Abstract
Imide-aryl ether oxadiazole copolymers were prepared and their morphology and mechanical properties investigated. A general method for the preparation of aryl ether oxadiazole-containing diamines has been developed in which the generation of the aryl ether linkage was the monomer-forming reaction. The electron-deficient oxazole ring activated a fluoro-substituent towards nucleophilic aromatic substitution. Facile displacement occurred at the para position of a 2-phenyl substituted oxadiazole heterocyclic since the oxadiazole can stabilize the negative charge developed in the transition state through a Meisenheimer complex, analogous to conventional activating groups (i.e. sulphone, ketone, etc.). The synthesis involved the reaction of 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole with either 3- or 4-aminophenol in an N-methyl-2-pyrrolidone N-cyclohexyl-2-pyrrolidone solvent mixture in the presence of potassium carbonate. These novel diamines were purified by recrystallization, polymerized with various compositions of 4,4′-oxydianiline (ODA) and pyromellitic dianhydride (PMDA) and thermally cured, producing two series of novel imide-aryl ether oxadiazole random copolymers. The resulting copolymers showed mechanical properties comparable to PMDA/ODA polyimide with elongations in the 40% range and moduli in the 2500 MPa range. The copolymers showed Tgs in excess of 300°C. However, the thermal stability was somewhat compromised by the incorporation of the oxadiazole-containing diamine relative to PMDA/ODA polyimide. © 1992.