High strength and high modulus polyimide (PI) fibers have outstanding mechanical properties, high and low temperature resistance, aging resistance, low water absorption, low dielectric, high insulation and other performance characteristics. They are high-performance organic fibers with excellent comprehensive performance and outstanding development potential. They can play an important role in aerospace and power electronics fields and are key materials with broad application prospects. Compared with existing high-performance organic fibers, PI fibers can compensate for the low heat resistance and poor creep resistance of ultra-high molecular weight polyethylene fibers. They do not have the problems of high water absorption and large thermal deformation of aramid fibers, and have better environmental resistance, especially UV radiation resistance, than PBO fibers.

While possessing the aforementioned advantages, PI fibers also have common issues with organic fibers. A large number of rigid conjugated cyclic structures composed of five membered rings and aromatic rings are alternately distributed in the polyimide polymer chain. The long and straight molecular chain segments connected by imide bonds have a regular structure, high stability, and strong intermolecular forces. The above structural characteristics make PI fibers have high strength and modulus, but also result in high orientation of PI fibers, inward reversal of polar groups in molecular chains, smooth and flat fiber surfaces, and overall low surface energy, high hydrophobicity, and chemical inertness of PI fibers. One of the main application directions of high-performance fibers is their use as reinforcing phases in composite materials. The surface characteristics of PI fibers are not conducive to the infiltration and composite of resin matrix. After curing with resin to form composite materials, the interface bonding effect between PI fibers and resin matrix will also be affected. The resin matrix cannot fully transmit the force to the reinforcing fibers, resulting in a low conversion rate of fiber to composite material properties. In addition, the morphological structure of PI fibers is similar to that of general organic fibers, with a skin core structure composed of original fiber arrangement. The performance differences and bonding states between the fiber skin and core layers can also lead to delamination and detachment of composite materials when subjected to interlayer shear and transverse tensile forces, which has adverse effects on the performance of composite materials. To address this issue, plasma treatment technology can be used to reconstruct the fiber surface and improve its surface properties.

Analysis of the Effect of Plasma Surface Treatment on Fiber Surface Morphology

The microstructure of the PI fiber samples before and after plasma treatment is shown in Figure 1. From the SEM image, it can be seen that the fiber surface of the untreated blank control group PI-0 is generally smooth and flat except for a small amount of adhesion, without obvious undulating and concave convex structures. The groove texture formed by the arrangement of the original fibers is not obvious; After plasma surface treatment, the adhesion on the fiber surface of the sample increased, and with the extension of treatment time, the groove texture deepened, micro cracks and partial delamination appeared in the original fibers of the cortex, and the fiber cortex cracked along the axial direction.

Figure 1 SEM images of PI fiber before and after plasma surface treatment

From the analysis of the structural characteristics of fiber-reinforced resin based composite materials, an increase in fiber surface roughness is beneficial for the resin matrix to infiltrate the fiber bundles, strengthen the interface bonding effect of the two after composite, and improve the performance of the composite material.

The Effect of Plasma Surface Treatment on the Wetting Effect of PI Fiber and Epoxy Resin

The changes in the infiltration effect of epoxy resin on PI fibers before and after plasma modification were characterized by dynamic contact angle testing, and the test results are shown in Figure 2. According to the test results, the contact angle of the blank control group PI-0 fibers without plasma treatment in epoxy resin liquid is 137 °; With the increase of plasma surface treatment time, the contact angle of modified PI fibers in epoxy resin liquid shows a decreasing trend; The contact angle of the PI-10 experimental group fibers treated with plasma for 10 minutes decreased to 44 °, with a decrease of 76.9%. The test results indicate that atmospheric plasma surface treatment can improve the infiltration effect of epoxy resin on PI fibers; As the plasma treatment time increases, the infiltration effect of epoxy resin on PI fibers becomes better.

Figure 2 Contact angle between PI fiber and epoxy resin before and after plasma treatment

To address the issues of low surface energy of polyimide (PI) fibers and poor interfacial properties of PI fiber/epoxy resin composites, plasma treatment technology can be used to modify the surface of PI fibers. Through plasma surface treatment, the infiltration and interfacial bonding state between PI fibers and epoxy resin can be improved, and the surface morphology of PI fibers can be reconstructed to optimize the composite effect of PI fibers and epoxy resin.