Rational study on the evolution mechanism of PVDF crystalline phase to prepare high-performance electroactive PVDF membranes

Compared with the routine anti-fouling methods, leveraging polarity and piezoelectricity of the electroactive PVDF membrane itself to improve the anti-fouling ability has been attracting increasing attention due to the better sustainability and stability. However, existing empirical research methods about the manipulation of crystal polymorphs lack in-depth scientific exploration regarding the formation and evolution mechanism of PVDF crystals, facing the trade-off dilemma between enhancing β-phase content and improving permeability of PVDF membranes. In this paper, from the viewpoint of intermolecular interaction during phase separation process, the effect of surfactant type on the crystal polymorphs, morphology and performance of PVDF membranes is thoroughly studied. A kind of biaxial traction effect (BTE) based on the polymer-solvent-coagulant interactions during the phase separation process is proposed. The introduction of ionic surfactants is discovered to enhance the BTE owing to the presence of ion-dipole interactions with –CH₂ groups of PVDF. As a result, the β-phase content of PVDF membranes is elevated to 97 %. Moreover, using ionic surfactants to enhance the BTE decreases the pore size while increases the number of through-pores, thus improving the perm-selectivity of the PVDF membranes (increased by 3.4 times). The continuous anti-fouling testing reveals that the vibration of piezoelectric PVDF membranes excited by AC signals has positive but limited effects on alleviating membrane fouling. This study inspires the rational manipulation of crystal polymorph and membrane performance of electroactive PVDF membranes.