The Correlation Research Of PAN Fiber-Forming, The Crystal Structure And The PRE-Oxidation Structure

Polyacrylonitrile (PAN), from fibrillation, precursor, pre-oxidation fiber to carbon fiber, its structure gradually formed and evolved, which is connected with environment of external field in the process of preparation, its microstructure affects macroscopic properties of carbon fiber. Crystal structure of PAN fiber as a hierarchical structure, its formation effects properties of fiber and subsequent reaction, which is a research topic of extensive concern.Crystallinity and crystal size of PAN precursor associated with the spinning process, by changing the condition of shaping and forming, obtain different crystal structure of PAN precursors and its pre-oxidation treatment. Content, crystallinity and crystal size of PAN fibers were analyzed by XRD, morphological structures of PAN fibers were characterized by body density and SEM, chemical structures of PAN fibers were tested by FT-IR, solubility of PAN fibers were studied by UV, thermal properties of PAN fibers were discussed by DSC, force-stroke curve of PAN precursors were detected by universal tested machine. By studying the crystal structure of the different characteristics of PAN precursors in the process of formation, and it effect properties of PAN precursors and pre-oxidation structures, resulting the correlation research of PAN fiber-forming, the crystal structure and the pre-oxidation process. Results showed:1. By heat and mass transfer concentrated solution of polyacrylonitrile occured to phase separation and polyacrylonitrile fiber-forming. Low-temperature coagulated into a fiber according to heat transfer, a large number of physical crosslinking points of primary gel network structure were formed, resulted in the larger crystallinity and the smaller crystal size of PAN fiber. High-temperature coagulated into a fiber according to mass transfer, diffusion capacity of water makes the smaller crystallintity and the larger crystal size of PAN fiber. By the thermal and force coupling, with first drawing improved and crystallinity of PAN fiber increased, but due to relative lower temperature of first drawing, the growth of crystal size was minimal. Total drawing constant, the different first and second drawing match, crystallinity changed little, but with second drawing ratio increased and crystal size significantly increased under high temperature and small molecules plasticization.2. Crystal structure of PAN precursor effects properties of precursor: The smaller crystallinity of PAN precursor, because binding force of its structure is as smaller as chains, with force increased and strain amplified. Structure looser, bulk density smaller and easy to dissolve. Conducive to the diffusion of oxygen, promoted the cyclized reaction and initial oxidation reaction, higher degree of pre-oxidation. At 60℃～160℃, due to the change of amorphous to crystalline much more, heat content was large. The smaller crystal size of PAN precursor, with force increased and stroke amplified.3. Crystal structure of PAN precursor effects reaction and process of pre-oxidation:Crystallinity of PAN precursor in the process of pre-oxidation, the relative cyclization index and the aromatization index of PAN fiber are high at every stages, the total heat release is low, relaxation reaction. Crystallinity is no differences but crystal size big, the relative cyclization index is small, the aromatization index changes little, the total heat release is low, relaxation reaction. The differences of crystallinity of PAN precursor play a conclusive effect in changing of crystal size in the process of pre-oxidation, the larger crystallinity of PAN precursor still keep a larger crystal size at the late pre-oxidation.Through the above research, revealed the correlation of PAN fiber-forming, the crystal structure and the pre-oxidation structure.