Lianghua Xu

The formation of conjugated structure and its transformation to pseudo-graphite structure during thermal treatment of polyacrylonitrile: Effect of molecular composition and configuration

Three types of polyacrylonitrile (PAN) were considered in order to investigate the effect of molecular composition and configuration on the formation of conjugated structures during stabilization and the conversion of that to pseudo-graphite sheets after carbonization. The stabilization process was performed in an inert or oxidative atmosphere with a temperature ramp from 180°Cto 280°C. The thermal behavior was studied by differential scanning calorimetry, and the change of chemical groups and conjugated structures was detected by in situ measurement of infrared (Fourier transform infrared) and ultraviolet–visible spectroscopy, respectively. The carbonization process of the stabilized samples was performed using a thermogravimetric analyzer under nitrogen atmosphere in the temperature range of 150°C–1200°C, and Raman spectra were applied to study the pseudo-graphite sheets of the residuals. It is suggested that the introduction of comonomer or the improvement of the isotactic regularity of the polymer chain are helpful to promote the stabilization reactions and accelerate the formation of conjugated structures rather than the extent of conjugation during stabilization in nitrogen. Moreover, they are also beneficial to obtain higher degree of graphitization and larger size of the pseudo-graphite sheets with less structural defects after carbonization. While stabilization is performed in air, atactic PAN copolymer has the highest extent of stabilization among these three PAN samples, but they are extremely close. PAN samples with comonomer or higher isotacticity still show a little advantage in the formation speed of the conjugated structures. After carbonization, PAN with higher isotacticity has the highest carbon yield and graphitization degree and the largest size of pseudo-graphite sheets with least structural defects. In addition, the presence of oxygen during stabilization is contributory to increase the extent of stabilization and generate some bigger conjugated structures, which leads to obtain higher graphitization degree and larger size of pseudo-graphite sheets, but it also brings more structural defects.

Evolution of the orientation structures in polyacrylonitrile precursors during stabilization revealed by in situ synchrotron wide-angle X-ray diffraction and polarized infrared spectroscopy

Evolution of the orientation structures of polyacrylonitrile (PAN) precursors during thermal stabilization was investigated on the basis of in situ temperature-dependent measurements including synchrotron wide-angle X-ray diffraction and polarized infrared spectroscopy. The results indicated that the Hermans orientation factor of PAN crystallites increased firstly and then decreased in the process of stabilization, while the orientation of molecular chains showed a two-stage decrease. These were mainly attributed to the thermal relaxation of molecular chains and the cyclization reactions, which also resulted in the physical shrinkage as well as the chemical shrinkage apparently observed from the thermal mechanical curves of the precursor.

Effect of In Situ Thermal Stretching during Oxidative Stabilization on the Orientation of Cyclized Ladder Structure and Its Carbon Fiber

The effect of in situ thermal stretching during oxidative stabilization on the orientation of cyclized ladder structure was investigated. Based on the structure evolution of PAN fibers with the increasing stabilization temperatures, the stabilization process was classified into three different stages, namely before the onset of cyclization, during cyclization in amorphous region, and during cyclization in crystalline region. The polyacrylonitrile (PAN) precursor fibers were stretched at the three stages with stretching ratios from 0 % to 8 % during continuous stabilization process. The results show that the orientation degree of cyclized ladder structure increases with the increase of stretching ratio at the three stages and the maximum orientation efficiency of cyclized ladder structure is obtained when PAN fibers are stretched at the stage of during cyclization in crystalline region. The orientation of resulting carbon fibers strongly depends on the orientation degree of cyclized ladder structure. The orientation efficiency of turbostratic graphite crystallite also agrees well with that of cyclized ladder structure. Meanwhile, the orientation efficiency of turbostratic graphite crystallite is higher than that of cyclized ladder structure and the difference values between orientation efficiency of the two structures decrease firstly then increase with the increase of degree of cyclization.

Stretching Deformation Mechanism of Polyacrylonitrile-based Carbon Fiber Structure at High Temperatures

In a high-temperature environment, polyacrylonitrile-based carbon fiber (PAN-CF) can be deformed by stretching, where the stretching deformation ability of PAN-CF is enhanced with the increase of the temperature. Further, the hightemperature stretching deformation of PAN-CF directly affects the control of the carbon crystalline orientation. Based on the techniques of high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray diffraction and in situ tension testing, the variation regularity and the intrinsic mechanism of high-temperature stretching deformation ability of PAN-CF obtained at different preparation temperatures were systematically studied in a high-temperature environment. The results indicated that the essence of PAN-CF high-temperature deformation was the relative motion of the carbon crystallite. Further, the main structural parameters that affected the high-temperature stretching deformation ability of PAN-CF were the degree of cross-linking between the carbon crystallites, the orientation angle(OA) of the carbon crystallite and the nitrogen content. When the testing temperature was lower than the preparation temperature, only physical structure changes were observed in the PAN-CF. For the PAN-CF tested undergoing physical structure changes, as the degree of cross-linking between the crystallites and the orientation angle decreased, the slipping of crystallites became easier. In the same environment, as the stretching tension decreased, the stretching deformation ability improved. When PAN-CF was tested under temperatures higher than the preparation temperature, the microcrystalline cross-linking in the PAN-CF was prone to fracture and slipping, and the high-temperature stretching deformation ability was enhanced. Also, for PAN-CF of lower preparation temperatures in PAN-CF containing no nitrogen (i.e., <0.15 wt%), the cross-linkages increased and the structures were more unstable, inducing an increase in the fracture of weak bonds and a reduction of the stretching tension. For nitrogen-containing PAN-CF, the removal of nitrogen led to severe shrinkage in the graphite layer and interlayer, and the fiber tension was thus increased, causing the high-temperature stretching deformation ability of the PAN-CF with less nitrogen content to be improved.