Thermal dechlorination induces chain extension to simultaneously improve the tensile strength and toughness of heterocyclic aramid

Abstract

The heterocyclic aramid (PBIA) has important applications in bulletproof protection and composites reinforcement. However, there are intrinsic conflicts to simultaneously improve the tensile strength and toughness. Herein, the chlorine groups are introduced into PBIA chains by copolymerization to prepare containing chlorine heterocyclic aramid (ClPBIA). The single crystal data and WAXD patterns revealed the effect of introducing chlorine groups on the fiber structures. ¹H-NMR and high-resolution mass spectra (HRMS) show that intermolecular radical-coupling reactions occur during thermal dechlorination of chlorine groups (≥340 °C). The crosslinking degree (DC) of ClPBIA fibers increases with increasing the heat-treatment temperature, which generates chain extension effects and crosslinking effects in sequence. In chain extension effects (DC≤15.6%), the molecular weight of ClPBIA fibers increases, and the tensile strength (27.75∼34.29 cN/dtex) and elongation at break (3.82%∼4.61%) increase simultaneously. In crosslinking effects (DC>15.6%), the polymer crosslinked network gradually forms. Then, the initial modulus begins to increase significantly from 642 cN/dtex to 737 cN/dtex, but the tensile strength and elongation at break decrease. The molecular dynamics simulation also demonstrated the dependence of chain extension effects and crosslinking effects on the DC, and the mobility of polymer chains decreases a lot from chain extension effects to crosslinking effects. Therefore, the tensile strength and toughness of ClPBIA fibers are simultaneously improved by controlling the DC in the chain extension effects. Compared with PBIA, the highest tensile strength (34.29 cN/dtex) and toughness (103.7 MJ/m³) of ClPBIA increase by 8.3% and 18.9%, respectively.