A systematic study of carbon fibre surface grafting via in situ diazonium generation for improved interfacial shear strength in epoxy matrix composites
Introduction
Increasing legislative pressure to reduce environmental emissions has resulted in a significant effort to light-weight transport in both the automotive and aerospace sectors. With their high strength to weight ratios contributing to reduction of structural mass, in this regard, carbon fibre reinforced polymers have already made a notable mark [1]. While carbon fibre is synonymous with high performance materials, microcracking, a known limitation of these materials can arise as a result of composite failure at a weak interface between fibre and resin. One means to obviate this limitation is to chemically modify the surface of carbon fibres to tailor the surface for resin compatibility, which typically results in concomitant increases in fibre/matrix adhesion [2], [3], [4], [5]. Surface treatments have become of intense interest, with a number of reviews published on the topic in the last year alone [6], [7]. Over the past decade many oxidative and non-oxidative techniques have been employed to etch [8], [9], [10], roughen [11], [12], and alter surface chemistry [13], [14] with varying degrees of success. Whilst much of this research demonstrates enormous promise, the remaining challenges are to replicate these results in larger scales and to quantify the degree of surface functionalization required to increase fibre/matrix adhesion and subsequently the interlaminar shear strength of composites [1].
Our effort has been focussed on using the graphitic surface of carbon fibres as a platform to which small molecules can be grafted via the generation of highly electrophilic intermediates (Fig. 1). Several methodologies have been investigated within our group, such as thermal degradation of azides [15], 1,3-azomethine cycloaddition [16], and in situ generation and grafting of organic diazo-species (Fig. 2) [17]. The latter of these techniques has shown the most potential for enhancing fibre-to-matrix adhesion and will be the focus of optimisation studies for this manuscript. The formation of diazonium species from the corresponding aniline (Fig. 2) has been shown to be an effective means of carbon/carbon bond formation in a wide variety of carbon materials including carbon fibre.
However, we have found that reports of carbon fibre surface modification are usually isolated and follow up investigations and optimisation of the procedure rarely reported. Additionally, a common feature in the literature is a lack of post surface treatment fibre characterisation, both physical and chemical, to ensure that the fibre retains the key properties (e.g. modulus, tenacity, etc.) which make carbon fibre desirable for use in advanced materials [18], [19], [20].
Previous studies [17], [21] which have utilised diazonium formation to functionalize carbon fibre have not included follow-up investigations. We seek to amend this by reporting an optimisation and fibre characterisation study in this work. Thus, the aim of this study was to examine a range of concentrations at which to perform carbon fibre surface grafting to determine if this had any major ramifications on the interfacial shear strength. These are very important aspects of surface modification, as being able to manipulate the surface of carbon fibres with minimal reagent while not sacrificing the benefit to interfacial adhesion is a means to make this process more viable on a large scale and/or more cost effective.
Section snippets
Materials
Carbon fibre samples (Panex 35 Unsized/Oxidized tow) were supplied by Zoltek carbon fiber, Hungary and used as supplied. All chemicals, reagents and solvents were purchased from Sigma–Aldrich Chemical Company and used as received.
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Ultra-DLD spectrometer (Kratos Analytical Inc., Manchester, UK) with a monochromated Al Kα source (hν = 1486.6 eV) at a power of 150 W (15 kV × 10 mA), a hemispherical analyser operating in the
Functionalisation of fibres
In our previous work [14], [15], [16], [17], we altered the surface chemistry of unsized carbon fibres by submerging them in a highly concentrated solution of reactants; this was to ensure that enough target molecule (compound 1, Fig. 3) to be attached to the surface was present for successful grafting. Taking this amount (2 g/30 cm tow) as our standard (i.e. 100%) we considered a range of dilutions at which to undertake the surface grafting experiment and correlate these to interfacial shear
Conclusion
We have shown that it is possible to dramatically dilute the reagents used in surface grafting techniques without major losses in the ultimate interfacial shear strength. This advance makes surface grafting larger volumes of carbon fibre possible which can potentially facilitate the fabrication of larger test specimens. With the exception of the highly concentrated (200%) sample the treatments used in this study had no major impact on fibre properties. Our study has concluded that a reagent
Acknowledgements
The authors would like to thank Abdullah Kafi for providing the fibre samples. We gratefully thank Carbon Nexus, Deakin University, CSIRO and the Australian Research Council for funding (DP140100165).
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