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

Abstract

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 [T_g^′: equivalent to softening temperature of the amorphous phase (T_s)]. Combinations of polymers showed either single (e.g., PAANa and dextran, DEAE-dextran and dextran) or double (e.g., PAANa and Ficoll or PVP) T_g^′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 T_g^′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 T_g^′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.