Critical behavior in a binary polymer blend as studied by static and dynamic light scattering

Abstract

We report static and dynamic light scattering experiments of an almost symmetric polymer mixture made up from poly(dimethylsiloxane) (PDMS), N=260, and poly(ethylmethylsiloxane) (PEMS), N=340, with N being the degree of polymerization, both below the entanglement molecular weights. The mixture exhibits an upper critical solution temperature Tc≂57 °C. The measurements were performed at the critical composition φc,PEMS = 0.465 in a broad temperature range in the one phase region above the spinodal point. The main results for the static case are: the temperature dependence of the static structure factor S(q=0) can be described by a mean field behavior. For T close to Tc, a crossover to an Ising behavior is observed according to a modified Ginzburg criterion. From the angular dependence of S(q), the static correlation length ξ is determined via an Ornstein–Zernike plot. Our experimentally determined values for limT→∞S(0) and limT→∞ ξ, respectively, are in agreement with theoretical predictions. For the dynamic case, the main results are summarized as follows: as expected, the mutual diffusion coefficient D̃, accessible by quasielastic light scattering, shows a critical slowing down for T→Tc. For qξ≥1, we observe that the q scaling of the Rayleigh linewidth Γ changes from a q2 to a q3 behavior, which is in agreement with mode coupled expressions. This occurs in a relatively broad temperature range, due mainly to the fact that polymer mixtures exhibit a larger ξ0 ∝√N, on the contrary to any other systems known, which allows us therefore to reach the region qξ≥1 even with light scattering easily. From the separation of the measured Rayleigh linewidth into a critical part and a background part, we have estimated the crossover between mode coupled to nonmode coupled dynamics. It is governed by the coil size. The scaling predictions for the critical part and the background part of the linewidth are in agreement with the predictions of the mode coupled theory by Kawasaki and subsequently by Fredrickson. We find that the mode coupled dynamics reaches far into the mean field regime which is not yet understood by theory. Furthermore, we can show that the critical part of the linewidth data is well represented by the Kawasaki shape function including the viscosity correction. Finally, we have estimated a segmental mobility W0∝D̃⋅S(q=0) which can be interpreted being a segmental quantity only down to characteristic lengths ξ(T)≂Rg. For ξ larger than the coil dimensions, W0∝ξ as predicted by mode coupled dynamics.