title: 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 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 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 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.
authors.affiliation: Department of Physics, Wesleyan University, Middletown, Connecticut 06459, USA.
authors.forename: Paul Z
authors.initials: PZ
authors.lastname: Hanakata
authors.forename: Jack F
authors.initials: JF
authors.lastname: Douglas
authors.forename: Francis W
authors.initials: FW
authors.lastname: Starr
mesh_terms: D016880:Anisotropy; D005898:Glass / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*; D044366:Transition Temperature
chemical_list: D011108:Polymers