(581a) Novel Characterization of Relative "Stiffness" and Its Distribution Across Nanoconfined Polymer Films Using Rigidochromic Fluorescence

Several years ago, our group showed that fluorescence spectroscopy applied to multilayer films in which one layer contains a fluorescence dye label such as pyrene or TC1 can yield very sensitive, quantitative details on the distribution of glass transition temperatures and physical aging rates across thin and ultrathin polymer films. We have extended this approach to provide information related to the average "stiffness" as well as the distibution of "stiffness" across thin and ultrathin polymer films. We demonstrate that several dyes exhibit fluorescence spectral shapes and/or a shift in their fluorescence spectra as a function of wavelength that is indicative of local rigidity in the polymer film on the time scale of the excited state lifetimes of the dyes (~ 1-200 ns), i.e., the dyes are rigidochromic. In films of poly(methyl methacrylate) or polystyrene that are either supported on silica substrates or freely standing, the rigidochromic dye fluorescence is consistent with a significant stiffening of the films with decreasing film thickness, increasing physical aging time in the glassy state, and proximity of the dye-labelled layer to a silica substrate when attactive polymer-substrate interactions (e.g., hydrogen bonding) are present. The rationale for the rigidochromic character of the dyes will be described along with other studies aimed at understanding how nanoscale confinement in thin films and nanocomposites can alter the rigidity of polymers.