Thermal fluctuations level inhomogeneities during gelation of polydimethylsiloxane: Observations employing the Rayleigh-Brillouin light-scattering technique

An isothermal end-link reaction process in polydimethylsiloxanes is studied by Rayleigh-Brillouin scattering. From the onset of gelation, the Rayleigh line shows increases in intensity which are of the Brillouin lines intensity level, and which are ascribed to the occurrence of static density fluctuations, i. e., of inhomogeneities. Using a density correlation function of an exponential type, the extent and magnitude of the inhomogeneities are estimated from the Rayleigh line intensity corrected for thermal fluctuation factors, and inhomogeneities with correlation length of the order of 100 å are found to appear in network structures during the gelation process. It is suggested that the inhomogeneities results from nonuniform progress of crosslink reactions. These inhomogeneities do not affect the propagation of hypersonic sound.