Complex Fluid Microstructure, Rheology and Glass Transitions:~ Effect of Continuous Phase Molecular Weight

The mechanical properties of suspensions depend dramatically on the suspension microstructure. Microstructure in turn depends on the nature of particle interaction potentials.~ For those systems that are thermally activated there will be an equilibrium microstructure and thus equilibrium transport properties.~ One of the model systems used to understand the links between interactions, microstructure and transport properties is that of hard spheres suspended in a Newtonian continuous phase.~ This model system can be studied experimentally and direct comparisons made with model predictions.~ With the increase in particle volume fraction, if the particles cannot crystallize, the suspension forms a glass where long range self diffusion is essentially eliminated.~ The approach to the glass transition has been studied experimentally and agreement with models is strong.~ In this talk we discuss what changes as the continuous phase takes on a granularity where the continuous phase molecules have substantial degrees of freedom. In particular, we investigate the mechanics and microstructure of hard sphere suspensions in polymer melts.~ The particles are composed of silica while we use polyethelene glycols of different weights ranging from small degrees of polymerization through the entanglement molecular weight.~ These studies are motivated by a desire to understand properties of polymer nanocomposites where the role of particle/polymer segment interactions is poorly understood but the state of particle dispersion is key to composite properties.~ In this talk I explore equilibrium and nonequilibrium phases of this system and compare with extant theoretical approaches. ~