POSITRONIUM LIFETIME IN AN ELLIPSOIDAL FREE-VOLUME HOLE OF POLYMERS

A new relationship between the ortho-positronium lifetime and the anisotropic dimension in an ellipsoidal free-volume hole is developed. The positronium wave function is solved in a model of the-particle-in-an-ellipsoid and the lifetime is obtained by a numerical integration method. The obtained positronium lifetime in an ellipsoidal hole is found to be shorter than that in a sphere with the same volume. The positronium lifetime in a stretched PEEK semi-crystalline polymer is measured to demonstrate the application of this new relationship to probe the anisotropic hole properties in polymers.