Improving the effects of plasma polymerization on carbon fiber using a surface modification pretreatment

https://doi.org/10.1016/j.compositesa.2021.106319Get rights and content

Abstract

Plasma and electrochemical treatments of carbon fibers for enhanced properties are often presented in opposition to each other. This work demonstrates the combination of these methodologies through the electrochemical attachment of nitroaryl moieties to the surface of the carbon fiber, prior to the deposition of plasma polymerized acrylic acid to the surface. Notably, the tensile strength of fibers having undergone both surface modification and plasma polymerization showed a significant increase (3.76 ± 0.08 GPa), relative to control fibers (3.31 ± 0.11 GPa), while plasma polymerization alone showed no change (3.39 ± 0.09 GPa). Additional benefits resulting from both treatments were observed when determining the fiber-to-matrix adhesion. Plasma polymerization of acrylic acid alone returned a 49% increase in interfacial shear strength (IFSS) compared to control (28.3 ± 1.2 MPa vs 18.9 ± 1.2 MPa, respectively). While the presence of nitrophenyl groups on the fiber prior to polymerization conferred an additional 24% improvement over plasma polymerization alone and a 73% improvement relative to control fibers (32.7 ± 0.5 MPa vs 18.9 ± 1.2 MPa, respectively). Finally, we present the first comparison of scanning electron microscopy (SEM) and helium ion microscopy (HIM) to visualize polymers on the carbon fiber surface. HIM shows a clear advantage over conventional SEM in visualizing non-conductive coatings on carbon fibers. Analysis of the samples by X-ray photoelectron spectroscopy (XPS) confirmed the desired chemistry had been imparted onto the surface, consistent with the plasma-polymerized acrylic acid coating and presence of nitro-aryl moieties.

Introduction

Improving the interfacial phenomena between dissimilar materials is a widely desired outcome across myriad disciplines from energy storage, nanocomposites, electronics, through to large-scale adhesives and rebar reinforced concrete [1], [2], [3]. Manipulation or modification of each material surface is one approach to address this problem. Treatment with plasma is common on carbonaceous surfaces including grapheme [4], [5], [6] and carbon nanotubes [7], and is routinely carried out on carbon fibers [8], [9]. Carbon fiber reinforced plastics (CFRPs) have great merit as structural components due to their excellent strength-to-weight ratio, although this is largely dictated by the interactions occurring at the interface [10]. Of the many routes to increase the polarity, and thus polymer ‘wet out’, plasma oxidation is one of the most commonly investigated methods, as it is both rapid and applicable on a large scale. Indeed, plasma oxidation of carbon fibers has been shown to improve composite strength for both pitch-based and PAN-derived carbon fibers in a polycarbonate matrix [11], and gains of ~40 MPa in interlaminar shear strength (ILSS) of an epoxy composite have been reported [12]. Although the exposure time and intensity of plasma treatment used on the fibers is often highly controlled, excessive oxidation can come at the expense of the inherent strength of carbon fibers.

Plasma treatment of fibers in N2-H2 or N2-H2-Ar atmospheres have been shown to improve the properties of the fibers themselves, as well as the ILSS and flexural strength of the resulting epoxy composites [13]. Oxygen plasma has been shown to improve the wear properties in a carbon fiber–epoxy composite, and increased interfacial adhesion [14] and improvements in interfacial shear strength (IFSS) have been reported due to increased graphitic structures following plasma oxidation [15]. Furthermore, plasma-enhanced chemical vapor deposition has improved IFSS by ~119% in a carbon fiber-epoxy composite through the vertical growth of graphene from the fiber surface [16]. As mentioned above, the mechanism by which plasma oxidation promotes adhesion can be attributed to the increasing surface polarity, due to the introduction of functional groups such as COOH, PhOH, and CO. This is usually combined with justifications such as increased mechanical interaction, via nano-texturing the surface, and removal of loosely bound surface contaminants [17].

The use of plasma to deposit polymers on surfaces has also be investigated. For example, the plasma polymerization of acrylic acid onto carbon fiber was demonstrated by Kettle et al. and resulted in a 53% increase in IFSS, relative to uncoated fibers [18]. This coating was the best performing monomer tested, likely due to the miscibility of the poly(acrylic acid) in the epoxy, and promotion of effects such as hydrogen bonding and ionic interactions with amine-derived hardeners.

Non-carbonaceous surfaces including polyester and polyamide fabrics have been modified by plasma-polymerized acrylic acid, and demonstrated improved wettability and increased hydrophilicity, with no detriment in breaking strength [19]. The mechanism of plasma polymerization to graft a polymer to a surface consists of several steps; the first is the generation of free radicals (Fig. 1, (a)) which then initiate the polymerization with the monomer. Also, any homolytic cleavage of the monomeric α,β-double bond can also be captured on the surface using these reactive radical species. This goes on to propagate the formation of the polymer, finally leading to a coating, in this instance, of poly(acrylic acid). A potential issue with this approach when treating carbon fibers is that the generation of radicals on the graphitic surface have, in effect, an infinite number of resonance structures, significantly reducing their reactivity. In addition, it is well known that the surface of carbon fibers is not all graphitic, and the presence of defects can potentially quench the radical species, negating polymerization. We therefore proposed that priming the carbon fiber surface with reactive aromatic rings should promote radical localization on these installed groups (Fig. 1, (b)), thus promoting in situ polymerization.

As an additional benefit, the surface modification pre-treatment used in this work also provides a plethora of potential anchoring points to covalently attach the polymer to the fiber. This latter point we have shown in numerous studies to be a critical factor in improving the IFSS in composites [20], [21], [22], [23], [24]. Effective transfer of stress from the matrix to the reinforcing fiber is a property consistently sought after in both academic and industrial settings. This has been elaborated by the use of metal-organic-frameworks (MOFs) to create an ‘armor’ on the fibre surface, significantly increasing tensile strength [25]. Mussel inspired nanoparticles by co-polymerization of polydopamine and poly(amidoamine) in varying ratios were attached to a carbon fiber surface by Zhang et al. [26] without compromising the fibers innate tensile strength and significantly increasing interfacial shear strength. Similarly, the use of anionic polymerization of glycidol, generating carbon fibers modified with polyglycerol as a means to enhance interfacial adhesion has also been reported [27]. That work showed the ability to modulate the size of the grafted polymer, which serves as an easy means to tune the interface and tailor composite properties to specific applications.

The use of electrochemical techniques has also shown great promise in increasing carbon fiber performance. Electrochemical oxidation of carbon fibers has been reported to increase the surface oxygen content and improve interfacial interactions within a composite [28], [29], [30], [31], [32]. This has been achieved with a variety of electrolytes with varying success, though many carbon fiber manufacturers have adopted this into their inline processes. Indeed, plasma has also been reported to stabilize PAN fibers, prior to carbonization in an effort to improve processing time [33], but this is yet to be adopted on an industrial scale by manufacturers. The carbon fiber surface has also been manipulated through electrochemical grafting of specifically designed compounds with great success [20], [22], [23], [34], [35], [36], [37], [38].

Treatments by plasma or electrochemical means are often presented in opposition, with many comparative studies concerning carbon surfaces [39]. However, the effect of cold plasma treatment has been shown to increase pore number and surface oxygen concentration on carbon fiber cloth activated by anodic polarization prior to exposure to plasma [40]. That outcome suggests that the effects of plasma treatment could potentially be enhanced by electrochemical pre-treatment. Given the demonstrated advantages of plasma treatments on energy storage [41], [42], [43], improvements in the efficacy of these techniques is pertinent to a range of conductive materials.

Therefore, the focus of this work was to investigate if the use of surface modifications, routinely used within our research group, via the irreversible reduction of aryldiazonium salts, could be used to enhance plasma polymerization processes.

Previously, acrylic acid has been electrochemically polymerized on carbon fiber resulting in greatly increased physical properties of the fibers themselves, as well as the IFSS of the fiber in an epoxy matrix (>300%) [21]. This treatment required the reduction of a diazonium salt (4-nitrobenzenediazonium tetrafluoroborate) to initiate the polymerization and provide a point of attachment for the polymer, generated in solution rather than gas phase. Following treatment, the fibers were imbued with a blue hue and able to maintain shape under load, without any supporting matrix or being put under tension.

The plasma polymerization of acrylic acid on the surface of carbon fiber has been shown to improve composite performance, although this increase is much more pronounced when polyacrylic acid films are electrochemically grown. This is likely due to the covalent nature of the latter, improving matrix-to-fiber stress transfer. However, electrochemically modifying the surface with anchor points may enrich the effects of plasma polymerization (Fig. 1). Herein, we report the electrochemical grafting of nitro-aryl groups to the carbon fiber surface, prior to plasma polymerization of acrylic acid and its effect on composite performance.

Section snippets

Materials and methods

For all these experiments, carbon fibres manufacture at Carbon Nexus, Deakin University, Australia were used. These fibres were sourced before surface treatment and sizing (i.e. directly from HT furnace) and thus no desizing process was required before our treatments were employed.

For greater detail regarding the data collection and analysis for electrochemical treatment, fiber mechanical properties and single fiber fragment test (SFFT), please refer to the Supporting Information (SI). Briefly,

Results and discussion

The electrochemical attachment of nitro-aryl moieties to the surface of pristine, unsized carbon fiber was achieved through the proximal irreversible reduction of 4-nitrobenzenediazonium tetrafluoroborate. This reaction expels nitrogen gas and results in the covalent bonding of the corresponding aryl radical to the surface of the fiber (Fig. 2). Due to the highly reactive aryl radical and the lack of any steric impingement, multiple aryl rings may attach to those already surface bound,

Conclusions

In this study, we have demonstrated advantageous outcomes in regard to composite strength of carbon fibers electrochemically decorated with nitro-aryl rings, prior to the plasma polymerization of acrylic acid onto the surface. These samples (E-ppAA) were visually different to the sample exposed to plasma polymerization of acrylic acid alone (ppAA), exhibiting a wider spectrum of color to the naked eye, which was confirmed by reflectance spectroscopy. HIM revealed a clear ‘sheathing’ effect for

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding sources

The authors gratefully acknowledge Deakin University for a scholarship for LS. This research was conducted with support from the Australian Government via the Australian Research Council World Class Future Fiber Industry Transformation Research Hub (IH140100018), the ARC Training Centre for Lightweight Automotive Structures (IC160100032) and Discovery projects (DP140100165, DP180100094). This work was also partially supported by the Office of Naval Research (N62909-18-1-2024).

CRediT authorship contribution statement

Daniel J. Eyckens: Conceptualization, Investigation, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration. Karyn Jarvis: Investigation, Writing - review & editing. Anders J. Barlow: Investigation, Writing - review & editing. Yanting Yin: Investigation, Writing - review & editing. Lachlan C. Soulsby: Investigation, Writing - review & editing. Y. Athulya Wickramasingha: Investigation, Writing - review & editing. Filip Stojcevski: Investigation,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was performed in part at the Institute for Frontier Materials (IFM) at Deakin University, the Biointerface Engineering Hub at Swinburne University of Technology, and the Materials Characterisation and Fabrication Platform (MCFP) at the University of Melbourne, all part of the Victorian node of the Australian National Fabrication Facility (ANFF): a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and microfabrication facilities for

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