Organic solvent-resistant and thermally stable polymeric microfiltration membranes based on crosslinked polybenzoxazine for size-selective particle separation and gravity-driven separation on oil-water emulsions

Polymeric membranes, compared with their inorganic counterparts, usually suffer from insufficient solvent resistance and thermal stability. In this study, an organic solvent–resistant thermally stable polymeric microfiltration membrane is prepared with a self-crosslinkable polybenzoxazine as a precursor. The obtained membranes have pore sizes of approximately 3.4 µm and superior antiswelling stability in organic solvents; thus, they exhibit excellent performance in terms of size-selective particle separation inorganic phases. The membranes are also effective in the gravity-driven separation of surfactant-stabilized water-in-oil emulsions. The water contents of the filtrate oil phases are close to the natural solubility of water in the solvents; for example, a water content of 350 ppm, which is comparable to the water solubility in toluene (330 ppm), has been found in the filtrate toluene from water-in-toluene emulsions. Moreover, the membrane demonstrates pH-induced changes in oil and water selectivity in oil–water separations. When treated with a strong base solution, the oil-selective membrane becomes water-selective and exhibits favorable performance in terms of separating organic solvents from surfactant-stabilized oil-in-water emulsions. These features effectively extend the application scope of the prepared membrane for oil–water separations.