Swelling and deswelling pathways in non-ionic poly(N-isopropylacrylamide) hydrogels in presence of additives

The behaviour of deswollen (D) and equilibrium swollen (S) non-ionic poly(N -isopropylacrylamide) (PNIPA) hydrogel samples in aqueous hydroxides and ch lorides of Na and K are investigated. The samples respond in dramatically d ifferent styles to increasing concentrations of the additive in the externa l aqueous medium. Both samples exhibit volume phase transition at additive concentrations characteristic of the anion. Thereafter, if left undisturbed in the same medium, the D samples shrink with increasing concentrations of the additive, while the S samples in a critical concentration range initia lly shrink to an opaque mass, then develop a lump on the surface which even tually turns into a clear transparent bubble. The bubble wall is permeable and the size and shape stable for months implying the contents within are i n osmotic equilibrium with the external fluid. (C) 2000 Elsevier Science Lt d. All rights reserved.