Electrostatically Active Polymer Hybrid Aerogels for Airborne

Feb 8, 2017 - The filtration efficiency is determined by passing airborne NaCl nanoparticles with diameter in the range 25–150 nm through the filter...
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Electrostatically Active Polymer Hybrid Aerogels for Airborne Nanoparticle Filtration Sung Jun Kim,† Prasad Raut,† Sadhan C. Jana,*,† and George Chase‡ †

Department of Polymer Engineering and ‡Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio 44325-0301, United States ABSTRACT: The role of electrostatic force on separation of airborne nanoparticles is evaluated in this work by considering a hybrid monolithic aerogel of syndiotactic polystyrene (sPS) and polyvinylidene fluoride (PVDF). The sPS part accounts for open pore structures in the monolith, while the PVDF chains contribute spontaneous polarity for particle capture by the electrostatic force. The hybrid aerogels are fabricated by thermoreversible gelation of sPS from a solution with PVDF in tetrahydrofuran followed by supercritical drying of the gel. sPS is present as the δ-form clathrate crystalline phase and PVDF as α- and γ-form crystalline phases in the hybrid. The presence of PVDF induces significant static charges on the surfaces of hybrid aerogels. The filtration efficiency is determined by passing airborne NaCl nanoparticles with diameter in the range 25−150 nm through the filter media. The experimental data reveal that air permeability of the hybrid system (∼10−10 m2) is close to that of sPS monoliths. The hybrid materials show filtration efficiency ≥99.999% in comparison to 98.889% observed for a sPS monolith with the same solid content. KEYWORDS: airborne nanoparticles, aerogels, air permeability, sPS, PVDF, electrostatic force

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surface area up to 1000 m2/g and 90% porosity with significant mesopore fraction of diameter 2−50 nm.11−23 The δ-form syndiotactic polystyrene (sPS) aerogel provides porosity up to 97% with a significant fraction of pores as macropores (diameter >50 nm).24−30 Despite strong promise, airborne nanoparticle filtration using aerogels has not been studied much. Only a few studies used aerogel granules or microspheres arranged in packed beds for the purpose of removal of particles.31−34 Several studies focused on the use of bioluminescent organisms contained in silica aerogels for viral particle detection35 and monolithic silica aerogel composites for removal of pollutants.1,19,36,37 In a recent work, macroporous monolithic sPS aerogels were used for high efficiency (>99.95%) removal of airborne nanoparticles of size 25−150 nm (mean size 75 nm) with air permeability of the order of 10−11− 10−10 m2.8 In another study, we evaluated the function of mesoporous silica particle networks grown inside the macropores of sPS in an organic−inorganic hybrid aerogel system on airborne nanoparticle filtration.7 It was found that the mesoporous silica particle networks increased the particle capture efficiency significantly without affecting much the values of air permeability. The filtration of airborne individual particles is governed by one or more of the five recognized mechanisms.38−43 Direct interception is the primary contributor of filtration of large particles by direct contact of the particles with the filter medium.

INTRODUCTION A number of health hazards such as nausea, birth defects, bronchitis, and weakened immune systems are tied to exposure to air pollution.1,2 According to a recent report published by the World Health Organization (WHO), the death of approximately 7 million people in 2012 is attributed to exposure to air pollution.3 The buoyant particulate matters in air generated from the natural activities or anthropogenic emissions are responsible for air pollution.4 The small buoyant particles can easily reach the pulmonary alveoli in the human body. Accordingly, small airborne particles are classified as fine and ultrafine particles with diameter of, respectively, 0.1−1 μm and 99.999

0.037 ± 0.001 1.349 ± 0.010 1.418 1.15 97.3 99.6 2.5 ± 0.7 37.3 1.40 ± 0.16 >99.999

The data were previously reported in ref 8. bPressure drop per unit thickness ΔP/L was calculated from the permeability data and air flow rate. G

DOI: 10.1021/acsami.6b14784 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

efficiency of 99%, 99.9%, and 99.99% are all in different categories as per EN 1822−1:2009 classification, and achieving high efficiency (>99.95%) is considered important. The increased filtration efficiency with an increase of static surface charge indicates that the electrostatic force contributed significantly to nanometric particle filtration.

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CONCLUSIONS Monolithic sPS and sPS/PVDF hybrid aerogels were prepared by thermoreversible gelation and supercritical drying in carbon dioxide. The δ-form sPS clathrate crystalline phase, and α- and γ- crystalline PVDF phases coexisted in the semicrystalline polymer strand networks of the hybrid aerogels. sPS/PVDF hybrid aerogels were electrostatically active from spontaneous polarity of the γ-crystalline phase of PVDF. The sPS/PVDF hybrid aerogels quickly responded to a piece of charged glass, but sPS aerogels did not. Also, measured static surface charges of sPS/PVDF hybrid aerogels were significantly higher than that of the sPS aerogel. The addition of PVDF did not alter air permeability of sPS/PVDF hybrid aerogels from its value of 10−10 m2 due to higher porosity of ∼97%. The filtration efficiency of nanometric NaCl particles increased significantly and reached a maximum value ≥99.999% in the case of sPS−PVDF hybrid aerogels with the PVDF:sPS mass ratios of 0.5 and higher.

Figure 10. Permeability as a function of macropore fraction of the total pore volume of an aerogel monolith.

and ΦM. The permeability data in Figure 8 show good correlation with the macropore fraction with R2 value of ∼0.823. These data indicate that most of the air flow was handled by the macropores in aerogels. The hybrid aerogels were electrostatically active as the surface charges were significantly developed. The surface charge of the sPS_PVDF-2 specimen was −1.0 kV, while that of the sPS aerogel was −0.02 kV (Table 2). A PVDF solid sheet with a thickness of 0.5 mm prepared by hot pressing showed much higher surface charge of −7 kV. It is evident that the values of the surface charge of the hybrid aerogels are much smaller than that of the solid PVDF sheet. We note here that the solid volume in the hybrid aerogels was less than 3%, while it was 100% for a sheet of PVDF. Accordingly, the net surface charge measured on the surfaces of the aerogels should be lower than that of solid PVDF. Figure 11 shows filtration efficiency and static surface charge of hybrid aerogels as a function of PVDF to sPS mass ratio. It is

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sadhan C. Jana: 0000-0001-8962-380X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS SCJ and SJK acknowledge financial assistance for this work from National Science Foundation in the form of grant number CMMI-1200484. The authors would like to thank Bojie Wang and Zhorro Nikolov of The University of Akron for assistance in obtaining EDX and XRD data.

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REFERENCES

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Figure 11. Filtration efficiency of nanometric NaCl particles at Vf ≈ 50 cm/s and static surface charges.

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DOI: 10.1021/acsami.6b14784 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX