Pre-drawing induced evolution of phase, microstructure and property in para-aramid fibres containing benzimidazole moiety

Abstract

Copoly(p-phenylene-benzimidazole-terephthalamide) (PBIA) fibre was spun by wet-spinning and drawn in a coagulating bath with different pre-drawing ratios (R). The evolution of phase, microstructure structure and conformation induced by pre-drawing were studied by 2D-wide angle X-ray diffraction (2D-WAXD), attenuated total reflection-Fourier transform infrared (ATR-FTIR) and scanning electron microscope (SEM). WAXD results indicate that mesomorphic/amorphous coexisting phases transform to crystal/mesomorphic/amorphous coexisting phases when R is higher than 75%. Even when R reaches an ultimate value of 100%, the content of crystalline is only 10.7% but the mesomorphic phase content is higher than 40%, which indicates that the 3D well defined crystalline structure is disturbed by the introduction of asymmetric benzimidazole units. The ATR-FTIR results indicate that the increase of R leads to conformational order, contributing to phase transition. The results of SEM show that PBIA fibres have a typical skin–core morphology. The sizes of micro-fibrils in the core part become more homogeneous with the increase of R. Moreover, with the increasing R from 0% to 100%, the tensile strength and initial modulus of the PBIA fibre significantly increase from 7.8 cN dt⁻¹ to 21.5 cN dt⁻¹ and from 238 cN dt⁻¹ to 884 cN dt⁻¹, respectively. Dynamic mechanical analysis (DMA) of PBIA fibres shows two transition peaks, and some amorphous phase is confined by the order phases.