Most of the earth’s water is in the oceans and only 3% is fresh water and the leading method for making potable water from seawater is reverse osmosis (RO). The polyamine (PA) barrier layer in commercial RO membrane systems is made through an interfacial polymerization process carried out on the surface of nano-porous polysulfone (PS). Despite the technical importance of the barrier layer (the smallest pore size component in a RO membrane), an understanding of the relationship between molecular level structure and function has yet to emerge. To determine the molecular level structure of the barrier layer, we have carried out grazing incidence wide angle x-ray scattering (GIWAXS) studies on BW30 and BWXLE membranes, commercially available products manufactured by DOW Filmtec. For the x-ray measurements, the non-woven fabric is first peeled off and the PS is dissolved with an organic solvent. At the end of the process, the PA film is in contact with a polished silicon wafer with the PA surface facing the wafer. The x-ray measurements were carried out at the CMS beamline at NSLS II (BNL) with varying incidence angles as this provides a measure of the depth dependence of the scattering. The GIWAXS patterns exhibit diffuse scattering features (see figure) characteristic of an amorphous structure with several characteristic length scales. The BWXLE membrane shows the existence of two peaks in q, each with about the same intensity whereas BW30 exhibits a single peak. For both samples, the anisotropy shows the existence of preferential surface induced alignment. Further, the dependence of the scattering profiles on the incident angle suggests that the molecular structure is depth dependent. The effects of using different solvents to dissolve the PS on the x-ray results will be discussed. On the basis of the scattering results, we present a structural model of the packing of the aromatic motifs. Further, we will discuss these results in the context of their permeance properties and published physical characteristics.

Molecular structure of commercial reverse osmosis polyamide barrier layers

2019 Spring National Meeting

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