Shear-induced permeability changes in a polymer grafted silica membrane

The hydrodynamic response of a graft-polymerized membrane was demonstrated for a microporous silica-poly(vinylpyrrolidone) (silica-PVP) membrane. The membrane pores were modified by graft polymerizing vinyl pyrrolidone onto the membrane pore surface, resulting in a polymer surface layer of covalently tethered polymer chains. The hydraulic permeability of the modified membrane increased with increasing transmembrane pressure owing to flow-induced deformation of the grafted polymer chains. The dynamics of the modified pores was investigated by membrane hydraulic permeability studies along with a two-region hydrodynamic pore flow model. The thickness of the grafted polymer layer decreased with increasing pore-wall shear rate by up to about 47%, relative to the thickness at the zero shear limit, depending on the surface density and length of the grafted chains. Although the effective pore size of the polymer-grafted membrane was reduced by 5–36% (at the zero shear rate limit), about 18–59% of the pore size loss was regained at high pore-wall shear rates. Increasing the degree of shear-induced permeability change is feasible by increasing the ratio of the polymer chain length/pore size ratio as well as the surface density of the grafted polymer phase. The present results suggest that hydrodynamic pore size control could provide an additional useful degree of freedom in operating polymer-modified filtration membranes.