Dynamic Light Scattering of Polymer/Solvent Solutions Under Pressure. Near-Critical Demixing (0.1 < P/MPa < 200) for Polystyrene/Cyclohexane and Polystyrene/Methylcyclohexane

Abstract

A conveniently sized high-pressure scattering cell used for 90° DLS (dynamic light scattering) measurements on polymer solutions is described (0 < P/MPa < 200; 283 < T/K < 373). DLS parameters are reported for semidilute solutions of polystyrene (PS) of various molecular weight in ϑ-solvents (cyclohexane (CH) and methylcyclohexane (MCH)), above and below Tϑ, and at high enough concentration to ensure that precipitation, when it occurs, follows spinodal decomposition/percolation. In the homogeneous region, close to and above Tϑ, the correlograms, initially monomodal and diffusive, split at high pressure. The intensity and the correlation radius of the diffusive mode diverge as critical demixing is approached during pressure or temperature quenches. That process is described in the (T,P)ψcr plane using a multidimensional reduced scaling formalism developed for the purpose. The analysis of the scattering and the phase equilibrium data for PS/CH is complicated by the pressure dependence of the CH freezing curve, but that for PS/MCH nicely exemplifies behavior across a good part of the (T,P)ψcr reentrant miscibility island showing multiple hypercritical points. It has been discussed in that context.