Interfacial glass transition profiles in ultrathin, spin cast polymer films

Abstract

Interfacial glass transition temperature ${\mathrm{(T}}_g)$ profiles in spin cast, ultrathin films of polystyrene and derivatives were investigated using shear-modulated scanning force microscopy. The transitions were measured as a function of film thickness (δ), molecular weight, and crosslinking density. The $T_g\mathrm{(\delta )}$ profiles were nonmonotonic and exhibited two regimes: (a) a sublayer extending about 10 nm from the substrate, with $T_g$ values lowered up to ∼10 °C below the bulk value, and (b) an intermediate regime extending over 200 nm beyond the sublayer, with $T_g$ values exceeding the bulk value by up to 10 °C. Increasing the molecular weight was found to shift the $T_g\mathrm{(\delta )}$ profiles further from the substrate interface, on the order of 10 nm/kDa. Crosslinking the precast films elevated the absolute $T_g$ values, but had no effect on the spatial length scale of the $T_g\mathrm{(\delta )}$ profiles. These results are explained in the context of film preparation history and its influence on molecular mobility. Specifically, the observed rheological anisotropy is interpreted based on the combined effects of shear-induced structuring and thermally activated interdiffusion.