Isolating the Impact of Reduced Domain Size on the Local Glass Transition Tg(z) Profile in PS/PnBMA Bilayers

Dynamical gradients have long been observed in confined polymer systems where interfacial effects perturb the local dynamics and propagate into the material. The range of the dynamical gradient varies for different interfaces, where broader polymer-polymer interfaces can perturb dynamics across particularly long length scales spanning hundreds of nanometers. A major open question associated with understanding these dynamical gradients is how a finite domain size alters its range. To isolate the impact of only the reduced domain size without confounding effects from any additional interfacial perturbations, we focus on a bilayer film of polystyrene (PS) capped with poly(n-butyl methacrylate) (PnBMA), where the underlying silica substrate is neutral to PS. Fluorescence is used to measure the local glass transition temperature Tg(z) profile arising from the PS/PnBMA interfacial perturbation as the PS layer thickness is reduced. We find that the Tg(z) profile across a 75 nm PS layer capped with PnBMA is significantly shortened compared to the dynamical gradient previously observed across the unconstrained PS/PnBMA system. We demonstrate that constraining the dynamical gradient alters both the shape of the Tg(z) profile, as well as the local dynamical perturbation at the interfaces.