Combined Mixing and Dynamical Origins of Tg Alterations Near Polymer–Polymer Interfaces

The extent to which altered glass formation behavior in block copolymers and layered polymers results from local compositional mixing vs from longer-ranged dynamical correlations has long been unresolved. Here, we perform molecular dynamics simulations at model polymer–polymer interfaces to understand the relative roles of these mechanisms. Results indicate a crossover from the high-χ regime, where near-interface Tg alterations are driven purely by dynamical gradient effects, to the low-χ regime, where both local mixing and dynamical correlations play a role. Observed Tg gradient ranges are asymmetric, exceed the range of composition gradients, and modestly grow with decreasing χ. These results provide new insight into the design of nanostructured polymers with targeted local dynamics. They also emphasize an apparent dichotomy between many studies, including ours, pointing toward Tg gradient ranges on the order of 10 nm, and a second set of studies reporting on longer ranges up to hundreds of nanometers.