POSITRONIUM FORMATION IN VARIOUS POLYIMIDES

Positronium (Ps) formation in various polyimides has been studied. It has been found that Ps yield is zero or small in the polyimides having pyromellitic dianhydride (PMDA) and 3,3',4,4'-benzophenonetetracarbox ylic dianhydride (BTDA) as acid anhydride moiety, while those having 3 ,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 2,2-bis(3,4-di carboxyphenyl)hexafluoropropane dianhydride (6FDA) form Ps with intens ities up to about 20%. This difference is well correlated with the ele ctron affinity of these moieties: PMDA > BTDA > BPDA approximately 6FD A. In another experiment o-Ps yields and its lifetimes were measured i n benzene solutions of monomeric model compounds (imide compounds prep ared from n-butylamine and the acid anhydrides). It has been found tha t the model compounds from PMDA and BTDA both inhibit Ps formation and quench o-Ps lifetimes but that those from BPDA and 6FDA have neither the inhibition nor the quenching effects. The results show that the sp ur model is applicable for Ps formation in the polyimides.