The Distribution of Glass Transition Temperatures in Ultrathin Polymer Films Controlled by Segment Density or Interfacial Interaction

Nowadays, the microscopic mechanism controlling the distribution of local glass transition temperatures (T_gs) across thin polymer films is still unclear and thus large‐scale applications of polymer films are restricted. Dynamic Monte Carlo simulations are performed to investigate the key factors dominating the distribution of layer T_gs in two kinds of capped ultrathin films with and without attractive polymer–substrate interactions, respectively. For the film without polymer–substrate interaction, the interfacial layer T_g is lower than the middle layer T_g. Additionally, the layer T_gs and the layer segment densities below T_g are linearly correlated, indicating that polymer density determines the distribution of layer T_gs. However, for the films with polymer–substrate interactions, the interfacial layer T_g increases dramatically with the raise of interfacial interactions, while the middle layer T_g decreases slightly. The interfacial layer T_g is proportional to the strength of interfacial interaction, while the middle layer T_g is linearly correlated with the segment density of the middle layer below T_g. Namely, interfacial interaction is the main factor dominating the interfacial layer T_g, while segment density controls the middle layer T_g.