Morphology and mechanical properties of chitosan fibers obtained by gel-spinning: influence of the dry-jet-stretching step and ageing.

The morphology and mechanical properties of chitosan fibers obtained by gel-spinning are reported. The objectives were both to understand how the microstructure of the fibers could be modified and how the mechanical properties were improved by means of a dry-jet-stretching step. A highly deacetylated chitosan (degree of acetylation=2.7%) from squid-pens, with a high weight-average molecular weight (M(w)=540,000g/mol) was dissolved in an aqueous acetic acid solution, spun using gaseous ammonia as the coagulant, and then directly dried under hot air. A "post-drying" of 1week was necessary to stabilize the fibers in ambient atmosphere. A dry-jet-stretching ratio applied during the monofilament coagulation (maximal value of 4.3) allowed us to increase the density and favor the orientation of chains along the fiber axis. This allowed us to improve the mechanical properties of the fibers (Young's modulus of 82g/denier and tenacity of 2g/denier). The ageing in ambient atmosphere played an important role in the crystalline microstructure in relation to: the kinetics of ammonium acetate hydrolysis, the formation of a weak fraction of the anhydrous allomorph of chitosan, and an increase of the crystallinity index, whereas the Young's modulus was increased and the tenacity was slightly lowered. In addition, gel-jet-stretched or dry-jet-stretched fibers could be stored at least 3months in ambient atmosphere without any significant degradation.