The dynamic mechanical response of polymer-based nanocomposites and network glasses
The structural entities, atomic or molecular, that constitute a material and their spatial organization within the material largely determine the properties of the material. Different classes of materials respond to external forces often in ways that are fundamentally connected to their structure. Polymer melts exhibit time-dependent viscoelastic behavior in response to external stresses. If the deformation is rapid then the response of the polymer is largely elastic, whereas for sufficiently slow deformations the response is time-dependent and dissipative. In the solid state, an amorphous polymer responds to sufficiently small forces through local relaxations of segments of the molecules. Inorganic network glass melts typically exhibit viscoelastic behavior in the vicinity of the glass transition temperature, Tg, whereas below Tg ionic entities that compose the structure undergo hopping processes in response the external perturbations. This dissertation is largely devoted to understanding the response, or relaxations, of two different classes of materials: polymer based nanocomposites and inorganic network glasses, to external periodic mechanical perturbations. Polymer nanocomposites, materials in which a polymer serves as the host for nanomaterials, may possess properties that differ substantially from those of the pure polymer. Interactions between polymer segments and the foreign particles and the geometry, size, and dispersion of the foreign particles profoundly influence the properties of the composite. Small concentrations (~ few percent) of particles of nano-scale dimensions have been shown to be particularly effective at modifying the properties of the polymer because of the large interfacial area. In this dissertation the influence of C60 fullerenes and of single walled carbon nanotubes on the dynamical mechanical and rheological properties of PS and PMMA is examined. The second problem examined was the mechanical response of inorganic network glasses based on tellurium oxide to mechanical perturbations. Often alkali oxides are included in the glass composition because they improve processability of the glass melt by decreasing Tg. Through a series of dynamic mechanical and rheological experiments, it is shown that the relaxations which characterize the response of the glass increase by up to an order of magnitude when two distinct types of alkali ions are present compared to one.