Effects of confinement on the glass transition of polymer-based systems
In recent years, considerable effort has been invested toward developing polymers for applications in which they serve as the active material component for devices such as transistors, light emitting diodes, and various sensors. Many of these applications require polymer thin films. Unfortunately, many of the properties of the thin films that impact device processing and performance are not well understood. Polymer films in this thickness range exhibit properties that are very different from the bulk. Properties such as the viscosity, the glass transition temperature, Tg, and phase transitions exhibit film thickness dependencies. This thesis examines three problems generally in the area of the glass transition temperature of polymer thin films. (1) The Tg of polymer-polymer mixtures: We examined the Tg of thin films of the miscible blend tetramethyl Bisphenyl-A polycarbonate (TMPC)/polystyrene (PS). Our results indicate that entropic, “chain packing,” effects and enthalpic effects associated with interactions between the dissimilar chain segments and the external interfaces (free surface and substrate) determine the Tg of these mixtures. (2) The glass transition of polymer-based nanocomposite films: We examined the film thickness dependencies of the glass transition temperatures of polystyrene based nanocomposite thin films containing small concentrations, 1-5 wt.%, of layered silicate clays and C60 fullerenes supported by silicon substrates, PS-LSi/Si and PS-C60/Si, respectively. Our results show that at these small concentrations the nanoscale particles can change Tg appreciably, particularly in films with thicknesses less than 45 nm. Shifts in Tg of up to 20 degrees were observed, regardless of the chemistry of the nanoparticles. (3) Polymer (polystyrene and polymethylmethacrylate) thin films in supercritical CO2: The effect of CO2 on the glass transition plays a central role in the processing of thin films for microelectronic applications. Our most significant finding is the phenomenon of retrograde vitrification, where the polymer in a CO2 environment exhibits a rubbery-to-glass transition as the temperature is decreased and surprisingly a glassy-to-rubbery transition with a further decrease in temperature. These findings have important implications on the processing of thin polymer films in CO2 environments.