Abstract:

Objective To address the challenges of the narrow processing window and high melt viscosity associated with high-melting-point naphthalene-ring-based liquid crystal polyarylates, this study aimed to prepare a series of liquid crystal copolyester fibers with tunable thermal and mechanical properties by precisely adjusting the monomer ratio of 2,6-Naphthalenedicarboxylic acid (NDA) to terephthalic acid (TA). The goal was to elucidate the structure-property relationship, clarifying how monomer composition influences the molecular chain sequence structure and crystallization behavior, ultimately determining the final material performance.

Method A series of liquid crystal copolyesters derived from p-hydroxybenzoic acid (HBA), 2,6-naphthalenedicarboxylic acid (NDA), terephthalic acid (TA), and 4,4'-dihydroxybiphenyl (BP) were synthesized via melt polycondensation, producing nascent fibers. The molar ratio of NDA to TA was systematically varied, with the NDA content increasing from 7.5% to 20% and the TA content decreasing correspondingly from 17.5% to 5%, while keeping the total content of NDA and TA constant at 25 mol%. The contents of HNA and BP were fixed at 50% and 25%, respectively. The thermal properties, crystallization behavior, and mechanical properties of the resulting copolyesters were thoroughly characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and a universal testing machine.

Results The results indicated that all copolyesters maintained similar lattice parameters (d=0.442-0.445 nm), suggesting an unchanged crystal structure. As the TA proportion increased, the melting temperature (T_m) of the copolyesters significantly decreased from 341 ℃ to 304 ℃, effectively broadening the processing window. Meanwhile, the char yield at 700 ℃ increased to 46.0%, demonstrating excellent thermal stability. The mechanical properties exhibited significant anisotropy, where the asymmetrically composed P-NDA10TA15 fiber achieved a tensile strength of 0.78 GPa, which was approximately 100% higher than that of the symmetrically composed P-NDA12.5TA12.5 (0.39 GPa). The high-TA-content fiber P-NDA7.5TA17.5 reached a modulus of 38.85 GPa. However, when the NDA content exceeded 17.5%, the crystallinity decreased drastically, leading to performance degradation. Rheological analysis confirmed the processing window was located in the viscous flow region (G″> G'), when G' and G″ represent the storage modulus and loss modulus, respectively, with the optimal spinning temperature not exceeding 390 ℃.

Conclusion The monomer composition is a crucial factor in regulating the hierarchical structure, thermal properties, and mechanical performance of liquid crystal polyarylates. Introducing TA units effectively modulates molecular chain packing and interactions, significantly lowering the melting temperature and broadening the processing window while maintaining the intrinsic high thermal stability of the material, thereby optimizing mechanical properties. The P-NDA10TA15 composition demonstrated the most balanced and superior overall performance. This research provides an effective strategy and a theoretical foundation for the molecular design of liquid crystal polyarylate fibers that combine excellent processability with high performance.

Key words: liquid crystal copolyester, naphthalene-ring polyacylates, melt polycondensation method, hierarchical structure, high-performance fiber, thermal property, mechanical property