Controlling Ultrafiltration Membrane Rejection via Shear-Aligned Deposition of Cellulose Nanocrystals from Aqueous Suspensions

Cellulose nanocrystals (CNCs) of 180 nm length and 8 nm diameter were deposited on porous supports by tangential flow filtration followed by salt permeation to form ultrafiltration membranes. At a high enough shear rate on the support surface, CNCs aligned in the direction of flow, showing a nematic order. The shear rates for transition to the nematic phase determined from rheology analysis, polarized optical microscopy, and membrane performance were consistent with one another, at ca. 10 s–1. Permeating an AlCl3 solution through the shear-aligned CNC deposit stabilized the CNC layer by screening repulsive electrostatic interactions, and the stable CNC layer was obtained. On changing the surface shear rate from 10 to 50 s–1, the order parameter of CNCs increased from 0.17 to 0.7 and the rejection for Blue Dextran (5 kDa) increased from 80.4 to 92.7% and that for β-lactoglobulin (18 kDa) increased from 89.6 to 95.4%. Hence, a simple and scalable method for controlling rejection properties of ultrafiltration membranes is developed, which uses aqueous CNC suspensions to form the selective layer.