title: Interfacial mobility scale determines the scale of collective motion and relaxation rate in polymer films.
abstract: Thin polymer films are ubiquitous in manufacturing and medical applications, and there has been intense interest in how film thickness and substrate interactions influence film dynamics. It is appreciated that a polymer-air interfacial layer with enhanced mobility plays an important role in the observed changes and recent studies suggest that the length scale ξ of this interfacial layer is related to film relaxation. In the context of the Adam-Gibbs and random first-order transition models of glass formation, these results provide indirect evidence for a relation between ξ and the scale of collective molecular motion. Here we report direct evidence for a proportionality between ξ and the average length L of string-like particle displacements in simulations of polymer films supported on substrates with variable interaction strength and rigidity. This relation explicitly links ξ to the theoretical scale of cooperatively rearranging regions, offering a promising route to experimentally determine this scale of cooperative motion.
authors.affiliation: Department of Physics, Wesleyan University, Middletown, Connecticut 06459, USA.
authors.forename: Paul Z
authors.initials: PZ
authors.lastname: Hanakata
authors.affiliation: Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
authors.forename: Jack F
authors.initials: JF
authors.lastname: Douglas
authors.affiliation: Department of Physics, Wesleyan University, Middletown, Connecticut 06459, USA.
authors.forename: Francis W
authors.initials: FW
authors.lastname: Starr