Critical Fluctuations in a Binary Polymer Mixture

The critical opalescence of a binary polymer mixture consisting of polyvinyl methyl ether and deuterated polystyrene was measured by small-angle neutron scattering. In accordance with a mean-field treatment, it was found that the critical exponents $\nu$ and $\gamma$ are 0.5 and 1, respectively; the Ornstein-Zernike scaling function describes the scattering data; and the spinodal is parabolically shaped. However, the dependence of the interaction parameter $\overline{\chi }$ on the composition of the polymer mixture is in conflict with the theoretical result.