Microstructural Evolution Of Polyacrylonitrile During Preoxidation

Carbon fiber is a kind of inorganic fiber material, in which the carbon content is higher than 90%. It has high specific strength, high modulus, high temperature resistance, corrosion resistance, creep, scour resistance and other excellent properties. Carbon fiber has been widely used in many fields. It has become an ideal alternative to traditional steel and aluminum alloy materials. So it plays a more and more important role in the mainstay industries. Pre-oxidation is the key step of the preparation of polyacrylonitrile-based carbon fiber. Although studies on pre-oxidation have been reported intensively, there are some problems are still controversial, such as priority region of cyclization reaction, the effect of oxygen on the cyclization reaction etc. This paper studies the microstructural evolution of polyacrylonitrile polymers during pre-oxidation, to understand the priority region of the cyclization reaction, the effect of oxygen on the cyclization reaction of the polyacrylonitrile homopolymer(PAN) and P(acrylonitrile/methyl methacrylate/itaconic acid) [P(AN/MMA/IA)] terpolymer.(1)The influences of treatment temperature, treatment time and atmosphere on the structural evolution of PAN homopolymer and related copolymer P(AN/MMA/IA) during heat treatment were studied by using fourier transform infrared spectroscopy(FT-IR) and wide-angle X-ray diffraction(WAXD).The results of FT-IR indicated that the cyclization reaction occurred in a large amount, but a small amout of dehydrogenation under nitrogen condition. Under air condition, both of cyclization and dehydrogenation happened in a large amount. Oxygen had a negative effect for the cyclization of PAN homopolymer, but had a positive effect of P(AN/MMA/IA). The results of WAXD indicated that the cyclization occurred in the amorphous region in PAN homopolymer at first and extended to the crystalline region as the proceeding of cyclization. Cyclization occurred mainly in the amorphous region at 240 oC for 1 h and occurred mainly in the crystalline region for t?1.5 h. The results of TG indicated that the cyclization reaction is the prerequisite for the oxidation reaction.(2) Kissinger method was used to calculate the kinetic parameters.of PAN and P(AN/MMA/IA) under nitrogen condition and air condition, respectively. The cyclization rate of PAN was faster under nitrogen condition than that under air condition. The results indicated that oxygen prevented PAN from free radical cyclization reaction While the cyclization rate of P(AN/MMA/IA) was faster under air condition than under nitrogen condition. The results indicated that oxygen has a positive effect on the cyclization of P(AN/MMA/IA) All these results also showed that oxygen has a positive effect on free radical mechanism,but. has a positive ef fect on ionic mechanism.