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Thin polyimide films with dispersed nano‐foam morphology have been prepared for the purpose of obtaining low dielectric polymer insulators for microelectronic applications. They were obtained by utilizing micro phase‐separated triblock copolymers where the thermally stable polyimide matrix component was derived from pyromellitic dianhydride (PMDA) with 1,1‐bis(4‐aminophenyl)‐1‐phenyl‐2,2,2‐trifluoroethane (3F) and a thermally labile poly(propylene oxide)(PO) component comprised the outside block of the ABA triblock architecture. TEM studies show that the initial irregular nanoscale phase‐separated morphology of polyimide triblock copolymers are mostly maintained in the final nano‐foam films upon thermal decomposition of the dispersed PO component. The nano‐foam polyimide films exhibit significantly lower dielectric constants ϵ′ (e.g., 2.3 at 19% porosity) as compared with ϵ′≊2.9 for the homopolymer, as predicted by Maxwell–Garnett theory, with the nano‐pore structures remaining stable at 350 °C. © 1996 American Institute of Physics.