Magnetically Responsive Membranes

Microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF) membranes have been modified by grafting polymer brushes from the membrane surface. Atom transfer radical polymerization (ATRP) has been used to graft two polymer: poly-hydroxyethyl methacrylate (PHEMA) and poly(N-isopropylacrylamide) (PNIPAm) from the surface of the membranes. PHEMA is very flexible while PNIPAm exhibits a lower critical solution temperature (LCST) at about 32 °C in DI water. Superparamagnetic nanoparticles were attached to the chain ends. In an oscillating magnetic field interactions between the external magnetic field and the magnetic moment of the nanoparticles will lead to heat generation as well as movement of the nanoparticles. Movement can be optimized my carefully selecting the oscillation frequency of the external field.

We show that for MF and NF membranes we can induce movement of the PHEMA brushes in an oscillating magnetic field. For membranes grafted with PNIPAm, we show that heating due to interaction of the oscillating magnetic field with the attached superparamagnetic particles can lead to collapse of the grafted chains most likely due to the temperature of the actual polymer chains being higher than the LCST. Both effects lead to significant change in membrane rejection and flux. In the case of NF membrane changes in membrane performance are due to polymer chains grafted from the barrier surface of membrane while for MF membranes, changes in performance are due to the chains grafted from the internal pore surface.

We have also modified UF membranes. We have developed a method to selectively graft PHEMA from the internal membrane pore surface or from the outer membrane surface. The method depends upon using an activator generated by electron transfer (AGET) ATRP procedure and limiting the ATRP initiator to the membrane pore or outer surface. Attachment of superparamagnetic nanoparticles to the polymer chains on the membrane surface can lead to a decrease in concentration polarization during protein filtration. On the other hand for membranes with nanoparticles attached inside the pores we can change membrane rejection in the presence of an external magnetic field.

Using PHEMA and PNIPAm as model polymers, we show how movement and heating induced by the attached nanoparticles can lead to changes in polymer conformation. We show that for NF, UF and MF membranes these changes in polymer conformation maybe used to tune membrane performance.