Electret Polyvinylidene Fluoride Nanofibers Hybridized by

Aug 23, 2016 - ... efficiency of 99.972%, a low pressure drop of 57 Pa, a satisfactory quality factor of 0.14 Pa–1, and superior long-term service p...
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Electret Polyvinylidene Fluoride Nanofibers Hybridized by Polytetrafluoroethylene Nanoparticles for High-Efficiency Air Filtration Shan Wang,†,‡,§ Xinglei Zhao,†,‡,§ Xia Yin,*,†,‡ Jianyong Yu,‡ and Bin Ding†,‡ †

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 S Supporting Information *

ABSTRACT: Airborne particulate matter (PM) pollution has become a severe environmental concern calling for electret fibrous materials with high filtration efficiency and low pressure drop. However, restraining the dissipation of the electric charges in service to ensure the stabilized electrostatic force of the fibers for effectively adsorbing particles is extremely important and also challenging. Herein, we report novel electret nanofibrous membranes with numerous charges and desirable charge stability using polyvinylidene fluoride (PVDF) as the matrix polymer and polytetrafluoroethylene nanoparticles (PTFE NPs) as an inspiring charge enhancer through the in situ charging technology of electrospinning. Benefiting from the employment of PTFE NPs and optimized injection energy, the fibrous membranes are endowed with elevated surface potentials from 0.42 to 3.63 kV and reduced decrement of charges from 75.4 to 17.5%, which contribute to the ameliorative stability of filtration efficiency. Significantly, an electret mechanism is proposed, while deepened depth of the energy level and incremental polarized dipole charges with increasing PTFE NP concentrations and injection energy have been confirmed through the measurement of open-circuit thermally stimulated discharge and surface potential decay. Ultimately, the resultant fibrous membrane exhibited a high filtration efficiency of 99.972%, a low pressure drop of 57 Pa, a satisfactory quality factor of 0.14 Pa−1, and superior long-term service performance. The successful fabrication of such an intriguing material may provide a new approach for the design and development of electret materials for PM2.5 governance. KEYWORDS: electrospinning, electret, surface potential, charge stability, filtration performance

1. INTRODUCTION

Benefiting from the ability of quasi-permanent reserving abundant charges and creating an external macroscopic electric field on the periphery of fibers, electret fibrous membranes have been proven to be a feasible, efficient, and promising material to intensively absorb PM2.5 by long-range electrostatic force.10,11 Meanwhile, their uncompacted structure endows electret fibrous membranes with very small resistance to air molecule motion.12 Heretofore, many electret fibrous materials, such as polypropylene,13,14 polyimide,15 polyethylene,16 polycarnonates,17 and so on, have been successfully created through various electret technologies, such as corona charging, tribocharging, and low energy electron beam bombardment. Nevertheless, some crucial drawbacks still remain: (1) slight dipole charges but enormous space charges trapped in shallow traps, resulting in undemanding dissipation of charges and eventually giving rise to the reduction of filtration efficiency and (2) a microsized fiber diameter and comparatively large

Due to the currently abominable levels of environmental particular matter pollution and the severe global energy crisis, ever-increasing research efforts have been committed to developing filter materials with high filtration efficiency to effectively wipe out particles and ultralow air resistance to attain the anticipated purpose of energy conservation.1−3 Among the available air filter materials, fibrous membranes with tortuous porous structures, which can serve as an intercepting unit for PM2.5, and comparatively high porosity to decentralize the applied airflow for the traverse of air molecules, have become the most pivotal materials for air filtration.4−6 However, conventional fibrous filtration materials based on mechanical filtration manner, such as spin-bonded fibers, melt-blown fibers, and glass fibers, are incapable of meeting the requirements of high-efficiency and low-resistance due to the compacted stacking structure or large aperture size.7,8 Although high filtration efficiency can be effectuated through unlimited increase of the content of fibers, the surge of the pressure drop will be engendered.9 © XXXX American Chemical Society

Received: July 6, 2016 Accepted: August 23, 2016

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DOI: 10.1021/acsami.6b08262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces aperture size, leading to an extremely low mechanical filtration efficiency (∼30%), which will bring about security hazards in the course of practical utilization. Electrospinning, as a burgeoning technology for nanofiberfabrication that provides a versatile method for in situ charge injection prior to the formation of fibers, could effectively address the bottleneck of common electret techniques.18,19 Benefiting from this extraordinary modality, the space charges of electrospun fibers with larger depth of energy level are more easily generated compared with that of fibrous membranes fabricated through other electret techniques.20 In addition, profiting from its nanoscale diameter, high tortuous porous structure, and adjustable porosity, improved mechanical filtration efficiency and reposeful pressure drop can be granted to electrospun membranes.21,22 Up to now, just a handful of efforts have been committed to investigate the electret performance and air filtration properties of electrospun membranes. Yeom et al. fabricated Polyamide-6 nanofibers incorporating boehmite nanoparticles with an absolute surface potential of 109 V, filtration efficiency of 96.400%, and pressure drop of 47 Pa.23 Heidi L. et al. fabricated electret polystyrene/ polyacrylonitrile fibrous membranes with large charge decay (from −1587 to −183 V within 20 h) and filtration efficiency of 98.260%.24 Previously, we reported a strategy for fabricating an electret poly(ether imide)-silica fibrous membrane, and the resultant membranes exhibited a comparatively high filtration efficiency of 99.992% and low pressure drop of 61 Pa.25 However, the resultant fibrous membranes mentioned above are still subjected to several deficiencies, such as inappropriate electret materials resulting in precarious space charges, relatively slight dipole charges and unsatisfactory charge stability, which seriously impeded its practical utilization. Accordingly, fabricating cost-effective electret materials for air filtration with abundant charges, particularly dipole and interfacial charges, and excellent charge stability by simple approach is still a big challenge to be resolved. Herein, we present a facile strategy to fabricate electret fibrous membranes with elevated surface potential and enhanced charge stability via electrospinning. The fibrous membranes are constitutive of polyvinylidene fluoride as matrix polymer and polytetrafluoroethylene nanoparticles as an inspiring charge enhancer, both of which are provided with substantial fluorine atoms and an extremely strong negative induction effect. The pore structure, mechanical properties, and charge stability of the fibrous membranes are thoroughly investigated. Significantly, the electret mechanism of electrospinning is proposed, meanwhile, the transformation of depth of the energy level and proportion of dipole charges with increasing PTFE NP concentration and injection energy were confirmed by surface potential decay and thermally stimulated discharge. Benefiting from the ameliorative electret features, the resultant membranes exhibited high filtration efficiency, low pressure drop, and superior long-term service performance, meaning that they would be a formidable candidate in ultralow resistance air filters, air purifiers, and respirators.

in Supporting Information (SI) Figure S1. In addition, performance comparison of PVDF fibrous membranes with various sizes of PTFE particles is presented in Figure S2. N,N-Dimethylfomamide (DMF) was provided by Macklin Biochemical Co., Ltd., China. The nonwoven substrate (polyethylene terephthalate) for the collection of nanofibers possessed insignificant filtration efficiency (2%) and pressure drop (0 Pa) when the airflow velocity was equal to 5.3 cm s−1 was supplied by Hainan Xinlong Nonwoven Co., Ltd., China. 2.2. Preparation of PVDF/PTFE Solutions. In a typical process, 22 wt % PVDF polymer solution containing 0, 0.01, 0.05, and 0.1 wt % of PTFE NPs were prepared by the following procedure: first, various weights of PTFE NPs (0, 0.006, 0.03, and 0.06 g) were put into four undefiled glass bottles, respectively, and then DMF was added to the corresponding bottles with vigorous stirring using magnetic stirrers. In addition, to ensure the homogeneous dispersion of PTFE NPs in DMF, an ultrasonic dispersion method was employed for 30 min. Finally, 12 g PVDF powder was added to the corresponding PTFE/ DMF dispersion solution, and afterward the multicomponent solutions were subjected to vigorous stirring for 24 h at 80 °C to guarantee the complete dissolution of polymer. The distribution of PTFE NPs in PVDF solution with the optimal PTFE concentration of 0.05 wt % are presented in Figure S3. 2.3. Fabrication of PVDF Nanofibers Doped with PTFE NPs. Electret PVDF nanofibers doped with PTFE NPs were fabricated using electrospinning equipment of DXES-3 (Shanghai Oriental Flying Nanotechnology Co., Ltd., China). Typically, the PVDF/PTFE NP polymer solution was sucked into plastic syringes and clamped to the supporting frame which can move right and left continuously. Afterward, the homogeneous PVDF/PTFE solution was extruded through 5 G metallic needles with a governable infusion velocity of 0.5 mL h−1 and a high DC voltage of 30 kV was simultaneously carried out in the pinpoints of the needles, giving rise to the formation of a stable jet flow. The obtained PVDF/PTFE fibrous membranes were assembled on the earthed metallic tumbling barrel covered by a nonwoven substrate, which rotated at a velocity of 50 rpm, and the distance of tip-to-collector was 15 cm. The ambient temperature and relative humidity were 23 ± 2 °C and 48 ± 4%, respectively. The resultant PVDF fibrous membranes containing various PTFE NP contents of 0, 0.01, 0.05, and 0.1 wt % were denoted as PVDF/PTFE0, PVDF/PTFE-1, PVDF/PTFE-5, and PVDF/PTFE-10. 2.4. Characterization of the Membranes. Scanning electron microscopy (SEM, TM 3000, Hitachi Ltd., Japan HQ) was employed to study the morphology of PVDF/PTFE fibrous membranes. The fiber diameter was gauged employing an image analysis software (Adobe Photoshop CS6). An automatic filtration performance tester purchased from Huada Instrument and Equipment Co. Ltd., China was employed to appraise the filtration property. The charge neutralized sodium chloride (NaCl) monodisperse aerosol particles possessing mass-average particular size of 0.3−0.5 μm and standard deviation less than 1.86, were generated by atomizing air pump and then passed through the test samples with an valid test area of 100 cm2. The NaCl aerosols were detected through a laser airborne particle counter at ambient temperature (25 ± 2 °C) and humidity (45 ± 5%). The filtration efficiency could be calculated by η = 1 − ε1/ε2, where ε1 and ε2 represented the quantity of the NaCl aerosol in the downstream and upstream of the filter, respectively. The accuracy of the test equipment for the evaluation of filtration efficiency could be read to three decimal places. The pressure drop of the sample was measured by a flow gauge and two electronical pressure transmitters. The filtration properties of the as-prepared membranes were measured with the nonwoven substrate together. In addition, detailed information about the test equipment for filtration properties can be seen in Figure S4. A tensile tester was used to test the mechanical properties of the membranes (XQ-1C, Shanghai New Fiber Instrument Co. Ltd. China), the size of the sample was 3 mm × 2 cm, the tensile speed was 5 mm min−1, and the applied force was 0.25 cN. The aperture sizes of the fabricated membranes were measured using a capillary flow porometer based on Laplace’s equation (CFP-1100AI, Porous Materials Inc., U.S.A.). Porosity of the fabricated membrane was obtained employing the following formula:

2. EXPERIMENTAL SECTION 2.1. Materials. Polyvinylidene fluoride (PVDF, DY-1, Mw = 680 000, density = 1.77 g cm−3) was obtained from Shandong DEYI New materials Co., Ltd., China. Polytetrafluoroethylene nanoparticles (PTFE NPs, average particles size 197 nm, purity 99.9%, density = 2.2 g cm−3) were supplied by Dongguan Yingde plastic cement materials Co., Ltd., China. The morphology and size distribution are presented B

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Figure 1. Morphology, pore structure, and mechanical properties of PVDF/PTFE electret fibrous membranes. SEM images of PVDF electret fibrous membranes containing various PTFE NP concentrations of (a) 0, (b) 0.01, (c) 0.05, and (d) 0.1 wt %, respectively. (e) Average diameter, average pore size, and porosity, (f) pore size distribution, and (g) tensile strength of PVDF/PTFE fibrous membranes fabricated from polymer solutions with various PTFE NP concentrations.

porosity =

(D0 − D1) × 100% D0

structure was obviously identifiable, as shown in Figure 1d and Figure S6, which was composed of slender fibers and micrometer-sized elliptical beads with an average diameter of 1.97 μm, attributed to the agglomeration of PTFE NPs.23,27 In addition, the TEM images demonstrated that PTFE NPs were well dispersed within PVDF/PTFE-1 and PVDF/PTFE-5, but some agglomeration appeared for PVDF/PTFE-10 (Figure S8). These results elucidated that the content of PTFE NPs would conspicuously influence the morphology of the nanofibers, which would bring about the structural transition of the resultant membranes. On account of the morphological transformation of fibers, the structure of relevant membranes would be influenced tremendously, which could be appraised by the characterization of porosity, average pore size, and pore size distribution. As presented in Figure 1e, plotting curves of average diameter, average pore size, and porosity versus concentration of PTFE NPs revealed that two noticeable intervals could be observed: a speedy descending interval at σPTFE < 0.05 wt %, which could be caused by the increased content of fibers in the unit basis weight and reinforced compact structure with the reduction of fiber diameter;28 a moderate ascending interval at σPTFE > 0.05 wt %, which may be the consequence of the formation of beadon-string structure. In addition, the relevant pore distribution curves were also studied, as shown in Figure 1f. The pore distribution presented a slightly narrow tendency until 0.1 wt % PTFE NPs, which could be elucidated by the ameliorative uniformity of fibers with increasing PTFE NP concentration from 0 to 0.05 wt %. However, the pore distribution broadened with the enhancement of the PTFE NP concentration, ascribing to the formation of bead-on-string structure for PVDF/PTFE-10 fibrous membranes. Mechanical character is a considerable factor in practical application of fibrous membranes for air filtration, which would be dramatically influenced by the addition of PTFE NPs. Figure 1g presents the representative curves of the tensile stress−strain of PVDF fibrous membranes containing varieties of PTFE NPs. The tensile stress of PVDF fibrous membranes containing

(1)

where D0 represents the density of raw PVDF, and D1 signifies the density of fibrous membranes. 2.5. Measurements of Charge Properties. The surface potential of the PVDF/PTFE electret fibrous membranes was measured through the method of vibrating electrode with compensation using a noncontacting electrostatic probe (TREK-542A-2-CE). The time used for the measurement of the decay of the surface potential was 300 min, and detailed information about the determination of the test time can be seen in SI S5. Meanwhile, an enclosure with a constant temperature of 25 ± 2 °C and a humidity 45 ± 3%, respectively, was employed. The measurement of the thermally stimulated discharge current with the mode of open-circuit was implemented using an oven controlled temperature at a heating rate of approximately 3 °C min−1 from 25 to 200 °C with an imposed external electric field, an electrometer (Keithley), and a computer collecting data. A couple of Teflon films with thicknesses of 20 μm were employed as blocking electrodes and electrospun PVDF/PTFE electret fibrous membranes were sandwiched between them.

3. RESULTS AND DISCUSSION 3.1. Morphologies and Structure of PVDF/PTFE Membranes. Figure 1a displays the representative SEM image of the as-prepared pristine PVDF fibrous membranes, revealing that the fibers with an average diameter of 622 nm oriented randomly to constitute a reticular structure (Figure S6), which would serve as an effective channel for the traverse of air molecules and had the capability of mechanical filtration for particles based on the mechanism of physical intercept, diffusion, and inertial impaction. Interestingly, by inclusion of 0.01 wt % PTFE NPs, the average diameter of fibers reduced to 510 nm, which could be interpreted by the increased conductivity of the solution (Figure S7), eliciting greater Coulombic interactions and, as a consequence, larger stretching of the electrospinning liquid jet.26 However, further increasing the concentration of PTFE NPs from 0.05 to 0.1 wt % gave rise to the noticeable enlargement of the average diameter of fibers from 380 to 480 nm, in the meantime, the bead-on-string C

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efficiency, which may be the result of decreased surface potential (Figure S11) and increased pore size caused by the agglomeration of PTFE NPs. Charge elimination using the method of isopropyl alcohol (IPA) soaking was employed to ascertain the ratio of mechanical filtration and electrostatic adsorption in the filtration efficiency. The filtration efficiency induced by the mechanical filtration mechanism gradually increased, achieving a maximum value of 47.894% when the concentration of PTFE NPs reached 0.05 wt % (Figure S12). This phenomenon could be ascribed to the decreased pore size caused by the reduced fiber diameter. However, decreased mechanical filtration efficiency could clearly be seen with further increase of PTFE NP concentration to 0.1 wt % (Figure S12). This could be contributed by the formation of bead-on-string structure and increased pore size. Appropriate pore size and porosity of PVDF/PTFE fibrous membranes would play a very important role in the formation of an interconnected passageway for the traverse of air molecules. As presented in Figure 2a, the pressure drops of the relevant membranes were 16, 17, 18, and 15 Pa, respectively, revealing that the larger distances of adjacent fibers and the fluffy structure of the membranes would be beneficial to the reduction of air resistance.31,32 In addition, the discriminate criterion of quality factor (QF) is extensively adopted to synthetically appraise the filtration performance of the prescribed obtained membranes, which could be defined as QF = −ln (1 − η)/Δp, where Δp and η represent pressure drop and filtration efficiency, respectively. Profiting from the remarkable force of electrostatic adsorption, PVDF/PTFE-5 membranes showed a quality factor of 0.1359 Pa−1, which was higher than that of other samples, further confirming the contribution of electrostatic adsorption force in improving the filtration efficiency without increasing the pressure drop for electret membranes. 3.3. Long-Term Stability of Filtration Efficiency and Charge Storage. One of the considerably great electret properties for fibrous membranes is the attenuation performance of filtration efficiency and charges, which directly presents the escape behavior of the charge. As illustrated in Figure 3a, the filtration efficiency of PVDF fibrous membranes containing various PTFE NPs exhibited a decreasing trend. In the initial 25 min, the decrements of the filtration efficiency were 6.4, 6.0, 3.2, and 5.6%, respectively (Table S1), illustrating that PVDF/ PTFE-5 membranes possessed superior stability of filtration efficiency. Furthermore, a relatively long decay time could be seen in the region of 25−240 min. The total attenuation in this region for PVDF/PTFE-0, PVDF/PTFE-1, PVDF/PTFE-5, and PVDF/PTFE-10 were 14.5, 8.2, 7.6, and 10.1%, respectively (Table S1), whereas the filtration efficiency of all the samples achieved a stabilized state in the region of 240−300 min. More importantly, good regeneration of the filtration efficiency for PVDF/PTFE-0, PVDF/PTFE-1, PVDF/PTFE-5, and PVDF/PTFE-10 could be achieved through high voltage treatment (Figure S13). To further provide insight into the mechanism of filtration efficiency decay, we performed the evaluation of charge decay using PVDF/PTFE fibrous membranes for 300 min via the measurement of surface potential decay. As demonstrated in Figure 3b, the overall trend of charge decay versus time was similar to the attenuation of filtration efficiency, indicating that the reduction of the filtration efficiency may be predominantly caused by the dissipation of charges. The greatest decay of

PTFE NP concentrations of 0, 0.01, and 0.05 wt % was 1.09, 1.21, and 1.55 MPa, respectively. The incremental tensile stress could be attributed to the reinforcement effect of nanoparticles.27 However, the tensile stress decreased to 1.31 MPa with further increasing PTFE NP concentration to 0.1 wt %, which may be the consequence of the formation of bead-onstring structure caused by the agglomeration of PTFE NPs. In addition, the detailed stress−strain curves of each composition with a sufficient number of measurements are presented in SI S9. Additionally, all fibrous membranes manifested identical nonlinear curve forms with diminutive elongation under small loadings of stress and large elongation with successive rising of stress until fracture, which may be caused by the reorientation of nonaligned fibers in the nonlinear region and the slipping of fibers in the large elongation region.29 Furthermore, the mechanism of the transformation of breaking elongation is presented in Figure S10. 3.2. Filtration Properties of Composite Electret Membranes. On the basis of the structural transition of fibrous membranes, we could reasonably speculate that the filtration characteristic may also be changed correspondingly, therefore the filtration properties of fibrous membranes were measured. As illustrated in Figure 2a, the filtration efficiency

Figure 2. Filtration performance of PVDF/PTFE fibrous membranes. (a) Filtration efficiency and pressure drop and (b) quality factor of PVDF/PTFE fibrous membranes.

presented an escalating tendency with increasing PTFE NP concentrations from 0 to 0.05 wt %, achieving a topmost value of 94.235% at 0.05 wt %. This could be attributed to the reinforced electrostatic force of nanofibers, which could charge the particles by induction and eventually adsorb the particles effectively.30 However, further increasing the concentration of PTFE NPs to 0.1 wt % conspicuously reduced the filtration D

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the deep energy level and stable dipole charges were left behind, which would maintain the stability of the space charge existence in the relevant membranes.35 More importantly, the regeneration property of surface potential for PVDF/PTFE-0, PVDF/PTFE-1, PVDF/PTFE-5, and PVDF/PTFE-10 also can be achieved through high voltage treatment (Figure S13). 3.4. Electret Mechanism of PVDF/PTFE Composite Membranes. During the process of electrospinning, a high positive voltage was applied to the polymer solution, meanwhile vast positive charges were generated and in situ injected into the whole system of the multicomponent solution, which would come into being as the volume charges and surface charges after the solidification of the nanofibers, as shown in Figure 4a,b.36 More importantly, benefiting from abundant fluorocarbon segments of PVDF and PTFE, abundant polarized dipoles were generated under the forceful effect of high voltage due to the strong electronegativity of fluorine atoms and, subsequently, the polarized dipoles were well established before the formation of nanofibers, as demonstrated in Figure 4c.33 In addition, as illustrated in Figure 4b, the in situ accumulation of charges would take place in the interfacial regions of PVDF and PTFE NPs due to the different electronic transport capabilities, resulting in the formation of interfacial polarization charges.37 For the sake of in-depth probing into the storage mechanism of charge, the thermally stimulated current was measured using a linear temperature program in the region of 25−200 °C. As illustrated in Figure 4e, the intense TSD peaks in the low temperature zone about 25−60 °C for PVDF/PTFE-0 fibrous membranes were the displacement current peak of the surface charges and the volume charges reserved in the shallow energy levels caused by the fast recapture effect of electrons, expounding the inferior charge stability of the sole PVDF. Furthermore, sharp current peaks in the region of 80−90 °C could clearly be seen. This phenomenon could be contributed by the depolarization of oriented dipoles caused by localized rotational fluctuations of the dipoles.38 The movement of dipoles may be due to the movement of ions, side chains, and dipolar relaxation.39,40 Interestingly, peaks of displacement current shifted slightly to higher temperature with the increase of PTFE NP concentrations, indicating more polarized dipole charges were generated due to the abundant fluorocarbon chains of PTFE NPs.41 More importantly, incisive peaks emerged in 165−185 °C due to the release of interfacial

Figure 3. (a) Filtration efficiency decay and (b) surface potential decay of PVDF fibrous membranes containing various PTFE NP concentrations.

charges appeared within the initial 25 min, and this noticeable decay of charge is mainly caused by the escape of charges existing in shallow traps and is easily subject to the effect of charge neutralization occurring through the recombination of opposite charge.33 Nevertheless, with the relatively long decay time of 25−240 min, the reduction of surface potential was mainly deduced to be from the segmental depolarization of orientated dipoles caused by molecular thermodynamic movement.34 Significantly, in the stable region, the space charges in

Figure 4. Schematic description of electric charge category existing in PVDF fibrous membranes containing various PTFE NP concentrations: (a) volume charges, (b) surface charges, (c) polarized dipole, and (d) interfacial polarization. (e) Thermally stimulated current spectra of PVDF fibrous membranes containing various PTFE NP concentrations. E

DOI: 10.1021/acsami.6b08262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Morphology, filtration properties, pore size distribution, and filtration efficiency decay of PVDF/PTFE electret fibrous membranes. SEM images of PVDF electret fibrous membranes fabricated at different voltage (a) 20, (b) 30, (c) 40, and (d) 50 kV, respectively. (e) Filtration efficiency, (f) pore size distribution, and (g) filtration efficiency decay of PVDF/PTFE fibrous membranes fabricated from polymers solution with various PTFE NP concentrations.

fabricated under various voltages. As illustrated in Figure 5e, it could clearly be seen that the filtration efficiency revealed a progressive rise and afterward achieved a metastable value when the voltage exceeded 40 kV. This observation was consistent with the fact that the fibrous membranes were endowed with enhancive initial surface potential through increasing voltage from 20 to 40 kV but appreciably reduced with further increase of the voltage to 50 kV (Figure S15). In addition, incremental mechanical filtration in the whole process also contributed to the change of filtration efficiency. As shown in Figure 5e, the filtration efficiency induced by the mechanical filtration mechanism gradually increased, achieving a maximum value of 63.019% when the voltage reached 50 kV, which could be elucidated by the transformation of the pore structure. As illustrated in Figure 5f, the average aperture size of the fibrous membranes fabricated under the voltages of 20, 30, 40, and 50 kV exhibited a gradually decreased mean pore size of 6.3, 4.8, 4.0, and 3.6 μm, respectively, which was beneficial to the promotion of the mechanical filtration efficiency but resulted in the incremental pressure drop (Figure S16). The implementation of high energy injection could effectively ameliorate the long-term attenuation property of filtration efficiencies and charges for PVDF/PTFE-5 membranes by aggrandizing the depth of energy level of charges. As illustrated in Figure 5g, the filtration efficiency of fibrous membranes fabricated under the voltage of 20 kV exhibited a maximum decrement of 17.1% in the whole testing process. This could be contributed by the dissipation of space charges existing in shallow traps caused by the lower energy implantation, which only required lower energy to overcome the potential barrier in the hopping process and eventually resulted in the reduction of surface potential (Table S2).44 Importantly, the decrement of filtration efficiency reduced progressively with the increase of voltage and achieved a minimum value of 4.8% for PVDF/PTFE-5 membranes fabricated under the voltage of 40 kV, which was consistent with the decay of the surface potential, as illustrated in Figure 6, revealing a deepened depth of energy level for charge storage.

polarization charges and volume charges, and an increscent peak area could be observed with increasing PTFE NP concentrations from 0 to 0.05 wt %, uncovering that more interfacial polarization charges were inspired by the increased interfacial area between PVDF and PTFE NPs. However, the peak area decreased with the increasing PTFE NPs to 0.1 wt %, attributed to reduced interfacial area inside fibers caused by the agglomeration of PTFE NPs.42 Moreover, current peaks shifted distinctly from 165 to 184 °C with increasing PTFE NPs, indicating that the energy level was effectively deepened due to the lower capabilities of electronic transport for PTFE NPs.37 On the basis of this, the best charge stability could be acquired when the content of PTFE NPs achieved 0.05 wt %, therefore the PVDF/PTFE-5 would be carried out in the following study. 3.5. Charge Stability Regulation through High Energy Injection. High energy injection strategy, which could effectively promote the depth of energy level, was adopted to further increase the quantity and reinforce the stability of charge under various electrospinning voltages of 20, 30, 40, and 50 kV using PVDF/PTFE-5 fibrous membranes. As demonstrated in Figure 5a, the fibrous membranes fabricated under 20 kV exhibited a mean fiber diameter of 556 nm, ascribed to the inadequately effective stretching of the jet flow caused by the low electric field force (Figure S14). However, with increasing voltage from 30 to 40 kV, the diameters of fibers ranging from 250 to 570 nm with more homogeneous distribution could be observed (Figure S14). This fascinating phenomenon could be related to the increased equilibrium of the electric field force and the surface tension force of the solution, which could result in improved stability of the jet flow. However, protrusions would be observed on the surface of fibers with increasing voltage to 50 kV, as shown in Figure 5d, ascribed to the increased instability of solution in the stretching stage of the jet flow caused by the higher voltage.43 Subsequently, considering that the filtration properties could be influenced tremendously by the transformation of fiber morphology, the initial and mechanical filtration efficiency was subjected to measurement using PVDF/PTFE-5 membranes F

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PTFE-5 fabricated under a voltage of 40 kV would be carried out in the following study. 3.6. Air Filtration Performance Evaluation. The ample electric charges and desirable charge stability of PVDF/PTFE-5 fibrous membranes fabricated under the voltage of 40 kV enabled us to further elaborately inquire into the air filtration performance. Figure 7a demonstrated that the filtration efficiencies of fibrous membranes with stepwise incremental gram weight were 88.958%, 97.113%, 99.166%, 99.923%, and 99.972%, meanwhile the corresponding pressure drops were 15, 27, 35, 45, and 57 Pa, respectively, uncovering the significant contribution of the electrostatic adsorption force on the promotion of filtration efficiency, further disclosing the inimitable characteristics of low air resistance for electret fibrous membranes. Additionally, the PVDF/PTFE-5 membranes could be facilely promoted to the specified value of high efficiency particulate air (HEPA) filters (>99.97%) by increasing gram weight of the material to 9 g m−2, while maintaining an ultralow pressure drop of 57 Pa, which was unfulfillable for the conventional filtration materials. The feature of airflow in the fibrous periphery is pivotal for filtration performance. On the basis of this principle, the filtration performance as a function of airflow velocity (1.67− 16.6 cm s−1) was systematically measured using PVDF/PTFE-5 membranes with a basis weight of 9 g m−2. As illustrated in Figure 7b, the filtration efficiency decreased laggardly with increasing airflow velocity, achieving the lowest value of 98.011%, attributed to the reduced retention time of particles in the fibrous membranes caused by enhancive airflow velocity and directly reducing the probability for particles to collide on

Figure 6. Surface potential decay of PVDF/PTFE fibrous membranes fabricated at various electrospinning voltages.

However, further increasing the voltage to 50 kV, both the decay of filtration efficiency and surface potential increased significantly, which could be the consequence of the formation of protrusions in structure on the fiber’s surface caused by the agglomeration of PTFE NPs resulting in the enhancive escape velocity of charge carriers.45 More importantly, the good regeneration property of filtration efficiency and surface potential for PVDF/PTFE-5 fibrous membranes fabricated under 20, 30, 40, and 50 kV could be achieved through high voltage treatment (Figure S17). On the basis of this, PVDF/

Figure 7. Filtration properties and long-term recycling performance of PVDF/PTFE-5 fibrous membranes fabricated under the voltage of 40 kV. (a) Filtration efficiency of PVDF/PTFE-5 membranes with various basis weight. (b) Filtration efficiency versus air flow of the PVDF/PTFE-5 (∼9 g m−2). (c) Long-term recycling performance. (d) Filtration performance of selected materials for air filtration under airflow velocity of 5.3 cm s−1. G

DOI: 10.1021/acsami.6b08262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces fibers through Brownian diffusion.46 However, larger reduction rates of filtration efficiency could be observed for commercial state-of-the-art materials due to larger pore size (Figure S18), resulting in attenuate function of the physical intercept and inertial impaction. More interestingly, the lower initial value but greater ascension rate of the pressure drop of the commercial samples was broadly identifiable (Figure S19).47 This phenomenon could be ascribed to reduced “slip effect” caused by a large Reynolds number under speedy airflow velocity.48 Furthermore, the long-term recycling performance of PVDF/ PTFE-5 membranes were investigated under a severe contamination level (PM2.5 index equals to 500) created by cigarettes (the details of the measurement are displayed in Figure S20). As shown in Figure 7c, the clean air delivery rate of PVDF/PTFE-5 membranes were 15 min without any change after 20 cycles (Figure S21), while the pressure drop increased only slightly (2 Pa) due to the excellent charge stability. In addition, both NaCl aerosol and real PM in air had little effect on the filtration efficiency of PVDF/PTFE-5 membranes (Figures S22 and S23 ). To compare with mainstream products such as glass fibers and electret melt-blown fibers, we plotted the filtration efficiency versus pressure drop under an airflow velocity of 5.3 cm s−1, as shown in Figure 7d. Despite the fact that commercial glass fibers can achieve comparatively high filtration efficiency profiting from the ultrafine diameter of the fibers (400−800 nm) and compacted stacking structure, it is inevitable to generate exceedingly larger pressure drop. Meanwhile, the characteristic of low air resistance is a typical feature for electret melt-blown microfiber materials with moderate filtration efficiency, however, comparatively lower mechanical filtration efficiency (∼30%) and piecemeal ineffective charge in the course of utilization would immensely deteriorate the filtration efficiency and give rise to potential safety hazards. Benefiting from the high mechanical filtration efficiency with its extraordinary stability of charge, the novel electret PVDF/PTFE-5 fibrous membranes were equipped with both a high filtration efficiency of 99.972% and a low pressure drop of 57 Pa, which is higher than that previously reported for electrospun fibrous materials, such as polyacrylonitrile/ fluorinated polyurethane,49 Polyamide-6,50 polylactic acid,51 and so on. In addition, the scale-up issue of the electrospinning process of the as-prepared air filters was discussed in the SI Discussion.

electret materials would open up an opportunity for the development of filters like ultralow resistance respirators, ultralow penetration air filters, and an air purifier which can be used in individual protection, clean room, and indoor air purification.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08262. The morphology and size distribution of PTFE NPs; the performance comparison of PVDF fibrous membranes with various size of PTFE particles; the size distribution of PTFE NPs in PVDF solution; experimental setup for property evaluation of filter media; the decay of surface potential within 660 min; the fiber diameter distribution of PVDF fibrous membranes with various concentration of PTFE NPs; the conductivity, viscosity, and surface tension of PVDF/PTFE polymer solutions; TEM images of PVDF/PTFE fibrous membranes; the thickness, stress−strain curves, elasticity modulus, initial surface potential, mechanical filtration efficiency, decrement of filtration efficiency and surface potential, and the regeneration properties of surface potential and filtration efficiency of PVDF fibrous membranes containing various concentrations of PTFE NPs; the fiber diameter distribution, initial surface potential, pressure drop, and decrement of filtration efficiency and surface potential of PVDF/PTFE-5 fibrous membranes fabricated under various voltages; the regeneration properties of surface potential, filtration efficiency of PVDF/PTFE-5 fibrous membranes fabricated under various voltage; the pore size distribution of commercial state-of-the-art materials; pressure drop versus airflow velocity of the PVDF/ PTFE-5 (∼9 g m−2) and commercial sample; the equipment of PM2.5 purification efficiency measurement; the time of removing PM2.5 from 500 μg cm−3 to 35 μg cm−3 using PVDF/PTFE-5 fibrous membrane and commercial sample, the long-term filtration efficiency for NaCl aerosol, and real PM particles using PVDF/PTFE5 fibrous membranes (∼9 g m−2) (PDF)

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4. CONCLUSIONS In summary, we have reported a synergistic assembly strategy for the fabrication of electret PVDF/PTFE nanofibrous membranes with high filtration efficiency and low pressure drop. The introduction of PTFE NPs and regulation of injection energy endowed the resultant membranes with improved surface potential and reinforced charges stability. Furthermore, the electret mechanism of electrospun PVDF/ PTFE composite membranes was proposed, meanwhile the deepened energy level of charges and the incremental dipole charges with the increase of PTFE NPs were confirmed by employing the methods of thermally stimulated discharge current and surface potential decay. Ultimately, the resultant electret PVDF/PTFE-5 fibrous membranes fabricated under the injection energy of 40 kV exhibited the largest surface potential of 3.63 kV, excellent charge stability, as well as high filtration efficiency of 99.972%, low pressure drop of 57 Pa, and long-term service performance. We envision that these novel

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.Y.). Author Contributions §

S.W. and X.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Key Technologies R&D Program of China (Nos. 2015BAE01B01 and 2015BAE01B02), the National Natural Science Foundation of China (Nos. 51273038, 51322304, and 51503030), the “DHU Distinguished Young Professor Program”, the Fundamental Research Funds for the Central Universities (Nos. 16D110115 and 16D310110), and the Shanghai Sailing Program (No. 15YF1400600). H

DOI: 10.1021/acsami.6b08262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.6b08262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX