Hot-drawing ionic liquid-spun lignin-poly(vinyl alcohol) fibres increases strength and polymer alignment

Abstract

Lignin is an attractive raw material for sustainable carbon fibres, however, the resulting mechanical properties require improvement before they can be implemented in sustainable composite applications. The mechanical properties of conventional polyacrylonitrile-based carbon fibres depend critically on the molecular alignment within the precursor polymer fibres, and its retention through subsequent thermal treatments. In this study, alignment was induced in high lignin content fibres wet-spun from low-cost ionic liquid and water mixtures by employing similar hot-drawing methods. 75/25 wt%/wt% lignin-poly(vinyl alcohol) (lignin-PVA) fibres were continuously wet-spun from a solvent consisting of N,N-dimethylbutylammonium hydrogen sulfate [DMBA][HSO₄] and water, with deionised water used as the coagulant. Hot-drawn fibres with high draw ratios of up to 20 were generated at 180°C. By careful selection of initial extrusion diameter, and subsequent draw ratios, the influences of fibre diameter and draw ratio were systematically distinguished. Draw ratio was found to dominate the mechanical properties of the precursor fibres, while the fibre diameter was more significant after stabilisation. The precursor fibres that experienced the highest draw ratios had tensile strengths of 235-249 MPa (up to four times higher than the undrawn lignin-PVA fibres) and 7.5-8.2 GPa tensile modulus, while fibre diameter was reduced from 64-106 µm to 15-23 µm. Wide-Angle X-ray Scattering (WAXS) studies showed that hot-drawing induced oriented crystalline PVA domains at high draw ratios. The crystallisation and orientation of PVA was lost during slow oxidative stabilisation at 250°C, associated with a plateau in tensile strength to around 110 MPa and modulus to 4 GPa for stabilised lignin-PVA fibres. Improvements to the stabilisation aimed at retaining alignment are proposed.