Formation Mechanism of Skin-Core Chemical Structure within Stabilized Polyacrylonitrile Monofilaments
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Although it has been half a century since polyacrylonitrile (PAN)-based carbon fibers were first developed, the exact formation mechanism of skin-core structure of PAN-based carbon fibers, especially the stabilized PAN fibers, was still not well clarified from the viewpoint of the chemical structure. In order to address this aforementioned challenge, a powerful tool with nanoscale resolution named photo-induced force microscopy was applied to map the chemical group distribution in the cross section of stabilized PAN fibers and reveal the evolution mechanism of skin-core structure throughout the whole stabilization process. The results indicated that the formation of skin-core structure of stabilized PAN fiber was attributed to the complex and overlapped chemical reactions caused by gradient of oxygen along radial direction and the formation of dense crystal layer at the interface between the skin and core part. Finally, the crystal layer was destroyed and the monofilaments tended to be homogeneous with further oxidation.
KeywordsSkin-core structure Stabilized PAN fiber Photo-induced force microscopy
Atomic force microscopy
Photo-induced force microscopy
PAN-based carbon fiber (CF) is a frontier material with high tensile strength and Young’s modulus, as well as excellent heat resistance. Due to its superior properties, it has been applied broadly as the reinforced structural material in aviation, aerospace, and other new industrial fields [1, 2, 3]. Currently, the strongest commercially available carbon fibers possess tensile strength of ~ 7 GPa. However, based on –C–C bond strength calculations with ideal graphite model, the theoretical strength of carbon fibers is around 180 GPa . The enormous gap between real and theoretical tensile strength is mainly attributed to the heterogeneous skin-core structure of carbon fiber. This structural heterogeneity results in uneven stress distribution within carbon fiber monofilament. Destruction tends to happen in the area, which suffers higher stress, thus leading to the breakage of carbon fiber [5, 6, 7]. Hence, it is of great significance to figure out the formation mechanism of this structural defect and minimize its effect on the properties of the resultant carbon fibers.
The manufacture of carbon fibers involves three steps including spinning of PAN precursors, thermal stabilization, and carbonization. Among these, thermal stabilization is the most complex step which involves reactions such as cyclization, dehydrogenation, and oxidation. Cyclization reaction leads to the generation of cyclized structures and the conversion from –C≡N to –C=N. Dehydrogenation reaction is associated with the formation of –C=C. Carbonyl groups are introduced after the precursor fibers undergo oxidation reaction [2, 8]. The stabilization process contributes to the transformation from linear PAN chains to infusible and heat-resistance ladder structure, which is necessary for the carbonization process [9, 10, 11]. The preparation of PAN-based carbon fiber is a continuous process, to put it another way, and the final heterogeneous skin-core structure in carbon fiber is mainly inherited from the stabilized PAN fiber. Therefore, revealing the formation mechanism of skin-core structure of stabilized PAN monofilament especially the chemical structural distribution is beneficial to minimize the structural heterogeneity within carbon fiber.
There have been numerous studies focused on the stabilization of PAN fibers. However, the investigations about skin-core structure of stabilized PAN fibers are very limited. Lv et al.  reported that the heterogeneous oxygen diffusion from skin to core results in the formation of a dense skin region, which retards further diffusion of oxygen and leads to the formation of skin-core structure. Nunna et al.  used Raman spectroscopy and elemental analysis to reveal skin-core structure of stabilized fibers. These elegant works have contributed greatly to the study of skin-core structure of the stabilized PAN fibers. However, they mainly focus on the radial mechanical property of stabilization PAN fibers rather than chemical structure, the detailed structural information is still not very clear. Hence, the equipment with high spatial resolution is necessary for the study of the skin-core chemical structure of stabilized PAN fibers at different stages of stabilization process.
Samples from different stages of stabilization under different ambient temperatures were collected. The PAN fibers used in this study are the 6 K precursor fibers of HENGSHEN T700 (HENGSHEN Co. Jiangsu, CHINA). The precursor fibers were continuously passing through five ovens with gradually increasing temperatures (210 °C, 220 °C, 230 °C, 240 °C, 250 °C). The samples were denoted to 01–05 sequentially. Stabilization time in each oven was 8 min, and the running speed of the tow was 30 m/h.
The procedure for preparing samples for PiFM measurements is as follows: Firstly, a fiber tow is attached straightly on the bottom of the model to ensure that the fiber axis is parallel and close to the epoxy block surface and then embedded in epoxy resin. To acquire the transverse section, the surface perpendicular to the fiber axis was mechanically grinded and polished by a polishing machine (Struers Inc.).
PiFM (Molcular Vista, USA) measurements were performed to investigate the changes of functional groups in different radial positions of the monofilament during the stabilization and were operated in non-contact to prevent the softest samples from damage and achieve higher spatial resolution than AFM topography.
Raman spectroscopy was performed with a × 100 objective by using the 532-nm laser of confocal Raman spectroscopy (RM2000, Renishaw, UK).
Results and Discussions
On the other hand, although the overall intensity at 1730 cm−1 showed almost no increase until sample 04, obvious skin-core difference was observed in samples 02 and 03. This is because the PAN was obtained by the copolymerization of acrylonitrile and itaconic acid which contains carbonyl group. At the initial stage, dehydrogenation reaction tended to happen in the skin part, so carbonyl group was eliminated in the form of H2O. Therefore, the core part has a higher concentration of carbonyl group. With the further stabilization, the higher temperature and the improved uniformity of oxygen content along radial direction promoted oxidation in the skin and dehydrogenation in the core simultaneously in samples 04 and 05. The oxidation not only involved the formation of –C=O bonds, but also enhanced the dehydrogenation by eliminating hydrogen in the form of H2O . As shown in Fig. 3, it is clear to observe that the conjugated and the oxidized structures tend to be homogenous in the samples 04 and 05 in terms of the absorbance intensities at 1600 and 1730 cm−1.
The ratio of I–C=O/I–C=N/−C=C from samples 01–03
I1730 cm−1/I1600 cm−1
This study shows that the skin-core structure of stabilized PAN fibers initially formed by cyclization occurred in a core region while the skin part underwent the oxygen-driven dehydrogenation domain. Then, with the higher degree of oxidation, the filaments could tend to be homogenous.
Financial support from the Beforehand Research Foundation Key program (6140922010102) and Joint Fund of Ministry of Education for Equipment Pre-research (BHJG2018007) is gratefully acknowledged.
Availability of Data and Materials
All data generated or analyzed during this study are included in this manuscript.
SY designed and engineered the samples. SY and LY performed the experiments. SY and LW conceived the formation mechanism of the skin-core chemical structure within monofilament. SY and LW wrote the article with support from CWY. All authors contributed to the general discussion. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- 14.Nowak D et al (2016) Nanoscale chemical imaging by photo-induced force microscopy. Sci Adv. https://doi.org/10.1126/sciadv.1501571
- 15.Oin Q et al (2008) Mechanism and kinetics of the stabilization reactions of itaconic acid-modified polyacrylonitrile. Polym Degrad Stabil 53:1415–1421Google Scholar
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