The research summarized in this thesis was primarily directed towards understanding the microscopic structure of polysiloxanes and their mechanical behavior in different time and length scales using optical and mechanical techniques. This thesis is comprised of two projects. The first was aimed at developing an experimental technique to probe polymer networks based on a mechanical pulse propagation idea. The theory of elastic wave propagation was used to provide a theoretical framework relating the wave speeds to the stress tensor. Molecular models for polymer networks were used to predict the relationship between the speed of propagation and the molecular constitution of networks under deformation. The main objective of this project was to achieve a better understanding of network topology using a propagating pulse as a probe while providing a fast non-destructive method for characterizing networks. These are low frequency measurements (typically ~ 1 KHz) and in these time scales the networks are essentially in the so-called rubbery regime.

At much higher frequencies (~ 5 GHz) the networks are almost frozen into a glassy state. At these frequencies Brillouin spectroscopy is a powerful technique to investigate the static and dynamic acoustic properties of polymers. In our second project we used this high-resolution optical technique to study inelastic light scattering studies from phonons in silicones. In these experiments, we probed the molecular-weight dependence, effect of deformation and cross-link dynamics in polysiloxanes. The Brillouin spectra of end-linked poly(dimethylsiloxanes) were obtained using a unique 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 were studied at 514.5 nm. From the measured Brillouin shifts, the molecular weight dependence of the velocity and the half-width of the phonons were obtained. The dispersion of the acoustic phonon was investigated over a limited frequency range to probe the acoustic nature of the phonon. Polarization studies confirmed that the observed phonons were longitudinal. Phonon propagation for networks under deformation (uniaxial tension or compression) was probed for high molecular weight networks. The results are interpreted using analytical arguments based on molecular models of networks.