Magneto-responsive ultrafiltration membranes

The discovery of polymers with tunable physicochemical properties led to the development of stimuli-responsive membranes, which have the potential to be applied in complex separation processes requiring adaptive control. Size-selective filtration is regulated by a reversible volumetric shift of the incorporated smart actuator (e.g., grafted polymer chains, hydrogel network, microgels), which opens and closes pore channels. A very popular stimulus for this action is heat. However, the dynamic barrier properties are usually induced by heating the entire feed, which limits the practicality of thermo-responsive composite membranes. A promising solution is the incorporation of nano-heaters. These are magnetic nanoparticles that generate heat when exposed to a high frequency alternating magnetic field (AMF). Pioneer works in the development of magneto-responsive membranes have successfully implemented this synergistic effect. However, the first designs of composite membranes with magneto-responsive sieving performance involved complex fabrication processes and/or the modification of the entire membrane, with the latter resulting in poor flux performance. In this work, magnetic microgel thin-film composite membranes were developed to address these challenges. Magnetic core-shell microgels with a particle diameter of a few hundred nanometers were successfully synthesized through seeded precipitation co-polymerization. Oleic acid bilayer coated iron oxide nanoparticles (IONPs, isoelectric point at pH 5.8) in monomer solution (pH 5.1) were hydrophobic due to the protonation of carboxylic end-groups, which allowed the adsorption and growth of precipitating poly(N-isopropylacrylamide) (PNIPAAm) on the particle surface. The different core sizes did not affect the dispersity of the magnetic microgels, volume phase transition or swelling behavior compared to the non-magnetic counterparts. The high degree of cross-linking (15 mol%) caused a gradual reduction in particle size, with an average volume phase transition temperature (VPTT, turning point of the sigmoidal curve) at 34 °C. The final microgel size in the collapsed state was reached at 40 °C, with the particle diameter decreasing by over a hundred nanometers. Polyethersulfone microfiltration and ultrafiltration support membranes with a sponge-like pore structure were utilized for microgel coating. To enable the adsorption of negatively charged microgels (-30 mV), the membrane surface was first pre-coated with the polycation polyethyleneimine (PEI) via drop coating. A homogeneous PEI coating and controlled PEI loading were achieved by adjusting the applied PEI concentration. Increasing the PEI concentration resulted in higher PEI loading, shifting the surface charge to higher positive values. The increase in PEI concentration was also associated with a reduction in pore size and permeance due to polymer swelling-induced thickening on pore walls. Based on that, the optimal PEI concentration for pre-coating was 0.3 g/L. The filtration of excess microgels caused concentration-polarization, which favored a dense coating. Ultrafiltration of swollen microgels (surface pores < microgels) resulted in poor thermo-responsivity (TR, N < 2), while microfiltration (surface pores > microgels) partially caused pore filling, resulting in moderate TR (N ∼4). Atomic force microscopy height profiles revealed packed spherical microgels but also irregular patterns of surface defects. Upper microgel layers were not electrostatically stabilized by PEI, resulting in partial washing out of microgels during post-coating cleaning or backwashing. Microfiltration of collapsed microgels increased the adsorption capacity, which resulted in higher film density, film growth within deeper pore sections, and ordered particle packing. The drawbacks included irreversible pore blockage and persistent defects. Controlled microgel layer adsorption on ultrafiltration and microfiltration membranes above the VPTT was conducted to analyze microgel film formation. In early adsorption stages, rapid flux decrease in pore-covered membranes was observed. As ultrafiltration progressed, thermo-responsive permeance became more apparent with a larger number of microgel layers. However, beyond a critical layer thickness, it plateaued or diminished, caused by irreversible shielding of sublayer voids by upper microgel layers. Rough surface topographies and agglomeration of hydrophobic microgels formed heterogeneous layers. The phase transition of the microgels affected the permeance. Microgel swelling improved gap filling because of increased particle diameter and flexibility below the VPTT. However, low shear rates weakened shape flexibility, causing them to mobilize into unbonded, repulsive top layers. These findings underscore the need for stabilizing multilayers and further exploration of concentration-polarization-based microgel adsorption. Optimal heating performance for superparamagnetic IONPs with a specific loss power (SLP) of 126 W/g was achieved at an input current of 15.6 A and a frequency of 745 kHz, corresponding to a field amplitude of 20 ± 10 kA/m. Water filtration experiments with external cooling successfully suppressed non-specific heating (0.06 W). However, it was greater than the SLP of embedded IONPs (0.03 W, six layers). Consequently, the magnetic composite membrane lost filtration dynamics upon cooling. With optimized cooling, magneto-heating could have contributed to a homogeneous heat distribution within the microgel layers, but further investigation is required to verify. The results imply that the specific heating power of IONPs should be in the range of the non-specific heating power (∼0.4 W) obtained without cooling to observe a pronounced VPT. Considering that the prepared superparamagnetic IONPs are of the weaker heating type, magnetic nanoparticles with significantly higher SLP may reach the required heating output on a localized scale. For further progress, refining various parameters is required. This work provides a deeper understanding of the formation of microgel multilayers and the behavior of microgels within these layers, which will be beneficial in optimizing the membrane performance of microgel thin-film composite membranes.