Investigating the Response Mechanism of Mechanical Properties in PA 66 Industrial Fibers to Posttreatment Temperatures

This study investigates the evolution of structure and properties in PA66 industrial fibers under varying post heat‐treatment temperatures. Samples were treated for 5 min at temperatures ranging from 120°C to 220°C under a pretension of 0.04 cN/dtex. The resulting multi‐scale structural changes were characterized by wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering (SAXS), and so on. The results delineate a distinct change in the trends of mechanical properties around a heat‐treatment temperature of 200°C, attributable to fundamentally different microstructural change mechanisms. When the heat‐treatment temperature was below 200°C, the tensile tenacity, tenacity at 5% elongation (Lase‐5), and initial modulus showed no significant changes, while the elongation at break increased with temperature. This mechanical behavior is primarily ascribed to the disorientation and relaxation‐driven restructuring of molecular chains within the amorphous regions. At heat‐treatment temperatures reaching or exceeding 200°C, tensile tenacity, Lase‐5, and initial modulus decreased sharply, accompanied by a substantial increase in elongation at break. Concurrently, the high temperature promotes chain reorganization and lateral crystal growth, resulting in increased crystallinity, and lamellar diameter. These findings indicate that heat treatment at high temperatures (≥ 200°C) induces substantial structural alterations in both the crystalline and amorphous domains.