Swelling, elasticity and spatial inhomogeneity of poly(N,N-dimethylacrylamide) hydrogels formed at various polymer concentrations

The equilibrium swelling degree, modulus of elasticity and the spatial inhomogeneity of poly(N,N-dimethyl-acrylamide) (PDMAAm) hydrogels were investigated over the entire range of the initial monomer concentration. The degree of dilution of the networks after their preparation was denoted by v(2)(0), the volume fraction of crosslinked polymer after the gel preparation. The linear swelling ratio of the gels increased linearly with increasing v(2)(0). Depending on the value of v(2)(0), three different gel regimes were observed: (1) For v(2)(0) < 0.3, increasing v(2)(0) decreases the extent of cyclization during crosslinking so that the effective crosslink density of gels increases with rising v(2)(0). (2) For 0.3 < v(2)(0) < 0.7, increasing v(2)(0) reduces the accessibility of the pendant vinyl groups during crosslinking due to steric hindrance at high polymer concentrations. As a result, the effective crosslink density of gels decreases with increasing v(2)(0). (3) For v(2)(0) > 0.7, the modulus of elasticity increases sharply with increasing v(2)(0) due to the increasing extent of chain entanglements in this high concentration regime. Static light scattering measurements on the gels show that the degree of spatial gel inhomogeneity in PDMAAm gels attains a maximum value at v(2)(0) = 0.06. The appearance of a maximum as well as the v(2)(0)-dependence of scattered light intensities from gels was successfully reproduced by the theory proposed by Panyukov and Rabin.