B-N-P-linked covalent organic frameworks for efficient flame retarding and toxic smoke suppression of polyacrylonitrile composite fiber

Abstract Covalent organic frameworks (COFs) recently exhibit great potential as flame-retardant additives for polymers, yet the limited choice of building blocks and skeletons have hampered the improvement of their flame-retardant efficiency. Herein, we develop a novel boron-nitrogen-phosphorus-linked COFs (BNPC) for the first time through a facile self-condensation reaction of hydroxyphenylboronic acid substituted hexachlorocyclotriphosphazene, which is employed as flame-retardant filler for polyacrylonitrile (PAN) fiber. As reflected by cone calorimeter test, by introducing 7 wt% BNPC into PAN fiber, the peak heat release rate (PHRR), total smoke production (TSP) and CO production of PAN@BNPC-7 fiber are significantly decreased by 72.6%, 67.2% and 76.3%, respectively. Simultaneously, the limiting oxygen index (LOI) value is improved from 17.5% to 27.8%. The detailed flame-retardant mechanism analyses demonstrate that the BNPC not only provides a kinetic advantage for initiating the cyclization of PAN chains via an ionic mechanism, which significantly reduces the chain scission/toxic gases (HCN, CO) emission and promote the intermolecular crosslinking of cyclized ladder-like structures, but also offers excellent cooperative catalytic effect to produce highly compact and crosslinked P-B-N-rich char layer barrier on fiber surface. Moreover, the resultant PAN@BNPC-7 composite fiber retains good spinnability and exhibits enhanced mechanical properties. This work offers a scalable strategy for designing COFs with multi-flame retardant elements, endowing its great application potential in functional polymer materials.