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(tetrafluoroet hylene) probe particles. The scattering is dominated by the probes, si nce 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 dec ay 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 la rge measured volume of the zero angle experiment, only one ''speckle'' of scattered light is observed after the gel is formed, so the measur ements do not represent an average over all possible configurations. O btaining 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 d iffered only in cross-linker content. Substantial differences were fou nd, both as regards the portion of particles retaining some mobility a nd their dynamics.