Light adjustable macromer-doped elastomers : the thermodynamics, transport, and photochemistry of silicones

Citation

Pape, Eric A (2006) Light adjustable macromer-doped elastomers : the thermodynamics, transport, and photochemistry of silicones. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/342T-6796. https://resolver.caltech.edu/CaltechETD:etd-12012005-203446

Abstract

Novel elastomeric photopolymer systems that exhibit macroscopic optical and mechanical changes when polymerized were designed and synthesized. These photopolymers consist of a well-defined polydimethylsiloxane matrix, an UV absorbing photoiniator, and a reactive macromer with methacrylate endgroups and a siloxane based midblock. Model elastomer matrices were synthesized, swollen with macromer, macromer was photopolymerized, and mechanical, transport, and thermodynamic behaviors of the resulting photoelastomers were measured. Modulus increases of 50 to 150% were seen for sequential interpenetrating networks (IPNs) formed by photopolymerizing different length macromers in model networks. Modulus change was independent of network structure but highly dependent on macromer length and extent of macromer doping. With increased macromer doping, the modulus increase seen on photopolymerization changed from a theoretical dispersed phase behavior to bicontinuous phase behavior. Unexpectedly, mechanical properties were independent of photopolymerization conditions; increased photoinitiator content and irradiation intensity did not alter cured photopolymer modulus. Swelling behavior for model networks and IPNs was independent of network precursor molecular weight and polymer volume fraction at network preparation. As macromer molecular weight increased, experimental scaling behavior approached theoretical values. For photopolymerized IPNs, macromer was a poorer solvent than in model networks. Macromer containing phenyl groups also showed significant decreases in solubility. Diffusivity values ranged from 2x10^-11 to 2x10^-12 m2/s for 1 to 5kg/mol macromer molecular weight in both model and interpenetrating networks, were dependent on penetrant molecular weight and host network modulus, and were independent of thermodynamic interaction parameters. Diffusivities of bimodal molecular weight distributions indicated dependence on host network modulus due to molecular sieving. Photopolymerization of macromer swollen in networks showed unusual reaction kinetics. Although reaction rates scaled linearly with reactive endgroup concentration for a specific macromer molecular weight, they exhibited variable dependence on initiation rate for different photoiniator concentrations and irradiation intensities. High irradiation intensity and photoinitiator concentration led to reaction rate independence on the initiation rate. Different macromer molecular weights showed unique reaction trajectories; maximum reaction rates and the time and extent of conversion at which they occur are not explained with simple scaling behavior. Chain length dependent termination measurements were used to describe reaction rate trajectories.

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