Preparation Of High Strength And Toughness Aramid Fiber By Introducing Flexible Asymmetric Monomer To Construct Misplaced-Nunchaku Structure (Vol 306, 2000814, 2021)

High performance fibers with high strength and toughness have great potential in composites, but contradiction between tensile strength and elongation at break makes the preparation to become a current challenge. Herein, an asymmetric structure of more flexible diamine, 3,4 '-diaminodiphenyl ether (3,4 '-ODA), is introduced into heterocyclic aramid (PBIA) fibers to replace rigid symmetric p-phenylenediamine (PDA). By studying the properties of copolymer (mPEBA) fibers with different ratios of diamine, it is found that the mPEBA fiber reached the optimal mechanical properties with the 30% content of 3,4 '-ODA. Compared with homopolymerized heterocyclic aramid fibers, the tensile strength and elongation at break of mPEBA fiber are improved by 26.2% and 38.7%, respectively. Results of X-ray diffraction show that the introduction of 3,4 '-ODA structure can increase stretchability of mPEBA fibers, improving the orientation degree during hot-drawing. Molecular dynamics simulations confirm that 3,4 '-ODA structure undergoes a conformation transformation to form a straightened chain during hot-drawing, while symmetrical 4,4 '-diaminodiphenyl ether (4,4 '-ODA) cannot form the same conformation. The misplaced-nunchaku structure is formed based on the special meta-para position of 3,4 '-ODA, achieving the synergy of high strength and high toughness.