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Carbon fibers were surface modified with acrylate-derived polymers with aromatic side chains, to protect the fiber when exposed to high temperatures. The surface modification process induced a significant increase in tensile strength (23.7%) and tensile modulus (8%), for the benzyl-bearing side chain and retained superior tensile strength (20%) and tensile modulus (7%) after heating to 600 °C. Commercial carbon fibres gave a significant decrease in tensile strength and modulus, 7% and 4%, respectively, when exposed to the same conditions. This suggests that the surface modification process provides a protective effect against thermal degradation, with possible application in carbon fibre recycling. The interfacial shear strength (IFSS) showed significant improvement before (up to 208%) and after (up to 84%) exposure to high temperatures. Analysis of the carbon fiber surface by XPS suggests that the surface bound polymer becomes more graphitic, potentially via the fusion of the aromatic side chains.

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.

The polymerisation and attachment of a covalent sizing to the surface of carbon fibres is demonstrated using an electrochemical surface treatment. This treatment proceeds in an aqueous environment at a constant potential of −1 V in 2 min, and thus is within the window of current industrial surface treatments (2–4 min). This treatment demonstrates the ability to significantly increase the tensile strength (up to 25%), Young’s modulus (up to 15%) and interfacial shear strength (IFSS, up to 199%) in an epoxy resin. Optimisation of this process gave similar improvements in carbon fibre properties and IFSS at lower monomer concentrations (0.2 M and 0.5 M vs 1.0 M). This treatment is demonstrated on both bundles of fibres and woven fabrics and the woven fabrics are able to be reversibly moulded into different shapes by simply wetting the surface with an appropriate solvent and allowing to dry.