The formation process and mechanism of the pseudo-graphite sheets of polyacrylonitrile (PAN) during thermal treatment in inert atmosphere were investigated by thermo-gravimetry (TG) and X-ray diffraction (XRD). According to the results, the conjugated plane originally formed during stabilization was proposed as the basic planar structure of the pseudo-graphite sheets, and it was connected and stacked to form pseudo-graphite microcrystal through chain scission reaction and non-carbon atoms elimination reaction in 280–450 °C and 800–1300 °C respectively. The in situ measurements for time dependence of ultraviolet-visible (UV-vis) and Fourier transform infrared spectra (FTIR) were applied to study the conjugated plane of PAN during isothermal stabilization. It was found that with the higher temperature the conjugated plane forms more rapidly, and it has higher conjugated extent with the extending of heating time. The FTIR data showed that both cyclization and dehydrogenation are beneficial to the evolution of conjugated plane in the first 3 h, but only cyclization continues after 3 h at 250 °C. Further investigation indicated that stabilized PAN with large quantity and high conjugated extent of the conjugated plane would contribute to get large size of pseudo-graphite microcrystal after carbonization, which is consistent with the former assumption. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43890.

Stabilized polyacrylonitrile (PAN) fibers pretreated under N₂ and air atmospheres were prepared and their thermal behaviors were compared by differential scanning calorimetry and thermogravimetry methods. The results indicated that the subsequent stabilization reaction of PAN pretreated in air was more obvious than that in N₂. In addition, the thermal stability of PAN pretreated in air is better than that in N₂. The structural analysis by Fourier transform infrared spectroscopy and solid state ¹³C nuclear magnetic resonance implied that oxygen promoted dehydrogenation and a compact conjugated structure was formed in PAN. In addition, the CO structures were generated in air and increased gradually with temperature. The contents of oxygen in PAN fibers studied by elemental analysis corresponded with the structural evolution. Further investigation indicated that the CO structures helped dehydration and also promoted formation of the cross-linked structures. A mechanism for structural evolution in PAN during stabilization in air was proposed. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

The thermal behavior and structural evolution during the thermal stabilization of polyacrylonitrile (PAN) fibers in N₂ and air were investigated using differential scanning calorimetry and solid-state ¹³C nuclear magnetic resonance. It was found that an oxidation reaction, that generated carbonyl (CO) groups could occur at 160°C which has not been reported in the literature. It is proposed that the cyclized structures in the PAN macromolecule chains are a prerequisite for the oxidation. Further investigations indicate that with more cyclized structures in the PAN macromolecule chains, the oxidation proceeds more readily, which is consistent with the proposed mechanism. The kinetic parameters for the oxidation and cyclization reactions were estimated using the Kissinger method. The activation energies for the reactions of oxidation and cyclization for PAN fibers are about 96.4 kJ/mol and 190.0 kJ/mol, respectively, which implies that the cyclization is the rate determining step during the thermal stabilization of PAN fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

The aggregation structure of polyacrylonitrile precursor evolves gradually with progress of cyclization. In this work, the variety of cyclization degrees were determined by Fourier transform infrared spectroscopy and the evolution of aggregation structure of PAN fibers were characterized by wide-angle X-ray diffraction. Experimental results showed that the cyclization occurred first in the amorphous parts when the heating temperature was below 200°C. After heated at 200°C for 30 min, molecular chains in the pseudo-crystalline regions started to pack into crystalline regions due to the increasing stress which was produced by cyclization occurred in the amorphous phase, and the crystallinity and crystallite size increased slowly. When the temperature reached to 220°C, pseudo-crystalline regions rearranged obviously under stress, while molecular chains in the crystalline region started to participate in cyclization, and the original crystalline structures were destructed. The two competitive processes induced that the crystallinity and crystallite size grew to the maximum values at 30 min. When the temperature up to 240°C, the cyclization occurred in the crystalline region became more intensely, while the crystallinity and crystallite size decreased out of synchronize. A scheme of evolution of aggregation structure in cyclization was modified based on the above results. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Secondary thermal shrinkage or chemical shrinkage involved in the thermal shrinkage of polyacrylonitrile (PAN) fibers was not only associated with the cyclization degree but also the thermal mobility of molecular chains in the aggregation structures during crosslinking. In this study, the cyclization process was monitored with differential scanning calorimetry and IR spectroscopy. The evolution of aggregation structures throughout cyclization and variations in the secondary shrinkage for the PAN fibers were characterized with wide-angle X-ray diffraction and thermal mechanical analysis, respectively. The results show that with increasing temperature, the cyclization degree increased; the cyclization occurred first in amorphous regions and then extended to the crystalline regions. Correspondingly, the secondary shrinkage also increased and could be separated into two stages: those of the amorphous and crystalline phases. The shrinkage of the crystalline regions was much bigger than that of the amorphous regions. For fibers with different aggregation structures, the crystallinity affected the cyclization degree in the amorphous and crystalline regions and resulted in the difference in total shrinkage. Furthermore, because the unoriented molecular chains in both the amorphous and crystalline regions shrank more after cyclization, the shrinkage of both regions was primarily decided by the level of orientated molecular chains participating in the cyclization. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

The thermal behaviors of polyacrylonitrile precursor during thermal stabilization in inert gas were investigated by differential scanning calorimetry, thermomechanical analysis, and thermogravimetry. Combining these methods with the tracing of chemical changes by Fourier transform infrared spectroscopy indicated that complex reactions, including cyclization and pyrolytic reactions occurred sequentially. An imine-enamine tautomeric structure was formed at around 240°C and was converted to a conjugated structure when the temperature was increased to 400°C. A thermal stabilization mechanism was proposed and confirmed experimentally by using a two-step heating process. The apparent activation energies and the pre-exponential factors for these stabilization reactions were also estimated by the Kissinger, Ozawa, and “Improved Coats-Redfern” methods. To obtain a fit to the experiment data, a new kinetic model, named the “Three Regions Kinetic Model,” was proposed using the Improved Coats-Redfern method. The applicability of this model and the prediction of the stabilization profile at a given heating rate were verified by plotting conversion rate against conversion profiles. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.