Particulate matter (PM) pollution has become a serious threat to public health, climate and ecosystems. Therefore, filtration materials with high filtration efficiency and low air resistance are in urgent need. Electret filters that can significantly improve filtration efficiency without increasing pressure drop are widely used for air filtration. However, long-term maintenance of the electrical charges on electret filters, particularly at high temperatures and in wet conditions, is still a challenge. Herein, we report the fabrication of a novel electret filter with a fluffy structure, high porosity and satisfactory filtration stability using polypropylene (PP) as the matrix polymer and magnesium stearate (MgSt) as the charge enhancer. Benefiting from the fluffy structure created by melt blown at a large die-to-collector distance (DCD), the pressure drop decreased from 67.0 Pa to 52.9 Pa at an air flow rate of 85 L min⁻¹. Furthermore, the introduction of MgSt changed the crystal structure of the melt blown nonwovens, thus improving the electret properties to achieve a high filtration efficiency of 99.22%, a low pressure drop of 92 Pa and a satisfactory QF of 0.054 Pa⁻¹. In particular, the electret filtration materials exhibited excellent filtration stability and loading performance. Thus, this electret material may become a good candidate for personal protective filtration materials against PM2.5.

Polyvinylidene fluoride (PVDF) is a significant polymer in the formation of nanofiber webs via the electrospinning technique. In this paper, three PVDF-wrinkled fiber webs with different molecular weights (M_Ws) (180000, 275000, and 530000) were generated via the electrospinning method by using tetrahydrofuran/N,N-dimethylformamide at the solvent ratio of 1:1 as a mixed solvent. The formation mechanism of the wrinkled electrospun PVDF fibers is demonstrated. Furthermore, the relationships between the M_W and the surface structure, mechanical properties, crystalline phases, and piezoelectric properties of electrospun PVDF fibers are comprehensively investigated. The results reported that the surface structure, mechanical properties, crystalline phases, and piezoelectric properties of wrinkled electrospun PVDF fibers can be affected intensely by maneuvering the M_W. We believe this study can be served as a good reference for the effect of M_W on the morphology and properties of electrospun fibers.

This work presents a simple and reliable method for directly generating polyvinylidene fluoride (PVDF) nanofibers with secondary surface morphology (e.g., porous surfaces, rough surfaces, grooved surfaces, and interior porosity) by using single/binary solvent systems and relative humidity. We clarified the mechanisms responsible for the formation of these morphologies by systematically exploring the molecular interactions among the polymer, solvent(s), and water vapor. Our results proved that the formation of secondary surface morphology needed the presence of water vapor, a non-solvent of the polymer, at an appropriate level of relative humidity. The formation of secondary surface morphology was dependent on the speed of evaporation of the solvent(s) (ACE, DMF, and their mixtures), as well as the inter-diffusion and penetration of the non-solvent (water) and solvent(s). The results of N2 physical adsorption-desorption isotherms showed that the macro-porous fibers (> 300 nm) exhibited the highest specific surface area of 23.31 ± 4.30 m2/g and pore volume of 0.0695 ± 0.007 cm3/g, enabling the high oil absorption capacities of 50.58 ± 5.47 g/g, 37.74 ± 4.33 g/g, and 23.96 ± 2.68 g/g for silicone oil, motor oil, and olive oil, respectively. We believe this work may serve as guidelines for the formation of different structures of macro-porous, rough, and grooved nanofibers with interior porosity through electrospinning.

The surface evolution of fibers has been attracting great attention in many areas. In this work, we demonstrated the feasibility of directly electrospinning grooved and porous polyvinylidene fluoride fibers by varying polymer concentration and using both single/binary solvent systems. We discovered that grooved fibers can be generated from a variety of binary solvent systems, whereas porous fibers can be achieved using low boiling point solvent (LBPS) systems. The results indicated that the key factors for the formation of grooved fibers are the polymer concentration, the difference of evaporation rate (the difference of boiling points between each of the two studied solvents) between the two solvents, and the solvent ratio. While the polymer concentration, the presence of LBPS, and the solvent ratio are the decisive factors for the formation of porous structures. The formation mechanism of grooved fibers was attributed to void-based elongation, wrinkle-based elongation, and collapsed jet-based elongation. The mechanism for the formation of porous fibers was ascribed to the coexistence of both thermally induced phase separation and vapor induced phase separation. These structures can be used in different applications, such as harvesting energy and filtration. Importantly, we believe our work can serve as guidelines for the preparation of grooved and porous nanofibers through electrospinning.