Investigation of the phase diagram and critical fluctuations of the system polyvenylmethylether and d-polystyrene with neutron small angle scattering

The system PVME/d-PS has been investigated by neutron small angle scattering. By extrapolating the critical scattering, the spinodal was determined and used to calculate the Flory-Huggins parameter χ(φ,T) in the frame of mean field theory. Separating the Gibbs potential of mixing ∆G in a Flory-Huggins term χ(φ,T) and a configurational term for the entropy ∆S, it turns out that χ is independent of the molecular weight, and of the weight distribution, i.e., it is a local or ``segmental'' quantity. The critical scattering was carefully investigated at the critical concentration. At temperatures T_c-T>2.4 K(T_c=141.9 °C), a mean field behavior of the susceptibility S(Q=0) was clearly observed, with a critical exponent γ=1. However, in a region of T_c-2.4 K≤T<T_c, a crossover occurs to a critical exponent γ≂1.26; it resembles the Ising behavior which is well established for normal liquid mixtures. The function S(Q) also describes the characteristic segmental length σ. Both σ and the range parameter of χ, r₀, are independent of molecular weight and weight distribution within experimental errors. The binodal, i.e., the line between the homogeneous and the two-phase region was identified by a change of critical scattering caused by the precipitated domains. The spinodal approaches the binodal near T_c in a characteristic way as predicted for polydisperse systems.