A LIGHT-SCATTERING STUDY OF 1-4-CIS-TRANS-POLYBUTADIENE

Light-scattering measurements on 1-4 polybutadiene of two molecular we ights have been carried out using two different tandem Fabry-Perot int erferometers in the back-scattering and 90 degrees scattering geometri es. The Raylejgh-Brillouin spectrum has been measured at different tem peratures through the glass transition. Relaxation times and sound vel ocities have been fined to a hydrodynamic formula, with as few indepen dent parameters as possible. Comparison with previous measurements sho ws that the Brillouin relaxation times do not follow the structural or alpha relaxation. Forcing the fit either with the times of the alpha process or with a high-frequency relaxation channel does not yield sat isfactory results. The only processes arising on a similar timescale a nd with comparable activation energies are intra-chain conformational changes rates as computed by molecular dynamics. A possible mechanism linking these conformational changes to the damping of the Brillouin l ines is suggested.