The Relevancy Research On Crstallization Of Electrospun PAN-Based Fibers With Oxidation And Carbonization Fibers

As-electrospun polyacrylonitrile fiber is an important precursor for preparing carbon nanofibers with high performance. When as-electrospun fiber draw in humid atmosphere with saturated steam, the molecular chain can fully stretch and orient in the fiber direction closely, which make the crystalline structure of as-electrospun fibers more perfect. And the macro properties of carbon fibers is dependent on precursor's various crystalline structure. Therefore the research on crystalline structure of as-electrospun fibers has great significance for pre-oxidation and carbonization process.Continuous and uniaxially aligned as-electrospun PAN nanofibers were prepared by electrospinning and Steam stretching with optimization process parameters via the preliminary experiment. Then we can obtain electospun fibers of various crystalline structure through changing draw ratio in steam stretching process. And morphology(SEM), density and crystalline parameters(XRD) of precursor were carried out as the foundation of further research. While the performance and structure of various electrospun carbon fibers were investigated for obtaining optimization pre-oxidation process parameters(240,260,274,280?; 5min per stage) used in this study. And high temperature, short heat treatment time makes it different from traditional pre-oxidation process. Differential scanning calorimetry(DSC), Fourier transform infrared spectrum(FT-IR) and elemental analysis were employed to investigate exothermic performance of as-electrospun fiber and calculate the cry stall inity, degree of orientation and oxygen content in the pre-oxidation process. Electrospun carbon fibers were characterized by WASD, Raman spectra and density test, then we evaluate their relevance to crystalline structure of as-electrospun fibers.The study shows that:(1) the steam stretching can effectively decrease diameter of as-electrospun fibers by ?49.0%, and improve fibers'density by eliminating voids and holes. WASD results reveal that when steam stretching was applied, the aggregation structure of as-electrospun fibers, the degree of orientation (61.4% increase), crystallinity (84.3% increase), the size of crystal (215.0% increase) significantly improves. But under 3.5-times draw ratio, the degree of oritation shows decline due to excess stretching. (2) This study explored out a new pre-oxidation process with high-temperature and short heat-treatment time, which can reduce more than half of the process time compared with conventional method. And the better process parameters (240, 260,274,280?; 5min per stage) were selected. By this method we improve the density of electrospun carbon fibers to 1.7623g·cm-3. (3) The DSC curves of as-electrospun fibers with various crystalline structure reflect that releasing heat was increased, the location of exothermic peaks were shifted to lower temperature and become sharper under better crystalline structure. In cyclization reaction, perfect crystal structure would help the formation of heat-resistant ladder structure. Finally a higher degree of cyclization can be achieved. The crystal structure and diameter of as-electrospun have synergistic impact on the change of oxygen content, in generally the final oxygen content would decrease with crystal structure of as-electrospun fiber improving. And variation of density were caused by both cyclization and oxidation reaction. (4) The results of electrospun PAN-based carbon fibers shows that under 3-times stretching we can obtain electrospun carbon fibers with compact and uniform structure, perfect graphite crystallite through same pre-oxidation and carbonization process.