Tuning Porous Networks in Polyimide Aerogels for Airborne

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TUNING POROUS NETWORKS IN POLYIMIDE AEROGELS FOR AIRBORNE NANOPARTICLE FILTRATION Chunhao Zhai, and Sadhan C Jana ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09345 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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ACS Applied Materials & Interfaces

TUNING POROUS NETWORKS IN POLYIMIDE AEROGELS FOR AIRBORNE NANOPARTICLE FILTRATION

Chunhao Zhai and Sadhan C. Jana*

Department of Polymer Engineering, University of Akron, Akron, OH 44325 *Corresponding author at [email protected]

Keywords: Aerogel; Nanoparticle filtration; Polyimide; Porosity; Air permeability; Mesoporous materials;

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Abstract The suitability of monolithic polyimide aerogels as filter media for removal of airborne nanoparticles was investigated in this work by considering two solvents, Nmethyl pyrrolidone (NMP) and dimethylformamide (DMF) for tuning of meso- and macropore content. Polyimide gels were synthesized from the chemical reactions between solutions of pyromellitic dianhydride, 2,2'-dimethylbenzidine, and 1, 3, 5triaminophenoxylbenzene. The gels were dried via supercritical drying in CO2 to obtain the aerogels. The porosity of polyimide aerogels was varied by changing the initial concentration of the solids in the solutions in the range of 2.5-10 wt.%. The resulting aerogels show high porosity (91%-98%), high specific surface area (473 m2/g-953 m2/g), low bulk density (0.025 g/cm3-0.12 g/cm3) and the solvent dependent macro- and mesopore content. The monoliths with bulk density >0.05 g/cm3 produced high values of nanoparticle filtration efficiency (>99.95%) with air permeability of the order of 10-10 m2. A strong proportional relationship was observed between the macropore content and air permeability and between the mesopore content and high filtration efficiency. Specimens prepared in DMF and NMP offered the same level of filtration efficiency, but the former provided a factor of 2 higher air permeability due to much greater proportion of macropores.


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1. Introduction It is widely known that inhaling airborne particles can cause respiratory diseases like cough, breathing difficulties, chronic bronchitis, and even cancer in human being.1,2 The hazard level of airborne particles shows an inverse relationship to their size. For example, particles with diameter of 5.5-9.2 µm are known to lodge in human nose and throat leading to breathing difficulties, while particles smaller than 5.5 µm can get into the breathing passages to cause more severe diseases. The particles of diameter less than 1 µm are most hazardous as they stay in the air sacs and significantly increase the risk of lung cancer.3 PM 2.5, defined as particulate matter with diameter less than 2.5 µm, is used as the air quality index (AQI) to characterize the concentration of inhalable particles in air.4 According to a report, for cities in developing countries, like Beijing, the PM 2.5 index reached a new high in the year 2015.5 Dealing with airborne particles has become an urgent task for governments all over the world. An important way to combat the hazards of nanoparticles before they enter human body is filtration. Commercial masks and high efficiency particulate absorption (HEPA) filters made of fiber mats show filtration efficiency as high as 99.95% for removal of PM 2.5. However, it is difficult to find filter products that consistently remove particles with diameter less than 300 nm. Note that airborne particles of size less than 300 nm are considered the most dangerous particles for human health. The use of porous materials as filter media has been widely investigated. Porous materials offer high pore volume and large internal surface area.6 In this context, aerogels are a unique class of materials with high porosity (>90 %), extremely low density (99.95 %) was obtained when the bulk density of sPS aerogels was at 0.034 g/cm3. Correspondingly, air permeability of sPS aerogels were found to be in the range of 5.299.74×10-10 m2. Kim et al.20 later evaluated the role of electrostatic charge by incorporating polyvinylidenefluoride (PVDF) in sPS aerogel. They reported that polarizability of PVDF produced substantial increases in filtration efficiency and negligible changes in air permeability compared to filtration media of only sPS aerogels. Kim and Jana 21 used air permeability data to estimate the thickness of higher density skin layers in sPS aerogels. Such higher bulk density skin layers can potentially contribute to an increase of particle filtration efficiency. Polyimide (PI) aerogels, first reported in 2004

22

, are a class of organic aerogels

with outstanding thermal stability and are fabricated by chemically reacting aromatic diamines with dianhydrides. In recent years, a variety of dianhydrides and diamines were used in synthesis of PI aerogels such as 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), 2,2'-dimethylbenzidine (DMBZ), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) pyromellitic dianhydride (PMDA), 2,2'-dimethylbenzidine (DMBZ), and 4,4'-oxydianiline (ODA)23-25. Different crosslinkers were also investigated.23-25


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Kawagishi et al. 26 obtained porous polyimide by supercritical drying using 1, 3, 5-tris (4aminophenyl) benzene (TPAB) as the crosslinker. Meador and co-workers

27

studied

mechanical properties of PI aerogels synthesized from select dianhydrides and diamines, crosslinked

with

1,3,5-tris(4-aminophenoxy)benzene

(TAB).

These

researchers

discovered that aerogels containing 2,2'-bis(phenoxyphenyl) propane dianhydride (PPDA) experienced shrinkage which in turn enhanced their moduli. Stronger PI aerogels find applications in aerospace technologies28,29 such as in insulation of aircraft to facilitate entry, descend, and landing processes and as acoustic absorbers.30 This study examined the potential of monolithic PI aerogels as filter media for removal of airborne nanoparticles of size 25-150 nm. PI aerogels were synthesized from a representative formulation involving pyromellitic dianhydride, 2,2'-dimethylbenzidine, and the TAB crosslinking agent. The route of dianhydride and diamine was chosen due to the high reactivity of these monomers at room temperature. N-methylpyrrolidone and dimethyl formamide were used as the solvents. Monolithic PI aerogels were obtained via drying of the gels in supercritical CO2. The resultant PI aerogel specimens showed low bulk density and large surface area. Filtration efficiency and air permeability are two key factors in quantifying the performance of filter media. Their functional dependence on the fractions of inherently generated micropores (dia < 2 nm), mesopores (dia 2-50 nm), and macropores (dia>50 nm) was studied. 2. Experimental Section Materials Pyromellitic dianhydride (PMDA) was purchased from Alfa-Aesar and 2,2ʹdimethylbenzidine (DMBZ) was purchased from Shanghai Worldyang Chemical Co., Ltd 5 ACS Paragon Plus Environment


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(Shanghai, China). The chemical crosslinker 1,3,5-tris(4-aminophenoxy)benzene (TAB) was provided by Triton Systems (Chelmsford, MA) as a dull brown powder. Anhydrous N-methylpyrrolidone (NMP) and anhydrous dimethylformamide (DMF) were obtained from Sigma-Aldrich. Pyridine, acetic anhydride, and acetone were purchased from Fisher Scientific. All reagents and solvents were used as received.

Preparation of Polyimide Aerogel Monoliths In this study, the amount of solid was accurately calculated according to the weight concentrations of polyimide products in the final solutions. Stoichiometric ratio of PMDA and DMBZ was set as 61:60 by mole to ensure that the length of repeat unit n was 60. The solutions with solid weight of 2.5%, 5%, 7.5% and 10% were prepared. Two different solvents, NMP and DMF, were used for comparison of aerogel materials properties, specifically the makeup of macro- and mesopores in the final aerogel materials. All reactions occurred at room temperature. The scheme of reactions in synthesis of PI chains is presented in Figure 1.

PMDA and DMBZ were separately added in the chosen solvent and magnetically stirred at 300 rpm until clear solutions were obtained. The process took 5-30 minutes depending on the amount of monomers. The dianhydride and diamine solutions were added together and mixed to obtain a translucent, homogenous mixture of poly(amic acid) oligomers. A solution of TAB was mixed with poly(amic acid) to form the poly(amic acid) networks to which acetic anhydride and pyridine were added respectively as the dehydrating agent and the reaction catalyst. Polyimide solution was poured into disk shaped molds of diameter of 32 mm and thickness 2.5-5 mm to cast the gel. The gelation 6 ACS Paragon Plus Environment

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Figure 1 Reaction mechanisms of preparation of polyimide. times were dependent on the concentration of the solid. For example, a solution with 2.5 wt.% solids needed more than three hours to gel while a 10 wt.% solution only needed 10 minutes to gel. All gels were kept in the mold for 24 hours for aging purposes. Afterwards, the solvent (DMF or NMP) was exchanged six times with acetone. The first three times, mixed solvents consisting of original solvent (NMP or DMF) and acetone in the volume ratio of 75/25, 50/50 and 25/75 were used. Afterwards, the wet gels were 7 ACS Paragon Plus Environment


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exchanged with 100% acetone. Each time, the exchange process took approximately eight hours. Finally, the acetone-filled gels were placed inside the chamber of the supercritical dryer for solvent exchange with liquid CO2 and subsequently supercritical drying at 50 ℃ and 7.3 MPa to obtain aerogels. Characterization The bulk density (ρb) of monolithic PI aerogel specimens were directly obtained from the mass and the volume of specimens, as in equation (1). The skeletal density (ρs) was measured by Accupyc 1340 helium pycnometer (Micromeritics Instrument Corp.). Porosity (Π) was obtained from the values of ρb and skeletal density (ρs) using equation (2). The diameter shrinkage (δd) of the aerogel specimen was calculated from the diameter of the gels and the dried aerogels using equation (3). The total specific pore volume (VT) was determined from equation (4).

=

(1)

Π = 100 × 1 −

(2)

= 1 −

(3)

= −

(4) In equations (1) and (3), m, h, d, and d0 refer respectively to mass of aerogel specimen, height of aerogel specimen, diameter of aerogel specimen, and diameter of the gel. Air permeability of aerogel specimens was obtained from modified Darcy’s law, shown in equation (5).


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=

!

"

(5) In equation (5), Q, k, G, A, ΔP, µ, and L correspond respectively to volumetric air flow rate, permeability constant, shape factor, cross-sectional area of the face normal to air flow direction, pressure drop across the specimen, viscosity of air, and the thickness of aerogel specimen. Frazier tester (Frazier Precision Instrument, Hagerstown, MD) was used here to obtain the values of air permeability constant k. Figure 2 illustrates the schematic of the set up for Frazier test. Polyimide aerogel specimens were placed at the center of the base plate with a hole of diameter 5 mm. The sample holder had a central hole of 30 mm diameter for air flow. Vacuum grease was used to eliminate the gap between the sample holder, the base plate, and the aerogel specimen. Afterwards, the specimen was placed on the Frazier permeability tester equipped with a pressure gauge and a vacuum pump. Different values of pressure drop ΔP and volumetric flow rate Q were recorded by controlling the power of the vacuum pump. The values of permeability constant k were calculated from Q vs.ΔP data with the help of equation (5). Since the

Figure 2 Schematic representation of the setup for air permeability measurement. 9 ACS Paragon Plus Environment


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size of the central hole on the sample holder and the base plate were different for some tests, the shape factor G (with values 1.5-3.0 in this work) was used to accommodate such differences.31 Filtration efficiency was measured using TSI-8130 filter tester (TSI Inc., Shoreview, MN). The instrument was able to generate sodium chloride nanoparticles in flowing air with size range 25-150 nm and average diameter 75 nm.31 Figure 3 illustrates the schematic for filtration efficiency test. The aerogel specimen was placed in the sample holder with 30 mm diameter on a wide-mesh metal net placed at the bottom of the holder to support the specimen. Two O-rings were placed tightly along the edge of the aerogel samples to avoid nanoparticles going through the gap shown in Figure 3. The filtration efficiency (E, equation 6) was obtained from the fraction of nanoparticles not Air Flow with Nanoparticles

O-ring

Aerogel Specimen

Sample Holder

Wide-mesh metal net

Figure 3 Schematic of specimen set up in TSI-8130 tester. captured by the aerogel monoliths. The maximum of filtration efficiency that can be measured by the instrument is 99.999%. Air flow rate in this test was controlled at 20 L/min. E=

$% &$'

(6)

%$10 ACS Paragon Plus Environment

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In equation (6), NA, NB respectively corresponds to the number of incident particles and output particles.

Micromeritics Tristar II Surface Area Analyzer was used to characterize the specific pore volume of aerogel specimens. Aerogel specimens were weighed and cut into small pieces with diameter about 5 mm to fit the sample tubes. The amount of sample needed was usually no more than 0.05 g. The specimens were subjected to 12hour degassing at room temperature for the purpose of removing the impurities. Nitrogen adsorption was carried out at 77 K to obtain the specific volume of the mesopores (Vms, dia 2-50 nm) and carbon dioxide adsorption was conducted at 273 K to obtain the specific volume of the micropores (Vmi, dia 50 nm) was obtained using equation (8) from total pore volume, VT. The fractions of micropores (φmi), mesopores (φms), and macropores(φma) in aerogel monoliths were determined by multiplying the overall porosity (Π) with the volume fraction of each pore category as is given in equation (9). The sum of φmi, φms, and φma equals to the value of porosity, Π reported in Table 1. ( = − ) − *

(8)

φmi=(ΠVmi)/VT; φms=(ΠVms)/VT; φma=(ΠVma)/VT

(9)

Morphology of polyimide aerogel specimen were observed via scanning electron microscope (JEOL JSM5310) at 8 kV. All specimens were sputter-coated with silver by



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ISI-5400 Sputter Coater. The images were taken of the internal network structures and the upper surfaces of the specimens collected after filtration tests.

3. Results and Discussion The values of bulk density, skeletal density, diameter shrinkage, porosity, filtration efficiency, and permeability are listed in Table 1. The average values listed in the table were calculated from at least three measurements. It is seen that the bulk density increased from 0.025 g/cm3 to 0.121 g/cm3 with an increase of the concentration of solids in the initial solution. The skeletal densities, however, did not change much (0.075 g/cm3) were found to be greater than 99.95% and showed strong dependence on the mesopore content. Due to outstanding thermal stability of polyimide aerogels, these monoliths show good potential for filtration of high temperature air. In this case, the cost of cooling of the hot exhaust gas can be eliminated if PI aerogels are used as filtration media.

Acknowledgment. The authors gratefully acknowledge Professor George Chase in Chemical and Biomolecular Engineering Department of University of Akron for allowing the use of TSI filter tester and Frazier tester. 5. References 1. Raaschou-Nielsen, O.; Andersen, Z. J.; Beelen, R.; Samoli, E.; Stafoggia, M.; Weinmayr, G.; Hoffmann, B.; Fischer, P.; Nieuwenhuijsen, M. J.; Brunekreef, B.; Xun, W. W. Air Pollution and Lung Cancer Incidence in 17 European Cohorts: Prospective Analyses from the European Study of Cohorts for Air Pollution Effects (ESCAPE). Lancet oncol 2013, 14, 813-822. 2. Marilena, K.; Castanas, E. Human Health Effects of Air Pollution. Environ. Pollut. 2008, 151, 362-367. 3. Mahowald, N. Aerosol Indirect Effect on Biogeochemical Cycles and Climate. Science 2011, 334, 794-796. 4. Querol, X.; Alastuey, A.; Ruiz, C.R.; Artiñano, B.; Hansson, H.C.; Harrison, R.M.; Buringh, E.T.; Ten Brink, H.M.; Lutz, M.; Bruckmann, P.; Straehl, P. Speciation and


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Graphical abstract

4.0

-10

m2

4.5

Permeability Constant *10

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3.5 3.0 2.5 2.0 1.5 1.0 0

20

40

60

80

100

φma(%)


Tuning Porous Networks in Polyimide Aerogels for Airborne

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