Microstructure And Properties Of PAN Precursor Fibers

The high quality of polyacrylonitrile (PAN) precursor fibers is a significant prerequisite for obtaining carbon fibers with excellent performance. It is important to study microstructures of PAN precursor fibers and relationship between microstructures and properties of PAN fibers, which is advantageous to improve quality of carbon fibers. In this paper, several technologies are employed, such as Ubbelohde dilution viscometer (UDV), nuclear magnetic resonance(NMR), wide angle X-ray diffraction (WAXD), polarizing microscope (PM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron probe microscopy analysis (EPMA), differential scanning calorimetry (DSC) and thermal gravimetry (TG), for the research of PAN precursor fibers from the following aspects: size and configuration of macromolecule, intermolecular packing, morphology, mechanical properties and thermal properties of PAN precursor fibers.Study on the size of PAN macromolecule and mechanical properties indicates that the molecular weight of PAN fibers has effect on the tensile strength but little effect on elongation at break of precursor fibers. When the molecular weight is below 150,000, the tensile strength of PAN fibers increases with the increase of molecular weight, but when the molecular weight exceeds 150,000, tensile strength of PAN fibers is retained at 7.0cN/dtex. The polymer chain end effect influences the tensile strength of PAN fibers. As the molecular weight gets smaller, polymer chain ends increase, and the intermolecular slippage becomes easier, so the tensile strength reduces. Contrarily, as the molecular weight gets larger, the intermolecular frictional force becomes greater, so the tensile strength increases. Inside the range of 135,000 to 165,000 molecular weights, the elongation at break of PAN precursor fibers fluctuates from 9.5% to 11.5%. It is attributed to the form of polymer chain. If polymer chains or segmers are warped, which have undergone sufficient heat processing, the elongation at break of PAN fibers increases. Contrarily, if chains or segmers are straight, the elongation at break of PAN fibers decreases.Finely resolved ¹³C NMR spectra of difference PAN precursor fibers are determined. The assignment of methane carbon spectra with triad obeys Bernoullian statistics in self-produced PAN fibers, Japanese PAN fibers and the PAN fibers produced in Jilin Chemistry Industrial Company. All the PAN fibers are atactic polymers. But the highest stereoregularity is found in self-produced fibers. The results of PAN solubility testing are consistent with NMR spectra analysis.The crystallization and orientation of PAN precursor fibers gradually increase during the spinning process. The crystallites act as physical nodes in PAN fibers. Therefore, as the crystallization increases, the tensile strength, initial modulus and fracture work of PAN fibers all increase. PAN fibers birefringent property feature arises from optical anisotropy, which can characterize the orientation of chains or segmers. When the absolute value of birefringence is lager, the orientation increases and the difference of length of chains or segmers in amorphous regions and of tensions that each chain or segmer bears become less, so tensile strength increases and stretching rate decreases.The internal textures and surface morphosis of PAN precursor fibers are observed by TEM, SEM and EPMA. There are a number of fibrils with diameter of about 250-400nm aligned along fiber axis inside fibers. The amorphous regions exist between fibrils and among the laminar microcrystallite regions. Layer structures arrange perpendicular or with certain angle to the axis. The mechanism of fibrils is not clear and need studing deeply. The microstructure is more compact and size of crystalline grain is smaller in skin than in core of PAN fibers. Therefore, the tensile strength of the skin is higher than that of the core. The fibrils on surface of PAN fibers are formed convex-concave on face and are conducive to the tensile strength of PAN fibers. There are some flaws like cavities inside fibers, which is vital to reduce tensile strength of PAN fibers.Close relationship is found between the microstructure and the mechanical properties of PAN precursor fibers. The PAN precursor fibers with high-tensile and high-module have high-crystallinity and high-orientation. Increasing rate of extension, stress-strain curve becomes steep and modulus of deformation increases and the elongation at break reduces and the tensile strength first increases then decreases. Relative hook joint strength is proportional to stretching rate of PAN fibers.The thermal properties are also associated with the microstructure of PAN precursor fibers. The initiation temperature of weight loss curve of PAN fibers with high crystallinity and high stereoregularity is higher, because the oxygen diffusion rate is lower. The shape of DSC exotherm, the exothermic amount and the heat releasing rate of PAN fibers are greatly influenced by the comonomer and composition of PAN precursor fibers. If the comonomer is low stereoregularity and high water absorption, the thermal shrinkage of PAN fibers in boiling water is higher than that in dry air. Disordered and comparative ordered regions are proved coexisted in amorphous region of PAN fibers by thermal shrinkage in boiling water. Undergoing thermal shrinkage in boiling water, the orientation degree of amorphous region reduces.