The Glass Transition and Physical Aging Behavior of Polymer Nanocomposites Studied via FluorescencePublic Deposited
The average and local glass transition temperatures (Tgs) and physical aging behavior of various confined polymers were studied in order to gain an understanding of these surface/interface effects and their propagation into the polymer. Using a novel multilayer/fluorescence method, the Tgs of layers at the free surface, substrate-polymer interface, and at known distances away from the interfaces of poly(methyl methacrylate) (PMMA) films supported on silica were measured. Since attractive hydrogen-bonding interactions are present, it was shown that perturbations to Tg can propagate over length scales of several hundred nanometers and overwhelm the free surface effect, thereby leading to an increase in the average Tg across the film. The effect of the addition of silica or alumina nanospheres to polystyrene, PMMA and poly(2-vinyl pyridine) (P2VP) on Tg was investigated using fluorescence. The results showed that the Tg and physical aging of polymer nanocomposites can be controlled by tuning the polymer-nanofiller interactions via choice of polymer, nanofiller, and the preparation procedure. The nanocomposite Tg may be either enhanced or depressed relative to bulk, depending on the presence of wetted interfaces with attractive interactions or free surfaces between the polymer and the nanofiller. In addition, model PMMA- and P2VP-silica nanocomposites consisting of polymer films confined between silica slides were designed in order to provide a simple way to gain insight into the effect of interparticle spacing on Tg. The model and real nanocomposites with identical Tg deviations yield similar dramatic suppressions of physical aging. This study has also resulted in the first determination of the distribution of Tgs within polymer nanocomposites. Different functionalized single-wall carbon nanotubes (SWCNTs) were grafted to PMMA in order to identify their effect on the local and average Tgs of the system. The results indicate a relationship between the length and flexibility of the side groups in the SWCNT and the extent of the increase on Tg. Even more important from a technological standpoint is that the incorporation of nanofillers in the polymer dramatically suppresses the physical aging behavior of the matrix. This provides a new application for nanofillers creating glassy-state polymer nanocomposites with long-term stable properties.