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

TLDR: In this article, the spinodal of the PVME/d−PS was determined by extrapolating the critical scattering, which was then used to calculate the Flory-Huggins parameter χ(φ,T) in the frame of mean field theory.

Abstract: 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 ΔG in a Flory–Huggins term χ(φ,T) and a configurational term for the entropy ΔS, 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 Tc−T>2.4 K(Tc=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 Tc−2.4 K≤T<Tc, 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 i...