Positron annihilation lifetime spectroscopy applied to the measurement of hole volumes in crosslinking polyimides

Part I: Positron Annihilation Lifetime Spectroscopy, which is applied to measure distributions of hole volume sizes in polymers, uses a model to relate the positronium (Ps) lifetime to hole volume dimensions. It is assumed in this model that the Ps remains in one site until annihilation. To investigate this assumption we calculated the rate for Ps in one potential well to transfer to a second. We found for appropriate, equal wells, that the transfer rate between sites was rapid on the timescale of Ps lifetimes, while for only slightly unequal wells the transfer is cut off. This cutoff is a consequence of the removal of the degeneracy of the single well energies. For multiple wells it is found that Anderson localization applies so that quite generally a Ps is trapped at only a single hole volume until it decays. Thus Ps-lifetime hole-size-distribution-measurements should be unaffected by tunneling. Part II: Thermally-induced crosslinking in polyimides has been studied by positron annihilation lifetime spectroscopy. A series of norbornene-endcapped polyimides with varied formulated molecular weights was exposed to 316 degrees C heat treatment for up to 2000 hours to obtain samples with a variety of crosslinking densities. It is found that measurements of the intensity of the o-Ps component are directly related to the crosslinking densities in non-highly crosslinked samples. However, in the highly crosslinked samples, o-Ps is no longer formed. The inhibition of o-Ps formation in these samples is attributed both to the small dimensions of the hole volumes in the crosslinked samples and to the presence of strong electron-trapping moieties in the highly crosslinked samples.