Morphological and hydrophobic modifications of PVDF flat membrane with silane coupling agent grafting via plasma flow for VMD of ethanol-water mixture

Membrane distillation (MD) has gained great attention during the last two decades. Meanwhile, polyvinylidene fluoride (PVDF) has also become one of the popular membrane materials for MD on account of its good hydrophobicity and excellent chemical stability. However, the additives remained in the membrane during the phase inversion in preparation usually reduce the hydrophobicity of membrane. In the present work, a versatile modification tactic that focused on increasing the porosity/pore size of membrane by plasma flow irradiation and subsequent enhancing of the hydrophobicity by grafting silane coupling agent was studied. The influences of plasma irradiation time and grafting conditions, including pH value, concentration of silane coupling agent and temperature of heat treatment, on the vacuum membrane distillation (VMD) performances for 5% (w/w) ethanol aqueous solution were investigated. The membranes were characterized by ATR-FTIR, contact angle, pore size, degree of grafting and SEM to test the effects of modification. The analysis results revealed the changes in the surface pore size/porosity, chemical composition and hydrophobicity of the membrane. A proper etching via plasma flow made the membrane surface more porous without damaging the membrane bulk structure. And the water contact angle increased greatly from 70.1° to 121.0° after modification. Therefore, the modified membranes showed significant increases in both permeation flux (from 3.19 kg/m2 h to 5.39 kg/m2 h) and separation factor (from 4.94 to 8.50) for VMD under a feed temperature of 50 °C and a vacuum degree of 86 kPa in permeates side.