Phase behavior of random and ABC triblock methacrylic polyampholytes with poly(vinyl alcohol) in water: effect of pH and salt

Abstract

ABC triblock and random methacrylic polyampholytes of molecular weight 4,000 g mol −1 with various base-neutral-acid compositions were examined for their phase separation behavior, each in an aqueous mixture with poly(vinyl alcohol) of molecular weight 10,000 g mol −1. Potassium chloride concentration and solution pH were varied and the phase behavior was mapped on the pH-(KCl) space. The block copolymers presented a much broader phase separation region than the random copolymers. This difference may reflect the effect of the structure of the block copolymers which leads to the formation of micelles in solution. The two-phase region of the block copolymers extended over the whole range of potassium chloride concentrations employed in this study, from 0.0 to 0.75 M, and covered the pH range from two pH units below the polyampholyte isoelectric point up to two units above, with the exception of the points at alkaline pH without added salt. The two-phase region of the random copolymers extended from 0.0 to 0.1 M KCl and covered at most one unit in pH, centered near the isoelectric point. The two-phase region of the block copolymers comprised two regimes: the low salt concentration regime characterized by a compact polyampholyte precipitate in equilibrium with a low-viscosity poly(vinyl alcohol) phase; the high salt concentration and alkaline pH regime characterized by two coexisting low-viscosity phases. The one-phase region of the block copolymers extended in the acidic pH range and covered the whole salt concentration range. The ion type effect on the phase behavior of block polyampholytes was related to the position of the ion in the Hofmeister series: high concentrations of salt of low salting out ability resulted in a transition from a two- to a one-phase system.