Study on ZIF-8@CNTs functionalized stereocomplexed poly(lactic acid) highly electroactive nanofibrous membranes for long-term PMs filtration

Particulate matter (PMs) in air pollution has become a global concern. These tiny particles are capable of carrying numerous harmful substances and entering the human blood circulation through the respiratory system, resulting in serious health effects, including respiratory and neurological diseases. However, conventional protective masks have significant limitations in filtration performance, and non-biodegradable filter masks often have a negative impact on the environment when discarded. Therefore, there is an urgent need to develop a biodegradable filtration fibrous membrane, in which polylactic acid (PLA) has great potential in the field of biodegradable air filtration materials. However, it is limited by its low electroactivity characteristics, which makes its ability in electrostatic adsorption of particulate matter insufficient to achieve efficient filtration of air particulate matter. In this paper, ZIF-8 nanocrystals were successfully synthesized on carbon nanotubes (CNTs) using a microwave-assisted method to obtain ZIF-8@CNTs nanohybrids with regular surfaces. In addition, poly (L-lactic acid) (PLLA) and poly (D-lactic acid) (PDLA) were blended, and the ZIF-8@CNTs NPs were embedded in the fibers based on electrospinning-electrospray technology to obtain the standing composite poly(lactic acid) fibrous membranes (ZIF-8@CNTs/SC-PLA fibrous membranes), the formation of protruding structures on the fiber surface increases the physical interception as well as the electrostatic adsorption properties of the fibers. The hydrogen bonding interaction between PLLA and PDLA chains can trigger the formation of stereocomplexed crystals (SCs), which in turn affects the crystallinity of PLA and improves the polarization properties and electrical activity of ZIF-8@CNTs/SC-PLA fibrous membranes. By adjusting the concentration of ZIF-8@CNTs and modulating the surface morphology of the fibers, the electrical signals generated under different respiration states were evaluated and the filtration performance as well as the pressure drop of the ZIF-8@CNTs/SC-PLA nanofibrous membranes were explored at different flow rates. The results showed that with the increase of ZIF-8@CNTs concentration, the surface of the fibers was rough, and the diameter of the fibers tended to decrease and then increase, the dielectric constant (2.5) and surface potential (7.7 kV) are significantly improved. The pressure drop of ZIF-8@CNTs/SC-PLA nanofibrous membranes was significantly lower than that of pure PLLA fibrous membrane at different flow rates, among which the pressure drop of 10%ZIF-8@CNTs/SC-PLA nanofibrous membranes was only 206.9 Pa at a flow rate of 85 L/min, which was much lower than that of pure PLLA fibrous membranes (465.2 Pa). And at a flow rate of 32 L/min, the filtration efficiencies of the 10%ZIF-8@CNTs/SC-PLA membrane for PM_0.3 and PM_2.5 were 86.9% and 96.1%, respectively, which were 13.2% and 8.6% higher than that of the pure PLLA fibrous membranes, respectively. Moreover, the 10%ZIF-8@CNTs/SC-PLA fibrous membranes can achieve different signal outputs under different respiratory states, showing excellent electrical activity, which provides a new design reference for self-powered portable respiratory monitoring and protective equipment.