Several authors have investigated the Rayleigh linewidth of light scattered from the gels of cross-linked flexible polymers such as polyacrylamide14. The width of the spectral broadening varies as sinL(O/2), and diffusion coefficients may be calculated in the same way as for a macromolecular solution5. According to the theory of Tanaka et al.1, this corresponds to freely diffusing fluctuations in polymer segment density and is equal to the ratio of the longitudinal elastic modulus to the force per unit volume required to maintain unit relative velocity between the polymer network and solvent. The degree of spectral broadening has, in general, been found to be small enough (ca. 10%) for the optical beating to be regarded as largely heterodyne, most of the scattered light arising from a stationary component in the density fluctuations.
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