The Effect of Nanoconfinement on the Glass Transition Temperature (Tg) and Mobility Associated with Diffusion in Polymer Films: A Fluorescence StudyPublic Deposited
In this dissertation, the Tg behavior of nanoconfined polymer films and 1-dimensional (1-D) patterned polymer nanostructures was studied. Using a novel fluorescence method, a reduction in Tg was observed upon confinement in PS films supported on silica, where free-surface (polymer-air interface) effects are dominant in modifying Tg. In contrast, Tg was observed to increase upon nanoconfinement in poly(methyl methacrylate) (PMMA) films supported on silica, where moderate attractive polymer-substrate interactions are present. By placing dye-labeled polymer films in a multilayer geometry with unlabeled polymer films, unique information was obtained on the effect of interfaces in modifying Tg upon confinement and the distribution of Tgs across the multilayer film thickness. In particular, with nanoscale confinement of PMMA films it was shown how the free-surface effect competes with and is overwhelmed by the substrate effect. In a first ever measurement, it was demonstrated that this competition leads to a free-surface layer Tg exceeding the bulk polymer Tg. This research also led to the first study of Tg-nanoconfinement effects in 1-D polymer nanostructures. As with films, interfacial interactions dictate the ultimate Tg behavior of nanostructures This research has contributed in other ways to understanding the effects of nanoscale confinement on polymer properties. Using small variations in the repeat unit structure and backbone rigidity or addition of low molecular weight diluents, it was demonstrated that the magnitude of the Tg-nanoconfinement effect is strongly tunable in polymers and can result in deviations from bulk Tg of about 45-50 K. Furthermore, this tunability can be understood qualitatively based on fundamental polymer physics related to cooperative segmental mobility. In this dissertation a new method was also developed based on simple fluorescence to determine the effect of confinement on the diffusion coefficients of small dye molecules and found that diffusion coefficients can differ by as much as an order of magnitude in 125-nm-thick films as compared with bulk films. Finally, a novel technique that uses intrinsic fluorescence to characterize the effect of confinement on Tg in thin and ultrathin films of styrene containing copolymers as well as the effect of stress relaxation on local chain conformations was also developed.