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Characterization of lignin molar mass and molecular conformation by multi-angle light scattering Ji, Lun


There are many obstacles that hinder the understanding and hence the utilization of softwood kraft lignin (SKL). The lack of reliable measurements for lignin molecular weight and the corresponding molecular conformation hampers proper elucidation of structure-property relationships. Conventional gel permeation chromatography (GPC) is unable to robustly measure the molecular weight because of a lack of calibration standards with a similar structure to lignin. Further, the potential for interactions between lignin and the column gel packing delays separation, changing the mechanism from a strict hydrodynamic radius interpretation. In the present work, the determination of absolute molar mass of technical lignin was conducted utilizing gel permeation chromatography (GPC) combined with multi-angle light scattering (MALS). In order to clarify the light scattering profile, six SKL fractions, homogeneous in both structure and size were obtained by a combination of ultrafiltration and organic solvent fractionation. Further information on the molecular structure was studied utilizing a differential viscometer combined with quantitative 1D and 2D nuclear magnetic resonance spectroscopy (NMR) methods for chemical and structural analysis of functional groups and interunit linkages, respectively. Separated lignin fractions were used to enhance the clarity of light scattering profiles by narrowing the molecular weight distribution of lignin fractions so the larger polymers would not dominate the scattering. For solvent fractionated materials, the acetone soluble fraction had a lower molecular weight than the acetone insoluble fraction. In addition, hydrodynamic behaviour was acquired based on viscosity and molecular mass of fractionated samples. Acetone soluble lignin was found to possess a more compact structure relative to the acetone insoluble fraction, due to a significantly lower “α” value in the Mark-Houwink-Sakurada (MHS) plot. This compact geometry was supported by the structural analysis from NMR showing the acetone soluble part contained fewer native linkages and aliphatic side chains, which suggests the samples were considerably degraded. The relative degree of compactness (branching degree) was quantified by comparing hydrodynamic behaviour of SKL fractions with a “linear” lignin reference, as represented by enzymatic milled acidolysis lignin (EMAL), and it was found that lower molecular mass samples contained more branches than higher molecular mass fractions.

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