Structure And Structural Formation Process Of Polyacrylonitrile Fibers

The preparation of high quality polyacrylonitrile (PAN) precursor fibers is the key to obtain carbon fibers with excellent performance. The intensive study of the microstructures and their evolution during spinning process and correlation of structure, properties and techniquesis quite important, which has significance in improving PAN-based carbon fiber properties. In this work, a series of experiments were performed on a PAN fiber spinning line. X-ray diffraction (XRD), small angle X-ray scattering (SAXS), polarizing microscope (PM), surface area and pore size analyzer, high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM), and Fourier transform infrared spectroscopy (FTIR) are employed. Agglomerate state structure, morphology and submicroscopic struture (fibrils, microfibrils and pores) of PAN precursor fibers and their structural formation were researched. Effect of drawing technology PAN fiber structure was discussed. And PAN fiber structure models were established.The crystal structure, orientation structure, pores, microfibrils and their structural evolution during spinning process were discussed. XRD curves showed a rather strong and relatively sharp diffraction with diffraction angles 2θ=16.8°and 2θ=29.45°on the equator, and two weak reflection with angles 2θ=36.1°and 2θ=39.2°on the meridian. Furtherly, it is evidenced that PAN structure was 2D paracrystal and PAN chains arranged hexagonally in a lattice. Unit-cell parameters were obtained. Crystal structures during wet-spinning process were studied. It was found that with wet-spinning progressing d(110), d(200) and d(211) reduced gradually, but d(002) increased gradually, and crystallinity and crystal size increased.The orientations in crystalline region and in whole fibers were studied. The orientation in crystalline region increased with wet-spinning progressing while crystal grains oriented with their growth. The whole orientation change was complex. Before collapsing, the whole orientation increased. But the whole orientation decreased after collapsing. Birefringence took place from negative to positive after drawing in vapor, because CN groups in PAN chain didn't adjust promptly with high drawing.The submicroscopic struture in PAN fibers was researched by SAXS. With wet-spinning progressing, the number of pores reduced, the pore size decreased, the pores became longer and narrower, and arranged directionally. Axial dimension and radial dimension of pores decreased slowly before collapsing, but they decreased rapidly after collapsing. Microfibrillar radii are about 20nm measured by SAXS, and their radii decreased with spinning progressing. The lamella structure appeared after collapsing and their size increased with spinning progressing.Effect of temperature and drawing on PAN crystal structure was studied. Crystallization was quite difficult from solution. The crystallinity and crystalline size increased slightly by heat treatment, and crystallization slowed down gradually. Increasing drawing multiple (2.0-4.0) in coagulation bath had a little influence on orientation in crystal region and in whole fibers and mechanics properties. Increasing drawing multiple (1.2-1.5) in boiling-water bath was effective for improving mechanical properties and was beneficial for crystallization and orientation. The crystallinity and crystalline size reduced with increasing drawing multiple (2.2-2.8) in vapor, while orientation in amorphous and in whole fiber decrease. As drawing multiple in vapor was smaller than 2.5, crystalline orientation increased with increasing of drawing multiple; as drawing multiple in vapor was bigger than 2.5, crystalline orientation decreased with increasing drawing multiple.Comparing the mechanism milling, the peel-back technique and the ultrasonic etching method for separating fibrils from PAN fiber, the ultrasonic etching method was selected, by which the fine structure of fibrils can be obtained by this method. In order to validate whether this method affected chemical and physical structure of PAN fibers, the difference of fiber structure and properties before and after ultrasonic etching treatment were studied. The experimental results indicated that ultrasonic etching had no effect on chemical structure of PAN fibers. Molecular weight of undissolved PAN did not change either. The crystallinity and crystal grains reduced slightly after ultrasonic etching. Breaking strength, breaking extension and initial modulus all decreased after ultrasonic etching.The optimum ultrasonic etching conditions of PAN fibers during wet spinning process were confirmed, which included concentration, treatment time and temperature of ultrasonic etching, It showed that resistance to ultrasonic etching of PAN fibers increased with spinning progressing, which is consistent with degree of microfibrillar compaction. Ultrasonic etching mechanism was presented, by which periodic changes of PAN fiber surface with treatment time prolonged was explained.Microfibrils were observed by FESEM and HRTEM, which are composed of periodic lamellae perpendicular to the fiber axis with thickness of 20-40nm. Fibrils were found in progressing fibers. The interface of fibrils and microfibrils are clear gradually as spinning progressing. Thickness of lamellae decreased and then increased with wet-spinning progressing. The lamellae arranged in parallel and then with an angle. The formation of fibrils was preliminarily discussed. It was considered that the embryo of fibrils formed in spinneret orifice, and the formation of fibrils was the result of shearing field in pipe and coagulation in coagulation bath.Fiber A, B and C respectively made in our lab by wet spinning, Japan by dry jet wet spinning and institute of coal chemistry by wet spinning were studied. For the crystal of fiber A, B and C, the power order of regularity and order is:A>B>C. (110) interplanar spacing is small while crystalline size and crystallinity are large in fiber A, but (110) interplanar spacing is large and crystalline size and crystallinity are small in fiber C. Syndiotactic sequences and isotactic sequences ratio was characterized by diffraction peaks integral intensity ratio (I211/I002). It shows tacticity of fiber A is highest, fiber B the second, and fiber C the third. The power order of orientation in crystal region and whole fiber of three fibers is:B>A>C. The pores axial size and distribution of three fibers are similar. But the pores and microfibrils radial size of fiber B are largest in three fibers. Lamella structure exists along fiber axis in three fibers, where long periodic of fiber B is largest and long periodic of fiber C is smallest. The power order of resistance to ultrasonic etching of three fibers is:B>A>C. The diameter of microfibrils in fiber A is about 100-300nm, which is the largest, the diameter of microfibrils in fiber C is about 75-150nm, which is the smallest, and the lamellar period of fibril B is largest among three fibers.Models of fibers prepared by wet-spinning and dry jet wet spinning are established. The PAN fiber is composed of fibrils with diameters of 300-900nm in parallel. Fibrils consist of microfibrils with about 50-200nm diameter, and microfibrils are composed of periodic lamellae perpendicular to the fiber axis with thickness of 20-40nm and amorphous interlayers arranging alternately. The main difference between wet-spinning fiber and dry jet wet-spinning fiber is that the skins of the former comprise are made up of ribbon fibrils and the skin of the latter are directly made up of microfibrils, and the diameter of fibrils in the latter is smaller than that in the former.