Surprising Impact of Chain Connectivity in Altering Local Glass Transition Temperature Near Interfaces

Our group has recently investigated how the local glass transition temperature T_g(z) changes across interfaces between two polymers with widely different bulk glass transition temperatures T_g^bulk. Starting with a single interface between two semi-infinite domains (ΔT_g^bulk ≈ 80 K), we used a localized fluorescence method to demonstrate broad profiles in T_g(z) across dissimilar polymer-polymer interfaces spanning hundreds of nanometers and observed to be asymmetric relative to the composition profile. A key finding of these results is the observation that the broad coupling of dynamics across the dissimilar polymer-polymer interface only occurs if this interface is annealed to equilibrium. Efforts to understand what factors during polymer interface formation cause these broad profiles in T_g(z) find that chain connectivity appears to be surprisingly important. This is confirmed by measurements near silica substrates with tethered chains where low grafting densities (~10 vol% tethered chains) are observed to cause large +50 K increases in local T_g. We now explore what property changes take place in multilayer systems during interface annealing using different experimental techniques with the goal of understanding how interfaces mediate dynamical coupling across dissimilar polymer domains. Advanced materials design is headed towards increasing amounts of interfaces with progressively smaller domain sizes where the geometrical arrangement of these interfaces can be utilized to alter local material properties for desired applications.