(643a) Novel Electrospun Membrane Structures for Reverse Osmosis, CO2 Stripping and Membrane Distillation

Novel electrospun membranes have been developed that show great promise across a range of different applications. Electrospun membranes are generally composed of randomly orientated nanofibers that form a highly porous structure. In the present case we form these fibres into an ordered crosshatched structure that both increases mechanical strength and reduces tortuosity, leading to increased flux. We also use electrospinning to customise the surface of the membrane, forming resilient, superhydrophobic surfaces that are long lasting.

In our initial studies we formed ultrathin fibres of ~50nm diameter from polysulfone within a unique crosshatched structure. The use of polysulfone, coupled with a polydopamine addition to increase hydrophilicity, facilitated the addition of a standard polyamide active layer using interfacial polymerisation. The resulting membrane was able to withstand transmembrane pressures of up to 55 bar during reverse osmosis, while the random electrospun structure failed at 14 bar. The flux through the crosshatched structure was similarly 150 to 180% higher than that of a random nanofibre membrane and more than 300% higher than a standard asymmetric membrane. The salt rejection through all three systems was comparable, as this is dictated by the polyamide active layer.

We have further tested electrospun structures for carbon dioxide stripping from a chemical solvent. This is a challenging application, requiring a superhydrophobic, highly porous membrane that is resistant to temperatures of 100^oC and above. We spin 200 nm fibres from poly(vinylidene fluoride-co-hexafluoro propylene), a polymer that is hydrophobic, but has greater alkali resistance than poly(vinylidene fluoride), which is needed for this application. Further, we form ‘bead-on-string’ and microparticle structures on the surface of the membrane to further increase hydrophobicity. All structures provide enhanced CO₂ flux relative to a standard asymmetric membrane, but the bead-on-string structure provides greater stability, with strong performance for up to ten days at 100 ^oC. In a further extension to this work, we spin cross-hatched and random nanofibre structures from hydroxyl polyimide and once formed, use thermal treatment to form TR nanofibers that also show strong performance in this application.

Most recently we have extended this approach to form cross-hatched nanofibre membranes formed from polytetrafluoroethylene (PTFE). The use of the cross-hatched structure and a microparticulate surface layer leads to very high water fluxes of close to 100 kg m^-2 hr^-1 during membrane distillation, readily exceeding that for both a commercial PTFE membrane and other electrospun structures in the literature, while maintain strong salt rejection.