Superhydrophobic membrane by hierarchically structured PDMS-POSS electrospray coating with cauliflower-shaped beads for enhanced MD performance

The construction of superhydrophobic surfaces for improving the anti-wetting performance of membrane distillation (MD) is still an on-going challenge. In this study, we fabricated a hierarchically structured electrospray coating with cauliflower-shaped beads by adjusting polymer chain entanglement. A series of poly(dimethylsiloxane)-polyhedral oligomeric silsesquioxane (PDMS-POSS) nanocomposites were synthesised by the chemical incorporation of rigid V-POSS into a flexible PDMS matrix. With the optimal incorporation content, we successfully obtained a homogeneous molecular-level dispersion of V-POSS into PDMS matrix, forming a more branched yet looser network, leading to the appearance of cauliflower-shaped beads when electrosprayed with polyvinylidene fluoride (PVDF). Moreover, the integrated V-POSS and its self-crosslinked clusters contributed to the formation of convexities and protrusion on the beads. This optimal coated membrane with PVDF as substrate exhibited superhydrophobicity, as evidenced by a water contact angle of ~166°; which resulted from the rough surface with well-defined ridges and valleys, composed of irregular beads. Once applied in direct contact MD (DCMD) processes, it demonstrated stable performance and high salt rejection (>99.9%) during the 200 h treatment of synthetic seawater containing 0.1 mM sodium dodecyl sulfate (SDS), along with great robustness under harsh feed condition containing 0.3 mM SDS, showing good applicability for preventing wetting.