Modification of membrane surfaces by grafting polymer brushes from the surface has been shown to impart unique surface properties. Here we focus on magnetically responsive membranes where magnetically responsive polymer chains are grown from the membrane surface. We have developed a range of microfiltration1, ultrafiltration2 and nanofiltration3 membranes by grafting magnetically responsive polymer brushes from the membrane surface. Atom transfer radical polymerization (ATRP) has been used to graft poly-hydroxyethyl methacrylate (polyHEMA) from the surface of the membrane. Superparamagnetic nanoparticles have been attached to the chain ends. In an oscillating magnetic field, movement of the magnetically responsive nanobrushes leads to suppression of concentration polarization resulting in higher permeate fluxes and better rejection. We have also grafted poly(N-isopropylacrylamide) a thermo-responsive polymer that exhibits a lower critical solution temperature, using ATRP, from the surface of the membrane4. By carefully choosing the frequency of the oscillating magnetic field, movement of the polymer chains can induce mixing. Using much higher frequencies, around 1,000 Hz, heating will lead to collapse of poly(N-isopropylacrylamide) layer as the temperature of the grafted polymer layer increase above the lower critical solution temperature of the grafted poly(N-isopropylacrylamide).

Unlike nanofiltration and microfiltration membranes where the majority the polymer chains are grafted from the barrier layer or the inside pore surface respectively, in the case of ultrafiltration membranes significant grafting can occur from both the barrier layer and the internal pore surface. In addition, given the smaller pore sizes compared to microfiltration membranes, pore plugging by the grafted polymer chains must be avoided. We have developed a novel technique to selectively graft from the external barrier layer and not the internal membrane pore surface. We show that the magnetically responsive polymer brushes can have a significantly different effect on rejection and flux of model feed streams consisting of proteins such as bovine serum albumin, depending on their location on the membrane barrier layer or in the pores. Our work highlights the importance of being able to control not only the three-dimensional structure of the grafted polymers but also their location; from the membrane barrier layer or from the inside pore surface.

References 1. H. H. Himstedt, Q. Yang, X. Qian, S. R. Wickramasinghe, M. Ulbricht (2012), Toward remote-controlled valve functions via magnetically responsive capillary pore membranes’, J Membr. Sci., 423, 257-266. 2. B. M. Carter, A. Sengupta, X. Qian, M. Ulbricht, S. R. Wickramasinghe (2018), Controlling external versus internal pore modification of ultrafiltration membranes using surface-initiated AGET-ATRP, J Membr. Sci, 554, 109-116. 3. Q. Yang, Q., H. H. Himstedt, M. Ulbricht, X. Qian, X., S. R. Wickramasinghe, Designing magnetic field responsive nanofiltration membranes, J Membr. Sc., 430 (2013) 70-78. 4. X. Qian, Yang, Q., Vu, A. T., Wickramasinghe, S. R. (2016), ‘Localized Heat generation from Magnetically Responsive Membranes’, Industrial & Engineering Research, 55 (33), 9015-9027.