Rubber Elasticity of Poly(N-isopropylacrylamide) Gels at Various Charge Densities

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 Δ G el ≈ α 4, thus deviating from Δ G el ≈ α 2 derived using Gaussian chain statistics. This expression for the elastic free energy explains the behavior of elastic modulus data of the equilibrium swollen hydrogels.