Dynamics of a Polymer Solution in a Rigid Matrix

We report dynamic light scattering measurements of solutions of a low molecular weight polymer (dextran M w = 70 000), both in the free state and inside agarose hydrogels of varying concentrations. The light scattered from the rigid agarose matrix strongly heterodynes the signal from the mobile component, thereby allowing measurements both of the diffusion coefficient D c and of the Rayleigh ratio R θ of the dextran. For dextran concentrations c less than or equal to the overlap concentration c*, D c decreases as the concentration of the gel matrix c g increases. The product D c R θ, which depends only on hydrodynamic factors, is, however, independent of the agarose concentration. It is concluded that the principal influence of the static gel matrix is to reduce the entropy and hence the osmotic pressure of the dextran solution. For concentrations of dextran greater than c*, the gel structure undergoes a phase separation.