Positron annihilation spectroscopy: a technique to probe the molecular level of nanofiltration membranes

摘要

Positron annihilation (lifetime) spectroscopy (PA(L)S) is a relatively new technique that has been used to probe structures at an atomic scale in semiconductors, metals, dielectrics, clays and polymers. It is based on the interaction of the electrons in these media with positrons. It is a mild, non-destructive method that can be applied to samples under ambient conditions. In membrane technology, PAS has already been applied in the characterization of commercial pervaporation and gas separation membranes. It has also been recently used to study the free-volume depth profile of commercial nanofiltration membranes for aqueous applications . Recent studies have been done to evaluate structural changes and free volume properties in thin film polyamide (PA) pervaporation membranes prepared by interfacial polymerization~(TM). Nevertheless, no clear correlation was found between the PAS data and the flux and selectivity in the pervaporation separation of 2-propanol aqueous solutions. From lifetime measurements, the effect of the concentration of the amine monomer in the synthesis of the selective PA top layer in the free volume could be related to the selectivity of these membranes. An in-depth study of the process and the parameters involved in the membrane synthesis via phase inversion has not been carried out so far. Such asymmetric structure is present in NF-membranes, where the skin determines the separation and the bottom structure gives mechanical support. Moreover, a need to link information provided by PAS to membrane performance is clearly still needed for a better understanding of separation processes.