Effect of the aggregation structure on the thermal shrinkage of polyacrylonitrile fibers during the heat‐treatment process

Abstract

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. V C 2012Wiley Periodicals, Inc. J Appl Polym Sci 125: [3545][3546][3547][3548][3549][3550][3551] 2012

Growth mechanism of bioglass nanoparticles in polyacrylonitrile-based carbon nanofibers

“…5, the majority of BG nanoparticles had migrated to the outside of nanobers and only a few of particles stayed inside the nano-bers. 35,36 The densication was determined by both the rate of denitrogenation and the rearrangement of carbon atoms, which were proportional to sintering temperature. The BG nanoparticles of CNF/BG-1200 presented cuboid-like geometry at sintering time points of 1 h and 2 h (Fig.…”

“…During carbonization, the microstructure of PAN-based carbon ber transformed from two-dimensional disordered graphite structure to three-dimensional laminar structure with loss of noncarbon elements, leading to structure densication at the same time. 35,36 The densication was determined by both the rate of denitrogenation and the rearrangement of carbon atoms, which were proportional to sintering temperature. The denser carbon network might create stronger expelling force to "squeeze out" BG nanoparticles due to their chemical incompatibility.…”

Effect of Polyacrylonitrile Precursor Orientation on the Structures and Properties of Thermally Stabilized Carbon Fiber

“…Considering the change in the orientation structure, it was considered that, on the one hand, the change in the orientation structure was affected by the mobility of the molecular segments. The aggregation structure could affect the mobility of the PAN molecular chain, and then affect the cyclization activity of the cyano group on the side of the PAN molecular chain [26]. On the other hand, the thermal stabilization reaction affected the evolution of the fiber orientation structure and the aggregation structure [27,28].…”

Influence of curing temperature on properties of the polyacrylonitrile/polyimide composite films

“…Comparing to the composite film cured at 350°C, the macropores of the composite film cured at 400°C increase and the size of the macropore becomes larger. These defects are most likely caused for the free shrinkage behavior of PAN during the process of cyclization and cross‐linking reaction …”

“…These defects are most likely caused for the free shrinkage behavior of PAN during the process of cyclization and cross-linking reaction. [45][46][47][48] Dielectric Results A complex dielectric permittivity consists of a real part and an imaginary part. The real part of the complex permittivity is often referred to as the dielectric constant (e 0 ) and the imaginary part is referred to as the dielectric loss (e 00 ).…”