Due to their unique structure and excellent mechanical, electrical, and other physical properties, carbon nanotubes (CNTs) have shown great potential in enhancing polymers, improving interlayer fracture properties of laminated composite materials, and enriching the functionality of composite materials in recent years. Introducing pure resin film or reducing resin viscosity is a simple and intuitive method to enhance the infiltration of resin in CNT films. However, the interlayer fracture toughness of composite materials has not been improved, which may be due to poor adhesion between CNTs and resin materials. Therefore, it is necessary to perform appropriate modification treatment on CNT thin films to enhance the adhesion between CNTs and resin, thereby improving the interfacial bonding strength between the two. At present, the commonly used methods for modifying CNTs include physical and chemical methods, with chemical methods being more effective in enhancing the properties of composite materials. For example, reactive groups (such as amino, hydroxyl, carboxyl, etc.) can be grafted onto the surface of CNTs using strong acids or oxidants to establish covalent chemical bonds between CNTs and resin; Alternatively, high temperature, high pressure, electron radiation, and other methods can be used to directly induce crosslinking between CNTs. These methods often require complex processes and extreme reaction conditions, which not only limit the size of CNT aggregates but also cause damage to the structure of CNTs and their aggregates. Low temperature plasma treatment technology has attracted much attention due to its unique advantages: firstly, this technology has a wide range of applicability and can treat the surfaces of the vast majority of materials; Secondly, its working temperature is moderate, which can effectively avoid thermal damage to materials caused by high temperatures; Again, the processing adopts a dry process, which does not require the use of solvents and is more environmentally friendly.

Low temperature plasma not only has physical properties but also chemical activity. Chemical activity is determined by the inelastic collisions in its microstructure, ultimately the components of atomic physics, including the excitation, ionization, and dissociation of molecules and atoms. However, the core of its chemical activity is the products of these inelastic collisions - active groups (high-energy electrons, metastable atoms, neutral groups, photons), which are extremely unstable and easily form new substances. Taking oxygen plasma as an example, oxygen molecules undergo ionization, excitation, and dissociation under the inelastic collision of electrons. Oxygen ions (O2+), excited oxygen molecules (O2 *), and free oxygen atoms (O) are generated, all of which are highly chemically active groups.

In addition, oxygen ions and oxygen atoms can react with water molecules to generate hydroxide ions and hydroxyl radicals. These active functional groups can affect the carbon carbon double bonds in CNTs through oxidation, and through a series of complex chemical reactions, introduce oxygen-containing functional groups (hydroxyl, carboxyl, carbonyl, etc.) on the surface of CNT films, as shown in Figure 1, thereby changing the chemical properties and structure of the CNT film surface.

Figure 1 (a) Schematic diagram of the mechanism of oxygen plasma modified CNTs thin film, (b) Schematic diagram of the interface bonding mechanism between CNTs and resin enhanced by oxygen-containing groups

The Effect of Plasma Treatment on the Structure and Properties of CNTs Thin Films

Surface morphology changes of CNTs thin films

Figure 2 shows the surface morphology of CNTs thin films treated with different plasma powers. In the original CNT film, multi walled carbon nanotubes are stacked in an irregular pattern, with amorphous carbon layer deposited on the surface of CNTs. Iron catalyst impurities (such as white particles in Figure 2) remain in the CNT film, and the network porosity is high. Under low discharge power conditions, there was no significant change in the CNT film, but as the plasma discharge power further increased, the amorphous carbon layer on the surface of the CNT film gradually disappeared, and the pore size slightly increased. However, when the power reached 100W, the edge of the CNT beam on the surface of the CNT film became blurred, and some areas appeared to be adhered. In addition, as the plasma power increases, the amount of residual iron catalyst in the CNTs film gradually decreases and impurities are removed.

Figure 2 SEM images of surface morphology of CNTs thin films under different plasma power treatment conditions

Surface Chemical Composition of CNTs Thin Films

Use X-ray photoelectron spectroscopy (XPS) to analyze the chemical composition of the surface of CNTs thin films before and after plasma treatment. The XPS full scan spectra of CNTs thin films treated with different powers are shown in Figure 3. From the figure, it can be seen that the surface of CNTs thin film mainly contains two elements: carbon (C) and oxygen (O). The C1s peak is located at 284.8eV, and the O1s peak is located at 532.5eV. The difference is that in the full spectrum, the peak heights of C1s and O1s peaks have changed, with the O1s peak showing a more significant change, indicating that oxygen plasma treatment has altered the surface elemental composition of CNTs thin films. According to the quantitative detection results of XPS analysis, the elemental content on the surface of CNTs thin films can be obtained, as shown in Figure 3. After oxygen plasma treatment, the C element content of CNTs thin films significantly decreased, while the O element content gradually increased. Especially when the plasma power was 100W, the oxygen element content on the surface of CNTs thin films increased from 1.85% before treatment to 42.87%, and correspondingly, the O/C ratio increased from 0.014 to 0.75. This may be because during the plasma treatment process, oxygen is discharged to form active particles such as atomic oxygen, molecular oxygen, oxygen ions, and peroxide radicals, which undergo a series of reactions with the surface of CNTs, thereby introducing a large number of oxygen-containing functional groups on the surface of CNTs, resulting in a significant increase in the O element content on the surface of CNTs.

Figure 3 XPS spectra of surface elements of CNTs thin films before and after plasma treatment

In order to determine the changes in functional groups on the surface of CNTs thin films, the Gaussian Lorentz function was used to perform corresponding peak fitting on the C1s peaks in the full spectrum, and the peak spectrum of C1s on the surface of CNTs was obtained, as shown in Figure 4. The carbon carbon double bond (C=C), carbon carbon single bond (C-C), hydroxyl group (C-O), carbonyl group (C=O), carboxyl group (O=C-O), and π - π * are located near the binding energies of 284.8eV, 285.5eV, 286.2eV, 287.2eV, 288.9eV, and 291.1eV, respectively. A small amount of hydroxyl (C-O), carbonyl (C=O), and carboxyl (O=C-O) functional groups appeared on the surface of the original CNTs film, which may be caused by contamination or impurities on the sample surface. After plasma treatment, the content of C=C on the surface of CNTs thin films gradually decreases, while the content of C-C gradually increases. This indicates that the graphene structure of CNTs is partially destroyed, and the degree of this destruction increases with the increase of plasma power. This indicates that the carbon carbon double bond is oxidized, and new C-O groups are generated on the surface of MWCNTs through plasma treatment. Especially when the power is 100W, the content of C=C bonds decreases from 69.15% before treatment to 31.42%, indicating that the structure of CNTs has been severely damaged, which will lead to changes in the overall performance, especially the mechanical properties, of CNT films. However, after plasma treatment, the content of oxygen-containing functional groups on the surface of CNTs was effectively increased, with the content of hydroxyl groups (C-O) gradually increasing from 4.86% to 10.45%; The content of carboxyl groups (O=C-O) increased from 1.37% to 12.42%; At the same time, the content of carbonyl groups (C=O) increased from 5.41% to 28.79%, which will effectively improve the surface polarity of CNT films and enhance the wettability of epoxy resin on CNT films and the interfacial properties between the two.

Figure 4: Cls peak fitting curve of XPS spectra of CNTs thin films before and after plasma treatment

Wettability of CNTs thin film surface

Good resin wettability is a prerequisite for ensuring that CNTs are infiltrated by resin. The resin contact angle on the surface of CNTs films before and after plasma modification is shown in Figure 5. The contact angle of the original CNT film is 106.6 °. After plasma modification, a large number of oxygen-containing functional groups such as hydroxyl and carboxyl groups are generated on the surface of the CNT film through oxidation. The presence of oxygen-containing functional groups improves the surface tension of the CNT film and enhances its wettability. With the increase of plasma discharge power, the content of oxygen-containing functional groups also increases, and the contact angle between the surface of the CNT film and the resin gradually decreases. When the discharge power is 100W, the contact angle is 29.5 °. The surface of the CNT film exhibits good resin wettability, which will be more conducive to the flow and wetting of the resin in the CNT film.

Figure 5 Contact angle of epoxy resin on the surface of CNTs film before and after plasma modification