Cooperative Hydration Induces Discontinuous Volume Phase Transition of Cross-Linked Poly(N-isopropylacrylamide) Gels in Water

The concept of cooperative dehydration, defined as the simultaneous dissociation of the bound water molecules from the polymer chains in correlated sequences, is applied to study the collapse of cross-linked poly( N -isopropylacrylamide) (PNIPAM) gels in aqueous solutions upon heating above the critical temperature. We examined the applicability of this concept in two situations: (i) free swelling of PNIPAM gels in water (in the absence of applied force), (ii) swelling of PNIPAM gels in water subjected to a uniaxial tension. The transition becomes sharper as the cooperativity parameter σ of hydration decreases (increasing cooperativity), and there is a critical value σ c ≃ 0.30 of the cooperativity parameter below which the volume transition is discontinuous. Hence, the real phase transition with thermodynamic discontinuity is shown to be possible for the nonionic polymer gels if the hydration is sufficiently cooperative. The calculations of the swelling ratio and the spinodal lines are compared with the experimental data of PNIPAM gels by Hirotsu for point i and by Suzuki for point ii.