β-Cyclodextrin-based star polymers for membrane surface functionalization: Covalent grafting via click chemistry and enhancement of ultrafiltration properties

This work provides the proof-of-concept that surface functionalization of porous membranes with well-defined star-like polymers of varied number of arms and arm length leads to a tunable effective thickness of grafted layers and a specific influence on size-selective sieving through ultrafilter pores. Alkyl-brominated beta-cyclodextrine with either 7 or 21 initiator sites per molecule was synthesized and further used for controlled atom transfer radical polymerization of copolymers of 2-dimethylamino(ethyl) methacrylate (DMAEMA) and propargyl methacrylate (PgMA), leading to star polymers with either 7 or 21 arms. Alkyne-containing PgMA segments enable the click coupling while DMAEMA segments provide the bulk of the polymer. Star polymers were characterized with respect to chemical structure and molecular weight (M). During ultrafiltration (UF) through cellulose membranes with different molecular weight cut-off, rejection was not simply correlated with star polymer M but was governed by macromolecular architecture, i.e. the smaller colloidal diameter for macromolecules of same M but 21 instead of 7 arms. Azide-functionalized poly(ethylene terephthalate) (PET) track-etched (TE) membranes and cellulose UF membranes were prepared by polymer-analogous surface functionalization so that the alkyne-substituted star polymers could be click-grafted. Isoporous PET TE membranes with a nominal pore diameter of 200 nm were used as model system to study the grafting and its effects onto pore size via the reduction of hydraulic permeability. Effective grafted layer thickness in the range of 10-50 nm correlated with macromolecular structure and architecture. For click-functionalized cellulose UF membranes, the effective pore size in the barrier layer was influenced by grafted star polymers, and a pronounced additional influence of the architecture and arm length of the grafted star polymer on macromolecular sieving was observed. Of particular interest are results with the more flexible 7-armed star polymers (compared to 21-armed counterparts); their grafting at the UF membrane pores of similar dimension leads to a large increase of test solute rejection at very low reduction of convective water flux, both compared to the unmodified membrane.