Electret Polyvinylidene Fluoride Nanofibers Hybridized by Polytetrafluoroethylene Nanoparticles for High-Efficiency Air Filtration

Transcription

1 Supporting Information 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 , China. Nanofibers Research Center, Modern Textile Institute, Donghua University, Shanghai , China. * Corresponding author: Prof. Bin Ding ( binding@dhu.edu.cn) S-1

2 Figure S1. (a) SEM image and (b) size distribution of PTFE NPs. Performance Comparison of PVDF Fibrous Membranes with Various Size of PTFE particles. The size of PTFE particles could have great influence on the electret property. Therefore, the performance comparison of PVDF fibrous membranes containing PTFE particles with various average size of 0.2, 1.5, 3, and 7 μm was carried out. As illustrated in Figure S2a-d, bead-on-string structure was clearly identifiable for all the PVDF fibrous membranes with micron-sized PTFE NPs. In addition, a few of PTFE NPs were outside the nanofibers when the average size of PTFE particles reached 7 μm. Furthermore, as demonstrated in Figure S2e, the filtration efficiency exhibited a decreased tendency with the incremental size of PTFE particles. This phenomenon could be contributed by the increased fiber diameter and decreased initial surface potential (as shown in Figure S2f). More importantly, incremental decrement of surface potential with the increase of the size of PTFE particles could be seen in Figure S2g. This could attributed to the reduction of interfacial charges between PVDF and micron-sized PTFE particles. These results enunciated that the nanoscale PTFE particles was optimal for the fabrication of PVDF/PTFE fibrous membranes. S-2

3 Figure S2. SEM images of PVDF fibrous membranes with various average sizes of PTFE particles: (a) 0.2 μm, (b) 1.5 μm, (c) 3.0 μm, and (d) 7 μm. (e) Filtration properties, (f) initial surface potential, and (g) surface potential decay of PVDF fibrous membranes with various sizes of PTFE particles. Figure S3. The size distribution of PTFE NPs in PVDF solution with the optimal PTFE NPs concentration of 0.05 wt% at different test time: (a) 8 h and (b) 32 h after finishing the preparation of solution. S-3

4 Figure S4. Experimental setup for property evaluation of filter media. Determination of Test Time. The test time is significate for the decay of surface potential. As shown in Figure S5, the test time was 11 h and the decrement of surface potential was almost zero after 5h. To unambiguously analyze the rapid attenuation zone, the data within 5h was preserved in the manuscript. Figure S5. The decay of surface potential within 660 min S-4

5 Figure S6. Histogram showing the fiber diameter distribution of PVDF fibrous membranes containing various concentrations of PTFE NPs: (a) 0 wt%, (b) 0.01 wt%, (c) 0.05 wt%, and (d) 0.1 wt%. Figure S7. (a) Conductivity and (b) viscosity and surface tension of PVDF polymer solutions containing various concentrations of PTFE NPs. S-5

6 Figure S8. TEM images of PVDF fibrous membranes with various PTFE NPs concentration of (a) 0 wt%, (b) 0.01 wt%, (c) 0.05 wt%, and (d) 0.1 wt%. S-6

7 Figure S9. (a) The thickness of PVDF fibrous membranes with various concentration of PTFE NPs. The stress-strain curves of PVDF fibrous membranes with various PTFE NPs concentration of (b) 0 wt%, (c) 0.01 wt%, (d) 0.05 wt%, and (e) 0.1 wt%. (f) The average tensile strength of PVDF fibrous membranes with various concentration of PTFE NPs. The Mechanism of the Transformation of Breaking Elongation. The transformation of breaking elongation is related to the change of fiber diameter, elasticity modulus and porosity of fibrous membranes. For pristine PVDF fibrous membrane, the large breaking elongation could be attributed to the low elasticity modulus according to the Hooke s law (as shown in Figure S10). By inclusion of 0.01 wt% PTFE NPs, the elasticity modulus of PVDF fibrous membrane increased to 7.98 MPa, S-7

8 resulting in the decrease of the breaking elongation. In addition, the elasticity modulus of PVDF fibrous membrane increased slightly to 8.13 MPa with further increasing the concentration of PTFE NPs to 0.05 wt%. However, both the fiber diameter and porosity of PVDF/PTFE-5 fibrous membranes decreased, leading to more fibers in unit volume and dense structure of fibrous membranes. This transformation of structure benefited to the increase of linear elasticity due to the incremental slip length between fibers. Furthermore, the decreased breaking elongation of PVDF fibrous membrane with the PTFE NPs concentration of 0.1 wt% could attributed to the increased porosity and bead-on string structure, which also could lead to the decrease of tensile strength. Figure S10. The elasticity modulus of PVDF fibrous membranes with various concentration of PTFE NPs. S-8

9 Figure S11. Initial surface potential of PVDF fibrous membranes containing various concentrations of PTFE NPs. Figure S12. The initial and mechanical filtration efficiency of PVDF fibrous membranes with various concentration of PTFE NPs. S-9

10 Table S1. Decrement of filtration efficiency and surface potential of PVDF fibrous membranes containing various concentrations of PTFE NPs. The regeneration experiment of filtration efficiency and surface potential Method: The electrospinning equipment of DXES-3 (Shanghai Oriental Flying Nanotechnology Co., Ltd., China) was used to regenerate the surface potential of PVDF/PTFE fibrous membranes. Typically, six empty plastic syringes with metallic needles were clamped to the supporting frame which can move right and left continuously. The samples were putted onto the earthed metallic tumbling barrel, which rotated at a velocity of 50 rpm, and the distance of tip-to-collector was 15 cm. Afterwards, a high DC voltage of 30 kv was carried out in the pinpoints of the needles, giving rise to high electric field between the needles tips and metallic tumbling barrel. The processing time of each sample was 10 min and the relative humidity were 48 ± 4%. Finally, each sample was measured the surface potential and filtration efficiency. Discussion: The regeneration of filtration efficiency and surface potential are really critical issues for electret filtration material. We tested the surface potential and filtration efficiency of the PVDF fibrous membranes with various PTFE NPs, all of which have stored almost 60 days. The filtration S-10

11 efficiency and surface potential exhibited identical values with that of fibrous membranes stored 5h (as shown in Figure 3a-b and Figure S13a-b), indicating the superior stability of charges. After the treatment by high voltage, as illustrated in Figure S13a-b, the incremental filtration efficiency and surface potential of all the fibrous membranes could be seen clearly. The recovery ratio of filtration efficiency of corresponding fibrous membranes were 89, 88, 95, and 93%, respectively. The recovery ratio of surface potential of corresponding fibrous membranes were 86, 90, 89, and 90%, respectively. These phenomena indicated that both the filtration efficiency and surface potential could be recovered. Figure S13. (a) The regeneration properties of filtration efficiency, (b) the regeneration properties of surface potential, and (c) the recovery ratio of filtration efficiency and surface potential of PVDF fibrous membranes with various concentration of PTFE NPs. S-11

12 Figure S14. Histogram showing the fiber diameter distribution of PVDF/PTFE-5 fibrous membranes fabricated under various electrospinning voltages: (a) 20 kv, (b) 30 kv, (c) 40 kv, and (d) 50 kv. Figure S15. Initial surface potential of PVDF/PTFE-5 fibrous membranes fabricated under various electrospinning voltages. S-12

13 Figure S16. Pressure drop of PVDF/PTFE-5 fibrous membranes fabricated under various electrospinning voltages. Table S2. Decrement of filtration efficiency and surface potential of PVDF /PTFE-5 fibrous membranes fabricated under various voltages. S-13

14 Figure S17. (a) The regeneration properties of filtration efficiency, (b) the regeneration properties of surface potential, and (c) the recovery ratio of filtration efficiency and surface potential of PVDF/PTFE-5 fibrous membranes fabricated under various voltages. Figure S18. Pore size distribution of commercial state-of-the-art materials. S-14

15 Figure S19. Pressure drop versus airflow velocity of the PVDF/PTFE-5 (~ 9 g m -2 ) and commercial sample. PM 2.5 purification efficiency measurement. With regard to all PM2.5 purification efficiency measurement, simulative particular matter were generated from cigarette by burning method. The generated white smoke particles own a broad granulometric distribution from < 0.3 μm to > 10 μm, with a large proportion of particular matter being < 1 μm. The entrance, concentration of particular matter was adjusted by diluting the smoke particular matter by clean air to a severe pollution level in which the PM2.5 concentration is equivalent to or greater than 500 μg m -3. The number concentration of particular matter in the airtight test cabin was detected with and without filters by a PM2.5 professional concentration detection instrument (SDL 301, Nova Fitness) and the PM2.5 purification efficiency was obtained by calculating the concentration difference before and after filtration. The clean air delivery rate and long term recycling operational performance was measured in an airtight cabin, which contains a small air purification machine with the airflow of 14 L min -1 and two PM2.5 professional concentration detection instrument (SDL 301, Nova Fitness). The filter materials was placed in the air inlet and sealed with the double sides adhesive tape to guarantee the air passing into the air purification machine through filter materials completely. The clean air delivery S-15

16 rate was obtained by calculating the time of PM2.5 concentration decrease from 500 μg m -3 to 35 μg m -3. The long term recycling operational performance was evaluated by test 20 cycles of PM2.5 concentration decrease from 500 μg m -3 to 35 μg m -3. Figure S20. Equipment of purification efficiency measurement for PM2.5. Figure S21. Time of removing PM2.5 from 500 μg cm -3 to 35 μg cm -3 using PVDF/PTFE-5 fibrous membrane fabricated under 40 kv and commercial sample. S-16

17 Figure S22. The long-term filtration efficiency of the PVDF/PTFE-5 fabricated under 40 kv (~ 9 g m -2 ) for NaCl aerosol. Figure S23. (a) The long-term filtration efficiency of the PVDF/PTFE-5 fabricated under 40 kv (~ 9 g m -2 ) for real PM. (b) Experimental setup for filter media property evaluation using real PM. Discussion: The Scale-up Issue of Electrospinning Process of the As-prepared Air Filters. Definitely, it is valuable for the massive production of the as-prepared air filters. However, some technical issues need to be addressed. First, the PVDF/PTFE NPs solution should be in the state of stirring to avoid the agglomeration of PTFE NPs due to the continuous process of electrospinning and much longer electrospinning time in the mass production. Second, uniform temperature and humidity in the S-17

18 electrospinning region must be guaranteed to ensure the uniformity of product. Third, the covering layer should be added onto the nanofibrous layer to avoid the break of nanofibers due to the inferior mechanical property. In addition, the testing equipment of air permeability and thickness of fibrous membranes should be installed to ensure the stability of quality. S-18