Francis W. Starr
Polymers are one of the most common glass-forming materials and they are ubiquitous in many industries, e.g., packaging, biomedical and construction industries. Thin polymer films that have properties that significantly deviate from those of the bulk polymer have attracted much attention from numerous researchers in the past two decades. The challenge is to understand how competing interfacial and confinement effects give rise to changes in the mean relaxation of the film (and thus the glass transition temperature Tg), the sensitivity of various Tg measurements to the gradient of molecular relaxation across the film, as well as the dynamical heterogeneity. Our findings also adds to the ongoing debate of the magnitude and even direction of Tg shifts in thin polymer films with the same chemistry, film thickness and supporting substrate. The implicit assumption of these studies is that methods used to estimate Tg in homogeneous bulk materials are relevant for inferring dynamical changes in these films.
In this thesis, we use an extensive set of molecular dynamics simulations to examine the nanoconfinement effects on the glass transition temperature Tg and heterogeneous dynamics of thin polymer films. In particular, we have shown that the slowing of polymer dynamics occurs only when the substrate is strongly attractive; a "bound layer" having a much lower mobility can form near the substrate, strongly influencing the thermodynamics and dynamics of the film. This bound polymer effectively insulates the remainder of the film from the strong interfacial interactions, and the resulting thermodynamically determined Tg is relatively insensitive to the polymer-substrate interaction strength when it exceeds that of the polymer-polymer interactions. Furthermore, we have shown that the disparate Tg measurements reflect differing sensitivities to the mobility gradient across the film. Our results emphasize the limitations of using single Tg to infer changes in the dynamics of polymer thin films. Additionally, we show that the thermodynamic and dynamic techniques to estimate of Tg can be combined to predict local changes in Tg near the substrate, providing a simple method to infer properties of the mobility gradient.
Another hallmark of the glass-forming system is the dynamical heterogeneity near Tg. We explore the confinement effects on the heterogeneous dynamics of the thin polymer film. Specifically, we examine ultra-stable vapor-deposited thin polymer films and compare their behavior to that of the ordinarily prepared thin films. Using common metrics to measure the extent and timescale of dynamical heterogeneity (e.g., non-Gaussian parameter, string-like cooperative motion), we have found that measurements for the film as a whole are not sufficient to distinguish significant differences between the ordinary film and ultra-stable vapor-deposited films, due to the substantial mobility gradient in the glassy films. Instead, it is crucial to separate the dynamics in the film center.
Broadly, our research examines the effects of nanoconfinements on dynamics and glass transition temperature of glass-forming materials; our findings show how various confinement conditions and sample preparation method affect different estimates of glass transition temperature and dynamical heterogeneity. Our findings can possible lead to an unified picture to explain the film dynamics and glass transition temperature under confinement for all polymers.
Zhang, Wengang, "Glass Transition and Dynamics of Polymers in Nanoconfinement" (2018). Dissertations. 91.
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