Abstract
We report the volume phase transition of poly(N-isopropylacrylamide-co-sodium acrylate) gel. The diameter decrement was observed under the continuous water flow (or exchange) for the cylindrical gels with different diameters at gelation. After the water flow, the diameter of the gel in the limited amount of water was measured as a function of temperature. The volume change of gel was classified into three different behavior, discontinuous, reswelling and continuous. When the gel was flushed by a small and large amount of water, the gel exhibits the well-known discontinuous and continuous phase transition. On the other hand, the gel exhibits a reswelling behavior after the sufficient water flow. The reswelling phase transition is discussed in terms of the exchange of counter ions by the water flow and of the creation and destruction of the intermolecular hydrogen bonding. A simple theoretical model is presented to qualitatively explain the condition for the appearance of the reswelling behavior on the basis of the Landau theory. It was concluded that the phase transition behavior of the present system depends on the initial swelling ratio at a swollen state (and not the gelation diameter), which is controlled by the degree of the water flow (or exchange).
