Alternative approaches toward thermo-responsive membranes for emulsion separation

Porous polyvinylidene difluoride (PVDF) microfiltration membranes with switchable wettability and controllable pore sizes were developed for separating oil-in-water and water-in-oil emulsions. The PVDF substrate membrane was fabricated using vapor-induced phase separation, with exposure times to water vapor ranging from 0.5 to 20 minutes and PVDF casting solution concentrations between 20% and 22% (wt/v). This process resulted in an interlaced mesh surface structure with an optimal mean pore size of 0.16 µm. A polydopamine coating was then applied to the PVDF substrate membrane to create a reactive platform for grafting poly(N-isopropyl acrylamide) (PNIPAm), a thermally responsive polymer with different degrees of hydrophilicity below and above its lower critical solution temperature (LCST). PNIPAm with a low molecular weight (Mn = 4 kDa) and low polydispersity (Mw/Mn = 1.6) was synthesized via free radical polymerization, using 2-aminoethanethiol hydrochloride (AET) as a chain transfer agent. The PNIPAm with amino end groups comprised approximately 40 mol% of the total polymer due to the influence of AET. This pre-synthesized PNIPAm was grafted onto the PVDF membrane after polydopamine coating via a Michael addition reaction (“grafting to” method). In contrast, the monomer N-isopropyl acrylamide (NIPAM) was grafted from the initiator-functionalized PVDF membrane (“grafting from” method) via surface-initiated atom transfer radical polymerization (ATRP). These methods were employed to prepare porous thermally responsive membranes with switchable wettability and controllable pore size, and the differences between the “grafting to” and “grafting from” methods were evaluated through morphology, chemical composition, contact angle, and water permeance analysis. The “grafting from” method resulted in more homogeneous spherical structures and higher grafting density of grafted PNIPAAm than the “grafting to” method, as confirmed by scanning electron microscopy as well as infrared and X-ray photoelectron spectroscopy. The PVDF-based thermally responsive membrane prepared by the “grafting from” method demonstrated superior thermal responsiveness in the controllable pore sizes, with water permeance increasing from 200 L m-2h-1bar-1 at 25°C to 480 L m-2h-1bar-1 at 45°C. In contrast, the membrane prepared by the “grafting to” method showed a less pronounced increase in water permeance, from 110 L m-2h-1bar-1 at 25°C to 190 L m-2h-1bar-1 at 45°C. Additionally, for the “grafting from” method, the water contact angle increased from 0° to 117° as the temperature exceeded the LCST, indicating switchable wettability between superhydrophilic and hydrophobic states. However, the membrane prepared by the “grafting to” method exhibited a smaller change in hydrophilicity, with the water contact angle increasing from 37° at 25°C to 46° at 45°C. To address the low permeability of the PVDF-based thermally responsive membrane prepared by the “grafting from” method, a single-side polydopamine coating combined with the “grafting from” technique was applied to the PVDF substrate membrane. This modification achieved a higher water permeance of 290 L m-2h-1bar-1 at 25°C while maintaining switchable wettability between hydrophilic and hydrophobic properties. This functionalized PVDF membrane successfully separated the toluene-in-water emulsion and water-in-toluene emulsion at 25°C and 45°C, respectively. Furthermore, the PVDF-based thermally responsive membrane prepared via single-side polydopamine coating and the “grafting from” technique demonstrated excellent anti-fouling properties due to its switchable wettability, with superior separation efficiency exceeding 94% in both oil-in-water and water-in-oil emulsions. This indicates its potential for application in oil-water emulsion separation. The single-side polydopamine coating combined with the “grafting from” technique offers a facile and convenient method for surface modification of thermally responsive membranes in oil-water emulsion separation.