We utilize recently introduced chemically specific but coarse-grained models of poly­(ethylene oxide) (PEO) and poly­(methyl methacrylate) (PMMA) to explore the influence of monomer architecture on the dynamics of supported thin polymer films based on molecular dynamics simulations. In particular, we contrast differences in the molecular packing and mobility gradients in these materials near the substrate and “free” interface regions. As expected, relaxation is generally enhanced in the free surface region relative to the film interior (and bulk), and the degree of enhancement is similar for both PEO and PMMA. However, the dynamical changes near the substrate are more sensitive to monomer structure, and are enhanced with increasing polymer–substrate interaction strength, ε. PMMA is relatively stiff compared to PEO and has a side group of appreciable size, and we find that the dynamics of PMMA near the substrate are slowed significantly more in comparison to PEO for the same substrate. Substrate interactions lead to a notable difference of local fragility near the substrate that appears to arise from a higher cohesive interaction strength of the PMMA chains in this region. Our data also reveal the inadequacy of the these coarse-grained polymer models to reproduce the experimentally known differences in the fragility of these materials. However, this technical shortcoming is not expected to alter our qualitative conclusions regarding the comparative effect of substrate interactions on relatively flexible polymers such as PEO versus a relatively stiff polymer such as PMMA.

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, T g , 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 T g and fragility. Dynamic fragility is key feature to know I would like to thank my advisor, Professor Francis Starr for helping me along this journey for the past two years. Thank you for being a listening ear and an endless source of encouragement without judgment. Many thanks to Wengang Zhang for collaborating with me on this project and for making polymers fun and spending many hours teaching me how to simulate them! I am also grateful to have had such an understanding and supportive committee, Professors Renee Sher and George Paily. I am very thankful for the support I've received from many faculty and staff members of the Wesleyan physics department over the past five years, with special mention to Dana Gordon-Gannuscio and Professors Tsampikos Kottos, Christina Othon, Thomas Morgan, and Greg Voth. Thank you to my fellow lab members for always keeping the zoo fun, Abe Kipnis, Jinpeng Fan, Hamed Emany, Chloe Thorburn, and Nathan Shankman. I would like to express my deepest gratitude for my friends Bardia Hejazi, Taryn Johnson, Kiana Dawkins-Autry, Nzinga Hall and the residents of 162 Church St (Class of 2018). You all are the embodiment of love and true friendship. Thank you to my "surrogate" family in the Registrar's Office, Rosie Villard, Tracey Stanley, Gladys Rodriguez, and Paul Turenne, for making me feel like Wesleyan was my home. Finally, no words suffice to explain how appreciative I am of my parents' endless love and support. Any dream I ever had was made possible because of their hard work. Being a woman of color is not easy in this world but, they taught me I could do anything I put my mind to and this is proof.