Surface Modification of Aramid Fibres with Graphene Oxide for Interface Improvement in Composites

The aramid fibres used in this research were type 248 from Bluestar (China). The aramid fibres were rinsed by acetone to remove surface sizing. Dopamine hydrochloride, epoxy (AY103) and hardener (HY951) were purchased from Sigma-Aldrich® (UK). The Tris(hydoxymethy)aminomethane (Tris) was purchased from Alfa Aesar Company (USA). The Hydrochloric acid solution was purchased from Fluka Analytical (UK) and used as received. Graphene oxide water dispersion (TM-01LI-06) was purchased from Institute of Coal Chemistry, Chinese Academy of Sciences (China).

Dopamine solution (2 mg/mL) was prepared by dissolving dopamine in Tris-HCl buffer solution (1.2 mg/mL) and the pH was controlled as 8.5. When dopamine was self-oxidising, the aramid fibre bundles were immersed in the solution at ambient temperature for 24 h to prepare the PDA-aramid fibre. For the GO-PDA-aramid fibre, after mixing dopamine with Tris-HCl buffer solution, aramid fibre bundles were immersed and 0.1 mg/mL GO solution was added into the mixture. The pH was controlled as 8.5. After 24 h, aramid fibres were taken out of the solution and rinsed by distilled water, and then dried in oven at 60^°C. The schematic of process was shown in the Fig. 1.

The UV-visible spectra of dopamine in solution were recorded on UV-Visible spectrophotometer (M550 Double Beam Scanning UV/Visible spectrophotometer, Camspec Ltd., UK) using H₂O as solvent. The scan range was 200–600 nm. All measurements were performed in quartz cuvettes.

The morphology of untreated and treated aramid fibres were studied by a Scanning Electron Microscopy SEM (Ultra-55 Scanning Electron Microscope, ZEISS, Germany). The aramid fibre samples were sputter-coated with gold before observation.

The chemical composition and functional groups on untreated and treated aramid fibre were characterised by X-ray Photoelectron Spectroscopy XPS (Kratos Axis Ultra Hybrid, Shimadzu, Japan) equipped with a monochromatic source of Al Kα, base pressure below 5 × 10⁻⁸ mbar, spot area of 300 μm × 700 μm. The binding energy peaks were calibrated with C1s at 284.8 eV as reference.

A microbond test was designed as depicted in Fig. 2 for the purpose of quantitatively identifying the interfacial properties between aramid fibres and resin matrix.

The interfacial shear strength (IFSS) between a single fibre and resin is given by Eq. (1):where F is the failure tensile load, d_f is the fibre diameter and L_e is the embedded length in the resin. 50 data were collected and average IFSS was calculated for each sample.

The single fibre tensile test was conducted on Instron 1122 with a 5 N load cell at a constant speed of 1 mm min⁻¹. 10 specimens of each type of fibres were tested.

The polymerisation process of dopamine in solution was detected and confirmed by UV-visible spectroscopy using H₂O as solvent and the results are shown in Fig. 3. From the UV-visible spectrum, an absorption band at 405 nm due to the formation of quinone is clearly shown, which is in agreement with the mechanism of polymerisation. A typical spectrum of polydopamine is obtained in the supernatant after 24 h. The results of UV-visible spectroscopy showed three main absorbance peaks, the sharp absorbance peaks at 265 nm (catechol group), shoulder peaks at 300 nm (dehydro-dopamine), and wide peaks at 405 nm (o-quinone). The intensity of these groups was observed to increase over time, which suggests the formation of various intermediates [8]. The photo in Fig. 3 demonstrates the colour change of the solution (from clear to light brown within 5 min, then dark brown after 24 h) during PDA formation.

The chemical compositions of aramid fibre surfaces were studied by X-ray photoelectron spectroscopy (XPS) and the fitting of C1s core level spectra results are presented in Table 1. For untreated aramid fibre, three peaks were exhibited in Fig. 4a, indicating C=C/C-C in aromatic rings (284.6 eV), C-N group (286.4 eV) and C=O group (288.8 eV), respectively. The PDA treated aramid fibre, three new peaks emerged as shown in Fig. 4b, representing the existence of O-C=O (289.1 eV), C-O (286.5 eV) and π – π* (290.5 eV). These three new peaks demonstrate the successful formation of PDA layer on the surface of aramid fibres. The ratio for oxygen-containing groups increase from 6.35 to 17.50% while the ratio for C=C/C-C group decreases from 78.91 to 55.14% after the PDA treatment. This is mainly attributed to the fact that the intramolecular bonding (hydrogen bonding) in aramid fibre is destructed which activates the originally inert fibre surface. For GO-PDA treated-aramid fibre, its XPS C1s core-level spectrum, shown in Fig. 4c displays six peak components with binding energies at about 284.6, 288.8, 286.3, 285.3, 287.4, and 290.1 eV respectively, associating to the sp²-hybridized carbon, O-C=O, C-O, C-N, C=O and shake-up satellite (π – π*) peak accordingly. Compared with the PDA treated aramid fibre, the ratio for C=O group increases from 3.37 to 4.16%. The increasing number of C=O group indicates potential hydrogen-bonding interaction with GO and PDA. The π – π* bonding also increases from 0.15 to 0.58%. The π – π stacking is a kind of van der Waals force relating to the molecules’ π-electron backbone system, to form a stable hybrid structure [9]. The aramid fibres are consist of aromatic molecules, which are proven effective for intermolecular π – π stacking through self-assembly processing [10, 11]. The strong π – π stacking between the graphene oxide surface and the aromatic molecules usually makes the surface stacking quite stable against rinsing or other solution processing. It became evident that the GO-PDA treatment increased the surface activation of aramid fibre and thus increasing their interfacial adhesion strength.

The surface morphology of aramid fibres before and after the treatment was studied using SEM. It can be seen in Fig. 5a, b that the pristine aramid fibres display a clean and smooth surface. After modification by polydopamine, a homogeneous PDA layer was successfully coated onto the aramid fibre. The PDA-aramid fibres showed an uneven surface with micropits, as shown in Fig. 5c,d. Figure 5e, f indicated that, the roughness of fibre surfaces was further increased by depositing GO onto the PDA coated aramid fibres. The typical thin paper-like wrinkled graphene oxide morphology was observed on the aramid fibre surfaces. The SEM images demonstrated uniform deposition of PDA and GO on aramid fibres, and both enhanced the roughness of the fibre surfaces.

The interfacial shear strength (IFSS) between aramid fibre and resin was investigated by pulling a fibre out of resin droplet through a knife-edge and the results are shown in Fig. 6. Compared with the untreated aramid fibre composites, the IFSS for the PDA-aramid composites increased from 1.9 MPa to 4.2 MPa by, representing a 121% increase, and for the GO-PDA-aramid composites, the IFSS was increased by 210% compared to that of untreated aramid fibre composites as shown in Fig. 6. The IFSS increase can be attributed to the increased roughness of the fibre surface caused by PDA treatment and GO-PDA treatment. In addition, the successful grafting of PDA and GO introduced chemical groups which can also enhance the surface reactivity and polarity. The results revealed that the modification applied to the aramid fibre surface by polydopamine and graphene oxide effectively enhances the interfacial property of aramid fibre/epoxy composites.

Some research literatures reported fibre surface etching when modified chemically or physically, causing decrease in tensile strength [12, 13]. To investigate the tensile strength of unmodified and PDA- and GO-PDA- modified aramid fibres, the single fibre tensile test was conducted. As shown in Fig. 7, the single fibre tensile strength of untreated aramid fibre was 2099.2 MPa. For the aramid fibre with PDA deposition, the tensile strength of single fibre was increased to 2470.9 MPa. Although only a thin layer of PDA was deposited on the fibre surface, the bulk structure of aramid fibre was protected. After adding GO onto the PDA deposited fibre surface the single fibre tensile strength became 2603.4 MPa, which is 24% higher than the unmodified aramid fibre. The reason for this increase is believed to be that the PDA and the PDA-GO coatings may have filled up the flaws on the surface of the untreated fibre. This method modified the fibre surface by coating an ultra-thin layer without etching or damaging the bulk structure of the fibre.