POLYIMIDE FOAMS PREPARED FROM HOMOPOLYMER COPOLYMER MIXTURES/

A new route has been developed for the synthesis of thermally stable p olymer foams with high glass transition temperatures. Compositionally asymmetric microphase-separated block copolymers were prepared where t he minor component, poly(alpha-methylstyrene), is thermally labile and the major component, polyimide, is thermally stable. The minor compon ent can unzip, to its monomer upon heating, and the decomposition prod ucts diffuse out of the him leaving behind pores embedded in a matrix of the thermally stable component. The addition of homopolymer of the thermally labile component to the block copolymer offered one means to increase the volume fraction of voids while maintaining a discrete ce ll structure. Mild decomposition conditions were required to avoid rap id depolymerization of the alpha-methylstyrene and subsequent plastici zation of the polyimide matrix. The foams showed pore sizes with diame ters ranging from 40 nm to just over 1 mu m, depending on the copolyme r used in the mixture, along with the expected increase in porosity.