A Brillouin scattering study of end-linked poly(dimethylsiloxane) networks

We have studied inelastic light scattering from acoustic phonons in end-linked poly(dimethylsiloxane) networks using a Brillouin scattering technique. A series of nearly monodisperse networks with molecular weights between cross links ranging from 800 g/mol to 30 000 g/mol are probed to study the effects of cross-linking and deformation on the phonon speed and attenuation. The Brillouin spectra are obtained using a combination of a three-pass tandem Fabry–Perot interferometer in conjunction with a CCD area detector. Longitudinal acoustic phonons of frequency 2–6 GHz propagating in the networks are studied at an excitation wavelength of 514.5 nm. The phonon frequencies and linewidths are measured as a function of the phonon wave vector by employing different scattering geometries. For highly cross-linked systems the phonon speed is shown to depend on the molecular weight. With decreasing cross-link density, the phonon velocity is observed to approach an asymptotic value. The dispersion of the acoustic phonon is investigated over a limited frequency range to probe the acoustic nature of the phonon. For poly(dimethylsiloxane) networks it is verified that at room temperature there is no measurable dispersion in the phonon speed at GHz frequencies. The phonon velocity is also measured as a function of strain, employing different geometries in elongation and compression experiments. These results are discussed in terms of the change in the compressibility and the local viscosity of the network.