Investigation Of The Viscoelastic Properties Of Polyacrylonitrile Spinning Solution

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. 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 fiber fabrication technics and relevant fundamental theories are important for improving the quality and production level of PAN precursor fiber. In this thesis, the viscoelastic properties of concentrated PAN/DMSO and PAN/DMSO/water solution, which are usually used as PAN fiber spinning solutions, have been systematically investigated under different conditions. New spinning methods for PAN precursor fiber based on the gelation behavior have been applied to form PAN precursor fibers and the structure and properties of the resultant fibers were characterized.The viscoelastic behavior of concentrated PAN/DMSO solutions at different temperatures was investigated via rheological measurements. The effect of three correlated parameters -polymer concentration, non-solvent content and temperature of the systems were discussed. It was found that the solution viscosity increased with the increase of concentration of Polymer/non-solvent or the decrease of the system temperature at a certain shear rate. PAN/DMSO solution showed typical Newtonian fluid properties at low shear rate in steady-state rheological measurements. Polymer concentration, non-solvent, temperature and other factors would make the solution deviate from Newtonian fluid with the increase of shear rate, showing a typical shear thinning behavior. And these factors would also affect the value of critical shear rate where shear thinning becomes pronounced. The viscous flow activation energy of different PAN/DMSO solutions were obtained through steady-state sweep. The spinnability of PAN/DMSO solution was influenced by temperature. The results of first normal stress difference measurements indicated that solution elasticity was very likely to increase with the increase of shear rate. The result of first normal stress difference measurements showed that polymer concentration, water and temperature of the system could significantly affect the steady-state elastic response of the system. Complex modulus |G*| was hardly affected by shear stress when the shear stress was in the range of 0.1 Pa～300Pa, implying the structure of PAN solution under certain conditions was hardly affected by applied stress. The creep behavior of PAN/DMSO solution was studied in the linear and non-linear cases. The rigidity of the solution was found to increase with increasing polymer solution, water content or decreasing solution temperature. In addition, the elasticity portion of the creep was found to increase with these changes. But in the nonlinear case, the results were significantly different in the concentration range of PAN-21%～23% and the temperature range of 40～55℃, which may be related to the existence of the critical value of polymer concentration and temperature. The zero shear viscosity values obtained from creep approximation, depending on the polymer concentration and molecular weight, were in good accordance with those determined by empirical formula, and the zero shear viscosity values linear increased with the increase of the non-solvent content. The results of thixotropic Theological measurements showed that PAN/DMSO solution possesses thixotropic properties. PAN/DMSO solutions with low polymer concentrations showed weak thixotropy at dissolving temperature. It was found that the thixotropy of solution increased with increased polymer concentration and water content in the system or decreased temperature.The viscoelastic behavior of concentrated PAN/DMSO solutions during ageing process was investigated via rheological measurements. The effect of correlated parameters - polymer concentration, non-solvent content and aging time of the systems were discussed. It was found that the solution viscosity increased with the increase of concentration of polymer/non-solvent or the increase of aging time at a certain shear rate. PAN/DMSO aging solution showed typical Newtonian fluid properties at low shear rate in steady-state rheological measurements after aging for 2 hours. The solution deviate from Newtonian fluid with the increase of aging time at a certain shear rate. These factors would also affect the critical shear rate. The creep behavior of PAN/DMSO solution during aging process was studied under the linear conditions. The rigidity of the solution was found to increase with increasing polymer solution, water content or increasing aging time. In addition, the elasticity portion of polymer creep corresponding was found to increase with these changes and water content was a major factor in a certain time.The results of thixotropic rheological measurements showed that the thixotropy of solution increased with increased aging time , polymer concentration and every sample exhibited doubled thixotropy loop area after aging for 2 hours. In addition, the thixotropy of solution increased when a small amount of water was added, but changed little when the water content further decreased, which is slightly different from the laws reported in the second chapter and needs further study.Gel spinning based on thermal-induced gelation mechanism was employed to produce PAN precursor fibers. The traditional dry-wet spinning was also used for comparison. Mechanical tests, wide-angle X-ray diffraction(WAXD), DMA were conducted to characterize the structure and properties of the resultant fibers. Compared with the fibers produced by dry-wet spinning, the fibers from the gel spinning had better mechanical properties and the gel spun fibers from the extraction bath had the best mechanical properties. However, the crystallinity of the three types of fibers was very close. The gel spinning was the best spinning method, as the fabricated fiber possessed the most favorable structure and best physical properties. For 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.