Bioelectrets in Electrospun Bimodal Poly(lactic acid) Fibers: Realization of Multiple Mechanisms for Efficient and Long-Term Filtration of Fine PMs.

Despite the great potential in fabrication of biodegradable and eco-friendly air filters by electrospinning poly(lactic acid) (PLA) membranes, the filtering performance is frequently dwarfed by inadequate physical sieving or electrostatic adsorption mechanisms to capture airborne particulate matters (PMs). Here, using the parallel spinning approach, the unique micro/nanoscale architecture was established by conjugation of neighboring PLA nanofibers, creating bimodal fibers in electrospun PLA membranes for the enhanced slip effect to significantly reduce the air resistance. Moreover, the bone-like nanocrystalline hydroxyapatite bioelectret (HABE) was exploited to enhance the dielectric and polarization properties of electrospun PLA, accompanied by the controlled generation of junctions induced by the microaggregation of HABE (10-30 wt %). The incorporated HABE was supposed to orderly align in the applied E-field and largely promote the charging capability and surface potential, gradually increasing to 7.2 kV from the lowest level of 2.5 kV for pure PLA. This was mainly attributed to HABE-induced orientation of PLA backbone chains and C═O dipoles, as well as the interfacial charges trapped at the interphases of HABE-PLA and crystalline region-amorphous PLA. Given the multiple capturing mechanisms, the micro/nanostructured PLA/HABE membranes were characterized by excellent and sustainable filtering performance, e.g., the filtration efficiency of PM0.3 was promoted from 59.38% for pure PLA to 94.38% after addition of 30 wt % HABE at a moderate airflow capacity of 32 L/min and from 30.78 to 83.75% at the highest level of 85 L/min. It is of interest that the pressure drop was significantly decreased, mainly arising from the slip effect between the ultrafine nanofibers and conjugated microfibers. The proposed combination of the nanostructured electret and the multistructuring strategy offers the function integration of efficient filtration and low resistance that are highly useful to pursue fully biodegradable filters.