Study On Structure And Properties Of Polyimide Fibers During Heat Treatment Abstact

Carbon fiber has high modulus, high strength, good resistance to temperature and chemical corrosion properties, so it is widely used in industrial goods and commodities. At present, the carbon matrix are polyacrylonitrile (PAN), asphalt, glue and wood fiber. Carbon fiber are obtained by the pre-oxidization, carbonization and graphitization of the precursor. Because the properties and structure are decided by the structure of precursor and process treatment, looking for more suitable precursor and improving processing technology are the main direction of the development for carbon fibers. Polyimide fiber has god mechanical performance, high dielectric properties, good resistance to high temperature, ultraviolet, corrosion and size stability performance, and it is widely used in aerospace, nation defense, machinery, chemical, energy conservation, environmental protection and atomic energy industry. In this research, polyimide-based carbon fibers were prepared because PI fiber has aromatic rings, diversitied chemical structure, and high carbon content after carbonization. The preparation of PI-based carbon fiber enriches the precursor for carbon fibers.First, the PI fibers with different structures (PMDA/ODA and BPDA/p-PDA/BIA) were prepared by wet-spinning method. From SEM photographs, the fiber had smooth surface and cross section morphology without holes. The thermal properties was analysed by TG and DSC:the oxygen gas can promote the decomposition of the structure, and the inert gas can promote the reorganization of molecular chain. This theory provides a basis for investigation of thermal stabilization in air and carbonization in nitrogen.Second, the two kinds of polyimide fibers were treated for stabilization in air and nitrogen at400-500℃. The maximum weight loss between500and600℃and residual weight content at800℃were charactered by TGA. The condition that had the smallest weight loss between500and600℃and had the highest residual weight content at800℃was chosen as the best. For PMDA/ODA, the best stabilization treatment is400℃in air for1h; for BPB, the best stabilization treatment is550℃in nitrogen for1h.Third, the stabilization treated fibers were carbonized at600,800,1000,1200, and1400℃with the heating rate of10℃/min and final holding time of1h. The carbon yield and diameter of the carbonized fibers declined with increasing temperature. IR spectrum shows that a new peak appears when carbonized at700℃, and that the peak at1595tends to be a platform with increasing temperature, indicating the growth of aromatic rings. XRD curves show that the diffraction angle and spacing of (002) decrease with increasing temperature, suggesting that the carbon layer got denser. The BPB system had the smaller weight loss, larger diffraction angle and spacing than PMDA/ODA. The Raman analysis of PMDA/ODA shows that the graphite degree increases with temperature. XPS analysis shows that both the pyrrole nitrogen and pyridine nitrogen exist in the structure. The content of pyridine nitrogen increased at1000℃. At last, the carbonization mechanism was proposed through a series of characterization of PMDA/ODA.