Phase separation kinetics of polyelectrolyte solutions

The kinetics of phase separation of aqueous solutions of sodium-poly(styrene sulfonate) (NaPSS) containing barium chloride (BaCl(2)) is studied by static and dynamic light scattering. We report a novel mechanism of phase separation, where an enrichment of polymer aggregates of well-defined size occurs in the very early stage of nucleation, which is then followed by a growth process in the formation of the new phase. In the latter stage, the polymer aggregates formed in the early stage act as the templating nuclei. Even in the homogeneous phase at higher temperatures above the upper critical phase boundary, polymer aggregates are present in agreement with previously reported results. Upon rapidly cooling the system below the phase boundary, the number concentration of the aggregates increases first by maintaining their size to be relatively monodisperse, before the growth process takes over at later times. The size and fractal dimension of aggregates in the homogeneous phase and the early nucleation stage of phase separation and the dependence of nucleation time and growth rate on quench depth and salt concentration are determined. The hydrodynamic radius (R(H)) of the unaggregated chains is of the order of 1-10 nm depending on the molecular weight of NaPSS, while R(H) of aggregates is of the order of 100 nm independent of the molecular weight of NaPSS. Unaggregated chains follow good solution behavior with a fractal dimension of 5/3 while the fractal dimension of aggregates is larger than 3.5 suggesting the branched nature of aggregates. Nucleation time is sensitive to quench depth and salt concentration. Increasing a quench depth or increasing BaCl(2) concentration shortens the nucleation time. After the nucleation time, during the growth period, the size of aggregates grows linearly with time, with growth rate being higher for deeper quench depths and higher BaCl(2) concentrations. The mechanism of phase separation of aqueous solutions of NaPSS and BaCl(2) is seen to proceed by utilizing the already-existing aggregates to nucleate the new phase, in marked contrast to hitherto known results on phase separation in uncharged polymer systems.