(4p) Dynamic Relaxation Characteristics of Polymer Nanocomposites and Aromatic Polyimides

Interest in polymer nanocomposite technology has grown because of the enhanced properties that can be achieved in the nanocomposite over those of the pure polymer. Particle-polymer interactions, as well as physical confinement, can lead to changes in polymer chain dynamics that are manifested in key performance properties, creating opportunities for new and unique material applications. In this work, the dynamic relaxation characteristics of both glassy (amorphous) and rubbery polymer nanocomposites have been examined; matrix polymers included poly (etherimide), poly (methyl methacrylate), and cross-linked poly (ethylene oxide) in combination with native and surface-modified metal oxide nanoparticles. Key material variables included nanoparticle size, surface chemistry, and loading. For nanocomposite formulations with favorable polymer-particle interactions, the inclusion of moderate to high levels of nanoparticles led to the emergence of a dual-T_g response that encompassed a bulk T_g event (matching T_g for the unfilled polymer), and a second elevated T_g corresponding to relaxations involving chain segments constrained in the vicinity of the polymer-particle interface. The combination of dynamic mechanical and dielectric methods provided a comprehensive view as to the influence of the particles both with respect to local sub-glass motions, as well as the larger-scale processes inherent to the glass transition.

A new class of thermally-modified aromatic polyimides (API's) has recently been identified as a promising material for membrane design. These membranes are based on soluble aromatic polyimides with ortho-positioned functional groups; exposure of the polyimides to thermal rearrangement at elevated temperature leads to fully-aromatic, insoluble polybenzoxazoles (PBO) with exceptional thermal and chemical resistance characteristics. The thermal conversion step produces fundamental changes in molecular connectivity and conformation that govern the distribution of free volume elements, and the ultimate transport properties of the membranes. A comprehensive study has been undertaken to examine the dynamic relaxation properties of the API's as a function of backbone structure and degree of thermal rearrangement, leading to insight as to the relative flexibility of the polymers, their local relaxation environment and corresponding free volume, and the influence of thermal rearrangement on segmental mobility. These results can be correlated with the static properties of the polymers and their separation characteristics for both gas separations and pervaporation applications.