Understanding the Biophysics of Plant Yarn Dissolution in Ionic Liquids

Abstract

This thesis investigates the dissolution behaviour of natural and pre-treated plant-based yarns (hemp, cotton, and flax) in the ionic liquid 1-ethyl-3-methylimidazolium acetate ([C₂mim][OAc]), with a focus on understanding the roles of molecular weight, composition, and crystallinity in their dissolution. A combination of mercerization (sodium hydroxide treatment) and electron beam irradiation (EBI) was applied to alter the structural and chemical properties of the yarns. While mercerization was found to affect both composition and molecular weight, electron beam irradiation was found to predominantly reduce molecular weight, allowing these two important factors to be separately evaluated. The dissolution behaviour of these yarns in the ionic liquid was evaluated across different dissolution times and temperatures. Optical microscopy, wide-angle X-ray diffraction (WAXD), and mechanical testing were employed to investigate structural and mechanical properties. The most important finding of this work is that molecular weight is the dominant factor governing both the rate of dissolution and the dissolution activation energy (Eα). A direct linear relationship was identified between Eα and molecular weight (MW). This correlation, consistent across all yarn types and treatment conditions, establishes molecular weight as the primary factor governing both the rate of dissolution and the energy barrier that must be overcome for dissolution to take place. The growth of a partially dissolved and coagulated fraction surrounding an undissolved core was tracked using optical microscopy. Time–temperature superposition was applied across all yarn types, allowing for the determination of the dissolution activation energies. Pre-treatments had a significant impact on dissolution behaviour: mercerisation led to reductions in both lignin content and molecular weight, whereas EBI primarily caused a notable decrease in molecular weight. EBI-treated yarns consistently exhibited the lowest activation energies compared to their natural counterparts. Optical microscopy confirmed that dissolution followed time–temperature superposition and diffusion-limited kinetics, with the growth of the coagulated layer being proportional to the square root of time. Overall, this work provides an understanding of the dissolution of different cellulosic yarns in ionic liquids and establishes molecular weight as a predictive parameter for dissolution kinetics. The findings offer valuable insights for optimising pre-treatment strategies to improve the processing of cellulose-based materials, with potential applications in sustainable textiles, all-cellulose composites, packaging, and biodegradable products.