Phase separation of polyelectrolytes and non-ionic polymers in frozen solutions

The miscibility of polyelectrolytes and non-ionic polymers in frozen aqueous solutions was studied using diethylaminoethyl (DEAE)-dextran, dextran sulfate, poly(acrylic acid, sodium salt) (PAANa), Ficoll, polyvinylpyrrolidone (PVP) and dextran as model polymers. Thermal analysis of single-polymer frozen solutions showed a glass transition of maximally concentrated solution at particular temperatures [Tg′: equivalent to softening temperature of the amorphous phase (Ts)]. Combinations of polymers showed either single (e.g., PAANa and dextran, DEAE-dextran and dextran) or double (e.g., PAANa and Ficoll or PVP) Tg′s, representing concentrated polymer mixtures and separated phases. The addition of salts altered the miscibility of some polyelectrolyte and non-ionic polymer combinations in frozen solutions. Freeze-concentration separates DEAE-dextran and dextran (5% w/w each, into two phases in the presence of low-concentration (20–50 mM) NaCl, NaSCN, sodium phosphate buffer or TRIS–HCl buffer in initial solutions. A high concentration (300 mM) of salting-in salt (NaSCN) merged the two Tg′s, indicating the inhibition of phase separation. Salts in some frozen solutions are distributed differently to separated phases (dextran sulfate and Ficoll or PVP), which shifts the Tg′s independently. The miscibility of polyelectrolytes and non-ionic polymers in frozen aqueous solutions varies depending on the nature of the polymers and the existence of co-solutes. Salts should change some polymers' miscibility by concealing the electrostatic effect that favors molecular interactions.