Approach to produce rigid closed-cell polyimide foams

Abstract

A new chemical approach to prepare rigid closed-cell polyimide foams (PIFs) has been developed by thermal foaming of nadimide end-capped imide oligomers (NAIOs) obtained by thermally treating an in situ polymerization of monomeric reactant (PMR)-type poly(amide ester) resin solution. The PMR poly(amide ester) was synthesized by the reaction of diethyl ester of 2,3,3′,4′-biphenyltetracarboxylic dianhydride and aromatic diamines using monoethyl ester of cis-5-norbornene-endo-2,3-dicarboxylic acid as reactive end-capping agent in ethyl alcohol. The thermal foaming properties of the NAIOs were investigated. It was found that thermal foaming of the NAIOs was attributed to the blowing of cyclopentadiene, which was evolved from thermal decomposition of the nadimide end-caps in the imide oligomer by the reverse Diels–Alder reaction during cross-linking of the bisnadimide oligomers. The resultant rigid PIFs showed uniform cellular structures with high closed-cell content ( Cc) as well as high thermal stability and mechanical properties. The PIF with a density of 100 kg m−3 and a Cc of 89% exhibited compression strength at 10% compressive deformation of 1.34 MPa and outstanding thermal stabilities, making it a desirable candidate as high temperature structural foam materials for applications in aviation and spacecraft industries.