Real-time determination of the induced optical anisotropy in dry-jet wet spun cellulose

Real-time monitoring provides valuable information for optimizing fiber spinning conditions. This work combines two self-developed devices, a monofilament dry-jet wet spinning system and an optical measuring device. The measuring protocol was validated and applied to capture the evolution of cellulose fibers during the Ioncell® process. Seven representative points were chosen to evaluate changes in diameter, birefringence and the mass transfer between the filament and the coagulation bath. When exiting the spinneret, the filament shows die swell in the air gap where axial strain induces polymer matrix rearrangement, accompanied by a diameter decrease before entering the coagulation bath where the solvent exchange occurs. It relies on the filament surface-to-volume ratio correlating linearly with the fiber diameter. Structurally, birefringence grows initially until the solvent exchange pushes fiber regeneration. After a minor decrease, Δn remains nearly constant. Polymer chain rearrangement occurs primarily in the air gap whose flow-induced birefringence is proportional to draw ratio reaching 1.88 ⋅ 10^-3 for DR=11 whereas Δn = 0.26 ⋅ 10^-3 for DR=1. As the filament regenerates Δn increases up to 12.7 ⋅ 10^-3 before entering a plateau. Characterization of the obtained fibers validates the protocol, reinforcing that process variables leading to a larger flow-oriented polymer chain yield fibers with higher tenacity and lower flexibility.