Novel effects of confinement and interfaces on the glass transition temperature and physical aging in polymer films and nanocomposites

Recently, it has become evident that the magnitude of the glass transition (T_g)-confinement effect depends strongly on the polymer repeat unit and that the magnitude of the physical aging rate can be dramatically reduced relative to neat polymer when attractive polymer-nanofiller interactions are present in well-dispersed nanocomposites. However, in neither case has a quantitative, fundamental understanding been developed. By studying polymers with different chain backbone stiffness, e.g., polystyrene (PS) vs. polycarbonate (PC) vs. polysulfone (PSF) and that lack attractive interactions with the substrate interface, we show that the T_g-confinement effect is the weakest in the polymer with the least stiff backbone (PS) and strongest in the polymer with the most stiff backbone (PSF). These results are consisten with the notion that, other things being equal, a larger requirement by the polymer for the cooperativity of the segmental mobility that is associated with the glass transition will result in a greater reduction of T_g near the free surface of the film and thus the average T_g across the film. A quantitative understanding of the causes behind the effects of confinement on the glass transition and physical aging of nanocomposites with dispersed nanofiller is prevented by the complex nanoparticle distribution. Here, we have developed model silica nanocomposites with a single, quantifiable interlayer spacing equal to the film thickness separating two silica slides. Comparisons show that the model nanocomposites yield results consistent with the complex real nanocomposites. This provides the possibility to conduct studies that will allow for an understanding of how the separation distance between nanofiller interfaces impacts the glass transition and physical aging.