Refining the Properties of Softwood Kraft Lignin with Acetone: Effect of Solvent Fractionation on the Thermomechanical Behavior of Electrospun Fibers

Abstract

The complexity and polydispersity of lignin, the most abundant aromatic polymer in nature, create great challenges for its valorization for value-added materials and products. Reducing the heterogeneity of industrial lignin and identifying the structure–property relationships are a necessity to achieve predictable and high performance materials for polymeric applications. In this study, a simple one-step solvent extraction method using an industrial solvent, acetone, was presented as a tool to improve the homogeneity of softwood kraft lignin (SKL). The effect of acetone fractionation on the molecular weight (GPC), chemical structure (13C and HSQC NMR), and thermal (DSC and TGA) and dynamical mechanical properties (compressive-torsion DMA) of soluble (ASKL) and insoluble (AISKL) lignin fractions was evaluated. Results showed that two fractions had significant differences in Mw, chemical structure, and functional groups, showing distinct thermal flow and viscoelastic behaviors along with large shifts in the glass-to-rubbery transition temperatures of nearly 80 °C. Electrospun submicron fibers composed of 99 wt % lignin were produced from ASKL and AISKL fractions, which had improved spinnability characteristics compared to their parent SKL lignin. Thermomechanical properties of fiber mats were studied by a tensile dynamical mechanical analyzer, and the heat-induced change in the morphology of fibers was evaluated by SEM. Fiber mats produced from ASKL fractions showed increased water resistance and hydrophobicity as revealed by water contact angle measurements.