Constructing Topological Constraints Selectively in the Amorphous Region to Reinforce and Toughen Heterocyclic Aramid Fibers

Abstract

Strength and toughness are intrinsically conflicting properties in materials science, especially in high-strength and high-modulus polymer fibers. Generally, increasing the amorphous content can improve toughness but often comes at the expense of tensile strength. Herein, a semi-interpenetrating network (semi-IPN) was in situ constructed selectively in the amorphous region to overcome this conflict in high-strength and high-modulus heterocyclic aramid (PBIA) fibers. An amorphous chlorinated PBIA oligomer with reactive phenylethynyl end groups (ClPA–PE) was blended into PBIA to prepare the composite as-spun fibers. During the subsequent high-temperature annealing process, the amorphous ClPA–PE underwent crystallization-induced phase separation into the amorphous regions of the fibers, where it simultaneously cross-linked via its phenylethynyl end-groups to in situ form a semi-IPN with PBIA chains. The semi-IPN reinforces the amorphous region through topological constraints while retaining limited PBIA chain slippage to promote energy dissipation. Consequently, incorporating 11.4 wt % ClPA–PE16 (polymerization degree = 16) oligomers raised the amorphous fraction of the PBIA fibers from 49 to 55%, yet the tensile strength, toughness, and modulus still increased to 36.4 cN/dtex, 132.8 MJ/m3, and 782 cN/dtex, displaying increases of 19.0, 27.9, and 10.3%, respectively. In addition, the cyclic loading measurements show that the ClPA–PE16/PBIA fibers exhibit superior fatigue resistance, with 13.0% higher tensile strength retention and markedly lower fatigue elongation and residual strain than those of the PBIA fibers.