POLYIMIDE FOAMS DERIVED FROM A HIGH T-G POLYIMIDE WITH GRAFTED POLY(ALPHA-METHYLSTYRENE)

A new route to high-T-g, thermally stable polyimide foams has been dev eloped. Foams were prepared by casting microphase-separated graft copo lymers comprising a thermally stable main chain, polyimide, and a ther mally labile graft, poly(ctl-methylstyrene). The copolymer composition s were designed so that the thermally labile block would be the disper sed phase. This can unzip to its monomer upon heating, and the decompo sition product diffuses out of the him, leaving pores embedded in a ma trix of the thermally stable component. The copolymers were synthesize d through either the poly(amic acid) precursor, followed by chemical c yclodehydration to the imide form, or the poly(amic alkyl ester) precu rsor followed by thermal imidization. The decomposition of the alpha-m ethylstyrene in the block copolymer was studied by thermogravimetric, dynamic mechanical and thermomechanical analyses. Mild decomposition c onditions were required to avoid rapid depolymerization of the alpha-m ethylstyrene and excessive plasticization of the polyimide matrix. The foams showed pore sizes with diameters ranging from less than 20 nm t o over 1 mu m, depending upon the synthetic route employed, and the re duction in the mass density was generally consistent with the starting composition. Copyright (C) 1996 Elsevier Science Ltd.