Phase separation *kinetics of polyelectrolyte solutions
The kinetics of phase separation of sodium Poly(styrene sulfonate) (NaPSS) in water with barium chloride (BaCl2) was studied by static and dynamic light scattering. This polyelectrolyte system shows an upper critical solution temperature behavior, such that the solutions are homogeneious at temperatures above the phase boundary. Using dynamic light scattering, two diffusive modes were detected, and unaggregated polyelectrolyte chains and aggregates coexist in the homogeneous phase.
The hydrodynamic radius (RH) of the unaggregated chains is of the order of 1 to 10 nm depending on the molecular weight of NaPSS, while RH 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. On the other hand, the fractal dimension of aggregates is larger than 3.5. The value of the fractal dimension suggests that the structure of aggregate is similar to branched polymers, such as star polymers.
Upon rapidly cooling the NaPSS sample below the phase boundary, phase separation takes place. During the initial stage, unaggregated chains were converted to aggregates. Newly generated aggregates have a similar fractal dimension and size to aggregates found above the phase boundary. The size of aggregates remained constant while the number of aggregates increased. During the initial stage, nucleation time is sensitive to quench depth and salt concentration. Either deepening a quench depth or increasing BaCl 2 concentration shortened the nucleation time. After the nucleation time, the size of aggregates grew linearly with time. This marks the growth period, at which time the growth rate is higher for deeper quench depths and higher BaCl2 concentrations. The mechanism of phase separation of NaPSS in water and BaCl2 is similar to the nucleation and growth mechanism where the aggregate serves as a nucleus. However, it is different from conventional nucleation and growth theory in that the nucleus already exists above the phase boundary.