Wrinkled silica doped electrospun nano-fiber membranes with engineered roughness for advanced aerosol air filtration
Graphical abstract
Introduction
Particulate matter (PM) pollutants cause great threats to human health [1]. Particles present in air, smaller than 2.5 µm (PM2.5), can penetrate through the respiratory system, and lead to lung disease and cancer [1], [2], [3], [4]. Although, conventional fibrous air membranes have been used widely to remove such contaminants from air, they are unable to remove effectively finer sub-micron particles due to the large fiber diameter and wide pore size distributions [5]. Membranes made from nanofibers, which have nanoscale diameter, high length to diameter ratio, and large specific surface area, have become promising materials for air filtration applications [6], [7], [8].
Various technologies have been developed to produce nanofiber membranes such as drawing and template synthesis [9]. Among them, electrospinning was shown to be more cost-effective to fabricate and design scalable fibers, with large specific surface area, interconnected open pores features, and high air permeability [10], [11], [12], [13], [14]. Electrospun nanofibers were fabricated from various polymeric materials, including polyimide (PI), polyvinylpyrrolidone (PVP), polyurethane (PU), polylactic acid (PLA), and polyacrylonitrile (PAN) [15], [16], [17], [18], [19]. Among them, PAN, a conjugated polymer, offers a large dipole moment with unique capture properties of PM pollutants from the atmosphere, high chemical stability, and ultraviolet resistance [4], [19]. These properties render electrospun PAN nanofiber membranes an appropriate choice for air filtration applications.
The morphological properties of electrospun nanofibers such as fiber diameter, packing density, and surface structure significantly affect the performance of fibers production and the performance of electrospun air membranes [1], [15], [16], [19], [20], [21], [22]. The fabrication of porous bead-on-string poly (lactic acid) nanofiber membranes has offered excellent air filtration performance, due to the enhanced specific surface area compared to dense fibers [20]. Preparing hybrid nanofibers can also provide a facile way to control the morphology and surface properties of electrospun nanofibers [23]. PAN/TiO2 electrospun nanofiber membranes and polysulfone (PSU)/TiO2 materials, with hierarchical structures, were found to improve filtration performance compared to counterparts without any additives [9], [24].
Wrinkled structures generated across the skin of nanofibers offer great promises in separation science for adsorption, capture or sieving [25], [26], [27]. Recently, surface modification of electrospun nanofibers was found to be an effective way to enhance pollutants capture. Ionic liquid with high viscosity (diethylammonium dihydrogen phosphate (DEAP)) were mixed with PAN solution to introduce roughness and modify the surface area of nanofiber. This strategy has contributed to utilize physical adsorption in addition to the chemical adsorption, resulting in higher PM2.5 removal efficiency compared to the smooth surface of bare PAN fibers [27]. In another study, the roughness and surface of electrospun PAN nanofibers were enlarged by oxygen plasma modification, leading to double the quality factor of rough electrospun membranes [28]. Wrinkled structures provide highly effective specific surface area, capable of increasing the air filtration efficiency, while simultaneously increasing the average distance between fibers, thus reducing the pressure drop across the material during filtration. The presence of such wrinkles across the surface of nanofibers supports gas adsorption and increases the non-slip and stagnation of aerosol particles on the surface of fibers, therefore providing high air filtration efficiency and obtaining strong aerosol particle adhesion on the surface of these fibers [29].
In this study, wrinkles generated across the skin of nanofibers for nano-scale aerosol filtration application is demonstrated from tetraethyl orthosilicate (TEOS) doped nanocomposite PAN electrospun nanofiber membranes. The impact of the membrane properties and fiber structure were systemically investigated and correlated to the air filtration performance of the electrospun nanofiber membranes. In addition, the electrospun nanofiber membranes were benchmarked against bare electrospun PAN membranes and commercial Glass Fiber (GF) air membranes.
Section snippets
Materials
Poly (acrylonitrile) (PAN) (Mw 150,000 g/mol) powder, tetraethyl orthosilicate (TEOS), N-N dimethylformamide (DMF) (99.8% purity) were used for the synthesis of the polymeric dope spinning solution. Potassium chloride (KCl) (99.9% purity) was used to generate the aerosol particles. All chemicals were purchased from Sigma-Aldrich and used as received without further treatments.
Results and discussion
Electrospun TEOS/PAN electrospun nanofiber membranes were investigated in terms of their structure, morphology, and air filtration performance properties to evaluate not only the performance of samples, but also to understand synergistic properties and the role of TEOS doping on the wrinkled nature of the fibres and thereafter filtration properties of the membranes.
The morphological properties of TEOS/PAN membranes at different concentrations were studied. Fig. 1 shows the SEMs and mean fiber
Conclusions
The potential of TEOS/PAN electrospun nanofiber membranes is demonstrated for the first time with outstanding air filtration performance, for air filtration application. Issues related to submicrometer particles filtration efficiency were alleviated by controlling the morphological properties of electrospun nanofiber membranes. By manipulating the polymeric solution properties, the filtration efficiency, pressure drop, and QF can be controlled. The introduction of TEOS dopant to PAN solution
Acknowledgement
Dr. Ludovic Dumée acknowledges the Australian Research Council for his Discovery Early Career Research Award (DECRA) 2018 DE180100130 fellowship. Riyadh Al-Attabi thanks the Scholarship from the Higher Committee for Education Development in Iraq (HCED).
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