Synthesis of soluble polyimide with high char yield for flame retardant poly(vinyl alcohol)

Polyimides (PIs) have been attracting increased attention due to their unique advantages, such as excellent thermal stability, good mechanical properties and low dielectric constants. However, poor solubility hinders their further application. To solve this problem, a series of PIs with different side units (–CH 3 , –CF 3 and –COOH) were synthesized, and the relationship between molecular structure and solubility was thoroughly investigated. In general, solubility has been significantly improved, and all the polymers could be dissolved in hot m-cresol and N-methylpyrrolidone (NMP). With increasing polarity of side units (–COOH > –CF 3 > –CH 3 ), the solubility of their corresponding polymers (PI-COOH, PI-CF 3 and PI-CH 3 ) improved, and PI-COOH could be dissolved in alkaline aqueous solutions. In addition, PI-COOH films showed an outstanding comprehensive performance with a high tensile strength greater than 100 MPa, a high initial decomposition temperature of 420.3 °C, a low coefficient of thermal expansion (CTE) of 9.62 ppm °C −1 and a low dielectric constant of less than 3. PI-COOH also possessed a good charring capacity, and a char yield of 40 % was achieved at a high temperature of 800 °C. Due to characteristics such as solubility in alkaline aqueous solutions, high mechanical properties and good charring capacity, when PI-COOH was added to the poly(vinyl alcohol) (PVA) matrix combined with l -aspartic acid phosphate ionic liquid (ILA), the PVA composite film passed the VTM-0 test and the limiting oxygen index increased from 18.9 % to 29.3 %. Tensile strength increased by 28.3 % from 60.7 to 77.9 MPa, and elongation at break increased by 167.4 % from 95.7 % to 255.9 %. Meanwhile, the high transparency was retained.