Exfoliation and modification of covalent organic frameworks by a green one-step strategy: Enhanced thermal, mechanical and flame retardant performances of biopolymer nanocomposite film

Covalent organic frameworks (COFs) synthesized from condensation reaction between 1,3,5-triformylphloroglucinol (TFP) and p-phenylenediamine (PDA) have been exfoliated and modified simultaneously through ultrasound of COFs and sodium ligninsulfonate in deionized water for 10 h and then incorporated into chitosan. It is inferred from nano-size sheets in Transmission electron microscopy (TEM) image and S and Na element in energy dispersive spectrometer (EDS) that modified COFs (mCOFs) nanosheets have been prepared successfully. The Young's modulus, fracture strength, and yield strength of chitosan/mCOFs nanosheets-1.6 wt% nanocomposite film increases by 94.3%, 27.9%, and 21.2%, respectively. The peak heat release rate (PHRR) and total heat release (THR) of chitosan loaded with 1.6 wt% mCOFs nanosheets also decrease by 41.3% and 33.4%, respectively. The well-dispersed mCOFs nanosheets not only function physical barrier and mechanical enhancement effect, but also contribute to graphitization degree of char residues, resulting in enhanced thermal, flame retardancy and mechanical performances.