Ionic liquids were used to prepare high-strength chitosan fibers by Li et al. Chitosan with a degree of acetylation of 86 % and molecular weight of 1.5 x 106 was dissolved using glycine chloride. Influence of dissolution and fiber-forming conditions on the properties of the fibers was studied [12Li1]. Filaments extruded from the spinneret were coagulated with dilute Na2SO4/C2H5OH and later freeze — dried. Table 25.2 provides a comparison of the properties of the chitosan fibers produced using the ionic solvent in comparison to the traditional approach of using acetic acid as the solvent. As seen from the table, the ionic solvent produced fibers with more than three times higher strength and more than 12 times higher modulus than the fibers obtained using acetic acid as the solvent. It was suggested that the higher strength of the fibers from the ionic solvents was due to the retention of the type I structure of chitosan which had stronger molecular forces compared to the type II amorphous structure formed when chitosan is dissolved using acetic acid. It was also proposed that glycine chloride could enter the chitosan network more easily, stretch the molecules to a more linear form by increasing the repulsion between the chitosan cations, and increase the strength of the fibers. Morphologi­cally, fibers obtained from glycine chloride were circular and smooth, whereas the fibers from the acetic acid solvent had a rough and uneven cross section.

In a recent study, binary ionic liquids composed of glycine hydrochloride and 1-butyl-3-methylimidazolium chloride (Gly-HCl-Bmimcl) were used to prepare high-strength chitosan fibers. Chitosan was dissolved in the ionic liquid by heating to 80 ° C for 1 h and later extruding the solution into a coagulation bath consisting of separate ethanol and 5 % sodium hydroxide troughs [13Ma]. Further separation of

Fig. 25.2 Longitudinal and cross-sectional views of chitosan fibers produced using the wet and dry-wet spinning approaches [13 Ma]. Reproduced with permission from Elsevier

the fibers into dry and dry-wet spun was done by an additional washing and air-drying step. Fibers produced by the wet spinning approach had striated surface and a circular cross section compared to the smooth surface and irregular cross section seen in the dry-wet spun fibers as seen in Fig. 25.2. Tenacity (2.4 g/den) and elongation (11.9 %) of the dry-wet spun fibers were considerably higher than the fibers obtained from the wet spinning (1.7 g/den, 8.1 %). Fibers obtained from both the dry and dry-wet spinning had good wet strength, measured after immersing the fibers in water for 5 min at room temperature. It was claimed that the fibers produced in this research had better tensile properties than any previous method of producing chitosan fibers.

To prevent hydrolysis of chitosan when acids were used as solvents, a combina­tion of LiOH and urea were used to dissolve chitosan and extrude fibers into a sulfuric acid and ethanol aqueous solutions [12Li2]. Fibers produced from the

LiOH-urea system had smooth and circular cross section, and the strength and elongation of the fibers were 1.3 g/den and 12 %, respectively, higher than the fibers produced using the conventional approach.