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

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 T-g 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 T-g and fragility, both as a function of h and film depth. We contrast changes in the relaxation dynamics with density rho and demonstrate the limitations of the commonly invoked free-volume layer model. As opposed to bulk polymer materials, we find that the fragility and T-g 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772402]