.5.2.3 Side group modified polysiloxanes
There are numerous routes to obtain side group modified polysiloxanes but three are most significant. The first route is modification of siloxanes by addition of donor reagents to reactive group for e.g hydrosilation, hydrosulfidation, addition to vinyl, allyl or oxirane group bound to siloxane. The second route is polycondensation of bifunctional monomers. The polycondensation can be either homofunctional polycondensation or heterofunctional copolycondensation. Homofunctional polycondensation may involve two dihalosilanes or two alkoxysilanes while the heterofunctional copolycondensation involves reactions between dihalosilanes, dialkoxysilanes or dihydroxysilanes etc. The third method is ring opening polymerization by cationic or anionic route.
The hydrosilation methodology was discussed in detail in section 2.4.1.
Hydrosulfidation reaction is used to react organic thiol with unsaturated functionalities attached to polysiloxane either linear or cyclic to introduce thioether moiety into polysiloxane polymer system120, 121, Figure 2-9a. These thiol-ene reactions are free radical mediated reactions resulting in monoaddition products in high yields. After introducing as side group to siloxane polymer, thiols can be oxidized to sulfoxide, which may have several interesting applications. Hydrophosphination reaction is analogous to
33 hydrosilation and hydrosulfidation, used to incorporate phorsphorus group into linear or
cyclic siloxanes. It is carried out by reacting secondary phosphines with vinyl or allyl
groups attached to polysiloxanes, Figure 2-9b.
SiO + H-R' SiO (a) R' = -SR, Hydrosulfidation n n (b) R'= - PR2, Hydrophosphination CH = CH CH R' 2 2 2
Figure 2-9. (a)Hydrosulfidation and (b) Hydrophosphination reactions with vinyl functional siloxanes
There are few other types of reactions involving addition of donor reagents to siloxane backbone. Addition of alkyl halide to vinyl or allyl functionality attached to polysiloxanes 122 is one of the methods which did not generate much interest.
Nucleophilic substitution of terminal halogen of haloalkyl sidechain by ester,
phosphonate esters, thiocyanate, thiourea, secondary amines etc and ROP of
cyclicsiloxanes bearing functional groups is another very useful route of getting homo
and copolymers of polysiloxanes. Generally anionic ROP is used but even cationic ROP yields high molecular weight polymers. Desired functional groups for e.g. epoxide, amine, haloalkyl, trihalopropyl may be incorporated into the cyclosiloxanes by one of modification techniques as hydrosilation, thiol-ene addition etc and then ROP may be carried out.
2.5.2.4 Polyorganosilesquioxanes
These are network polymers or polyhedral clusters, having the generic formula
(RSiO3/2)n. Each silicon atom is bound to an average of one and a half (sesqui) oxygen
atoms and to one hydrocarbon group (ane). Polyorganosilesquioxanes are synthesized by
hydrolytic condensation of tri-functional silanes with acid or base catalyst. Typical 34 functional groups that may be hydrolyzed or condensed include alkoxy or chlorosilanes,
silanols, and silanolates. Due to the trifunctional nature, silesquioxane can have several
structures for e.g. oligomeric cages, ordered ladder structures, and 3D network structures.
The structure of resultant silesquioxane depends on monomer concentration, nature of
solvent, nature of hydrolysable substituents, catalyst, temperature, concentration of water
etc. The general synthesis of silesquioxane is shown in Figure 2-10. The complex
interdependence of hydrolysis and condensation results in formation of variety of
structures. For e.g. rate of condensation may be retarded by presence of hydrogen
bonding between reactants and solvents making either intra or inter molecular
condensation more favorable.
RSiX3 +3H2O RSi(OH)3 +3HX hydrolysis
3RSi(HO) condensation 3 RSiO3/2 + 1.5H2O
RSi(OH)3 +RSiX3 RSiO3/2 +3HX condensation
X= OR', OAc, Cl etc
Figure 2-10. General synthesis of silesquioxanes
The ability to selectively and rationally prepare a silesquioxane system in a manner that affords strict control over structure, functionality and yield is still beyond the capability of current methods. Unlike in all other applications of siloxane polymers, silesquioxane derived from methyltrichlorosilane were not the first to reach commercialization. This is due to the fact the their hydrolysis led to brittle intractable gelled solids. These inorganic-organic hybrids offer a unique set of physical, chemical, and size dependent properties that could not be realized from just ceramics or organic 35 polymers alone. Silsesquioxanes are therefore depicted as bridging the property space
between these two component classes of materials. Many of these silsesquioxanes hybrid materials also exhibit an enhancement in properties such as solubility, thermal and thermomechanical stability, mechanical toughness, optical transparency, gas permeability, dielectric constant, and fire retardancy, to name just a few.
2.6 Backbone substitution and functionalization of polysiloxanes
As has been documented before, organosiloxanes have attracted lot of attention
owing to the potentially useful properties. Some of the Si atom based polymers apart
from polysiloxanes 56 are polysilanes123, polycarbosilanes124, and polycarbosiloxanes.125
Functionalization of siloxanes can be carried out by either by polymerizing the silane monomers (Figure 2-8) and cyclic oligomers 126 after functionalization or by
functionalizing the siloxane polymer. The presence of functional groups such as amino,
amide or carboxyl in some polymers can allow compatibilization of the polymeric phase
domains with inorganic matrices at a sub-microscopic level through hydrogen bonding
with silanol groups. Hydrosilation is one of the most widely used reaction technique to
functionalize silane monomers or siloxane polymers containing Si-H group. The
popularity of hydrosilation technique is due to the fact that if stoichiometry of starting
monomers are nearly exact, this reaction usually proceeds cleanly with a minimum of
side reactions and yields polymers of reasonable high molecular weight.127 Apart from
hydrosilation, another route to synthesize telechelic siloxanes is ring opening
polymerization of cyclic siloxanes in presence of disiloxanes128, equation (2-23). Mostly acid catalyzed cationic ring opening polymerization is used for the above functionalization process.
36 R CH3 CH CH3 R CH3 3 ROP 2-23 Si O + X Si O Si X X Si O Si O Si X n n R CH3 CH3 CH3 R CH3
X = H, Amine, Hydroxyl, Vinyl, Acryl etc
Incorporation of the reactive groups at the end or as side group in polysiloxane backbone increases new avenues of applications. In the current study functionalization of siloxanes with glycidyl epoxide, aliphatic amine and acrylic group will be discussed in detail. Amino siloxanes can be copolymerized with polyurethanes to form block copolymers that are useful in medical applications for contact lens, catheters and medical implants.129 Secondary amine functionalized siloxanes can be reacted with silanols to
generate copolymers with high thermal stability.130 Aminosiloxanes can be used for
curing and forming block copolymers with epoxide siloxanes and epoxide resins. Novel
block copolymers capitalizing on the unique properties of aminosiloxanes include
fluropolymers, peptides, aromatic esters etc. Since fluoropolymers possess a wide
working temperature range and good fuel and chemical resistance, block copolymers with
fluoropolymers can be used as seals. Block copolymers of aminosiloxanes with different
N-carboxy anhydrides are used for applications requiring good oxygen permeability and
antithrombogenecity.131 Aminosiloxanes are also used as mold release agents, adhesive
polyimide films for use in the manufacture of flexible printed boards, primers for rubber-
based automotive components. Epoxide silicone systems are mostly used in UV curable
applications since these systems can combine fast UV cure response with stable release
properties unlike styrene-butadiene rubber, and hot melt adhesives.
37 Certain onium salts as iodonium and sulfonium salts are capable of initiating
photopolymerization. One of the performance advantage inherent to onium salt-Epoxide
siloxane photocurable systems are the non free radical nature of the crosslinking. This
mechanism is not subject to oxygen inhibition, making UV-curable siloxane based
release agents well suited to wide variety of converting operations not requiring nitrogen
blanketing. 60 Heat resistant coatings are becoming popular with introduction of more
sophisticated automotive and aircraft exhaust equipments, smoke stacks, stoves, furnaces,
and incinerators. Conventional organic coatings fail when used at high temperature. The
exact requirements are now met mostly by the use of silicone-based coatings. However,
the silicone-based paints have many intrinsic disadvantages because of the high cost
factor and therefore the search for some alternatives is relevant. An alternative route was
to make paint combinations having major part of silicone resins blended with Epoxide resins. It was apparent that the Epoxide resin properties could be improved by mixing small amounts of silicone resin. One disadvantage of this systems was that the blend was never uniform. One solution to the problem was to functionalize silicones with Epoxide group. Siloxane forms the non-stick, hydrophobic, lubricious component. Epoxide forms component possessing hardness and adhesion.
UV-cured silicone acrylates are very robust systems whose cure is unaffected by
impurities of the substrates. This allows the use of any substrates for siliconizing. High solids, low VOC, and highly weatherable topcoats can be developed from siloxane acrylates. These class of compounds are developed primarily as alternatives to isocyanate systems. Several products employing siloxane acylates systems may be self-curing, one- component or two components. One part siloxane acrylic hybrid system usually includes
38 alkoxysilanes are cured by hydrolysis of alkoxysilanes functionalities. Two component
siloxane acylates are formulated with an aminosilane. The proposed cure mechanism of these two part system is by Michael addition of amine and acrylate functionality and hydrolytic condensation of the silicone resin intermediate.132 The most usual route to
synthesize acrylic functional siloxane133, 134 is by reacting omega- hydroxyalkyl
acrylates/methacrylates with silanol in presence of condensation catalysts such as
tetraisopropyltitanate. An alternative route to acryalted siloxanes is by reacting epoxide
siloxanes with ethylenically unsaturated hydroxylic or carboxylic compounds.135, 136
2.7 Hybrids of siloxanes with other polymers
The reaction of siloxanes with organic polymers under mild chemical conditions has been used for the preparation of polymeric organo-inorganic hybrids. These hybrids can be used in the form of films for membranes or coatings, or as precursors for the preparation of porous materials.137 Binders made of polysiloxane alone are insufficient in
providing the all round properties required for a protective coating. Organic modification
of the polysiloxane is necessary to achieve a balance of film properties, such as adhesion
and flexibility. The formation of hybrids of siloxane with a large range of organic
polymers are possible due to versatility of siloxane chemistry. The initial hybrids of
siloxanes with epoxide and acrylic resins showed excellent performance as protective
coatings. Later combinations of siloxanes with vinyls, alkyds, acetoacetates,
fluoropolymer, elastomeric epoxide and phenol binder systems have also been developed.
Hybrids of siloxanes with alkyds will be discussed in details. Self-curing siloxane hybrids
show potential not only to improve the current polysiloxane coatings but also expand the
39 usage in areas such as tanklinings, adhesives, flooring systems, laminates and single pack
high solids, high build maintenance coatings, further expanding the performance limit.
Alkyd resins are widely used in applications as enamels, lacquers, textile finishes,
protective coatings and films etc. The wide usage of this class of resins is due to good
adhesion of these resins to various substrates, high gloss and reasonable cost.138 Some of the disadvantages of alkyd based paints are the lack of weathering resistance, low resistance to degradation by hydrolysis and UV radiation. The exterior durability of coatings based on silicones were also found to be less than certain other coating compositions, such as two part urethane finishes. One approach to these drawbacks was to use silicone-alkyd hybrid resins. Silicone-alkyd combinations are mostly used as lacquer resins and as protective coating in electric industry. To prepare silicone alkyd hybrid with ≡−−≡Si O C bonds, the silicone component having ≡−Si OH groups are mixed with alkyd resin, oil modified alkyd resins or linear polyester and heated.139, 140
Since homocondensation of ≡ Si− OH takes place readily, linkage of alkyd with silicone is loose in the end product. Homocondensation reaction can be suppressed, and yield of heteropolymer can be improved by using catalysts.141 More homogeneous structures can
be obtained if the organosilicon component is reacted with monomer form of alkyd which
is still in the process of alkyd preparation.142 Thus polyol may be reacted with a silicone component such as organoalkoxysilicone, and the products can be reacted with organic dicarboxylic acids, dialkylesters or anhydrides. A modification of this process is also known, in which polyester prepolymer of polyols and polybasic organic acid is reacted with silicon component.143 By employing numerous modifications to the basic chemistry
40 of coupling silicone and alkyd component, a multitude of hybrid resins has been possible.
Since silicone-alkyd coatings have greater flexibility, hardness and thermal stability than
alkyds, and the lack of high thermal and oxidative resistance of the unmodified silicones,
so the hybrids must be considered intermediate in properties.144
2.8 Curing of siloxanes
Curing of siloxanes can be broadly classified as radiation curing, thermal curing,
moisture curing and addition curing. Radiation curing can further be distinguished as
ultraviolet (UV) and electron beam (EB) curing. Since siloxanes are used in multitude of
applications, use of more than one curing mechanism in a multi-component system offer
advantages over use of one mechanism. An example of a multi-component system
applying silicones is an electronic potting system. In electronic applications, siloxanes are
used for the flexibility, low temperature stability, and good electrical properties.
Elastomeric or gell type materials are used to protect these electronic parts. The use of
thermal curing may cause problems with sensitive electronic components. Conventional
moisture curing needs a long duration for complete cure. Radiation curing is fast and one
component but does not give the elastomeric properties that are needed for this
application. Besides shadow areas are not well cured by the UV exposure. Hence a
combination of radiation and moisture cure offers solution for this application. 145
Polyorganosiloxanes can be developed which utilizes a combination of curing mechanism in order of radiaton, thermal and moisture cure. Drawback of one curing techniques can be eliminated by other techniques.
41 2.8.1 Radiation curing
Radiation curing can be divided into UV-curable and electron beam coatings.
Radiation curing is due to excitation of photoinitiator by absorption of electromagnetic radiation. An electron beam curing is due to ionization and excitation of resin by high energy electrons.
2.8.1.1 UV initiated cross-linking
The applications of photosensitive resins that polymerize readily under intense
UV radiation are increasing rapidly in various sectors, mainly in coating industry,
microelectronics, and graphic arts. Photosensitive resins usually consist of
multifunctional monomers and/or oligomers, and a photoinitiator that yields a reactive
radical, cationic or anionic species upon UV-exposure. Most commonly, medium
pressure mercury vapor lamps are used as a source of high intensity UV-radiation.
Tubular electrode lamps emit radiation in all directions lowering its efficiency. The
elliptical reflectors can be housed over the bulb to focus the radiation intensity directly
onto the surface being cured. A very short cure time even in ambient temperature and
minimum energy requirement are the main advantages of UV- curing. The disadvantage
of UV-curing is that UV-radiation is hazardous and may burn the coatings. UV-curing
also requires the distance of the lamp to all parts of the substrate to be same. As a result
UV-curing cannot be used to cure complex shaped parts. Since thermal energy is
produced the lamp must be air and water cooled. Due to the toxicity of ozone the, the UV
chamber unit must be ventilated.
The two types of crosslinking mechanisms catalyzed by UV radiation are free
radical and cationic polymerizations. The rate of crosslinking is directly proportional to
42 concentration of photo-initiator. A photo-initiators convert light energy to chemical energy by absorbing photons and generating free radicals or cations. Rate of initiation and penetration of the incident light depend on the type, absorption wavelength, and efficiency of the photoinitiator. The different route of deactivation of the excited photoinitiator can be demonstrated by Jablonsky diagram 146, Figure 2-11. The triplet state shown is formed through intersystem crossing from the excited singlet state. In the triplet state electrons have the same rotation.
H-donor Excited Singlet State Intersystem Crossing Triplet State radicals CLEAVAGE Crosslinked Polymer Internal Conversion monomer QUENCHING (O2, monomer)
Photoinitiator Ground State
Figure 2-11. Deactivation pathways of the excited photoinitiator
The formation of triplet state by direct absorption of a photon by photo-initiator ground state is a forbidden transition. An advantage of photo-curing over thermal curing is that rate of initiation in UV-curing can be easily adjusted by changing the light intensity. The optimum photo-initiator concentration is dependant upon the thickness of the film, thicker films requiring a lower optimum concentration. However since photo-initiators absorb UV-light, therefore prohibits the penetration of light to lower parts. Above an optimum initiator concentration, the cure will not be complete and overall film properties will decline. The half-life of the active species generated in cationic curing are
43 significantly longer compared to free radicals. Migration of these species is restricted by
diffusional constraints as cross-linking progresses. The coating components as pigments,
also absorbs and scatter UV-radiation and hinder UV absorption by photo-initiator. In
practice, liquid formulations containing reactive monomers or polymers, a photo-initiator,
and other components, for example fillers, pigments, or additives are coated onto a
substrate which may be glass, plastic, paper, leather, wood, or metal. The coating is then
exposed to UV-radiation of an appropriate wavelength to form a solid coating.
2.8.1.1.1 Free radical initiated UV-curing
Free radical UV curing can be initiated either due to unimolecular photo-cleavge or bimolecular hydrogen abstraction. The bond dissociation energy for unimolecular
photo-initiatiors is lower than excitation energies of the reactive excited state. Benzoin
ethers were the first commercially used class of unimolecular photoinitiators and it
cleaves into benzoyl radical and benzyl ether radical by UV absorption, Figure 2-12.
O OR O OR hv C C C. + C. H H
Figure 2-12. Free radical generation from unimolecular photoinitiators
In bimolecular initiation, the photoexcited initiators such as benzophenone, thioxane or other related dialkyl ketones donot cleave to give free radicals but can abstract hydrogen from hydrogen donors to yield excited complex that initiate polymerization as depicted in Figure 2-13. The main advantage of bimolecular initiator in the reaction with amine is that it reduces oxygen inhibition 147 unlike unimolecular
44 initiators. The limitation of bimolecular initiation is that the excited state of the initiators are more rapidly quenched by oxygen as well as acrylic or vinyl monomers.
ΟΗ O O N CH2 R hv . . C C C + N CH R *
Figure 2-13. Free radical generation from bimolecular photoinitiators
(Meth)acrylated monomers and oligomers are the most frequently used resins for
UV-free radical curing. This is due to higher reactivity and lower volatility of UV-cure
coatings based on acryalted oligomers. Acrylated resins are preferred over methacrylated
ones due to higher cure rates at room temperature and lower oxygen inhibition. The main
types of acrylate oligomers are epoxide acrylates, polyether acrylates, urethane acrylates,
polyester acrylates, and silicone acrylates. Low modulus, higher elascticity films are
generally obtained with aliphatic monomers whereas aromatic ones results in hard and
glassy networks. Siloxane acrylates have been prepared through four different
approaches, condensation reactions, polymerization of silane acrylates, displacement
reactions148, and hydrosilation reaction.149 Acrylate monomers and polymers are usually
introduced as a reactive diluent to reduce viscosity and increase crosslink density.
2.8.1.1.2 Cationic UV curing
A reaction route of cationic UV curing is summarized as below.150
UV Absorption Proteolysis LIGHT INITIATION LEWIS/ BRONSTED ACID
Initiation Propagation CATIONIC SPECIES POLYMERIZATION
Figure 2-14. The reaction route for cationic UV curing
45 In cationic photo-polymerizations, initiators 151 generally onium salts can yield
strong protic acids of the corresponding counter anions as well as radical cations as
shown in Figure 2-15. A key feature of these photo-initiators is that the anions produced
upon UV-exposure do not terminate the cross-linking process. Therefore, a majority of
− the cationic photo-initiators are salts of complex, non-nucleophilic anions such as BF4 ,
− − − PF6 , AsF6 , or SbF6 . Anions, such as halides, are not suitable counterions for cross-
linking since the intermediate oxonium or carbonium ions may get trapped and yield a
irreversible compound as alkyl halide. Complex anions do not form stable alkyls and
may act as chain transfer reagents. Employing such photo-initiators are beneficial since
no termination counterions occurs and the extent of polymerization is driven
thermodynamically by concentrations and temperature. The decrease in volume resulting
from formation of polymers from monomers is partially offset by an increase in volume
from ring opening. Adhesion of cationic cure coatings can be enhanced by surface
treatment of the substrate.152
+ ...... + Ph3S Ph2S Ph Ph SPh + H+
+ ortho and meta isomers
...... + . Ph2S Ph + . Ph2S + Ph
Figure 2-15. Usage of onium salts for free radical photoinitiation
The advantages of UV-induced cationic curing are that, it is rapid, and not
inhibited by oxygen (unlike free radical induced cure), and a variety of polymers can be
prepared. The disadvantages are that, the curing mechanism can be inhibited by bases
and the final cured product may contain acids.
46 2.8.1.2 Electron beam (EB) curing
EB curing can be used mainly to polymerize acrylate coatings.153 The high energy
. electron cause direct excitation of the coating resins(P) ionization into radical cation (P+ ) and secondary electron. The combination of radical cation with secondary electron yields an additional excited state resin (P*). The P* undergoes homolytic bond cleavage to form
free radicals, which initiates the polymerization of acrylate resins. The details of
excitation of resins by EB is shown in Figure 2-16.
P + EB P* + P+ . + e- P+ . + e- P*
* I . P
Figure 2-16. Excitation of resins by e-beam
Since the polymerization reaction is oxygen inhibited, it is essential to do EB
curing in inert atmosphere. The advantages of EB curing over UV curing are that, no
photoinitiator are needed and pigments do not interfere with curing. The advantage of the
ability to cure pigmented system is real but have limited importance in coatings since the
flow problem is not removed by EB curing.154
2.8.2 Thermally induced cross-linking
Almost all the thermal curing mechanisms employ two part mechanism. The
cross-linking reaction can be catalyzed by free radical initiators for e.g. azo and peroxide,
transition metals for e.g. tin, platinum and rhodium, or acid/base catalyzed for e.g. curing
of epoxy-amine, anhydride-amine. Generation of free radicals depend on the minimum
energy needed to break the covalent bond i.e the bond dissociation energy (D). Thermal
47 initiators having D in the range of 120-170 kJ/mol are preferable. If the D value is out of
this range the free radical generation is either too fast or too slow. Compounds with O-O,
S-S, O-N single covalent bonds are ideal.155 The most widely used thermal initiators are
peroxides due to availability, stability, efficiency, and rate of dissociation. Peroxides
generate radicals at much faster rate in presence of amine due to decreased rate of oxygen
inhibition. Free radicals can also be generated using an electron transfer mechanism using
redox initiators. Organic peroxides in conjunction with ferrous, cobalt, copper,
manganese or chromium are capable of producing radicals at ambient temperature.
2+ ROOR + Fe RO- . 3+ + RO + Fe
Siloxanes polymers having vinyl, allyl, acrylic, methacrylate functionality can be
cured both by UV and thermal curing. Thermal latency of these siloxanes are achevied by
compounding with free radical initiators as peroxide curative. These curatives have good
stability at room temperature but becomes reactive at processing temperature which is
100-170 °C. The reaction in case of vinyl functionalized siloxanes consists of both vinyl
. addition as in homopolymerization as well as addition of SiCH 2 to vinyl unsaturation forming two to three carbon linkages, Figure 2-17.
R R R
SiO SiO Si ROOR
R R R Δ x
O
R R R CH CH CH2 R R 2 SiO SiO SiO Si SiO Si CH R RO R R CH2 R R CH CH x 2 CH CH SiO 2 SiO R
Figure 2-17. Peroxide initiated vinyl polymerization
48 Epoxide amine curing is one of the most important thermal curable systems.
Depending upon the reactivity of the epoxide and amine groups, the reaction is either
non-catalytic, catalytic or autocatalytic, Figure 2-18.
H H H H R N R N.. .. R' R' O O H O
R'' non catalytic catalytic autocatalytic
H H H H R N.. R N .. H H R' R' .N. O CH O 2 CH R' H O O H O R' R''
CH R' R H2C N .. O H H2C CH R' H O
Figure 2-18. Mechanism for epoxide amine curing
The concept of isothermal time temperature diagram is a useful tool under thermal cure conditions.156 The diagram shows the time to reach certain event during isothermal
cure versus cure temperature. Mostly in industries thermal curing is not used alone.
Automotive industries which are one of the biggest consumers of coatings generally
combine UV with thermal curing process to get a better cure. As a result automotive paints have thermal initiators and photoinitiators. In thermal curing, thermoset polymers 49 may undergo gelation, vitrification or devitrification. Gelation is formation of infinite network, vitrification occurs when Tg rises to cure temperature (Tcure) of the coating
components and devitrification occurs when Tg falls below Tcure due to degradation.
Curing reactions proceed beyond vitrification causing Tg to be higher than the cure
temperature (Tg > Tcure). In thermal curing the rate of heating depends on the substrate
thickness. To achieve good coating properties, some minimum time at a particular
temperature is required but over baking can lid to excessive cross-linking. There is a cure
window for any baked coating which sets the time and temperature range to obtain
satisfactory properties. In conventional thermal coatings the cure window is relatively
large i.e. it makes little difference if the baking temperature is off by ± 10 oC or by ±
20%.157
2.8.3 Moisture induced cross-linking
In moisture curing or RTV (room temperature vulcanization), the monomers or
prepolymeric materials used are highly branched in nature and polymerize via reaction
with ambient moisture.154
OZ OZ OZ H2O ROSi SiO SiOR SiOH + ZOH x OZ OZ OZ
OSi
Si SiO SiOZ + ZOH x OSi OSi
OSi
Z = CH3COO-, R'CONH-, R'R''C=NO-, R'O-
Figure 2-19. A general moisture curing mechanism of silicones
50 The nature and amount of branching control the crosslink density, and hence physical and
mechanical properties of the polymer. If rate of curing is slow, the rate depends on the
rate of diffusion of moisture through the polymer.
Depending on the end groups and propagation of moisture from surface down, it
takes a long time to develop the final properties. Very reactive endgroups such as, acetoxysilyl, amidosilyl groups donot need any catalysts. Mechanism for moisture curing of siloxanes is shown in Figure 2-19. R’, R’’ in Figure 2-19 are generally alkyl groups.
With increase in steric hindrance of the these alkyl groups, the rate of moisture curing
decreases. There are two main concerns related to moisture curable systems. One is
outgassing of volatile cure products generated through network formation, such as
alcohol and acid. The other is evolution of acid in presence of moisture, limits the
applicability of this curing systems to well ventilated areas and corrosion resistant
substances. Non-volatile substances are not the remedy because there is a probability of
phase separation in the polymer matrix causing detrimental results. Systems with leaving
groups having low volatility can partially solve this problem.
2.8.4 Addition curing
Siloxane elastomers produced from a multi-pack system are usually based on
hydrosilation-addition curing. 154 Unlike condensation crossliking where Si-O-Si bonds
are formed, equation (2-22), addition curing by hydrosilation usually forms Si-C linkages
between two siloxane polymers. Addition across the unsaturated bond may also occur to
form Si-C-Si linkage, but due to steric hindrance involved, such addition yields minority
by-products. Though different transition metals compounds for e.g. Co, Rh, Pt, Pd etc
have been used, Pt is the metal of choice. The thermal latency in the pre-polymeric
51 formulations is difficult to obtain. A possible solution was to use a ligand which can couple with the hydrosilation catalyst by thermally reversible complexation. On warming the formulation, the complex with ligand is exchanged for reactive hydridosiloxanes and vinyl, allyl or acetylene functionalized siloxanes, Figure 2-20.
H R OH R + PtLa bL + La-b Pt R' H Δ O R' H
Active catalyst Inactive catalyst
Figure 2-20. Complexation of hydrosilation catalyst
52 CHAPTER III
EXPERIMENTAL
3.1 Materials
Octamethylcyclotetrasiloxane (D4), dichlorosilane, dimethyldichlorosilane, and
1,1,3,3- tetramethyldisiloxane were purchased from Gelest, Inc (Morrisville, PA). The potassium hydroxide, methanol (99.93 %), and methylene chloride (99.9 %) was purchased from Fischer Scientific. Magnesium sulfate (98 %), perchloric acid, xylene, glacial acetic acid (99.7 %), epicure 9551, sodium bicarbonate, sodium sulfate, dichloromethane, and toluene were purchased from EMD chemicals (Gibbstown, NJ).
Allyl glycidylether (99 %) was obtained from TCI America. Allylamine (98 %), trifluoroacetic acid (99 %), di-t-butyldicarbonate (97 %), phenolphthalein, deuterium oxide (99.96 %), chloroform-D (99.8 %), platinum(0)- 1,3-divinyl -1,1,3,3 –tetramethyl disiloxane 3 wt % solution in xylenes (Karstedt catalyst), crystal violet indicator, amberlyst 15 ion- exchange resin, methacrylic acid (99 %), phenothiazine (98+ %), hexanediol dimethacrylate (HDDM) (98 %), cyclopentene (99+ %), cyclohexene (99+
%), phthalic anhydride (99 %), pentaerythritol (98 %), lithium hydroxide monohydrate
(98+ %), and tris(triphenylphosphine)rhodium(I) chloride (Wilkinson’s catalyst)were purchased from Aldrich (Milwaukee, WI). Chromium (III) acetate was purchased from
Strem Chemicals. DGEBA (EPON Resin 828) was purchased from Miller-Stephenson
Chemical Company (Connecticut, OH). Heloxy Modifier 48 and epicure 9551 were
53 purchased Hexion Speciality Chemical. Irgacure 250 (97 %), daracure 1173 (97 %) was obtained from Ciba Speciality Chemicals, NY. 5 wt % Cobalt hydrocure II, 5 wt % calcium and 12 wt% zirconium hydro-cem were purchased from OMG Americas, Inc.
(Westlake, OH). Aluminum panels (type A, alloy) (3 x 6 in) were obtained from Q-panel lab products. All the above chemicals were used as received with no further purification.
The chemical structure of photoinitiators, reactive diluents, and functional siloxane are shown in Figure 3-1.
O
CH2 O CH2 CH CH2 O
CH3 CH C 2 CH2 OCH2 CH CH2 O CH O CH CH 2 2 CH2
HELOXY Modifier 48 (Trimethylol propane triglycidyl ether)
+ + I O - OCH3 PF6 F F C OH - C P FF F F OCH3
Irgacure 250 Daracure 1173
Figure 3-1. Structure of reactive diluents and photoinitiators used
3.2 Synthesis
The synthesis of dicyclopentyldichlorosilane and dicyclohexyldichlorosilane monomers are as follows:
54 3.2.1 Synthesis of dicycloaliphatic dichlorosilane
The synthetic pathway have been reported previously by Soucek et al.61 A clean
and dry stainless steel PAAR Reactor was cooled via dry ice/acetone bath and charged
with cyclopentene (204 g, 3 mol) or cyclohexene (246 g, 3 mol) and Karstedt catalyst
(0.2 mmol) with a nitrogen purge. The temperature was maintained <-10 °C using liquid
nitrogen/acetone bath. Dichlorosilane gas (150 g, 1.5 mol) was condensed in a round bottom flask and then transferred to the reactor through a canula. The inlet valve of the
reactor was sealed and the pressure was increased to about 2 MPa. The bomb was allowed to warm to room temperature, and then heated for 48 h at 180 oC. The reactor
was allowed to cool to ambient temperature to produce a clear yellow to light brown
liquid. Any unreacted cyclopentene or cyclohexene was removed in vaccuo.
The spectra for dicyclopentyl dichlorosilane are as follows: FTIR (cm-1): 785-820
1 (Si-C), 2800 (-CH2- ), 3000 (-CH3-); H NMR (CDCl3) δ (ppm): 0.85 – 1.9 ( Si – CH),
29 1.2-1.7 (-CH2-), 1.8-2.5 (-CH-); and Si NMR (CDCl3) δ (ppm): -10.5.
The spectra for dicyclohexyl dichlorosilane are as follows: FTIR (cm-1): 790 - 830
1 (Si-C), 2800 (-CH2- ), 3000 (-CH3-); H NMR (CDCl3) δ (ppm): 1.1 – 2.05 ( Si – CH),
29 1.0- 1.25 (-CH2-), 1.6-2.0 (-CH-); and Si NMR (CDCl3) δ (ppm): -12.0.
3.2.2 Synthesis of cyclic oligomer of polydicycloaliphaticsiloxane
To a three neck round bottom flask, equipped with a reflux condenser, nitrogen inlet/outlet, and a dropping funnel, aqueous KOH solution was added. A solution of cyclopentyl dichlorosilane (15 g, 0.06 mol) or cyclohexyl dichlorosilane (15.92 g, 0.06 mol) in diethyl ether was taken in the above round bottom flask and aqueous KOH solution was added via dropping funnel and the solution was allowed to stir for several
55 minutes at 35 °C. This was continued until hydrochloric acid evolution stops. The
aqueous and organic layer were separated using separatory funnel after removing the
precipitated KCl by decanting the liquid reaction mixture to a separate glass beaker.
From the organic layer, diethyl ether was removed under vacuum yield a clear yellow
oily liquid, 10 g (66.67 % yield) for cyclopentyl derivative and 8.2 g (54.67 % yield) for
cyclohexyl derivative. Number average molecular weight of 1200 dalton was obtained by
gel permeation chromatography.
The spectra for cyclic oligomer of polydicyclopentylsiloxane are as follows: FTIR
-1 1 (cm ): 1010- 1147 (Si-O-Si), 2848-2942 (-CH2-), 2929-2980 (-CH3 -); H NMR (CDCl3)
29 δ (ppm): 0.01 ( Si – CH), 1.25-1.7 (-CH2-), 1.8-2.5 (-CH-); and Si NMR (CDCl3) δ
(ppm): -24.69 ( small cyclics, n= 3-5), -30.64 to -31.56 ppm (larger cyclics, n => 5).
The spectra for cyclic oligomer of polydicyclohexylsiloxane are as follows: FTIR
-1 1 (cm ): 1016- 1122 (Si-O-Si), 2835-2887 (-CH2-), 2895-2990 (-CH3 -); H NMR (CDCl3)
29 δ (ppm): -0.05 ( Si – CH3), 1.0- 1.25 (-CH2-), 1.6-2.0 (-CH-); and Si NMR (CDCl3) δ
(ppm): -19.19 ( small cyclics), -30.55 ppm (larger cyclics).
3.2.3 Activation of ion exchange resin
Amberlyst-15, ion exchange resin is activated before use.158 It was done by
placing 100g of resin in a 1000 ml Erlenmeyer flask with 500 ml of THF and stirring it
for 18 h. Then the resin is filtered off, air dried and placed in a 1000 ml Erlenmeyer flask with 400 ml of 0.1 N HCl. This mixture was stirred for 3 h and then acid was decanted.
The resin was washed with distilled water until a neutral pH is obtained, and then dried.
56 3.2.4 Synthesis of hydride terminated polydimethylsiloxane
The cyclic oligomers used for synthesizing hydride terminated poly
(dimethylsiloxane) was octamethylcyclotetrasiloxane. In a three neck round bottom flask,
cyclic oligomer ( 30 g, 0.1 mol), 1,1,3,3-tetramethyldisiloxane (7 g, 0.05 mol ) and
Amberlyst-15 ion exchange resin (20 wt %) were taken. The flask was fitted with a
nitrogen inlet/outlet valve, reflux condenser and a thermometer. The reaction mixture was
stirred at 60 °C under nitrogen atmosphere for 15 h. Vacuum filtration was performed on
the viscous H-terminated polymer to remove low molecular weight oligomers and unreacted starting materials. The spectra for hydride terminated polydimethylsiloxane are
-1 as follows: FTIR (cm ): 989 - 1147 (Si-O-Si), 2912 (-CH2 -), 2966 (-CH3 -), 2144 (-Si-
1 H); H NMR (CDCl3) δ (ppm): 0.01- 0.2 (–Si – CH3), 4.71 (–Si–H).
3.2.5 Synthesis of hydride terminated polydicycloaliphaticsiloxane
Cyclic polydicyclopentylsiloxane (30 g, 0.03 mol) or cyclic
polydicyclohexylsiloxane ( 30 g 0.029 mol) were used to prepare hydride terminated
polydicyclopentylsiloxane or hydride terminated polydicyclohexylsiloxane respectively.
In a three neck round bottom flask, cyclic oligomer, 1,1,3,3-tetramethyldisiloxane (2.1 g,
0.015 mol ) and Amberlyst-15 ion exchange resin (20 wt%) were taken. The flask was
fitted with a nitrogen inlet/outlet valve, reflux condenser and a thermometer. The reaction
mixture was stirred at 60 °C under nitrogen atmosphere for 15 h. Vacuum filtration was
performed on the viscous H-terminated polymer to remove low molecular weight
oligomers and unreacted starting materials.
57 The spectra for hydride functional polydicyclopentylsiloxane are as follows: FTIR
-1 1 (cm ): 1010- 1126 (Si-O-Si), 2862 (-CH2 -), 2960 (-CH3 -), 2135 (-Si-H); H NMR
(CDCl3) δ (ppm): -0.01 to 0.20 ( Si – CH), 1.43- 1.7 (-CH2-), 4.71 (-Si-H).
The spectra for hydride functional polydicyclohexylsiloxane are as follows: FTIR
-1 1 (cm ): 1020- 1114 (Si-O-Si), 2859 (-CH2 -), 2916 (-CH3 -), 2133 (-Si-H); H NMR
(CDCl3) δ (ppm): 0.01-0.25 ( Si – CH), 1.2- 1.36 (-CH2-), 4.73 (-Si-H).
3.2.6 Synthesis of glycidyl epoxide terminated PDMS, PDPS, and PDHS
Hydride terminated PDMS, PDPS or PDPS were taken in a round bottom flask
equipped with a reflux condenser, nitrogen inlet/outlet and a thermometer. Allyl-glycidyl
ether (5.7 g, 0.05 mol) and Karstedt catalyst (0.04 g) were added to it. The reaction was
held at 60 °C in an oil bath and mechanically stirred under nitrogen. Unreacted starting
materials were removed under vacuum. Glycidyl epoxide functionalized polysiloxanes
were confirmed through 1H NMR, FT-IR and epoxide equivalent weight (EEW) titration.
The epoxy equivalent weight for glycidyl epoxide terminated PDMS, PDPS, and PDHS
were 203, 796, and 817, respectively. The yield of hydrosilation reaction for PDMS,
PDPS and PDHS were ~ 100 %, ~ 91 %, and ~ 85 %, respectively.
The spectra for glycidyl epoxide functional polydimethylsiloxane are as follows:
-1 1 FTIR (cm ): 997- 1136 (Si-O-Si), 916 (epoxide), 2883 (-CH2-), 2964 (-CH3 -); H-NMR
13 δ (ppm): 0.08 -0.11 ( Si-CH3), 2.6- 2.82 (epoxide CH2 ), 3.37-3.44 (epoxide CH); C-
29 NMR δ (ppm): 44.03 (CH2 of epoxide), 50.74 (CH of epoxide); and Si-NMR δ (ppm): -
21.6 to -22.42 ( Si-O-Si ), 7.53 (Si-CH3).
The spectra for glycidyl epoxide functional polydicyclopentylsiloxane are as
-1 follows: FTIR (cm ): 1004- 1118 (Si-O-Si), 910 (epoxide), 2870 (-CH2-), 2960 (-CH3 -); 58 1 13 H-NMR δ (ppm): 0.19 ( Si-CH), 2.57- 2.8 (epoxide CH2 ), 3.4-3.67 (epoxide CH); C-
29 NMR δ (ppm): 42.85 (CH2 of epoxide), 46.73 (CH of epoxide); and Si-NMR δ (ppm):
-22.17 to –24.89 ( CH-Si-H, DH ), -19.87 to -20.86 (CH- Si- CH), 6.1-8.02 (M,
H (CH3)2(CH2)(O) Si), -11.15(M , (CH3)(CH2)(H)(O) Si)).
The spectra for glycidyl epoxide functional polydicyclohexylsiloxane are as
-1 follows: FTIR (cm ): 1010- 1130 (Si-O-Si), 898-914 (epoxide), 2860 (-CH2-), 2937 (-
1 CH3 -); H-NMR δ (ppm): -0.04-0.14 ( Si-CH3), 2.53- 2.79 (epoxide CH2 ), 3.11-3.48
13 (epoxide CH); C-NMR δ (ppm): 44.36 (CH2 of epoxide), 51.05 (CH of epoxide); and
29Si-NMR δ (ppm): -20.69 to -24.15 ( CH-Si-H, DH ), -18.39 (CH- Si- CH), 6.05-7.69
H (M, (CH3)2(CH2)(O) Si), -11.48(M , (CH3)(CH2)(H)(O) Si)).
3.2.7 Epoxide equivalent weight determination (EEW)159
Epoxide equivalent weight was determined according to ASTM D 1652-97.
Perchloric acid (0.1 N) was prepared by adding 13 ml of 60 % perchloric acid and 50 ml
of acetic anhydride in 250 ml of glacial acetic acid and thoroughly mixing it. It was
allowed to stand for atleast 8 h. Tetraethylammonium bromide solution was made by
dissolving 100g of tetraethylammonium bromide in 400 ml of glacial acetic acid at room
temperature. Diglycidyl Ether of Bisphenol-A (DGEBA) was used as a standard. 0.25-
0.40 g of DGEBA was weighted out and dissolved in 15 ml of methylene chloride. 10 ml
of tetraethylammonium bromide reagent and 8 drops of crystal violet indicator solution
was added to it and titrated to sharp blue-to-green end with perchloric acid reagent
solution. Perchloric acid reagent factor (F), was calculated as follows:
* EW F = d (3-1) V
59 Wd = Weight of DGEBA standard used (g)
E = Epoxide of the standard used
V = Volume of percloric acid used (ml)
The epoxide equivalent of synthesized epoxide siloxane is measured by titrating it in
similar way using the factor F.
*VF E = (3-2) We
We = weight of epoxide siloxane taken
3.2.8 Synthesis of t-butoxycarbonyl (BOC) protected allylamine
Ice-cooled solution of di-t-butyldicarbonate (6.35 g, 29.1 mmol) was taken in a
two neck round bottom flask, fitted with a condenser and a dropping funnel. The solution
was magnectically stirred and a solution of allylamine ( 2.5 ml, 35.8 mmol ) was added
dropwise. The mixture was stirred overnight at room temperature and then the solvent
was removed. Yield ~ 75 %. The spectra for t-butoxycarbonyl (BOC) protected
-1 allylamine are as follows: FTIR (cm ): 986-1144 (Si-O-Si), 2943 (-CH2-), 2970 (-CH3-),
1724 (C = O ), 3267- 3493( NH).
3.2.9 Synthesis of amine terminated PDMS, PDPS, and PDHS
A 125 ml flask equipped with an addition funnel was charged with hydride
terminated PDMS, PDPS or PDHS and 2 μl of 3 wt % solution of platinum (0)- 1,3-
divinyl -1,1,3,3 –tetramethyl disiloxane in xylenes. BOC protected allylamine was dissolved in THF and slowly added to the reaction mixture. After the completion of
reaction, the temperature was raised to 60 °C. The reaction was monitored through the
transmittance of Si-H band and disappearance of it at 2144 cm-1 suggested the completion
60 of reaction. Solvents and excess BOC-protected allylamine was removed by extraction
with acetonitrile and vacuum filtration. Deprotection of t-BOC group was effected by adding the BOC protected polymer in 60 ml of dichloromethane (DCM) and mixing in ice-cooled waterbath for 15 min. Then 10 ml of TFA was added drop wise and the reaction was continued for 30 min. Then, the ice bath was removed and the reaction was continued for 24 h more at room temperature. The reaction mixture was neutralized with cold super saturated sodium bicarbonate solution till the pH becomes neutral or slightly alkaline. The reaction mixture was extracted with distilled water (2-3x, 300 ml). The organic layer was separated and anhydrous sodium sulfate was added and then filtered off. The DCM was removed to get amino terminated PDMS, PDPS or PDHS.
The spectra for amine functional polydimethylsiloxane are as follows: FTIR (cm-
1 1 ): 986-1144 (Si-O-Si), 2943 (-CH2-), 2970 (-CH3-), 3284-3409 (NH2); H-NMR δ
13 29 (ppm): 0.08 ( Si – CH3), , 5.41 (NH2). C-NMR δ (ppm): 45.65 (CH2- NH2); and Si-
H NMR (CDCl3) δ (ppm): -21.6 to -22.26(Si-O-Si, D ), 7.53(CH3- Si).
The spectra for amine functional polydicyclopentylsiloxane are as follows: FTIR
-1 1 (cm ): 1006-1124 (Si-O-Si), 2868 (-CH2-), 2956 (-CH3-), 3219-3481 (NH2); H-NMR δ
13 (ppm): 0.2-0.36 ( Si – CH), 1.05- 2.01 (-CH2-), 5.49 (NH2). C-NMR δ (ppm): 43.49
29 H (CH2- NH2); and Si-NMR (CDCl3) δ (ppm): -25.0 to -27.5 (CH-Si-H, D ), -19.5 to -
21.0 (CH- Si- CH), 6.5-7.5 ((CH3)(CH2)2(O) Si).
The spectra for amine functional polydicyclohexylsiloxane are as follows: FTIR
-1 1 (cm ): 991-1134 (Si-O-Si), 2927 (-CH2-), 2966 (-CH3-), 3315-3450 (NH2); H-NMR δ
13 (ppm): 0.19-0.34 ( Si – CH), 1.32- 1.86 (-CH2-), 5.43 (NH2). C-NMR δ (ppm): 42.85
61 29 H (CH2- NH2); and Si-NMR (CDCl3) δ (ppm): -21.0 to -22.5 (CH-Si-H, D ), -20.5 (CH-
Si- CH), 7.5 ((CH3)(CH2)2(O) Si).
3.2.10 Synthesis of methacrylate terminated PDMS, PDPS, and PDHS
The glycidyl epoxide functionalized polydimethylsiloxane (PDMS), poly(dicylo pentyl)siloxane (PDPS) and poly(dicyclohexyl)siloxane (PDHS) were used as a starting material to synthesize functionalized acrylated siloxane. The epoxide equivalent of the three epoxides were measured according to ASTM D1652, test B. The reaction between glycidyl epoxide functional siloxane and methacrylic acid was carried out in a 300 ml 3- neck round bottom flask in stoichiometric ratio. Methacrylic acid (0.16 mol, 13.77 g) and epoxide siloxane having 0.16 mol of epoxide group were charged in the reaction vessel.
The details of reagent amount is given in Table 3-1.
Table 3-1 Reagent amounts for methacrylated siloxane synthesis
Siloxane Epoxide Methacrylic Acid Chromium (III) Acetate Phenothiazine Amount EEW 32.59 g, 203.7 13.77 g, 8 mg, 12 mg, R= Methyl 0.031 mol 0.16 mol 0.013 mmol 0.06 mmol R = 31.85 g 3.44 g, 2 mg, 2.56 mg, 796.3 Cyclopentyl 0.025 mol 0.04 mol 0.003 mmol 0.013 mmol
R = 32.71 g 817.6 3.44 g, 2 mg, 2.56 mg, Cyclohexyl 0.021 mol 0.04 mol 0.003 mmol 0.013 mmol
The temperature was kept constant at 100 °C using an oil bath. A chromium (III) acetate catalyst at 0.01 mol % of the reactants was used. Phenothiazine (0.037 mol %) was added as free radical polymerization inhibitor. The reaction was monitored through measurement of acid value, and was continued till the acid value was ≤ 10. The product was characterized through FT-IR, 1H, and 13C-NMR as given in Table 3-2 and Table 3-3.
62 Table 3-2. FTIR assignments of methacrylic functional PDMS, PDPS and PDHS
Methacrylic Methacrylic Methacrylic Units functional PDMS functional PDPS functional PDHS (cm-1) (cm-1) (cm-1) Si-O-Si 997- 1136 1004- 1118 1010- 1130 (C=O)O 1725 1730 1736
CH2 = CH(C=O)O- 1609-1622 1610-1625 1604-1616
Table 3-3. 1H & 13C-NMR Resonance- assignments of Methacrylic Functional PDMS, PDPS and PDHS
Methacrylic functional Methacrylic functional Methacrylic functional PDMS PDPS PDHS
1H-NMR 13C-NMR 1H-NMR 13C-NMR 1H-NMR 13C-NMR (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) Si- CH3 0.03 1.14 0.01 0.12 0.04 0.12
SiMe2 CH2 0.50 14.23 0.46 14.28 0.50 7.89
SiMe2 CH2CH2 1.60 18.37 1.23-1.7 18.08 1.42-1.78 18.08
SiMe2 (CH2)2CH2O 3.32-3.57 70.74 3.28-3.67 71.43 3.30-3.64 65.91
OCH2CH(OH) 3.32-3.57 65.93 3.28-3.67 72.29 3.30-3.64 68.84
OCH2CH(OH) 3.32-3.57 71.6 3.28-3.67 73.68 3.30-3.64 70.40
CH(OH)CH2O(C=O) 3.32-3.57 74.65 3.28-3.67 74.88 3.30-3.64 74.54
(CH3)(C=O) C=CH2 1.94 17.94 1.89 14.45 1.90 14.28
(CH3)(C=O) C=CH2 5.53 & 6.09 127.88 5.57 & 6.12 127.72 5.58 & 6.15 127.55
(CH3)(C=O) C=CH2 - 135.95 - 136.01 - 136.01
(CH3)(C=O) C=CH2 - 173.26 - 172.96 - 172.79
SiCH cyclic - - 0.85 17.62 0.91 18.65
CH2 of cyclic ring - - 1.23- 1.7 25.85-29.30 1.42-1.78 24.81-28.26
3.2.11 Synthesis of linseed oil based short oil, medium oil and long oil alkyds
Long (LOA), medium (MOA) and short (SOA) oil alkyds were synthesized by two stage triglyceride process. The reaction was conducted in 500 ml four-neck round
63 bottom flask equipped with an inert gas inlet, thermometer, reflux condenser, and a mechanical agitator. The transesterification step involved reacting linseed oil and excess of pentaerythritol. These two materials were purged with argon for ~15 min. The amounts of reactants are given in Table 3-4. The mixture was then heated to 120 °C and lithium hydroxide catalyst was introduced into the flask. The temperature was gradually increased to 240 °C. After approximately one hour, a small aliquot was removed and mixed in three parts 95 % ethanol. This was repeated until the resulting solution was clear.
The reaction mixture obtained is cooled to 100 °C and a Dean-Stark trap was introduced to the reaction setup. The reaction was then charged with phthalic anhydride and 100 ml of xylene for use as reflux solvent. The reaction mixture was then slowly heated to 220 °C. After every hour, an aliquot was removed and the acid number was determined. One the acid number is below 10, the reaction was stopped. Acid number determination was based on ASTM D4462-98 with 1M KOH and phenolphthalein indicator. The product was then cooled to room temperature and stored under argon atmosphere. The product were characterized through FTIR and 1H-NMR.
Table 3-4. Reagent amount for long, medium and short oil alkyd synthesis
Linseed Pentaerythritol Phthalic anhydride Lithium hydroxide Oil (LO) (PE) (PA) (LiOH) LOA 100 g 15.11 g 16.44 g 67 mg MOA 100 g 30.22 g 32.88 g 100 mg SOA 50 g 30.26 g 16.46 g 50 mg
3.2.12 Synthesis of block copolymer of PDMS with PDPS (PDMS-block-PDPS) and PDHS(PDMS-block-PDHS)
A 300 ml three neck round bottom flask was taken and was fitted to a condenser and a nitrogen inlet valve. To the flask, 10 g (0.034 mol) of D4 and 30 g (0.034 mol) of cyclic 64 oligomers of PDPS or 35.7 g (0.034 mol) of cyclic oligomers of PDHS and 2 g of activated
amberlyst catalyst was added. The reaction mixture was heated with an oil bath at 60 °C for
15 h to enable ring opening of cyclic oligomers. The reaction mixture was filtered to remove
the amberlyst resin. The final reaction product was a clear viscous fluid.
The spectra for PDMS-block-PDPS are as follows: FTIR (cm-1): 1020-1090 (Si-O-
1 Si), 796 (C-H stretching), 2950 (CH3 asymmetrical stretch). H-NMR δ (ppm): 0.01-0.16
(Si-CH3), 1.34-1.80 (-CH2-), 1.80-1.96 (-CH-).
The spectra for PDMS-block-PDHS are as follows: FTIR of (cm-1): 987-1090 (Si-O-
1 Si), 795 (C-H stretching), 2960 (CH3 asymmetrical stretch). H-NMR δ (ppm): 0.0-0.22 (Si-
CH3), 1.09-1.41 (-CH2-), 1.66-1.93 (-CH-).
3.2.13 Synthesis of homopolymers of PDPS and PDHS
A 500 ml three neck flask was fitted with a condenser and nitrogen inlet valve.
Dimethyl dichlorosilane (0.12 mol, 15.48 g), dicyclopentyl dichlorosilane (0.12 mol, 28.5 g)
or dicyclo- hexyldichlorosilane (0.12 mol, 31.8 g) was charged into the flask. 100 g of
toluene was added to the flask. With stirring at 60 °C, 4.4 gms (0.24 mol) of aqueous KOH
solution was added dropwise over 30 minutes, and then the reaction mixture was aged 6 h at
60 °C. The reaction mixture was then cooled to 30 °C and 20.16g (0.24 mol) of NaHCO3 was
added. Stirring was continued for 3 more hours at 30 °C. The reaction mixture was filtered to
remove inorganic salt present at the flask bottom. The organic layer was washed several
times with distilled water to remove until the pH was neutral. Toluene was then removed
under vacuum to get viscous oily hydroxyl functional PDPS and PDHS.
65 The spectra for homopolymer of PDPS are as follows: FTIR of (cm-1): FTIR (cm-1):
1 1000-1100 (Si-O-Si), 791-868 (C-H stretching). H-NMR δ (ppm): 1.31-1.74 (-CH2-), 1.74-
1.96(-CH-).
The spectra for homopolymer of PDHS are as follows: FTIR of (cm-1): 989-1110 (Si-
1 O-Si), 845 (C-H stretching). H-NMR δ (ppm): 1.08-1.44 (-CH2-), 1.66-1.94(-CH-).
3.2.14 Synthesis of random copolymer of PDMS with PDPS (PDMS-ran-PDPS) and PDHS (PDMS-ran-PDHS)
A 500 ml three neck flask was fitted with a condenser, nitrogen inlet valve, 200 g of
toluene and dimethyl dichlorosilane (0.084 mol, 10.8 g) was taken. Dicyclopentyl
dichlorosilane (0.084 mol, 20 g) or dicyclohexyl dichlorosilane (0.084 mol, 22.3 g) was charged into the flask. With stirring at 60 °C, 3 g (0.17 mol) of aqueous KOH solution was added dropwise over 30 min, and then the reaction mixture was aged 6 h at 60 °C. The reaction mixture was then cooled to 30 °C and 14.1 g (0.17 mol) of NaHCO3 was added.
Stirring was continued for 3 more hours at 30 °C. The reaction mixture was filtered to remove inorganic salt present at the flask bottom. The organic layer was washed several times with distilled water to remove until the pH was neutral. Toluene was then removed under vacuum to obtain viscous oily hydroxyl functional siloxane.
The spectra for PDMS-ran-PDPS are as follows: FTIR of (cm-1): 997-1090 (Si-O-
1 Si), 775-812 (C-H stretching), 2954 (CH3 asymmetrical stretch). H-NMR δ (ppm): 0.03-
0.22 (Si-CH3), 0.88-1.09 (Si-CH2), 1.37-1.70 (-CH2), 1.70-1.89 (-CH-).
The spectra for PDMS- ran -PDHS are as follows: FTIR of (cm-1): 993-1080 (Si-
1 O-Si), 777-818 (C-H stretching), 29650 (CH3 asymmetrical stretch). H-NMR δ (ppm):
0.04-0.25 (Si-CH3), 0.65-0.88 (Si-CH2), 1.13-1.37 (-CH2), 1.65-1.89 (-CH-).
66 3.3 Characterization
The chemical, and thermal characterization of the synthesized polysiloxanes were
performed. Mechanical and coating properties of the cured siloxanes were determined.
3.3.1 Proton, Carbon and Silicon Nuclear Magnetic Resonance (1H NMR, 13C NMR, and 29Si NMR)
Proton and carbon NMR spectra were obtained from a Mercury-300 spectrometer
(Varian), while silicon NMR spectra were recorded on a Gemini-400 spectrometer
(Varian). All NMR samples were prepared in CDCl3 and recorded at room temperature.
Tetramethylsilane was used as internal standard for silicon NMR.
3.3.2 Fourier Transform Infrared Spectroscopy (FT-IR)
Fourier transform infrared spectroscopy was obtained on a Nicolet 380 FTIR
instrument (Thermo Electron Corp).
3.3.3 Gel Permeation Chromatography (GPC)
Gel Permeation Chromatography measurements were performed on a Waters
GPC instrument equipped with a series of six Styragle columns (HR 0.5, HR 1, HR 3,
HR 4, HR 5 and HR 6) calibrated with narrow-MWD polystyrene standards. A
refractive-index (RI) detector (Optilab, Wyatt Technology), and a dual-ultraviolet
absorbance detector (Waters 2487), and a laser light scattering detector (Minidawn,
Wyatt Technology) were used to obtain number average molecular weights ( M n ),
weight average molecular weights ( M w ) and polydispersity index (PDI).
Tetrahydrofuran was used as the eluent at the flow rate of 1 ml/min. All molecular weights are reported relative to polystyrene standards.
67 3.3.4 Differential Scanning Calorimetry (DSC)
DSC measurements were performed on a Thermal Analysis Q2000 with a heating
rate of 10 °C/min and under a nitrogen atmosphere.
3.3.5 Photo-Differential Scanning Calorimetry (PDSC)
Samples were analyzed on a Thermal Analysis Q 1000 DSC equipped with a
photocalorimetric accessory. The photocalorimetric accessory included transfer optic
cables to produce UV light of varying intensity and a monochromatic filter to produce
light at a specific wavelength. The initiation light source was a 100 W mercury arc lamp.
The samples were placed in uncovered hermetic aluminum DSC pans and polymerization reactions were run isothermally at various temperatures and exposure times, to regulate the total heat released during the polymerization.
The rate of propagation (Rp) is directly proportional to the rate at which heat is
released from the reaction. As a result, the area of the DSC exotherm can be used in
conjunction with other sample information to quantify the rate of polymerization. The
rate of photopolymerization was given as:
Rp = ((Q/s) · M) / (n · ΔHR · m) (3-3)
where (Q/s) is the heat flow per second released during the reaction in J/s, M is the molar
mass of the reacting species, n is the average number of epoxide groups per polymer
chain, and m is the mass of the sample.
3.3.6 Dynamic Mechanical Thermal Analysis (DMTA)
The viscoelastic properties were measured on a dynamic mechanical thermal
analyzer (DMTA V, Rheomtrics Scientific, Piscataway, NJ). Compression mode was
used at the frequency of 1 Hz and a heating rate of 2 oC/ min over a range of - 160 to 150
68 oC. The testing conditions and methodology were as per ASTM D 4065-95. A minimum
preload force of 200 mN was applied by the instrument. For each formulation, there were
4 replicates, and the average values are reported. The crosslink density (υe ) of the films
was obtained through the elastic modulus in the rubbery plateau region. The relationship
between rubbery plateau modulus and crosslink density is given by equation (3-4):
' Emin υ = (3-4) e 3RT
' where υe is the crosslink density of elastically effective network chains, Emin is the
minimum value of the storage modulus (Pa) above Tg. R is the gas constant (J/K moL)
and T is absolute temperature ( >> TT g ) in Kelvin. At the temperature much below Tg, loss modulus ( E ’’) is very low so modulus ( E ) is approximately equal to storage
modulus ( E ’).
3.3.7 Oxygen permeation analysis
Oxygen permeability were measured using Model 8001 Oxygen Permeation
Analyzer (Illinois Instruments, Inc). This analysis is performed to provide accurate
measurements of oxygen transmission rates (OTR) through flat films and packages.
Three replicates were tested for each sample. Flat film samples were clamped in a
diffusion chamber and pure O2 was introduced into the upper half of chamber while an oxygen free carrier gas flows through the lower half. 160 The schematic representation of the chamber cross-section is shown in Figure 3-2. Molecules of oxygen diffusing through the film into the lower chamber are conveyed to the sensor by the carrier gas. This allows
a direct measurement of the oxygen without using complex extrapolations. The OTR rate
of the test film is displayed as either as cc/100 in2/ day or cc/m2 /day. 69
O2 in O2 out
N2 in N2 /O2 out
Figure 3-2. Sample chamber cross-section
3.3.8 X-Ray diffraction
Wide-angle X-ray diffraction (WAXD) patterns of cured specimens were generated using Bruker AXS D8 Discover X-ray diffractometer.
3.3.9 Fracture toughness
The fracture toughness measurements (ASTM D5045-96) of the cured polymeric films were performed to determine how a polymeric film will perform after it has been damaged. The sample, crack geometry, and stress distribution along the crack tip are shown in Figure 3-3.
F
σy
w
y σx σx b a r τxy
σz crack σy θ x
F z (a) (b)
Figure 3-3. (a) A thin film with single edge notch in tension (b) Stress distribution in the vicinity of a crack tip.
70 The stresses at the crack tip in cylindrical and Cartesian coordinates are given in equation 3-5 to 3-7:161
K θ θθ3 σ =+cos (1 sin sin ) (3-5) y 2π r 222
K θ θθ3 σ =−cos (1 sin sin ) (3-6) x 2π r 222
K θ θθ3 τ = (sin cos cos ) (3-7) xy 2π r 22 2 where the origin for the cartesian (x, y, z) is located at the crack tip, and the polymeric film and the crack are in x-y plane. r is the distance from the crack tip to the point in film under consideration. θ is the angle that r makes with x axis.
The stress field in the vicinity of crack tip is identical for all loadings, and reflected through stress intensity factor (K). The critical value of K is known as fracture toughness of the material (KIC). The stress intensity factor is a function of stress (σ), and crack length (a), expressed as: 162, 163
KY= σ a (3-8) where Y is a geometric configuration factor that accounts for the load application and crack location. For rectangular specimens with single edge notch geometry, the geometric configuration factor and the stress at the crack tip are given by equation 3-9, and 3-10 respectively:
1/2 ⎡ 2wa⎛⎞π ⎤ Y = ⎢3.94( ) tan ⎜⎟⎥ (3-9) ⎣ π aw⎝⎠2 ⎦
F σ = (3-10) ()wab−
71 where w, a, b, and F are sample width, crack length, sample thickness, and the force at which the crack propagates. Hence, the fracture toughness was given by equation 3-11:
1/2 ⎡⎤2wa⎛⎞π F KaIC = ⎢⎥3.94( ) tan ⎜⎟ (3-11) ⎣⎦π awwab⎝⎠2()−
The critical energy release rate (GIC) was calculated from KIC based on linear elastic fracture mechanics assumptions, i.e. the sample is isotropic and linear elastic. The critical energy release rate is expressed as:
K 2 G = IC (3-12) IC E where E is the tensile modulus of the sample. Six replicates were tested for each sample.
The samples dimensions were 60 mm x 15 mm x 0.09-0.14 mm (length x width x thickness). Each film were cut with a razor blade to create a notch at approximately half the length of the specimen. The notch length was approximately 10% of the sample width. The fracture toughness equipment was mounted on a microscope stage and equipped with a 25 lbf load cell and a variable speed motor. Crosshead speed of 5 mm/min was used to deform the specimen in tensile mode. The variation of load versus displacement was recorded. The crack tip region was viewed on the computer screen at the magnification of 10x and the onset of propagation was marked on load-displacement curve.
3.3.10 Tensile testing
The tensile properties (ASTM D 2370-92) of the coatings were measured to determine the elongation, tensile strength, and tensile modulus of the coatings. The tests
72 were performed on an instron 5567 (Instron Corp., Grove City, PA). The tests specimens were rectangular free films, 15 mm wide, 0.09-0.14 mm thick and with gauge length of
60 mm. Six replicates were tested for each sample. A crosshead speed of 2.0 mm/min was applied to determine the tensile strength, elongation at break and tensile modulus.
The jaws of the tensile tester was set at the gage length selected and a specimen of uniform thickness was placed between the grips of the machine. The specimens were elongated at a particular crosshead speed until the films are ruptured. The elongation of the specimens were determined by measuring the increase in jaw separation from the point of original load application to the point of rupture. The tensile load required to rupture the films were noted.
3.3.11 Contact angle
Contact angle measurements were performed with a Rame-Hart contact angle goniometer, model 100-00. Six replicates were tested for each sample. Images of advancing and receding angles were taken using image-capturing equipment (Dazzle
DVC, Dazzle media). Contact angle on both the side of the droplet were measured using
Scion Image at ambient conditions (1 atm, ~25 oC ). An average value of all the contact angles is reported. 164
3.3.12 Release Measurements
Release testings were performed by peel-off adhesion according to ASTM D3330.
In order to measure adhesion of Al plates to substrate film, the following procedure has been optimized. 165 Six replicates were tested for each sample. The silicone methacrylate formulations were cast on Al plates and cured thermally at 120 oC. Then the mold release
Scotch Tape 249 was applied on the coated substrate and a 1lb load was rolled over it 5
73 times. Then, the release force of the tape from the cured silicone layer was measured at
180 o peeling angle using tensile tester. The release energy (G) is given by: F G 1( −= cosθ) (3-13) b where F is steady-state peel load, b is the width of the scotch tape and θ is the peel angle.
166 After the tape was removed from the silicone layer, the subsequent adhesion
experiments was repeated by reapplying the tape to a clean steel panel, rolling 5 times
with 1 lb roller and again measuring the force required to remove the tape at an angle of
180 o. A minimum of five trials was conducted on each sample and the mean was
reported. The effect of thermal ageing on UV-cured samples were observed by putting
the UV-cured aluminium panels in an oven at 70 oC for 24 h. Then, the tape was applied
as described before.
3.3.13 Drying Time
Drying time measurements were conducted using Gardco quadracycle drying time recorder as per ASTM D5895.
3.4 Film preparation
Two types of substrate aluminum and glass panel were used as substrate for film
preparation. The substrates were cleaned with distilled water and acetone and dried.
3.4.1 Film preparation from glycidyl epoxide and amine terminated siloxanes
The coatings formulations were made by taking the synthesized glycidyl epoxide
polysiloxane and HELOXY Modifier 48 in a glass vial and adding 0.1 % wt acetic acid,
then mixing thoroughly for 20-30 min at room temperature. Then amine functionalized
polysiloxane and epicure 9551 was added to the glass vial and mixed again for about 15
min. The amount of the four components used is shown in Table 3-5. The films were cast
74 on the substrates with a thickness of 200 μm (8 mil) by a drawdown bar. The films were cured at 120 oC for 6 h and stored in a dust free cabinet for testing purposes.
Table 3-5. Formulation for thermal cured film formation
Glycidyl Heloxy Amino Epicure Epoxide Modifier 48 Polysiloxane 9551 Polysiloxane R= Methyl 2.5 g, 2.6 g 10.32 g 1.2 g 0.0123 moL 0.018 moL 0.02 moL of 0.01 moL of epoxide grp epoxide grp amine grp amine grp R=Cyclo Pentyl 10 g 2.6 g 8 g 1.2 g 0.0126 moL of 0.018 moL 0.02 moL of 0.01 moL of epoxide grp epoxide grp amine grp amine grp R= Cyclo Hexyl 10 g 2.6 g 8 g 1.2 g 0.0122 moL of 0.018 moL 0.02 moL of 0.01 moL of epoxide grp epoxide grp amine grp amine grp
3.4.2 Film preparation from methacrylate terminated siloxanes
The formulations were made by taking the synthesized methacrylate functionalized polysiloxane and mixing them with 10 wt % hexanediol dimethacrylate
(HDDM) and 0.1 wt % photoinitator D1173 in a glass vial at room temperature thoroughly for 20-30 min. The formulation is shown in Table 3-6. The films were cast on either aluminium or glass substrates with a thickness of 200 μm (8 mil) by a drawdown bar. The films were UV- cured and stored in a dust free cabinet for testing purposes.
Table 3-6. Formulation for UV-cured film formation
Siloxane methacrylate HDDM Photo-initiator (D1173) R = Methyl 10 g 1 g 0.01 g
R = Cyclopentyl 10 g 1 g 0.01 g
R= Cyclohexyl 10 g 1 g 0.01 g
75 3.4.3 Film preparation from hydride functional siloxane and alkyds
The synthesized alkyds having 0.01 mol of hydroxy functionality were mixed
with hydride terminated siloxanes having 0.01 mol of Si-H functionality in toluene as
given in Table 3-7. The Wilkinson’s catalyst was added and the reaction mixture was
heated to 60 °C.
Table 3-7. Formulation for film formation of long oil (LO), medium oil (MO) and short oil (SO) alkyds with PDMS, PDPS and PDHS
Alkyd (free HO-) Hydride terminated Wilkison’s Catalyst Siloxane (Si-H) PDMS 9.3 mg, 0.01 mmol 6 g, 0.01 mol 10 g, 0.01 mol
LO PDPS 6 g, 0.01 mol 17.5 g, 0.01 mol 9.3 mg, 0.01 mmol PDHS 6 g, 0.01 mol 26.7 g, 0.01 mol 9.3 mg, 0.01 mmol PDMS 3.6 g, 0.01 mol 10 g, 0.01 mol 9.3 mg, 0.01 mmol
MO PDPS 3.6 g, 0.01 mol 17.5 g, 0.01 mol 9.3 mg, 0.01 mmol PDHS 3.6 g, 0.01 mol 26.7 g, 0.01 mol 9.3 mg, 0.01 mmol PDMS 2.2 g, 0.01 mol 10 g, 0.01 mol 9.3 mg, 0.01 mmol SO PDPS 2.2 g, 0.01 mol 17.5 g, 0.01 mol 9.3 mg, 0.01 mmol PDHS 2.2 g, 0.01 mol 26.7 g, 0.01 mol 9.3 mg, 0.01 mmol
The reaction was monitored through the FTIR stretching of Si-H functionality.
The reaction was stopped on complete disappearance of Si-H stretchings at 2170 cm-1.
The 1H and 13C- NMR of reaction mixture was also taken. Drier formulation having 33
wt % each of cobalt hydrocure II, zirconium hydro-chem and calcium hydro-cem was
prepared. The drier was mixed with siloxane-alkyd hybrid, and stirred for 5 min for
76 complete homogeneity. The mixture was cast on glass and aluminum substrates with a
thickness of 200 μm (8 mil) by a drawdown bar. The films were cured at 120 °C for 6 h
and stored in a dust free cabinet for testing purposes.
3.5 Film Characterization
The pencil hardness (ASTM D3363-74) measurement was used to determine the
hardness of organic coatings. A set of wood pencils meeting the following scale of
hardness were used for the testing:
6B - 5B - 4B - 3B - 2B - B - HB - F - H - 2H - 3H - 4H - 5H - 6H
Softer Harder The tests were performed at room temperature, by holding the pencil at 90o to a
abrasive paper and rubbing it maintaining an exact angle of 90o, until a flat, smooth and circular cross section was obtained. Now the coated panel was placed on a level, firm, horizontal surface. Starting with the hardest lead, the pencil was held at a 45o angle
(pointing away from the operator), and pushed away from the operator. The process was repeated down the hardness scale until a pencil was found that would not cut through the film to the substrate for a distance of at least 3 mm. The hardest pencil that keeps the coating uncut was reported as the pencil hardness of the film.
The reverse impact resistance (ASTM D 2794-84) measurement was used for determining the energy required to rupture the coatings due to the impact from a falling weight. This test method uses a falling weight having a specified diameter impact surface restrained vertically, and dropped from varying heights to produce impact energies over the required range. The apparatus consists of a tub, a drop tube and a specimen holder.
The tub is made from a tub body, and a hemispherical tub nose having a combined fixed
77 weight of 3 lb is used over a drop range of 2 to 4 ft. The drop tube, 5 ft long, contains the
tub and guide it during the free fall. A scale is attached to the guide tube for measuring
the height of drop to the nearest 2.54 mm. The specimen holder contains a base plate for
positioning and holding the pipe specimen on line with the axis of the vertical drop tube.
For a reverse impact measurement, the coated side of the panel is kept facing down. The
impact resistance corresponds to the highest height of fall resulting in a non-failure.
The pull off adhesion (ASTM D 4541-02) measurements were used for evaluating
the pull-of strength of a coating on a rigid substrates such as metal, concrete, or wood.
The test determines the greatest perpendicular force (in tension) that a surface area can
bear before a plug of material is detached, or whether the surface remains intact at a
prescribed force (pass/fail). The test was performed by fixing a dolly perpendicular to the surface of the coating with an adhesive. After the adhesive was cured, a adhesion testing apparatus was attached to the loading fixture aligned to apply tension normal to the test surface. The force applied to the loading fixture was gradually increased and monitored until a plug of material gets detached. When a plug of material was detached, the exposed surface represents a plane of limiting strength within the system and was reported as the pull-of strength of the specimen.
The crosshatch adhesion (ASTM D3359-87) measurements were used for
assessing the adhesion of coating films to metallic substrates by applying and removing
pressure sensitive tape over cuts made in the film. An area free of surface defects were
chosen on the coated surface. Two straight sets of cuts about 40 mm long were made
that intersects at 90o near their middle. A sharp razor blade, scalpel or any other cutting
device having straight edge was used to ensure straight cuts on the films. A piece of
78 pressure-sensitive tape about 75 mm long was placed at the intersection of the cuts. The
tape was smoothened by finger press or rubbing with an eraser. Within 90 ± 30s of tape application, the tape was removed by pulling off backward at nearly 180o angle. The
intersection was inspected for removal of coatings from the substrate. The scale of
measurement ranged from 5B for no removal of coatings to 0B for more than 65 % removal.
79 CHAPTER IV
SYNTHESIS OF AMINE AND EPOXIDE TELECHELIC SILOXANES
4.1 Introduction
Basic building blocks of silicones and polysiloxanes are chlorosilanes.
Chlorosilanes can be reacted with organometallic reagents, such as organolithium
compounds, Grignard reagents or organic zinc compounds to form alkyl or aryl
functional silanes.167, 168, 169, 170 Ring opening polymerization of cyclic silicones with
hydride functional disiloxanes can generate higher molecular weight hydride functional
silicones. Silicones 170, 171 can be further modified via hydrosilation of silicon hydride
with a variety of alkenes. The hydrosilation reaction have been used in industry 172, 173 to
synthesize functionally terminated siloxane oligomers and various linear segmented copolymers. 174 Silicones can be end-capped with various reactive groups such as e.g.
epoxides,175, 176 amines, 177, 178 vinyl, 179 and sulfides. 121, 180
To expand and improve properties of silicones, it has been proposed to replace the
phenyl or methyl substituent with cycloaliphatic group. Soucek. et. al. recently reported
the synthesis of cyclopentyl and cyclohexyl substituted silane monomers. These silane
monomers were used to prepare cycloaliphatic silicon oligomers which were ring opened
and functionalized with cycloaliphatic epoxide and alkoxy silane groups for cationic UV-
curing. 181 The effect of temperature, UV light intensity, sol-gel precursor concentration,
80 and exposure time were studied on cycloaliphatic epoxides. Kinectics of curing was
studied for both UV 182, 183 and thermally cured systems. 184
Amino functionalized siloxanes have been extensively studied as a component of
a large number of segmented materials. 185, 186 , 187, 188 , 189 Amine terminated siloxanes are
used to synthesis polyureas with superior mechanical strength properties due to
segregated morphologies. A typical synthetic approach which has been reported recently by Soucek. et. al. involves the reaction of cyclic polysiloxane oligomers with 1,3-bis(3-
aminopropyl)tetramethyldisiloxane and employing tetramethylammonium hydroxide as
the catalyst. 190 Also, silicon-imides which have good optical, mechanical, processibility
and thermo-oxidative stability can be made or modified through amine siloxanes and
some other crosslinkers. 191
In the present study, a series of glycidyl epoxide and amine functionalized
siloxane have been prepared. Cyclopentene and Cyclohexene substituted dichlorosilane
were synthesized and used for making cyclic oligomers i.e. cyclic dicyclopentyl siloxane and cyclic dicyclohexyl siloxane through hydrolytic condensation. Aliphatic amine functional siloxane was obtained by hydrosilation of hydride terminated dicyclopentyl siloxane and dicyclohexyl siloxane with t-butoxycarbonyl protected allylamine and then
deprotecting the t-butoxycarbonyl group. The functionalized siloxanes were
characterization via infrared IR, 1H-NMR, 13C-NMR, 29Si-NMR and GPC. Glass transition temperature of the cured epoxide and amine terminated siloxane samples were observed by Differencial scanning calorimetry (DSC).
81 4.2 Result and Discussion
The objective of this study was to synthesize functionalized siloxane oligomers to be used as thermosetting silicones. These oligomers are supposed to have intermediate vapor pressure and glass transition temperatures between methyl and phenyl substituted silicones. The siloxanes were modified with glycidyl epoxy functional groups for either homopolymerization or copolymerization with other functional groups. The reaction pathway for preparation of glycidyl epoxy functionalized siloxane is described in Figure
4-1.
H R 180 oC, 300 psi , Cl Si Cl + 2 HCl ClSi Cl + 48 hrs R H
(1) R = (2) R = (a)
R R H2O/ KOH ClSi Cl Si O (3) R = (4) R = RT n R R n = 3-5
(b)
(c)
82 O CH3 R CH3 O H Si OOSi Si H n CH R CH Karstedt Catalyst 3 3
CH R CH O 3 3 O Si OOSi Si n O O CH3 R CH3
(8) R = M e (9) R = (10) R = (d) (continued)
Figure 4-1. Synthesis of glycidyl epoxy functional siloxane (a) Hydrosilation of cycloaliphatic alkene with dichlorosilane gas (b) Hydrolytic condensation to prepare cyclic oligomer (c) Hydride functional polysiloxane synthesis (d) Hydrosilation to give glycidyl functional polysiloxane.
Aliphatic amine functionality was introduced into the siloxane moiety by
hydrosilation of hydride functional siloxane with BOC protected allylamine. Attempts
were made to react amine functionalized siloxanes with glycidyl epoxy functionalized
siloxanes for thermal curable applications. The schematic representation of the synthesis
is given in Figure 4-2.
O O O + RT O O NH2 O NH O
(11) (a)
83 CH3 R CH3 HSSi O iO Si H n CH R O 3 CH3 NH O Karstedt Catalyst 0 60 C, N2
O CH3 R CH3 O
O HN ( CH2 )3 Si OOSi Si ( CH2 )3 NH O n CH3 R CH3
(12) R = M e (13) R = (14) R =
(b) (continued)
CH3 R CH3 TFA, CH2Cl2 H N ( CH ) Si OOSi Si ( CH2 )3 NH (12), (13), (14) 2 2 3 n 2
CH3 R CH 3
(15) R = M e (16) R = (17) R =
(c)
Figure 4-2. Synthesis of aliphatic amine functional siloxane (a) t-BOC protection of allylamine (b) Hydrosilation of t-BOC protected allylamine with hydride functional siloxane (c) Deprotection of hydrosilated product.
The addition of cyclopentene and cyclohexene to dichloro-silane gas via hydrosilation reaction was performed at higher temperature and pressures. The synthesis of dicycloalkyl dichlorosilanes as shown in Figure 4-1a was followed by hydrolytic condensation under alkaline conditions to produce low molecular weight cyclic oligomers as represented in Figure 4-1b. Ring opening polymerization (ROP) of cyclic oligomer was used to produce hydride terminated polysiloxane as given in Figure 4-1c.
84 Hydrosilation of the hydride functional siloxanes i.e compound 5, 6 and 7 was performed
to make glycidyl epoxide terminated polysiloxane is shown in Figure 4-1d.
A three step amine functional polysiloxane synthesis is given in Figure 4-2. It is
known that platinum catalyzed addition cure systems are prone to catalyst poisoning in
presence of amine systems. Also these systems are prone to produce hydrogen gas as by- product during cross-linking reaction; this can result in the unintentional entrapment of
gas bubbles within the cross-linked matrix.192 Thus, direct hydrosilation of unsaturated
compound containing primary amines is difficult. Hence, if primary amines are used, the
blocking and then deblocking avoids side reactions and reduces the potential of catalyst
poisoning. The tert-butoxycarbonyl (t- BOC) group is often used for protection of amino
group in bio-macromolecules specially in amino acids in peptide synthesis. 193, 194 The
two advantages of blocking with a t-BOC group is that it is possible to de-block even in
the presence of other acid sensitive functionalities such as t-butyl esters and trityl
(triphenyl methyl) groups 195 and unlike other carbonyl groups 196, t-BOC groups are
essentially inert to Si-H bonds. Deblocking of t-BOC group was afforded by using
trifluoroacetic acid (TFA), CH2Cl2 followed by neutralization with sodium bicarbonate.
The reason for this neutralization step is to remove the TFA, which could otherwise lead
to the formation of triflouroacetamides when a coupling agent is added later in
formulation. 197
Spectroscopic Characterization
The cyclic oligomer of polydicyclopentylsiloxane (PDPS) and polydicyclohexyl
siloxane (PDHS) are clear yellow oily liquid. The FTIR stretchings of cyclic PDPS are:
1010- 1147 (Si-O-Si), 2848-2942 (-CH2-), 2929-2980 (-CH3 -). The stretchings at 1010-
85 1147 cm-1 is due to siloxane (Si-O-Si) bond, at 2848-2980 cm-1 due to C-H alkylene
group of cycloaliphatic moiety and at 2929-2980 cm-1 due to C-H stretch of methyl
group. The FTIR of cyclic PDMS and PDHS also show similar spectra . The 29Si-NMR of the cyclic oligomers show three sets of resonances. The resonance at ~ 25 ppm corresponds to siloxy units attached to the two tertiary carbons of cyclopentyl group
( −− CSiC ). Partial hydrosilation occurs as a side reaction giving rise to mono substituted product e.g poly (cyclopentyl) siloxane, having resonance at δ -32 ppm.
Separation of partially unreacted siloxanes becomes difficult so a resonance appears at δ
32 ppm.
The synthesis of hydride terminated siloxane was accomplished through ring
opening polymerization (ROP) of cyclic oligomers. The structures were verified through
FTIR and 1H-NMR. FTIR stretchings of hydride functional PDPS (Figure 4-3) are 1010-
1126 (Si-O-Si), 2862 (-CH2 -), 2960 (-CH3 -), 2135 (-Si-H); In FTIR, the presence of a strong absorption at 2160 cm-1 corresponds to Si-H group, whereas the two stretches at
1090 and 1110 cm-1 are indicative of silicon oxygen bond of siloxane. In 1H-NMR, there
are characteristic resonances for Si-Me at δ 0-0.07 ppm and Si-H at 4.67 ppm. Alkyne
and alkylene protons of cyclopentene group are observed at δ 1.7 - 2 ppm, as a very broad
resonance. Two smaller resonances were observed at δ 0.9 and δ 1.18 ppm. These resonances were attributed to differences in stereoconfigurations of the substituents along the polymer backbone. Hydrosilation of H-terminated siloxanes, compounds PDMS (5),
PDPS (6) and PDHS (7) were performed using Karstedt catalyst.
86 Completion of the reaction was monitored through the disappearance of the absorption at
~2127 cm-1 in FT-IR (Figure 4-3). The formation of epoxide can be monitored by observing the 950-800 cm-1 region.
Hydride terminated PDPS
2127 cm-1
CH3 CH3 HSi O Si O Si H
Transmittance (%) n
CH3 CH3
3900 3400 2900 2400 1900 1400 900 400 Wavelength (cm-1)
Figure 4-3. FTIR of hydride terminated polydicyclopentylsiloxane
Glycidyl Epoxy Functional PDPS
CH CH O 3 3
(%) Transmittance O Si OSiO Si O n O CH CH 3 3
3900 3400 2900 2400 1900 1400 900 400 Wavelength (cm-1)
Figure 4-4. FTIR of glycidyl epoxy functional polydicyclopentylsiloxane
87 The presence of a broad Si-O-Si stretching between 1000- 1100 cm-1 may overlap
with the epoxy band. Proton, carbon, and silicon NMR spectra of glycidyl epoxide
functional PDPS (9) are shown Figure 4-5a, 4-5b and 4-5c respectively.
In Figure 4-5a, a group of alkylene and alkyne resonances appear between δ 2.0 -
5.0 ppm. The two protons of CH2 group of epoxy appears at δ 2.65 and δ 2.8 ppm, while
the proton of CH group from the epoxide occurs at δ 3.2 ppm. The methylene CH2 units of the cycloaliphatic ring exhibits proton resonances around δ 0.85-1.25. The proton resonances of the methylene groups formed from the hydrosilation reaction appears downfield. The proton resonance due to three methylene units attached to oxygens of glycidyl groups are observed at δ 3.9 - 4.8 ppm. The proton resonance due to methine CH of cycloaliphatic group appears between δ 1.25-1.2 ppm. The integration ratio of 4 protons of CH2 unit of epoxy unit to that of 12 methyl protons attached to silicon atom gives functionality of epoxy group of about 75 %.
In Figure 4-5b, the characteristic 13C-NMR resonance of epoxy appears at δ 44.50 ppm (C1) and δ 46.22 ppm (C2). 29Si-NMR spectra in Figure 4-5c show 4 major groups of
resonances. The signals at around δ -20 ppm correspond to siloxy groups attached to two
tertiary carbons from cyclopentane group. The group of resonances at δ -22.5 to -24 ppm is due to hydrosiloxy units attached to one carbon, either methyl or an tertiary carbon.
The resonances observed in Fig 4-5c is in agreement with that reported in the reference
198 i.e the resonance due to silicon atoms adjacent to methylhydrosiloxy units are shifted
slightly downfield relative to the resonances for silicon atoms between dimethylsiloxy
units.
88 12 a. a. 9 11 9 1 CH3 10 CH3 8 O 3 6 4, 5 7 1 O Si OSiO Si O 2 n 7 O 6 8 3 2 CH3 CH 4 3 5 9
7, 8, 10
5 11, 12 1 2 3 64
0.07 0.27 5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -0.5 -
10 b. b. 7 9 7 CH3 8 CH3 3 O 6 4 1 2 5 2 O Si OSiO Si O 1 n 5 O 3 4 6 CH3 CH3
2 5, 9 7
1 8 3, 4 6, 10
200 180 160 140 120 100 80 60 40 20 0
c. c.
CH3 CH3 O 1 23 O Si O Si O O Si Si n O O CH3 H CH3
1 2
3
40 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 -40
Figure 4-5. NMR spectra of glycidyl epoxy functional PDPS a. 1H-NMR b. 13C-NMR c. 29Si-NMR 89
Amine Functional PDPS
) e (%
ittan
CH3
CH3
sm c
O O H 2NSSi Si iNH2 Tran n
CH3 CH3
3900 3400 2900 2400 1900 1400 900 400 Wavelength (cm-1)
Figure 4-6. FTIR of amine functional polydicyclopentylsiloxane
The reaction pathway to synthesize amine functional siloxane by a three step
process is shown in Figure 4-2. The FTIR, 1H-NMR, 13C-NMR and 29Si-NMR of amine functional PDPS is given in Figure 4-6, 4-7a, 4-7b, and 4-7c respectively. In FTIR of
Figure 4-6, the characteristic vibration stretch of primary amine can be observed as a broad band at 3200-3400 cm-1.
In 1H-NMR spectra of Figure 4-7a, the proton resonance of amine appears at δ
5.4-5.6 ppm. methyl resonance appears at δ 0.1 ppm. Methylene CH2 resonances of
cycloaliphatic group occurs between δ 1.0-1.2 ppm. The resonances due to propyl chain
proton and alkyne hydrogen of cycloaliphatic group appears between δ 1.3- 2.0 ppm.
The integration ratio of 2 protons of CH2NH2 unit at 2.48 ppm to 2 protons of
Si(CH3)CH2 at 0.75 ppm is 0.67. This gives the functionality of amine group of about 67
%. The rest of the functionality may be attributed to hydroxyl group.
In 13C-NMR spectra of Figure 4-7b, the characteristic carbon resonance connected to terminal amine is at δ 42 ppm. The resonance at δ 0 ppm is the carbon of methyl
90
a. 8 5
CH3 7 CH3
1 6
O 3 O H2N Si Si Si NH2 n 3, 6, 7, 8 2 4 CH3 CH3
5 1
2 4
-00..44 0 -0.0.660
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -0.5 -1.0
b. 7 7
4 6 4 CH3 6 CH3
5 3 1
O 2 O H N Si Si Si NH 2 2 2 n 1 3 CH3 CH3
2
3 7 5 1 6 4
200 180 160 140 120 100 80 60 40 20 0 c. CH 3 CH3
1 O 1 2 O H NSSi Si i 2 NH 2 n
CH3 CH3 2
1
Figure 4-7. NMR spectra of amine functional PDPS a.1H-NMR b. 13C-NMR c. 29Si-NMR 91 group. The resonances at δ 10 and 22 ppm are due to carbon of methylene group of cyclopentyl ring. The tertiary carbon resonance is around δ 42 ppm. The resonances between δ 28 to 32 ppm are due to three methylene groups of propyl amine. In Fig. 4-7c, the 29Si-NMR, the resonance at δ -22 ppm is due to Si atom in –O-Si-O- group attached to two tertiary carbons. Partial hydrosilation byproduct, having one tertiary carbon and a hydrogen attached to a –O-Si-O- gives a resonance at δ 22.5 ppm. The resonance at δ 7.5
ppm is due to the –CH2-Si-O- group.
The molecular weight of cyclic oligomers of PDMS, PDPS and PDHS were
measured by GPC and were found to be in the range of M n ~ 700 to 1000 with PDI ~
1.15 for all the cyclic oligomers. The epoxy equivalents for epoxy terminated
polydimethylsiloxane, polydipentylsiloxane and polydihexylsiloxane were calculated
according to ASTM D1652 and found to be 203.7, 796.3 and 817.6, respectively. From
the epoxy equivalent values, it was found that epoxy value of methyl siloxane is
substantially lower than cyclopentyl and cyclohexyl siloxane.
R = Cyclohexyl -81.51 °C
-92.55 °C R = CycloPentyl
-103.85 °C Heat Flow(W/g)) R = Methyl
-120 -100 -80 -60 -40 Temperature (C)
Figure 4-8. DSC of siloxane epoxy/ siloxane amine cured systems 92 Figure 4-8 illustrates that glass transition temperature of a thermally curable
system can be controlled by substituting different pendant group in the backbone. The
cycloaliphatic groups on siloxane backbone to raised the Tg above the methyl substituted
silicones.
Methyl and phenyl substituted siloxanes comprise the majority of silicones used
as additives in industries, as a result the properties are at the extremes in terms of
flexibility or glass transition temperature. 199 It is proposed that cycloaliphatic silicones
have better resistance and weathering properties than resins containing aromatic groups.
200 It was anticipated that hydrophobicity properties can be varied by different
substitution on the cycloaliphatic group. Silation with bulky groups is important in the
making of many bio-macromolecules synthesis.
The siloxane alkoxylates modified with amino group at terminal position have the
potential to enhance the efficacy of agrochemicals on plants compared to conventional
trisiloxane alkoxylates (TSE) alone. 201 Amino siloxane oligomers are also suitable for
covalent coupling to a bio-affinity adsorbent, as compatibilizers in cosmetic industries,
and in microlithography. 202 On account of sensitivity to water, oligomeric siloxanes
cannot be used in conventional water-based coatings, however, introduction of amino
groups allows the product to be produced which are emulsions. Siloxane diamines form
block copolymers of PDMS-polyamide or PDMS-aramide when reacted with
bifunctional oxazolones, dicarboxylates etc. These polymers having thermally stable units along the backbone as well as siloxane units have long term stability at temperatures
above 200 °C, but still low Tg .
93 The introduction of epoxy group on to siloxane polymers has a wide range of
applications. 203, 204 The epoxy resins have excellent mechanical, electrical and adhesion
properties due to which it is used as high performance thermosetting materials in many
industrial fields. The desirable properties of epoxy functional siloxanes resins which
makes them attractive to coatings, adhesives, paper release agents, additives for printing
inks etc are low viscosity, excellent clarity, high gloss and high reactivity.
Most of the studies in literature dealing with enhancement of properties of epoxy
resins focus on either blending or copolymerizing epoxy resins with other polymers i.e
carboxy- or amine- terminated acrylonitrile-butadiene rubbers functionally terminated
acrylates , poly(phenylene oxide) and alkylene oxides. 205, 206 But usually the above process leads to increment in the crosslink density and hence increases the hardness and decreases the tensile strength and modulus. The introduction of cyclopentyl and cyclohexyl groups in the siloxane backbone keeps the crosslink density of the epoxy siloxane system much lower than in case of PDMS. Some of the potential uses of epoxy functional siloxanes are in coatings, encapsulants, molding compounds, matrix resins for fiber reinforced composites. Due to lower viscosity of these epoxy siloxanes, it can be used in adhesives, elastomers, liquid injection molding, and in room temperature vulcanizable rubbers. 207
4.3 Conclusions
Glycidyl epoxy and aliphatic amine functionalized cyclopentyl and cyclohexyl substituted polysiloxane were synthesized. Hydrosilation of cyclo-alkene was performed at high temperature and pressure. There was a decrease in epoxy equivalent weight as the
steric hinderance of pendant group in the siloxane backbone increases. The thermal cured
94 system derived from reacting glycidyl epoxy functionalized di(cyclopentyl) and di(cyclohexyl) siloxane with the corresponding amine functional siloxane have higher glass transition temperature as compared to methyl siloxane. The advantage of t-BOC chemistry is that cleavage and deprotection times are rapid. BOC protection deprotection is mostly used in peptide synthesis due to higher selectivity of the process.
95 CHAPTER V
MECHANICAL AND FILM PROPERTIES OF THERMAL CURABLE
POLYSILOXANES
5.1 Introduction
Epoxide siloxane offers the benefits of both silicone resin and epoxide resin.
Epoxide siloxane can be synthesized from double bond containing epoxide and silane by hydrosilation.208 The siloxane bond is stable in response to heat and ultraviolet light, and
epoxide resin has high adhesive strength. Cationic polymerization of epoxide siloxane
were studied by Crivello et. al. 182, 209 The synthesized monomers consisting of epoxycyclohexyl groups exhibited excellent reactivity in cationic ring opening polymerization. The properties of the cross-linked resins depend on the length of the siloxane backbone separating the pendant epoxide groups. Silicones are also used for toughening of epoxides. The phase separation of the siloxane component from the epoxide matrix results in a rubber toughening mechanism that effectively retards the fracture, thus improving the fracture toughness. 206, 210 , 211 However, the phase
incompatibility of siloxanes and BPA-epoxide leads to problems when compounding.
One of the desirable characteristics of epoxide siloxane monomer is high
reactivity. The ideal monomer can be cured with minimum catalyst concentration, and as
a result the matrix will have good color stability. 212 It was reported that epoxide siloxane
96 could be easily polymerized by either photo-indued polymerization or thermally-induced polymerization. 208 Mechanical and thermal properties of polydimethylsiloxane epoxide were investigated extensively in literature. 213, 214 The mechanical properties of the resin can be directly related to the length of polydimethylsiloxane. As the length of siloxane increases the storage modulus of the polymer decreased progressively.215, 216, 217
The properties of films obtained by thermal curing of glycidyl epoxide functional polysiloxane and alkyl amino polysiloxane are reported in this study. A set of three glycidyl epoxide terminated polysiloxanes were prepared with methyl, cyclopentyl and cyclohexyl substituents and photopolymerized with an epoxide reactive diluent using a cationic photoinitiator. The telechelic siloxane epoxides were also thermally reacted with amine terminated methyl, cyclopentyl and cyclohexyl substituted siloxanes. The viscoelastic and mechanical properties were evaluated using dynamic mechanical thermal analysis, stress strain measurements, and fracture toughness. Coating properties, adhesion testings, release properties, were performed. Oxygen permeability, contact angle, X-ray diffraction, chemical resistance, and impact resistance were also studied.
5.2 Results and Discussion
The cycloaliphatic substituted telechelic siloxane epoxides were homopolymerized via a cationic UV-curing mechanism as shown in Figure 5-1. The epoxides were also thermally cured with the corresponding telechelic amine functionalized siloxanes as shown in Figure 5-2. The synthetic approaches for attaching the glycidyl epoxide and amine groups onto the ends of the siloxane have been recently reported. 61, 218 The addition of phosphoric acid, diethylenetriamine (DETA), diazabicyclo[2.2.2]octane (DABCO) or triethylamine (TEA) gave either very weak or
97 almost no exotherm, and thus were not effective in catalyzing cure the silicone-
epoxide/silicone-amine reactions. Acetic acid was observed to accelerate the thermal cure
at 120 oC and consequently was chosen as a curing catalyst for this study. One of the target usages of the thermal curable siloxanes are for release coatings. The degree of cure as determined by MEK double rubs of silicone-epoxide/silicone-amine were used to optimally choose the catalyst for silicone-epoxide/silicone-amine curing. When silicone- epoxide resins were cured with the silicone-amine, very soft films were obtained. To obtain continuous films for mechanical and coating tests, HELOXY MODIFIER 48 and
Epicure 9551 were added as reactive diluents in film formulations. The amount of reactive diluents was however, minimized to ensure that it did not dominate the end properties of the siloxane films.
O H X O O + O X + 2H+ X O O + H
O X +O OH H X H HO O + X O
CH3 R CH3 X = O Si Si OO Si n O CH3 R CH3
where R = Me, ,
Figure 5-1. Schematic representation of telechelic epoxide siloxane homopolymerization
98 O O ...... O + H N X 2 Y NH2 X NH Y NH2 O HO X
OH .. .. O X NH Y NH X HO HO OH
CH3 R CH3 CH3 R CH3 Si O Si Si( CH ) X = O Si Si OO Si Y = ( CH2 )3 O 2 3 n O n CH R CH CH3 R CH3 3 3
R = Me , ,
Figure 5-2. Schematic representation of reaction between telechelic epoxide and amine siloxane
The thermal curing of glycidyl epoxide and amine system were observed through
DSC. Each of the siloxane systems showed exothermic cure curve as shown in Figure 5-
3. This thermogram is similar and representative of the other two systems (methyl and cyclohexyl siloxanes). The heat of reaction is given in Table 5-1.
1.5
PDMS + Reactive Diluents 1.0
) g 0.5 Reactive Diluents
Cured PDMS
0.0 Heat Flow (W/
-0.5
-1.0 0 50 100 150 200 25 Exo Up Temperature (°C) Universal V4.3A T
(a)
99 0.4
PDPS + Reactive Diluents 0.2
) 0.0 Reactive Diluents
-0.2 Heat Flow(W/g Cured PDPS
-0.4
-0.6 0 50 100 150 200 25 Exo Up Temperature (°C) Universal V4.3A T
(b) (continued)
0.3 PDHS + Reactive Diluents
0.1 Reactive Diluents
(W/g) -0.1 Cured PDHS Heat Flow Heat -0.3
-0.5
-0.7 0 50 100 150 200 25 Exo Up Temperature (°C) Un ivers al V4.3A T
(c) Figure 5-3. Curing exotherm of (a) telechelic epoxide and amine PDMS, reactive diluents, and cured siloxane with reactive diluents (b) telechelic epoxide and amine PDPS, reactive diluents, and cured siloxane with reactive diluents (c) telechelic epoxide and amine PDHS, reactive diluents, and cured siloxane with reactive diluents
The heat of curing reaction was determined for epoxide amine systems with and without reactive diluents. The heat of reaction for the combination of the siloxanes and
100 reactive diluents are much higher than cured siloxane systems alone. Not surprisingly, the reactive diluents being small molecules aided in the completeness of the cure.
Consequently, films for characterizations were formed by curing siloxane with reactive diluents to obtain representable mechanical properties.
Table 5-1. Heat of curing for polysiloxanes and reactive diluents
Sample Name Cured Composition Heat (J/g)
PDMS Polydimethylsiloxane epoxide and 251 Polydimethylsiloxane amine PDPS Polydicyclopentylsiloxane epoxide and 97 Polydicyclopentylsiloxane amine PDHS Polydicyclohexylsiloxane epoxide and 127 Polydicyclohexylsiloxane amine Reactive Diluents 208 Heloxy 48 + Epicure 9551 PDMS Polydimethylsiloxane epoxide 562 + Polydimethylsiloxane amine Reactive Diluents Heloxy 48 Epicure 9551 PDPS Polydicyclopentylsiloxane epoxide 277 + Polydicyclopentylsiloxane amine Reactive Diluents Heloxy 48 Epicure 9551 PDHS Polydicyclohexylsiloxane epoxide 285 + Polydicyclohexylsiloxane amine Reactive Diluents Heloxy 48 Epicure 9551
The rate of homopolymerization of the epoxidized siloxanes was obtained using photo- DSC. The overall heat of reaction, including initiation, propagation and termination can be measured as:
ER = EP + EI - ET (5-1)
For the validity of the above equation, production of active cationic centers must be distributed throughout the reaction. 181 The rate of propagation of a photosensitive
101 reaction is proportional to the height of PDSC exotherm measured. Figure 5-4, shows the
overlay of exotherm for cationic polymerization of PDMS_Ep, PDPS_Ep, and
PDHS_Ep, respectively. It was found that when the exposure time was kept constant at
10 sec and temperature was varied between -10 °C and 60 °C, the rate of polymerization
increases. It was observed for UV-cured epoxides telechelic siloxanes that as the bulk of
the pendant group increased at the siloxane backbone, the rate of polymerization also
increased. With increase in bulk of the pendant group, the epoxy equivalent increases, i.e.
the number of epoxide group present at a given weight of siloxane polymer decreases.
· · · · · · · H· yEp_I250_60C_10sec H yEp_I 25 0_ 60 C_1 0se c.0 01 5.6 – – – – PyEp_I25 0_ 60 C_1 0se c.0 01 –– –– –– – M eEp_ I2 50 _6 0C_ 10 sec. 00 1 a – – – – PyEp_I250_60C _10s ec ––––––– MeEp_I250_60C _10sec
3.6
b Heat Flow (W/g) Flow Heat
1.6 c
-0.4 01 2 3 4 Exo Up Time (m in) Univer sal V3.9A T A
Figure 5-4. Exotherm for cationic photopolymerization of glycidyl epoxide functionalized (a) PDHS (b) PDPS (c) PDMS at 60 °C for 10 s
The free volume between each epoxide group was more, leading to an increase in
mobility, and thus an increase in the efficiency of conversion. A similar effect was observed in the polymerization of multifunctional acrylates.219
102 Tensile, DMTA, and fracture toughness were performed to determine the general
mechanical properties of the three thermal cured and UV-cured siloxane systems. A
DMTA was used to determine the cross-link density and glass transition temperature (Tg) of the cured systems as given in Table 5-2 and Table 5-3. The cross-link densities were calculated from the modulus on the rubber plateau and the corresponding temperature (T
>> Tg) using equation 3-4. The Tg was obtained as the maximum of tan δ. The same trend
in Tg was observed in both DSC and DMTA. As the size of the backbone substituents
increased the cross-link density decreased, thus sterics and packing had an effect on the
cross-link density. It was found that glass transition temperatures of polymers can be
controlled by adding suitable pendant group through hydrosilation. As the pendant groups
were varied from methyl to cyclopentyl to cyclohexyl, the Tg was observed to increase
from -104 to 82 oC. With increase in the bulk of pendant group rotation along Si-C bond
becomes more hindered. Thus, a more rigid system results and cross-linking reactions are slowed.
The plane stress fracture toughness (KIC) was also found to be increasing with
increase in size of pendant group. This may be due to the fact that as the backbone
pendant group of a polymer becomes bulkier, the rotational freedom of the substituents
along Si-O- Si is inhibited, flexibility reduces, and toughness increases. With increase in
size of the substituents in the backbone, a reduction in cross-link density was observed.
Young’s modulus of the thermally cured matrix increased, more energy was released for
the crack to propagate, hence GIC increased with increase in size of the substituents.
The overall mechanical properties of UV-curable and thermal curable siloxanes
showed a mixed trend as given in Table 5-2 and Table 5-3, respectively. The tensile
103 modulus of UV-cured polydimethylsiloxane and polydicyclohexylsiloxane siloxanes were higher than the corresponding thermally cured siloxanes whereas PDPS showed the opposite trend. The tensile strength for all three UV-cured siloxanes were higher than the thermal cured siloxanes. The fracture toughness, elongation-to-break and energy release rate of UV-cured siloxanes were lower than thermal cured siloxanes. Though the number of moles of epoxide functionality in both UV-cured and thermal cured systems were the same, the cross-link density of UV-cured siloxanes were considerably less than the thermally cured siloxanes. This may be attributed to the fast UV-curing process, which do not allow all the reactive groups to participate in the cross-linking process. Thus, the films are cured only at the surface, and have high modulus. Crosslink density of UV- cured siloxanes are lower than the thermal cured systems. The tensile strength of the cyclohexyl siloxane system was highest at 5.4 MPa, more than 5 times higher than dimethylsiloxane system.
Table 5-2. The mechanical properties of UV-cured siloxanes
MS_Ep PS_Ep HS_Ep Tensile Modulus 89 ± 5.1 143 ± 12.7 204 ± 17.4 (MPa) Tensile Strength 1.8 ± 0.5 6.2 ± 2.4 6.9 ± 1.9 (MPa) Elongation-to- 0.70 ± 0.01 0.019 ± 0.008 0.01 ± 0.005 break (%) υ (mol/m3) e 1562 671 598
K [MPa.m1/2 ] c 0.008 ± 0.002 0.05 ± 0.009 0.08 ± 0.01
-2 GIC (J m ) 20 ± 2.6 57 ± 6.1 68 ± 7.7
104 Tensile modulus of the cyclohexyl system was 2.5 times higher than dimethylsiloxane at 187 MPa. The elongation-to-break (%) of methyl siloxane was 9 %, six times higher than the dicyclohexylsiloxane. It was observed that as the bulk of
pendant group increases, the tensile modulus and strength was increased and the
elongation-at-break decreased.
Table 5-3. The mechanical properties of thermally cured siloxanes
MS_Ep_NH PS_Ep_NH HS_Ep_NH Tensile Modulus 73 ± 1.68 156 ± 2.15 187 ± 2.02 (MPa) Tensile Strength 0.79 ± 0.05 4.8 ± 0.4 5.4 ± 0.15 (MPa) Elongation-to- 1.8 ± 0.87 0.4 ± 0.07 0.3 ± 0.04 break (%) υ (mol/m3) e 2935 228 115
K [MPa.m1/2 ] c 0.07 ±0.01 0.15 ± 0.05 0.26 ± 0.06
-2 GIC (J m ) 67.12 ± 5.6 144.2 ± 9.2 361.5± 15.4
0.6 PDPS -27 oC o PDHS 0.5 -36 C PDMS 0.4 o
δ -64 C 0.3
Tan Tan 0.2
0.1
0 -150 -100 -50 0 50 Temperature (o C)
Figure 5-5. Tan δ plot of thermally cured PDMS, PDPS, and PDHS
105 The general film properties for the three thermally cured system is summarized in
Table 5-4. Usually, the larger the organic substituents on the siloxane backbone, lower release properties are observed, thereby improving the adhesion of PSAs.220
Table 5-4. Coating properties of thermally cured PDMS, PDPS and PDHS
PDMS PDPS PDHS
K [MPa.m1/2 ] c 0.07 ±0.01 0.15 ± 0.05 0.26 ± 0.06
-2 GIC (J m ) 67.12 144.2 361.5
Adhesion 40 ± 7.5 120 ± 3.69 130 ± 4.99 Release Force (N/m) Readhesion 60 ± 6.78 160 ± 4.03 220 ± 9.13
Pencil Hardness B 2H 2H
Cross-hatch adhesion B 4B 4B
Pull-Off Adhesion (N/mm2) 0.375 0.5 0.5
MEK resistance 20 ± 1.29 40 ± 2.9 45 ± 0.82 ( Double Rubs)
Impact Direct > 40 > 40 > 40 Resistance (lb/in) Reverse 10 ± 0.5 25 ± 1.26 30 ± 1.7
Advancing 90° ± 0.75 104° ± 0.40 115° ± 0.31 Contact Angle Receding 70° ± 2.08 77° ± 2.52 90° ± 4.44
O2 Permeability (barrer) 0.022 ± 0.004 0.098 ± 0.01 0.625 ± 0.07
106 As the pendant group in silicone backbone was varied from methyl to cyclopentyl
to cyclohexyl, the backbone becomes more rigid. Hence, the segmental mobility reduces
resulting in a denser network. Re-adhesion values increase significantly for all the
systems. Pencil hardness of a cured film is related to the elongation-at-break, i.e. the
coating is broken only when the maximum stress due to the pencil or indenter scratching
exceeds the tensile strength of the coating film. Therefore, the pencil hardness shows the
same trend as the tensile properties. With increase in substituent size, the pull-off
adhesion is increased. This may be attributed to increase in toughness.
The falling weight impact test was performed to determine the ability of the
coating to resist damage caused by rapid deformation (impact). The resistance of the
coating to the penetration by the falling weight is directly proportional to strength of the
coating matrices. So, in this case both for reverse and direct impact testing energy that the
coating can withstand increased with increase in bulkiness of the pendant group attached
to silicone backbone. Impact resistance was found to be directly proportional to the
fracture toughness. A high value of fracture toughness and impact resistance in the absence of crack, is the reflection of good resistance to crack initiation and crack propagation. It was found that as the bulkiness of the pendant group in the silicone backbone increased, the adhesion strength, and MEK resistance increased, and crosslink density decreased. Crosshatch adhesion values of thermally cured PDPS and PDHS was observed to be much higher than the PDMS system.
Oxygen permeability values were found to increase with increase in the bulk of
the pendant group. This was due to the fact that as the steric bulk of the organic groups
attached to the silicone backbone increases, the sites of cross-linking become further
107 apart. The free volume of the cured polymer matrix increases and hence the oxygen
transmission rate rises. Both the advancing and receding contact angle increase with the
increase in hydrophobicity of the thermally cured siloxane layer on the silicone wafer.
The methyl substituted siloxane is the least hydrophobic and cyclohexyl substituted
siloxane is the most hydrophobic.
The X-ray diffraction pattern of the three thermally cured siloxanes is shown in
Figure 5-6. The packing density of the cured siloxane specimens were studied by
WAXD.
R = Cyclohexyl
R = Cyclopentyl
Intensity R = Methyl
5 1015202530 Degree
Figure 5-6. WAXD exotherm of thermally cured siloxanes
All the samples exhibit a broad peak indicating amorphous nature of these samples. The d-spacing in the diffraction pattern, which characterize the chain to chain distance in the polymer matrix was calculated using Bragg’s equation as shown below:
ndλ = 2sinθ 5-1
108 where θ is the angle of maximum intensity of the peak observed in the sample spectrum and λ is the wavelength of the X-ray radiation. As the bulkiness of the pendant group on
the siloxane main chain is increasing, the peak intensity is found to decrease.
As previously reported, an unusual relationship between free-volume, Tg, and curing exists for cycloaliphatic substituted siloxanes. It has been surmised that due to packing of the cycloaliphatic group. For both the methyl and phenyl group the packing is driven either by size or dipole-dipole effects, respectively. The cycloaliphatics bring the unique feature of a higher usage temperature (Tg), and greater free volume. This will be
useful for a number of applications, i.e. membranes, 221, 222 and coating applications. 223,
224 5.3 Conclusion
The synthesized glycidyl epoxide and amine functionalized siloxane with varying
backbone substitution were cured thermally using reactive diluents. The thermal,
mechanical, X-ray, coatings and release properties of the cured silicones were studied.
Cationic polymerization of the glycidyl epoxide functionalized siloxanes was also
observed. It was found that overall mechanical and coatings properties of thermally cured
siloxanes improve with increase in bulk of the pendant group in the backbone. Due to
increase hydrophobicity contact angles and release forces also increase with increase in
steric bulk of the side groups. Also crosslink density increased and oxygen permeability
decreased with increase in the size of pendant group. PDSC studied showed that rate of
cationic polymerization increases with increase in bulk of the side groups, with increase
in exposure time and with increase in temperature.
109 CHAPTER VI
SYNTHESIS OF TELECHELIC METHACRYLIC SILOXANES WITH
CYCLOALIPHATIC SUBSTITUENTS FOR UV-CURABLE APPLICATIONS
6.1 Introduction
The radiation curing process is gaining importance as an effective alternative to all
other technology in coatings industry as environmental and public health concerns have become major issues in the films and composites. 225, 226 Radiation curable silicone release
compositions successfully address both energy and environmental problems inherent in the
use of traditional solvent-dispersed silicones.227 It is an advantage for the UV-curable
silicones to be cured without generation of VOCs. 228 Performance advantages of radiation
curing systems over thermally cured coatings include scratch, abrasion, solvent, and chemical
resistance. 229, 230, 231 The UV-curable system also offers the advantages of a rapid cure, and high gloss.
Most UV-cure technology involves acrylics or vinyl monomers via free radical
mechanism.232 Radical polymerization of unsaturated monomers or polymers is strongly
dependent on the reaction atmosphere and in particular oxygen inhibition. 233 This effect is
maximized in case of siloxane acrylates since not only is oxygen highly soluble in silicones,
but it also exhibits a high diffusion coefficient. 234 Silicone based monomers are typically
formulated with vinyl ether or acrylic functionalized silicone oligomers to address viscosity
and balance the end film properties. 235 It was reported that UV-curable silicone acrylates and
110 vinyl ethers were very robust whose cure is unaffected by impurities of the substrates 236 which may act as inhibitors e.g., heavy metals, sulfo and carboxy groups. The UV-curable silicones have wide temperature usage, good adherence to most substrate due to inherent low surface energy, and good oxidation resistance.237 Soucek. et. al. studied free radical photopolymerication of vinyl238 and acrylic functionalized239 siloxane colliods. In order to
expand and improve properties of existing silicones, the phenyl or methyl substituent were
replaced with cycloaliphatic group. Soucek. et. al. 61, 240 recently reported the synthesis of
cyclopentyl and cyclohexyl substituted siloxanes.
In the present study, a series of telechelic methacrylic siloxane having methyl,
cyclopentyl or cyclohexyl substituents on the silicone backbone have been synthesized. The
methacrylated silicones were characterized by FT-IR, 1H-NMR, and 13C-NMR. The curing of
methacrylated silicones were performed via UV-light using a free radical photoinitiator.
Hexanediol dimethacrylate (HDDM) was used as reactive diluent during UV-curing. The
mechanical, viscoelastic, X-ray, oxygen-permeability and release properties of cured films were evaluated. Photodifferential scanning calorimetry (PDSC) was used to obtain relative rates of photopolymerization.
6.2 Results and Discussion
New cycloaliphatic silicone monomers were recently reported and as a
consequence, the exploitation of these monomers into functionalized oligomers is
ongoing. 218, 241 Methacrylic functionalized siloxanes couples the properties of siloxanes
with the free radical UV-curing of a methacrylic system. Methacrylic telechelic siloxanes
were synthesized by ring opening of epoxide functionality of the corresponding glycidyl epoxide telechelic siloxanes as shown in Figure 6-1. Chromium (III) acetate has been
111 previously reported as an effective catalyst for nucleophilic attack of carboxylic acid on a
glycidyl ether group.242, 243 It proved useful for catalyzing the reaction for the methyl,
cyclopentyl and cyclohexyl substituted siloxanes with carboxylic functionality. 244
Hexanediol dimethacrylate (HDDM) was used as a reactive diluent at 10 wt% for the
UV-curing the methacrylated siloxanes. Without the HDDM not all the siloxanes formed continuous films nor had adequate flexibility for removal from the substrate. The amount of HDDM was minimized to ensure that it did not dominate the properties of the cured methacrylated siloxane films.
CH R CH O 3 3 O Si OOSi Si O n O CH3 R CH3 + (1) R = Me (2) R = (3) R = O Chromium (III) Acetate OH Phenothiazine o 100 C, N 2
CH3 R CH3 O O O Si OOSi Si O n O OH O CH3 R CH3 OH
(4) R = Me (5) R = (6) R =
Figure 6-1. Schematic representation of methacrylic functionalized siloxane synthesis
Characterization of methacrylate functional siloxanes
The molecular weight of the synthesized methacrylic functionalized dimethylsiloxanes, dicyclopentylsiloxanes and dicyclohexylsiloxanes are between 1500-
112 2000 dalton. The polydispersities are in the range on 1.2-1.7. Figure 6-2 shows the 1H and 13C NMR spectra for glycidyl epoxide functionalized polydicyclopentyl siloxanes.
Figure 6-3 shows the 1H and 13C NMR spectra of the methacrylic functionalized
siloxanes. For brevity, only the dicyclopentylsiloxane is shown as representative spectra.
All the NMR data assignments of resonances are tabulated in Table 3-3 of chapter III. In
Figure 6-2a, the resonance at δ 3.7-4.0 is due to CH, and at δ 2.8-3.25 is due to CH2 of
the terminal epoxide moiety of PDPS. The resonances at δ 44 and 48 ppm in Figure 6-2b
were attributed to the epoxide carbons of PDPS.
In Figure 6-3a, the resonances at δ 5.6 and 6.2 ppm were attributed to
methacrylic protons.245, 246 The resonance near δ 128 and 131 ppm in Figure 6-3b were
attributed to the double bond of methacrylic functionality. The epoxide carbons at δ 45
ppm and δ 52 ppm shift as expected reflecting the opening of epoxide ring. The δ 173
ppm resonance was ascribed to the carbonyl carbon of methacrylate. The ratio of
integration of four methylene protons attached to Si atom (C 7 in the Figure 6-3a) to the
unsaturated protons of methacrylate group is Figure 6-3a is equal. So, it was concluded
that the any side product formed by esterification reaction of secondary alcohol with
methacrylic acid is almost negligible.
113
12
9 11 a. CH3 10 CH 3 O 3 7 1 O Si OSiO Si O n O 6 8 2 CH3 CH 3 4 5 9 7, 8, 10
5 11, 12
6 1
2 34
7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
10 b. 7 9 CH3 8 CH3 O 2 5 O Si OSiO Si O 1 n O 3 4 6 CH3 CH3
2 5, 6 7 1 8 3, 4 9, 10
20 0 180 160 140 120 100 80 60 40 20 0
Figure 6-2. NMR spectra of glycidyl epoxide functional PDPS a) 1H-NMR b) 13C- NMR
114
11 a. 10 8 1, 1' CH3 CH3 5 9 O O OH 7 4 O 2 Si O Si O Si 3 O n 6 O OH O CH3 CH3 8
2
6, 9, 10, 11
1 3, 4, 5 1' 7
0.50 0.50 8. 0 7. 5 7.0 6. 5 6.0 5.5 5.0 4. 5 4.0 3.5 3. 0 2.5 2.0 1.5 1. 0 0.5 0 -0.5 -1.0
14 1 11 13 11 b. CH3 CH3 O OH 12 8 7 O 9 6 4 5 O Si O Si O Si 10 O 5 3 2 2 3 n 9 4 O 6 8 10 7 OH O CH3 CH3 1
13 , 14
5
6, 7, 8 4 1 12 11 10 , 2 3 9
200 180 160 140 120 100 80 60 40 20 0
Figure 6-3. NMR spectra of methacrylated polydicyclopentyldisiloxane (PDPS) a) 1H-NMR b) 13C-NMR
115 The extent of reaction was determined by calculating the decrease of area under
the curve of epoxide functionality in FTIR. The siloxane bond at 1000-1100 cm-1 was
taken as internal standard. The conversion of epoxide group can be given by: 247
A ()Epoxy A t =∞ α =−(1 Si−− O Si ) * 1 0 0 6-1 A Epoxy ()t = 0 A Si−− O Si
where AEpoxy and ASi-O-Si are the area of absorbance of epoxide and siloxane functionality,
respectively. The epoxide conversion for PDMS, PDPS and PDHS were found to be 85
%, 74 % and 71 %, respectively.
A Epoxy functional PDPS methacrylated PDPS
))
B
Transmittance (%)
Epoxide
3900 3400 2900 2400 1900 1400 900 400
Wavelength (cm-1)
Figure 6-4. FTIR spectra of A) epoxide and B) methacylate functionalizated PDPS
A qualitative idea of effect of change of time, temperature and backbone
substitution on rate of reaction was obtained using PDSC technique. Overall heat of
reaction, including initiation, propagation and termination can be measured by PDSC
using:
116 ER = EP + EI - ET 6-2
According to equation 6-2, the production of free radical is in a steady state. The rate of propagation of a photosensitive reaction is proportional to the height of PDSC exotherm measured in watts per gram. The UV-initiated free radical polymerization of methacrylate functionalized siloxanes were studied keeping the photoinitiator (D1173) concentration and light intensity constant at 5 wt % and 300 mW/ cm2, respectively.
Figure 6-5 is a comparison of exotherms for free radical polymerization of methacrylic functional polydicyclohexylsiloxane at two different exposure times and temperatures.
The area under curve are 115.0 J/g for exposure time of 10 sec at 60 °C, 105 J/g for exposure time of 10 sec at -10 °C, and 163 J/g for exposure time of 20 sec at -10 °C. It was observed that if exposure time is kept constant at 10 sec and temperature is varied
––––––– HyAc_ D1173_60C_10sec.001 ––– – – HyAc_ D1173_-10C_10sec.001 a · · · · HyAc_ D1173_-10C_20sec.001 7 b
5
c 3
Heat Flow (W/g)
1
-1 1234 Exo Up Time (min) Universal V3.8B TA
Figure 6-5. Temperature and time effect for methacrylic functionalized PDHS (a) at 60 °C for 10 sec (b) at - 10 °C for 10 sec and (c) at -10 °C for 20 sec
117 between -10 °C and 60 °C, the rate of polymerization and conversion increases from 45% to 73 %. If the time of exposure is increased isothermally, the rate of polymerization also increased.
Figure 6-6 shows the overlay of exotherm for free radical polymerization of
methacrylic functionalized siloxane having methyl, cyclopenyl and cyclohexyl groups on the siloxane backbone. It was observed that as the bulk of the pendant group increases, the rate of polymerization increases, however the final conversion decreases from 88 % for methacrylated PDMS, to 77 % for methacrylated PDPS and to 73 % for methacrylated
––––––– HyAc_ D1173_60C_10sec.001 · · · · MeAc_D1173_60C_10sec.001 11 – – – – PyAc_ D1173_60C_10sec.001
9 c 7
b 5
Heat Flow(W/g) 3 a
1
-1 1234 Exo Up Time (min) Universal V3.8B T
Figure 6-6. Effect of pendant group variation on siloxane backbone on reaction rate measured at 60 °C at 10 sec (a) methacrylated PDMS (b) methacrylated PDPS (c) methacrylated PDHS
PDHS. With increase in bulk of the pendant group, the number of methacrylic group
present at a given weight of siloxane polymer decreases. Hence, space between each
118 reactive group is more, hence increased efficiency of conversion by photoinitiated
species. Table 6-1 shows the summary of PDSC experiments.
Table 6-1. Summary of PDSC results for methacrylic functional PDMS, PDPS and PDHS
Height of Sample Exposure Temperature Exotherm R Exotherm p Name time (s) (oC) Area (Jg-1) (mol/Ls) (Wg-1) PDHS 10 60 7.74 108.8 0.131
PDHS 10 -10 7.06 105.4 0.092
PDHS 20 -10 6.88 159.1 0.119
PDPS 10 60 10.34 158.7 0.216
PDMS 10 60 5.44 79.85 0.350
The thermal and viscoelastic properties of the cured films were investigated using
DSC and DMTA, respectively. The crosslink density is calculated from the modulus on
the rubber plateau and the corresponding temperature (T >> Tg) using the equation 3-4.
The Tg was obtained as the maximum of tan δ. The DMTA results are summarized in
Table 6-2. The glass transition temperatures for the polydimethylsiloxane, polydicyclopentylsiloxane and polydicyclohexylsiloxane films are -115 oC, -83 oC, -76
o 248 C. Comparing the Tg of the three systems obtained from DSC and DMTA, it was
observed that Tg increases as a function of bulk of substituents on siloxane backbone with
o the cyclohexyl substituted siloxane having the highest Tg at 38 C. A similar trend was
observed for the previous reported UV-cured cyclohexaneoxide functionalized siloxanes
where the size of the backbone substituents controlled the Tg of cured film. It can be
119 observed from the Table 6-2 that as the bulkiness of the backbone substitution increases,
the crosslink density is reduced. This is due to the fact that as the bulkiness increases, the
distance between reactive sites, and hence the cross- linking sites increase, so the free
volume in the cured matrix also increases. The contribution of backbone substituents
outweighs the effect of crosslink density on the Tg of the matrix. It is noted that the
240 difference in Tg between the thermally curable siloxanes and corresponding UV-cured
siloxanes is minimal.
Table 6-2. Storage modulus, crosslink density and glass transition temperature of methacrylic functionalized PDMS, PDPS and PDHS
' 3 ° Substitution E (Pa) at Tg ( C) ° υe (mol/m ) Tg + 20 C
Methyl 1.27 E +07 1740 -54
CycloPentyl 5.51 E +06 754 20
CycloHexyl 1.44 E +06 196 38
The X-ray diffraction pattern of the three cured siloxane is shown in Figure 6-7.
The packing density of the cured siloxane specimens were studied by WAXD. All the
samples exhibit a broad peak indicating amorphous nature of these samples. The d- spacing in the diffraction pattern, which characterize the chain to chain distance in the polymer matrix was calculated using Bragg’s equation as shown below:
120 ndλ = 2sinθ 6-3
where θ is the angle of maximum intensity of the peak observed in the sample spectrum and λ is the wavelength of the X-ray radiation. As the bulkiness of the pendant group on
the siloxane main chain is increasing, the peak intensity was found to decrease. There is
a small shift in the peak position, as the pendant group varies from methyl (2θ = 12.45°,
d-spacing value of 3.57 A°), to cyclopentyl (2θ = 10.05, d-spacing value of 4.41A°), to
cyclohexyl (2θ = 9.8°, d-spacing value of 4.52 A°). The increase in d spacing supports
the DMTA and oxygen permeability data. It suggests that the cycloaliphatic substituents
have more free space by virtue of rigidity and poor packing. Thus, the oxygen
permeability increased and the crosslink density decreased. The cured films were flexible
and had excellent adhesion properties among others.
R= Methyl
R= Cyclopentyl
Intensity R= Cyclohexyl
5 10152025 Degree
Figure 6-7. WAXD spectrum of UV- cured siloxanes
Tensile, DMTA and fracture toughness were performed to determine the general
mechanical properties of the three UV-cured silicone methacrylates. The fracture
toughness of the films is shown in Figure 6-8. The fracture toughness was also found to
121 increase with increasing substituents size. The fracture toughness is closely related to the
free volume. The poor packing of the cycloaliphatic substituents result in a lower
crosslink density, and thus a higher free volume. The increase in free volume with
substituents size is proved by d-spacing of the X-ray Figure 6-7. The Tg is related to the
rotational freedom of the substituents along Si-O-Si. The mobility of the polymer
molecular chain is inhibited by the cycloaliphatic groups, thus the higher Tg systems also
249 have higher fracture toughness. The energy release rate (GIC) of the films is given in
Figure 6-9. The energy release rate is given as the ratio of square of fracture toughness
and elastic modulus and is related to crosslink density also. With increase in size of the substituents in backbone the crosslink density reduces but modulus of the cured matrix is
increased, so more energy is released for the crack to propagate, hence Gc increases.
0.18
0.16
0.14
0.12 ] 1/2 0.1
0.08 [MPa (m) [MPa c K 0.06 0.04
0.02
0 Methyl Cyclo Pentyl Cyclo Hexyl
Figure 6-8. Plain stree fracture toughness methacrylic functionalized PDMS, PDPS, and PDHS
122 200 180
160
140 ) -2 ) 120 -2
(Jm 100 IC (Jm
c
G 80 G 60 40 20 0 Methyl Cyclo Pentyl Cyclo Hexyl
Figure 6-9. Energy release rate per unit crack area at fracture (GIC) of methacrylic functionalized PDMS, PDPS, and PDHS
The tensile properties of the films are shown in Figure 6-10. It was observed that as the bulk of pendant group increases in the backbone tensile modulus and strength increases while elongation-at-break reduces. The tensile strength and tensile modulus of
polydicyclohexylsiloxane (PDHS) is 12.6 MPa and 335 MPa, respectively. This represents
six times increase in the strength and modulus of the cycloaliphatic substituted siloxanes
than the corresponding values for polydimethylsiloxane. The % elongation-to-break for
polydicyclohexylsiloxane is 2.9, a 50 % decrease than polydimethylsiloxane. With the increase in bulk of the substituents the system becomes brittle, so the stretching ability decreases, and hence also the elongation-to-break decreases.
123
14 a. 12
10
8 6
4 Tensile Strength (MPa) Strength Tensile 2
0 Methyl Cyclo Pentyl Cyclo Hexyl
400 b. 350
300 250
200
150
100 Tensile Modulus (MPa) 50
0 Methyl Cyclo Pentyl Cyclo Hexyl
7 c. 6
5
4
3
2 Elongation at Break (%) Break at Elongation 1
0 Methyl Cyclo Pentyl Cyclo Hexyl
Figure 6-10. Tensile properties of UV- cured methacrylic functionalized PDMS, PDPS, and PDHS : (a) Tensile strength (b) Tensile modulus (c) elongation at break
124 The oxygen permeability, contact angles and coating properties of the UV- cured
PDMS, PDPS, and PDHS dimethacrylate cured systems are given in Table 6-3. Oxygen
permeability values were found to increase with increase in the bulk of the pendant
group. This can be attributed to the steric bulk of the organic groups attached to the silicone backbone increases, the sites of crosslinking becomes further apart. The free volume of the cured polymer matrix increases, and hence the oxygen transmission rate increases.250 The result indicated that increase in steric hindrance due to pendant group
on the backbone prevailed over that of crosslink density. Similar phenomenon has been
observed in literature 251 with permeability of polymer possessing larger substituents.252
Pencil hardness of a cured film is related to the elongation at break, i.e. the coating is broken only when the maximum stress, due to the pencil or indenter scratching, exceeds the tensile strength of the coating film. 253 So, the variation in pencil hardness
shows the same trend as the tensile properties i.e. hardness increases with increase in the
bulkiness of the backbone.
The reverse and direct impact test was performed to determine the ability of the
coating to resist damage caused by rapid deformation (impact). It was found that both for
reverse and direct impact testing, energy that the coating can withstand increased with
increase in bulkiness of the pendant group attached to silicone backbone. In methyl ethyl
ketone (MEK) double rub test, resistance of the film surface was found to increase as the bulkiness of the pendant group in the silicone backbone increases.
125 Table 6-3. Oxygen permeability, contact angle and coating properties of methacrylic functionalized PDMS, PDPS and PDHS
PDMS PDPS PDHS
Pencil Hardness B H H
Direct 30 ±2.55 40 ±2.07 40 ± 1.58 Impact Resistance Reverse 25 ± 1.92 30 ± 0.83 40 ± 1.14 (lb/in)
Cross-hatch adhesion 1B 4B 4B
MEK resistance (Double Rubs) 30 ± 2 50 ± 2 57 ± 3
O Permeability 2 0.11 ± kmol. m 4.67 ± 0.64 6.23 ± 0.82 Barrer( .3*1015 ) 0.02 mskPa2 ..
Pull-Off Adhesion 0.375 0.625 ±0.06 0.75 ±0.03 (N/mm2) ±0.05 72° ± Contact Advancing 80° ± 1.8° 90° ± 3.4° 2.1° Angle Receding 57° ± 5.6° 60° ± 3.3° 65° ± 2.7°
It was observed that as the bulkiness increases the pull-off adhesion value also increases. This is due to the increase in the toughness of the resin matrices. It was found that as the bulkiness of the pendant group in the silicone backbone increases, the adhesion strength increases. Crosshatch adhesion values of UV cured PDPS and PDHS methacrylate is much higher than UV cured gets PDMS methacrylate.
126 Both the advancing and receding contact angle increases with the increase in hydrophobicity of the thermally cured siloxane layer on silicone wafer. The hydrophobicity and steric size of the substituent is as follows: methyl < cyclopentyl < cyclohexyl. The surface-water-air contact angle measurements are more surface sensitive and probably respond to the outermost monolayer of the surface. The contact angle increases with increasing hydrophobicity of the surface, indicating decreasing surface polarity. Thus, the value of both the advancing and receding contact angle increases as the group in siloxane backbone changes from methyl to cyclopentyl to cyclohexyl. The low surface tension and film formation tendencies of the polysiloxane were result of rotation.
Table 6-4. Release properties of UV-cured methacrylated siloxanes
Methyl Cyclopentyl Cyclohexyl
Pull-Off Adhesion 0.375 0.625 ±0.06 0.75 ±0.03 (N/mm2) ±0.05 Adhesion 78 ± 4.3 125 ± 9.4 170 ± 6.7 Release Force (N/m) Readhesion 93 ± 8.7 140 ± 5.0 185 ± 9.8
Adhesion 75 ± 1.5 131 ± 3.2 167 ± 1.9 Aged Release Force (N/m) Readhesion 90 ± 13.3 139 ± 9.7 180 ± 11.4
It was observed that pull-off adhesion value increased as a function of substituents size. This can be attributed to the increase in the fracture toughness of the resin matrices.
With increase in size of alkyl substitution in polysiloxane backbone, a suppression of release properties but increase in adhesion strength was observed of silicones is expected,
127 thereby improving the adhesion of PSAs as given in Table 6-4. As the pendant group in silicone backbone is varied from methyl to cyclopentyl to cyclohexyl, the backbone becomes more rigid and hence the segmental mobility reduces. It was observed that the thermal ageing had almost no effect on release force values of UV-cured samples. The pull off adhesion results of photo-cured PDMS 254 is comparable with the photo-cured methacrylated PDMS.
The physical and mechanical properties of silicone acrylates or methacrylates cured films has not been widely reported. Ozgumus. et.al.255 have prepared silicone modified acrylic resin and studied its thermal and film properties. The crosslink density of cationic photocured polydimethylsiloxane (PDMS) and polydicyclohexylsiloxane (PDHS) cured by cationic UV-curing were found to be higher than the crosslink density of the corresponding free radically photo-cured methaylated PDMS and PDHS. This may be due because rate of chain transfer of the initiated radical, thereby reducing the initiator efficiency. Silicone
(meth)acrylates have been predominantly studied as a component of copolymers and copolymers, and in particular for interpenetrating network.256, 257 The application of silicone
(meth)acrylates in waterborne emulsion polymerization is also increasing.258, 259, 260
The uniqueness of the cycloaliphatic substituted siloxanes is that, these have potential for a number of applications including membranes , UV-curable resins, emulsions, and release coatings. A small addition of siloxane acrylates can cause substantial reduction in interfacial energy, useful in embossing techniques 261 and as impact modifiers. Silicone acrylates have potential usage as reactive additives in various applications such as phase separated, stratified systems. In coating and printing technology, the silicone additives provide surface properties such as increased slip and mar resistance, substrate wetting,
128 improved flow, leveling and air release. The adherence of silicone acrylates is superior and in
particular the cycloaliphatic substituted siloxanes offer advantages with respect to Tg and other physical properties. Due to its excellent flexibility and extensibility properties, particularly at low operating temperatures, these can also be used for protection of optical fibers.262
6.3 Conclusion
Low molecular weight methacrylic telechelic siloxanes having different substituents
in the siloxane backbone were successfully synthesized. It was found that with increase in
bulk of pendant group in siloxane backbone, the mechanical, and release properties
improves, crosslink density and oxygen permeability increases. With increase in size of the
pendant group mobility of the end functional reactive groups is hindered. This effect
becomes more predominant as the curing progresses. The rate of free radical polymerization
increases with time of exposure, temperature and the bulk of pendant group in the methacrylic functional siloxane backbone.
129 CHAPTER VII
NEW APPROACH TO GRAFT SILOXANES TO ALKYDS
7.1 Introduction
Alkyds were the predominant binders for paints, throughout 1940s until the early
1960s. 263 Due to the development of synthetic polymers, alkyd usage have diminished
over the past 40 years. 264 Severe restrictions on VOCs, and the push for greener biomass derived material have led to resurgence in alkyd usage. Synthesis of alkyds requires naturally occurring oils as raw materials. Coatings derived from alkyds are more environmentally compatible than the coating binders from other systems.265 Alkyds have
several benefits, for e.g. gloss retention, auto-oxidative curing, and are derived from a
renewable source. But the lack of hardness, hydrolytic stability, and alkali resistance
generally diminished alkyd usage. Alkyds also have poor outdoor weatherability and
color retention.
The ability of the alkyds to be modified in wide variety of ways accounts for their
use in more type of paints than any other binder. This versatility can be utilized by
synthesizing alkyd hybrids with various other resin systems, thus avoiding the
disadvantages of alkyd alone. One approach is to blend or graft the silicones onto alkyds.
The first siloxane alkyd hybrid system was reported in 1946, and since then many have
266 -271 been extensively reported in literature. 267 268 269 270 Silicone-alkyd copolymers were one of
the earliest examples of the hybrid organic-inorganic silicone binders.272 The
130 importance of silicon-alkyd system is derived from the synergistic properties of the two
resins enabling the resultant copolymer to have higher temperature resistance and better
exterior durability. ,273
The properties of alkyd-silicone copolymer are substantially improved over the
alkyd resins.274 Silicones possess excellent resistance to heat, oxidation, and chemical
inertness. Silicones also possess excellent moisture and UV-degradation resistance. On
the other hand, the mechanical properties of the siliconized alkyds by virtue of glass
transition temperature of many organic resins are superior to silicones. The miscibility of
silicones in alkyds is high, hence all the early silicone-alkyd hybrids systems were
blends. Due to microphase separation of components involved, polymer blends usually
exhibited multiple glass transition temperatures.275 In the grafted hybrid resins, the
microphase separation was controlled leading to better overall properties. Alkyd-silicone hybrids are used as lacquer resins and protective coatings in the electric industry due to the low-dielectric constants and, find use as coatings for lawn furniture. 276, 277
Silicone-modified alkyd resins can be prepared by both one-stage and two-stage process. The resins are usually prepared by reacting the hydroxyl or acid groups of the medium or long oil alkyd resins with hydroxyl functional groups of silicones. 278
Homocondensation of silanol groups occur as a side reaction in silicone-alkyd synthesis,
However, enough grafting occurs to compatibilize the rest of the mixture. A more
homogeneous structure may be obtained by reacting alkoxy functional silanes with the
free hydroxyl groups of alkyds.279 Low molecular weight silicone oligomers must be
used since condensation results in considerable increase in molecular weight.280
Homogeneous rhodium (I) complexes e.g. Wilkinson’s catalyst (Ph3P)3RhCl, are very
131 efficient catalysts for dehydrocoupling reaction of hydroxyl groups (R-OH) with
hydridosilanes (R’-Si-H) to form (R-O-R’). 281
In the present study, the dehydrocoupling reaction has been used to couple hydride
terminated silicones with hydroxyl groups of alkyds. Three alkyds having similar
compositions, but different oil lengths have been synthesized and reacted with three
hydride terminated silicones. Methyl, cyclopentyl and cyclohexyl groups have been
substituted on the silicone backbone. The silicone-alkyd hybrids that were synthesized
were characterized via FTIR, 1H-NMR, 13C-NMR, and GPC. After curing, the glass
transition temperature, drying time, and mechanical and coatings properties of the nine
thermally cured hybrids were determined.
7.2 Result and Discussion
The properties of siliconized-alkyd hybrids are a function of oil length and of
polyester backbone. Hence, the effect of variation of oil length on properties of the silicone-alkyd hybrids was studied. A combination of Co, Ca and Zr based drier is used to catalyze the auto-oxidative curing. 282, 283 Drier combinations having different ratios of
these three driers were tested, but the combination having equal weight ratios of the three
controlled the surface wrinkling.
Hydride functional silicones were reacted with long oil (LO), medium oil (MO)
and short oil (SO) alkyds with oil lengths of 77.2, 62.8, and 52.8. The HO- groups of the
alkyds were stoichiometrically reacted with Si-H group via dehydrocoupling reaction
evolving hydrogen gas. The dehydrocoupling reaction was catalyzed by Wilkinson’s
catalyst. 284 The dehydrocoupling reaction is useful for introducing a variety of functional
groups on siloxanes under catalytically mild conditions by the elimination of hydrogen
132 molecules. 285 The gaseous byproduct, hydrogen gas is easily removed from the system.
The dehydrocoupling reaction can be schematically depicted as in Figure 7-1.
O CH3 R CH3 O C O OH O O O O + H Si OOSi Si H O O O O n O OH O C CH3 R CH3
Alkyd Resin Wilkinson's Catalyst
CH3 R CH3 O Si OOSi Si CH n 3 CH3 R O O C O O O O O O O O O O O O O C R CH3 Si OOSi Si CH3 n CH3 R CH3
Figure 7-1. Reaction pathway for dehydrocoupling reaction
The reaction of a hydride terminated siloxane with the hydroxyl group of the alkyd was monitored through disappearance of silicone hydrogen by FTIR. The hybrid formed between hydride terminated poly(dicyclohexyl) siloxane and long oil alkyd was taken as an example for depiction of FTIR and 1H-NMR in Figure 7-2a-c, and 7-3a-c, respectively. In Figure 7-2a, the FTIR of hydride terminated PDHS is shown, with Si-H 133 stretching appearing at 2150 cm-1. The Si-H resonance was found to disappear (Figure 7-
2c), after reacting with LO alkyd.
a
Si-H
))
b
Transmittance (%) c
no Si-H
3530 3030 2530 2030 1530 1030 530
Wavelength (cm-1)
Figure 7-2. FTIR spectra of (a) hydride functional polydicyclohexylsiloxane (PDHS) (b) long oil (LO) alkyd (c) hydride functional PDHS-LO alkyd hybrid
Figure 7-3 shows the 1H-NMR spectra of hydride functionalized long oil alkyd, polydicyclopentyl siloxane and siliconized alkyd. In Figure 7-3a, the 1H-NMR, of hydride terminated PDHS is shown, with Si-H resonance appearing at δ 4.74 ppm. The ratio of integration of methyl protons attached to Si to Si-H protons is 6. In Figure 7-3b, the doublet appearing at δ 3.2-3.4 ppm was due to the methylene protons attached to the hydroxy group of the alkyd. Upon reacting the hydroxyl groups of alkyds with Si-H
134 protons, the doublet protons (CH2-OH) disappeared and a singlet due to methylene protons attached to the siloxane group (CH2-OSi) appeared as shown in Figure 7-3c.
Since equimolar Si-H and hydroxyl groups were reacted, the ratio of integration of the methylene proton attached to siloxane ether and the methyl proton of the siloxane is 2.82.
Si-H
a
9 8 7 6 5 4 3 2 1 0 -1 1 6.47
b
9 8 7 6 5 4 3 2 1 0 -1
c
1 2.82. 9 8 7 6 5 4 3 2 1 0 -1 9 8 7 6 5 4 3 2 1 0 -1
Figure 7-3. 1H-NMR spectra of (a) hydride functional polydicyclohexylsiloxane (PDHS) (b) long oil (LO)alkyd (c) hydride functional PDHS-LO alkyd hybrid
135 Figure 7-4 represents the 13C-NMR spectra for hydride functional PDHS, long oil
alkyd, and the siloxane alkyd hybrid. In Figure 7-4a, the resonances at δ 0.0 and 1.15 ppm are attributed to carbon atoms of methyl and cycloaliphatic group attached to siloxane polymers. In Figure 7-4b, the resonance due to the primary carbon atom attached to alcohol (CH2- OH) appears at δ 57. 45 ppm and the ether (C- O- C) resonance appears
at δ 62.13 ppm. Figure 7-4c shows how the alcohol resonance disappears. Due to ether
formation (C-O- Si), the resonance at δ 62.13 ppm becomes larger. The 1H and 13C NMR
spectra for medium and short oil alkyds modified with silicones were very similar.
a.
b.
c.
200 180 160 140 120 100 80 60 40 20 0
Figure 7-4. 13C-NMR spectra of (a) hydride functional polydicyclohexylsiloxane (PDHS) (b) long oil (LO)alkyd (c) hydride functional PDHS-LO alkyd hybrid
136 The drying time for the nine siloxane-alkyd hybrids are given in Table 7-1. From the dry time measurements, it was observed that the dry time increases as a function of oil length and size of bulk substituent on siloxane backbone. An alkyd with a higher oil length has a larger proportion of plasticizing fatty acid. Thus, for long oil alkyds most of the drying occurs via an autoxidative and not an evaporative process. For a medium oil
alkyd, drying has both an autoxidative and an evaporative component. For short oil
alkyds, drying is dominated by the Tg of the polyester backbone, and thus is almost entirely an evaporative process. The autoxidative process is based on a relatively slow radical chain mechanism, thus is instrinsically slower than the evaporation mechanism.
Thus, drying time is inversely proportional to the oil length of the alkyd component of the hybrid. With an increase in size of the pendant group on the siloxane, the curing is retarded, due to the presence of larger amount of non-reactive component. The mobility of chains reduces faster, and as a result frequency of collision of reactive sites reduces; hence, cure time increases.
Table 7-1. The drytime measurement data for hybrids of LO, MO and SO alkyds with PDMS, PDPS and PDHS
Alkyd-Siloxane Drytime Hybrid (hr)
LO_PDMS 8 LO_PDPS 17 LO_PDHS 21 MO_PDMS 5 MO_PDPS 10 MO_PDHS 13 SO_PDMS 2 SO_PDPS 6 SO_PDHS 6.5
137 The tensile properties of the siloxane-alkyd hybrid is shown in Figure 7-5. Tensile strength, elongation-at-break and tensile modulus of the films are shown in Figure 7-5 a, b and c respectively.
a 6.00E+06
5.00E+06
4.00E+06 M P 3.00E+06 H
2.00E+06
Tensile Strength (Pa)
1.00E+06
0.00E+00 LO MO SO (a)
6.00E+06 b
5.00E+06
4.00E+06 M
P 3.00E+06 H
2.00E+06
Tensile Modulus (Pa) 1.00E+06
0.00E+00 LO MO SO
(b)
138
c 60
50
) 40 M
on (
ti30 % P H onga 20 El
10
0 LO MO SO
(c) (continued)
Figure 7-5. Tensile properties of alkyd-siloxane hybrid (a) Tensile strength (b) Tensile modulus (c) Elongation-at-break
M - Polydimethylsiloxane P - Polydicyclopentylsiloxane H - Polydicyclohexylsiloxane
The alkyd-siloxane hybrids containing short oil alkyds were found to have higher tensile strength than corresponding hybrids containing long oil alkyd. This is due to the larger number of pendant fatty acids, in long oil alkyds. 286 The auto-oxidative curing
introduced in long oil alkyd, affords more flexibility than the rigidity of the polyester
backbone of the short oil alkyds. The tensile strength also increased as a function of
substituent size on the siloxane backbone. It has been previously reported that the
cycloaliphatic substitutions on the siloxane backbone improved the mechanical properties
of the siloxane hybrid, and thus exhibited the same trend. 287
The tensile modulus of the three alkyd-siloxane hybrids derived from short and
medium oil differ only slightly from each other. For the long oil alkyd-siloxane hybrids,
139 the cyclopentyl siloxane derivatives had the highest tensile modulus value. The elongation-to-break percent varied from 26 to 54 %. Upon comparing the hybrids derived
from the same alkyd, it was observed that the elongation-to-break percent increased with
increasing in bulkiness of the pendant group on the siloxane backbone and did not appear
to be dependent on the oil length of the alkyd.
The thermal and viscoelastic properties of the coatings were investigated using
DSC and DMTA as shown in Table 7-2. The crosslink density was calculated from the modulus on the rubber plateau and the corresponding temperature T >> Tg using equation 3-4. The Tg was obtained as the maximum of tan δ. Figure 7-6 gives the Tan δ
curves for the nine hybrids.
a. 1 PDMS_MO o PDMS_SO 0.9 24 C PDMS_LO 0.8 0.7 0.6 15 oC δ 0.5 o
Tan Tan 0.4 -1 C 0.3 0.2 0.1
0
-100 -50 0 o 50 100 Temperature ( C) (a)
140
b. 0.7 PDPS_SO o 0.6 27 C PDPS_MO o 7 C PDPS_LO 0.5 0.4 16 oC δ 0.3
Tan Tan 0.2
0.1
0 -100 -50 0 50 100 Temperature (oC)
(b) (continued)
c. 0.7 30 oC PDHS_SO 28 oC PDHS_MO 0.6 o 10 C PDHS_LO 0.5 0.4 δ 0.3 Tan Tan
0.2
0.1
0 -100 -50 0 50 100 Temperature (oC)
(c)
Figure 7-6. The Tan δ of (a) PDMS (b) PDPS and (c) PDHS based alkyd- silicone hybrids
It is observed when keeping the oil length constant, i.e. long Oil (LO), that Tg increases with increasing bulkiness of pendant group on siloxane backbone. With an increase in steric bulk of the backbone substituents, the segmental motion along the
141 polymer decreases, which in turn increases Tg. If hybrids of a particular siloxane, i.e
PDPS, with the three different oil length alkyd is considered, it is observed that with the
increase in oil length, Tg of the siloxane-alkyd system decreases. It can be explained by
the fact that with increase in oil length, the proportion of aliphatic chain in the system
increases, so the flexibility of the system also increases. The same trend in Tg values are
obtained from DSC values.
Considering the hybrid of an alkyd, i.e. medium oil, it was observed that crosslink
density decreases with increase in steric bulk of the pendant group on the siloxane
backbone. This is due to the fact that as the bulkiness increases, the distance between reactive sites i.e. free volume increases and hence the cross link density decreases. From the Tan δ plots of the hybrids, the presence of only one Tg confirms the phase miscibility
of the alkyds with siloxanes in all nine hybrid systems.
The theoretical crosslink density was calculated by eq (7-1):288
cfii ν t = ∑ (7-1) i 2
where, νt is the theoretical crosslink density
th ci is the concentration of reactive functionality of i component
th fi is the functionality of each component of i component
289 The molecular weight between the crosslink (Mc) is given in equation (7-2): ρ M = (7-2) C 3ν
where ρ is the density of the compound and ν is the crosslink density 142 Table 7-2. The crosslink density of silicone alkyd hybrids
o o Tg ( C) Tg ( C) Experimental Theoretical Mc 3 3 DSC DMTA υe (mol/m ) υe (mol/m ) (gm/mol)
PDMS-LO 16 -1 20984 1117 227
PDPS-LO 21 7 9709 783 324
PDHS-LO 49 10 5543 575 440
PDMS-MO 25 15 16241 1356 187
PDPS-MO 40 16 8430 896 283
PDHS-MO 52 28 684 632 401
PDMS-SO 46 24 5737 1550 164
PDPS-SO 47 27 514 973 260
PDHS-SO 66 30 616 671 377
On relating the Tg and crosslink density from Table 7-2 of a particular series, i.e cyclopentyl based hybrids, it was observed that the Tg was dependent on the oil length.
As the oil length increased, the crosslink density increased and the glass transition temperature decreased. This was somewhat puzzling at first. Theoretically, glass transition temperature of a system is directly proportional to the rigidity of the overall system. This can be explained by the fact that as the oil length increased, Tg reduced due to an increase in flexibility, but the number of double bonds are increased. Due to the increase in the concentration of double bonds, the crosslink density increased. Since the fatty acid groups add more flexibility than the crosslinking takes away, there is no increase in the rigidity of the overall system.
143 The plane stress fracture toughness of the nine siloxane-alkyd hybrids is shown in
Figure 7-7. Irrespective of the siloxane used, fracture toughness of the hybrids was found to be inversely proportional to its oil length. This may be attributed to the polyester
character of the alkyd backbone. As a secondary factor, the fracture toughness was also
dependent on the siloxane substitution. For a particular oil length alkyd, as the bulkiness
of the pendant group on the siloxane backbone increased the toughness also increased.
9 Long Oil Medium Oil 8 Short Oil
]
1/2 7
[MPa(m) 6 c K
5
4 Methyl Cyclopentyl Cyclohexyl
Figure 7-7. Plane stress fracture toughness of alkyd-siloxane hybrids
The coating properties for the siloxane-alkyd hybrids are given in Table 7-3. The
pull off adhesion test is a simple empirical test for assessing the bond between single and
multilayer coatings with the substrate and in between individual layers. It was observed
that as the oil length of the alkyds increased, the hybrid films had less cohesive force. A
part of the film was retained on the dolly and the other part was retained on the aluminum
substrate. With a given oil length, the hybrid alkyds with bulkier siloxane groups had
more adhesive strength. This may be due to the increase in toughness with increasing size
144 of siloxane substitution. Overall, the type of alkyd used had more impact on the pull-off adhesion than the siloxane used. The crosshatch adhesion and direct impact resistance was found to be independent of oil length or siloxane substitution.
Table 7-3. Coating properties of alkyd siloxane hybrid
Pull Off Crosshatch Pencil Impact Resistance (lb/in) Adhesion adhesion hardness (MPa) Direct Reverse
PDMS-LO 0.25 5B 7B > 40 38
PDPS-LO 0.375 5B F > 40 > 40
PDHS-LO 0.375 5B F > 40 > 40
PDMS-MO 0.75 5B F > 40 33
PDPS-MO 1.00 5B 3H > 40 39
PDHS-MO 1.25 5B 3H > 40 39
PDMS-SO 1.5 5B H > 40 31
PDPS-SO 1.625 5B 4H > 40 37
PDHS-SO 1.625 5B 4H > 40 38
Pencil hardness was inversely dependent on oil length. Short oil alkyd have more polyester character with less flexibility and impact resistance than the long oil alkyds.
Again, a general trend of increasing pencil hardness and reverse impact resistance was observed with increasing the size of the siloxane substituent.
Generally there are two ways to prepare siliconized alkyds. One is to react silanol or alkoxy functional polysiloxanes with alkyds to form copolymers. 29, 280 The other is to add silicon compounds to an intermediate or oligomer, and continue the reaction until the 145 desired properties are achieved.290 The latter method provides materials with a more
homogeneous and wider distribution of silicon atoms.
This is the first reported attempt in academic literature to prepare a siliconized alkyd via a dehydrocoupling reaction. The dehydrocoupling reaction gives more control of the grafting reaction and limits side reactions in comparison with random siloxane condensation reactions. It is presumed that the siloxane condensation grafting approach results in more self condensation of the siloxane than actual grafting. There is enough grafting, however, to compatibilize the alkyd with the ungrafted siloxane polymer resulting in a system with reasonable miscibility.
The second unique feature of the work is the utilization of the cyclopentyl and cyclohexyl substituted siloxane monomers and oligomers which were developed in the
Soucek laboratory. 61, 218 As in previously related work, the end properties of the coatings
were enhanced by the cycloaliphatic substituents. The drying time and the overall
coatings properties were similar to commercially prepared siliconized alkyds. 291, 292 The
hardness is close to phenyl substituted siloxanes without the drawback of the poor
photoxidative stability. 293 More work needs to be performed to optimize the formulations for commercial usage.
7.3 Conclusion
Nine different siloxane-alkyd hybrids were prepared using a dehydrocoupling reaction
from linseed oil based alkyds of three different oil lengths and three siloxanes with
different backbone substitutions. The coatings properties were dependent on both oil
length and siloxane substitution. Crosslink density, flexibility, and reverse impact
resistance were found to be directly proportional to oil length. Tensile modulus,
146 elongation-at-break, glass transition temperature, through dry time, and fracture toughness were inversely proportional to oil length. The cycloaliphatic substituents on the siloxane backbone enhanced all the mechanical and coating properties of the hybrids while decreasing crosslink density.
147 CHAPTER VIII
SYNTHESIS AND CHARECTERIZATION OF HOMOPOLYMERS AND
COPOLYMERS OF CYCLOALIPHATIC SILOXANES
8.1 Introduction
The two commercially dominant siloxanes are the polydimethylsiloxane (PDMS),
the polydiphenylsiloxane (PDPhS) and the copolymers thereof.294, 295 The PDMS is mechanically very weak and show cold flow at high molecular weight. Also, PDMS has
crystallization properties at low temperatures, which can be disrupted by introduction of
other siloxane segments having different backbone substitutions as alkyl, cycloaliphatic
or aromatic groups, thus keeping the siloxanes amorphous over much wider temperature
range.296 The PDPhS has a higher glass transition temperature, but the phenyl group
introduces poor photoxidative stability into the system which limits its usage. 297, 298
A diverse variety of linear, random, graft and block polymers have been synthesized during last 40 years by either ring opening polymerization or anionic polymerization.299 Block copolymers are synthesized through reactions between
organfunctionally terminated siloxanes or by ring opening of cyclic siloxanes. The block
polymers are needed to compatabilize inorganic and organic polymers. The majority of
copolymer synthesis involving siloxane polymers centers on siloxane-organic block
copolymers which can be utilized as surfactants and compatibilizers. 300, 301
148 An important approach to the synthesis of siloxane containing block or segmented
copolymers is the step growth or condensation reactions involving reactive, telechelic
siloxane oligomers with organic difunctional monomers or oligomers. 302 Usually the
route for synthesizing random block copolymers are through the mutually reactive
hydroxyl end groups under non-equilibrium kinetic conditions. Siloxane oligomers are
generally synthesized via anionic or cationic ring opening polymerization of cyclic
siloxane monomers, such as octamethylcyclotetrasiloxane (D4) in the presence of
functionally terminated disiloxanes which acts as end blockers.16, 303 , 304 Ring opening
polymerization (ROP) is another way of synthesizing reactive oligomers or block
copolymers. 305, 306 This is due to the availability of wide range of cyclic monomers,
versatility of the initiation/termination systems and mechanisms and also to some extent
the ease of the control of the end group functionalization.
In the present study, siloxane homopolymers having the cyclopentyl and cyclohexyl groups were synthesized to be used as control for comparison with the copolymers. Hydrolytic condensation of a dimethyl, dicyclopentyl and dicyclohexyl dichlorosilanes in toluene medium at 60 oC was performed to obtain the corresponding
homopolymers. Random copolymers were obtained by hydrolytic condensation of
dimethyldichlorosilane with dicyclopentyldichlorosilane or dicyclohexanedichlorosilane.
Cyclic oligomers were used as precursors for ring opening polymerization (ROP) of
cyclic oligomers to obtain block copolymers. Amberlyst-15 was used as catalyst for ROP.
Then, 1H, 13C-NMR, FTIR, DSC and ARES techniques have been used to compare the
block, and the random copolymers of polydimethylsiloxane (PDMS), polydicyclopentylsiloxane (PDPS) and polydicyclohexylsiloxane (PDHS).
149 8.2 Results and Discussion
The objective of this article is to prepare random and block copolymers of
polydimethylsiloxanes with cyclopentyl or cyclohexyl siloxanes. The copolymers were
compared to homopolymers of the corresponding cycloaliphatic siloxanes through NMR
and FTIR. The glass transition temperatures of the polymers were also compared through
DSC and rheological measurements. Hydrolytic condensation of organoalkoxysilanes and/or oligomers is the usual methodology to prepare polyorganosiloxane resins. Figure
8-1, shows the synthetic route to get the homopolymers and copolymers. By adding
weakly acidic compound to the reaction mixture for adjusting the reaction mixture to pH
2-5, the alcohol by-product can be distilled off from the reaction mixture without altering
the polyorganosiloxane resins.
R R
NaHCO ClSi Cl 3 Si O H2O n R R
(1) R = , (2) R =
(a)
R CH3 R R CH3 Amberlyst A-15 HO Si O Si O H Si O + Si O 0 Si O n m N2 , 60 C, 15 hr m n p CH3 R R CH3 R
, (3) R = (4) R =
(b)
150 R NaHCO3 R H O Cl Si Cl 2 0 HO Si O H Toluene, N2 , 60 C n R 6 hr R
(5) R = CH3 (6) R = , (7) R =
(c) (continued)
R NaHCO CH3 3 H2O (d) Cl Si Cl + Cl Si Cl Toluene, N , 60 0C 2 CH3 R 6 hr R R CH R CH R R R CH3 R CH3 CH3 CH3 R CH3 CH3 3 3 HO Si O Si O Si O Si O Si O Si O Si O Si O Si O Si O Si O Si O Si O Si O Si O Si OH R R R R R R CH3 R CH3 CH3 CH3 R CH3 CH3 CH3 CH3
(8) R = , (9) R =
(d)
Figure 8-1. Route for synthesis of homo and copolymers (a)Cyclic oligomer synthesis by hydrolytic condensation of dichlorosilanes, (b) Block copolymer synthesis by ROP of cyclic oligomers (c) Homopolymer synthesis by condensation of di substituted dichlorosilane (d) Random copolymer synthesis by condensation of two different di substituted dichlorosilane
Characterization
The cyclic oligomer of polydicyclopentylsiloxane (PDPS) and polydicyclohexylsiloxane (PDHS) are clear yellow and white oily liquid, respectively. In
Figure 8-2 shows the FTIR of homopolymers and copolymers of polydicyclohexylsiloxane (PDHS). The three main stretching in FT-IR spectra of cyclic oligomers are present at 1000- 1100 cm-1 are due to the siloxane (Si-O-Si) bond
151 stretching of C-H alkylene group of cycloaliphatic moiety occurs at 2800 cm-1, and C-H
stretch of methyl group occurs around 300 cm-1. A sharp absorptions can be found at 795
cm-1 for copolymers. It is due to the C-H stretching vibrations of methyl group of PDMS
present in either the random or block copolymer with PDHS. In PDHS homopolymer,
-1 due to the absence of the CH3-Si bond, this absorbtion is absent. At 2960 cm , there is presence of an absorption in the random and block copolymers which is absent in the
120 a. ------PDHS-b -PDMS PDHS-ran -PDMS PDHS homopolymer 100
Transmittance (%)
80
60
40 ))
20
0 3000 2900 2800 270015 0 0 14 0 0 13 0 0 1200 110 0 10 0 0 900 800 700 600
Wavelength (cm-1)
Figure 8-2. FT-IR of homopolymer and copolymer of polydicyclohexylsiloxane (PDHS)
PDHS homopolymer. This is due to asymmetrical stretching of methyl group (νas CH3)
group of PDMS present in the copolymers, which is absent in the PDHS homopolymer.
The corresponding spectra for polydicyclopentylsiloxane is very similar and hence not shown given.
152 Figure 8-3 shows 1H-NMR of homopolymers and copolymers of polydicyclohexyl- siloxane (PDHS). The random and block copolymers were synthesized
from equimolar proportion of PDHS and PDMS.
a. 4 3 3 2 2 1 O Si O 1, 2 n
3, 4
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -0.5 -1.0 Chemical Shift (ppm)
(a)
b. 4 5 33 5 22 CH 1 3 Si O Si O n m 1, 2 CH3
3, 4
3.3 1 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -0.5 -1.0 Chemical Shift (ppm)
(b)
153 c. 4 3 5 2 CH CH3 CH3 CH3 CH3 3 1 5 O Si O Si O Si O Si O Si O Si O Si O Si O Si O Si O CH CH CH CH 3 3 3 CH3 3
3, 4 1, 2
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -0.5 -1.0 Chemical Shift (ppm)
(c) (continued)
Figure 8-3. 1H-NMR of (a) polydicyclohexylsiloxane (PDHS) homopolymers (b) block copolymer of PDHS and PDMS (c) random copolymer of PDHS and PDMS
In the block copolymer of PDHS and PDMS i.e. PDHS-b-PDMS, the ratio of
integration of three protons of methyl to eleven protons of cyclohexyl group is 3.3, the
theoretical being 3.6. Spectral differences between homopolymers, block and random
copolymers are good and clear indications for distinguishing between the polymers. The
NMR spectra of block copolymers are expected to be almost the superimposition of the spectra of the corresponding homopolymers. In functionally terminated polymers, the block copolymers can be distinguished from the mixture of the corresponding homopolymers by the resonances of the end groups, even though the predominant part of the spectra of the block copolymer and homopolymer mixture is the same. But in this case since the block copolymers are not end capped so the 1H-NMR spectra are
indistinguishable from homopolymer mixtures.
In random copolymer PDMS-random-PDHS, there is appearance of additional
resonance like at δ 0.71 and δ 2.39 due to random distribution of cyclohexyl and methyl
154 group along the backbone. Methylene and methine units of cycloaliphatic group showed complicated splitting around the methyl (Si-CH3) resonance (δ 0.04 to 0.25 ppm) owing to the presence of different types of monomer sequences. Similarly in Figure 8-3b, the proton NMR spectra of block copolymer of PDPS and PDMS, is a superimposition of spectra of homopolymer of PDMS and PDPS, respectively. There is a presence of additional resonance at 1.0 and 2.37 ppm
Figure 8-4 gives 13C-NMR of homopolymers and copolymers of polydicyclohexyl- siloxane (PDHS).
4 a. 33 2 2 1 O Si O n
200 180 160 140 120 100 80 60 40 20 0 (a)
b. 4 33 5 5 2 2 CH 1 3 Si O Si O 1, 2, 3, 4 m n CH3
200 180 160 140 120 100 80 60 40 20 0 (b) 155
4 c. 3 5 2 CH3 CH CH3 CH3 CH3 1 3 Si O Si O Si O Si O Si O O Si O Si O Si O Si O Si O 5 CH CH CH CH 3 3 3 CH3 3
1, 2,3, 4
200 180 160 140 120 100 80 60 40 20 0
(c) (continued)
Figure 8-4. 13C-NMR of of (a) polydicyclohexylsiloxane (PDHS) homopolymers (b) block copolymer of PDHS and PDMS (c) random copolymer of PDHS and PDMS
As in the case of 1H-NMR, 13C-NMR of block copolymer is indistinguishable
from the superposition of the two homopolymers PDMS and PDHS in Figure 8-4b. The
PDHS homopolymer 13C-NMR spectrum, consists of four resonances appearing at δ
26.25, 26.63, 27.51 and 28.69 ppm. The four resonances corresponds to the four types of
carbon in dicyclohexylsiloxane. In the random copolymer of PDHS and PDMS, there are complicated splitting of cyclohexyl and methyl carbon resonance occurs and appearance
of additional resonances at δ 37.8, 39.5, 51.0, and 53.6 ppm. The carbon NMR spectra
of homopolymers and copolymers of PDPS, shows similar trait.
Figure 8-5 gives 29Si-NMR of homopolymers and copolymers of
polydicyclohexyl- siloxane (PDHS). The resonance at δ 0 ppm is due to TMS standard.
The Si resonance due to dicyclopentyl in PDHS homopolymer appears in δ -39.5 ppm.
The Si-NMR spectra of the block copolymer, shows no splitting of resonance due to Si
156 atom of PDMS. The random copolymer shows complicated splitting around PDMS Si (-
20 ppm ) and PDHS Si (-39.5 ppm), due to random monomer sequences.
a. 1 O Si O 1 n
60 40 20 0 -20 -40 -60 -80 -100 (a)
b. CH3 2 1 Si O Si O m n
CH3 2 1
60 40 20 0 -20 -40 -60 -80 -100 (b)
157
c. CH3 CH CH3 CH3 CH3 3 1 2 11 2 2 1 2 1 2 O Si O Si O Si O Si O Si O Si O Si O Si O Si O Si O CH CH CH CH 3 3 3 CH3 3
60 40 20 0 -20 -40 -60 -80 -100
(c) (continued)
Figure 8-5. 29Si-NMR of (a) polydicyclohexylsiloxane (PDHS) homopolymers (b) block copolymer of PDHS and PDMS (c) random copolymer of PDHS and PDMS
The differential scanning calorimetry thermo- grams of homo, random and block
copolymers of PDHS and PDPS is given in Figure 8-6a and 8-6b respectively. Since
PDMS is completely miscible with PDPS and PDHS, so DSC curves show single and
narrow glass transition temperature (Tg). The Tg (-65 °C) of random copolymer PDHS-
ran-PDMS was found to be very close to the Tg (-56 °C) of homopolymer PDHS. The bulky cyclohexyl group of PDHS dominates over the thermal transition of the constituent
PDMS in the random copolymer and hence the Tg of random and homopolymer of PDHS
are so close. The Tg of block copolymer PDHS-b-PDMS is (-82°C ) intermediate between those of the homopolymers but much towards PDHS than towards PDMS.
158
Heat Flow (W/g) Flow Heat
Temperature (oC)
(W/g) Flow Heat
o Temperature ( C)
Figure 8-6. DSC curves of homopolymers and copolymers of (a) PDHS (b) PDPS
159 The Advanced Rheometric Expansion System (ARES, TA Instruments) was used at a constant angular frequency of 1 rad/s for dynamic temperature sweep experiments.
The Tg was obtained as the maximum of tan δ. Figure 8-7a and 8-7b shows the tan delta plot for homopolymers and copolymers of polydicyclohexylsiloxane (PDHS) and
δ δ
Tan Tan
Temperature (oC)
δ δ
Tan Tan
Temperature (oC)
Figure 8-7. Glass transition temperature (α-transition) of homopolymers and copolymers of (a) PDHS (b) PDPS
160 polydicyclopentylsiloxane (PDPS) respectively. As can be observed by comparing Figure
8-6a with 8-7a or Figure 8-6b with 8-7b, that the trend of increase of Tg remains the same irrespective of the measurement techniques used.
The relationship between Tg of random and block copolymers of polydimethylsiloxane (PDMS) with polydicyclopentylsiloxane (PDPS) and polydicyclohexyl siloxane (PDHS) were compared w.r.t the homopolymers using Fox equation. 307 The Fox equation is based on free-volume concepts can be expressed as follows:
1 w i = ∑ (1) Tg Tgi where wi is the weight fraction of monomer i and Tgi and Tg are the glass transition temperatures of homopolymers i and the copolymers, respectively.
Table 8-1. Comparison of Tg values obtained experimentally and predicted by fox equation
o Sample Name Weight Fractions Tg ( C) Experimental Calculated (DSC) by Fox eqn PDPS-b-PDMS 0.75 : 0.25 -90 -94
PDHS-b-PDMS 0.78 : 0.22 -82 -86
PDPS-ran-PDMS 0.65 : 0.35 -50 -95
PDHS-ran-PDMS 0.67 : 0.33 -56 -88
It is observed that the Tg values predicted by Fox eqn is in good agreement with the experimental Tg values of the block copolymers found by DSC measurements but not for the random copolymers.
161 Hydroxy terminated polysiloxanes are of great importance due to their reactivities
308, 309 towards isocyantes and carboxylic acids (and the derivatives). DPTP TD This reactivity allows
for subsequent synthesis of segmented polyurethanes, polyesters and polycarbonates. 310, 311,
312 Block copolymer seem to be particularly interesting because of an unusually high
mobility of the silicon-oxygen chain.313 These polymers are soluble in various organic solvents making siloxanes useful in many applications 314, 315 as additives for solvent-
containing and also low-solvent finishes 316, surface coatings and pastes, for coating textiles
317 and paper (tissues) 318, as starting materials for cross-linking reactions 319, as antifoams
320, as emulsifiers 321, as additives in cosmetic formulations 322 etc. Linkage of different
siloxane blocks in a one molecule has unusual and surprising effects in various application
tests. The various siloxane blocks can be linked to one another via coupling agents. 323, 324
The blocks may differ from one another in polarities 325, hydrophilic / hydrophobic balance
321, 326, or in proportions of reactive groups. 327
8.3 Conclusion
Synthesis of random and block copolymers of PDMS was accomplished with either
PDPS and PDHS. Control of copolymerization can be achieved for copolymers of dimethyl dicycloaliphatic groups. The synthesized polymers were characterized and
1 13 29 compared using H-NMR, C-NMR, Si-NMR, FT-IR, DSC and ARES. The Tg of random copolymers were found to be higher than the corresponding block copolymer Tg.
There was very little difference in Tg of homopolymers PDPS and PDHS from the Tg of the corresponding random copolymers synthesized with PDMS i.e PDPS-ran-PDMS and
PDHS-ran-PDMS.
162 CHAPTER IX
CONCLUSIONS
A novel route was developed to synthesize functionally terminated cycloaliphatic
substituted siloxanes from dicycloaliphatic silane monomers. A high temperature and
high pressure hydrosilation reaction of gaseous dichlorosilane with cycloalkene ( in
present study, cyclopentene and cyclohexene) produced dicycloaliphatic dichlorosilane.
Hydride telechelic siloxane was prepared from the silane monomers subsequently by hydrolytic condensation and ring opening polymerization. This hydride end functionalization is a very useful precursor for preparing siloxanes with a wide range of functionalities. In the present study glycidyl epoxide, and aliphatic amine telechelic siloxanes were obtained by hydrosilation reaction from the hydride terminated siloxanes.
A decrease in epoxide equivalent weight of the glycidyl epoxide telechelic siloxanes was observed as the size of pendant group in the siloxane backbone increased.
Blocking and deblocking chemistry was utilized for synthesizing amine functionalized siloxane. Primary amine were found to poison the Pt based hydrosilation catalyst. Hence, direct hydrosilation of a primary amine containing aminating agent with –Si-H moiety was unsuccessful. Hence, blocking of –NH2 functionality of allylamine, followed by
hydrosilation and de-blocking was employed and was proved to be effective.
The epoxide terminated siloxanes were thermally cured with amino terminated
siloxanes. UV curable siloxanes were obtained by ring opening of epoxide functionality
163 of glycidyl epoxide siloxanes by methacrylic acid. The incorporation of cycloaliphatic
moiety at the siloxane backbone increased the glass transition temperature with respect to corresponding polydimethylsiloxanes. The thermal and UV-cured siloxanes system showed similar trend in all the properties with respect to the change of substitution at the polymer backbone. Some of the noticeable property trends were, decrease in conversion and crosslink density but increase in oxygen permeability and release properties as substituents size on siloxane backbone increased.
To extend the applications of siloxanes in outdoor applications hybrids of
siloxanes with alkyds were synthesized. These hybrids showed no phase separation.
Increase in oil length of alkyds in the hybrid, resulted in increased crosslink density and
flexibility, while the mechanical properties deteriorated. With the increase in steric bulk
of the pendant group on the siloxane backbone, coating and mechanical properties
improved while cross-link density decreased. Random and block copolymer of
polydimethylsiloxanes and polydicycloaliphaticsiloxanes were synthesized. The glass transition temperatures (Tg) of random copolymers were found to be higher than the
corresponding block copolymers. There was very small difference in Tg between cycloaliphatic siloxanes homopolymers and the corresponding random copolymers with polydimethylsiloxanes.
From this study, it can be concluded that the cycloaliphatic substitutions can lead to intermediate properties between polydimethyl- and polydiphenyl siloxanes. The salient feature of the cycloaliphatic substituted siloxanes is the unique relationships between crosslink density, free volume and glass transition temperature. For cycloaliphatic substituted siloxanes, the relationship between glass transition temperature and crosslink
164 density is inversely proportional as a consequence of packing. The phenyl group, although capable of rotation can pack together more closely in the solid/ rubber state on account of the incipient planar and dipolar forces, both of which are absent in the cycloaliphatic groups. As a consequence, the cycloaliphatic substituted silicones offer a unique opportunity to have higher Tg and thus usage temperature with high permeability.
165
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