Effect of solvent quality on the diffusion of polystyrene latex spheres in solutions of poly(methyl methacrylate)

Dynamic light scattering has been used to study the diffusion of polystyrene (PS) latex spheres (approximate to 0.2 mu m radius) in poly(methyl methacrylate) (PMMA) solutions at 25 degrees C. The weight-average molecular weight of the PMMA was 350 000. The sign and magnitude of the deviations from the Stokes-Einstein (SE) equation varied dramatically with solvent quality. Positive deviations from SE behavior (eta D-sphere/eta(0)D(0) >1) were observed in good solvents for PMMA, such as dimethylformamide (DMF) and tetrahydrofuran (THF). We argue that these positive deviations are a result of a layer of solution locally surrounding the latex spheres that is rich in solvent and deficient in PMMA. This ''depletion layer'' is likely caused by a combination of entropic repulsion between the matrix PMMA and the latex spheres, and most importantly the immiscibility of PMMA and PS. A negative deviation (eta D-sphere/eta(0)D(0)<1) by almost of factor of 3 from the SE equation was observed in a dioxane-water mixed solvent, which is a Theta-solvent for PMMA at 25 degrees C. PMMA adsorption onto the latex spheres is argued to occur under these unfavorable solvent conditions. At high matrix concentrations, entanglements of the adsorbed PMMA with free PMMA in solution likely occur. A very slow relaxation mode, in addition to the mode associated with sphere diffusion, was present in the CONTIN analyses of the PMMA/PS latex/dioxane-water system. It is possible that this mode is due to PMMA clusters, or less likely, bridged PS latex moieties.