Kink-bands as drivers of hygroscopic response in flax fibres: A multi-scale investigation

Flax fibres are increasingly explored as sustainable reinforcements in high-performance composites due to their remarkable stiffness and strength. However, their hygroscopic nature and complex hierarchical organisation pose challenges for dimensional stability and long-term durability. Among the intrinsic microstructural heterogeneities, kink-bands, localised deformations associated with cellulose misalignment and increased porosity, may affect water uptake and swelling. Despite their potential impact, the effect of kink-bands on water sorption and moisture-induced structural changes has not been directly investigated. This study investigates the influence of kink-band density on the hygroscopic behaviour of flax fibres using an integrated multi-scale approach combining Dynamic Vapour Sorption (DVS), Environmental Scanning Electron Microscopy (ESEM), X-ray microtomography, and solid-state Nuclear Magnetic Resonance (ss-NMR). Three flax fibre batches with distinct kink-band frequencies were prepared through controlled mechanical processing. DVS results revealed no significant difference in overall water sorption capacity between batches, indicating that kink-bands do not markedly affect global hygroscopicity. However, ESEM and X-ray tomography analyses highlighted heterogeneous swelling behaviour at the bundle and fibre scales, with kink-band regions exhibiting lower expansion than intact zones, suggesting a compensating effect of internal pores on local deformation. Solid-state NMR analyses revealed that, although fibril dimensions and overall crystallinity were unchanged, fibres with higher kink-band content exhibited increased water accessibility and subtle nanoscale polymer reorganisation. This suggests that kink-bands promote local water penetration and interactions with surrounding non-cellulosic components, without altering overall bulk hygroscopicity. Together, these results indicate that kink-bands modulate local moisture dynamics, providing new insights into the structural–functional relationships governing flax fibre behaviour in humid environments. They are also relevant for natural fibre composites, as understanding how kink-bands influence local moisture dynamics can inform predictions of dimensional stability, fibre–matrix interactions, and long-term durability. • A multi-scale study links kink-band defects to flax fibre moisture behaviour • Kink-bands do not change global water sorption but affect local swelling • ESEM and X-ray tomography reveal heterogeneous fibre deformation • Solid state NMR shows enhanced water mobility in kink-band-rich fibres • Defect morphology governs hygro-mechanical stability in plant fibres