Polymer Analysis by Thermal-Diffusion Forced Rayleigh Scattering

Abstract

The holographic grating technique of thermal-diffusion forced Rayleigh scattering (TDFRS) is used for the study of Fickian and thermal diffusion in simple liquids and polymer solutions. All three diffusion coefficients D th , D, D t and the Soret coefficient S t can be obtained from a single experiment. Due to the short diffusion length of the order of a few μm, the whole system is very stable against perturbations like convection. TDFRS and photon correlation spectroscopy (PCS) are compared in detail. In case of polydisperse solutes, TDFRS does not suffer from the high scattering power of heavy components, which dominate the PCS correlation function. Quantities of interest for polymer analysis are distribution functions and averages of diffusion coefficients and molar masses. The weight distribution of the hydrodynamic radius follows directly from the heterodyne diffraction efficiency, without the need to resort to scaling relations. Pseudostochastic binary sequences with a broad power spectrum allow for the direct measurement of the linear response function, to which the individual molar masses contribute strictly concentration proportional, with a high spectral power density. Certain diffusive modes can be suppressed or enhanced by random binary sequences with colored power spectra, which are tailored to the specific experimental problem.