Durability and Free Volume in Polymeric Coatings Studied by Positron Annihilation Spectroscopy

A simple positron annihilation lifetime (PAL) spectrometer using secondary electrons as the starting signal has been constructed inside the existing slow positron beam at the University of Missouri-Kansas City (UMKC). This instrument has been employed for investigating photo-degradation of amine-cured epoxy coatings exposed to UV radiation. The lifetime results are compared with the S-parameter changes obtained from positron annihilation spectroscopy (PAS). A significant decrease of the S-parameter and of ortho-positronium (o-Ps) intensity in polymers is observed as a function of exposure time and of depth from the bulk to the surface. This is interpreted as due to a loss of free volume and holes resulting from photo-degradation in the samples. UV irradiation in epoxy produces a 'dead layer' near the surface up to a thickness of 0.3 *mm, where a substantial change in the o-Ps intensity and the S-parameter is observed. (Materials: aluminum plates coated with cross-linked epoxy.)