| Poly(p-phenylene benzobisoxazole)(PBO)fiber shows fascinating properties including excellent mechanical performance,high crystallinity,and fairly good heat resistance as a kind of polymer fiber.Due to its properties and a theoretical carbon content of 71.71 wt.%,it can be a possible candidate as a kind of precursor of carbon fiber.This thesis investigates the thermal behavior of PBO under high temperature with thermogravimetric analysis(TG),thermogravimetry coupled with Fourier transformed infrared spectroscopy(TG-FTIR)and thermogravimetry coupled with mass spectroscopy(TG-MS).After the production of PBO-based carbon fiber with a kind of aged domestic PBO fiber as the raw material and a self-built production line,further study has been conducted on those products with a universal testing machine,a multimeter,an elemental analyzer,an X-ray diffraction analyzer and a Raman spectroscope.Moreover,in order to improve the mechanical performance of PBO-based carbon fiber,a preliminary research has been conducted where PBO fiber was firstly submerged into boron-containing solution and then carbonized continuously,with the mechanical performance and microstructure of those product analyzed.The results are as follows.(1)The heat-induced mass loss of PBO fiber under inert atmosphere contained two steps.The first step occurred around 168?,which was not significant.The second step happened at 722.3?,with a large mass loss.After the second step,the mass loss still continued with the further increasing in heat treatment temperature(HTT).(2)The pyrolysis mechanism was announced according to the results of TG-FTIR and TG-MS.Under the condition of pyrolysis,the homolytic scission of C-C?C-O and C=N bonds in the oxazole ring structure and C-C bonds in phenylene structure in PBO chains occurred,which resulted in the generation of various small molecules including H2,C,CH4,NH3,H20,C2H2,HCN,CO/N2,NO,02,C02,N02,benzene and benzonitrile.Due to this situation,not only did some PBO chains broke,but also new structure formed,which prevented the fiber from failure or melting down,and made it have fairly high mass remaining.(3)PBO-based carbon fiber was successfully obtained by optimizing the continuous carbonization technique.The result shows that PBO fiber was able to sustain a HTT as high as 1400? under an inert atmosphere and the shape of the fiber was still preserved without failure.At first,the mechanical performance of the fiber dramatically decreased with the increase in HTT due to the rupture of the microstructure in PBO fiber.After that,because of the formation of the new graphite sheet structure,the mechanical performance improved.Furthermore,the electrical performance changed significantly,for an electrically insulating material as PBO becoming an electrically conductive material.Also,the electrical resistivity dramatically dropped along with the increase in HTT.Specifically,the electrical resistivity even almost reached that of some kinds of high-performance carbon fibers,such as T300 carbon fiber produced by Toray Industries Co.,Ltd.Because of the manufacturing technique of PBO fiber,ditch-like structure was not observed on the surface of PBO-based carbon fiber,which is common on PAN-based carbon fiber.Moreover,skin-core structure was also not observed in PBO-based carbon fiber.It could be inferred that double diffusion which could result in the skin-core structure did not happen during the carbonization of PBO fiber.(4)No significant effect of boron-doping on both mechanical performance and microstructure of PBO-based carbon fiber obtained after 1400?-treated could be detected in the experiment.The performance of carbon fiber depends on the graphite-like structure in it.The purpose of boron doping was to improve the performance of carbon fiber,because boron can promote the formation and development of graphite-like structure.But due to the limitation of the experiment,HTT didn't reach and go above 2000? where boron can work significantly,which led to this situation.Further research on this is needed. |
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