Preparation of Thermosetting/Thermoplastic Polyimide Foam with Pleated Cellular Structure via In Situ Simultaneous Orthogonal Polymerization

Miscible thermosetting/thermoplastic aromatic polyimide foam is prepared by in situ simultaneous orthogonal polymerizations, where the linear polyimide (PI) is formed by condensation polymerization from polyester ammonium salt (PEAS) and the cross-linked bismaleimide (BMI) is synthesized through addition polymerization from 4,4′-bismaleimide diphenylmethane (BDM). A unique pleated cellular structure is formed after the polyblend foam is cured at high temperature; here, 2D FT-IR correlation analysis is employed to detect the detailed chemical reactions during the thermal foaming progress. The simultaneous orthogonal polymerization is confirmed, and the cross-linking reaction of BDM is found to be divided into two stages. The pleated structure formed in the second stage significantly improves the thermal insulation property of the polyblend foam—the effective thermal conductivity decreases from 427.5 to 77.5 mW·m–1·K–1 at 300 °C with 15 wt % BMI. Meanwhile, the polyblend is miscible and foams only have one single glass transition temperature (Tg), which increases with the content of BMI. This polyblend polyimide foam will be a promising porous material for thermal insulation applications in extreme environments, and the method to prepare the pleated cellular structure in this work should provide a strategy to develop advanced polymeric foam materials.