Influence of Rough and End-Tethered Substrates on the Local Glass Transition Temperature Tg(z) of Polystyrene

Modifying interfacial interactions are an efficient way to tune polymer properties, however, the mechanism of how a given interface perturbs local material properties is poorly understood, with efforts often complicated by competing factors. We have recently studied the local glass transition temperature profile Tg(z) in polystyrene (PS) near end-tethered substrates, finding a maximum local Tg increase of 50 K next to the substrate at an optimum grafting density of σ = 0.011 chains/nm², coinciding with the "mushroom-to-brush" crossover regime. This optimum grafting density also corresponds to when the Tg(z) perturbation extends the furthest from the interface, comparable to our group's recent work on PS next to a higher-Tg polymer such as polysulfone. More recently, we have investigated the local Tg in PS next to silica substrates with varying roughness, using hydrogen fluoride vapor to etch silica without changing the surface chemistry. We find a local Tg increase of ~10 K with increasing RMS roughness from 0.5 to 28 nm, consistent with predictions from computer simulations. Interface roughness has been heavily studied in computer simulations where the specific details of the substrate must be constructed at its most basic level, but virtually unexplored experimentally.