“…For example, O, CH 2 O, SO 2 , and CMe 2 units have been used for this purpose. 15 Other approaches to modify the properties of PBIs are to make PBI copolymers, 16 to make mixed polymers such as PBI-polyimides 17 or PBI-polyamides, 18 or to blend them with other polymers. 19 As co-monomers for PBI synthesis, the aromatic bis(o-diamines) are also expensive and difficult to purify.…”