Tailoring Supramolecular Nanofibers for Air Filtration Applications

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Tailoring Supramolecular Nanofibers for Air Filtration Applications Daniel Weiss, Dominik Skrybeck, Holger Misslitz, David Nardini, Alexander Kern, Klaus Kreger, and Hans-Werner Schmidt ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04720 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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ACS Applied Materials & Interfaces

Tailoring Supramolecular Nanofibers for Air Filtration Applications Daniel Weiss, Dominik Skrybeck, Holger Misslitz, David Nardini, Alexander Kern, Klaus Kreger and Hans-Werner Schmidt*

Macromolecular Chemistry I, Bayreuth Institute of Macromolecular Research, and Bayreuth Center for Colloids and Interfaces, University of Bayreuth, 95440 Bayreuth, Germany

Keywords: supramolecular materials, nanofibers, microfiber-nanofiber composites, particulate matter, air filtration

Abstract

The demand of new materials and processes for nanofiber fabrication to enhance the performance of air filters is steadily increasing. Typical approaches to obtain nanofibers are based on topdown processes such as melt blowing, centrifugal spinning, and electrospinning of polymer materials. However, fabrication of polymer nanofibers is limited either with respect to a sufficiently high throughput or to the achievable smallest fiber diameter. This study reports 1 ACS Paragon Plus Environment


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comprehensively on a fast and simple bottom-up process to prepare supramolecular nanofibers in situ inside viscose/polyester microfiber nonwovens. Here, selected small molecules of the materials class of 1,3,5-benzenetrisamides are employed. The microfiber-nanofiber composites exhibit a homogeneous nanofiber distribution and morphology throughout the entire nonwoven scaffold. Small changes in molecular structure and processing solvent have a strong influence on the final nanofiber diameter and diameter distribution and, consequently, on the filtration performance. Choosing proper processing conditions, microfiber-nanofiber composites with surprisingly high filtration efficiencies of particulate matter are obtained. In addition, the microfiber-nanofiber composite integrity at elevated temperatures was determined and revealed that the morphology of supramolecular nanofibers is maintained compared to the utilized polymer nonwoven.

Introduction Exposure to airborne particulate matter can lead to adverse effects to human health causing, e.g., lung cancer and cardiovascular and respiratory diseases.1,2 Improvements in air quality can be achieved either by reducing the emission of particulate matter or by filtration.3 In particular, nanofibrous materials are a topic of high interest in the air field filtration applications. Nanofibers are highly beneficial, because of a greatly increased probability of particle deposition originating from the larger surface to volume ratio compared to microfibers.4–8 Commonly, such micro- and nanofibers are based on polymer materials. Typically, fabrication of polymer nanofibers can be achieved by top-down processes from melt or solution through nozzles such as melt blowing, centrifugal spinning, and electrospinning.4,9–17 The demand for nanofibrous filter media is steadily increasing and the preparation of such polymer nanofibers is established in academia, however, the transfer of spinning technologies to industrial scale cannot be easily accomplished. 2 ACS Paragon Plus Environment

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From a technical point of view, with decreasing fiber diameter the rate of production is significantly reduced. Despite these issues in polymer nanofiber production, steadily growing improvements in the productivity were made during the last decade for example by the development of multiple nozzle setups and spinnerets.18 In contrast to the nanofiber preparation with polymers, here we utilize small molecules and a self-assembly process uniquely allowing the in situ formation of nanofibers inside nonwoven scaffolds. Supramolecular chemistry provides the opportunity to build fibrous nanostructures via secondary interactions with properties and functions beyond the individual molecular building blocks. For instance, selfassembly of small molecules into supramolecular nanofibers in organic solvents or water has been utilized to prepare thermoreversible functional organo-

19–22

or hydrogels.23–28 One of the

most prominent and simplest supramolecular motif capable of forming nanofibers are 1,3,5benzenetrisamides (BTA).21,29–31 Mostly, BTAs form columnar stacks during self-assembly processes driven by three directed self-complementary hydrogen bonds.32–34 Due to the complexity and hierarchical structured morphologies of this materials class, BTAs can be used in various applications, e.g., as additives for the nucleation35–44 and clarification35 of thermoplastic semi-crystalline polymers or to improve the electret performance of polypropylene.45–47 These applications require the presence of solid supramolecular nanostructures formed by the additives in the polymer melt. Investigations on individual supramolecular nanofibers prepared from solution by atomic force microscopy (AFM) bending experiments yielded Youngs moduli in the range of 2-3 GPa.48,49 Recently, we have reported for the first time that introducing supramolecular nanofibers in a nonwoven scaffold results in a stable microfiber-nanofiber composite, which can potentially be used in air filtration applications.50 For example, the specific surface area of the microfiber nonwoven scaffold is about 0.12 m2/g and increases in the microfiber-nanofiber composites to about 0.47 m2/g with introduction of 7 wt.-% of 3 ACS Paragon Plus Environment


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supramolecular nanofibers. The underlying in situ preparation process is schematically depicted in Figure 1 and illustrated with photographic images in Figure 2. A homogenous immersion solution of the BTA at a given concentration is prepared at elevated temperatures and the nonwoven is immersed into the clear solution until the microfiber scaffold is fully soaked. Upper limitations are BTA concentrations at which the molecules cannot be fully dissolved and the maximum temperature defined by the boiling point of the solvent. Upon removing the nonwoven, the remaining solvent within the nonwoven starts to cool and evaporate. At a certain concentration and temperature, the BTAs self-assemble into supramolecular nanofibers within the entire nonwoven scaffold. Thus, the in situ formation of supramolecular nanofibers inside the nonwoven support is different to the fabrication of, e.g., electrospun polymer nanofibers, which are typically deposited on top of a support.51

Figure 1. Schematic representation of the in situ preparation process of supramolecular microfiber-nanofiber composites (nonwoven scaffold: grey fibers; solvent: blue dots; dissolved 1,3,5-benzenetrisamide: red dots; supramolecular nanofibers: red fibers). Nonwoven microfiber scaffold containing a 1,3,5-benzenetrisamide solution at elevated temperatures. (A) Upon cooling and simultaneous solvent evaporation, self-assembly occurs and supramolecular nanofibers are 4 ACS Paragon Plus Environment

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formed within the nonwoven microfiber scaffold (B). Complete solvent evaporation yields the microfiber-nanofiber composite (C).

Figure 2. Photographic images showing the process and equipment for the preparation of microfiber-nanofiber composites. A 9.5 cm x 9.5 cm nonwoven is placed in a metal frame and fixed (a). Subsequently, the metal frame is dipped into an immersion bath containing the BTA solution at elevated temperatures (b). The nonwoven scaffold remains for a period of time of 30 s in the BTA solution ensuring complete solution uptake (c). After removal from the solution, the samples are cooled and simultaneously dried to yield microfiber-nanofiber composites (d). Here we report on a comprehensive investigation of microfiber-nanofiber composites providing a deeper understanding of the preparation process and the resulting properties. For that, we have chosen two selected BTAs based on trimesic acid, which differ in the peripheral side groups, i.e., 5 ACS Paragon Plus Environment


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branched alkyl chains of different lengths. We demonstrate that small changes in molecular structure and different polar processing solvents, such as 2-butanone, isopropanol and ethanol, have a strong influence on the nanofiber diameter, diameter distribution, homogeneity and pore size distributions, which can be correlated to the filtration performance. By optimizing the processing conditions, microfiber-nanofiber composites with very high filtration efficiencies of particulate matter are obtainable. Determination of the microfiber-nanofiber composite integrity at elevated temperatures revealed that the morphology of the composites is maintained. Materials and Methods Materials The synthesis and characterization of N,N’,N’’-tris(6-methylheptyl)-1,3,5-benzenetricaboxamide (BTA 1; CAS: 1402460-30-3) and N,N’,N’’-tris(3-methylbutyl)-1,3,5-benzenetricaboxamide (BTA 2; CAS: 436149-10-9) are described in detail elsewhere.52 The microfiber scaffold is a commercially available viscose/polyester nonwoven (source: AmPri). The thickness of the nonwoven is about 0.15 mm with a basis weight of 32.2 g m-2. The nonwoven was cut into squares of 9.5 cm x 9.5 cm, corresponding to an area of 90.25 cm2. For the filtration experiments, a fine test dust from Powder Technology Inc. (ISO 12103-1, A2; Saharan desert sand) was used, whereas most of the particle diameters are in the range of 0.2 – 2.0 µm. All solvents were commercially available from Aldrich and used without further purification.

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Substituent R BTA 1

*

BTA 2

*

Preparation of supramolecular nanofibers in nonwovens 1 wt.-% solutions of BTA 1 and 2 were prepared at a temperature of 50 °C in 2-butanone, isopropanol, and ethanol, respectively. Each individual solution was filled into an immersion bath at a constant temperature of 50 °C. The nonwoven was placed in a frame with an open accessible area of 72.25 cm2 (8.5 cm x 8.5 cm) and immersed vertically in the hot BTA-solution for 30 s. The soaked nonwoven was removed and dried in a horizontal position at ambient conditions for at least 30 min to ensure complete solvent removal. Scanning electron microscopy (SEM) For SEM investigations, samples of the microfiber-nanofiber composites were fixed via doublesided adhesive conductive carbon tape on a SEM sample holder and sputtered with carbon utilizing a MED 010 sputter-coater from Balzers Union. SEM micrographs were recorded by means of a Zeiss 1530 FESEM equipped with a SE2 detector with an acceleration voltage of 2 kV.



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Fiber diameter histograms The fiber diameters of the supramolecular nanofibers were determined by utilizing AxioVision Software provided by Zeiss. The composites were analyzed at not less than four different positions and at least 150 fibers were taken into account to generate the nanofiber diameter histograms. Capillary Flow Porometry Pore-size distributions of supramolecular microfiber-nanofiber composites were measured by means of a Capillary Flow Porometer (PSM 165, manufacturer: TOPAS). Samples were placed into the sample holder with a circular opening with 6 mm diameter. Volumetric flow rates ranging between 0 and 7.5 L/min were applied using compressed air and the corresponding pressure was recorded. As wetting liquid TOPOR (supplier: TOPAS) was used. To determine pore size distributions, two measurements (with and without the wetting liquid) were performed under identical parameters. Each pore size distribution is based on 5 individual samples. Poresize distributions were calculated using PSM Win software (TOPAS). Filtration test All filtration tests were performed utilizing a filter test rig with a filtration area of 28.3 cm2 (MFP 2000, PALAS GmbH) equipped with a white light-scattering spectrometer Welas digital 2100 (detection range: 0.2 µm – 10 µm particle size). Iso fine test dust (Saharan desert sand) was applied to the filter test rig by means of a RBG 1000 aerosol generator (PALAS GmbH). To minimize the influence of electrostatic effects, the dust particles were discharged with a CD 2000 8 ACS Paragon Plus Environment

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(PALAS GmbH). The microfiber-nanofiber composites were fixed with a pneumatic sample holder in the test rig and a particle concentration of 30000 particles cm-3 was applied for 30 s at a constant flow velocity of 0.25 m s-1. Filtration efficiencies were determined by recording particle concentrations and particle-size distribution without any sample (upstream concentration) before and after the filtration experiment and with the microfiber-nanofiber composite material (downstream concentration). The pressure drop, which is defined as the difference in pressure before and after the filter, was recorded online. Corresponding particle-size dependent filtration efficiency curves are based on an average of at least six independently prepared samples. Microfiber-nanofiber composite integrity at elevated temperatures The microfiber-nanofiber composites were fixed with a double-sided adhesive conductive carbon tape on a SEM sample holder and placed in an oven at 80 °C, 100 °C, 120 °C, and 140 °C for 1 h, respectively. After each annealing step, the specimens were investigated at the same positions, without prior sputtering by means of a Desktop SEM (Phenom G1 from FEI).

Results and discussion Influence of the solvent on the preparation of supramolecular microfiber-nanofiber composites N,N’,N’’-tris(6-methylheptyl)-1,3,5-benzene tricarboxamide (BTA 1) and N,N’,N’’-tris(3methylbutyl)-1,3,5-benzenetricaboxamide (BTA 2) were selected as supramolecular building blocks. The synthesis and thermal characterization of these compounds are described elsewhere. 52

BTA 1 and 2 differ in the length of the peripheral side groups by three methylene units, which 9 ACS Paragon Plus Environment


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have a strong influence on the thermal properties of the compounds. Both compounds feature a sufficient solubility of at least 1 wt.-% at a temperature of 50 °C in the solvents, 2-butanone, isopropanol and ethanol. These solvents were selected to be nonhazardous to the environment and therefore suitable to be used in filter applications. The use of nonpolar solvents such as hydrocarbon solvents often leads to the gelation of the BTA solution even at very low concentrations. Gelation has to be avoided to obtain supramolecular microfiber-nanofiber composites with suitable morphologies for air filtration applications. To gain more insight into the solubility and self-assembly behavior of the selected BTAs in the polar solvents, turbidity measurements upon cooling of solutions with a constant concentration of 1 wt.-% of BTA in a temperature range from 50 °C to 0 °C were performed. The results are shown in Figure S1. The turbidity temperatures, which are determined at 50% transmittance, are 30 °C for BTA 1 in 2-butanone, 6 °C in isopropanol and 6 °C in ethanol. For BTA 2 the turbidity temperature is 25 °C in 2-butanone. In isopropanol and ethanol no turbidity temperatures were detected upon cooling until 0 °C. In those samples in which turbidity occurs, the behavior is fully reversible upon heating. This finding shows significant differences in the self-assembly and solubility behavior with respect to the selected solvents and slight differences with respect to the molecular structure of the BTAs. For the preparation of the microfiber-nanofiber composites, solutions in each solvent comprising BTA 1 or BTA 2 at a concentration of 1 wt.-% were prepared resulting in six different systems. To achieve microfiber-nanofiber composites as described in Figure 1, a nonwoven (physically and chemically bonded sheet of fibers) with a thickness of 0.15 mm was immersed into these solutions at a temperature of 50 °C, removed from the immersion bath and dried at ambient conditions. The average content of the BTAs of the six different microfiber-nanofiber composites 10 ACS Paragon Plus Environment

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were determined by means of gravimetric analysis. The gravimetric values and the corresponding deviations are based on nine individual samples and were found to be 7.3 ± 0.6 wt.-% (2butanone), 7.0 ± 0.4 wt.-% (isopropanol), and 6.3 ± 0.6 wt.-% (ethanol) for BTA 1 in the microfiber-nanofiber composites. The corresponding gravimetric values for BTA 2 in the composites are 7.3 ± 0.7 wt.-% (2-butanone), 6.1 ± 0.5 wt.-% (isopropanol), and 6.4 ± 0.5 wt.-% (ethanol). These findings suggest an insignificant difference between the contents of BTA 1 and 2 being in good agreement with previous results.50 The nanofiber-morphology within the resulting composites was studied by means of scanning electron microscopy (SEM). Figure 3 shows SEM-micrographs of an overview and a corresponding magnified section of each microfiber-nanofiber composite containing BTA 1 and 2 processed from the different solvents. The thick fibers correspond to the polymer nonwoven featuring an average microfiber diameter of 13 µm. As shown in Figure 3 (top), both microfibernanofiber composites processed from 2-butanone exhibit a very homogeneous filling of the nonwoven scaffold with supramolecular nanofibers. However, in the magnified image of the composite comprising BTA 2, small openings within the supramolecular nanofiber web are visible. The corresponding fiber diameter histograms (Figure 3, right) are based on the evaluation of more than 150 fiber diameters from the SEM-micrographs. The average fiber diameter of the supramolecular nanofibers is 453 nm ± 182 nm (2-butanone) for BTA 1 and 512 nm ± 219 nm (2-butanone) for BTA 2. Compared to composites processed from 2-butanone, SEM-micrographs of composites obtained from isopropanol containing BTA 1 (Figure 3, middle) show an inhomogeneous filling with supramolecular fibers. In this case, some openings are fully covered with supramolecular fibers, while others remain open.



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Figure 3. SEM-micrographs of composites containing either supramolecular nanofibers of BTA 1 or 2. The composites were prepared from solutions consisting of 1wt.-% BTA in 2-butanone, isopropanol and ethanol, respectively. The amount of supramolecular nanofibers in the composite prepared from 2-butanone is 7.3 wt.-% for BTA 1 and 2. Processed from isopropanol the amount 12 ACS Paragon Plus Environment

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is 7.0 wt.-% for BTA 1 and 6.1 wt.-% for BTA 2. Processed from ethanol the amount is 6.3 wt.% for BTA 1 and 6.4 wt.-% for BTA 2. The corresponding histograms of the fiber diameters are based on the evaluation of at least 150 fibers.

In the composite containing BTA 2 (isopropanol) almost all openings remain unfilled. The corresponding fiber diameter is 1257 nm ± 578 nm for BTA 1 and 1026 nm ± 578 nm for BTA 2 if processed from isopropanol. SEM-micrographs of the composites prepared from ethanol (Figure 3, bottom) exhibit thick supramolecular fibers compared to those processed from 2butanone or isopropanol. For BTA 1, most openings of the nonwoven are penetrated with a few supramolecular fibers resulting in a loosely covered nonwoven. Composites containing BTA 2 exhibit supramolecular fibers with a similar diameter. The determined average fiber diameters are 1922 nm ± 894 nm for BTA 1 and 2389nm ± 1313 nm for BTA 2 if processed from ethanol. These findings reveal significant differences in the resulting fiber diameters with respect to the selected solvents and subtle differences with respect to molecular structure of the BTAs. These results correlate to those found in the turbidity measurements. Infrared spectroscopic investigations on the microfiber-nanofiber composites revealed the formation of supramolecular columns. Specific IR-vibrations can be assigned to directed threefold hydrogen bonds between the individual building blocks inside a columnar stack.

53,31

As shown in Figure S2 exemplarily

for BTA 1, the supramolecular nanofibers inside the composites exhibit the characteristic IR bands at 1560 cm-1 (amide II stretch), 1640 cm-1 (C=O stretch) and 3240 cm-1 (N-H stretch), proving the columnar stacking of the BTA molecules. The same results were found for BTA 2. The length of the supramolecular nanofibers cannot clearly be determined, since the nanofibers are interlaced in the polymer nonwoven and have strong tendency to adhere to the microfibers. 13 ACS Paragon Plus Environment


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To demonstrate the reproducibility and robustness of this process, the preparation of the composites for all BTA-solvent combinations were repeated several times. Exemplarily, Figure S3 shows the fiber diameter histograms of three independently prepared samples of microfibernanofiber composites processed from solutions with BTA 1 in 2-butanone. The deviation in the average fiber diameters as well as in the fiber diameter distributions is marginal. Similar results were found for all investigated BTA-solvent combinations. An important issue is the homogenous insertion of supramolecular nanofibers over the entire nonwoven, since inhomogeneity will provide pathways for the particulate matter to pass the filter medium during the filtration test. Therefore, we prepared a microfiber-nanofiber composite based on BTA 1 at a concentration of 1 wt% in 2-butanone, cut pieces of a size of approximately 1 x 1 cm at nine different positions of the dried composite and investigated these samples by means of SEM. The cut nonwoven and a series of SEM micrographs at the corresponding positions are shown in Figure 4 demonstrating that each piece of the viscose/polyester nonwoven is completely covered with supramolecular nanofibers. No significant differences in the homogeneity or nanofiber morphology were found. Thus, we assume that this method allows for the preparation of composites with supramolecular nanofibers on large scale.


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Figure 4. Left: photograph of a microfiber-nanofiber composite (8.5 x 8.5 cm) with BTA 1 prepared from a 1 wt.-% solution in 2-butanone. Samples (size: approximately 1 x 1 cm) were taken at nine different positions for SEM investigations. Right: SEM micrographs correspond to the different positions of the composite. Capillary flow porometry was performed to determine the pore size distributions of the microfiber-nanofiber composites. Five samples with an area of 0.28 cm2 at different positions of each microfiber-nanofiber composite and the neat nonwoven were measured to determine representative pore size distributions. Figure 5 shows the pore size distributions of the neat reference nonwoven and of microfiber-nanofiber composites containing BTA 1 and BTA 2, respectively. The nonwoven features large pore sizes with more than 99% of the pores in the range from 20 to 92 µm. About 70% of these pores have a diameter of 60 to 70 µm. The microfiber-nanofiber composite containing BTA 1 exhibits a small and narrowly distributed pore sizes ranging from 6 to 20 µm, whereas more than 80% of the pores have sizes between 6 and 10 µm. These results show that nanofibers of BTA 1 in the corresponding composites densely cover all pores of the nonwoven. Composites containing BTA 2, however, exhibit a very broad pore 15 ACS Paragon Plus Environment


size distribution with remaining pores of 8 to 86 µm and around 62% are in the range from 18 to 38 µm. This result indicates that the pore size distribution arises from a superposition of nanofibrous pores with a smaller amount of pores originating from the neat nonwoven. In addition, the main part of the pores in these composites is not as densely covered as the pores in composites with BTA 1. These slight alterations in the morphology of both microfiber-nanofiber composites may be attributed to the complex drying process and the differences in the temperature-dependent solubility behavior of BTA 1 and BTA 2 as revealed by turbidity measurements (see Figure S1).

50

relative frequency / %

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nonwoven BTA 1 BTA 2

40 30 20 10 0 0

20

40

60

80

100

pore size / µm Figure 5. Pore size distributions of supramolecular microfiber-nanofiber composites containing BTA 1 (blue) or BTA 2 (red) prepared from solutions of 1 wt.-% in 2-butanone and the nonwoven reference (black) obtained by capillary flow porometry. Each of the shown pore size distribution is an average of 5 individual samples.


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Filtration performance of the microfiber-nanofiber composites To investigate the filtration performance and to reveal the correlation between filtration efficiency and fiber morphology, composites were subjected to filtration experiments of particulate matter from air. An Iso fine test dust was used as particulate matter featuring particle diameters in the range of 0.2 – 2.0 µm. All composites shown in Figure 3 were investigated by means of a filter test rig at a flow velocity of 0.25 m s-1. The applied particle concentration of the aerosol was 30000 particles cm-3. Important parameters to evaluate a filter media are the filtration efficiency, the pressure drop (∆p) and the quality factor (QF). The filtration efficiency of a filter medium is defined as the percentage of removed particles from an air stream. As the filtration efficiency varies with the particle size, typically the filtration efficiency is specified at a given particle size. The pressure drop is defined as the difference in pressure before and after the filter. The filtration efficiencies of the different composites and of the neat nonwoven are shown in Figure 6. Each curve represents an average of at least six independently prepared samples. The neat nonwoven shows no significant filtration efficiency at any particle size of the fine dust and features an average pressure drop of 3 Pa. All composites containing supramolecular fibers based on BTA 1 exhibit an increased filtration performance. Though the diameter of the supramolecular fibers in the composites processed from ethanol were close to 2 µm, the filtration performance was doubled and featured an average pressure drop of 18 Pa. The composites processed from isopropanol resulted in further increased filtration performance with an average pressure drop of 31 Pa. Composites processed from 2-butanone show remarkable filtration efficiencies of 95% for 0.2 µm particles and approximately 100 % for 2.0 µm particles at an average pressure drop of 1529 Pa. Considering the applied high flow velocity of 0.25 m s-1, the value for the differential 17 ACS Paragon Plus Environment


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pressure demonstrates that self-assembled nanofibers inside the nonwoven scaffold are strongly fixed to the individual microfibers and form a mechanically stable composite. This result also indicates a homogeneous distribution of the supramolecular nanofibers throughout the microfiber scaffold correlating to the results obtained from SEM and capillary flow porometry investigations. The quality factor (QF), another parameter to assess the filter performance, is defined as -ln(P)/∆p, where P is the penetration of the particles (with P = 1-filtration efficiency). The quality factor is indirectly influenced by the kind of particulate matter, the particle size, the size distribution and most importantly the flow velocity. Under our conditions with a particle size from 0.2 to 2.0 µm and a rather high flow velocity of 0.25 m s-1, we calculated for these microfiber-nanofiber composites processed from butanone, quality factors in the range of 2 - 4 kPa-1. These values are acceptable compared to literature data with similar flow velocity.5 In contrast, the filtration performance of microfiber-nanofiber composites containing BTA 2 deviates significantly. For instance, the microfiber-nanofiber composites, which are processed from isopropanol or ethanol, show no reasonable improvement in the filtration efficiency and, consequently, no significant increase of the pressure drop compared to the neat nonwoven. Composites processed from 2-butanone with BTA 2 show filtration efficiencies of 60% for 0.2 µm particles and 75 % for 2.0 µm particles at an average pressure drop of only 348 Pa. Even though the amount of supramolecular nanofibers, the mean fiber diameter and the diameter distribution of both BTAs in the composites are very similar, the deviation in the filtration performance of composites processed from butanone containing BTA 1 or BTA 2 is attributed to the slightly less dense nanofiber web of BTA 2 as observed by SEM and the broader pore size distribution in composites with BTA 2 as revealed by capillary flow porometry.


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filtration efficiency / %

120

BTA 1

100 2-butanone isopropanol ethanol nonwoven

80 60 40 20 0 0.4

0.8

1.2

1.6

2.0

particle size / µm 120

filtration efficiency / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BTA 2

100 80 2-butanone isopropanol ethanol nonwoven

60 40 20 0 0.4

0.8

1.2

1.6

2.0

particle size / µm Figure 6. Filtration efficiencies of supramolecular microfiber-nanofiber composites with BTA 1 (top) and BTA 2 (bottom) prepared from 1 wt.-% concentration in 2-butanone (triangles down), isopropanol (triangles up) and ethanol (open circles). As reference the nonwoven scaffold without supramolecular nanofibers is shown as filled squares. The curves are based on an average of at least six independently prepared samples. Test conditions are: Filter area: 28.3 cm2; flow velocity: 0.25 m s-1; test aerosol: iso fine dust; upstream aerosol concentration: about 30000 Particles cm-3; measuring time: 30 s. Owing to this highly promising filtration efficiency obtained with composites with BTA 1, we compared under the here described filter testing conditions the system based on N,N’,N’’-tris(219 ACS Paragon Plus Environment


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ethylhexyl)-1,3,5-benzenetricaboxamide BTA 3 (see Supporting Information Figure S6).50 Both BTA 3 and BTA 1, are constitutional isomers featuring different branching of the side groups. The corresponding turbidity measurements, nanofiber morphology, nanofiber diameter histograms and filtration performance of BTA 3 are shown in Figure S4-S6. The results obtained with BTA 3 are very similar to the results obtained with BTA 1. This finding confirms that properly selected side groups of BTAs and processing solvent have a strong influence on nanofiber morphology and filtration performance. Microfiber-nanofiber composite integrity at elevated temperatures The removal of particulate matter from air, especially in industrial applications, may be performed at elevated temperatures.54 Therefore, a sufficient thermal stability and integrity of the filter media is required. To validate the integrity of the supramolecular nanofibers, composites containing BTA 1 and BTA 2 were exposed at elevated temperatures. Both samples were annealed at 100 °C, 120 °C, and 140 °C for one hour, respectively. After each annealing step, the non-sputtered composites were investigated via Desktop-SEM at the same position at room temperature. At temperatures above 140 °C the viscose/polyester nonwoven starts to soften and the dimensional stability of the microfibers is lost. Figure 7 displays a series of SEM images of the composites after each annealing step. For composites containing BTA 1, the nanofiber morphology is maintained till 100 °C. However, this morphology alters at a temperature of 120 °C. This finding is attributed to the thermal behavior of BTA 1, which features a plastic crystalline phase till 240°C. 52 In contrast, no change of the nanofiber morphology was observed in the composite containing BTA 2. This result indicates that the supramolecular nanofibers maintain their dimensional shape till the melting point of BTA 2 at 211 °C.52


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Figure 7. SEM-micrographs (Desktop-SEM) of non-sputtered supramolecular microfibernanofiber composites after exposure to different temperatures such as 25 °C, 100 °C, 120 °C, and 140 C for 1h; The micrographs were taken at the same position for BTA 1 (left column) and BTA 2 (right column). Both microfiber-nanofiber composites were prepared from butanone solutions with a concentration of 1 wt. %.



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Conclusions We have demonstrated a fast and simple bottom-up process to prepare supramolecular nanofibers based on selected 1,3,5-benzenetrisamides in viscose/polyester microfiber nonwovens. Small changes in the molecular structure and the use of different polar processing solvents have a strong influence on the supramolecular fiber diameter and diameter distribution. The resulting nanofiber diameter distribution in the composites can be reproduced with high accuracy. Composites processed from 2-butanone feature a high homogeneity with respect to the nanofiber distribution and the nanofiber morphology throughout the entire nonwoven scaffold. This finding also demonstrates a potential processability on large scale and implicates the possibility to employ this solution-based bottom-up process in any complex porous structure. Capillary flow porometry revealed that depending on the molecular structure of the BTAs the pore size distribution can vary significantly, which is also reflected in the filtration experiments of particulate matter. Choosing tailored BTAs and proper processing conditions, microfiber-nanofiber composites with filtration efficiencies of more than 95% for particles of 0.2 µm can be reproducible obtained. In addition, temperature-dependent exposure of microfiber-nanofiber composites showed that the dimensional integrity of supramolecular nanofibers is maintained at elevated temperatures compared to the viscose/polyester nonwoven. Therefore, supramolecular nanofibers based on 1,3,5-benzenetrisamides are a potential materials class offering a broad scope of air filtration applications.


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ASSOCIATED CONTENT Turbidity measurements of BTA 1 and 2 in different solvents. Preparation method of supramolecular microfiber-nanofiber composites. Infrared spectroscopy of composites containing BTA 1. Fiber-diameter histograms of repeated experiments. Corresponding data of N,N’,N’’tris(2-ethylhexyl)-1,3,5-benzenetricaboxamide (BTA 3) (turbidity measurements, SEM-images and filtration efficiencies). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Funding Sources Deutsche Forschungsgemeinschaft (German Research Foundation), Collaborative Research Center 840 (SFB 840), project B8. Notes The authors declare no competing financial interest. Acknowledgements We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (German Research Foundation) within the Collaborative Research Center 840 (SFB 840), project B8.



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Tailoring Supramolecular Nanofibers for Air Filtration Applications

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