Large-Scale Nanofiber Manufacturing| Large polystyrene microsphere embedded3D polycaprolactam-6 nanofiber mats withhigh filtration performance for ultrafineparticulates

Prof. Fang Kuanjun at Qingdao University: Polystyrene Microsphere-Embedded 3D Polyamide-6 Nanofibers for High-Efficiency Ultrafine Particle Filtration

Environmental pollution in China has become increasingly severe, with haze pollution causing significant economic losses. Haze is primarily formed by suspended particles, smoke, and water vapor in the air, among which high concentrations of PM2.5 (particulate matter with aerodynamic diameter ≤2.5 μm, also known as respirable particles) are one of the main causes. Due to their small size, PM2.5 can remain suspended in the atmosphere for extended periods and be transported over long distances, easily carrying toxic and harmful substances. Once inhaled, they can directly enter the bronchi, alveoli, and bloodstream, causing irreversible damage to human health, including asthma, bronchitis, cardiovascular diseases, pneumoconiosis, and lung cancer.In recent years, various influenza viruses such as SARS-CoV-2 (2019-nCoV), mycoplasma, and pertussis (whooping cough) have spread nationwide, primarily through droplet transmission, aerosol transmission, and close contact. Virus particles, with diameters less than 0.1 μm, mix with secretions and droplets to form clusters approximately 5 μm in size. Therefore, developing a high-efficiency, low-resistance nanofiber membrane to effectively block various viruses and PM2.5 is not only a social issue related to people's quality of life and health but also an important environmental challenge and a critical-related in China.

Prof. Fang Kuanjun's team at the Collaborative Innovation Center for Eco-Textiles (co-established by the province and ministry) at Qingdao University published their latest research成果 in Separation and Purification Technology titled "Large polystyrene microsphere embedded 3D polycaprolactam-6 nanofiber mats with high filtration performance for ultrafine particulates". This study developed a novel three-dimensional (3D) nanofiber filtration material specifically designed for filtering ultrafine particles with diameters ≤PM1. Through gas-jet electrospinning, polystyrene (PS) microspheres were successfully embedded into PA6 nanofibers to create a "bead-on-string" PA6/PS microsphere nanofiber membrane with a fluffy 3D stacked structure.

Fig. 1: Preparation and performance of PA6/PS nanofiber membranes

To optimize the filtration performance of the PA6/PS microsphere nanofiber membrane, the effects of PS microsphere size and concentration on the pore structure and filtration performance were thoroughly investigated. Through a series of comparative tests on PS microsphere sizes and a single-variable experimental design with PS concentration gradients, various tests and characterizations were conducted. The finally conclusion was that when the PS microspheres were 10 μm in size and added at 1.5 wt%, the microspheres achieved the most uniform dispersion in the nanofiber membrane, resulting in optimal fiber morphology.

Fig. 2: SEM images (1000×) of PA6/PS membranes with (a) 0 wt%, (b) 0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt%, (e) 2.0 wt% PS; (f) Macroscopic view

As shown in Figure 3, the PA6/PS microsphere nanofiber membrane achieved a filtration efficiency of 99.58±0.09% for ultrafine particles ≤1 μm under high airflow velocity, with a pressure drop of 105±2 Pa. This achive high filtration efficiency with low pressure drop while exhibiting excellent durability and quality factors, providing new insights for air filtration technology.

Fig. 3: (a) Size-dependent efficiency, (b) filtration performance, (c) quality factor, (d) durability

Airflow velocity and membrane basis weight also affected the filtration performance. As shown in Figure 4, at 1.5 wt% PS microsphere concentration, when the airflow velocity increased from 3.7 cm/s to 9.7 cm/s, the filtration efficiency remained stable at around 99.5%, while the pressure drop increased from 85.67 Pa to 173 Pa. When the membrane basis weight increased from 5 g/m² to 20 g/m², the filtration efficiency improved from 70.99% to 99.7%, and the pressure drop increased from 46 Pa to 105 Pa.

Fig. 4: (a) Efficiency/pressure drop vs. airflow velocity at 1.5 wt% PS; (b) Pressure drop vs. PS content; (c) Efficiency/pressure drop vs. basis weight; (d) Pressure drop vs. basis weight

Figure 5 shows that the PA6/PS microsphere nanofiber membrane simultaneously exhibited excellent breathability, moisture permeability, and mechanical properties. Additionally, the microspheres helped to hold the cavity structure of the nanofiber membrane, resulting in high porosity, which was a key factor in reducing pressure.

Fig. 5: (a-b) Air permeability, (c) moisture permeability, (d) visual demonstration, (e) porosity, (f) pore distribution, (g) stress-strain curves

Figure 6 presents the chemical characterization of different nanofiber membranes. The FTIR spectra in Figure 6a confirmed the successful incorporation of PS microspheres into the nanofiber membrane, with gradually increasing microsphere content. The XPS survey spectra in Figure 6b analyzed the chemical environment of surface atoms, confirming the membrane's拦截effect on PM particles and the successful incorporation of PS microspheres.

Fig. 6: (a) FTIR spectra, (b) XPS survey, (c-e) high-resolution C 1s, O 1s, N 1s spectra

Figure 7 shows the diameter distribution of nanofiber membranes with different PS microsphere concentrations. As the microsphere concentration increased, the fiber diameter first decreased and then increased. This was because during membrane formation, when the spinning solution was stretched by high-pressure airflow to form jets, the relatively large size and inertia of the PS microspheres exerted a drawing effect on the PA6 fibers, refining the fiber diameter to 472.96 nm.

Fig. 7: Diameter distributions (a-e) and trends (f) with varying PS content

As illustrated in Figure 8a, under the propulsion force, the spinning solution containing PS microspheres formed a Taylor cone at the inner nozzle tip. The high-pressure airflow from the outer nozzle were deprived the PA6/PS suspension from the Taylor cone surface into fine jets. These jets were whipped by the airflow and stretched by the large PS microspheres, ultimately forming thin nanofibers embedded with PS microspheres (Figure 8b). When the PA6/PS nanofibers were used for air filtration, PM particles were effectively captured by the fluffy 3D nanofiber structure (Figure 8c). As shown in Figures 8d and 8e, the nanofiber structure prepared in this study was finer and fluffier than pure PA nanofibers, thereby enhancing the diffusion and interception effects of filtration (Figure 8f) and achieving excellent filtration performance.

Fig. 8: (a) Bead formation mechanism, (b) fiber-PS structure, (c) particle capture SEM, (d) pure PA6 cross-section, (e) 1.5% PS cross-section, (f) filtration mechanism