Local variation of fragility and glass transition temperature of ultra-thin supported polymer films

Abstract

Despite extensive efforts, a definitive picture of the glass transition of ultra-thin polymer films has yet to emerge. The effect of film thickness h on the glass transition temperature Tg has been widely examined, but this characterization does not account for the fragility of glass-formation, which quantifies how rapidly relaxation times vary with temperature T. Accordingly, we simulate supported polymer films of a bead-spring model and determine both Tg and fragility, both as a function of h and film depth. We contrast changes in the relaxation dynamics with density ρ and demonstrate the limitations of the commonly invoked free-volume layer model. As opposed to bulk polymer materials, we find that the fragility and Tg do not generally vary proportionately. Consequently, the determination of the fragility profile—both locally and for the film as a whole—is essential for the characterization of changes in film dynamics with confinement.