Publication Date

5-2019

Advisor(s)

Francis Starr; Meng-Ju Sher; George Paily

Department

Physics

Abstract

Polymers are ubiquitous in our everyday lives and have uses in a wide range of industries, from electronics to food goods. Understanding properties of ultra-thin films has become a major interest in polymer science due to their use in semiconductors, adhesives, and artificial tissues. There has been a sustained interest in confinement since work by Keddie et al. (1994 EPL 27 59) published over 20 years ago suggesting the reduction in the glass transition temperature, Tg, is due to the presence of a \liquid-like" layer at the air-polymer interface. Confinement effects result from a difference in the dynamics at the interfaces or the interference of a characteristic length scale; therefore, it is our aim to understand how confinement effects work in conjunction with complex polymer structures, which also affect dynamic properties like Tg and fragility. Dynamic fragility is key feature to know about polymer glasses for both their processing and application. However, there is still much debate on what polymer properties specifically contribute to the fragility. Much work has been done investigating this challenge using simulations, however prior studies have focused primarily on bead-spring type models instead of models that are more molecularly realistic. In this thesis, we employ molecular dynamics (MD) simulations of a coarse-grained polymer models that are chemically specific to polyethylene oxide (PEO) and polymethyl methacrylate (PMMA) with the aim to understand the role of molecular architecture on the structure and dynamics of bulk polymers and supported thin films. Our results show that when film thickness is reduced to 50 Å near Tg, there are minor reductions in Tg for PEO and PMMA indicating the dynamics are only weakly impacted by the differing molecular architecture. The structure, however, is highly affected due to the presence of the side group in PMMA. We also probe the film sensitivity to changes in the substrate interaction strength and observe that PMMA is more sensitive to changes of the substrate interaction strength because of the resulting disruption in monomer organization persisting nearly completely throughout the film.

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