Nanoscale Hydrophobicity and Chemical Distribution of Surface Modified Polyethersulfone (PES) Membranes

Chemical modifications bring unique properties into polymeric membranes and enhance filtration or separation efficiencies, antifouling, antimicrobial activity and selectivity. However, there is a lack of nanoscale characterization for these chemical modifiers or additives to better understand their surface distribution and impacts on membrane surface properties. In this study, a series of polyethersulfone (PES) membranes modified with polyvinylpyrrolidone (PVP) were analysed systematically using powerful hybrid atomic force microscopy (AFM). Particularly, hydrophobicity and chemical distribution were measured by regular AFM and AFM coupled with infrared analysis capability (AFM-IR) for the first time, which successfully resolved nanoscale structural and chemical properties of the chemically modified PES membranes as shown in the attached image. Our results indicated the heterogeneous spatial distribution of PVP based on their characteristics IR bands and the resulting hydrophobicity distribution on modified membrane surfaces at nanoscale. Particularly, we established a linear correlation (R²=0.994) between the measured adhesion force and water contact angles, which enabled the examination of local surface hydrophobicity. The PES membranes became more hydrophilic with the increasing blend of PVP. With AFM-IR, trace amounts (1-4%) of PVP could be identified sensitively on PES membranes based on their unique characteristic IR bands, which were not achieved by conventional FTIR or IR mapping. Overall, these novel characterization approaches hold paramount importance for the design and quality control of polymer membrane modification and manufacturing.