Abstract:

Objective Aramid nanofibers (ANFs) have become a favored composite skeleton reinforcement material in recent years due to their large aspect ratio, specific surface area, high surface energy, good dispersibility. It has been reported that the preparation cycle of aramid nanofibers using deprotonation is relatively long, and the reports on the deprotonation reaction cycle or reaction endpoint are not sufficiently uniform. The evolution mechanism of ANFs under different reaction cycles is not clear, and there is still a lack of research on their wet spinning processability. In order to fully grasp the characteristics of ANFs and their wet spinning processability, this research carried out a series of studies on ANFs with different reaction cycles.

Method Using poly-p-phenylene terephthamide(PPTA) fibers as raw materials, ANFs dimethy sulf-oxide(DMSO) dispersions were prepared through an alkaline solution deprotonation method. SEM was utilized to observe and analyze the evolution of PPTA fibers across the reaction cycles during the deprotonation process. The influence of the preparation period on the microstructure of ANFs was analyzed using SEM, TEM, Raman, and XRD tests. A rheometer was employed to test the rheological properties of the ANFs/DMSO dispersion, and the impact of reaction cycles on these properties was analyzed. Furthermore, the wet spinning method was used to assemble and prepare pure spun ANFs fibers, and the effect of the preparation cycle of ANFs in the spinning solution on the mechanical strength of pure spun ANFs fibers was investigated.

Results The ANFs produced exhibited a typical branching morphology with a large aspect ratio. It was found that as the protonation reaction cycle prolonged, the aspect ratio of nanofibers was decreased. The average diameter of ANFs prepared with a reaction period of 3 d was about 10.46 nm, and the aspect ratio was relatively large. However, excessively extending the reaction period to 9 d will significantly damage the main chemical structure of the fibers. The modulus of dispersed solutions at different reaction cycles exhibited no frequency dependence, and G'/G″ was greater than 1, making it suitable for assembly using wet spinning processing. As the reaction cycle prolonged, the apparent viscosity of the prepared dispersion showed a significant decline, indicating that excessive reaction would deteriorate its spinnability. Observing the microstructure of pure spun ANFs fibers assembled by wet spinning, it was found that PPTA polyanions in the dispersed solution were able to reconstruct their structure through protonation reduction reactions during the spinning solidification process. The formed aramid nanofibers was able to be arranged in an orderly manner along the fiber axis, with very good orientation regularity. As the preparation period of ANFs in the spinning solution prolonged, the strength of the assembled pure spun ANFs fibers showed a significant decrease. Among them, the AF-3 pure spun fibers assembled with a preparation period of 3 days correspond to the best mechanical properties, with fracture strength and modulus reaching 151.84 MPa and 6.23 GPa, respectively.

Conclusion The ANFs/DMSO dispersion prepared by deprotonation method has good wet spinning processability. During the wet spinning process, ANFs are oriented in an orderly manner along the fiber axis under the action of jet shear, and their structure can be reconstructed through protonation reduction during solidification. The assembled pure spun fibers have been proven to have good mechanical properties. The preparation cycle has an impact on the length and fineness of the prepared ANFs. The larger the aspect ratio of ANFs, the higher the strength and modulus of the assembled pure spun ANF fibers. Research has shown that ANFs can serve as reinforcing skeletons and material composites, laying the foundation for the design and development of composite fibers.

Key words: aramid nanofiber, high performance fiber, wet-spinning, spinnability, alkali dissolution deprotonation, rheological property