Abstract:

Objective Polyimide fiber is a high-performance material characterized by its highly regular molecular chain structure. These structural features impart superior properties, enabling broad applications in multiple industries, including aerospace, military, transportation sectors, sports equipment, and special protective clothing. However, polyimide exhibits a smooth surface and lacks active functional groups along its macromolecular chains, which imposes certain limitations for practical applications. Consequently, modifying polyimide fibers can significantly expand their potential applications. Conventional fiber modification methods often affect fiber performance and may raise environmental concerns. Growing sustainability demands have driven research toward more environmentally friendly and cleaner production technologies.

Method Supercritical carbon dioxide (scCO₂) fluid combines liquid-like density with gas-like transport properties, exhibiting exceptional mass transfer characteristics and penetration capacity that enable effective fiber modification. Polyimide fibers were initially washed to remove surface contaminants, and the purified fibers were loaded into a high-pressure autoclave reactor. The system was heated to 40 ℃ and pressurized with CO₂ to pressures of 8-12 MPa. The treatment was maintained for 60 min. The changes of fiber structure and properties before and after treatment were analyzed through various characterization methods including mechanical property testing, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA).

Results The surface roughness (R_a) of the original fiber is 5.29 nm. When the pressure was under 10 MPa, the surface of the fibers was similar to and even smoother than the original fibers. The R_a of the fibers was reduced to 3.35 nm at 8 MPa. When the pressure increased above 10 MPa, uniformly and continuously distributed tiny concave structures appeared, and the R_a was improved to 6.72 nm. It was found that scCO₂ had no effect on the chemical structure of the fibers, indicating that no molecular-level structural changes occurred in the fibers, thereby ensuring the structural stability and good performance of the modified fibers. Also, scCO₂ showed no significant effect on the mechanical properties of fibers, suggesting that the modification effect of scCO₂ on fibers was mainly on the surface layer of the fibers but not the interior of the fibers. After scCO₂ treatment, the mass losses within the range of 200-500 ℃ were significantly reduced. The scCO₂ fluid has excellent diffusion and mass transfer effects, which can remove the residual solvents or small molecular substances inside the fibers. In addition, the residual carbon content of the treated fibers slightly increased, which also indicated that the fiber structure was more stable, confirming that the supercritical fluid was beneficial to the improvement of the fiber microstructure. The orientation of fibers was analyzed separately in both radial and axial directions. The results revealed relatively low structural ordering in the radial direction. In contrast, well-aligned crystalline structures with high orientation indices were observed along the axial direction, which accounts for the fibers' superior mechanical properties. Meanwhile, when the processing pressure exceeded 10 MPa, the diffraction peaks shifted to a lower angle, indicating an increase in the crystal plane spacing. Finally, the interfacial shear strength (IFSS) value between the fibers and the epoxy resin before and after scCO₂treatment was investigated. Due to the smooth surface of the original polyimide fibers, the average IFSS was 33.98 MPa. When the pressure was less than 10 MPa, due to the reduction of the surface roughness of the fibers, the average IFSS value decreased. When the pressure was higher than 10 MPa, the average IFSS value increased significantly, with the maximum increase reaching 78.2%.

Conclusion scCO₂ treatment does not change the chemical structure of the fibers but can improve the surface properties of fibers. When the pressure is low, the surface of the fibers becomes smooth and the roughness decreases from 5.29 nm to 3.35 nm. When the pressure exceeds 10 MPa, the surface roughness of the fibers increases to 6.72 nm. The slight changes in the surface structure do not significantly affect the mechanical properties or thermal stability of the fibers, but the interfacial properties between fibers and resins have been significantly improved. After scCO₂ treatment, the interfacial shear strength between fibers and resins has increased by a maximum of 78.2%. scCO₂ fluid leads to the expansion of the internal lattice of the fibers or the formation of new crystalline structures. These results provide a theoretical basis for the future application of supercritical fluid modification technology in polyimide fibers.

Key words: polyimide fiber, high-performance fiber, supercritical carbon dioxide fluid, modification, interface characteristic, aggregation structure