Abstract
Closed-cell rigid polyimide foams with excellent thermal stability and combined properties were prepared by thermal foaming of a reactive end-capped polyimide precursor powder in a closed mold. The precursor powder was obtained by thermal treatment of a polyester-amine salt (PEAS) solution derived from the reaction of the diethyl ester of 2,3,3′,4′-biphenyl tetracarboxylic dianhydride (α-BPDE) with an aromatic diamine mixture of p-phenylenediamine (PDA) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) in the presence of an end-capping agent (mono-ethyl ester of nadic acid anhydride, NE) in an aliphatic alcohol. The effect of polymer mainchain structures on the foaming processability and combined properties of the closed-cell rigid polyimide foams were systematically investigated. The polyimide foams (100–300 kg/m3) with closed-cell rates of 91–95% show an outstanding thermal stability with an initial thermal decomposition temperature of ≥490 °C and a glass transition temperature of 395 °C. Polyimide foams with density of 250 kg/m3 exhibited compression creep deformation as low as 1.6% after thermal aging at 320 °C/0.4 MPa for 2 h.
Highlights
Polyimide (PI) foams [1,2,3,4], due to the presence of the imide groups in the polymer mainchain, display excellent thermal stability, thermal insulating properties, nonflammability, radiation resistance, etc. and have found widespread application in aerospace, aviation, orbiting vehicles, and transportation [5,6,7]
The PI foams were produced by the thermal foaming of the nadimide end-capped polyimide oligomers (NPOs), which were prepared by the copolymerization of an aromatic dianhydride (α-BPDA) and an aromatic diamine mixture containing BIA and PDA using cis-5-norbornene-endo-2,3-dicarboxylic anhydride (NA) as the end-capping agent
The peak intensity increased with increasing BIA content in the NPOs, indicating the presence of benzimidazole moieties in the polymer backbones
Summary
Polyimide (PI) foams [1,2,3,4], due to the presence of the imide groups in the polymer mainchain, display excellent thermal stability, thermal insulating properties, nonflammability, radiation resistance, etc. and have found widespread application in aerospace, aviation, orbiting vehicles, and transportation [5,6,7]. The resulting rigid PI foams contained a uniform cellular structure with a closed-cell rate as high as 89%, along with a high thermal stability and good mechanical properties. Heteroaromatic diamine monomers containing benzimidazole or the benzoxazole structure have been employed as co-monomers to improve the closed-cell rigid PI combined properties. A heteroaromatic diamine, 2-(4-aminophenyl)-5-aminobenzimidazole (BIA), was employed as a comonomer to improve the combined properties of closed-cell PI foams possessing a crosslinked structure. The PI foams were produced by the thermal foaming of the nadimide end-capped polyimide oligomers (NPOs), which were prepared by the copolymerization of an aromatic dianhydride (α-BPDA) and an aromatic diamine mixture containing BIA and PDA using cis-5-norbornene-endo-2,3-dicarboxylic anhydride (NA) as the end-capping agent. The BIA content and the molecular weight (Mw ) of the PI precursors had a significant effect on the cell morphology and overall performance of the PI foams
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