Polyimide foams derived from poly(4,4'-oxydiphenylpyromellitimide) and poly(α-methylstyrene)

An alternative route for the synthesis of high- Tg thermally stable polymer foams with pore sizes substantially smaller than those produced by other methods is described, using compositionally asymmetric microphase-separated block copolymers where the minor component, poly(α-methylstyrene), is thermally labile and the major component, a polyimide derived from poly(4,4'-oxydiphenylpyromellitimide), is thermally stable. Upon thermal treatment, the thermally unstable block can unzip to the monomer, leaving behind pores, the size and shape of which should, in principle, be identical to the initial block copolymer morphology. Triblock copolymers were prepared where the imide component was derived from poly(amic alfkyl ester) precursor using either the meta- or para-isomers with 4,4'-oxydianiline, and the α-methylstyrene component comprising the outside block of the ABA architecture. Dynamic mechanica] analysis confirmed microphase-separated morphologies for each of the copolymers. The decomposition of the α-methylstyrene block was studied by thermogravimetric, dynamic mechanical and thermomechanical analysis. Mild decomposition conditions were required to avoid rapid depolymerization of the α-methylstyrene and subsequent plasticization of the imide block. The resulting foams showed the expected reduction in density with pore sizes ranging from ∼200Å to ∼1 µm in size, depending on the synthetic route and processing conditions employed.