Characterization of molecular motions in polymer gel networks using intensity-fluctuation spectroscopy

This work seeks to investigate the intrinsic dynamics of synthetic polymer gel networks in terms of cooperative submolecular motions, which give rise to autocorrelation functions, observed by intensity fluctuation spectroscopy. Poly(dimethyl siloxane) samples were purified and chemically crosslinked to three levels of crosslink density. The resulting networks were swollen in toluene. Light scattering data were obtained over a range of swelling ratios from each sample as well as from the uncrosslinked polymer in solution. Due to a large amount of extraneous scattering, indicating the presence of a large component of static scattering, a "background" normalization technique was devised which allowed extraction of the desired correlation function. Also, a computer program utilizing the "method of cumulants" was developed to obtain diffusion coefficients from nonlinear autocorrelation decay data. The "diffusion coefficient" DT calculated for the uncrosslinked polymer in solution indicates scattering entities of about 6 A. This corresponds to a molecular weight of about 16,, considerably less than the molecular weight of the polymer (16,). The values for the diffusion coefficient were found to increase with increasing concentration, contrary to intuition. A similar increase in DT with polymer concentration was observed in swollen crosslinked gels. Also, there was evidence that the diffusion coefficient decreases with increasing degrees of crosslinking. A qualitative model was developed which describes a system of cooperative submolecular scatterers, consistent with the observed changes in D. The results of the light scattering measurements were also compared to mechanical measurements of the tensile elastic modulus for the different crosslinked systems. These tests indicate that not all of the available sites were utilized in the crosslinking reactions