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 ${\mathrm{\phi }}_{\mathit{c},\mathrm{P}\mathrm{E}\mathrm{M}\mathrm{S}}$=0.456 in the temperature range 8×${10}^{\mathrm{-}4}$<ɛ<4×${10}^{\mathrm{-}2}$, with ɛ=T-${\mathit{T}}_{\mathit{c}}$/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 ${\mathit{T}}_{\mathit{c}}$ 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 ɛ< 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 ${\mathit{x}}_{\mathrm{\eta }}$ which determines the divergence of the viscosity while approaching the critical point to be ${\mathit{x}}_{\mathrm{\eta }}$=0.06±0.03 in agreement with theory. We further establish the crossover of the relaxation rate Γ in the mode coupled regime (ξ>${\mathit{R}}_{\mathit{g}}$${\mathit{N}}^{1/2}$) from Γ∝${\mathrm{\varepsilon }}^{\nu }$ for qξ<1 to Γ∝${\mathrm{\varepsilon }}_{\mathrm{\eta }}^{\nu \mathit{x}}$ for qξ>1 thereby considering the weak divergence of the viscosity. The proposed crossover of Γ from Γ∝${\mathit{q}}^4$${\mathrm{\varepsilon }}_{\mathrm{\eta }}^{\nu \mathit{x}}$ (nonmode coupled) for ${\mathit{qR}}_{\mathit{g}}$<${\mathit{N}}^{\mathrm{-}1/2}$ to Γ∝${\mathit{q}}^3$${\mathrm{\varepsilon }}_{\mathrm{\eta }}^{\nu \mathit{x}}$ (mode coupled) for ${\mathit{qR}}_{\mathit{g}}$>${\mathit{N}}^{\mathrm{-}1/2}$ was not observed due to the small background contribution. We conclude that a polymer mixture where viscoelastic effects are small behaves for ɛH of Hohenberg and Halperin. Anomalies in the viscosity exponent ${\mathit{x}}_{\mathrm{\eta }}$, as has been reported for polymer-solvent systems [Phys. Rev. B 12, 368 (1975)], were not detected.

• Received 31 October 1994

DOI:https://doi.org/10.1103/PhysRevE.51.5776