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.