Colloidal deposition on polymer-brush-coated NF membranes

Abstract

Fouling control remains a challenge in membrane separation. Here, nanofiltration (NF) membranes have been modified with poly-N-iso-propylacrylamide (PNIPAM) brushes, fabricated using surface-initiated atom transfer radical polymerization (ATRP). PNIPAM, a thermo-responsive polymer, exhibits a phase transition at its lower critical solution temperature (LCST) of ∼32 °C in DI water, which reduces to ∼20 °C at high-ionic strength. Direct microscopic observation was employed to investigate, in-situ, the deposition and release patterns and kinetics of colloidal fouling on NF membranes with and without a PNIPAM brush layer. Membrane surfaces were characterized by atomic force microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS), and attenuated total reflection–Fourier transform infrared (ATR-FTIR) analysis. Changes in surface chemistry and morphology confirmed the grafting of PNIPAM with a rougher surface morphology. The permeate flux of grafted membranes with DI water at ∼20 °C and ∼40 °C indicate reversible conformational changes of PNIPAM. Cross-flow experiments revealed that even though the surface hydrophobicity of the PNIPAM-grafted membranes increased, the brush-coated membrane surfaces hinder particle deposition regardless of particle surface charge, exhibiting considerably lower deposition, compared to the unmodified membrane. Deposition was lower, for both membranes, in the case of negatively charged particles. When the particles had a small positive charge, a significant increase in deposition was observed for the unmodified membrane, compared with the modified membrane, presumably due to the former’s larger negative charge. However, particles, once deposited, could generally not be removed by physical rinsing, nor – in the case of the PNIPAM-coated membranes – by switching polymer conformation. Nevertheless, the results of the direct observation can provide valuable insight for further improving fouling-resistant membranes based on polymer brush coatings.