A novel vacuum-assisted method for fabricating flexible polyimide foams from 3,3′,4,4′-oxydiphthalic anhydride/4,4′-oxydianiline

In this research, a novel method for fabricating flexible polyimide foams was developed. The foam was derived from the precursor synthesized via esterification of 3,3′,4,4′-oxydiphthalic anhydride and 4,4′-oxydianiline. After drying process, the remaining solvent acted as blowing agent. By adjusting the synthesis condition, low-molecular weight precursor was obtained and can be foamed under environment temperature. With this novel type of precursor and vacuum-assisted foaming process, we could easily obtain any shape of foams, which was easy to process under relatively low temperature. The foam could be imidized later in order to improve mechanical strength. During the imidization process, in situ Fourier transform infrared (FTIR) analysis was applied to analyze the variation of the imidization percentage with time. The density of the foams could be adjusted from 12 to 30 kg m⁻³ with uniform cell structure. Other properties of foams were also investigated. The glass transition temperature of the precursor was 70°C, which is obviously lower than that of the usual previously reported. The 5% and 10% weight loss temperatures of the finally imidized foam can reach up to 567°C and 593°C, respectively. The foams belong to open cell structure with 98% open cell content. The limiting oxygen index of the foams varied from 38.6 to 41.9, depending on foam densities. The compressive strength of the foam ranged from 30.6 to 90.6 kPa when the density changed from 15 to 28 kg m⁻³. In addition, the in situ FTIR analysis was applied to analyze the imidization percentage of the foam during imidization process.