STATIC AND DYNAMIC CRITICAL-BEHAVIOR OF A BINARY POLYMER BLEND IN THE STRONG FLUCTUATION LIMIT - A LIGHT-SCATTERING STUDY

Based upon previous work [G. Meier, B. Momper, and E. W. Fischer, J. Chem. Phys. 97, 5884 (1992)], we report light scattering experiments of the binary polymer blend poly (dimethylsiloxane), PDMS (N=225), and poly (ethylmethylsiloxane), PEMS (N=325), with N being the degree of polymerization at the critical composition φc,PEMS=0.456 in the temperature range 8×10-4<<4×10-2, with =T-Tc/T being the reduced temperature. Using an Ornstein-Zernike scattering law we extract the static structure factor S(q=0) from the angular dependence of the scattered intensity thereby taking the correction for turbidity into account which has become necessary due to the proximity to Tc in contrast to the previous study. From an analysis of S(q=0) with a crossover function describing the change from mean field to three-dimensional Ising behavior close to the critical point, we calculate the Ginzburg number Gi which now quantitatively allows us to identify the Ising regime for < Gi. In this region, the strong fluctuation limit, we obtain the critical exponents γ=1.24±0.02, ν=0.62±0.01, and η=0.036±0.002 in agreement with values from renormalization group calculations for the three-dimensional Ising universality class. From the angular dependence of the Rayleigh linewidth we obtain the critical exponent xη which determines the divergence of the viscosity while approaching the critical point to be xη=0.06±0.03 in agreement with theory. We further establish the crossover of the relaxation rate Γ in the mode coupled regime (ξ>RgN1/2) from Γν for qξ<1 to Γηνx for qξ>1 thereby considering the weak divergence of the viscosity. The proposed crossover of Γ from Γq4ηνx (nonmode coupled) for qRg<N-1/2 to Γq3ηνx (mode coupled) for qRg>N-1/2 was not observed due to the small background contribution. We conclude that a polymer mixture where viscoelastic effects are small behaves for <Gi exactly like a simple ordinary binary liquid belonging to the model H of Hohenberg and Halperin. Anomalies in the viscosity exponent xη, as has been reported for polymer-solvent systems [Phys. Rev. B 12, 368 (1975)], were not detected. © 1995 The American Physical Society.