Ionic liquid treatment of flax fibers and the effects on morphology and mechanical properties

The structure of the cell walls of plant fibers, comprising cellulose microfibrils embedded in a matrix (hemicelluloses, lignin, and/or pectin) plays a crucial role for their mechanical behavior. However, the direct measurement of the cell wall structure remains challenging, and the effects of their partial dissolution on mechanical properties are poorly understood. Although, with growing environmental awareness more industrial applications and production routines based on dissolution of lignocellulosic materials are being explored. Here, we demonstrate the time-dependent dissolution of flax fibers using the ionic liquid 1-ethyl-3-methylimidazolium acrylate [EMIM][ACR] to better understand the influence of the fiber structure and its dissolution on the mechanical properties. The treatment progressively dissolves the outer layers (primary wall and S1 layer) of isolated single fibers, providing access to the S2 layer. Atomic force microscopy (AFM) revealed cellulose microfibril angles closely aligning with wide-angle X-ray scattering (WAXS) results (AFM: 5.7 ± 1.9°; WAXS: 5.0 ± 1.6°). Tensile testing showed significant reductions in mechanical properties with treatment duration, while Brillouin spectroscopy revealed small changes in axial stiffness (C₃₃), dominated by the microfibrils, but a substantial decrease in shear modulus G = 12.0 GPa–4.6 GPa, highlighting matrix dissolution. Combining AFM, WAXS, tensile testing, and Brillouin spectroscopy provides a comprehensive understanding of the dissolution process and its effects on flax fiber structure and mechanics.