Dynamics of a Polymer Solution in a Rigid Matrix

Abstract

Dynamic light scattering measurements of the diffusion coefficient D and the Rayleigh ratio R θ are reported for dextran molecules confined in agarose gel networks of various concentrations. In this condition, the light scattered by the dextran is some 2 orders of magnitude less intense than that from the agarose. Three molecular weights of dextran were investigated: 7 x 10 4 , 5 x 10 5 , and 2 x 10 6 g mol -1 . For the lowest molar mass it is confirmed that, below the dextran overlap concentration c * , the product DR θ is independent of the agarose concentration, showing that the reduction of the rate of diffusion inside the gel is the result of a decrease in the osmotic pressure in the confined geometry. For the higher molar masses, entanglement effects between the dextran and the network become noticeable in the more highly concentrated gels. The dynamic light scattering intensity measurements are also found to yield reasonable estimates of the molar mass M w and radius of gyration R G of the trapped dextran molecules. The second virial coefficient A 2 is positive, indicating that the agarose-water matrix acts as a good solvent for dextran, but the ratio of R G to the hydrodynamic radius is less than 1.5. These results are interpreted in terms of branching of the dextran molecule.