Molecular Weight Effects on the Glass Transition and Confinement Behavior of Polymer Thin Films

Nanoscale polymer thin films exhibit strong confinement effects on T_g arising from free surfaces. However, the coupled influence of molecular weight (MW) and surface effects on T_g is not well understood for low MW film systems below the entanglement length. Utilizing atomistically informed coarse-grained molecular dynamics simulations for poly(methyl methacrylate) (PMMA), it is demonstrated that the decrease in free-standing film T_g with respect to bulk is more significant for low MW compared to high MW systems. Investigation of the local interfacial properties reveals that the increase in the local free volume near the free surface is greater for low MW, explaining the MW dependence of T_g-confinement behaviors. These findings corroborate recent experiments on low MW films, and highlight the relationship between nanoconfinement phenomena and local free volume effects arising from free surfaces. The aim of this contribution is to explore the impact of molecular weight (MW) on confinement behavior of free-standing thin films of poly(methyl methacrylate). Simulation results show that low MW films exhibit a greater T_g depression, which can be attributed to a stronger interfacial effect on local free volume for low MW films.