Manish K. Mundra

Effects of Nanoscale Confinement and Interfaces on the Glass Transition Temperatures of a Series of Poly(n-methacrylate) Films

We use fluorescence from dye-labelled polymer to measure the glass transition temperatures (Tgs) across single-layer films and near surfaces and silica interfaces in bilayer films for a series of poly(n-methacrylate)s. With nanoscale confinement, the average Tg across a film supported on silica increases for poly(methyl methacrylate) (PMMA), decreases for poly(ethyl methacrylate) (PEMA) and poly(propyl methacrylate), and is nearly invariant for poly(iso-butyl methacrylate) (PIBMA). These trends are consistent with the relative strengths of local perturbations to Tg caused by surfaces and substrates as measured in bilayer films. The substrate effect, which increases Tg via hydrogen-bonding interactions between the polymer and hydroxyl groups on the silica surface, is stronger than the free-surface effect in PMMA. The free-surface effect, which reduces Tg via a reduction in the required cooperativity of the glass transition dynamics, is stronger than the substrate effect in PEMA. The substrate and free-surface effects have similar strengths in perturbing the local Tg in PIBMA, resulting in a net cancellation of effects when measurements are made across single-layer films.

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 (Tg)‐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 Tg‐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 grea...