A means of generating high temperature polymer foams which leads to pore size in the nanometer regime has been developed. Nanofoams were prepared by casting block copolymers comprised of a thermally stable block and a thermally labile material, where the thermally stable material forms the matrix in which thermally labile material is dispersed. Triblock copolymers comprised of high Tg, amorphous polyimide matrices with poly(propylene oxide), as the thermally decomposable coblock, were prepared. The copolymer synthesis was carried out through either the poly(amic alkyl ester) or poly(amic acid) precursor and subsequent cyclodehydration to the polyimide by either thermal or chemical means, respectively. Microphase separated morphologies were observed for all copolymers, irrespective of the block lengths surveyed, by dynamic mechanical analysis. Upon heating, the poly(propylene oxide) underwent thermolysis leaving pores where the size and shape of the pores were dictated by the initial copolymer morphology. A 5 to 15% reduction in density was observed consistent with the generation of a foam.