Morphology of thin film composite membranes explored by small-angle neutron scattering and positron-annihilation lifetime spectroscopy

The morphology of thin film composite (TFC) membranes used in reverse osmosis (RO) and nanofiltration (NF) water treatment was explored with small-angle neutron scattering (SANS) and positron-annihilation lifetime spectroscopy (PALS). The combination of both methods allowed the characterization of the bulk porous structure from a few Å to µm in radius. PALS shows pores of ~ 4.5 Å average radius in a surface layer of about 4 μm thickness, which become ~ 40% smaller at the free surface of the membranes. This observation may correlate with the glass state of the involved polymer. Pores of similar size appear in SANS as closely packed pores of ~ 6 Å radius distributed with an average distance of ~ 30 Å. The main effort of SANS was the characterization of the morphology of the porous polysulfone support layer as well as the fibers of the nonwoven fabric layer. Contrast variation using the media H2O/D2O and supercritical CO2 and CD4 identified the polymers of the support layers as well as internal heterogeneities.