(324a) Invited Talk: Molecular Simulation of the Dynamics in Thin Polymer Films

After more than 15 years of study since the original article by Keddie et al. demonstrating the effects of confinement on the glass transition temperature (Tg) in nanoscopic polymer films supported on a silicon substrate, there is not yet a consensus on the origins of the Tg shift. Understanding and controlling the effects of confinement on glass-forming polymers is essential to further development of photolithography and semiconductor manufacturing, as well as several emerging technologies that will depend on the properties of confined glasses, such as stable glasses, flexible displays, and responsive materials. A growing body of experimental literature exists that suggests that the dynamics near a free surface are not only enhanced but are fundamentally different in their nature compared to a bulk glass-forming material. However, despite the fact that the experimentally-relevant length scales can be easily captured by molecular simulation, here are few simulation studies examining the dynamics of glass-forming polymers in confined geometries compared to the extensive experimental work. In this talk, I will describe some of our recent efforts to understand how the dynamics of glass-forming polymers change near free surfaces. We have identified several cases where the magnitude of the confinement effect can depend strongly on the property under investigation, which can be due to changes in the molecular packing near an interface. In addition, we have shown how physical aging can vary depending on the property measured. Finally, I will describe some of our work trying to understand how the view of confinement effects found in model systems studied with molecular dynamics compares with recent experiments.