Side-group size effects on interfaces and glass formation in supported polymer thin films

Recent studies on glass-forming polymers near interfaces have emphasized the importance of molecular features such as chain stiffness, side-groups, molecular packing, and associated changes in fragility as key factors that govern the magnitude of T_g changes with respect to the bulk in polymer thin films. However, how such molecular features are coupled with substrate and free surface effects on T_g in thin films remains to be fully understood. Here, we employ a chemically specific coarse-grained polymer model for methacrylates to investigate the role of side-group volume on glass formation in bulk polymers and supported thin films. Our results show that bulkier side-groups lead to higher bulk T_g and fragility and are associated with a pronounced free surface effect on overall T_g depression. By probing local T_g within the films, however, we find that the polymers with bulkier side-groups experience a reduced confinement-induced increase in local T_g near a strongly interacting substrate. Further analyses indicate that this is due to the packing frustration of chains near the substrate interface, which lowers the attractive interactions with the substrate and thus lessens the surface-induced reduction in segmental mobility. Our results reveal that the size of the polymer side-group may be a design element that controls the confinement effects induced by the free surface and substrates in supported polymer thin films. Our analyses provide new insights into the factors governing polymer dynamics in bulk and confined environments.