STUDIES OF LIQUID-LIQUID DEMIXING OF POLYSTYRENE SOLUTIONS USING DYNAMIC LIGHT-SCATTERING - NUCLEATION AND DROPLET GROWTH FROM DILUTE-SOLUTION

Dynamic light scattering (DLS) measurements of liquid-liquid demixing from dilute polystyrene (PS)/cyclohexane and PS/methylcyclohexane solu tions are described. Phase transitions were induced by temperature que nches into the metastable region and beyond using concentrations low e nough to ensure liquid-liquid demixing by a nucleation growth (NG) mec hanism. In a companion paper the NG results are compared with demixing studies at higher concentration, where the mechanism is spinodal deco mposition (SD), and with DLS measurements in a good solvent. The prese nt measurements employed polystyrenes of low or medium molecular weigh t, M-w,M-1 = 30 000, rho(1) = M-w/M-n = 1.05, M-w,M-2 = 90 000, rho(2) = 1.04 and M-w,M-3 = 400 000, rho(3) = 1.05 In the NG region DLS meas urements yield well-behaved correlation lengths for quenches well into the metastable region, but that pattern ends in the immediate vicinit y of the cloudpoint where it splits to a bimodal distribution. The lar ger (slow) component is associated with the nucleation and growth of d roplets of the precipitating phase. The rate of growth of the slow com ponent was measured for the M-w 400 000 sample but growth was too rapi d to follow at the lower M-w's. The splitting process is sensitive to both quench depth and quench rate. (At higher concentrations where liq uid-liquid demixing is via an SD mechanism no splitting is observed. I nstead, one finds well-behaved monomodal DLS correlograms which show e xponentially increasing intensity and correlation length as the cloudp oint is approached.).