PVDF membranes for membrane distillation prepared by VIPS – A combined study of artificial water channel effectiveness and membrane performance prediction

Background A combined VIPS-NIPS technique was used to investigate the potential of self-supporting flat-sheet polyvinylidene fluoride (PVDF) membranes containing amphiphilic I-quartet Artificial Water Channels (AWC) for Direct Contact Membrane Distillation (DCMD). The AWC are formed in situ by self-assembly of HC6 molecules upon contact with water during membrane formation. Methods A Design of Experiment (DoE) was used to investigate the influence of various experimental parameters and their interactions on membrane performance, namely polymer and AWC concentrations in the dope solution, relative humidity and duration of the VIPS treatment. Pure water permeability (PWP), Liquid Entry Pressure (LEP) and DCMD flux were used to characterize membrane performance. A Response Surface Methodology (RSM) was used to evaluate the DoE results and a second order model was fitted. Based on the model predictions several multiple response-optimized (MRO) membranes were prepared. Results The performance improvement of the MRO membranes was 7.0 times for PWP (M-T4) and 3.3 times for MD flux (M-T3) over the average performance of the DoE membranes, while maintaining LEP of 1.9 bar and Salt rejection of 99.9%. In addition, hybrid PVDF/HC6 MRO membranes improved PWP by 3.2 times and MD flux by 1.6 times over pure PVDF MRO membranes. Conclusions DoE in conjunction with RSM enabled the prediction of membrane PWP, LEP and MD flux performance by varying the levels of the experimental parameters within predefined limits. The HC6 additive improves PWP and MD flux without loss of LEP or selectivity. HC6 promotes the formation of porous skin and asymmetric cross-sections with finger-like structures. Probably by affecting diffusion during phase separation due to its amphiphilic properties.