A new strategy for ultralow biofouling membranes: Uniform and ultrathin hydrophilic coatings using liquid carbon dioxide

Highly stable, uniform and ultrathin hydrophilic polymer coatings on the surface as well as in the pores of a PVDF microfiltration (MF) membrane are obtained by coating a hydrophilic monomer in liquid carbon dioxide (l-CO₂) followed by subsequent crosslinking reaction. Polyethylene glycol diacrylate (PEGDA, M_n ~258g/mol) is used as the l-CO₂ soluble hydrophilic monomer source and azobisisobutyronitrile (AIBN) was used as a radical initiator. The extremely low surface tension and the low viscosity of l-CO₂ result in ultrathin and uniform PEG coatings on the hydrophobic polyvinylidene fluoride (PVDF) microfiltration membrane. The chemical composition, morphology, and the depth profiles of the PEG-coated membranes are characterized in detail using X-ray photoelectron spectroscopy, scanning electron microscopy, electron probe microanalysis and energy dispersive X-ray microanalysis. Long-term permeation flux test using a bovine serum albumin solution shows that the 1.0wt% PEGDA-coated membrane using l-CO₂ exhibits 1.34 times larger BSA solution flux than that of the uncoated PVDF membrane, and 1.3 times larger flux than that of a commercial hydrophilic membrane. Fouling resistance estimation shows that the 1wt% PEGDA-coated membrane exhibits ~30% lower internal fouling resistance than the pristine membrane, and ~24% lower internal fouling resistance than the commercial hydrophilic membrane.