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.
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