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Anti-deformed Polyacrylonitrile/polysulfone Composite Membrane with Binary Structures for Effective Air Filtration Shichao Zhang, Hui Liu, Xia Yin, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00359 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 10, 2016

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ACS Applied Materials & Interfaces

Anti-deformed Polyacrylonitrile/polysulfone Composite Membrane with Binary Structures for Effective Air Filtration Shichao Zhang,† Hui Liu,‡ Xia Yin,‡ Jianyong Yu,§ and Bin Ding*,†,‡ †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of

Materials Science and Engineering, Donghua University, Shanghai 201620, China. ‡

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles,

Donghua University, Shanghai 201620, China. §

Nanofibers Research Center, Modern Textile Institute, Donghua University, Shanghai 200051,

China.

*

Corresponding author. E-mail: [email protected]; Phone: +86-21-62378202; Fax: +86-21-

62378202

ACS Paragon Plus Environment

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ABSTRACT

Airborne particle filtration proposed for fibers requires their assembly into porous structures with small pore size and low packing density. The ability to maintain structural stability upon deformation stress in service is essential to ensure a highly porous packing material that functions reliably; however, it has proven extremely challenging. Here, we report a strategy to create anti-deformed polyethylene oxide@polyacrylonitrile/polysulfone (PEO@PAN/PSU) composite membranes with binary structures for effective air filtration by combining multi-jet electrospinning and physical bonding process. Our approach allows the ambigenous fiber framework including thin PAN nanofibers and fluffy PSU microfibers, through which run interpenetrating PEO bonding structures, to assemble into stable filtration medium with tunable pore size and packing density by facilely optimizing the bimodal fiber construction and benefiting from the PEO inspiration. With the integrated features of small pore size, high porosity, and robust mechanical properties (8.2 MPa), the resultant composite membrane exhibits high filtration efficiency of 99.992%, low pressure drop of 95 Pa, and desirable quality factor of 0.1 Pa-1; more significantly, it successfully gets rid of the potential safety hazards caused by unexpected structural collapsing under service stress. The synthesis of PEO@PAN/PSU medium would not only make it a promising candidate for PM2.5 governance, but also provide a versatile strategy to design and develop stable porous membranes for various applications.

KEYWORDS binary structure, composite membrane, anti-deformed, high filtration efficiency, low pressure drop, air filtration


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1. INTRODUCTION Particulate matter (PM) pollution, especially the notorious PM2.5 which refers to particle sizes below 2.5 µm, causes growing impact on people’s living quality and production efficiency, as well as influencing visibility, climate, and ecosystems.1-3 With rapid urbanization and industrialization, both the deteriorating air quality and rising living standards have contributed to the urgent need for high performance air filters. Nowadays, there are two types of air filtration media in common use.1,4 One is a porous film filter, which is made by creating pores on solid substrate, and it usually has very small pore size and low porosity, resulting in a relatively high filtration efficiency and a terribly large pressure drop.5 Another type of air filter is the fibrous filter, including melt-blown fibers, glass fibers, and spun-bonded fibers, which are attractive for particle filtration because of their integrated characteristics of energy-efficient, cost-effective, and ease of scalable synthesis.6,7 Although the existing fibrous filters usually have a higher porosity which endows the filters with outstanding air permeability, they still suffer from many performance disadvantages due to their micro-sized fiber diameter, such as relatively low filtration efficiency, low quality factor (QF), and bulkiness. They are also incapable of capturing the fine particles.8,9 Benefiting from the small diameter, nanofiber-based filters evoke more and more attentions due to their high porosity to disperse the applied airflow by the enhanced “slip effect” and tortuous channels made of interconnected open pores for excellent particle separation.10-12 Various strategies including drawing, template synthesis, phase-separation, sea-island spinning, plasma treatment, etc. have been created to fabricate the nanofibers. Among them, the electrospinning, as the mainstream of large-scale nano-architecture constructional technology, stands out and gains special attention in view of its ability of producing nanofibers with


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controlled dimensions, morphologies, and functional components.13-15 To date, a series of electrospun nanofiber membranes have been fabricated and employed as air filtration media, such as PAN,10 polyamide-6 (PA-6),16 polyurethane (PU),17 polyvinyl chloride/PU,18 PA-66,12 and so on. Also reported were the hierarchically structured nanofibers doped with nanoparticles for enhancing their specific surface area or electret effect.19,20 However, generally speaking, in most cases, the fine particle capture efficiency, air permeability, and structural stability of the aforementioned nanofiber media are still far from satisfactory due to their large pore size, uncontrollable packing density, and easy-collapsed cavity structures. Inspired by the air permeability of melt-blown microfibers and the high capture ability of superfine glass fibers, binary structures combining the advantages of microfibers and nanofibers provide a facile way to construct filtration media with controllable packing structures and enhanced filtration performance by their heterosis effect.21,22 Both PAN and PSU are regarded as fine raw polymer for producing the fibrous filtration media owing to their desirable properties of chemical stability, resistant to abrasion, and ease of processing; and considerable efforts have been paid to the variables in these two electrospun fiber membranes.23,24 Despite numerous studies, no effort has been made to develop the PAN/PSU fiber composite filter, let alone this membrane with controllable pore size, packing density, and stable cavity structures for fine particle filtration. Herein, we have presented a robust and cost-effective strategy to fabricate binary structured and anti-deformed PEO@PAN/PSU fibrous air filtration medium with small pore size, low packing density, and stable cavity via multi-jet electrospinning. With this aim in mind, PAN/PSU bimodal sized fiber membranes with controllable packing density were carefully constructed by regulating the jet ratio of PAN and PSU solution. The key to our development design was that


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the fluffy and stable membranes with interpenetrating bonding/non-bonding structures were created and optimized based on the in situ bonding agent PEO inspiration. Furthermore, the morphology, mechanical properties, pore structure, packing density, and filtration performance of the composite membranes were thoroughly investigated; and three-dimensional (3D) computer simulation was applied to graphically display the filter structure, particle interception, and airflow penetration of the composite membranes with/without PEO bonding structures for elaborating the contribution of the anti-deformed property on enhancing the filtration performance of air filters.

2. EXPERIMENTAL SECTION 2.1. Materials. PAN (Mw = 90,000) was obtained from Kaneka Co., Ltd., Japan. PSU (Udel® P-1700LCD) was purchased from SOLVAY Shanghai Technology Park, China. PEO (Mw = 100,000) was supplied by BASF Co., Ltd., Germany. N, N-dimethylformamide (DMF) was purchased from Shanghai Chemical Reagents Co., Ltd., China. The nonwoven polypropylene substrate with negligible filtration ability was kindly provided by Shandong Huaye Nonwoven Fabric Co., Ltd., China. All chemicals were of analytical grade and were used as received without further purification. 2.2. Preparation of fibrous membranes. PAN/PEO solutions were prepared by using PAN, PEO, and DMF as starting materials by a vigorous stirring process at 40 oC. The PEO concentration in the precursor solutions were 0, 0.5, 1, 1.5, and 2 wt %, respectively, while the PAN concentration was maintained at 9 wt %. Additionally, 22 wt % PSU solution was obtained by dissolving PSU chips in DMF with stirring for 24 h at ambient temperature.


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A typical procedure for the fabrication of fibrous filters via multi-jet blend electrospinning was demonstrated in Scheme 1. The fabrication of fiber membranes which were deposited on the nonwoven polypropylene substrate was performed by using the DXES-3 spinning equipment (Shanghai Oriental Flying Nanotechnology Co., Ltd., China). Typically, the homogeneous solutions were loaded into four 10 mL plastic syringes fixed on the slipway, and injected through 6-G metal needles with a controllable feed rate of 1 mL/h for electrospinning under a high voltage of 30 kV. The grounded stainless roller was rotating at a speed of 50 rpm keeping a tipto-collector distance of 20 cm. For the PAN/PSU membranes, the PAN and PSU solutions were loaded with the PAN/PSU jet ratio of 4/0, 3/1, 2/2, 1/3, and 0/4, respectively. While for the PEO@PAN/PSU composite membranes, the PAN/PSU jet ratio remained 3/1 and the PEO concentration in the PAN solution was adjusted to 0.5, 1, 1.5, and 2 wt %, respectively. To guarantee the uniformity of fibrous membranes including their thicknesses and basis weights, the four plastic syringes were uniformly placed on an injection pump, and the pump horizontally moved backwards and forwards at a speed of 200 cm/min within a fixed distance by using the mechanical slide unit. Furthermore, electric shield devices on the needles were also employed to ensure that the jets were flying forward, thus the resultant fibers could be uniformly deposited during the quadrature motion process caused by the synchronous movement of the stainless roller and mechanical slide unit. The ambient temperature was 25 oC, and the relative humidity was kept at 50% by using a CH948B humidity controller (WGI Inc., USA). The composite membranes with various basis weights could be obtained by carefully regulating the depositing time. All samples were vacuum-dried at 100 oC for 1 h to remove the residual solvent and charges, especially for the PEO@PAN/PSU membranes to form the bonding structures based on


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the physical melting and solidification of in situ bonding agent PEO component during the heating and annealing process.

Scheme 1. Schematic illustration of the fabrication procedure of the PEO@PAN/PSU composite membranes and an anti-deformed PEO@PAN/PSU fiber based filter for effective air filtration. To test the structural stability and long-term service performance of the composite membranes, all the vacuum-dried membranes with the size of 25 × 60 cm were placed under a steel plate with weight of 15 kg for 48 h, which meant the membranes were under a sustained load of 1000 Pa (ultimate pressure drop in the filter industry). Then the membranes were taken out and transferred to a dry box for further use. 2.3. Characterization. The morphology of the composite membranes was examined by scanning electron microscopy (SEM) (TM3000, Hitachi Ltd., Japan). And, the microstructures on the fibers were examined by field emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi Ltd., Japan). The fiber diameters in the membrane were measured by the image analyzer (Adobe Photoshop CS6), while the thickness of each membrane was measured by a highprecision thickness gauge (CHY-C2, Labthink Co., Ltd., China). The mechanical properties of


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the relevant samples were tested on a tensile tester (XQ-1C, Shanghai New Fiber Instrument Co., Ltd., China). The porous structures of the membranes were characterized through a bubble-point test performed on a capillary flow porometer (CFP-1100AI, Porous Materials Inc., USA). An automated filter tester (LZC-K, Huada Filter Technology Co., Ltd., China) was used to evaluate the filtration performance of the fibrous media before and after the 1000 Pa pressure treatment. 300,000-500,000 charge neutralized monodisperse solid sodium chloride (NaCl) aerosol particles with mass mean diameter of 300-500 nm and geometric standard deviation < 1.86, were delivered through the membrane which was clamped by a filter holder with an effective area of 100 cm2. The whole operation time for performance test was 2 min to ensure the stable output of NaCl particles and avoid the external disturbance. Then the number of NaCl particles in the upstream and downstream of the airflow could be accurately measured by two laser particle counters, and the filtration efficiency was calculated by the data processing system. Similarly, the air resistance could be tested by two electronic pressure transducers that could detect the air pressure before and after the filter under a controlled airflow speed which can be adjusted with purpose between 20 and 100 L/min. To further measure the dust holding capacity, the NaCl testing particles are substituted by the hydrophobic silicon dioxide nanoparticles in the size range of 4-70 nm to shorten the testing time, as reported in our earlier work.4

3. RESULTS AND DISCUSSION 3.1. PAN/PSU composite membranes. Creating the air filter with high particle interception efficiency and robust air permeability requires the precise regulation of the structure of the filtration medium. We designed the PEO@PAN/PSU fiber filter based on three criteria: (i) the fibers in membrane should possess thin fiber diameter which can facilitate the formation of small


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pores, (ii) the membrane should have high porosity and low packing density, (iii) the fibers must assemble into uniform and stable 3D structure without collapsing during the filtration process, especially for high airflow speed and long service. The first two requirements were satisfied by a versatile, readily accessible, and ease of scalable synthesis technology of multi-jet electrospinning, which caused the thin PAN nanofibers and porous PSU microfibers to uniformly assemble into membrane with small pore size and low packing density. To satisfy the third criterion—the formation of stable bonded anti-deformed fibrous networks—we employed the low-melting PEO as a novel in situ bonding agent to yield stable and porous networks with robust mechanical properties. The PAN nanofibers and PSU microfibers were selected as the major building blocks to construct the composite fibrous filter. The representative SEM images of electrospun PAN/PSU composite membranes obtained by varying the jet ratio of PAN/PSU were shown in Figure 1a-e, revealing the randomly aligned 3D nonwoven structures, which could be suitable for the requirement of tortuous structure for particle intercept and air transmission. The sole PAN membrane was composed of continuous and smooth nanofibers with an average diameter of ~ 270 nm; while the microfibers in sole PSU membrane exhibited porous, wrinkled surface, and an average diameter of ~ 1.3 µm, as shown in Figure 1a and e, respectively. For the PAN/PSU composite membranes, they were composed of fibers with bimodal sized diameter distributions, as displayed in Figure 1f. From Figure 1b-d we can know that, with decreasing PAN/PSU jet ratio from 3/1, 2/2 to 1/3; the content of PSU microfibers that acted as a skeletal framework for the thin PAN nanofibers in the membranes constantly increased, resulting in the formation of the looser packing structures. What was more interesting, dense and obvious micropores and wrinkles formed on the surface of PSU microfibers, as demonstrated in the insets of Figure 1e.


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The formation of these pores and wrinkles which were beneficial for improving the surface area of the fibers, could be attributed to the fast phase separation and solidification of the charged liquid jets.25,26

Figure 1. SEM images of PAN/PSU composite membranes obtained with various jet ratios of PAN/PSU (a) 4/0, (b) 3/1, (c) 2/2, (d) 1/3, and (e) 0/4. (f) Fiber diameter distribution of the relevant membranes. The filtration performance of the resultant PAN/PSU composite membranes was systematically investigated by using the charge neutralized NaCl particles in the size range of 300-500 nm under a standard airflow speed of 32 L/min. As demonstrated in Figure 2, the filtration efficiency of the PAN/PSU membranes obtained with PAN/PSU jet ratios of 4/0, 3/1, 2/2, 1/3, and 0/4 were 82.414%, 81.211%, 76.018%, 74.758%, and 72.063%; while the relevant


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pressure drop were 7, 4.8, 3.8, 3.3, and 2.9 Pa, respectively, indicating a synchronous decrease upon increasing PSU microfibers content in the membranes. The trade-off parameter of QF is widely employed to comprehensively evaluate the filtration performance of a given filtration medium, and it can be expressed as: QF = -ln(1-η)/∆p, where η and ∆p represent the filtration efficiency and pressure drop, respectively.27 Benefiting from the supporting effect of PSU microfibers, the PAN/PSU composite membrane with PAN/PSU jet ratio of 3/1 possessed a sharply increased QF value of 0.348 Pa-1 in contrast to the sole PAN nanofiber membrane (QF value of 0.248 Pa-1), further confirming the contribution of the appropriate amount of PSU microfiber on enhancing the air permeability of the filter, as shown in Figure S1.

Figure 2. Filtration efficiency and pressure drop of the pressure-treated and untreated PAN/PSU composite membranes (~ 0.5 g/m2) by service loading of 1000 Pa fabricated from various jet ratios of PAN/PSU. Considering that the filters in real application would suffer from a constant extrusion stress caused by the airflow, which could lead to the collapsing of the loose structures during the service process; then their filtration performance would deteriorate rapidly and irretrievably.28 Here, by introducing the weight pressure (1000 Pa) to equivalently simulate the airflow pressure,


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we evaluated the filtration performance change of the composite membranes for the first time. From Figure 2 we can know that, the filtration efficiency of the pressure-treated PAN/PSU membranes with PAN/PSU jet ratios of 4/0, 3/1, 2/2, 1/3, and 0/4 were 62.732%, 65.320%, 62.995%, 66.543%, and 74.396%; while the relevant pressure drop were 4.5, 3.2, 2.8, 2.3, and 3.5 Pa, respectively. Obviously, both the filtration efficiency and pressure drop of all the treated membranes except for sole PSU microfibers, exhibited significantly decreased trend compared with the corresponding initial membranes, especially for the sole PAN nanofibers. These phenomena could be explained by that, after pressure treatment the membranes collapsed and became densely packed. And this explanation of the structural change could be easily confirmed by the SEM images of the cross-sectional morphology of the PAN/PSU membrane before and after pressure treatment, as demonstrated in Figure S2. The former change would damage the capture ability for the ultrafine particles due to the reduced collision probability between the particles and fibers in the thinning membranes based on Brownian diffusion; while the latter caused the membranes to turn thinner, facilitating the air pass through the membranes.17,29 Obviously, the filtration efficiency of the pressure-treated PAN/PSU membrane with PAN/PSU jet ratio of 3/1, was much higher than that of the membrane with PAN/PSU jet ratio of 2/2. This phenomenon could be explained that, by virtue of the initial advantage of the filtration efficiency, the former membrane remained higher even though it suffered a more serious deterioration after pressure treatment (from 81.211% to 65.320% for 3/1, from 76.018% to 62.995% for 2/2). What was worth mentioning, the pressure treatment synchronously endowed the sole PSU microfibers with slightly increased filtration efficiency and pressure drop for air filtration. That could be attributed to the drastic densifying of the fluffy PSU packing structures, resulting in much smaller pore size; thus the permeability of air and particles was both inhibited,


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indirectly revealing the contribution of PSU microfibers on improving the porosity of the membranes. Additionally, although the initial composite membrane with PAN/PSU jet ratio of 3/1 exhibited good filtration performance in short service period, they still suffered from the relatively weak mechanical properties (tensile strength of 2.43 MPa, Young's modulus of 83 MPa, and toughness of 0.81 MJ/m3) that must be addressed before realizing their subsequent processing and real application, as shown in Figure S3. 3.2. PEO@PAN/PSU composite membranes. To maintain the fluffy and porous structure, we employed the low-melting PEO as in situ bonding agent to form physical bonding structures among the fibers based on a vacuum-dry process (100 oC for 1 h), endowing the resultant composite membranes with anti-deformed properties against high wind pressure and other service stress. Figure 3a-d presented the SEM images of PEO@PAN/PSU composite membranes loaded with various PEO concentrations. Upon the PEO melting-solidification process, the bonding structures formed among the adjacent fibers, as indicated in the dotted circle. With increasing the PEO concentration (from 0.5 to 1.5 wt %), the number of point-shaped bonding structures increased dramatically, while the fiber diameter remained almost unchanged. Further increasing the PEO concentration to 2 wt %, the ribbon-shaped bonding structures formed, as shown in the inset of Figure 3d. This change could be a consequence of the excessively increased binding agent content, which could flow along the fibers when heated to its melting point, and pulled the adjacent and parallel nanofibers together during the solidification process.30 Additionally, the bonding structures only appeared among the PAN nanofibers, which was consistent with the original composition of PAN/PEO solutions. Thus, it would not damage the pore structures on the PSU microfibers, enhancing the stability of PEO@PAN/PSU packing


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structure without sacrificing the particle capture ability based on the adsorbability of surface pores.

Figure 3. SEM images of PEO@PAN/PSU (PAN/PSU jet ratio of 3/1) composite membranes formed with PEO concentration of (a) 0.5, (b) 1, (c) 1.5, and (d) 2 wt %. As expected, the fiber bonding strategy that was employed to build the stable fluffy structures, also greatly enhanced the mechanical properties of PEO@PAN/PSU composite membranes. As presented in Figure 4, the composite membranes with a certain thickness of 20 ± 2 µm fabricated from the solution with various PEO concentrations of 0.5, 1, 1.5, and 2 wt % possessed a tensile strength of 3.83, 5.95, 8.20, and 9.26 MPa, respectively, revealing a significant improvement compared with the PAN/PSU membranes without bonding structures (2.43 MPa). The Young’s modulus of the relevant membranes with PEO concentration range from 0 to 2 wt %, which was generally considered to be one of the indicators of their anti-deformed ability, were 83, 110, 157, 204, and 222 MPa; while their toughness were 0.81, 0.71, 1.66, 2.44, and 2.70 MJ/m3, respectively, further confirming the PEO bonding contribution on the stable cavity structures. Under the tensile stress, the bonding structures of PEO@PAN/PSU membranes would inhibit the


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fiber slipping and shorten the elongation at break. Thus, the PEO@PAN/PSU membrane from solution with 0.5 wt % PEO showed a slightly lower toughness compared with the PAN/PSU membrane. And, it was clearly visible that the mechanical properties including tensile strength, Young’s modulus, and toughness of PEO@PAN/PSU composite membrane were all at least 2.5 times higher than that of PAN/PSU membrane, fully indicating the anti-deformed ability against the external stress. Additionally, all the mechanical properties of PEO@PAN/PSU composite membrane including tensile stress, Young’s modulus, and toughness exhibited a slightly decreasing trend with increasing the thickness of the membranes, as shown in Figure S4. This change could be explained by that, with prolonging the electrospinning time, the thickness of the membranes and the amount of residual charge increased simultaneously, thus the membranes became much more fluffy which meant the weaker interaction among fibers, resulting in the slightly decreased mechanical properties of the thicker membranes. Considering the postprocessing and service requirement for filter membranes, the excellent mechanical properties of this composite membranes made them an ideal candidate for the real air filtration applications, especially in the harsh environment applications, such as high wind speed, large ventilation volume, high particle concentration, etc.


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Figure 4. Mechanical properties of PEO@PAN/PSU composite membranes formed with various PEO concentrations. Figure 5 exhibited the filtration performance of the pressure-treated and untreated PEO@PAN/PSU composite membranes formed with various PEO concentrations. It could be clearly seen that, with increasing the PEO concentration (from 0 to 2 wt %), the filtration efficiency of the composite membranes before and after the service loading treatment both showed an increased trend at first, and then decreased obviously; and the gap between the two gradually reduced. While, the pressure drops of the pressure-treated and untreated membranes exhibited the same variation trend: they increased at first, then decreased until 1.5 wt % PEO and finally increased again. Obviously, all the pressure drops of PEO@PAN/PSU membranes were higher than that of PAN/PSU membrane due to the introduction of PEO bonding structures, which sacrificed the porosity of the membrane to some extent even though endowing them with anti-deformed property. As the PEO concentration increased from 0.5 to 1.5 wt %, the pressure drop of the PEO@PAN/PSU membranes gradually decreased by virtue of their growing stability and bulkiness. Especially, the filtration efficiency of the original and treated membranes were almost the same (80.579% and 80.575%) when PEO concentration exceeded 1.5 wt %, revealing that 1.5 wt % PEO was enough to endow structural stability to the composite membranes; meanwhile, their pressure drops showed the same value as well. However, further increasing the PEO concentration to 2 wt %, the composite membranes exhibited decreased filtration efficiency of 74.707% and increased pressure drop of 8 Pa, resulting from the formation of compact structure due to the superabundant ribbon-shaped bonding structures, which corresponded very well with the SEM observation. Additionally, benefiting from the thin PAN nanofibers, fluffy PSU microfibers, and stable bonded structures without collapsing, the PEO@PAN/PSU


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composite membrane with 1.5 wt % PEO possessed a high QF value of 0.273 Pa-1, revealing the promising potentials in real filtration application, as demonstrated in Figure S5.

Figure 5. Filtration efficiency and pressure drop of the pressure-treated and untreated PEO@PAN/PSU composite membranes (~ 0.5 g/m2) by service loading of 1000 Pa formed with various PEO concentrations. 3.3. Porous structure analysis. The fascinating fiber morphologies and various filtration performances enabled us to intensively investigate the pore structure of PEO@PAN/PSU composite membranes with various PEO concentrations, and further to discern the interrelationship among porous structure, apparent density, and filtration performance. Although the incorporation of PEO caused a slight increase of the diameter of PEO@PAN nanofibers (as exhibited in Figure S6), here we mainly focused on investigating the integrated effect of fiber diameter and packing structure, namely the pore structures, to explain the change of filtration performance of the filters. The pore structures involving pore size and pore size distribution were measured through a capillary flow porometer, while the apparent density was calculated based on the basis weight and thickness of the membranes, as demonstrated in Figure 6. The composite


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membranes from solutions with PEO concentration of 0, 0.5, 1, 1.5, and 2 wt % showed the pore size distribution in the range of 1.5-3.5 µm, with well-developed peaks centered at 2.8, 2.6, 2.1, 2.0, and 1.9 µm, respectively. What was more, the pore distribution exhibited an obviously narrow trend until 1.5 wt % PEO, then it widened with increasing PEO concentration. These phenomena could be explained by that with increasing the PEO content, the bonding structures enhanced, thus no liquid extrusion caused nanofiber slippage would occur during the bubblepoint test. However, the membrane with 2 wt % PEO showed a relatively wider pore distribution due to its non-uniformly distributed ribbon-shaped bonding structures, as shown in Figure 3d. Additionally, the pore size distribution of the pressure treated PEO@PAN/PSU membrane (1.5 wt % PEO) remained almost unchanged, further confirming the anti-deformed property contributed by the PEO bonding structures, as demonstrated in Figure S7. While, some of the pore size of the pressure treated PAN/PSU membrane decreased from ~2.8 to ~1.0 µm and became non-uniform after pressure treatment due to the structural collapse. These results corresponded very well with the change of the filtration performance of the fibrous membranes mentioned above. Meanwhile, the apparent density of the composite membranes with various PEO concentrations exhibited the same variation trend as their pressure drops, as shown in Figure 6b. All the PEO@PAN/PSU membranes possessed a higher apparent density than the PAN/PSU membrane, which could be ascribed to structural stabilization process based on the introduction of PEO binder.31 For the PEO@PAN/PSU membranes, the apparent density exhibited an obviously decreased tendency until PEO concentration of 1.5 wt %, then it increased slightly, further confirming the optimum PEO content (1.5 wt %) for guaranteeing the fluffy structures without collapsing during structural stabilization process. While for 2 wt % PEO, the apparent


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density of the membrane increased again, which could be ascribed to the reduced void in the membrane caused by the ribbon-shape bonding structures. Furthermore, the packing condition of the PEO@PAN/PSU membranes from solutions with various PEO concentrations could be vividly indicated by their cross-sectional morphologies, as demonstrated in Figure S8. Obviously, the SEM results possessed the same variation trend as the calculated apparent density, further confirming the structural explanation for the filtration performance change. Additionally, the PEO@PAN/PSU membrane from solution with 1.5 wt % PEO possessed a high porosity of 88.7%, indicating that this tortuous porous structure could not only provide numerous channels for airflow to pass through the media in a shorter path obeying the minimal resistance principle, but also significantly improve the Brownian diffusion, electrostatic interaction, aerodynamic slip, and sieve effects between ultrafine particles and the fibers.32,33 Therefore, this stable and fluffy membrane could exhibit an almost unchanged particle interception ability and much more robust air permeability compared with other PEO@PAN/PSU membranes, which matched very well with the aforementioned filtration performance of the composite membranes.


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Figure 6. (a) Pore size distribution curves and (b) apparent density of PEO@PAN/PSU composite membranes from solution with various PEO concentrations. 3.4. Filtration progress simulation. To elucidate the unique features of the novel PEO@PAN/PSU composite membranes, the FilterDict procedure with the SEM images, obtained pore structure parameters, and designed fluid parameters was used to create the 3D structure models and analyze their filtration process, as illustrated in Figure 7. Throughout the literature, the particle capture was mainly based on two major deposition patterns (surface


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filtration and deep bed filtration) and five capture mechanisms (physical sieving, interception, diffusion, inertial separation, and electrostatic attraction) according to the respective filtration features of the filters.34-36 From Figure 7a and a’ we can know that, by introducing the PEO in situ binding agent the two membranes showed different bulk statistical properties: the PEO@PAN/PSU membrane possessed much more fully structure with larger cavities, while the PAN/PSU membrane exhibited compact structure with low porosity. Fig 7b and b’ presented the interception process of airborne particles for these two filters. Obviously, with the same generation quantity of airborne dust, the PEO@PAN/PSU membrane could trap all the aerosol particles based on the synergistic effect of multi-capture manners, while the small size particles could easily penetrate the PAN/PSU membrane. This result could be explained by that the stable and fluffy structure prolongs the transmission time for airborne particles to pass through the filtration medium, thus significantly improve the collision probability between the fine particles and the fiber surface, especially for the particles with smaller size.32 The air permeation processes of PEO@PAN/PSU and PAN/PSU filters under various airflow speeds were simulated by the FilterDict procedure with the same fluid parameters, and the pressure distributions were vividly displayed in Figure 7c and c’. Noticeable conclusion could be drawn that the pressure drop of PEO@PAN/PSU membrane was obviously lower and its distribution was more uniform than that of PAN/PSU membrane under the same airflow speed. During the filtration process, the larger cavity structure in PEO@PAN/PSU membrane greatly reduced the friction between the airflow and fibers, favoring the air penetration and resulting in an extremely low pressure drop.37 Overall, this exclusive particle deposition and air penetration change of the PEO@PAN/PSU substantiates the fact that the filtration process is inseparably related with fiber, porous, and stack structures.


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Figure 7. Illustration of the concept of (a) PEO@PAN/PSU and (a') PAN/PSU membrane filter based on their bulk statistical properties. Schematic diagram illustrating the interception process of airborne particles in (b) PEO@PAN/PSU and (b') PAN/PSU membrane filter. Schematic diagram illustrating the air pass process in (c) PEO@PAN/PSU and (c') PAN/PSU membrane filter under various airflow speeds. 3.5. Filtration performance evaluation. The filtration performance of PEO@PAN/PSU composite membranes with various basis weights under a face velocity of 32 L/min was presented in Figure 8a. The filtration efficiencies of the composite membranes with increased basis weight were 80.579%, 90.435%, 94.99%, 98.678%, 99.289%, 99.971%, and 99.992%, while the relevant pressure drops were 6, 15, 31, 52, 68, 80, and 95 Pa, respectively, indicating a


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synchronous improvement upon increasing the basis weight and confirming the benefit-to-cost role of basis weight. It was clearly visible that, the filtration efficiency showed a sharp rise and then reached a relatively steady high-grade value when the basis weight excessed 2.1 g/m2, and it could be easily improved to the standard of high efficiency particulate air (HEPA) filters (>99.97%) by endowing the membrane with a basis weight of >3.1 g/m2, which was unavailable for the traditional filters with such a superlight weight, revealing the key role of small pore size and fluffy structures for enhancing the filtration performance.

Figure 8. (a) Filtration efficiency and pressure drop of PEO@PAN/PSU composite membranes (PEO concentration of 1.5 wt %) with various basis weights. (b) Filtration efficiency and pressure drop, and (c) quality factor versus air flow of the PEO@PAN/PSU (~ 3.5 g/m2) composite fiber filter. (d) Comparison of the filtration performance between current filtration media and PEO@PAN/PSU membrane under high airflow speed of 90 L/min.


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To elaborate the desirable performances and encouraging perspectives of PEO@PAN/PSU composite membrane, the filtration efficiency, pressure drop, and QF as a function of airflow speed (20-100 L/min) were systematically studied (Figure 8b and c). The filtration efficiency remained virtually unchanged with increasing the airflow speed, ranging from 99.999% to 99.985%, even under the highest speed of 100 L/min it could achieve the HEPA standard as well, further confirming the contribution of PEO bonding structures on enhancing the antideformed performance of the composite membranes. Apart from that, according to the previous studies the Stokes flow regime was considered to prevail in fibrous filters, thus the Darcy’s law for viscous resistance was applicable,38 indicating that the pressure drop should be directly proportional to face velocity. And, our experimental results matched very well with Darcy’s theory; the slope of the curve of pressure drop versus face velocity was only about 3.1, much smaller than the ever reported data (e.g., 3.7 for PA-6/PAN/PA-6, 3.4 for PA-56, 3.37 for PAN/PU, 5.4 for PA-6),4,10,39 indicating the more robust air permeability in practical applications. By virtue of the outstanding filtration efficiency and low pressure drop which could be attributed to the thin PAN nanofibers and fluffy structures, the QF of PEO@PAN/PSU membranes exhibited much higher level of 0.1 Pa-1 under 32 L/min (even 0.027 Pa-1 under 100 L/min) compared with previously reported electrospun fiber based filter media, such as 0.09 Pa-1 for PA-56,4 0.02323 Pa-1 forγ-Al2O3,40 0.0299 Pa-1 for PU,41 0.063 Pa-1 for polylactic acid,26 etc. Furthermore, the comparison of dynamic separation behavior between the PEO@PAN/PSU and PAN/PSU membranes were systematically investigated to further reveal the robust air permeability of the PEO@PAN/PSU filter, as shown in Figure S9. With increasing the treatment time by service loading, both the filtration efficiency and pressure drop of these membranes showed an increased trend until 36 h due to the growing structural compactness, then they


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remained almost unchanged. Especially, benefiting from the anti-deformed and fluffy structures even under large service loading, the PEO@PAN/PSU membrane exhibited a significantly low pressure drop and much more stable filtration performance compared with the PAN/PSU membrane. Additionally, both the mechanical properties and filtration performance of the PEO@PAN/PSU membrane possessed good stability in the high humid (>95%) environment, as exhibited in Figure S10 and S11. Overviewing the background of air filtration, several high performance fibrous materials including superfine glass fibers, electret melt-blown fibers, and electrospun nanofibers have evoked wide attentions whether in industrial or academic sectors and serve as the mainstream products in the practical application, especially the former two media.42 Figure 8d presented the comparison of the filtration performance between current filtration materials and novel PEO@PAN/PSU membrane under high airflow speed of 90 L/min. Although the glass fiber can achieve a rather high filtration efficiency benefiting from its small fiber diameter (500-700 nm) and unlimitedly increased basis weight, it still suffers from the extremely high airflow resistance. While a few bulky melt-blown microfiber media work well at capturing the fine airborne particles at relatively low pressure drop depended on the electret effect, but the electret strategy also brings the potential menace of performance degradation when charge loss occurs (exposed to certain chemicals, aerosols, or high humidity, etc.), which is now strictly constrained by the latest European standard for air filters (EN779: 2011).43 Electrospun nanofibers, as an emerging nanomaterial for air filtration have drawn tremendous attention due to their small diameters and ease of fabrication. Among these mainstream filtration media, the novel PEO@PAN/PSU composite membrane stands out from the rest filters in view of its superlight weight of 3.5 g/m2, anti-deformed properties, together with excellently stable filtration performance under 90 L/min


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airflow, including high filtration efficiency of 99.986% and low pressure drop of 290 Pa. Furthermore, the PEO@PAN/PSU membrane exhibited a much higher dust holding capacity (42 g/m2) than H&V glass fiber medium (28 g/m2) reported in our previous work,4 indicating the longer service life and the larger filtration volume.

4. CONCLUSIONS In summary, we have for the first time fabricated the anti-deformed PEO@PAN/PSU composite membrane for effective air filtration by combining multi-jet electrospinning and physical bonding process. The fluffy PAN/PSU composite membranes with small pore size and controllable packing density were successfully constructed via regulating the jet ratio of PAN and PSU solution. And, by employing the PEO in situ bonding agent incorporation, the aforementioned composite membranes were endowed with stable cavity and anti-deformed property due to their interpenetrating bonding/non-bonding structure. By virtue of the small pore size provided by PAN nanofibers, large cavity supported by PSU microfibers, and stable porous framework supplied by PEO bonding points, the resultant PEO@PAN/PSU composite membrane possessed robust mechanical properties (tensile strength of 8.2 MPa, toughness of 2.44 MJ/m3, and Young’s modulus of 204 MPa), high filtration efficiency of 99.992%, low pressure drop of 95 Pa, and desirable QF of 0.1 Pa-1 for 300-500 nm airborne particle filtration. More significantly, this PEO@PAN/PSU membrane with anti-deformed property successfully got rid of the potential safety hazards caused by unexpected structural collapsing under service stress. We anticipate that the PEO@PAN/PSU composite membrane will provide a novel promising candidate to diminish the negative impact of PM2.5 in wide applications of individual


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protection and industrial filtration including respirator, protective clothing, engine intake, and clean room.

ASSOCIATED CONTENT Supporting Information. Quality factor (Figure S1) and mechanical properties (Figure S3) of PAN/PSU composite membranes fabricated with various jet ratios of PAN/PSU. Cross section images of PAN/PSU fibrous membranes before and after pressure treatment (Figure S2). Mechanical properties of PEO@PAN/PSU composite membranes with various thicknesses (Figure S4). Quality factor (Figure S5), average diameter of PEO@PAN nanofibers (Figure S6), and cross section images (Figure S8) of PEO@PAN/PSU composite membranes formed from solutions with various PEO concentrations. Pore size distribution curves of PEO@PAN/PSU membranes with various PEO concentrations after pressure treatment (Figure S7). Filtration performance of PEO@PAN/PSU and PAN/PSU membranes treated by service loading of 1000 Pa for various time (Figure S9). Mechanical properties (Figure S10) and filtration performance (Figure S11) of PEO@PAN/PSU membrane after treated for various time under high relative humidity of >95%. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Key Technologies R&D Program of China (No. 2015BAE01B01), the National Natural Science Foundation of China (No. 51322304 and 51503030), the ‘DHU Distinguished Young Professor Program’, the Fundamental Research Funds for the Central Universities, and the Shanghai Sailing Program (No. 15YF1400600).


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Table of Contents/Abstract Graphic


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Polysulfone ... - ACS Publications

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