Correlation Study between Free-Volume Holes and Molecular Separations of Composite Membranes for Reverse Osmosis Processes by Means of Variable-Energy Positron Annihilation Techniques

The nanoscopic structure of three kinds of commercially available composite membranes (LF10, NTR729HF, and NTR7250) for water purification has been studied by energy-variable positron annihilation. The membranes consist of a functional polymer film and porous polysulfone substrate supported on a nonwoven polyester fabric. Positron annihilation γ-ray and positron lifetime techniques were employed to probe the layer structure of the composite membranes and subnanometer-sized free-volume holes in the functional films, respectively. It was found that the functional film on the porous substrate in LF10 contains two different layers on the substrate, while that in NTR729HF and NTR7250 consists of a single top layer. Comparison of the rejections of different uncharged solutes, determined by a pressure-driven experiment, with the hole size of the functional film, obtained from the positron lifetime data, revealed that the holes probed by positrons can well explain the hindering effect of molecular transport. Moreover, the higher rejections by LF10 were found to arise from the denser top layer than the top layers of the other membranes. A combined use of the present positron techniques was demonstrated to be useful for examining the hole structure as well as the layer structure of the composite membranes.