Glass Transition of the Surface Monolayer of Polystyrene Films with Different Film Thicknesses and Supporting Surfaces
Jinsong Yan, Jianquan Xu, Lu‐Tao Weng, Fengliang Wang, Xinping Wang, Hailin Yuan, Tong Wang, Ophelia K. C. Tsui
Abstract
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 > ∼60 nm, Tgsurf approached a plateau value of ∼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.