Abstract:

Objective Thermotropic liquid crystalline polyarylate （TLCP） fibers exhibit excellent properties including high strength， high modulus， and low dielectric loss. After the preparation of TLCP as-spun fiber via melt spinning， a high-temperature annealing treatment under nitrogen atmosphere is typically employed to achieve a significant enhancement in its mechanical properties. The mechanism by which structural changes in the aggregated state of TLCP as-spun fibers during annealing affect their mechanical properties remains incompletely understood. By investigating the performance and changes in multi-scale aggregated structure of liquid crystal polyarylate fibers during thermal annealing， this study aims to reveal the structure-property relationships during the annealing process， thereby providing guidance for the efficient thermal treatment of liquid crystal polyacrylate.

Method As-spun TLCP fibers were subjected to thermal annealing under various temperatures and for different durations to produce a series of finished fibers for required mechanical properties. The aggregated structure of the fibers， both before and after annealing， was systematically characterized and analyzed using X-ray Diffraction， Small-angle X-ray diffraction， Thermogravimetric Analysis， Scanning Electron Microscopy， Atomic Force Microscopy， Fourier Transform infrared spectroscopy， Differential Scanning Calorimetry， shear rheology tests， sonic velocity orientation measurements， and tensile testing.

Results By treating TLCP fibers under different thermal annealing conditions， we investigated their effects on the structural evolution and mechanical properties of the aggregated state of fibers. It was found that as thermal annealing time and temperature increased， the mechanical properties of the fibers was gradually improved. Shear viscosity testing revealed that the shear viscosity of the TLCP system gradually increased with extended annealing time and elevated temperature. Furthermore， SEM and AFM analyses showed that after thermal annealing， more densely packed adjacent fibrillar structures were formed along the fiber axis. Calculations from AFM two-dimensional geometric morphology maps showed that the average distance between adjacent valley bottoms decreased from 0.406 μm to 0.368 μm， indicating that a more tightly arranged structure formed on the fiber surface after thermal annealing. This densely packed structure enhanced intermolecular forces， leading to an increase in the average transverse Young's modulus of the primary fiber from 1 340.8 MPa to 1 569.8 MPa after 12 h of thermal annealing at 260 ℃. XRD analysis revealed that crystallinity and grain size exhibited an initial increase followed by a decrease during thermal annealing with no new peaks appearing， indicating no crystal transition occurred during thermal annealing， and the crystalline structure was disrupted as the degree of heat annealing increased. In summary， solid-phase polycondensation reactions in liquid crystalline polyacrylate fibers during thermal annealing led to further molecular chain growth along the axial direction， resulting in increased molecular weight. The orientation of TLCP fibers gradually decreased after thermal annealing， resulting in a corresponding increase in fiber elongation at break. It was also discovered that higher annealing temperatures could achieve superior mechanical properties in a shorter duration.

Conclusion During the thermal annealing process of TLCP as-spun fibers， the synergistic effects of molecular chain changes， crystalline structure alterations， orientation variations， and increased intermolecular packing density simultaneously enhance the fiber's breaking strength， elongation at break， and elastic modulus. At lower annealing temperatures and for shorter durations， increased crystallinity

Key words: thermotropic liquid crystalline polyarylate fiber, thermal annealing, mechanical property, transverse modulus, solid-state polycondensation, intermolecular interaction, multi-scale aggregate structure