Research On Polyvinylidene Fluoride Porous Membrane Microstructure Adjustment And Oil-Water Separation Performance

Polyvinylidene fluoride（PVDF）is one of the most widely used membrane materials for oily wastewater separation due to its outstanding oxidation resistance,high thermal stability,highly selectivity,excellent chemical resistance as well as mechanical strength.However,normal PVDF membrane is hardly to achieve high fluxes,high selectivity and long-term stability in emulsion separation.Optimizing the surface structure and wettability of the membranes is the key to improve its oil-water separation performance.In this paper,a number of PVDF membranes were prepared by phase inversion method.The influence of fabrication process on membrane structure and performance,the influence of surface structure on nanoparticle loading,wettability and emulsion separation were systematically studied.Furthermore,Janus membrane with asymmetric wettability was prepared by pre modifying the substrate.The main work of this paper is as follows:pH responsive PVDF/PAA porous membranes were prepared via in-situ polymerization of PAA in PVDF solution and vapor induced phase separation process.After characterization,it was found that being exposed in vapor resulted in the special structure forming on membrane surface,increasing of pore size,porosity and the concentration of carboxyl on the surface.PVDF/PAA had p H-responsive ability,the membranes with 5 minutes exposure time had the highest flux 613.9±45.7 L·m^-2·h^-1·bar^-1.By adjusting p H value,the pore size of PVDF/PAA membrane decreased with the increase of p H value.PVDF/PAA membrane had the copper ions adsorbing ability,after 20 minutes static adsorption,the maximum adsorption capacity is 78.2±0.4 mg/g,and it can desorb with the change of p H.β-Fe O（OH）nanorods were successfully modified on the PVDF/PAA membrane surface,which made PVDF/PAA membrane equipped with oil-water separation and photocatalytic dual ability.It is found that VIPS process effectively increased the pore size,porosity of the membrane and the loading ofβ-Fe O（OH）nanorods.When the exposure time in the air is 5 minutes,the water contact angle reached 33.9±5.5°,the pure water flux was 1300 L·m^-2·H^-1·bar^-1,the flux in the oil-water separation process was 604.8 L·m^-2·H^-1·bar^-1 and stable in three cycles.The non-woven fabrics substrates were pre-modified PDMS as prevent layer and fabricated PVDF+PEI membranes on it,further grown a hydrophilic nano Si O₂ layer on membrane surface,then Janus membranes were obtained by mechanical peeling of the non-woven fabric substrate.The PDMS on non-woven fabrics transferred to Janus membrane bottom surface after mechanical peeling process,besides the roughness of bottom surface increased due to the micro-nano structure on the non-woven fabric"printed"to the bottom surface of the membrane as a template.The Janus membranes had asymmetric wettability,the contact angle of top and bottom surface was 35.3°and152.4°,respectively.The membrane shown different floating states between different oil-water interface.Janus membranes shown good separation performance for surfactant stable emulsion.The water flux of toluene-in-water emulsion was 172.8±4.8 L·m^-2·h^-1,and the oil rejection was 99.72±0.01%.meanwhile it is possible to separate water-in-hexane emulsion,the hexane flux was 237.6±12.0 L·m^-2·h^-1,and the water rejection was 92.25±0.98%.The Janus membranes exhibited good stability in the three times emulsion-water cycle test and owned excellent directional transmission capacity.