Aqueous Synthesis, Structure And Properties Of PAN Polymers For Making Carbon Fibers

Polyacrylonitrile(PAN) is the most important precursor polymer for producing carbon fibers. Homogeneous solution polymerization and aqueous precipitation polymerization are the two main methods for synthesis of PAN polymers. The former is the most popular method used today for producing high-performance carbon fibers, but it is difficult to satisfy the huge demand for low cost carbon fiber in auto industry in the future due to its low production efficiency and high cost. Conversely, the latter has the advantage of high production efficiency and high production flexibility, and seems to be more suitable for developing large-scale and low-cost technology for producing carbon fibers.In this thesis, PAN polymers were prepared by an aqueous precipitation polymerization technique using redox initiators. Ubbelohde viscometer(UVM), gel permeation chromatography(GPC), nuclear magnetic resonance(NMR), fourier transform infrared spectroscopy(FT-IR), differential scanning calorimetry(DSC), thermalgravimetric analysis(TGA) and X-ray diffraction(XRD) were employed to characterize the molecular weight and its distribution, molecular chain structure, crystalline structure and thermal properties of the PAN polymer. Effects of comonomers, initiators and content of itaconic acid(IA) on the structure and thermal stabilization behavior were studied. The main achievements are as follows:1. Acrylonitrile homopolymer(PAN) and copolymers with vinyl acetate(P(AN-VAc)) and methyl acrylate(P(AN-MA)) were prepared using K2S2O8-NaHSO3 as initiator, and the effect of ester comonomers on the structure and thermal stabilization behavior of PAN polymers was investigated. The results showed that although the ester comonomers of VAc and MA could not initiate the cyclization reaction by ionic mechanism at lower temperature, they all could alleviate the violent heat release and reduce the weight loss during the thermal stabilization in the air atmosphere; and P(AN-MA) had higher relative cyclization index and aromatization index under the same thermal stabilization condition, which proved that MA was more effective for improving the thermal stabilization extent.2. Acrylonitrile homopolymer(PAN) and copolymers with itaconc acid(P(AN-IA)) and β-monoethyl itaconate(P(AN-MIA)) were prepared using K2S2O8-NaHSO3 as initiator, and the effect of the α, β-carboxylic acid groups of IA on the thermal stabilization behavior of PAN polymers and the corresponding mechanism were discussed. The results showed that: both of IA and MIA significantly increased the relative cyclization index and aromatization index, decreased the initiation temperature of stabilization reactions, moderated the heat-releasing and weight-losing behaviors. However, IA showed a better effect due to both of the α, β carboxylic acid groups could initiate the ionic cyclization reaction, making PAN convert into more stable ladder structures. The oxidation reaction of P(AN-MIA) was promoted due to the larger volume β-ester group of MIA, which was conducive to the diffusion of oxygen.3. PAN homopolymers and P(AN-IA) copolymers were prepared using K2S2O8/NaHSO3,(NH4)2S2O8/(NH4)2SO3 and AIBN as initiators, and the effects of polymerization methods and initiators on the thermal stabilization behavior were studied. The results showed that(NH4)2S2O8/(NH4)2SO3 was the best initiator for improving the stabilization of PAN polymers, which may attribute to that the ammonium(NH4+) could release ammonia(NH3) under high temperature, and NH3 was able to accelerate the thermal stabilization reactions. Under the same heat treatment condition, P(AN-IA) synthesized by K2S2O8/NaHSO3 possessed the lowest relative cyclization index, the cyclization reaction was possibly hindered by the metal ions.4. P(AN-IA) copolymers of different proportion of AN/IA were prepared using(NH4)2S2O8/(NH4)2SO3 as initiator, and the effect of the content of IA on the thermal stabilization behavior was studied. The results showed that with the increase of IA, more free radical cyclization reaction was converted into ionic cyclization, resulting the intensity of the first ionic cyclization exothermic peak was enhanced. However, with the increase of heating rate, the DSC exothermic peaks shifted to higher temperature, and the intensity of the second free radical exothermic peak was obviously enhanced, indicating the free radical cyclization changed to be more violent.