Coupled effects of substrate adhesion and intermolecular forces on polymer thin film glass-transition behavior

Intermolecular noncovalent forces between polymer chains influence the mobility and glass-transition temperature (T_g), where weaker interchain interactions, all else being the same, typically results in lower bulk polymer T_g. Using molecular dynamics simulations, here we show that this relation can become invalid for supported ultrathin films when the substrate-polymer interaction is extremely strong and the polymer-polymer interactions are much weaker. This contrasting trend is found to be due to a more pronounced substrate-induced appreciation of the film T_g for polymers with weaker intermolecular interactions and low bulk T_g. We show that optimizing this coupling between substrate adhesion and bulk T _g maximizes thin film T_g, paving the way for tuning film properties through interface nanoengineering.