Correlating local versus global measures of glassy polymer dynamics by comparing different thermal analysis methods

摘要

Advanced materials performance through the strategic use of interfacial interactions to perturb local properties, combined with an optimized morphology, could lead to numerous designer materials where the desired global properties are obtained from an amalgam of local property changes. However, to achieve this grand goal, a detailed understanding of how interfaces perturb local properties are needed, along with knowledge of the global macroscopic characteristics resulting from these local effects. This presentation will discuss our efforts to understand how various interfaces perturb local material properties and how different experimental techniques contribute to this picture. Recently our group employed a localized fluorescence method to measure the profile in local glass transition temperature Tg(z) across glassy-rubbery polymer interfaces. For a single interface between semi-infinite domains of polystyrene (PS) and poly(n-butyl methacrylate) (PnBMA), the local Tg(z) profile was found to be extremely broad and asymmetric, spanning 350-400 nm as the local Tg(z) transitioned the 80 K difference in bulk Tgs [1], with subsequent studies finding this profile to be common to a range of weakly immiscible systems [2]. Related studies on polymer films have demonstrated similar long range Tg(z) profiles near PS end-grafted silica substrates, with the largest Tg(z) increase of 50 K next to the substrate corresponding to a low grafting density coinciding with the "mushroom-to-brush" crossover regime [3], and having a Tg(z) profile comparable to that observed for a polymer-polymer interface. We discuss here efforts to correlate these local measures of Tg with global measures of glassy physical aging dynamics using ellipsometry. By studying single layer PS films across a wide range of molecular weights, we have observed variations in glassy dynamics with chain connectivity [4], supporting the notion that chain connectivity may play an important role in modifying how dynamical perturbations propagate in the material. We also describe our attempts to develop a similar ellipsometric thermal analysis method to extract the physical aging response of glassy PS domains in contact with rubbery PnBMA domains. In doing so, we address the role that finite domain size has on this phenomenon [5].