Dynamic light scattering of polymer/solvent solutions under pressure. Near-critical demixing (0.1 < P/MPa < 200) for polystyrene/cyclohexane and polystyrene/methylcyclohexane

A conveniently sized high-pressure scattering cell used for 90 degrees DLS (dynamic light scattering) measurements on polymer solutions is described ( 0 < P/MPa < 200; 283 < T/K < 373). DLS parameters are reported for semidilu te solutions of polystyrene (PS) of various molecular weight in Theta-solve nts (cyclohexane (CH) and methylcyclohexane (MCH)), above and below T-Theta , and at high enough concentration to ensure that precipitation, when it oc curs, follows spinodal decomposition/percolation. In the homogeneous region , close to and above T-Theta, the correlograms, initially monomodal and dif fusive, split at high pressure. The intensity and the correlation radius of the diffusive mode diverge as critical demixing is approached during press ure or temperature quenches. That process is described in the (T,P)(psi cr) 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)(psi cr), reentrant miscibility island showing multiple hypercritical p oints. It has been discussed in that context.