Thermoreversible swelling behaviour of hydrogels based on N-isopropylacrylamide with a zwitterionic comonomer

The swellability in water of poly(N-isopropylacrylamide) (PNIPA) is reduced by the presence of salts in the aqueous medium. Poly(NIPA-co-acrylic acid) is more swellable, but its swelling ratio r (=mass hydrogel/mass xerogel) is also reduced by the presence of salts. To retain the advantage of reversible thermosensitivity without reducing swellability in aqueous salt solutions crosslinked copolymers of NIPA with the zwitterionic comonomer 1-(3-sulphopropyl)-2-vinyl-pyridinium-betaine (SPV) have been prepared and swollen in water, aqueous KCl and aqueous KSCN. The values of r have been measured and the effects of content of SPV, temperature, concentration and nature of salt have been noted and discussed. Swellability is enhanced at low [salt] and is then reduced at higher ionic strength. However, at [salt] extending to ∼1 mol dm−3 the value of r⩾the value of r in pure water. The presence of salts has virtually no effect on the value of the lower critical solution temperature.