Dynamics of a polymer solution in a rigid matrix. 2

Dynamic light scattering measurements of the diffusion coefficient D and th e Rayleigh ratio Re are reported for dextran molecules confined in agarose gel networks of various concentrations. In this condition, the Light scatte red by the dextran is some 2 orders of magnitude less intense than that fro m the agarose. Three molecular weights of dextran were investigated: 7 x 10 (4), 5 x 10(6), and 2 x 10(6) g mol(-1). For the lowest molar mass it is co nfirmed that, below the dextran overlap concentration c", the product DRthe ta 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, ent anglement effects between the dextran and the network become noticeable in the more highly concentrated gels. The dynamic light scattering intensity m easurements 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 mat rix acts as a good solvent for dextran, but the ratio of R-G to the hydrody namic radius is less than 1.5. These results are interpreted in terms of br anching of the dextran molecule.