Studies on pH and temperature dependence of the dynamics and heterogeneities in poly(N-isopropylacrylamide-co-sodium acrylate) gels

Dynamic light scattering study has been carried out on weakly charged, temperature-sensitive polymer gels made by copolymerization of N-isopropylacrylamide (NIPA) and sodium acrylate (SA) as a function of pH and temperature. The ensemble average light scattering method [Macromolecules 29 (1996) 6535] was employed to extract the thermal component, 〈IF〉T, of the scattered intensity from the ensemble-average scattered intensity 〈I〉E as well as to evaluate the collective diffusion coefficient, D. The following facts were disclosed: (1) The NIPA/SA gels swelled in 4<pH<10, as a result of ionization of acrylic acid (AAc). (2) When pH was varied, 〈I〉E exhibited a maximum around pH 5. This was accounted for by the interplay of the two antagonistic effects on scattered intensity due to ionization of the gel, i.e. (i) an increase in heterogeneities by swelling and (ii) a decrease in heterogeneity due to an increase in osmotic pressure. (3) On the other hand, 〈IF〉T decreased in the pH region between 5.2<pH<10, due to an increase in osmotic modulus by ionization of the network chains. (4) The variation of D was found to be highly correlated with that of polymer concentration, φ, and was scaled to be D∼φ1.