Preparation,Rheological Properties And Spinnabilityof Ultrahigh-high Molecular Weight Polyacrylonitrile Copolymer Solution

High-quality polyacrylonitrile (PAN) carbon fiber precursors play key role in fabricating high-performance carbon fibers. The quality of PAN precursors is mainly affected by the chemical and physical structures of PAN macromolecules. The rheological properties, spinnability, moldability of spinning solution and other parameters of precursor structure can be precisely controlled by changing multiple parameters of the PAN molecules. Due to the direct relation between the quantities of molecule ends and micro-defects of the precursor, the molecular weight and molecular weight distribution of PAN have become the dominant factors that influence the strength of the precursors and the resultant carbon fibers. In recent years, a new process of preparing carbon fiber precursors by mixing high and ultra-high molecular weight PAN has been developed. Using this technique, the spinning rate and the quality of precursors and the resultant carbon fibers can be improved greatly. Preparation of uniform high and ultra-high molecular weight of the polymer is the key point to obtaining the high-performance carbon fibers.In this paper, the high and ultra-high molecular weight PAN (UHMWPAN) copolymers were firstly synthesized by aqueous suspension polymerization. The effects of the concentration of the initiator, total monomer mass, monomer ratio, reaction temperature and time on the conversion ratio of the polymerization reaction and relative molecular mass of the copolymer were investigated. The optimum conditions of the polymerization process were determined as follows: the total monomer mass (25%), AN:IA= 98:2,0.01wt% AIBN,0.15wt% PVA, reaction temperature (70?) and reaction time (2h). The viscosity average molecular weight (M?) of the obtained UHMWPAN was 1.47×10. The gel permeation chromatography (GPC), elemental analysis, Fourier infrared spectroscopy (FTIR), X-Ray Diffraction (XRD), Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) were used to investigate the physical and chemical structures and properties of the different polymer component.Next, Dimethyl sulfoxide was used as the solvent to swell and dissolve UHMWPAN. Orthogonal experiments were applied to analyze the effects of solid content, swelling time and swelling temperature on the swelling performance. The optimal process was as follows:160 g UHMWPAN were grinded into powdered with 200 mesh and then dispersed evenly in 7.13L DMSO at 25?. The swelling system need to be heated gradually and remained at 38?,44? and 52? for 30 min, respectively. After that, the mixture system was remained at 60? for 8h with continuous stirring to obtain UHMWPAN-DMSO dilute solution with 2% solid content. The optical microscope and rotational viscometer were employed to examine the appearance and the viscosity change of the polymer particles in the swelling and dissolving process. Meanwhile, the fundamental mechanism of the swelling process of UHMWPAN in DMSO was explored.Then, AN and IAco-monomer were added into the above-mentioned UHMWPAN-DMSO diluted solution through solution polymerization. This was the first time to prepare a new kind of ultrahigh-high molecular weight PAN spinning mixed solution using this strategy. The effects of the content of dilute UHMWPAN-DMSO solution, polymerization temperature, polymerization time, the content of initiator and co-monomer on the molecular weight, conversion and viscosity of copolymer were investigated. The optimal process of solution polymerization was obtained as follows:2% dilute UHMWPAN-DMSO solution(0.5%), AN:IA=98:2, total monomers(22%), AIBN (0.1%), continuous stirring rate (480rpm), thermostatic reaction (at 60? for 20h under the protection of nitrogen). Based on the optimal process of solution polymerization, the weight-average molecular weight (Mw) of the obtained PAN spinning mixed solution is 4.37×105 and the polydispersity (Mz+1/Mw) is 3.61.The shear rheology of above-mentioned novel ultrahigh-high molecular weight mixture PAN spinning solution was investigated. The results of the rheology characteristic of the solution revealed that the presence of UHMWPAN at relatively low concentration have a remarkable influence on the steady state and dynamic rheological properties of mixed spinning solutions. On one hand, the steady-state rheological results show that the viscosity of all the samples decreased as the temperature grew, indicating a typical shear-thinning or pseudoplastic fluid. Further study was performed to demonstrate that the initial viscosity of the solution increased with the increase of the content of UHMWPAN, while the rheological curve trend of the solution did not change obviously. The flow activation energy of the spinning solution indicated that the viscosity of the solution became more sensitive to the temperature and the non-Newtonian index of the solution decreased. These phenomena showed that the solution strayed far from the newtonian fluid. On the other hand, the dynamic rheological experiments indicate that the complex viscosity (?*), storage modulus (G') and loss modulus (G") all increased with the increase of UHMWPAN content; the visco-elasticity of PAN solutions became more apparent. While all the parameters did not change as the strain increased and were in the range of linear viscoelastic region. It should be noted that the physical stability of the spinning solution had not changed by UHMWPAN. As shown in the experiments, the complex viscosity (?*), storage modulus (G') and loss modulus (G") of the spinning solution were all linearly decreased with the rise of the temperature. However, it showed the opposite tendency for the loss angle tangent (tan?). As the temperature increased, a higher content of UHMWPAN would make the effects more obvious. Based on the results of shear rheological tests, it was found that PAN spinning solutions containing appropriate content of UHMWPAN not only play an important role in keeping the physical stability of the mixed spinning solutions, but also have significant effects on the shear thinning behavior and elasticity of the solutions. The existance of UHMWPAN facilitated maintaining fine fluidity at high shearing and stretching rates.The extensional rheology measurements of the above-mentioned novel spinning solutions were also studied. Results showed that the content of UHMWPAN also exerted great effects on three crucial parameters of the solutions:the diameter changes during the stretch, the tensile viscosity and the Tr ratio. When the content of UHMWPAN increased, the diameter of the spinning trickle containing UHMWPAN became smaller and had a longer hold-up time. Meanwhile, the tensile viscosity of the spinning solution and Tr ratio increased during the stretch process. These results indicated that UHMWPAN indeed strengthened the anti-stretch properties of PAN solution. The meaningful findings could provide theoretical guidance for the relevant parameters setting of the spinning solution to obtain high stability in the process of stretching.The dry-wet spinning based on the gelation mechanism was carried out using the spinning solutions containing UHMWPAN as mentioned above. It revealed that the addition of UHMWPAN could improve the spinnability of the spinning system and the mechanical properties of the as-spun fibers. The optimal content of UHMWPAN was 0.5%. At this level, the spinning speed (V1,m) could reach 43.61m/min,2.31 times higher than that of common spinning solution (18.84 m/min); the air gap reached 5 mm, a 67 %increase over that of common spinning solution (3mm). Compared with the surface of the precursor without adding UHMWPAN, the as-spun fibers containing UHMWPAN had a smooth surface and semi-round section. As to mechanical properties, the breaking elongation increased as the content of UHMWPAN increased; the breaking strength was 17.77% higher than that of common precursor. All the results above showed that the addition of UHMWPAN could optimize the morphological structure and mechanical properties of as-formed fiber.This study gives an insight to the synthesis, swelling and dissolution of ultrahigh-high molecular weight PAN copolymer, as well as the relevant technological and theoretical study of the rheological properties and spinnability of the mixture. The developed techniques meet the international standards and can guide the fabrication of high-quality PAN-based carbon fibers.