pH and salt concentration dependence of the microstructure of poly(N-isopropylacrylamide-co-acrylic acid) gels

The structure of environment-sensitive gels consisting of poly(N-isopropylacrylamide-co-acrylic acid) (NIPA/AAc) hydrogels was investigated by small-angle neutron scattering (SANS). Effects of the variation of ionic circumstance were observed both on the volume phase transition and on the structure factors: (1) The volume phase transition temperature, Tc, increases with increasing pH from 4 to 9 and then decreases with further increasing pH. (2) the volume gap between the shrunken and swollen gels at Tc is highest around pH 7 to 9. (3) Though the structure factor, I(q), has a maximum above the Tc 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 T≳Tc. However, strong ionic screening leads to a lower swelling power and a decrease in Tc, 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.