Novel magneto-responsive ultrafiltration membranes for remote controlled switchable molecular sieving
Stimuli-responsive separation membranes with tunable molecular scale pore size, which are desirable for on-demand sieving of targeted macromolecules, have attracted increasing attention in recent years. It is well-known that magnetic nanoparticles (MNP) can generate heat when exposed to an alternating magnetic field (AMF), due to hysteresis and relaxation losses. When combined with thermo-responsive polymers, the heat generated by MNP can trigger the phase or conformation change of polymer and therefore by their synergistic effect, magneto-responsive material systems can be designed. In this study, by using iron oxide MNP as localized heater and thermo-responsive Poly(N-isopropylacrylamide) (PNIPAAm) hydrogel as sieving medium and actuator, two kinds of magneto-responsive membranes with excellent responsivity and tunability for molecular sieving have been developed. One type is magneto-hydrogel pore-filled membrane constructed by post modification of commercial polyethylene terephthalate (PET) track-etched membrane. Such membranes use the hydrogel mesh as the ultrafiltration sieving medium, and the iron oxide entrapped in the hydrogel network as localized heater. It has been demonstrated that by manipulating AMF and thus controlling the heat generated by the incorporated MNP, the micro-syneresis of PNIPAAm network can be locally triggered and magneto-responsive molecule sieving can be achieved. The other type is polyethersulfone (PES) mixed matrix membrane blended with prefabricated PNIPAAm nanogel and iron oxide MNP fabricated by non-solvent induced phase separation (NIPS) process. It has the PNIPAAm nanogels as responsive gates and the co-blended iron oxide nanoparticles as local heaters. By manipulating the swollen/shrunken state of PNIPAAm nanogels triggered by the heat generation of nearby MNP, the molecular sieving performance of the membrane can be adjusted. Overall, important parameters affecting the construction of both kinds of membranes have been studied, their stimuli-responsive barrier properties including thermo- and magneto-responses have been investigated, and their structure and molecular sieving effect in the ultrafiltration range have been evaluated. Based on the results, correlations between synthesis, structure and separation performance of the novel magneto-responsive membranes and their potential for applications have been discussed in this work.