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

Abstract

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 θ_w, the degree of methanol binding θ_m, the total coverage of polymer chains by the bound solvent molecules, the excess (selective) adsorption Δξ^E of methanol, the spinodal region, are theoretically calculated as functions of temperature and the volume fraction ξ of methanol outside the gel. The results are compared with the experimental data. At room temperature T = 26.1 °C, the gel exhibits a discontinuous collapse (Q ≈ 0.15) at ξ ≈ 0.18 by the cooperative dehydration of the polymer chains. For higher ξ, the gel approaches the swollen state (Q ≈ 1.5), while in other temperature regions the coverage changes continuously.