Brillouin scattering from cross-linked gels

In this letter we report Brillouin scattering measurements on methyl-methacrylate (MMA) gels crosslinked with varying amounts of ethylene-dimethacrylate (EDMA). We find that the k dependence of the phase velocity changes on increasing the cross-link content. For higher concentrations of crosslink we observe maxima and minima in the k dependence of the phase velocity. We associate these minima and maxima With spatial inhomogeneities in the gel, with the formation of regions of low and high cross-link density, respectively. This micro-phase separation is frozen in by the presence of the already existing spanning network.