HYDRODYNAMICS OF KRAFT LIGNINS

The self-diffusion coefficients of kraft lignin preparations in aqueous and nonaqueous solvents have been measured by using pulsed field gradient NMR (PFGNMR). The diffusion coefficients of kraft lignin fractions conform to the relationship D(s) approximately M-alpha, where alpha is 0.39 for unacetylated lignin in aqueous 1.0 M NaOD and 0.30 for acetylated lignins in CDCl3. These exponents and the measured D0 values indicate a branched macromolecule. Measurements of the concentration dependence of the self-diffusion coefficients for both the solvent and solute are consistent with an extremely compact and flat conformation for acetylated lignin in CDCl3. A diffusion coefficient corresponding to a hydrodynamic radius of 380 angstrom in pH 6.5 aqueous buffer indicated significant association under these conditions. However, there was no evidence of association for unacetylated lignin preparations in pH 9.0 D2O, aqueous 1.0 M NaOD, and DMF-d7 and for acetylated lignins in CDCl3 and acetone-d6. A kinetic interpretation based on free-volume theory is given for diffusion of branched macromolecules in concentrated solution.