Evolution of The Geometry and Optical Anisotropy as Cellulose is Wet-Spun

It is critical to monitor the structural evolution of complex fluids for an optimal manufacturing performance. Thus, we increase the understanding of the underlying physics which drives the structural modifications and contribute to identify the variables triggering changes in the material properties. Also, in situ measurements facilitate the continuous optimization of the manufacturing process.<br/><br/>Since cellulose undergoes thermal degradation before melting, its processing requires a solvent media with the ability of dissolving cellulose without causing major harm to the polymer chains. The resulting dissolution, often named spinning dope, show slow dynamics with a significant shear thinning behavior which becomes larger as we increase cellulose concentrations. Under extension, spinning dopes experience strain softening behavior followed by a strain hardening trend as the strain grows larger (Sanchez et al, Biomacromolecules, 2022). The spinning dope is extruded through a nozzle as it is pulled by a rotating rod, so it undergoes combined shear and extensional strains. Immediately after, it is coagulated in water which acts as an anti-solvent and triggers the formation of a hydrogen bond network. While cellulose regenerates into a fiber, polymer chains aligned, and the formation of the hydrogen bond network induces a birefringent response in the material which depends on the spun material and the process variables. Traditionally, the performance of the spinning process is evaluated by measuring the target properties of the fiber (e.g. tenacity or elongation) and iterating over the spinning parameters in a heuristic loop which tends to be time consuming and neglects the insights of the different stages involved in the fiber formation. This approach seeks to establish a relation between the source material and the produced fiber and to optimize the process variables. However, information about the chemical and conformational changes undergone by the spinning dope are not accessible.<br/><br/>In this work we propose a new framework to determine the evolution of the fiber geometry and optical anisotropy along the spinning process by measuring diameter and birefringence at the selected points along the process. With this aim, we have built a customized spin-line enabling an easy-control of the critical variables involved in the spinning process: draw ratio and residence time. Then, a self developed optical device comprised of a charge couple device (CCD) and a liquid-crystal (LC) compensator with tunable retardation for rapid and accurate birefringence measurements was designed and built together with the spin line (Sanchez et al, Carbohydrate Polymers, 2023). Results substantiate the microstructural variation as the extensional strain increases. Noticeably, the drawing process occurs almost instantaneously at the nozzle while the fiber coagulation takes several minutes to be completed. Birefringent response grows linearly with the draw ratio and diminishes tenderly with the residence time proving the material relaxation when strain is not imposed. Superposition techniques were successfully applied to capture and describe the effect of draw ratio and coagulation time in terms of shifting factors. Finally, the fiber geometry and optical anisotropy obtained “in situ” are discuss together with the rheological analysis of the spinning dope and the mechanical properties of the obtained fibers.