Dynamic light scattering from polyacrylamide-water gels

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Abstract
Rayleigh light scattering measurements are reported of the elastic modulus G1 and collective motion diffusion constant Dc of a series of polyacrylamide-water gels as a function of concentration. The gels were copolymerized with N,N'-methylene bisacrylamide which acts as a cross-linking agent. While G1 varies only slightly with cross-linking density at constant polymer concentration c (in the region where multiple scattering is negligible), the mechanically measured elastic modulus Gm increases strongly with increasing cross-linking, almost attaining the value of G1, then falls as the gels become opalescent and inhomogeneous. For concentrations between 0.025 and 0.12 g cm-3 G1 varies as c2.35±0.06 at room temperature. At the cross-linking densities used for the mechanical measurements, Gm is generally smaller than G1 by a factor of 2, but at lower polymer concentrations the difference increases as the effects of loose chains become pronounced. Dc is found to vary as c0.65±0.03, in agreement with observations on other polymer systems. The difference between this exponent and that expected for the inverse screening length is consistent with a free volume behaviour of the solvent viscosity