Morphology and porous structure of standalone aromatic polyamide films as used in RO membranes – An exploration with SANS, PALS, and SEM

This report presents a study about the morphology of a cross-linked aromatic polyamide (PA) film in its dry and wet state using small-angle neutron scattering (SANS), positron annihilation lifetime spectroscopy (PALS), and scanning electron microscopy (SEM). PA is used as active skin layer for reverse osmosis membranes and determines the selectivity of the membrane with respect to water and salt molecules. This selectivity of PA is largely determined from its morphology. The PA film scatters the neutrons much stronger and shows a different profile as expected from randomly distributed nanoscopic large pores. SANS contrast variation, using supercritical CO2 as contrast medium, confirms that pores are the only scattering centers. The analysis shows that interconnected pores of an average radius of about 16 Å determine the scattering. The pores are formed as a network of channels showing a fractal structure. PALS determines pores of ∼6 Å diameter representing the diameter of such channels. The volume fraction of the pores is estimated as ∼10% which after water up-take increases by ∼30% due to slightly swelling of the pores.