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 get volume V-eq with the charge density IN was considerably weaker (V (eq) approximate to fN(0.75)) than expected assuming Gaussian statistics (V-eq approximate to fN (1.5)). This deviation is interpreted as the nonGaussian elasticity of the hydrogels swollen to equilibrium. The exponent 0.75 found indicates that the elastic free energy DeltaG(el) scales with the linear swelling ratio a as DeltaG(el) approximate to alpha(4), thus deviating from DeltaGel approximate to alpha(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.