Finite extensibility and deviation from Gaussian elasticity of dimethylacrylamide-based gels with different charge density: Insight into pH/solvent-dependent swelling and surfactant interactions

The quasi-binary ionic gels composed of non-ionic N,N-dimethylacrylamide (DMAAm) and ionic comonomer sodium acrylate (NaA) were prepared to investigate the charge density dependence of the equilibrium swelling and elasticity under various environmental conditions. By altering the chemical composition, highly ionic poly(N,N-dimethylacrylamide-co-sodium acrylate) P(DMAAm-co-NaA) hydrogels and cryogels were obtained which were highly sensitive to pH- and ion-concentration of the external solution containing surfactant or halide ions. The mechanical properties and the corresponding degree of swelling in water/solvent mixtures both for hydrogels and cryogels were followed by the pulsatile swelling/deswelling experiments. The swelling behaviors of the prepared hydrogels and cryogels with specific network structures were dependent on the pH of external solution, NaA content, associative interactions between polymer and surfactant species and the concentration of the monovalent-salt solutions. By the results of dynamic swelling test, the influence of NaA content on the swelling rate, swelling coefficients and diffusional behavior of water in the prepared gels were determined. The variation in the charge density in P(DMAAm-co-NaA) hydrogel networks was theoretically explained in the framework of model which treats to explain the dependence of the elastic free energy on their swelling ratio. Although the introduction of electrostatic interactions decreased the reduced elastic modulus at low swelling degree, the deviation from Gaussian elasticity due to finite chain extensibility was clearly detected at higher ionic comonomer concentration in which the reduced modulus increases with increasing swelling. The elasticity data showed that Gaussian statistics fails to describe the statistical properties of PDMAAm chains at higher swelling degree and the equilibrium swollen P(DMAAm-co-NaA) hydrogels are in the non-Gaussian regime. Developed theory containing adjustable parameters allows to obtain semiquantitative matching of experimental results on swollen state properties of highly charged gels and non-Gaussian elasticity.