| Preoxidation process plays an important role and it is a critical step curing the conversion from polyacrylonitrle(PAN) fibers to carbon fibers. Excellent preoxidized fiber can be prepared carbon fiber with good performance. The relations between thermal properties of PAN precursor and the preoxidation parameters have been deeply studied in this thesis. Stretching is an indispensable parameter in preoxidation process, so studying positive stretching and negative stretching(especially at high temperature) could be very meaningful to the future work. Several technologies, such as differential scanning ealorimetry(DSC), thermal gravimetry(TG), Fourier infrared spectroseopy(FTIR), elemental analyzer, wide diffraetion(WAXD), scanning electron microscopy(SEM), and strength testing were used to systemically investigate thermal properties of PAN precursor, determination of preoxidation parameters, and the effect of positive and negative stretching on the crystal structure, chemical structure and mechanical properties. Thermal analysis of PAN precursor can be referred as to as a guide of preoxidation process and the results indicated that comonomer can improve thermal properties of PAN precursor including broadening exothermic peak, slowing down heat release rate and reducing weight loss and also initiate cyclization reaction at low temperature. For PAN copolymer, the exothermic peak was wider and the weight loss was lower in air than that in nitrogen atmosphere. With the increasing of heating rate, the overall DSC curve moves to the higher temperature and the shape of exothermic peak become narrower. So the preoxidation should be carried out with lower heating rate or smaller temperature gradient in air. At the initial stage of preoxidation, there is little variation in C, H, N, O content. At later stage, due to the acute oxidation reaction, the content of O increases obviously along with the gradual decrease of C, H and N.Experimental results of stretching show that whatever positive or negative stretching, there is no obvious influence on the shape and displacement of the FTIR spectrum and the XRD patterns. At positive stretching, the d-spacing decreased at first around 20=17°and then increased, while d-spacing around 20=25°increased with the increasing positive stretching ratio, meanwhile the tensile strength decreased gradually. When negative stretching was carried out, the trend of d-spacing around 20=17°was opposite to that around 29=25°. The tensile strength was higher at low negative stretching ratio, but the strength was lower are high negative stretching ratio. The competive process of amorphous and crystal phase and the relationship between stretching and strength have been investigated.The FTIR spectrum showed that the original functional groups such as C=N and CH2 disappeared, but new groups such as C=N, C=C, C=O appeared in preoxidation process. Meanwhile, as shown in XRD patterns, the intensity of peak around 26=17°increased because of the stretching at low temperature, but it decreased with the preoxidation, and a new peak appeared around 20=25°. Cyclization rate and aromatization index both increased with increasing temperature. Cyclization rate increased at lower temperature but aromatization index increased conspicuously at higher temperature, which indicated that cyclization was initiated first in the amorphous region and then propagated to the boundaries of the orderedphase. At high temperatures, the reactions developed to the crystalline region. |
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