Swelling equilibrium of hydrogels of (N-isopropyl acrylamide + anionic and cationic comonomers) in aqueous solutions of sodium chloride: Experimental results and modeling

Abstract

Experimental results are reported for the swelling equilibrium of some ionic hydrogels consisting of N-isopropyl acrylamide (N-IPAAm) and one or two comonomers – the anionic monomer 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) and/or the cationic monomer 3-(methacrylamido)propyl trimethylammoniumchloride (MAPTAC) – in water and in aqueous solutions of sodium chloride at 298K. The degree of swelling of such ionic hydrogels increases with the net charge (i.e., the difference in the mole fractions of anionic and cationic comonomers) as well as with the charge density (i.e. the mole fraction of a single comonomer). The mole fraction of a single comonomer was varied between 0 and 0.05. The (absolute) difference between the mole fractions of the anionic and cationic comonomers was varied between 0 and 0.03. Two characteristic transitions in the degree of swelling of the hydrogels were observed when sodium chloride is added to the coexisting liquid phase. The first transition depicts the conversion of the ionic comonomer into the neutral state. In that transition the degree of swelling of a charged ionic hydrogel reduces to the degree of swelling of a nonionic hydrogel. For the investigated hydrogels, that transition occurs when the mass fraction of NaCl in the coexisting aqueous phase is increased beyond approximately 5×10−5. The second transition occurs at much higher sodium chloride mass fractions (around 5×10−3). In that transition the hydrogel collapses, i.e., that transition resembles the phase transition observed when non-ionic N-IPAAm hydrogels are dissolved in aqueous solutions of sodium chloride.The experimental results for the degree of swelling are quantitatively described by a thermodynamic framework that combines an expression for the Gibbs energy of a liquid phase (a chemical reactive, aqueous electrolyte solution of the non-crosslinked polymer chains) with an expression for the Helmholtz energy of an elastic network (that accounts for the crosslinking effects).