Properties and Structural Anisotropy of Gel-Spun Lignin/Poly(Vinyl Alcohol) Fibers Due to Gel Aging

In contrast to traditional techniques for fiber spinning, the gel-spinning of biobased lignin/poly(vinyl alcohol) (PVA) fibers yields higher mechanical performance. To further improve upon the mechanical properties and molecular anisotropy of gel-spun lignin/PVA fibers, gel-fibers were aged in chilled solvents prior to fiber drawing. Fibers, containing 5% and 30% lignin to PVA, were spun from gel-fibers aged in 25/75 (v/v) water/acetone for up to 30 days at 5 °C. The best values of mechanical performance were obtained from 5% lignin fibers, where the average tensile strength was 1.4 GPa after 1 day of gel aging in water/acetone. After 14 days of aging 5% lignin fiber, the Young's modulus was 54 GPa, and toughness was 25 J/g. Gel containing 30% lignin yielded high mechanical properties after 30 days of aging; however, some lignin gradually diffused into the aging solvent. The strongest fibers had the highest degrees of crystallinity and molecular anisotropy among PVA and lignin functional groups. Interestingly, 1 day of 25/75 water/acetone aging greatly improved the molecular alignment of lignin benzyl groups (Herman's orientation factor f = 0.5) as compared to what was previously reported with 15/85 methanol/acetone aging (f = 0.1). Overall, gel aging studies revealed the influential role of aging solvents and time on the mechanical performance and structure of gel-spun lignin/PVA fibers.