Composition and temperature dependence of monomer friction in polystyrene/poly(methyl methacrylate) matrices

The composition (phi) and temperature dependence (T) of the monomeric frict ion factor (zeta) has been examined for styrene (S) and methyl methacrylate (MMA) in five SMMA diblock copolymers and the corresponding homopolymers. In all cases the degree of polymerization was approximately 150, sufficient ly low to ensure that the chain dynamics follow the Rouse model and that th e copolymers are far above the order-disorder transition. The five diblock copolymers had styrene compositions of 91, 70, 39, 19, and 9 wt %. Values o f an effective friction factor, were obtained from measurements of the stea dy flow viscosity via the Rouse model, whereby zeta (eff) represents an ave rage over the S and MMA contributions. Values of the component friction fac tors, zeta (PS) and zeta (PMMA), were extracted from forced Rayleigh scatte ring tracer diffusion measurements of dye-labeled PS, PMMA, and SMMA chains . In accord with previous studies, zeta (PMMA) in pure PMMA is significantl y greater than ps in pure PS, whether compared at equal T or equal T - T-g, an effect attributable to specific details of local relaxation in PMMA. Th is difference between zeta (PMMA) and ps persists in a common SMMA matrix, an effect which can be quantitatively accounted for by the recently introdu ced concept of self-concentration. The values of zeta (eff) increase monoto nically with MMA content at fixed T and with decreasing T at fixed phi. The phi dependence of zeta (eff) is consistent with both "Rouse" and "Arrheniu s" mixing rules, utilizing the pure component friction factors as input. Th e T dependence of zeta (eff) follows the standard Williams-Landel-Ferry rel ation, but the data for different phi do not collapse to a single curve whe n plotted against T - T-g. In this respect, the PS/ PMMA system is more com plicated than the previously studied PS/polyisoprene system.