FOLLOWING POLYMER GELATION BY DEPOLARIZED DYNAMIC LIGHT-SCATTERING FROM OPTICALLY AND GEOMETRICALLY ANISOTROPIC LATEX-PARTICLES

The formation of polyacrylamides gels has been followed by zero angle dynamic depolarized light scattering from colloidal poly(tetrafluoroethylene) probe particles. The scattering is dominated by the probes, since polyacrylamide does not strongly depolarize light. Before gelation, the zero angle depolarized intensity time correlation function g((2))(tau) lies in the fully homodyne limit with a high optical coherence parameter f = g((2))(0) - 1 approximate to 0.94. As gelation proceeds, f decreases and the correlation functions take on a wide spectrum of decay rates centered about a decreasing average value. The average decay rate does not decrease to zero, however. Assuming a mixed homodyne-heterodyne model, the decrease in f was used to measure the population of ''mobile'' vs ''frozen'' colloidal latex particles. Despite the large measured volume of the zero angle experiment, only one ''speckle'' of scattered light is observed after the gel is formed, so the measurements do not represent an average over all possible configurations. Obtaining a proper average at zero angle would be, at best, tedious. At finite scattering angles, ensemble averaged correlation functions for depolarized scattering were generated over many speckles by rotating the sample. This was performed for ''soft'' and ''hard'' gels, which differed only in cross-linker content. Substantial differences were found, both as regards the portion of particles retaining some mobility and their dynamics.