Effect of spatial gel inhomogeneity on the elastic modulus of strong polyelectrolyte hydrogels

Abstract

The effect of spatial inhomogeneity on the elastic modulus of ionic poly(acrylamide) (PAAm) hydrogels has been investigated with the static light scattering measurements. The gels were prepared by free-radical crosslinking copolymerization of acrylamide and 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) monomers and N,N’-methylenebis(acrylamide) crosslinker. The crosslinker concentration of the hydrogels was fixed in our experiments while the mole fraction of AMPS in the comonomer feed (x i ) was varied over a wide range. Elasticity measurements show that the modulus of elasticity of gels increases with increasing x i from 0 to 0.2. The excess scattering of gels, that is, the degree of spatial gel inhomogeneity rapidly decreases with increasing x i and approaches to zero at x i =0.05. Debye-Bueche analysis of the light scattering data indicates frozen concentration fluctuations in gel; the size of the static structures increases while the extent of concentration fluctuations reduces with increasing x i . It was shown that the macroscopic elastic properties of PAAm gels are mainly controlled by the microscopic gel structure determined by the scattering measurements.