Material PropertiesRadial structure and property relationship in the thermal stabilization of PAN precursor fibres
Introduction
The manufacture of carbon fibres from Polyacrylonitrile (PAN) and its copolymers involves initial thermal stabilization followed by high temperature carbonization [1]. The thermal stabilization of PAN fibres is a crucial step in the carbon fibre manufacturing process as the ultimate properties of the carbon fibres are influenced by the structure of the stabilized fibres [2], [3], [4]. The thermal stabilization process in the presence of oxygen is essential to achieve stabilized state for the fibres, as exposure of non-stabilized fibres to low temperature carbonization will result in pyrolysis and filaments fusing together [5]. However, the development of radial heterogeneity (skin-core effect) during thermal stabilization of fibres in air atmosphere persists as a major concern, as the carbonization of stabilized fibres with radial heterogeneity leads to defective carbon fibres [6], [7].
In the literature, various studies have proposed that during stabilization of fibres in air, the diffusion of oxygen from skin to core of the fibres causes the formation of a dense structure in the skin which impedes further diffusion of oxygen to the core of the fibres leading to the development of radial heterogeneity in the stabilized fibres [6], [8], [9], [10]. However, no confirmation of the role of oxygen on defining the structure and properties in skin and core of the fibres. In 2009 Sun et al. [11] mentioned that compared to fibres treated in nitrogen atmosphere the skin-core is only visible in the air treated samples however the conclusions of these studies were drawn based on optical microscopic analysis, which, while useful, is unable to provide detailed functional group identity and physical property variation details along the cross-section of the fibres. Lv et al. [12] have performed transmission electron microscopy (TEM) analysis on the fibre cross-section and concluded that the structure of the skin region is compact when compared to core region of the fibres. However, quantified data on the variation of radial properties of the fibres was not provided. Nonetheless, it is very difficult to get useful information on the microstructure of stabilized fibres using TEM [13]. Recently, Liu et al. [14] used nano DMA and showed that the storage modulus of the skin is higher than the core regions of the fibres. They have concluded that the difference in storage modulus of skin and core regions of the fibres improved however reduced in spatial scale during the transformation of stabilized fibres to carbon fibres. To the best of our knowledge, there are limited studies on the details of modulus distribution along the cross-section of the fibres during the stabilization process. Moreover, no clear evidence was found in the literature that highlights the role of oxygen in the evolution of radial structure and property variations in the fibres during thermal stabilization of PAN fibres.
Raman spectroscopy is one of the characterization techniques that was extensively used to understand the relative variation of sp3 (D-peak) to sp2 (G-peak) bonded carbon atoms in carbon based materials [15], [16], [17], [18]. Interestingly, D and G peaks can also exist in carbon materials which do not possess ordered or graphite like structures and G-peak can arise from any sp2 sites or C=C either in chain or aromatic ring structures [19]. Since Raman spectroscopy is sensitive to sp3 and sp2 hybridized carbon atoms in polymer structures we believe that it can be applied on the fibre cross-sections to understand the evolution of radial structures during the thermal stabilization process. However, up to our knowledge, apart from the study that was published recently [6], this technique has never been widely used to understand these conversions in the cross-section of the stabilized fibres. Nano indentation is another technique that provides information on the modulus distribution in the fibre cross-sections. The combination of these two techniques will provide evidence on the influence of treatment atmosphere on structure and property variations in the fibre cross-sections during the thermal stabilization process.
Hence, we have conducted a comparative study on the samples treated at various temperatures in air and vacuum to understand the influence of oxygen on the development of radial heterogeneity, variation of radial modulus and structure of the fibres. The evolution of radial heterogeneity was assessed by optical microscopy and differences in radial modulus of skin and core of the fibres was obtained by nano-indentation. This was further complemented with the variation in the structure from the skin to core of the fibres using a combination of Raman spectroscopy and FTIR techniques. Moreover, the influence of heterogeneous modulus distribution in the fibre cross-section on the fracture morphology of the stabilized fibres was analysed.
Section snippets
Sample preparation
The precursor fibre used for this study was obtained from Blue Star co Ltd, China. The precursor consists of polymerised acrylonitrile 93%, methyl acrylate 6% and itaconic acid 1% [6], [7]. The thermal behaviour of the precursor was initially examined using Differential Scanning Caloriemetry (DSC). A 5 mg fibre sample was enclosed in an aluminium pan and heated from 30 to 450 °C in air atmosphere at a heating rate of 10 °C/min. According to DSC exothermic curve shown in Fig. 1, there are two
Effect of atmospheric oxygen on the development of radial heterogeneity
The fibre samples treated in air and vacuum atmospheres at various temperatures were initially examined for the existence of radial heterogeneity using optical microscopy. The cross-sections of the fibre samples are shown in Fig. 2. The radial heterogeneity was visible in the fibres treated in air from 240 °C onwards, conversely it was not observed in the fibres treated in vacuum. Moreover, morphological changes in the cross-sections of the vacuum treated samples was visible as shown in Fig. 2
Conclusion
The influence of oxygen on the development of radial heterogeneity during thermal stabilization and its effects on the radial structure and modulus of the fibres were studied. The atmospheric presence of oxygen during fibre stabilization plays a critical role in the formation of radial heterogeneity in PAN fibres. In air treated samples, nano-indentation clearly showed heterogeneous modulus distribution in the skin and core regions, with the skin of the fibres possessing a higher modulus
Acknowledgements
We would like to acknowledge the advanced characterization facility in Deakin University for their kind support and operational team of Carbon Nexus carbon fibre manufacturing facility for providing precursor fibre samples. We also thank federal and state governments of Australia for providing the funds to conduct this research, including the Australian Research Council Discovery Programme (DP 14 0100165). We would like to thank Mr. Srikanth Mateti for his kind support for the sample
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