Temperature Dependence of Density of Polymer Gels: Effects of Ionizable Groups in Copoly(N-isopropylacrylamide/acrylic Acid Or Sodium Acrylate)-water Systems


Effects of ionizable groups in hydrogels of copolymer networks on the volumetric contraction-expansion process were investigated. Polymer networks used were: copoly[N-isopropylacrylamide (NIPA)(1 − x)/acrylic acid (HAc) or sodium acrylate (NaAc)(x)] with mole fraction of minor component (x) assuming 0.0114 and 0.0457. From the temperature (T) dependence of total volume of gels, densities of the polymer and solvent (water) components, and stoichiometry, we evaluated (1) the volume of gels occupied by a single mean polymeric residue and associated water molecules (expressed in units of nm3), mean vsp(gel), and (2) number of water molecules per single mean polymeric residue, mean Ns(gel), from near 273 K to 323 K. These quantities (1) and (2) listed above showed how acid and salt forms affect differently on volumetric changes of gels over 50 K. We developed an approach to evaluate volumetric changes of gels solely caused by a single polymeric residue of a minor component (x < 0.05) plus associated water by applying thermodynamic first-order perturbation theory. They are specific vsp(gel)(T) for a single HAc or NaAc polymeric residue plus associated water and the corresponding specific Ns(gel)(T). Specific vsp(gel)(HAc or NaAc)(T) and the corresponding specific Ns(gel(T)) revealed specific characteristics in thermal behavior near their respective transition temperatures from the swollen to shrunken states. We found these thermal changes shown at the nano-scale match very well with specific changes in the molality(T) of both ionizable groups. In fact, these are directly triggered by varying contents of water in gels. Based on the understanding of dissociative equilibrium attained by ionizable groups, we successfully replaced Na+ in hydrogels of copoly[NIPA(1 − x)/NaAc(x)] (x = 0.0457) by hydrogen ions. Absence of Na+ in treated hydrogels was experimentally verified by 23Na NMR and Na atomic absorption flame photometry. Discontinuity in the volumetric contraction-expansion process from the swollen to shrunken states and vice versa was not observed in contradiction to the previous reports [Hirotsu S, Hirokawa Y, Tanaka T. J Chem Phys 1987;87:1392-5. Matsuo SE, Tanaka T. J Chem Phys 1988;89:1695-703.] obtained by the conventional swelling experiments.




United States. Department of Energy

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