An Investigation into the Glass Transition Temperature of Vapor Phase Infiltrated Organic-Inorganic Hybrid Materials
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Glass transition temperature (Tg) is a fundamental property of a polymer that defines its upper service temperature for structural applications and is reflective of its physicochemical features. We are interested in how vapor phase infiltration (VPI), which infuses polymers with inorganic species to create hybrid materials, affects the glass transition temperature of a material. We examine Al2O3 VPI into poly(styrene-co-2-hydroxyethyl methacrylate) (PS-r-PHEMA) using trimethylaluminum (TMA) and water precursors. Our VPI precursors are selected to be unreactive towards the styrene monomer units and highly reactive towards the HEMA monomer units. Experiments were conducted on PS-r-PHEMA thin films (200 nm) spun-cast onto silicon wafers and infiltrated at 100°C with 4 hr. exposure times. Copolymers with varying fractions of HEMA units were investigated, from 0 mole % to 20.2 mole % HEMA. Volumetric swelling of the films after VPI and aluminum oxide film thicknesses after pyrolysis both confirmed higher metal oxide loading with higher fraction HEMA units. Tg was measured using a spectroscopic ellipsometer with a heating unit. We find that the glass transition temperature increases significantly with metal oxide loading. Copolymers with 0.0%, 3.0%, 7.7%, 11.5%, and 20.2% HEMA units experienced 6°C, 8°C, 22°C, 37°C, and 46°C increases in Tg respectively. Changes in Tg at low HEMA composition fit the Fox-Loshaek model for crosslinking phenomena which, along with a dissolution study on these materials, suggests that VPI alumina crosslinks PS-r-PHEMA. We conclude that VPI may be useful as a crosslinking process for designing the thermophysical and thermochemical properties of polymer thin films, fibers, and fabrics.