Physico-chemical characterization of a fractionated kraft lignin

A kraft lignin was leached from a softwood pulp and fractionated by ultrafiltration. The fractions were characterized in respect to phenolic group content, molecular weight distributions and self-diffusion coefficients. The H-1-Pulsed Field Gradient (PFG) NMR self-diffusion measurements and the High-Pressure Size Exclusion Chromatography (HPSEC) analysis of the fractions, were seen to correlate fairly well. From the self-diffusion measurements, the mass-weighted median hydrodynamic radii of the diffusants in the fractions, were calculated assuming spherical fragments. Furthermore, the content of phenolic groups in the fractions, was found to decrease by increasing hydrodynamic radius and molecular weight, but the calculated median surface charge densities of the macromolecules, were determined to be constant in the range of oligomers up to at least 65 structural units.