The mechanical properties and the swelling behavior of a series of N-isopropylacrylamide copolymers with the sodium salt of 2-acrylamido-2-methylpropanesulfonic acid were investigated. The increase of the equilibrium gel volume V eq with the charge density fN was considerably weaker (V eq ≈ fN 0.75) than expected assuming Gaussian statistics (V eq ≈ fN 1.5). This deviation is interpreted as the non-Gaussian elasticity of the hydrogels swollen to equilibrium. The exponent 0.75 found indicates that the elastic free energy Δ G el scales with the linear swelling ratio α as ΔG el ≈ α4, thus deviating from ΔG el ≈ α2 derived using Gaussian chain statistics. This expression for the elastic free energy explains the behavior of elastic modulus data of the equilibrium swollen hydrogels.

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