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How To Fabricate TFNC Membranes On A Large Scale By Electrospinning Nanofibers Technology?

2020-05-20



Schematic illustrations of the fabrication process for thin-film composite membranes.

The preparation of an appropriately thin barrier layer on the highly porous electrospun nanofibrous scaffold often faces a series of challenges. For example, using the conventional casting method, penetration of the coating solution into the nanofibrous scaffold is unavoidable, which would result in a much thicker barrier layer. Various methods, such as adjusting the viscosity of the cast solution through gelation, soaking the nanofibrous scaffold with coagulant of the cast solution, and photo-crosslinking of a UV-reactive coating solution (e.g., photo-crosslinkable PVA), have been demonstrated to minimize the penetration of the cast solution to the electrospun nanofibrous support and to decrease the thickness of the barrier layer. Wang and coworkers demonstrated a unique technique to prepare an ultrathin barrier layer on the nanofibrous scaffold and further improved the performance of the TFNC UF membrane . In their approach, a double-layer electrospun nanofibrous mat, containing a thin hydrophilic nanofibrous top layer and a nanofibrous support layer, was manufactured via electrospinning or electrospraying. The top electrospun layer was subsequently fused using solvent vapor or solvent solution or hot-pressing treatment to form a thin integrated barrier layer without the fibrous structure. An alternative TFNC UF membrane was prepared by remelting the deposited PVA electrospun nanofibers on the PAN electrospun membrane with water vapor (Fig. 14.11A). The filtration performances of the PVA/PAN composite membranes were evaluated by the oil/water emulsion separation system, and the highest permeate flux of 210L/m2h was achieved with the rejection of 99.5% for the composite membrane under the operating pressure of 0.3 MPa. In another of their studies, an electrosprayed PVA top layer was swollen in a water/acetone mixture to form an intact filmlike barrier layer on the electrospun PAN support layer (Fig. 14.11B). The resultant TFNC membrane, containing a crosslinked PVA barrier layer, possessed very high flux (347.8 L/m2h) and a high rejection ratio (99.6%) at low pressure (0.2 MPa) for separation of oil emulsion and water. The system was able to maintain a much higher flux value than a typical commercial UF membrane, with no loss of oil rejection capability under a long cross-flow UF operation. To precisely control the compactness of the top-layer film, Wang and coworkers improved the strategy by a hot-press top-layer formation process (Fig. 14.11C). Specifically, the electrosprayed PVA layer was moistened and subsequently softened by hot-pressing treatment to form an integrated barrier film on the supporting layer. The obtained TFNC membrane was used to filter molecules smaller than in an oil emulsion, such as in a bovine serum albumin (BSA) solution. Specifically, the optimized PVA/PAN TFNC membrane possessed a high UF performance in BSA filtration tests, with a water flux of 173.0 L/m2h and rejection above 98.0% at a low feeding pressure of 0.3 MPa. A novel three-tiered arrangement of composite membranes consisting of an ultrathin PAN-co-acrylic acid (PAN-AA) barrier layer based on a PAN nanofibrous support layer was developed (Fig. 14.11D). Due to its hydrophilic and negatively charged barrier layer, the TFNC UF composite membranes exhibited excellent permeate flux (221.2 L/m2h) and rejection efficiency (97.8%) for BSA aqueous solution at 0.3 MPa. These methods all overcome the typical challenge of easy penetration of the coating solution into the porous substrate and eliminate the bottleneck of the barrier thickness, thus offering great potential in fabrication of TFNC membranes on a large scale.

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