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 chlorides of Na and K are investigated. The samples respond in dramatically different styles to increasing concentrations of the additive in the external 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 initially shrink to an opaque mass, then develop a lump on the surface which eventually turns into a clear transparent bubble. The bubble wall is permeable and the size and shape stable for months implying the contents within are in osmotic equilibrium with the external fluid. (C) 2000 Elsevier Science Ltd. All rights reserved.