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Three samples of polystyrene in cyclohexane were studied at the theta temperature by photon correlation spectroscopy. The photon correlation data were analyzed using the moments (or cumulants) method which, under appropriate conditions, yields the Z−average diffusion coefficient ?Z of the solute and one or two higher moments of the diffusion coefficient distribution D.E. Koppel, J. Chem. Phys. 57, 4814 (1972). ?Z can be combined in the Svedberg equation with the weight−average sedimentation coefficient to yield the weight−average molecular weight. The samples studied comprised an essentially monodisperse fraction of polystyrene, a mixture of two monodisperse fractions and a single fraction of broader molecular weight distribution; they had been extensively characterized by other techniques. Our results indicate that, indeed, the Z−average diffusion coefficient is provided by the moments−analysis method. The values of ?W obtained from the Svedberg equation were, within experimental error, in good agreement with those determined by other techniques. The second moment of the diffusion coefficient distribution provides a useful index of polydispersity.
Brown et al. (Sat,) studied this question.