(75e) Stability Of Polymer-Grafted Nanoparticles In Semidilute And Concentrated Polymer Solutions

Engineered composites are often formulated by grafting polymer brushes to the surfaces of nanoparticles to disperse them into polymer solutions and melts. In these suspensions it is well established that nanoparticles can exhibit a wide variety of behaviors through depletion flocculation at low-to-moderate free polymer concentrations. However, few studies fundamentally elucidate the fate of polymer-grafted particles, or soft spheres, in concentrated polymer solutions and melts. Researchers who have studied these systems predict that both hard and soft spheres, once destabilized by depletion flocculation, should restabilize in concentrated polymer solutions. In contradiction, we will show the results of recent light scattering experiments on polydimethylsiloxane (PDMS)-grafted silica nanospheres in PDMS/cyclohexane mixtures which indicate that at high free polymer volume fractions (0.6 ? 1.0 v/v) the wetting of the grafted brush is needed to stabilize the nanoparticles against aggregation. We correlate this behavior to phase diagrams and self-consistent field calculations that predict the regions of stability and instability for the PDMS-g-silica nanoparticles. Moreover, we fundamentally examine the effect of solvent quality on nanoparticle dispersion. Overall, these studies represent new ways of quantifying the factors that control the dispersion of polymer-grafted nanoparticles in concentrated polymer solutions and melts ? systems that are ubiquitous in formulating engineered composites.