Light scattering molecular weight dispersity

The experimental estimate of 37.5 nm for p may be compared with the predictions of the rotational isomeric state model using the result for C of Yathindra and Rao mentioned above, for which p lies in the range 13 to 22 nm. The difference is small enough to be attributed to error in experimental light scattering, the effects of residual intermolecular association, the effects of molecular weight dispersity, or solvation effects, which are neglected in the theoretical estimate of p. 

Light-Scattering Photometer. In one type, suspended particles are determined (counted). Another type is used to determine the molecular weight of macromolecules dispersed in solution. The former type operates on the basis of a nephelometer the latter is of much higher precision and uses monochromatic light. 

The molecular weight obtained by application of Eq. (4.82) to a poly disperse polymer will be some average over the molecular weight distribution characteristic of the polymer. It can be easily shown that the molecular weight determined from light scattering measurement is Mw (see Problem 4.13). 

Finally, when studying dilute solutions of high-molecular-weight flexible coil macromolecules, an additional contribution to the spectral dispersion of the scattered light can arise from the dynamic behavior of the low frequency, long wavelength internal vibrational motions of the chains (70). Additional Lorentzian spectral components (or exponential components of the correlation function) arise through this mechanism characterized by halfwidth (time constant) ... 

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