Surface modifications of PVDF membrane applying non-thermal RF plasma and grafting reaction for MD desalination of highly saline brine: Surface properties and performance

Traditional membranes commonly used in MD (membrane distillation), such as PP (polypropylene), PVDF (polyvinylidene fluoride), or PTFE (polytetrafluoroethylene), have certain limitations: PP is not thermally stable, PTFE is easily fouled and difficult to process, and PVDF exhibits good properties but has poor hydrophobicity. Therefore, this study employed TEOS (tetraethyl orthosilicate), OTS (octyltrichlorosilane), and DTS (decyltrichlorosilane) to graft onto PVDF membranes, thereby enhancing their hydrophobicity and improving their MD performance. Before grafting at 70 ± 5°C for 1 h with different silane concentrations (5 %, 10 %, 15 %, 20 %, and 25 %), PVDF membranes were surface-activated by O 2 RF (radio frequency) plasma treatment. The membranes were then characterized using advanced methods, including WCA (water contact angle) measurement, ATR-FTIR (attenuated total reflectance Fourier-transform infrared spectroscopy), XPS (X-ray photoelectron spectroscopy), SEM-EDX (scanning electron microscope-energy-dispersive X-ray spectroscopy), and AFM (atomic force microscopy). The results showed that WCAs could reach 144 ± 6° for 25 % concentrations of OTS and DTS, while that of pristine PVDF was 124 ± 4.0°. FTIR and XPS spectra confirmed the successful grafting through the characteristic C-H vibration in 2900 − 3000 cm⁻¹ and the C-Si-O-C peak with binding energy of 102.0 ± 0.1 eV. SEM and AFM images showed that the grafted membrane surface became rougher with increasing silane concentration. The membranes grafted with OTS (10 %–25 %) showed lower permeate water conductivity (1.0 ± 0.1 – 1.6 ± 0.1 µS/cm) and higher MD flux than that of the pristine PVDF membrane. The MD performance of these grafted membranes was also quite stable during MD times of 16 and 24 h and at high NaCl concentrations (15 %). • Eco-friendly non-perfluoroalkylation of PVDF membrane for water resistance in MD. • Facile plasma treatment followed by grafting reaction in membrane hydrophobilization. • The highest WCA of 144 ± 6° (PVDF of 124 ± 4°) achieved for silane concentration of 25 %. • OTS (10 %–25 %) grafting showed lower permeated water conductivity and higher MD flux. • MD salt rejection and permeate flux were stable in 24 h in high salinity of 15 wt%.