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Tuning the Morphology and Activity of Electrospun Polystyrene/ UiO-66-NH2 Metal-Organic Framework Composites to Enhance Chemical Warfare Agent Removal Gregory W. Peterson, Annie Xi Lu, and Thomas H. Epps, III ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09209 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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ACS Applied Materials & Interfaces

Tuning the Morphology and Activity of Electrospun Polystyrene/ UiO-66-NH2 Metal-Organic Framework Composites to Enhance Chemical Warfare Agent Removal Gregory W. Peterson,*,†,‡ Annie X. Lu, †,# and Thomas H. Epps, III‡,± †

‡

Edgewood Chemical Biological Center, 5183 Blackhawk Rd., APG, Maryland 21010-5424,

Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716

‡

Defense Threat Reduction Agency, 8725 John J. Kingman Road, Stop 6201, Fort Belvoir, VA

22060-6201; ±Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716

*Corresponding author: Gregory W. Peterson, Email: [email protected]; Phone: (410) 436-9794

Abstract This work investigates the processing-structure-activity relationships that ultimately facilitate the enhanced performance of UiO-66-NH2 metal-organic frameworks (MOFs) in electrospun polystyrene (PS) fibers for chemical warfare agent detoxification. Key electrospinning processing parameters including solvent type (dimethylformamide [DMF]) vs. DMF/ tetrahydrofuran [THF]), PS weight fraction in solution, and MOF weight fraction relative to PS were varied to optimize MOF incorporation into the fibers and ultimately improve composite

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performance. It was found that composites spun from pure DMF generally resulted in MOF crystal deposition on the surface of the fibers, while composites spun from DMF/THF typically led to MOF crystal deposition within the fibers. For cases in which the MOF was incorporated on the periphery of the fibers, the composites generally demonstrated better gas uptake (e.g., nitrogen, chlorine) due to enhanced access to the MOF pores. Additionally, increasing both the polymer and MOF weight percentages in the electrospun solutions resulted in larger diameter fibers, with polymer concentration having a more pronounced effect on fiber size; however, these larger fibers were generally less efficient at gas separations. Overall, exploring the electrospinning parameter space resulted in composites that outperformed previously reported materials for the detoxification of the chemical warfare agent, soman. The data and strategies herein thus provide guiding principles applicable to the design of future systems for protection and separations as well as a wide range of environmental remediation applications. Keywords Metal-organic framework, UiO-66-NH2, polymer, polystyrene, electrospun nanofiber, chemical warfare agent


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1. Introduction The remediation and detoxification of chemical warfare agents (CWAs) has recently regained interest due to current world events.1 In addition to the extreme toxicity of these chemicals, nerve and blister agents also can be environmentally persistent.2 Thus, materials solutions are sought for the detoxification of, and physical protection against, CWAs. Metal-organic frameworks (MOF)s have been studied extensively over the past 10-15 years and show promise in numerous applications such as gas storage,3 separations,4,5 catalysis,6-8 filtration,9,10 and sensing.11 Due to the ability to tune the linkers and metal secondary building units (SBUs), MOFs can be competitive with, and in many cases exceed, the performance of other porous materials in such applications.12 Additionally, several MOFs have been identified as promising candidates for nerve agent degradation in powder form,8,13,14 yet many applications require functional forms other than powders, such as MOFs integrated into a fabric for use in protective clothing. Only relatively recently have engineered particles and hierarchical/composite structures, such as mixed matrix membranes (MMMs), received significant attention.15,16 Indeed, even the most promising materials from a chemistry and performance standpoint must be properly integrated into hierarchical systems to have significant industrial impacts. While many avenues exist for engineering MOFs into useful forms, polymers offer a robust platform due to the wide variety of chemistries and physical properties that are accessible in macromolecular systems. Polymers have been used on numerous occasions as substrates for the incorporation of MOFs,16,17 and several reviews are available on the subject.15,18,19 Although polymer/MOF composites have been investigated for use in various forms (e.g., films, binders),



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fibrous substrates offer numerous additional opportunities. For example, the ability to develop fiber-based polymer/MOF composites allows for materials solutions in areas such as separations,17 active wear (e.g., protective or odor resistant clothing),20 wound treatment,21 flexible filters for pollution or toxic gas control,22 and decontamination wipes.23 Electrospinning is a well-known technique for generating polymeric nano/micro-fibers and is conducive to the incorporation of active particles such as MOFs.22,24,25 Many different polymers have been mixed with MOFs, with one of the lesser-studied materials being polystyrene (PS). PS is a common commodity polymer that offers mechanical stability and low cost, while also being compatible with a wide range of common polymers and solvents.26 A few efforts have investigated PS/MOF electrospun composites;22,27-29 however, none have done so systematically. Thus, a fundamental understanding of the effects of solvent, polymer concentration, and MOF concentration on processing-structure-activity relationships of these composites is still necessary. In this study, we investigated the above relationships, with relevance to CWA detoxification, using PS/UiO-66-NH2 electrospun from two solvent systems, dimethylformamide (DMF) and DMF/tetrahydrofuran (THF), with THF used to increase the relative volatility of the spinning solution. The use of PS in this study has a substantial benefit over previous reports that employed poly(vinylidene fluoride) (PVDF) as the polymeric carrier due to the significantly lower cost of PS.30 We employed a variety of techniques to understand how the MOF is deposited within vs. on the surface of the fibers, and we studied the resulting activity of the fibers using various chemical probes. The MOF (UiO-66-NH2) and PS system were chosen for multiple reasons. First, UiO-66-NH2 is one of the most stable MOFs and is resistant to a wide range of conditions, including high temperature, high humidity, solvents, and even a wide pH range.31 Second, UiO-66-NH2 is a highly reactive material useful for removal of chlorine,8 4 ACS Paragon Plus Environment

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nitrogen dioxide,9 and chemical warfare agents.13,14 Third, the amphiphilic nature of the MOF coupled with the hydrophobic nature of PS should facilitate the tailoring of the MOF dispersion behavior in addition to tuning the wetting and wicking properties of the system during and after processing. Overall, the systematic studies herein led to the generation of MOF-containing electrospun PS mats that exhibited superior CWA detoxification relative to previously reported electrospun MOF/polymer systems.30

2. Experimental Section 2.1 Materials PS (Mn ~170,000 g/mol and Mw ~350,000 g/mol), THF, and DMF were purchased from Sigma-Aldrich and used as received. The MOF UiO-66-NH2 was synthesized by TDA, Inc. (Wheat Ridge, Colorado) and used as received. PS pellets were dissolved in DMF and 50/50 volume ratio of DMF/THF. The polymer solution was stirred for approximately 16 h or overnight, after which the appropriate amount of MOF was added corresponding to 10 wt%, 25 wt%, and 50 wt% relative to the polymer content. The resulting solution then was stirred magnetically at 250 rpm for approximately 24 h prior to use.

2.2 Electrospinning Electrospinning was conducted using a programmable floor-stand electrospinning unit (MTI Corporation MSK-NFES-4). Solutions were loaded into 6 mL plastic syringes equipped with a 20-gauge needle. The solutions were pumped at a flow rate of 3 mL h-1 onto a rotating mandrel 5 ACS Paragon Plus Environment


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operating at 300 rpm. The electric field was set at approximately 12 kV but was modified when necessary according to perceived solution viscosity. 2.3 Scanning Electron Microscopy Scanning electron microscopy (SEM) images were obtained using a Phenom GSR desktop SEM. Samples were supported on double-sided carbon tape and sputter coated with gold prior to analysis. Typical settings for the instrument used an accelerating voltage of 5 kV at a nominal working distance of 10 mm. Specific operating conditions are listed with each image for clarity. Fiber diameters were automatically analyzed using the Phenom Fibermetric software package. 2.4 Powder X-ray Diffraction Powder X-ray diffraction (PXRD) measurements were conducted using a Rigaku Miniflex 600 X-ray powder diffractometer with a D/Tex detector. Samples were scanned at 40 kV and 15 mA using Cu Kα radiation, a scan rate of 5° min-1 over a 2θ range of 3° to 50°. Data are plotted as absolute values and offset on the y-axis to illustrate the differences between each sample. 2.5 Nitrogen Isotherms Nitrogen uptake was measured at 77 K using a Micromeritics ASAP 2040. Samples were off-gassed at 60 °C overnight under vacuum. Surface area measurements were calculated using the Brunauer-Emmett-Teller (BET) method, and total pore volumes were calculated at a relative pressure of 0.975.


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2.6 Thermogravimetric Analysis Thermogravimetric analysis (TGA) measurements were collected on a Netzsch TG 209 F1 Libra analyzer to determine the mass of MOF deposited within the fiber mat. Measurements were obtained over a temperature range from 25 °C to 600 °C at a rate of 10 °C/min. Data were corrected for solvent as described in the Materials Characterization Section. 2.7 Microbreakthrough Testing Samples were evaluated for chlorine loading to determine accessibility of MOFs deposited on/in the fibrous mats. The system has been described previously.32,33 Briefly, a specific amount of chlorine was injected into a ballast that was subsequently pressurized. Then, the ballast contents were mixed with a diluent air stream. The air stream rate was set such that a challenge concentration of 2,000 mg m-3 chlorine was achieved. The mixed stream was passed through a sorbent bed submerged in a temperature-controlled water bath. Approximately 55 mm3 of each sample was packed into a 4 mm length and 4 mm diameter geometry, and was tested as a powder under approximately 0% relative humidity (RH) (-40 °C dew point) conditions. Samples were pre-equilibrated at the test RH for approximately 1-2 h. The effluent stream was monitored using a photoionization detector (PID) and a 11.7 eV lamp. Loadings were calculated in mol kg1

by integrating the breakthrough curves at saturation. An error of approximately 15% exists for

loadings based on the standard deviation calculated from twenty control breakthrough tests. 2.8 Nuclear Magnetic Resonance Testing The organophosphorus nerve agent O-Pinacolyl methylphosphonofluoridate (soman) was evaluated using 31P solid state nuclear magnetic resonance (SS NMR) spectroscopy. Approximately 20 mg of composite was packed into a NMR rotor and humidified at 50% RH for 7 ACS Paragon Plus Environment


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16 h. Next, 2.6 µL of soman was dropped via syringe onto the composite, which then was transferred into the NMR instrument and monitored for the disappearance of soman according to established procedures.34 The production of the non-toxic hydrolysis product pinacolyl methylphosphonic acid (PMPA) also was monitored when possible. The natural log of the integrated area of the soman peak was plotted as a function of time to determine the half-life for each composite.

3

Results and Discussion

3.1 Materials Characterization Images of fibers taken with SEM are shown in Figure 1. Fibers spun from a pure DMF solution are displayed in Figure 1a, and fibers electrospun with a 50/50 solution of DMF/THF are shown in Figure 1b. Differences in fiber morphology are apparent, with fibers spun from DMF generally exhibiting smooth features, while those spun from DMF/THF solutions exhibiting multiple fibrils that aggregated into larger fiber bundles. These features are particularly apparent for the initial study of five different DMF/THF solutions as shown in Figure S1. It was expected that by adding a lower volatility chemical (THF), the solvent mixture would evaporate more rapidly and allow for selective deposition of MOF on the surface of the fiber. Instead the opposite occurred, with the MOF being selectively deposited within the fiber (or fiber bundles as discussed below) in the mixed solvent system. While the resulting composite structure does not appear to support the above hypothesis, closer examination of the fibers spun from the mixed DMF/THF solvent suggests what appear to be either striations and/or even


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bundled fibers. This apparent “yarn” is most noticeable in the 20% PS, 25 wt% UiO-66-NH2 sample (20PS-25U-DMF/THF), see Figure S2. Thus, it appears that the MOF is indeed on the outside of individual fibers, but a thin sheath around the yarn bundle results in the MOF inside the larger structure. Additionally, the UiO-66-NH2 appears to be better distributed throughout the fiber mat, and in smaller particles sizes, for the DMF system. This phenomenon is most apparent in the 10% PS, 25 wt% UiO-66-NH2 sample (10PS-25U-DMF, see Figure 1b) – larger SEM images can be found in the Supporting Information (Figures S2-S4).

Figure 1. Scanning electron microscopy images of composites electrospun from (a) DMF and (b) DMF/THF solutions. The rows correspond to the wt% of PS in the solution, and the columns correspond the wt% of MOF in the solution (relative to the polymer). The scale bar represents 8 µm in all images. All materials discussed in the text refer to % polymer - % MOF - solvent. For example, 10PS-10U-DMF refers to 10 wt% PS and 10 wt% UiO-66-NH2 electrospun from DMF.

For both sets of samples MOF loadings up to 25 wt% do not affect the fibers in terms of structure and diameter. At loadings of 50 wt% MOF, however, the fiber morphology changes for all samples, but most noticeably for mats spun from solutions with higher PS content. For the 10% PS composites with 50 wt% MOF, although fibers are still present, they begin coalescing and the MOFs clump together. Visually, the composites at the highest PS/UiO-66-NH2 loadings are no longer mats but rather large diameter strands of composite, as seen in Figure S5. At the



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microscopic level, the composites spun from DMF appear pea-pod-like (Figure S3, red dashed circle), with MOF crystals clustered in highly defective hollow fiber strands. On the other hand, the composites spun from DMF/THF appear to resemble continuous films as opposed to discrete fibers, and the MOF is well-dispersed throughout (Figure S6). The fiber diameters resulting from electrospinning the composites are shown in Figure 2. Fiber diameter is generally unchanged within each subset of samples up until 50 wt% UiO-66NH2, although standard deviation increases as the MOF content increases. The larger standard deviations indicate more polydisperse samples as a result of the additional MOF in the composites. For example, the samples spun from 10% PS in DMF all have approximate diameters of 500 nm; however, increasing the polymer percent increases the fiber diameter, in several cases linearly (i.e., doubling the polymer in solution approximately doubles the fiber diameter). The exception is the highest polymer/MOF loadings, where significant deviation occurs from this trend. When considering the solvent used for electrospinning the composites, the DMF/THF mixture generally leads to slightly larger fiber diameters than the pure DMF solvent. The exception is once again for 50 wt% MOF samples with 20 wt% PS. Although UiO-66-NH2 is one of the more stable MOFs,34,35 it was important to confirm that the MOF remained intact after the electrospinning process, as polymer, solvent, and high electric charges could impact the self-assembled structure. PXRD results for all composites are shown in Figure 3. All data are plotted on absolute scales and are not corrected relative to the most intense peak. PS fibers electrospun without MOF exhibit no diffraction peaks. For all samples containing UiO-66-NH2, peaks at approximately 7.6° and 8.8° 2θ are present, representing the long-range order of the MOF commensurate with the pure material. For composites with higher MOF loadings, higher 2θ peaks also are present. All materials show increasing intensity with 10 ACS Paragon Plus Environment

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increasing MOF content. In most cases, the peaks of the composite are slightly wider than those in the parent MOF, which is likely due to reduced crystal size and less agglomerates in the composites in comparison to the pure MOF.36 Thus, UiO-66-NH2 appears to remain intact after the electrospinning process and is present in all composites.

6.0 10PS-DMF 5.0

Avg Fiber Diameter (µm)

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20PS-DMF 10PS-DMF/THF

4.0

20PS-DMF/THF

3.0 2.0 1.0 0.0 0

10

25

50

Weight Percent MOF

Figure 2. Average fiber diameter of electrospun composites generated under various processing conditions. Error bars represent one standard deviation from the mean. TGA data were collected to determine the amount of MOF within each composite for comparison to the expected values based on initial solution concentrations. Results are shown in Figure S7, and Table 1 contains a summary of the MOF content calculated for each composite. To determine the amount of MOF, the mass loss for each composite was normalized on the basis of the mass loss for the parent UiO-66-NH2. However, solvent also must be taken into account in these calculations, as the amount adsorbed varies greatly depending on the amount of MOF within each composite. This behavior has been investigated in depth by Lillerud and coworkers on pure UiO-66-type MOFs and is thus sufficient to determine mass of MOF in the swatch;37 however, the approach must be further refined for composite systems. Therefore, Equation 1 was used to calculate the overall MOF loading in each composite sample,



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% =

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%, ° # %, ° "

!

$%&'%()*+%, ° # $%&'%()*+%, ° "

(1)

!

in which MOFwt%,250 °C is the weight percent for the pure MOF at 250 °C from the TGA curve, etc. Equation 1 assumes that all (or at least a majority) of solvent is evacuated at 250 °C. This assumption likely is valid, as the normal boiling points for DMF and THF are 153 °C and 66 °C, respectively. However, it may not account for strongly bound DMF, which may be more difficult to evacuate. In any case, solvent represents a very small percentage of total sample mass. Another assumption of the equation is that the only substance left after 600 °C is zirconia (ZrO2) from the secondary building unit of UiO-66-NH2.37 From these calculations, it is apparent that the fibers electrospun from the DMF/THF solution are more consistent in terms of expected MOF weight percent in comparison to the fibers spun from pure DMF. Samples spun from both solvents generally show slightly lower mass of MOF than expected based on the amount in the original spinning solution, with the exception being the samples with lower MOF weight percent spun from DMF. This behavior is to be anticipated, as settling occurs during the electrospinning process, resulting in less MOF in the polymer solution. The deviation noted for the lower weight DMF samples could simply be an anomaly due to measurement error, or a locally dense portion of the fiber mat. Large deviations are found for the 20PS-50U-DMF sample, as this sample was difficult to spin, resulting in poor, inconsistent fibers. In general, samples spun from higher percent polymer solutions resulted in less consistent fibers for both solvent systems. 12 ACS Paragon Plus Environment

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160,000

140,000

UiO-66-NH₂ 10PS-50U-DMF 10PS-25U-DMF 10PS-10U-DMF 10PS-0U-DMF

Intensity (Offset)

Intensity (Offset)

120,000 100,000

b

140,000

UiO-66-NH₂ 20PS-50U-DMF 20PS-25U-DMF 20PS-10U-DMF 20PS-0U-DMF

120,000

80,000 60,000 40,000 20,000

Intensity (Offset)

a

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100,000 80,000 60,000 40,000 20,000

0

0 0

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2Θ (°) 180,000 UiO-66-NH₂ 10PS-50U-DMF/THF 10PS-25U-DMF/THF 10PS-10U-DMF/THF 10PS-0U-DMF/THF

Intensity (Offset)

140,000 120,000 100,000

d

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UiO-66-NH₂ 20PS-50U-DMF/THF 20PS-25U-DMF/THF 20PS-10U-DMF/THF 20PS-0U-DMF/THF

140,000

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Intensity (Offset)

c

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20 2Θ (°)

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Figure 3. PXRD traces of electrospun composites. (a) 10% PS spun from DMF, (b) 20% PS spun from DMF, (c) 10% PS spun from DMF/THF, and (d) 20% PS spun from DMF/THF.

Nitrogen isotherm data are shown in Figure S8, and surface areas calculated using the BET method are summarized in Table 2. The samples spun from DMF indicate generally increasing trends in nitrogen uptake as mass of MOF increases; however, the samples spun with 50 wt% MOF have lower overall uptake than those spun with 25 wt% MOF for the mats spun from higher polymer concentrations. On the other hand, the samples spun from DMF/THF solutions behave as expected, with nitrogen uptake directly proportional to the amount of MOF in the composite. Yet, there is limited uptake for the 20PS-10UDMF/THF and 20PS-25U-DMF/THF samples. It is clear that MOFs are in the composite 13 ACS Paragon Plus Environment


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from both SEM and TGA data; thus, data indicate that a significant mass percent of UiO66-NH2 is inaccessible to nitrogen, further pointing to sheaths formed by PS MOF/fibrils within the samples spun from higher polymer concentration. In Table 2, the “%” refers to the amount expected when comparing the surface area of the composite to the baseline UiO-66-NH2, after correcting for surface area of the fiber. There are limited trends among the data. For the 10% PS samples spun from DMF, the expected surface area deviates on the high side. Likewise, the 20% PS samples spun from DMF are also on the high side of theoretical values, with the exception of 50 wt% MOF sample. This exception corresponds to other data and the overall poor sample quality for this specific composite. For the DMF/THF samples, the 10% PS composites demonstrate a reasonable comparison to expected values. This behavior is likely a result of the small size of the fibers in comparison to the MOF crystals. Due to the large agglomerate size of the MOF, the fiber diameter is small enough such that the MOF is exposed and available for sorption, even though it is embedded in, and not on, the fiber. Conversely, the 20% PS samples have lower-than-expected surface areas. In this case, the significantly larger diameter of the fibers totally encases the MOF agglomerates, rendering them relatively inaccessible to the nitrogen molecules.


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Table 1. MOF weight percent calculated in composites using TGA data.

Sample 10PS-0U-DMF 10PS-10U-DMF 10PS-25U-DMF 10PS-50U-DMF 20PS-0U-DMF 20PS-10U-DMF 20PS-25U-DMF 20PS-50U-DMF 10PS-0U-DMF/THF 10PS-10U-DMF/THF 10PS-25U-DMF/THF 10PS-50U-DMF/THF 20PS-0U-DMF/THF 20PS-10U-DMF/THF 20PS-25U-DMF/THF 20PS-50U-DMF/THF

MOF wt% In From Solution TGA 0 -0.6 10 12.3 25 26.5 50 42.7 0 -0.9 10 9.0 25 21.0 50 17.2 0 -2.7 10 9.1 25 20.8 50 47.7 0 -1.1 10 4.4 25 20.3 50 44.4


% Deviation 23.0 6.0 -14.6 -10.0 -16.0 -65.6 -9.0 -16.8 -4.6 -56.0 -18.8 -11.2


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Table 2. N2 isotherm results for electrospun composites. Surface %* Area (m2/g) UiO-66-NH2 726 100.0 10PS-0U-DMF 51 10PS-10U-DMF 95 14.1 10PS-25U-DMF 192 28.4 10PS-50U-DMF 481 71.3 20PS-0U-DMF 64 20PS-10U-DMF 104 15.7 20PS-25U-DMF 322 48.6 20PS-50U-DMF 148 22.4 10PS-0U-DMF/THF 19 10PS-10U-DMF/THF 50 7.1 10PS-25U-DMF/THF 174 24.6 10PS-50U-DMF/THF 329 46.5 20PS-0U-DMF/THF 3 20PS-10U-DMF/THF 15 2.1 20PS-25U-DMF/THF 45 6.2 20PS-50U-DMF/THF 241 33.3 * Percentage for composites are calculated on the basis of the surface area of the pure MOF. Sample

3.2 Materials Performance In addition to understanding how the properties of electrospun composites change due to solvent type and polymer and MOF loading, it also was important to understand how these variables affect accessibility of and reactivity towards toxic chemicals. Chlorine microbreakthrough curves of the electrospun composites are shown in Figure S9, and the loadings calculated via mass balance are summarized in Figure 4. Data are plotted on a weighted basis to account for differences in mass and density. In each case, the pure UiO-66-NH2 powder has the longest breakthrough time. The 10% PS samples spun from both DMF and DMF/THF show expected trends, with breakthrough time and loading generally increasing with MOF weight percent. The 20% PS samples spun from DMF show almost no difference in loading 16 ACS Paragon Plus Environment

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between 10 wt% and 25 wt% UiO-66-NH2, and a decrease in the 50 wt% sample. These data are consistent with nitrogen isotherm data, which also indicate a decrease for the same 50 wt% UiO66-NH2 sample. The 20% PS sample does show increasing chlorine loading with increasing MOF percent; however, for the similar 10 wt% and 25 wt% samples, materials spun from DMF significantly outperform those spun from DMF/THF solutions. This behavior is consistent with the UiO-66-NH2 being located on the outside of the fibers for the composites spun from DMF, and hence, remaining more accessible than UiO-66-NH2 in DMF/THF-processed composites. 6.0 5.0

Cl2 Loading (mol/kg)

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4.0

10PS (DMF) 20PS (DMF) 10PS (DMF/THF) 20PS (DMF/THF) UiO-66-NH2 Loading = 5.2 mol/kg

3.0 2.0 1.0 0.0 0

10

25

50

Wt% MOF

Figure 4. Chlorine microbreakthrough loading of composite. Error bars represent a 15% standard deviation, which is typical for Cl2 microbreakthrough tests.

Finally, composites containing 25 wt% UiO-66-NH2 also were evaluated to determine the ability to hydrolyze the nerve agent soman. For these experiments, neat soman was dosed onto composites within an NMR rotor and degradation was followed using 31P SS NMR spectroscopy. This technique is routinely used to establish reactivity of solids towards chemical warfare agents.38,39 Conversion data of the hydrolysis of soman to non-toxic PMPA are shown in Figure 5. Evidence of PMPA (non-toxic byproduct) formation is shown in NMR traces in Figure S10. Half-lives calculated from the data assuming first order kinetics are presented in Table 3; the 17 ACS Paragon Plus Environment


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table includes data for the composites from this study as compared to recently published composites of PVDF and UiO-66-NH2. Plots of the natural log of area as a function of time used in the half-life calculations are presented in Figure S11. On the basis of the data in Table 3, the removal of soman is not directly related to the amount of MOF in the composite, the location of the MOF in/on the fiber, or the fiber diameter. Instead, as shown in Figure S12, it appears that the composites with the most homogeneous dispersion of MOFs perform the best, likely due to agent spreading and access to active sites. For example, MOFs in Figures S12a and S12d are well dispersed and exhibit less agglomeration than those in Figure S12b, while there appears to be less MOF (and fibers) per unit volume in Figure S12c. The materials in this study perform better than previous PVDF-based composites, in some cases by up to 30% with less total MOF (i.e., have significantly lower soman half-lives), as noted by the half-life data in Table 3.30 Furthermore, all composites perform better than the powder form of UiO-66-NH2. This behavior possibly is due to the ability of the composite to wick soman throughout the fiber, providing better diffusion and access to active sites as compared to the powder MOF. This performance indicates the potential importance of MOF dispersion, polymer substrate, and electrospinning/composite processing conditions, in CWA removal. Furthermore, these data are directly relatable to the behavior of other organophosphonate compounds such as pesticides.


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100

80

% Soman Removal

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60

40 10PS-25U-DMF 20PS-25U-DMF 10PS-25U-DMF/THF 20PS-25U-DMF/THF UiO-66-NH₂

20

0 0

100

200 Time (min)

300

400

Figure 5. Soman conversion as a function of time for composites.

Table 3. Soman half-lives of MOF/fiber composites. t1/2 (min)

Reference

UiO-66-NH2

315

This work

10PS-25U-DMF 20PS-25U-DMF 10PS-25U-DMF/THF 20PS-25U-DMF/THF MOFabric-7% MOFabric-13% MOFabric-19% MOFabric-33%

98 144 154 95 1616 1155 161 131

This work This work This work This work Lu et al.30 Lu et al.30 Lu et al.30 Lu et al.30

Sample

4

Conclusions The development of materials used to detoxify CWAs continues to be an important area of

research. In this work, we have developed composites that detoxify the nerve agent soman faster than previously published fiber-based materials. To accomplish this task, electrospun nanofiber composites consisting of PS and UiO-66-NH2 were systematically processed using multiple solvents and compositions of polymer and MOF to maximize performance through greater 19 ACS Paragon Plus Environment


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understanding of processing-structure-activity relationships. It was found that using DMF as the spinning solvent generally led to deposition of the MOF on the surface of the fibers, whereas using a 50/50 volume ratio of DMF/THF led to selective deposition within the fibers, with the latter forming yarn-like composites. Increasing the polymer and MOF weight percentages increased the fiber diameters, with the weight percent polymer in the spinning solution having a more prominent effect. Due to the selective deposition on the outside of the fibers using DMF, the DMF-processed composites generally showed enhanced nitrogen uptake and surface area, as well as increased chlorine removal, in comparison to the DMF/THF-processed materials. Finally, the electrospun composites exhibited better hydrolysis rates for the super-toxic chemical warfare agent soman as compared to other composites published in the literature indicating their potential usefulness in protective clothing against toxic chemicals.

Acknowledgements G.W.P. and A.X.L. acknowledge the U.S. Army for funding of this work through Edgewood Chemical Biological Center’s Individual Laboratory Independent Research program (PE 0601101A 91A). The opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government. T.H.E. acknowledges the Thomas & Kipp Gutshall Professorship at the University of Delaware for supporting this research effort. The authors also thank Morgan Hall for performing the soman tests.


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Supporting Information Available: SEM images, TGA, nitrogen isotherm, microbreakthrough, soman NMR spectra. This information is available free of charge via the Internet at http://pubs.acs.org.

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