Analysis of the Dissolution Behaviour of Flax Based Yarn in an Ionic Liquid

Flax based yarn has been dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate [C2mim][OAc] and the dissolution behaviour studied as a function of time, temperature, co-solvent and anti-solvent concentration. With pure ionic liquid, dissolution is seen to occur from the outside in, with a coagulated outer layer forming and growing around the fibrous core with time. Following dissolution, yarns were coagulated in water and left to dry at room temperature. Three key methods of analysis have been used to study the resultant yarns: optical microscopy, X-ray diffraction and mechanical testing- with each method allowing for a distinct means of tracking the amount of dissolution. The resulting data displays an equivalence between dissolution time and temperature, verified by the creation of master curves via the shifting of data sets in the time domain, much like time-temperature superposition in rheological systems. The shift factors used to generate master curves have been plot against the inverse of their respective dissolution temperatures and upon doing so, a linear relation has revealed Arrhenius behaviour in the system. As a result, an activation energy has been calculated describing the energy needed for the dissolution of flax yarns in [C2mim][OAc]. This energy was found to be 100 ± 10 kJ/mol, with agreement across the various experimental techniques. With the introduction of the co-solvent dimethyl sulfoxide (DMSO), the growth of the coagulated material is again found to follow an Arrhenius equation with a single activation energy of 98 ± 2 kJ/mol. This value is, surprisingly, independent of the weight fraction of DMSO. The addition of DMSO does, however, strongly alter the time-scales taken for dissolution, but interestingly not the temperature dependence of these time-scales. The composition for the peak dissolution rate is determined, which occurs at an equal weight fraction of DMSO and [C2mim][OAc]. The inclusion of small amounts of the anti-solvent water has also been found to drastically alter the time-scales of dissolution, with a sharp exponential decay in the dissolution rate seen as a function of H₂O concentration. Despite the substantial effect of water on the dissolution rate, the activation energy is found to be remarkably constant- within error of the activation energies determined when using both pure ionic liquid and ionic liquid/co-solvent mixtures. As little as 8 % water by weight is shown to be sufficient to halt the dissolution process on experimental time-scales.