Investigation of molecular isomerism in polydimethylsiloxane by Raman scattering

High optical quality samples of polydimethylsiloxane rubber have been prepared, crosslinked by gamma radiation. The Raman depolarization ratios, rho, of two highly polarized bands, the CH3 stretching at 2907 cm(-1) and the Si-O skeletal mode at 491 cm(-1), have been measured as a function of temperature and strain. The changes in rho are related to changes in the relative populations of trans and gauche isomer populations, and 2 value for Delta E. the energy difference, is found to be 820(-20)(+50) cal/mole. The proportionality constant in a theory connecting strain and isomer populations D-2 is measured to be 10 +/- 2. Computer calculations based on statistical weight matrices yield Delta E approximate to 875 cal/mole and D-2 approximate to 1.18. Reasons for the discrepancy in the D-2 values are discussed.