Glass Transition of the Surface Monolayer of Polystyrene Films: Effect of Thermal Preannealing
Jinsong Yan, Zongyi Ma, Jianquan Xu, Haoran Nie, Hailin Yuan, Xinping Wang, Tong Wang, Lu‐Tao Weng, Ophelia K. C. Tsui
Abstract
In a previous experiment, we observed that the glass transition temperature of the surface monolayer glass transition temperature ( T g surf ) of unannealed polystyrene (PS) films spin-coated on oxide-coated silicon decreased with decreasing film thickness ( h < ∼60 nm). However, this h dependence could be removed by thermal preannealing. In this experiment, we investigated the origin of this preannealing effect. We found that T g surf of the thin films increased with preannealing time ( t ANN ) and reached the steady-state thick-film value at a characteristic time (τ surf ) following the τ surf ∼ h 2 scaling for diffusion. Moreover, chain adsorption occurred alongside the increase in T g surf during thermal preannealing and reached the steady-state adsorbed amount at a time similar to τ surf . We tentatively suggest that the observed increase in T g surf with t ANN was caused by diffusion of free-volume holes from the polymer–substrate interface to the free surface due to densification of the interfacial polymer upon chain adsorption.