Quasielastic light scattering measurements of self-diffusion and mutual diffusion in gelatin solutions and gels

Abstract

There has been considerable interest in comparing the mutual diffusion coefficients, measured by quasielastic llghtscattering, for a concentrated or semidilute uncross-linked polymer solution and for a cross-linked gel having identical or nearly identical chemical structure and the same polymer concentration. In several cases, the mutual diffusion coefficients have been found to be the same (JARRY and PATTERSON 1982) or closely similar (MUNCH, et al. 1977a, 1977b). In the study reported here, quasielastic light scattering measurements have been made on semidilute solutions of gelatin and the corresponding gels that are formed from these solutions at lower temperatures. Although gelatin is in a sense a poor choice of polymer because of its complex structure, uncertain gelation mechanism, and broad molecular weight distribution, it behaves above the gel temperature as a random coil polymer (VEIS 1964) and the viscoelasticity of its gels corresponds to that of a network of flexible strands (LAURENT, et al. 1980). In the present study, it has the unique advantage that viscoelastic measurements (LAURENT, et al. 1980) of the same gels have previously provided values of the translatory monomeric friction coefficient ~oas well as accurate values of the macroscopic shear modulus G, so the relation between scattering and mechanical measurements can be examined. From the scattering measurements on solutions above the gel temperature, the mutual diffusion coefficient D c is determined as well as a much smaller diffusion coefficient D 2 which can be identified as the self-diffusion coefficient of the polymer. For the gels, only D c is obtained, and its dependence on temperature and solvent viscosity (varied over a wide range by incorporation of glycerol up to 91% by weight) is examined. Over the range studied, both the friction coefficient ~o and the shear modulus G were previously observed to depend on both these variables (LAURENT, et al. 1980).