Deciphering the “Performance Paradox” of TLCP Fibers: Competition between Chain Extension and Gas-Induced Structural Instability

Abstract

A “performance paradox” in thermotropic liquid crystalline polymer (TLCP) fibers is identified and resolved: although solid-state polycondensation (SSP) drives a monotonic increase in molecular weight during heat treatment, tensile strength exhibits a sharp decline beyond a critical time–temperature threshold. To systematically investigate this phenomenon, a comprehensive evaluation matrix comprising 30 time–temperature conditions was established, with injection-molded disks serving as macroscopic signal amplifiers. We propose a gas-induced damage mechanism, in which heat treatment entails a dynamic competition between chain-extension strengthening and condensed-state physical degradation. Two-dimensional wide-angle X-ray diffraction (2D-WAXD) analysis reveals that while molecular orientation remains largely stable, crystallinity undergoes a stepwise reduction, signaling progressive structural destabilization. The underlying mechanism is attributed to a kinetic mismatch between the generation and diffusion of SSP byproducts (acetic acid, H2O, CO2): the accumulation of trapped gases loosens noncrystalline domains and disrupts stress-transfer pathways between microfibrils. This microstructural damage is amplified into directly measurable macroscopic phenomena in disk specimens─including a volume expansion of up to 132.58% and a catastrophic density collapse─providing strong support for the proposed damage mechanism. Based on these findings, we introduce the inflection point in fiber surface roughness and the plateau region in disk apparent density as physically grounded criteria for process control. This work reframes the optimization of TLCP heat treatment within a multidimensional coupling framework that integrates chemical reaction kinetics, physical structure evolution, and macroscopic morphological integrity.