Investigation of the phase diagram and critical fluctuations of the system polyvenylmethylether and dpolystyrene with neutron small angle scattering
Abstract
The system PVME/dPS has been investigated by neutron small angle scattering. By extrapolating the critical scattering, the spinodal was determined and used to calculate the FloryHuggins parameter χ(φ,T) in the frame of mean field theory. Separating the Gibbs potential of mixing ∆G in a FloryHuggins 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, r_{0}, are independent of molecular weight and weight distribution within experimental errors. The binodal, i.e., the line between the homogeneous and the twophase 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.
 Publication:

Journal of Chemical Physics
 Pub Date:
 November 1987
 DOI:
 10.1063/1.453482
 Bibcode:
 1987JChPh..87.6078S