Preparation of a Phenolic-Resin-Based Polymer Sponge Composed of Intertwined Nanofibers with Tunable Wettability for High-Efficiency Separation of Oil–Water Emulsions

Nowadays, as the combination of water pollution and water shortage causes severe environmental and social issues, the special wettable materials, which can be selectively wetted by either water or oil, attract tremendous attention for high-efficiency separation of oil–water mixtures. Herein, we prepare a phenolic-resin-based sponge composed of intertwined nanofibers via a simple hydrothermal method. The wettability of the as-prepared polymer is tuned readily by controlling only the hydrothermal temperature. In the case of the hydrothermal temperature below 210 °C, the polymer sponge demonstrates superhydrophilic and underwater superoleophobic properties, affording the separation of oil-in-water emulsions. However, as the hydrothermal temperature increases above 220 °C, the resulting bulk phenolic-resin-based material becomes superhydrophobic and underoil superhydrophobic, realizing a high filtration flux of 6147 L m–2h–1for the separation of water-in-oil emulsions driven by an external pressure of 40 kPa. This provides a feasible platform for future practical applications. The wettability transition depending on the hydrothermal temperature is discussed in terms of the reaction mechanism. In addition, the stability and breakthrough pressure are also addressed from the viewpoints of thermodynamic and fluid mechanics, respectively.