Elsevier

Polymer

Volume 55, Issue 16, 5 August 2014, Pages 4270-4280
Polymer

In-situ microstructural changes of polyacrylonitrile based fibers with stretching deformation

https://doi.org/10.1016/j.polymer.2014.06.073Get rights and content

Highlights

  • Crystalline phase, transition layer and amorphous matrix coexist in the PANFs.

  • There is partial structure transmissibility from PANFs, to SFs, then to CFs.

  • Transition layer, ladder and turbostratic structures are relatively rigid.

  • We obtain the microstructural changes of PANFs, SFs, and CFs with tensile strains.

  • A model is proposed to describe the microstructural changes with fiber strains.

Abstract

In-situ wide-angle X-ray scattering technique was used to investigate the microstructural changes of polyacrylonitrile (PAN) based fibers with the macroscopic fiber strain. Crystalline phase, transition layer, and amorphous matrix were found coexistence in the PANFs. Tensile deformation induced the PAN-chain transformation from amorphous matrix to crystalline region through the transition layer. After the PANFs were oxidized into stabilized fibers (SFs), the SFs inherited the amorphous and crystalline phases of the PANFs, but the transition layer disappeared and a cyclized ladder structure was formed. Partial PAN chains were anchored to the relatively rigid ladder structure, leading PAN chains folding with tensile deformation. After the SFs were further carbonated into carbon fibers (CFs), two rigid turbostratic structures were formed in the CFs. A new structure model was proposed to describe the microscopic structure changes with macroscopic fiber strains as well as the structural development from PANFs to SFs then to CFs.

Introduction

Carbon fiber (CF) is an important material because of its outstanding mechanical and thermal properties. Its low mass density, high strength, low thermal expansion coefficient and high thermal conductivity make it be widely applied to aeronautic and space industry. It is well known that the unique performance of carbon fibers (CFs) is tightly related to the interior microstructure of CFs. However, the preparation of CFs needs to experience a series of technical processes. The final microstructures of CFs must be affected by the early technical processes. Therefore, comparing the structural features in different preparation stages of CFs is very important to understand their unique performance.

In preparation of CFs, there are three typical stages that are, respectively, the initial wire-drawing process, the subsequent stabilization process, and the final carbonization process. In the initial wire-drawing process, the protofilaments are obtained. Similarly, in the stabilization process as well as the final carbonation process, the stabilized (or oxidized) fibers and the final CFs are obtained, respectively. Polyacrylonitrile (PAN) is widely used as the precursor in the preparation of CFs. A number of researches on the structure of PAN fibers (PANFs) have been performed by using X-ray diffraction (XRD) technique. Based on the XRD pattern, the information of the intramolecular and intermolecular structures of PANFs was reported [1], [2], [3], [4]. Two distinct regions, i.e., the amorphous (disordered) and the partially ordered regions, were found existing in the PAN protofilaments. The PAN molecular chains were also reported being aligned in parallel with the fiber axis in the ordered region, but being entangled together in the amorphous region [5], [6], [7].

When PAN protofilaments were stabilized into oxidized fibers, the fibrous PAN molecular chains were found to transform partially into a cyclized ladder structure. X-ray diffraction pattern of the stabilized fibers (SFs) showed two pairs of reflection arcs. The inner arcs were attributed to the reflection of the residual PAN molecular chains, and the outer arcs were ascribed to the reflection of the ladder polymer. Therefore, both residual PAN molecular chains and cyclized ladder polymer were thought coexisting in the SFs [8], [9], [10].

As for the final CFs, ribbon-like crystallites constructed by turbostratic honeycomb graphene planes [11], [12], [13] were considered as their basic structural units. These ribbons were thought having thickness of some atomic layers and presented a certain inclined angle towards the long axis of the fiber. It was these ribbons that formed the wrinkled and interlinked crystallites. At the same time, these crystallites were separated by needle-like voids.

Despite lots of researches focus on the microstructures of these fibers, the microstructure changes with macroscopic tensile deformation are still unclear. The correlation between microstructures and mechanical behaviors of these fibers has not been understood yet. In this study, in-situ wide-angle X-ray scattering (WAXS) technique is used to characterize the structural changes of PANFs, SFs and CFs induced by a stretching deformation. Based on the information of structural changes, the microscopic deformation processes of PANFs, SFs and CFs are expected to be better understood. By comparing the structural features among the PANFs, SFs and CFs, we expect to have a clearer cognition for the structural evolution from PANFs, to SFs, then to CFs, as well as the structural influence on the mechanical behavior of the final CFs.

Section snippets

Experimental

PANFs, SFs and CFs were prepared at the Institute of Coal Chemistry, Chinese Academy of Sciences. Synchrotron radiation WAXS measurements were performed at beamline 1W2A of Beijing Synchrotron Radiation Facility (BSRF). A Mar CCD165 detector was used to record the 2D-WAXS patterns. The sample-to-detector distance was set to 59 mm. The incident X-ray wavelength was fixed at 1.54 Å. During the WAXS measurements, the axial direction (or the meridional direction) of the fibers was held in

Data analysis

2D scattering background without sample was first subtracted from each of the 2D-WAXS patterns. As an example, the background-removal WAXS patterns of the PANFs, SFs and CFs without stretching load are, respectively, shown in Fig. 2(a), (b) and (c). From Fig. 2, it can be found that two discrete and sharp diffraction rings always appear on the WAXS patterns of all the three fibers, and their positions have no change with the stretching load. A careful check confirms that these two discrete

PANFs

Previous studies [19], [20] demonstrate that PANFs have an orthorhombic crystalline structure with space group Cc2m. On the basis of Cc2m symmetry, all the reflections from the PANFs have been indexed as shown in Fig. 2(a). We notice that the distribution of the diffraction spots on the WAXS patterns are almost unchangeable with the increase of the applied stretching load, which means that there is no new phase appearance during the tensile deformation of the PANFs. But the intensity

Conclusions

In summary, the results can be concluded as follows:

  • (1).

    The PANFs are a three-phase system. Crystalline PAN regions surrounded by a transition layer are embedded in the amorphous matrix. The crystallite sizes of crystalline regions are decreasing in the equatorial plane, but increasing along the meridional direction of the PANFs with the increment of PANFs strain. However, the size of the transition layer is almost unchanged.

  • (2).

    There is partial structure transmissibility as the PANFs are oxidized into

Acknowledgments

This work was supported by the National Natural Science Foundation (Nos. U1232203, 10835008) of China.

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