Abstract:

Objective The mechanical properties of high-performance polymer fibers such as Kevlar (poly(p-phenylene-terephthamide) (PPTA) fiber) and Zylon (poly(p-phenylenebenzobis-oxazole) (PBO) fiber) can deteriorate severely when exposed under an UV irradiation condition. Polyimide (PI) fiber is of great interest to researchers with excellent irradiation resistance, high thermal stability and good mechanical property. Generally, the preparation of PI fiber adopts a two-step process, i.e., fabrication of polyamic acid (PAA) fibers, and then the conversion of PAA fibers into PI fibers by a thermal imidization process which is one of the key processes affecting the microstructure and the overall properties of resultant PI fibers. This research investigates the thermal imidization process aiming to prepare high performance PI fibers.

Method The precursor PAA fibers were fabricated through a dry-jet wet spinning process (a classic two-step method). The PI fibers were prepared by thermal imidization of PAA fibers under a relaxation state and satress state, respectively. The evolution of chemical structure and aggregation structure of PI fibers under different states were compared and analyzed using Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, thermogravimetric analysis and so on. The relationship between the structure and properties of PI fibers was established.

Results The thermal imidization process was accelerated by the applied stress. The degree of imidization of fibers reached 90% at 300 ℃ for 120 s under a stress of 30 cN, which was 17% higher than that in the relaxation state, thus contributing to the enhancement of fibers' properties. Under a stress, the axial crystal plane spacing of fibers increased after thermal imidization, leading to an increase in orientation and crystallinity of the fiber. Compared to the relaxation state imidization, the orientation degree of the (004) plane increased from 0.63 to 0.80, and crystallinity increased from 14.20% to 16.73% after the stress-state thermal imidization, indicating a more perfect crystal structure formed inside the fiber. However, most of the fiber remained amorphous. Owing to enhanced molecular chain orientation during imidization, both strength and modulus of fibers increased significantly while the elongation at break decreased. For the PI fiber thermally imidized under a stress condition, the 5% and 10% weight loss temperatures reached 529 ℃ and 565 ℃, respectively, higher than the relaxation samples.

Conclusion The stress applied in the thermal imidization process improves the degree of imidization, orientation and crystallinity of the resultant PI fiber, and significantly improves the mechanical and thermal properties of the PI fiber. It makes PI fibers more prominent in the advantages of high-performance fibers. Some results in this paper can serve as a foundation for optimizing the thermal imidization conditions of PAA fibers and producing high-performance PI fibers. It is believed that in the future, with the in-depth study of fiber forming process and post-treatment process, PI fibers with a better comprehensive performance will be widely applied in aerospace, military and composite material fields.

Key words: high-performance fiber, polyimide fiber, dry-jet wet spinning, thermal imidization, stress, aggregation structure