Effect of the Coagulation Bath on the Structure and Mechanical Properties of Gel-Spun Lignin/Poly(vinyl alcohol) Fibers

Gel spinning was investigated as an alternative approach to the melt spinning of lignin-based fibers. Lignin/poly(vinyl alcohol) (PVA) composites with various weight percentages of lignin were gel-spun into high-strength fibers. Although lignin is an amorphous biopolymer, incorporation of the rigid filler enhanced the mechanical properties of the PVA fibers and affected their structure. Lignin stabilized the gel structure of the thermoreversible PVA gel, as noted by higher gel melting temperatures. Methanol/acetone coagulation baths with high acetone content rendered the gel-spun fibers more drawable, helped to maintain lignin within the gel fibers, and increased the gel melting point. The best mechanical performance was observed for fibers containing 5% lignin, which had an average tensile strength of 1.1 GPa, a Young's modulus of 37 GPa, and a toughness of 17 J/g. Structural analysis of the 5% lignin fibers showed them to possess the highest index of PVA crystallinity. The fibers were more drawable at higher weight percentages of lignin. This plasticizing behavior at elevated temperatures of drawing led to stronger lignin-based fibers. Evidence of hydrogen bonding between lignin and PVA within gel-drawn fibers was observed by infrared spectroscopy, and substituents of the lignin biopolymer were mildly aligned along the fiber axis. Lignin/PVA fibers resisted dissolution in boiling water, unlike neat PVA fibers. However, fiber swelling increased with lignin content. Structurally, gel-spun composites must have regions of PVA laced between lignin. In summary, biobased fibers containing 5–50% lignin were gel-spun into composites having mechanical properties that are suitable for industrial and high-performance fiber applications.