Dissolution of hemp yarns by 1-ethyl-3-methylimidazolium acetate studied with time-temperature superposition

Abstract

This study investigated the dissolution of hemp yarns in the ionic liquid 1-ethyl-3-methylimidazolium acetate. The yarns were submerged in the ionic liquid at various temperatures and times, then coagulated in water. This resulted in the formation of a composite yarn where two optical microscopy methods were employed to track the growth of the coagulated matrix. In the first, the submerged yarns within water were measured from a side view. In the second, the yarns were dried then analysed by encapsulating in epoxy resin. In both methods, the growth of the swollen ring thickness and the coagulated fraction was tracked as a function of time and temperature. It was found to obey time-temperature superposition, giving a dissolution activation energy of \(78\pm 2 \text{k}\;\text{J}/\text{m}\text{o}\text{l}\). In water the yarn is swollen; the swelling ratios of the different regions were calculated to be \(4.4\pm 0.2 \; \text{a}\text{n}\text{d} \; 1.4\pm 0.1\) for the outer dissolved ring and the undissolved core, respectively. A novel finding in this study was that the growth of the coagulated region follows a diffusion process, increasing with the square root of time, and so could be modelled to give a diffusion coefficient for the ionic liquid of \(6.74\times {10}^{-13}\;{\text{ m}}^{2}/{\text{s}}\). This compared well with previously published NMR data for a saturated cellulose solution (23.5%). This strongly suggests that the diffusion of the ions through a saturated layer of cellulose solution controls the dissolution of the yarn. This finding has significant implications for cellulose-based composite production and thus the recycling of cellulose textiles.