Surface Characterization of Membrane Surfaces Nanostructured with Tethered Hydrophilic Polymers

Membrane surface modification with end-tethered hydrophilic polymers is an effective approach for reducing fouling propensity and improving cleaning efficiency of RO and UF membranes. It has been suggested that, in addition to chemistry, swelling of the polymer layers play a crucial role in minimizing foulant attachment, yet detailed characterization of swollen tethered polymer chains is still needed in order to elucidate the mechanism by which swelling minimizes fouling. In this study, AFM was used to characterize swelling of surface tethered polymer chains by utilizing poly(acrylic acid) (PAAc) and poly(2-hydroxyethyl methacrylate) (PHEMA) chains end-grafted onto polysulfone (PSf) membrane surrogate surfaces via atmospheric pressure plasma-induced graft polymerization (APPIGP). Impact of polymer chain swelling on surface topography of PAAc and PHEMA layers was evaluated by AFM imaging in various solvents (i.e., air, deionized water, saline water). The thickness (or extension) of the swollen polymer chains was assessed by determining contour length distributions of the tethered polymer chains via single molecule force spectroscopy performed in aqueous solutions of different salinity. The adhesion force measurements were performed with a functionalized colloidal probe in order to quantify fouling potential of the polymer layers of different swelling degrees. The results of the present study provide a direct measure of the potential effectiveness of tethered chains for membrane fouling reduction. In this regard, direct surface force measurement can guide the optimization of surface modification for synthesizing high performance low fouling membranes.