Swelling and stress-relaxation of poly(N-isopropylacrylamide) gels in the collapsed stateThe structure of environment-sensitive gels consisting of poly͑N-isopropylacrylamide-co-acrylic acid͒ ͑NIPA/AAc͒ hydrogels was investigated by small-angle neutron scattering ͑SANS͒. Effects of the variation of ionic circumstance were observed both on the volume phase transition and on the structure factors: ͑1͒ The volume phase transition temperature, T c , increases with increasing pH from 4 to 9 and then decreases with further increasing pH. ͑2͒ the volume gap between the shrunken and swollen gels at T c is highest around pH 7 to 9. ͑3͒ Though the structure factor, I(q), has a maximum above the T c of the corresponding NIPA homopolymer gels, the peak intensity decreases with increasing pH, where q is the magnitude of the scattering vector. When NaCl is added, a similar scattering maximum appears for TϾT c . However, strong ionic screening leads to a lower swelling power and a decrease in T c , resulting in precipitation at a lower temperature than for the gel at a moderate pH. The roles of pH and salt are discussed on the basis of Donnan membrane equilibrium.

The swelling/shrinking behavior of chemically cross-linked poly(vinyl alcohol) (PVA) gel complexed with borate ions in aqueous medium and the intrinsic viscosity behavior of the corresponding polymer solutions were compared as a function of the boric acid concentration, b. The equilibrium swelling ratio of the gels, Q/Q 0, and the intrinsic viscosity ratio of the solutions, [η]/[η]0, were in good accordance when the NaOH concentration, [NaOH], is high enough, where Q 0 and [η]0 are the equilibrium swelling volume of the gel and the intrinsic viscosity of the solution without borate ions, respectively. NaOH was added so as to ionize boric acid. However, a significant difference was observed when [NaOH] was lowered. The comparison of Q/Q 0 and [η]/[η]0 allowed one to elucidate the essential difference in the swelling (or expansion) behavior of polymer chains, which is the presence (in a gel) and absence (in a dilute polymer solution) of the Donnan potential. A free energy function for ion-complexed polymer gels is proposed.