Structure And Structural Evolution Of PAN Fibers During Spinning Progress

Obtaining high quality polyacrylonitrile (PAN) precursor fibers is one of the prerequisites for the preparation of carbon fibers with excellent performance. Having further study about the microstructures of PAN fibers and the formation and evolution of it during spinning progress is significant. And to define the effect of the spinning progress on the structure and properties of the PAN fibers based on this is important to improving the spinning progress and obtaining high quality PAN precursor fibers. In this paper, three series of self-made PAN fibers which were spun separately by wet spinning progress, quasi dry-jet wet spinning progress and dry-jet wet spinning progress were chosen to be analyzed. High resolution transmission electron microscopy (FESEM), X-ray diffraction (XRD), fineness testing, tensile strength testing and elongation testing were carried out to detect the microstructures and properties of PAN fibers. The microfibrils, fibrils, crystal structure, compactness and mechanical properties of the PAN fibers from different spinning stages were studied. Differences of PAN fibers from different spinning progresses were researched by studying the evolution of structure and properties.Surfaces morphologies of different PAN fibers were observed. It was found that PAN fibers from wet spinning progress and quasi dry-jet wet spinning progress had trench structure on the surface. And the uniformity, parallelism and orientation of the trench structure were improved during the spinning progress. As the trench structure represented the fibrils of the PAN fibers, the changes of it reflected that the fibrils inside the PAN fibers grew gradually and became completely oriented at last. Besides, the trench structure of the wet spinning fibers was more apparent than that of the quasi dry-jet wet spinning fibers. While PAN fibers from dry-jet wet spinning progress had smooth surface without grooves.Ultrasonic etching method was used to separate fibrils from the PAN fibers, and the optimum ultrasonic etching conditions for different PAN fibers were confirmed by series of experiments based on some early research. The morphology of PAN fibers after ultrasonic etching was observed with SEM. The results showed that skin-core structure existed in all the three series of PAN fibers, and that of the dry-jet wet spinning fibers was relatively unobvious. The thickness of the skin layer of PAN fibers from wet spinning and dry-jet wet spinning progress is about200-300nm. When the fibers coagulated at the beginning, a gel net work first formed. Under the action of series of drawing, the PAN molecular chain was stretched, slipped and arranged to form oriented structures. The fibrils and microfibrils formed and evolved at the same time. For the PAN fibers from wet spinning and quasi dry-jet wet spinning progress, the bonding between fibrils and microfibrils was untight, so the fibrils and microfibrils could be separated easily by ultrasonic etching. While for the dry-jet wet spinning fibers, there were large amount of physical entanglements and chemical crosslinking points among different fibrils and microfibrils. And the microfibrils interspersed through different fibrils and connected them to form network skeleton structure. As a result, it became hard to separate the fibrils from the PAN fibers even when the ultrasonic etching conditions were improved. It also could be observed that the microfibrils of all the PAN fibers were consist of lamellar structures. But as the dry-jet wet spinning fibers were compact and the microfibrils of them were linked closely, to obtain the details of its lamellar structure morphology was hard than that of the other two series of PAN fibers.During the spinning progress, the crystallinity and tensile strength of the PAN fibers went up overall, while the linear density and elongation rate at break decreased. But during the heat setting stage, internal stress was eliminated and the orientation of PAN molecular chains released. So the four parameters mentioned above changed slightly in this stage. Especially for the crystallinity of the fiber, it even reduced after this stage.By the comparison of the three series of PAN fibers, it was showed that the dry-jet wet spinning PAN fibers had almost perfect surface morphology which is rather smooth and of wh ich the uniformity of the structure is the best among three series of PAN fibers. And the tensile strength and compactness of dry-jet wet spinning precursor fibers were better than the other two. So it could be inferred that the dry-jet wet spinning progress was more suitable for the preparation of high quality PAN precursor fibers.