Polyimide nanofoams based on ordered polyimides derived from poly(amic alkyl esters): PMDA/4-BDAF

A means of generating foams of high-temperature polymers, polyimides, has been developed for use in dielectric layers in microelectronics. In these systems, the pore sizes generated are in the tens of nanometers range, thus the term ''nanofoams''. The foams are generated by preparing phase-separated block copolymers with the majority phase comprised of polyimide and the minor phase consisting of a thermally labile block. Films are cast, solvent is removed, and the copolymers are cured, causing phase separation of the two blocks. The labile blocks are subsequently removed via thermal treatments leaving pores having a size and shape commensurate with the size and shape of the original copolymer morphology. The polyimide derived from pyromellitic dianhydride (PMDA) and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF) was used as the matrix materials for the generation of nanofoams, and poly(propylene oxide) oligomers were used as the thermally labile constituent. The synthesis and characterization of the copolymers were performed, and the process for obtaining nanofoams was optimized. The foams were characterized by a variety of techniques including TEM, SAXS, WAXD, DMTA, density, and refractive index measurements. Thin-film, high-modulus foams with good mechanical properties can be synthesized using the copolymer/nanofoam approach.