QUASI-ELASTIC LIGHT-SCATTERING - SEPARABILITY OF EFFECTS OF POLYDISPERSITY AND INTERNAL-MODES OF MOTION

The influence of a size distribution on the angular dependence of the quasielastically scattered light is studied for (i) large hard spherical particles and (ii) large flexible chain molecules. For the spherical particles the angular dependence is shown to depend solely on the size distribution and the particle scattering factor. Combination of conventional elastic light scattering with quasielastic light scattering allows the determination of the z-average radii moments r n ̄z (n = -1, 1, 2, ...) which define the size distribution. Flexible chains - linear and branched ones - show in any case a linear dependence of the apparent diffusion constant Dapp = G{cyrillic} q2 on q = ( 4π λ) sin θ 2, when q becomes large. This behaviour represents the flexibility of the spring-bead model with strong hydrodynamic interaction. The initial part on the other hand form a straight line when Dapp is plotted against q2. The intercept of this straight line is the z-average diffusion constant while the slope is proportional to the z-average mean square radius of gyration. Thus, the polydispersity can be estimated from Dz and 〈S2〉z while the asymptote at large q-values is determined by the internal flexibility of the molecule. © 1979.