Glass Transition of the Surface Monolayer of Polystyrene Films: Effect of Thermal Preannealing

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.

Physical aging and vitrification in polymers and other glasses: Complex behavior and size effects

“…Since the model of McCaig et al 214 was introduced, different experimental efforts have been devoted to the application of variously developed FVHD models to describe physical aging of thin polymer films, 197,215,216 and polymer nanocomposites. 217,218 Furthermore, the FVHD model was also successfully applied to the description of vitrification kinetics, specifically the previously discussed T g depression with reducing sample size, in polymer thin films, 197,[219][220][221][222][223] and metallic glasses with micrometric size. 190 An important aspect highlighted in all these studies concerns the importance of free unburied sample surface to allow free volume holes to diffuse out of the sample and maintain equilibrium, thus delaying vitrification to lower temperatures; or to recover it once in the glassy state.…”

Advancements in Novel Mechano-Rheological Probes for Studying Glassy Dynamics in Nanoconfined Thin Polymer Films

“…84 Such an irreversibly adsorbed layer has the potential to even perturb the T g of the surface monolayer in the polymer film. Yan et al 85 showed that the confinement effect observed for surface monolayer T g (T g surf ) in PS thin films could be eliminated by performing thermal preannealing. The adsorption kinetics of polymer chains matched well with the time required for the saturation of T g surf value to the thick film T g .…”