Preparation and characterization of novel hydrophobic/hydrophilic polyetherimide composite membranes for desalination by direct contact membrane distillation

Two different types of hydrophobic surface modifying macromolecules (SMMs) were synthesized and characterized for fluorine content, average molecular weight and glass transition temperature. The synthesized SMMs were blended into polyetherimide (PEI) hydrophilic host polymer to form porous hydrophobic/hydrophilic composite membranes by the phase inversion method. The prepared membranes were characterized by the contact angle measurements, X-ray photoelectron spectroscopy test, gas permeation test, liquid entry pressure of water and scanning electron microscopy. Finally, these membranes were tested for desalination by direct contact membrane distillation (DCMD). Different parameters affecting the membrane preparation process were studied and their effects on the membrane morphology as well as on the membrane performance in desalination by DCMD were identified. These parameters include the SMMs type, SMMs concentration, solvent type and solvent evaporation time before gelation. An attempt to link the membrane morphology to its performance in DCMD is presented. This leads to better understanding of the effect of the membrane preparation on its performance. It was found that increasing the solvent evaporation time before gelation decreased the membrane flux since smaller pore sizes were observed. The membranes with higher contact angles and fluorine contents (more hydrophobic) exhibited smaller permeate fluxes. The membranes having sponge-like structures at the hydrophilic layer exhibited higher fluxes than those having finger-like structure at the hydrophilic layer. Moreover, the results were compared to a commercial polytetrafluoroethylene (PTFE) membrane. It was observed that most of the SMMs blended PEI membranes achieved better DCMD fluxes than those of the commercial membrane. A permeate flux 55% higher than that of PTFE was achieved. For both PTFE commercial membrane and all SMMs blended PEI membranes, the NaCl separation factor was found to be higher than 99% except for the PEI membrane prepared without SMMs.