Study of the nonlinear phase‐separation behavior of a polystyrene/poly(vinyl methyl ether) blend using small‐angle light scattering

The thermally induced phase‐separation behavior of a polystyrene/poly(vinyl methyl ether) (PS/PVME) blend was studied mainly using time‐resolved small‐angle light scattering, as a function of temperature and heating rate. Under a non‐isothermal field, the dependence of the critical temperature on heating rate deviated obviously from linearity, even at very low heating rates. Such a nonlinear dependence was consistent with the deviation from linearity of the temperature dependence of the isothermal phase‐separation behavior in a wider temperature range from 100 to 140 °C. It was also found that a Williams–Landel–Ferry (WLF)‐like equation could be employed to describe the temperature dependence of the apparent diffusion coefficient ( D app ) and the relaxation time (τ) of normalized scattering intensity at the early stage of spinodal decomposition (SD), as well as τ of phase behavior at the late stage of SD for the PS/PVME blend. The equilibrium phase‐separation temperature could hardly be established through the conventional linear extrapolation of heating rate or D app to zero at the early stage of SD. The successful use of the WLF‐like function for PS/PVME blends extends the applicability of the time–temperature superposition principle for describing the phase‐separation behavior of binary polymer mixtures over a relatively large temperature range. Copyright © 2010 Society of Chemical Industry