A means of generating high temperature polymer foams which leads to pore sizes in the nanometre regime has been developed. Foams were prepared by casting block copolymers comprising a thermally stable block and a thermally labile material, such that the morphology consists of a matrix of the thermally stable material with the thermally labile material as the dispersed phase. Upon thermal treatment, the thermally unstable block undergoes thermolysis leaving pores of which the size and shape art dictated by the initial copolymer morphology. Triblock and graft copolymers comprising high glass transition temperature 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 decomposition of the thermally labile coblock, a 5-15% reduction in density was observed, consistent with the generation of a foam.