Abstract
The positron annihilation method has been used to evaluate the unoccupied space of epoxies aiming at the molecular design of epoxy for cryogenic use. The fracture toughness of the commercially available epoxies was measured and was compared with the life time τ3. The epoxy was found to be classified into two types that is (i) those shows the increase of fracture toughness with free volume and (ii) those do not present large change with increasing free volume evaluated by positron annihilation. The former was understood as the system with disordered network and the latterr as that with different crosslinking density. The discussion suggests that the latter system is preferable for cryogenic use and was confirmed experimentally.
