STATIC AND DYNAMIC LIGHT-SCATTERING SOLUTION BEHAVIOR OF PULLULAN AND DEXTRAN IN COMPARISON

Abstract

The conformational and dynamic behavior of fractionated samples of pullulan and dextran has been studied by means of static and dynamic light scattering. Measurements were carried out in water and in some THETA solvents at various temperatures. Static light scattering yields the molar mass, M(w), the radius of gyration, [S2]z0.5, and the second virial coefficient, A2. From dynamic light scattering we obtained the translational diffusion coefficient, D(z,0), and the second hydrodynamic virial coefficient, k(D,2). Analysis of the molar mass dependencies of [S2]z0.5, A2, and D(z,0) clearly exhibits that pullulans are linear and dextrans are branched molecules. The ratio g = [S2]z,b/ [S2]z,l suggests that branching becomes more pronounced when the molar mass increases. However, the g factor says nothing about the exact branching structure. To obtain such information, the particle scattering factors, P(q)z, of dextran were compared with model calculations of nonrandom ABC polycondensates and soft spheres. Using Kratky plots, where q2P(q), is plotted vs q, we find that the soft-sphere model describes the experiment satisfactorily. Interestingly, the same result is proposed by the discussion of the ratio p = [S2]z0.5/R(h), where R(h) represents the hydrodynamic radius. Analysis of the second virial coefficients shows that the A2 values of pullulan are larger than those of the corresponding dextrans of the same molar mass. This effect is discussed in terms of the interpenetration function, psi, and the g factor. It is found that the intra-and intermolecular segment interactions can be described simultaneously by a universal function psiBAR. For the linear pullulans, a two-parameter theory is satisfactory, while for the branched dextrans one more parameter is needed. Finally, the concentration dependence of the diffusion coefficient, D(z,0), was investigated. It is found that there exists a ratio XBAR = SBAR/R(h) at which the volume fraction virial coefficient, k2,D(V), becomes zero. This value of XBAR separates the good solvent behavior, where D(z,0) increases with concentration, from the poor solvent behavior, where it decreases with concentration. Note that SBAR is the thermodynamically effective radius of interaction. At the THETA state the k2,D(V) values become identical for pullulan and dextran, and the limiting value -2.2 of the soft sphere is reached. However, refined theories are needed to explain these effects in more detail.