Reentrant swelling behavior of poly(4-vinyl phenol) gel in aqueous organic solvent mixtures

Swelling behaviours of poly(4-vinyl phenol) (P4VPh) hydrogel in various kinds of mixed solvents (methanol, ethanol, 2-propyl alcohol, t-butyl alcohol, acetone, acetonitrile, dioxane, THF and DMSO) have been investigated. With increasing the organic solvent content, the gel first significantly deswelled and then reswelled via lowest swelling points (10-60 vol.% depending the solvent species), and finally showed a second deswelling in the highest content region (typically >= 80 vol.%). The rather unique swelling-deswelling sequence was not reproduced when the solvent content was decreased; namely, the backward swelling curves obtained with increasing water composition in the mixed solvents first followed the forward one until the lowest swelling points, then, however, continued the deswelling trend without recovering the initial swelling state in water. This remarkable hysteresis as well as the reentrant swelling with the extra second deswelling has been interpreted in terms of the intermolecular interactions, i.e., hydrogen bonding between the phenol OH groups and the p-p stacking of the phenol rings, by referring to the corresponding results of poly(vinyl alcohol) (PVA) gel system.