Observation of chain dynamics in depolarized light scattering spectra of polymers

A theoretical model for the chain modes' contribution to the depolarized light scattering (DLS) spectrum of polymers is developed. It is shown that the frequency dependence of the DLS susceptibility spectrum is similar to that of the shear loss modulus. Specifically, the contribution of chain modes to the DLS spectrum is composed of a series of Lorentzians (single-exponential processes) having the same amplitude. The relaxation time associated with each mode follows the Rouse prediction, but with a value equal to one-half the mechanical Rouse time. DLS spectra of poly(dimethylsiloxane) melts with different molecular weights have been analyzed in the framework of the model. Reasonable agreement on both qualitative and quantitative levels is observed. In particular, the relaxation time of the longest Rouse mode extracted from DLS spectra is consistent with that from viscosity measurements.