DYNAMIC AND STATIC LIGHT-SCATTERING BY AQUEOUS POLYACRYLAMIDE GELS

Dynamic light scattering (DLS) and static light scattering (SLS) experiments on cross-linked polyacrylamide (PAA) gels show that these systems exhibit nonergodic behavior; i.e., the time-averaged intensity correlation function (ICF), the quantity obtained from a single DLS experiment, is not equal to the ensemble-averaged ICF. Treating the gels as arbitrary nonergodic media, the intermediate scattering function (ISF) is extracted from single DLS experiments. The initial decay of the ISF yields a diffusion coefficient that takes into account all density fluctuations. Particularly, at high cross-link density or high total monomer content the ISF's show a nondecaying component resulting from frozen-in density fluctuations. The ensemble-averaged scattered intensity shows a clear angular dependence, implying long-range (almost-equal-to 500-nm) correlation lengths in the gels. By combining DLS and SLS the fluctuating part of the scattered light can be separated from the total scattered intensity. This fluctuating component is hardly angle dependent and surprisingly appears to be somewhat larger for gels than for polymer solutions at the same concentrations, implying smaller osmotic moduli for gels than for solutions. Furthermore, the analysis shows that the mobility of the polymer is larger in gels than in analogous solutions.