Gelation Behavior Of Concentrated Polyacrylonitrile Solution And Its Application In Fabrication Of Polyacrylonitrile-based Carbon Fiber Precursor

Polyacrylonitrile (PAN)-based carbon fiber is the most widely used and promising carbon fiber. As the precursor of PAN-based carbon fiber, the quality of PAN precursor fiber determines the performance of PAN-based carbon fiber to a large extent. At present, the demand for carbon fiber is growing fast in the world. However, domestically fabricated PAN-based carbon fibers are generally at low quality level compared with those produced by some developed countries. The low-quality PAN precursor fibers are not qualified for producing high-performance carbon fiber, which has been restricting the improvement of PAN-based carbon fiber. Studies on relevant fundamental theories and fiber fabrication technics are important for improving the production level of PAN precursor fiber. In this dissertation, the gelation behavior of concentrated PAN/DMSO and PAN/DMSO/water solution which are usually used as PAN fiber spinning solutions has been systematically investigated. New spinning methods for PAN precursor fiber based on the gelation behavior and the characteristics of PAN gel have been applied to the formation of PAN precursor fibers and the structure and properties of the resultant fibers were characterized.The viscoelastic behavior of concentrated PAN/DMSO solutions during thermal-induced gelation was investigated via dynamic rheological measurements. The effect of two correlated parameters-gel concentration and gelation temperature on the gelation behavior of the systems were discussed. The loss tangent tanδand the dynamic moduli G’andG" of different samples were obtained through frequency sweep, based on which the critical gel concentration cg was determined using scaling laws. It was found that the cg value increased with the increase of temperature. According to tanδ, G’andG" obtained from temperature sweep and frequency sweep at different temperatures, the critical gelation temperature Tg was determined and found to be proportional to the PAN concentration of the system. Also, the critical gel strength S increased with the increase of the concentration. Although cg changed with temperature, the value of the relaxation exponent n at the gel point hardly changed. The same phenomenon occurred at the critical gelation temperature Tg. It was thus concluded that PAN gel formed under certain conditions has unique structure, irrespective of the PAN concentration and the gelation temperature.The gelation behavior of concentrated PAN/DMSO solution during aging process was investigated through dynamic rheological measurements. The system would gel after a certain period of aging at a constant temperature below its gelation temperature Tg. In the aging process, the gelation rate increased with decreasing aging temperature, which meant the critical gelation time decreased with decreasing aging temperature. Scaling laws could be applied to PAN systems. The critical n values obtained from the scaling laws were in good accordance with those obtained directly from experimental results, and the n value kept almost constant when the critical gelation time tg changed. For the systems with the same PAN concentration, the critical gel strength S of the gels formed in aging process was smaller than that of the gels formed in the cooling process, likely ascribed to the different gel forming conditions.The results of cloud point titration and intrinsic viscosity measurements indicated that phase separation was more likely to occur when the water level was higher. The result of dynamic rheological measurements showed that water in the system could significantly affect the gelation behavior of the system. Tg was found to increase with increasing water content and S decreased a little when the water content grew. In addition, n was hardly affected by the water content, implying the structure of PAN gel formed under certain condition was hardly affected by the water level in the system. The presence of water could accelerated the gelation process, reflected by the decreased critical gelation time tg. The results of dynamic rheological measurements as well as atomic force microscopy (AFM) and scanning electron microscopy (SEM) showed that, though the presence of water made the gel structure more compact, it could lead to phase separation during the gelation process of the system. The gel thus became inhomogeneous and its strength reduced.The fractal characteristics of PAN gels formed from PAN/DMSO/water systems were studied via dynamic rheology and AFM image analysis. The possible ranges of the fractal dimension df value for the PAN gels with different water levels were determined using Winter’s theory. By applying Wu and Morbidelli’s model to our gel systems, the df values for the two PAN gels of different water level were obtained and found to agree well with the results from Winter’s theory. According to Wu and Morbidelli’s theory, the contributions of intrafloc and interfloc links to the gel stiffness were comparable for PAN gels. Also the fractal analysis based on AFM image gave df values consistent with those obtained from the rheological measurements. The amount of water in PAN solution was found to have little effect on the df value and hence on the gel structure, and the conclusion has been confirmed by both rheology and image analyses.Dynamic rheological measurements, differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) were employed to investigate the thermoreversibility of PAN/DMSO and PAN/DMSO/water systems, and the crystallization of the systems in varied states. The changes of G’and G" during heating and cooling processes showed the thermoreversibility of PAN gel. However, the viscoelasticity changes of the systems in heating and cooling processes were different, reflected by the difference in the traces of G’and G". Besides, the viscoelasticity changes in the cooling-heating process and heating-cooling process were also different. It was found from the dynamic strain sweep and the frequency sweep after shear that the PAN gel structure could recover immediately after it was broken, which indicated that the dipole-dipole interaction between nitrile groups rather than PAN crystallites acted as the cross-linking points in PAN gels. The results of DSC tests for the solvent-containing PAN gels showed that PAN gels formed only by cooling could not crystallize, so crystallization was not the cause for the formation of PAN gel. Phase separation may occur when PAN solution underwent gelation. The results of WAXD agreed well with those of rheological measurements and thermal analyses, i.e., cooling alone could not make the resultant gel crystallize. However, the water-induced PAN gel could crystallize and the average crystallite size increased with reduced water content in the gel.Two new spinning methods-not-pre-gelled gel spinning and pre-gel gel spinning which were on the basis of thermal-induced gelation mechanism in both cooling and aging processes were employed to produce PAN precursor fibers. The traditional dry-wet spinning was also used for comparison. SEM, mechanical tests, WAXD, small angle X-ray scattering were conducted to characterize the structure and properties of the resultant fibers. In the aging process, it was found that aging the spinning solution for 112 to 190 minutes could make the solution gel but not lose flowability, which guaranteed the feasibility of pre-gel gel spinning. Compared with the fibers produced by dry-wet spinning, the fibers from the not-pre-gelled gel spinning had more compact structure and fewer pores inside the fiber. The skin-core difference between the two types of fibers was, however, not pronounced. In pre-gel gel spinning, since the pre-gelled spinning solution had formed somewhat stable interconnected network structure, the as-spun fibers formed from the pre-gelled spinning solution possessed comparatively circular cross-section, and spinnability of the spinning solution was better. Of the fibers produced with the three methods, the pre-gel gel spun fiber had a smallest total volume of pores, smallest fractal dimension of pores and best mechanical properties. The not-pre-gelled gel spun fiber took the second place. However, the crystallinity of the three types of fibers was very close. The pre-gel gel spinning was the best spinning methods, as the fabricated fiber possessed the most favorable structure and best physical properties. For the pre-gel gel spinning, the fiber obtained from the extraction bath had better mechanical properties than that obtained from the coagulation bath. In addition, the fiber from the extraction bath had more easily oriented and better packed micro-structure.