Importance of Polymer-Polymer Interfaces on the Glass Transition Temperature in Polymer Multilayer Films and Nanostructured Blends

摘要

Polymer multilayer films and nanostructured blends possess large amounts of polymer-polymer interfacial regions (‘interphase’) which can lead to significant deviations from bulk properties not anticipated from measurements on analogous systems with larger domain sizes. Recent advances in polymer processing and compatibilization are yielding blends and multilayers with domain/layer sizes down to tens to hundreds of nanometers in size. Extensive research on nanoconfined polymer systems have demonstrated the large impact that the presence of a free surface or strong substrate interactions can have on the glass transition temperature (T_g) and physical aging (structural relaxation). Strong substrate interactions, such as hydrogen bonding between the hydroxyl groups on the surface of silica and the nitrogen atom on the repeat unit of poly(2-vinylpyridine) (P2VP), tend to result in large increases in T_g of several tens of Kelvin. In contrast, the presence of a free surface can lead to significantly enhanced mobility at the air-polymer interface. The largest effects are observed for freely-standing polystyrene (PS) films which show decreases in T_g of up to 80 Kelvin. However, the extent to which a free surface can modify the dynamics depends on the chemical structure of the polymer, with poly(methyl methacrylate) (PMMA) films demonstrating a significantly weaker free surface effect compared to PS. In addition, we have recently demonstrated that a polymer-polymer interface can modify the dynamics of adjacent domains/layers to a greater extent than a free surface.