Vitrification of Thin Polymer Films: From Linear Chain to Soft Colloid-like Behavior

Abstract

We show that the vitrification of star-shaped polystyrene (PS), of functionality f and molecular weight per arm Mwarm, thin films supported by silicon oxide, SiOx, is strongly dependent on Mwarm and f. When f is small, the vitrification behavior is similar to that of linear-chain PS where the average glass transition, Tg, decreases with decreasing film thickness (ΔTg < 0). However, for sufficiently large f and small Mwarm, Tg becomes independent of film thickness (ΔTg ≈ 0). In this region, where ΔTg ≈ 0, the star-shaped macromolecules self-assemble into ordered, periodic structures, similar to that of soft spheres or colloids, as revealed by simulations and experiments. This is identified as the soft-colloidal region. The transition from the linear-chain-like to the soft-colloidal-like region occurs over an intermediate range of functionalities and arm lengths; throughout this transition range ΔTg > 0. We show that the overall vitrification behavior of these thin film star-shaped polymers is due to competing entropic interactions associated with changes in f and Mwarm. The vitrification behavior of thin star-shaped PS films on SiOx is summarized in terms of a “diagram of states”.