Nonstop Monomer-to-Aramid Nanofiber Synthesis with Remarkable Reinforcement Ability

Abstract

Aramid nanofibers (ANFs), typically produced by exfoliating aramid microfibers (Kevlar) in alkaline media, exhibit excellent mechanical properties and have therefore attracted increased attention as nanoscale building blocks. However, the preparation of aramid microfibers involves laborious and hazardous processes, which limits the industrial-scale use of ANFs. This work describes a facile and direct monomer-to-ANF synthesis via an as-synthesized intermediate low-molecular-weight poly­(p-phenylene terephthalamide) (PPTA) without requiring the environmentally destructive acids and high-order shearing processes. Under the employed conditions, PPTA immediately dissociates and self-assembles into ANFs within a time period of 15 h, which is much shorter than the time of 180 h (not including the Kevlar preparation time) required for the Kevlar-to-ANF conversion. Interestingly, the fabricated ANFs exhibit nanoscale dimensions and thermoplastic polyurethane (TPU) reinforcing effects similar to those of Kevlar-derived ANFs; i.e., a 1.5-fold TPU toughness improvement and a maximum ultimate tensile strength of 84 MPa are achieved at an ANF content of only 0.04 wt %. Remarkable reinforcement ability investigated by comprehensive analytical data comes from ANFs, which disturb ordered hydrogen bonding in hard segments and induce strain hardening along the elongation pathway. Thus, the developed approach paves the way to industrial-scale production of ANFs and related nanocomposites.

Allyl and Benzyl Modified Aramid Nanofibers as an Enhancement in Polystyrene-Based Composites

“…Aramid fiber is a synthetic fiber composed of a long-chain polyamide with at least 85% of the amide bond directly connected to the benzene rings (Downing and Newell, 2004 ). Among them, para-aramid fiber, also known as (poly-p-phenylene terephthamide) or PPTA fiber, is particularly widely used (Koo et al, 2019 ). Aramid fibers have high modulus and high strength, which makes them an ideal mechanical reinforcement for compositing with various materials (Nie et al, 2017 ).…”

Fabrication, Applications, and Prospects of Aramid Nanofiber

“…C) Nonstop monomer‐to‐ANF synthesis from low‐molecular‐weight PPTA in KOH/DMSO/cosolvents shortened the preparation cycle to 15 h. Reproduced with permission. [ 66 ] Copyright 2019, American Chemical Society. D) Fibrillation, ultrasonication, and proton donor‐assisted deprotonation reduced the preparation time to 3 days, 24 h, and 4 h, respectively.…”

“…In order to improve the lengthy preparation cycle of the traditional deprotonation method, as shown in Figure 8C, Koo et al [ 66 ] proposed a direct monomer‐to‐ANF synthesis strategy by deprotonating the as‐synthesized low‐molecular‐weight PPTA intermediate in KOH/DMSO/cosolvents. The preparation cycle could be reduced to 15 h by decreasing the molecular weight of the as‐synthesized PPTA intermediate and diluting DMSO with suitable cosolvents.…”

“…However, utilization of the low‐molecular‐weight PPTA intermediates and cosolvents also decreased the weight‐average molecular weight ( M w ) of ANF to 1.0 kg·mol −1 , which is much lower than that of ANF prepared by the traditional deprotonation method from PPTA fiber (15 kg·mol −1 ). [ 66 ] The lower weight‐average molecular weight results in the decreased strength and thermal stability. Alternatively, in our recent work, [ 49 ] we have proposed three kinds of time‐saving, high‐efficiency strategies (fibrillation, ultrasonication, and proton donor‐assisted deprotonation) to reduce the preparation time.…”

Mechano-responsive hydrogen-bonding array of thermoplastic polyurethane elastomer captures both strength and self-healing

“…As evidenced by differential scanning calorimetry (DSC), C-IP-SS is poorly microphase separated, with non-crystalline hard segments in the static state because the non-planar SS and IP moieties interfere with hard-segment stacking ( Supplementary Fig. 11) 49 . This behavior is unusual because typical TPUs have distinctly separated amorphous soft-domain and crystalline hard-domain microphases induced by the intermolecular stacking of mesogenic MD.…”