LIGHT-SCATTERING OF ACETYLATED LIGNIN

Organosolv lignin was fractionated on a Sephadex G 75 column with 0.1 M aqueous NaOH resulting in 14 fractions. These fractions were acetylated and a high-molecular-weight fraction (F3) was investigated by means of combined static and dynamic light scattering (LS) in solvents acetone, tetrahydrofuran (THF), and trifluoroethanol (TFE). The measurements were found to be reproducible, and recycling of lignin by freeze drying caused slight but unessential changes in solution properties. Depending on the solvent used, weight average molecular weights M(w). between two and ten million were found. By contrast, M(n) of the fractions, measured by vapor pressure osmometry (VPO), was in the range of a few thousands only. Analysis of the angular dependence in static LS by means of a Casassa - Holtzer plot and the fractal dimensions showed the presence of chain stiffness, most distinct in TFE. Also, the dynamic light scattering results in TFE are typical for semiflexible chains, while in THF, and to some extent in acetone, the dynamic measurements including viscosity suggest the presence of spherical structures. These findings are being explained by large lignin clusters that consist of stiff subunits.