Multi-faced investigation for pH-sensitivity and solvent polarity in highly charged thermo-responsive hydro- and cryogels with strongly dissociated groups: A comparative evaluation of physico-chemical properties

Different series of strong polyelectrolyte poly(N-isopropylacrylamide-co-sodium acrylate) P(NIPA-co-NaA) hydrogels and cryogels were prepared in aqueous solution. With changing the ionic comonomer NaA ratio in reaction mixture, the resulting physico-chemical and swelling properties varied significantly. The findings elucidated that ionic P(NIPA-co-NaA) gels exhibited multi-stimuli performances with pH-, temperature-, solvent-, surfactant- and salt-responsivity, showing more sensitivity than non-ionic PNIPA. Their swollen/collapsed behaviors were strongly dependent on various synthesis/external parameters, including the content of ionic comonomer, temperature and solvent polarity. The effects produced by non-solvents were qualitatively compared considering the aspects related to co-nonsolvency phenomenon and re-entrant swelling transition on the basis of weakening of PNIPA-water interactions. Measurements of the elastic properties and swelling degree of samples having different NaA content were treated by combining Flory-Rehner theory and rubber elasticity to interpret the experimental observations. The polymer-solvent interaction parameter χ of hydrogels was calculated from the swelling studies in water at different temperatures and was related to the composition and temperature. The ionic strength-reversibility and on-off switching kinetics of hydrogels and cryogels were studied in 10⁻⁵ M and 1.0 M of NaBr solutions. Diffusion kinetics showed that the swelling of ionic hydrogels is of non-Fickian-type transportation, however, non-ionic PNIPA hydrogels and P(NIPA-co-NaA) cryogels showed pseudo-Fickian diffusion. The resulting hydrogels and cryogels containing both temperature and pH-sensitive units are able to interact with oppositely charged groups via hydrophobic and electrostatic interactions.