Reentrant volume phase transition of cross-linked poly(N-isopropylacrylamide) gels in mixed solvents of water/methanol

The reentrant volume phase transition of cross-linked poly( N-isopropylacrylamide) gels in mixed water/methanol solvents is shown to be induced by the competition between polymer-water hydrogen bonds and polymer-methanol hydrogen bonds by means of the theoretical analysis of the experimental data. On the basis of a statistical-mechanical model for the competitive hydrogen bonds with cooperativity in forming sequences of hydrogen bonds on a polymer chain, the swelling ratio Q, the degree of hydration theta(w), the degree of methanol binding theta(m), the total coverage (theta) over tilde of polymer chains by the bound solvent molecules, the excess (selective) adsorption Delta xi(E) of methanol, the spinodal region, are theoretically calculated as functions of temperature and the volume fraction xi of methanol outside the gel. The results are compared with the experimental data. At room temperature T = 26.1 degrees C, the gel exhibits a discontinuous collapse (Q approximate to 0.15) at xi approximate to 0.18 by the cooperative dehydration of the polymer chains. For higher xi, the gel approaches the swollen state (Q approximate to 1.5), while in other temperature regions the coverage (theta) over tilde changes continuously.