Pyridine

2, 6-naphthalenedicarboxylic acid (purity: 99%, J&K Scientific Ltd., Beijing, China), 4-nitroaniline (purity: 99%, Aladdin Reagent Co., Ltd., Shanghai, China), 10% palladium on carbon (10 wt% Pd/C, Alfa Aesar Chemical Co., Ltd., Shanghai, China), triphenyl phosphate (TPP, purity: 98%, Macklin Biochemical Co., Ltd., Shanghai, China), argon (purity: 99.999%, Hualong Special Gas Co., Ltd., Zhuzhou, Hunan, China), nitrogen (purity: 99.999%, Hualong Special Gas Co., Ltd., Zhuzhou, Hunan, China) and hydrazine monohydrate (NH₂NH₂·H₂O, purity: 95%, J&K Scientific Ltd., Beijing, China) were used directly without further treatment. Pyromellitic dianhydride (PMDA, Alfa Aesar Chemical Co., Ltd., Shanghai, China) and 4, 4′-diaminodiphenyl ether (ODA, Alfa Aesar Chemical Co., Ltd., Shanghai, China) was heated at 125 °C in vacuum for 4 h prior to use. N-Methyl pyrrolidone (NMP), dimethylformamide (DMF), methanol, ethanol and pyridine (Py) were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, Guangdong, China), before use, all solvents was dehydrated with molecular sieves.

Polyamide 6 (PA6, TP4208) was purchased from Zig Sheng Industrial Co., Taiwan, China. Red phosphorus (RP, 70 wt% content of phosphorus) was supplied by Guangzhou YS Flame Retardant Materials Co., Ltd., Guangzhou, China. Tween−80 and β−cyclodextrin (β−CD) were purchased from Beijing HWRK Chem Co., Ltd., Beijing, China. Toluene diisocyanate (TDI) was purchased from Wanhua Chemical Group Co. Ltd., Yantai, China. Pyridine and acetone were purchased from Guangzhou Chemical Reagent Factory, Guangzhou, China. Microcapsules based on melamine borate coated red phosphorus (Mic−MBP) were prepared by our laboratory [23 (link)].