Annealing of grain-like poly (vinylidene fluoride-trifluoroethylene) membranes with a single-crystalline electroactive phase and high anti-fouling activity

Electroactive membranes are attracting attention due to their piezo-, pyro-, and ferro-electric properties. Here we report the fabrication of electroactive membranes from poly (vinylidene fluoride-trifluoroethylene) P(VDF-TrFE), i.e. co-polymers of vinylidene fluoride and trifluoroethylene. A new P(VDF-TrFE)/Polyvinylpyrrolidone (PVP)/Dimethylacetamide (DMAc)/water system was used to tailor the membranes’ structure. Since hot treatment in the air could induce defect on membrane structure (i.e., fracture, shrinkage, and rolling), the annealing was conducted in the glycerin. Thanks to the high boiling point and moderate surface tension of glycerin, the integrity of P(VDF-TrFE) membranes was preserved during the annealing process. X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry, and scanning electron microscopy experiments revealed that the relative abundance of the β–crystalline phase increased with the annealing temperature when the latter was above the Curie temperature. P(VDF-TrFE) membranes annealed at 130 °C exhibited high crystallinity with grain-like surface, which resulted from multiple stacks of edge-on lamellae, and possessed excellent physicochemical properties of filtration. The anti-fouling performance of pristine and annealed P(VDF-TrFE) membranes was tested by dead-end filtration with a 13.5 mg/L humic acid (HA) solution. Annealed P(VDF-TrFE) membranes achieved greater fluxes and had superior anti-fouling properties, which was attributed to the weaker hydrophobic attraction between HA and the aligned β–phase crystals. This work provides a facile method for designing highly crystalline P(VDF-TrFE) membranes with potential applications in filtration systems, smart wearable devices, and medicine.