Application of positron annihilation: Study of pervaporation dense membranes

Separation of liquid mixtures, particularly azeotropic mixtures, is being tried out by pervaporation process, which utilizes dense membranes. Such membranes, although are of non-porous type, still requires determination of free volume sizes which is crucial to the understanding of the process. Positron annihilation technique has been developed into a powerful characterization tool for the study of free volume and free volume fraction in polymers. In the present work, an attempt has been made to estimate the free volume sizes of commercial and laboratory made pervaporation membranes with the application of positron annihilation lifetime spectroscopy. The positron lifetime spectra were analyzed, assuming 3 or 4 exponential components. Long-lived components (lifetimes: in the range of 1.4–3 ns) were found, which were attributed to ortho-positronium (o-Ps) pick-off annihilations in free volumes. Accordingly, free volume size determinations were carried out, considering shapes for free volumes as spherical as well as cylindrical. Further, utilizing such free volume data, results of hydrazine hydrate separation by pervaporation were interpreted leading to a better understanding of the process.