Phase Separation in Low Molecular Weight Polymer Mixtures

The initial stages of phase separation in low molecular weight mixtures of polystyrene (Mw 2000) and polybutadiene (Mw= 1000 and 2500) have been examined by using time-resolved small-angle X-ray scattering. The high flux from a synchrotron source was utilized due to the rapid kinetics and the weak contrast at the early stages of phase separation. Over the scattering vector range studied (from q = 0.07 to 0.97 nm-1) no evidence of a scattering maximum, characteristic of a periodic variation in the concentration, was observed. At the onset of phase separation, the intensity at a constant scattering vector was found to depend exponentially on time with an amplification factor R(q) that changed from positive values at low q to negative values at high q. According to the Cahn-Hilliard theory,1, 2 these crossover points were used to estimate the spinodal temperatures and Flory-Huggins interaction parameters. Deviations of the scattering vector dependence of R(q) strongly suggest that thermal density fluctuations not taken into account by the Cahn-Hilliard theory contribute significantly to the observed scattering. © 1985, American Chemical Society. All rights reserved.