Cooperative Dynamics in Supported Polymer Films

Ultra-thin polymer films have ubiquitous technological applications, ranging from electronic devices to artificial tissues. These nanoconfined polymer materials, typically with thickness less than 100nm, exhibit properties that are different from their bulk counterparts. Despite extensive efforts, a definitive picture of nanoconfinement effects on dynamics, such as the glass transition temperature and fragility (two of the most important properties for amorphous polymer processing) has yet to emerge. In particular, property changes in the dynamics of supported polymer films in comparison to bulk materials involve a complex convolution of effects such as boundary thermodynamic interactions, boundary roughness and compliance, in addition to finite size effects due to confinement. In this thesis, we consider molecular dynamics simulations of substrate-supported, coarse-grained polymer films where these parameters (e.g polymer-substrate interaction) are tuned separately to determine how these variables influence polymer film molecular dynamics. All these variables significantly influence the film dynamics, but all our observations can be understood in a unified framework through a quantification of how these constraining variables influence string-like collective motion within the film. This scale serves a measure of the scale of cooperatively rearranging regions, hypothesized in the Adams-Gibbs theory to describe molecular relaxation. A challenge to this framework is that this cooperative dynamical scale is not readily accessed in experiments. Therefore, we investigate the relatively high mobility interfacial layers near the polymer-air interface, whose thickness ξ grows in a similar fashion to the scale of collective motion within the film. We find the precise scaling relation between ξ and the average length L of string-like particle exchange displacements (strings). This is the first direct evidence that the thickness of the interfacial mobile layer is related to the scale of collective motion within the film. Moreover, this relation links ξ to relaxation time via the AdamGibbs relation, so that changes in ξ can be directly connected to the changes in film glass transition temperature. Our findings are consistent with other recent studies, theoretically predicting or providing indirect evidences regarding the relation of ξ to the scale of collective motion.