Finite extensibility and deviation from Gaussian elasticity of dimethylacrylamide-based gels with different charge density: Insight into pH/solvent-dependent swelling and surfactant interactions. (6th December 2017)

• Record Type: Journal Article
• Authors: Orakdogen, Nermin; Boyaci, Talin
• Is Part Of: Polymer. Volume 132(2017)
• Page Start: 306
• Page End: 324
• Publication Date: 2017-12-06
• Subjects: N, N-dimethylacrylamide -- Non-Gaussian elasticity -- Counterion -- pH-dependent swelling -- Surfactant interaction -- Diffusion coefficient; Polymers -- Periodicals; Polymerization -- Periodicals; Polymères -- Périodiques; Polymérisation -- Périodiques; 547.7
• DOI: 10.1016/j.polymer.2017.11.013
• Languages: English
• ISSNs: 0032-3861

Abstract

Abstract: 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. Graphical abstract: The prediction of the effect of swelling on the stress-strain behavior from small to large deformation levels remains a challenging issue for true characterization of rubber elasticity. Highly-swollen P(DMAAm-co-NaA) hydrogels showed deviation from Gaussian behavior with a scaling parameter 0.31. Due to the presence of the charges on the hydrogel matrix and the osmotic pressure of counter-ions, the polymer chains are highly extended and thus far from the Gaussian coil state assumed in the classical theory. Highlights: Joint theoretical and experimental study presents semiquantitative matching of results on highly charged PDMAAm hydrogels. Deviation from Gaussian elasticity due to the finite chain extensibility was detected. Gaussian statistics fails to describe the statistical properties of PDMAAm chains at higher swelling degree. Charge density dependence of equilibrium swelling and elasticity under various environmental conditions was investigated. Compressional elastic properties of strongly ionic P(DMAAm-co-NaA) hydrogels and cryogels were discussed.