Glass Transition of the Surface Monolayer of Polystyrene Films with Different Film Thicknesses and Supporting Surfaces

Surface glass-transition temperature (Tgsurf) and transition width (W) within 1 nm of the surface were measured by monitoring a qualitative change in the contact angle or density of end-groups (by time-of-flight secondary ion mass spectrometry) with temperature. Polystyrene (PS) films with various thicknesses (h) and molecular weights were studied. For unannealed PS supported on oxide-coated silicon or poly(dimethyl siloxane) with h > similar to 60 nm, Tgsurf approached a plateau value of similar to 25 K below the bulk Tg; below 60 nm, Tgsurf decreased with decreasing h. Separately, W exhibited a stepwise increase when h was decreased below the radius of gyration of the polymer. Upon thermal preannealing or deposition on a PS brush or adsorbed layer, the films ceased to exhibit Tgsurf reductions or stepwise change in W. We discuss how an out-of-equilibrium density profile with a deficit near the substrate and de Gennes' sliding mode may explain these observations.