Effect Of Spinning Technology On The Structure And Properties Of Polyacrylonitrile Fibers Processed With Ionic Liquid As Solvent

As one of the important industrially products, polyacrylonitrile fibers are not only used as textile but also as industrial fibers, ionic liquid because of its good stability, lower steam pressure and good dissolve ability can be used as a good solvent for processing polyacrylonitrile fibers, but the influence of the spinning technology on structure and properties of the fibers is not very clear so far. In this paper the effect of spinning technology on structure and properties of polyacrylonitrile fibers processed with1-butyl-3-methyl imidazole chloride ([BMIMJCl) as solvent was studied.Firstly, the PAN/[BMIM]Cl solutions were prepared by dissolving polyacrylonitrile powder in ionic liquid standing in vacuum oven, stirring and kneading, restpectively with the same concentration. The rheological behavior of the PAN/[BMIM]C1solutions were investigated by Rotational Rheometer. It is found that the zero-shear-rate viscosity and relaxation time are smaller and the non-newtonian index is larger of the solution prepared by kneading. The polyacrylonitrile fibers were prepared by dry-jet wet spinning technology from the PAN/[BMIM]C1solutions prepared by stirring and kneading, respectively. And a higher draw ratio can be achieved of the fibers processed with the solution prepared by kneading. The supramolecular structure and properties polyacrylonitrile fibers were studied with Synchrotron wide-angle X-ray diffraction (WAXD) technologies, birefringence test, dynamic mechanical analysis (DMA) and mechanical tests. It is shown that the structure of the fibers processed with the solution prepared by kneading is more uniformly, which is helpful to improve the mechanical properties. Only a single glass transition temperature (Tg) was observed from DMA but the Tg is higher of the fibers processed with the solution prepared by kneading that is due to the ability of chains movement in the amorphous phase.Then, the structure and properties of polyacrylonitrile fibers on spinning line were studied. It is shown that crystallinity, crystalline orientation of the fiber enhanced obviously and also the macromolecular chain arrange more order along the fiber axis gradually but the crystalline grain sizes downtrend at first, then increased during the fiber-forming process, which are the reasons why the tenacity of the fibers were improved but the initial modulus first decrease but then increase. The glass transition temperature (Tg) was shown downtrend at first but then remained, which is due to the of macromolecular chain entanglement degree and the ability of chains movement in amorphous phase.Furthermore, the influence of draw ratio in boiling water on the structure and properties of polyacrylonitrile fibers was investigated. From the WAXD pattern, it is shown that the crystalline structure become more complete with increasing draw ratio, which is helpful for improving the mechanical properties. Only a single Tg was observed from DMA and the peak of Tg is very broad which indicates that the structure is not a very completed ordered one. After drawn in boiling water, a more ordered amorphous phase structure was formed with the higher Tg, but the impact of draw ratio was not obviously.Finally, the influences of post-drawing in boiling water on the structure and of polyacrylonitrile fibers were investigated. There was still4.38wt%residual solvent in polyacrylonitrile fibers after being coagulated in coagulation bath, pre-drawn and drawn in boiling water. The residual ionic liquid provides plastic action for post-drawing in boiling water. The analysis of tension on the acrylic fibers during drawing in boiling water show that the stress increased but the fineness stayed the same with the drawing speed. Therefore, the structure became more ordered, the chains packed more regularly along the chain axis, the mechanical properties and the glass transition temperature (Tg) increased with the increasing drawing speed. The structure also became more ordered and the mechanical properties were improved with the increasing of ratio, and at last the tenacity achieved at4.95cN/dtex, but if the draw ratio exceeds the threshold, the fibers would break during the drawing process. Two Tg were observed from DMA of the fibers with draw ratio of2, and the lower Tg is due to the movements of macromolecules in crystal phase.