Understanding the Dissolution Behaviour of Flax Yarn in Ionic Liquids

Abstract

The purpose of this thesis is to study the dissolution of flax fibres in imidazolium based ionic liquids and anti-solvent mixtures. This is an important area of study, helping to understand the mechanism of cellulose solvation and the ways in which the properties of ILs (in particular different anion and cation combinations) can influence their ability to dissolve cellulose at the micro- and macro level, and how different IL features affect the dissolution process. This study investigates the dissolution behaviour of flax yarns in three distinct imidazolium based ILs:1-ethyl-3-methylimidazolium acetate ([C2mim]+[OAc]- ),1- butyl-3-methylimidazolium acetate ([C4mim]+[OAc]-), and 1-ethyl-3-methylimidazolium octanoate ([C2mim]+[Oct]-). The first two of these had the same anion ([OAc]-) but a different cation, while the third had the same cation ([C2mim]+), as the first but a different anion. This work was able to reveal the role of the cation and the anion on the dynamics of cellulosic yarn dissolution. The dissolution process involved submerging the yarns in the pure ILs for a range of temperatures and times, followed by coagulation in water. The coagulated material called coagulated fraction (CF) produced an outer ring that surrounded the centre yarn fibre. Optical microscopy was used to follow the growth of this ‘dissolved’ region and it showed an Arrhenius behaviour, enabling the determination of the dissolution activation energy from this simple measurement. The dissolution activation energies of the ILs [C2mim][OAc], [C4mim][OAc] and [C2mim] [Oct] were found to be 64 ± 5 kJ/mol, 67 ± 1 kJ/mol and 79 ± 1 kJ/mol, respectively. In addition, the growth of the outer coagulated ring's thickness of the coagulated material was investigated, enabling the IL's diffusion coefficients to be determined. NMR study (pulsed- field gradient self- diffusion measurements), viscosity, density, and Stokes-Einstein analysis provided further understanding of the properties of the pure ILs. The calculated diffusion activation energies of the ILs [C2mim][OAc], [C4mim][OAc] and [C2mim][Oct], were found to be 64 ± 5 kJ/mol, 69 ± 5 kJ/mol and, 77 ± 3 kJ/mol, respectively. The resultant data shows that the dissolution rate goes from fastest to slowest in the order [C2mim][OAc] >[C4mim][OAc] >[C2mim][Oct]. Our key result is that the dissolution of the flax yarns (in all three ILs) is controlled by the diffusion of the IL, through a region of swollen cellulose/IL solution around each fibre as the thickness of the dissolved and coagulated layer increases with the square root of time and so is diffusion controlled. The effect of adding small amount of water on the activation energy and dissolution speed of ionic liquids ILs [C2mim][OAc] and [C4mim][OAc] was investigated separately. For the IL [C2mim][OAc], three different water contents have been used 1%, 2% and 4% by weight and for the IL [C4mim][OAc], four different water concentration have been used 1%, 2%, 4%, and 8% by weight. The resultant data has also been compared to the results from chapter 3 (the pure IL [C2mim][OAc] was found to consist of 0.2% water), and chapter 4 (the pure IL [C4mim][OAc] was found to consist of 0% water). As expected, the coagulated outer layer was seen to form around the undissolved core fibre for the water systems of 1%, 2%, and 4%. However, there was no sign of dissolution showed by the IL [C4mim][OAc]-water system of 8%. For the IL[C2mim][OAc], the activation energies were found to be 77 ± 5 kJ/mol, 97 ± 3 kJ/mol and 116 ± 6 kJ/mol for the system containing 1%, 2% and 4% water respectively. For the IL [C4mim][OAc], the activation energies were found to be 78 ± 7 kJ/mol, 83 ± 7 kJ/mol and 110 ± 6 kJ/mol for the system containing 1%, 2% and 4% water respectively. The dissolution rate was found to exponentially decrease as a function of water content for [C2mim][OAc]; however, the dissolution rate at 1% water was found to be higher than that of 0% water for [C4mim][OAc]. This shows a level of effectiveness at 1% water could make it a viable option for both research and industrial use.