APPLICATION OF THE POSITRON-ANNIHILATION METHOD FOR EVALUATION OF ORGANIC MATERIALS FOR CRYOGENIC USE

The positron annihilation method was used to evaluate the unoccupied s paces in epoxies with the aim of molecular design of epoxies for cryog enic use. The fracture toughness of commercially available epoxies was measured and compared with the lifetime tau(3). Evaluation by positro n annihilation enabled the epoxies to be classified into two types: (1 ) those whose fracture toughness increases with free volume, and (2) t hose that do not change much with increasing free volume. The former t ype was understood to have systems with disordered networks, the latte r, to have different crosslinking densities. The discussion suggests t hat the latter system is preferable for cryogenic use; it was confirme d experimentally.