Abstract:

Objective Polyimide (PI) fibers are widely applied in high-temperature filtration and protection fields by virtne of their excellent thermal stability and chemical resistance. However, the mechanical properties of commercially available PI fibers are often insufficient for high-load applications. Tension heat-setting is a critical post-treatment process to improve fiber performance, yet the evolution mechanism of the hierarchical structure during this process remains unclear. This study aims to investigate the influences of heat-setting time and draft ratio on the aggregation structure (including crystallization, molecular orientation, and microfibril evolution) and mechanical properties of PI fibers derived from 2-(4-aminophenyl)-5-aminobenzimidazole (BIA), p-phenylenediamine (PDA), and 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA). The goal is to establish a process-structure-property relationship to guide the production of high-strength and high-modulus PI fibers.

Method High-viscosity polyamic acid (PAA) solution was synthesized by copolymerizing BIA, PDA, and BPDA in DMAc. PI fibers were prepared by a dry spinning process with a spinneret temperature of 250 ℃ and a winding speed of 200 m/min. The nascent fibers were thermally imidized at 300 ℃ (draw ratio 1.15) and then drawn at 450 ℃ (draw ratio 2.0) to obtain precursor fibers (PI-2). Subsequently, tension heat-setting was performed at 450 ℃ under different conditions: time periods (12, 20, 30, and 60 s) and draft ratios (0.9, 1.0, and 1.1). The chemical structure was characterized by attenuated total reflectance fourier transform infrared spectroscopy (FT-IR). The aggregation structure, including crystal parameters and microfibril dimensions, was analyzed using synchrotron radiation wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Mechanical properties were tested using a single fiber strength tester.

Results FT-IR spectra confirmed that the nascent fibers had a low degree of imidization (35.7%), while fibers treated at 450 ℃ achieved complete imidization. Mechanical testing revealed that the tensile strength and modulus were sensitive to both heat-setting time period and draft ratio. The optimal mechanical performance was achieved at a heat-setting temperature of 450 ℃, a draft ratio of 1.0, and a time period of 20 s. Under these conditions, the tensile strength and initial modulus reached 2.35 GPa and 142.8 GPa, respectively, representing increases of 14% and 8% compared to the fibers without heat-setting, respectively. WAXD analysis indicated that heat-setting significantly improved the crystal perfection and molecular orientation. The appearance of distinct diffraction spots corresponding to the (004) crystal plane of the BPDA-PDA unit confirmed the formation of ordered crystalline regions. The orientation factor increased monotonically with the draft ratio. SAXS results, analyzed using the Grubb model, elucidated the evolution of microfibrils. Under negative draft (0.9 draft ratio), the misalignment angle (B_f) increased significantly, indicating a disordered arrangement of microfibrils due to entropy elasticity. Under positive draft (1.1 draft ratio), the fibers exhibited the lowest average length of microfibrils (l_f), suggesting the best orientation; however, the decrease of l_f is likely due to the breakage of microfibrils under high tension. In contrast, the constant length heat-setting (1.0 draft ratio) facilitated the synergistic effect of thermal and stress fields, resulting in the maximum microfibril length and the most perfect crystal structure, which contributed to the highest mechanical strength.

Conclusion Tension heat-setting is an effective method to enhance the mechanical properties of BIA-modified PI fibers. The evolution of the fiber structure is governed by the competition between molecular chain disentanglement, crystallization, and degradation. While higher draft ratios improve orientation, they may cause structural damage to microfibrils. The constant length heat-setting (1.0 draft ratio) at 450 ℃ for 20 s provides the optimal balance, promoting the growth of long, well-oriented microfibrils and perfect crystals. These findings offer valuable experimental data and theoretical support for the industrial manufacturing of high-performance polyimide fibers.

Key words: polyimide fiber, tension heat-setting, aggregation structure, dry spinning, tensile strength, elastic modulus