Effects of salt on the temperature and pressure responsive properties of poly(N-vinylisobutyramide) aqueous solutions

Abstract

 We examined the effects of salt on the lower critical solution temperature (LCST) and lower critical solution pressure (LCSP) of aqueous solutions of poly (N-vinylisobutyramide), polyNVIBA, and compared them with those on poly(N-isopropylacrylamide), polyNIPAAm. We found that the addition of salt (such as Na₂SO₄, NaCl, or KCl) decreased the LCST of aqueous polyNVIBA from 45 °C to below 20 °C, almost linearly with the salt concentrations and dependent on the type of salt. We observed a similar concentration-dependent decrease in LCST for polyNIPAAm. When KI or NaSCN was added to each aqueous polymer solution, some smaller increases in LCST were observed at relatively low salt concentrations; higher concentrations of salt gave an almost linear decrease in LCST. As for LCSP, the addition of most types of salt lowered the transition pressure, but the effects were much more dependent on the type and the valence of the salt (especially of anion) in both polymers. Salt with divalent anion showed a larger decrease in LCSP, but those with mono valent anion showed a relatively small decrease, even showed a slight increase at lower salt concentrations in the case of polyNVIBA. Salt with I^- or SCN^- showed evident increases in LCSP up to 1 M and was maintained higher than the control even at 2 M. We discuss the interactions of the amide group in the side chains of polymers and water and their perturbation by ions.