Research Note pubs.acs.org/IECR
A Highly Breathable Organic/Inorganic Barrier Material that Blocks the Passage of Mustard Agent Simulants Yeny C. Hudiono,† A. Lee Miller, II,†,‡ Phillip W. Gibson,§ Andrew L. LaFrate,†,‡ Richard D. Noble,*,† and Douglas L. Gin*,†,‡ †
Department of Chemical & Biological Engineering, and ‡Department of Chemistry & Biochemistry, University of Colorado, Boulder, Colorado 80309, United States § U.S. Army Natick Soldier Research, Development, and Engineering Center, Natick, Massachusetts 01760, United States S Supporting Information *
ABSTRACT: Garment materials that provide protection against exposure to toxic chemical warfare agents (CWAs) not only require the ability to block the passage of these toxic compounds in vapor form but also the ability to transport water vapor to allow cooling for the wearer. Only a very limited number of examples of such “breathable” CWA barrier materials are known. A new type of reactive organic/inorganic composite film material is presented that has a very high water vapor transport rate (>1800 g m−2 day−1 for a 220-μm-thick film) and the ability to completely block penetration of the mustard agent simulant, 2chloroethyl ethyl sulfide (CEES), after 22 h of continuous exposure. This new composite material is based on two components: (1) a cross-linked, diol-functionalized room-temperature ionic liquid polymer that serves as a dense, flexible hydrophilic matrix, and (2) a basic zeolite (sodium zeolite-Y (NaY)) that serves as an inexpensive, nucleophilic additive that chemically degrades the CEES as it enters the film. Preliminary FT-IR studies on this new reactive barrier material suggest that the OH groups on the ionic polymer not only facilitates water vapor transport but may also help activate mustard-type vapors for reaction with the imbedded NaY.
Effective individual protection from exposure to toxic chemical warfare agents (CWAs) in vapor form is of great importance in the U.S. military and in civilian defense.1,2 One major class of CWAs are blister agents, such as sulfur mustard (i.e., agent HD) and its analogues (Figure 1). They readily alkylate and
the desired water vapor transport rate range for chemical protective suits.1 Some “breathable” protective garment systems have been developed that provide good rejection of CWAs and varying levels of water vapor permeability. The majority of these are porous fabrics containing reactive or absorptive entities that physically trap and/or chemically degrade CWAs upon contact, while allowing water vapor to pass. Examples of such materials include the semipermeable fabric systems currently used by the U.S. military that utilize activated charcoal or carbon microspheres to adsorb CWAs.1,2 More advanced, reactive CWA barrier materials have also been explored that are based on selectively permeable membranes containing metal oxides,7 oxidation8 and hydrolysis catalysts,9,10 or enzymes to chemically degrade the CWAs.2−4 Several nonreactive/nonabsorptive, breathable CWA barrier materials have also been explored that take advantage of the physical differences (i.e., solubility, molecular size) between CWAs and water. Many of these materials are based on selectively permeable membranes used in water purification, gas separation, and medical applications.2,11 Recently, lyotropic liquid crystal polymers with uniform, molecular-size pores have been developed that can size-separate CWA simulants from smaller water molecules.12,13 Herein, we present a new type of reactive organic/inorganic composite film material with a very high water vapor transport rate and the ability to completely block vapor penetration of the
Figure 1. Sulfur mustard (agent HD), its simulant CEES, and their main mode of reaction with nucleophiles.
cross-link mucus membranes, skin, and DNA via reactions with nucleophilic sites, causing severe and painful tissue damage.3,4 These compounds spontaneously undergo reversible intramolecular cyclization to form a strained cyclic sulfonium intermediate that is extremely reactive to nucleophiles.3,4 Traditional CWA-protective garments based on impermeable, dense polymer materials (e.g., cross-linked butyl rubber and coated fabrics) protect the wearer by completely blocking the passage of all vapors and liquids.5,6 However, they easily lead to heat stress in the wearer, since personal cooling from water evaporation is no longer possible.3,4 The ideal protective garment material should block CWAs while being highly water vapor permeable (i.e., “breathable”) to avoid discomfort and heat stress. 1500−2000 g m−2 day−1 is commonly accepted as © 2012 American Chemical Society
Received: Revised: Accepted: Published: 7453
December 19, 2011 May 15, 2012 May 17, 2012 May 17, 2012 dx.doi.org/10.1021/ie202977e | Ind. Eng. Chem. Res. 2012, 51, 7453−7456
Industrial & Engineering Chemistry Research
Research Note
mustard agent simulant, 2-chloroethyl ethyl sulfide (CEES, halfmustard). This new material is based on a cross-linked (for better film properties) hydrophilic, hydroxylated, room-temperature ionic liquid (RTIL) polymer (1) containing a basic zeolite (sodium zeolite Y (NaY)) (Figure 2). The dense, ionic polymer
Table 1. Water and CEES Vapor Transport Results on Test Filmsa polymer matrix x-linked x-linked x-linked x-linked x-linked
1 1 1 4 4
x-linked 1 x-linked 4
zeolite added none 10 wt % NaY 20 wt % NaY 10 wt % NaY 10 wt % HSAPO-34 10 wt % HSAPO-34 none
H2O vapor flux (g m−2 day−1)
% CEES vapor penetration
6200 1850 2870 0 0
100 0 0 0 50
770
50
660
100
All films were 220 μm thick with a test area of 5.00 cm2 for the water vapor tests and 2.85 cm2 for the CEES vapor tests. a
Figure 2. Synthesis of the cross-linked 1/NaY composite material, and the structures of its components and starting materials.
experiments, films of similar thickness made of (a) cross-linked 1 (no NaY); (b) a nonhydroxylated, more hydrophobic poly(RTIL) (i.e., cross-linked poly(styrylmethylimidazolium bis(trifluoromethyl)sulfonimide) (cross-linked 4)) containing NaY;18 and (c) cross-linked 4 containing an acidic zeolite (HSAPO-34)18 (to confirm that basic and not acidic zeolites are required for effective CEES reactive blocking) were also prepared and tested in the same apparatus (Table 1). The cross-linked 4/zeolite composites were previously explored as light gas separation membranes.18 As can be seen in Table 1, films of cross-linked 1 (no NaY) are highly effective at transporting water vapor but completely lack the ability to block CEES vapor. When 10 wt % NaY is incorporated into cross-linked 1, the resulting composite acts as a dual-mode material that completely blocks CEES vapor while transporting water vapor. Although the water vapor flux drops to about one-third of its original value with 10 wt % added NaY, it is still above the >1500 g m−2 day−1 U.S. military guideline for water vapor breathability in a protective garment.1 Interestingly, increasing the amount of NaY to 20 wt % increases water vapor flux while still retaining complete CEES vapor blocking. When cross-linked 1 is replaced with the more hydrophobic, alkyl-functionalized cross-linked poly(RTIL) 4 (i.e., no OH groups present), the resulting blend with 10 wt % NaY acts as an excellent CEES vapor barrier but exhibits essentially no water vapor flux. This result shows that the OH groups on cross-linked 1 are essential for good water vapor breathability and more important than just having ionic repeat units present. Also, control samples of cross-linked 4 containing 10 wt % H-SAPO-34 (an acidic zeolite) exhibited only 50% CEES vapor penetration and essentially no water vapor transport. This result is consistent with the need for hydroxylated cross-linked 1 and a basic zeolite to both be present to simultaneously provide good water vapor transport and effective CEES vapor blocking. Additional permeation studies on control membranes of cross-linked 1 + 10 wt % HSAPO-34 and cross-linked 4 (no zeolite) also support these conclusions, albeit with a lower than expected water vapor flux for the former and a higher than expected value for the latter. As mentioned before, basic zeolites are known to react with HD and CEES to form a number of reaction products via some sort of hydrolysis, and/or nucleophilic attack on the C−Cl sites by the anionic zeolite framework O atoms.15−17 In our system, polymer 1 introduces two new components (i.e., polymerbound OH and ionic groups) that can potentially participate in these reactions. Unfortunately because of the complex, heterogeneous nature of the cross-linked 1/NaY material,
provides the desired high water vapor breathability, while the imbedded basic zeolite reacts with CEES to prevent it from passing across the film. Initial vapor transport studies performed at the U.S. Army Natick Soldier Research, Development, and Engineering Center showed that thick (220 μm) films of this composite exhibit water vapor fluxes of >1800 g m−2 day−1 and CEES vapor penetration values that are below the detection limits of the test system (i.e., ca. 0%) even after 22 h of continuous CEES exposure. Polymer 1 was recently synthesized by our groups and found to be highly permeable to water vapor in film form under a variety of test conditions.14 It is believed that the diol group on the repeat units of 1 hydrogen-bonds with water vapor on the surface of the polymer, and that water is shuttled across the film by interacting with other diol groups within the polymer. Prior work has shown that basic forms of zeolites (e.g., Na+exchanged zeolite X or Y) react with mustard-type agents to detoxify them.15−17 From product analysis, the degradation of HD and CEES by these basic zeolites has been speculated to be (1) hydrolysis of the C−Cl sites on HD and CEES by residual water molecules or OH groups in the zeolite pores; or (2) nucleophilic attack at the C−Cl sites by the negatively charged zeolite framework O atoms to form zeolite-bound ethyl ethyl sulfide units that can be hydrolyzed off if water is present.16 We thought that by blending together these two very different materials, a new type of breathable, reactive, composite barrier film for blister agent protection could be obtained that may be fabricated into protective overgarments. To show proof-ofconcept for this approach, films of cross-linked 1 containing NaY, as well as several control samples, were prepared and tested for water and CEES vapor permeability. Test films of the cross-linked 1/NaY composite were prepared by mixing appropriate amounts of diol-RTIL monomer 2, cross-linker 3 (5 mol % vs 2), NaY particles, and a small amount of radical photoinitiator. This mixture was then pressed between glass plates, and 2 and 3 were then radically photopolymerized together under UV light to form a binder matrix of cross-linked polymer 1 around the NaY particles (see Figure 2 and the Supporting Information). The heterogeneous composite films made by this process are typically 220 μm thick. These films were then tested separately for water vapor flux and CEES vapor permeation (Table 1) at the U.S. Army Natick Soldier Research, Development, and Engineering Center using a direct moisture permeation cell method (see the Supporting Information). For control 7454
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broad 3400 cm−1 O−H band similar to that of the cross-linked 1 and cross-linked 1/NaY films exposed to CEES. This O−H IR band shift is not observed when just H2O vapor or just dry NaCl (undissolved ionic lattice, no free ions) is added to crosslinked 1, but it is manifested when NaCl and H2O vapor are both present to form solvent-separated ions (see Supporting Information). These results suggest that it is free (solvated) Cl− ions that are interacting with the OH groups on cross-linked 1 via H-bonding, even in the absence of NaY. This implies that when CEES enters the film, the OH groups on cross-linked 1 can coordinate with the free Cl− anions formed by the reversible self-cyclization of CEES, which in turn can shift the initial equilibrium toward the cyclic sulfonium intermediate. When NaY is present, this means that degradation of CEES can be enhanced due to the formation of more reactive cyclic sulfonium intermediate (see Supporting Information for a proposed reaction scheme). Although the mechanisms of how HD and CEES react with basic zeolites have not been fully elucidated,15−17 it is clear that residual H2O or surface OH groups in the zeolites play an important role, since HD and CEES hydrolysis products are detected.15−17 Water has also been found to lower the nucleophilic reactivity of the zeolite framework O anions via H-bonding.17 Our present work shows that added OH moieties can also play other roles if they are bound to a matrix polymer. The polymer-bound OH groups on cross-linked 1 facilitate water vapor transport and may help “activate” HD and CEES for reaction with basic zeolites via H-bonding to the Cl− ions formed in the initial HD or CEES self-cyclization preequilibrium. In summary, a new, highly breathable barrier film material for effectively blocking sulfur mustard-type vapors has been developed that is based on a diol-functionalized poly(RTIL) (cross-linked 1) blended with NaY. The hydroxylated polymer provides high water vapor permeability while the basic zeolite provides reactive blocking of CEES vapor. IR analysis indicated that the OH groups on cross-linked 1 may also help activate mustard-type vapors for reaction with NaY upon entry into the composite film. Clearly, more studies are needed to better understand the mechanism of reactive CEES blocking in this new composite. Studies are also planned to determine the effect of higher NaY loadings on water vapor transport, CEES/water selectivity, and film mechanical properties; the reactive capacity of the cross-linked 1/NaY films; and whether the rate of CEES reaction with NaY is enhanced by the hydroxylated, ionic polymer matrix (since RTILs are known to accelerate certain reactions.19)
detailed mechanistic studies on the activity of this system could not be easily performed. However, we were able to probe the nature of the some of the chemical interactions between crosslinked 1, NaY, and CEES in this composite material using IR analysis. FT-IR spectra of films of cross-linked 1 (no NaY), the crosslinked 1/NaY composite, and several control samples are shown in Figure 3. As can be seen in Figures 3a and 3b a film of
Figure 3. FT-IR spectra of (a) films of cross-linked 1, cross-linked 1 + CEES, and pure CEES; and (b) films of cross-linked 1 + 10 wt % NaY with added CEES, Et2S, and aq NaCl.
cross-linked 1 and a film of cross-linked 1/10 wt % NaY have very similar IR spectra with a relatively weak O−H stretching band centered at ca. 3500 cm−1. This suggests that 1 and NaY do not interact appreciably. However, when a film of crosslinked 1 is exposed to dry CEES vapor, a more intense and broader O−H stretching band appears at ca. 3400 cm−1. The same thing happens when a film of cross-linked 1/NaY is exposed to CEES vapor (Figure 3b). In both cases, this O−H stretching band wavenumber shift can be attributed to weakening of the O−H bond via increased H-bonding with CEES or a reaction product of CEES. To test this supposition, cross-linked 1/NaY films were combined with diethylsulfide (to mimic the thioether moiety on CEES), and separately with added aq NaCl (to mimic the free Cl− released during CEES self-cyclization and CEES degradation by NaY). As can be seen in Figure 3b, the addition of diethylsulfide results in no change in the IR spectra compared to a pristine 1/NaY film. However, the cross-linked 1/NaY film containing added aq NaCl shows a
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ASSOCIATED CONTENT
S Supporting Information *
Details on the synthesis and characterization of NaY, the monomers, and the cross-linked polymer/zeolite composite films; procedures for membrane film fabrication, water and CEES vapor transport testing, and control experiments. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. 7455
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Author Contributions
(15) Bellamy, A. J. Reaction of Gaseous Sulfur Mustard with Zeolite 13X. J. Chem. Soc., Perkin Trans. 2 1994, 2325. (16) Wagner, G. W.; Bartram, P. W. Reactions of VX, HD, and Their Simulants with NaY and AgY Zeolites. Desulfurization of VX on AgY. Langmuir 1999, 15, 8113. (17) Kanyi, C. W.; Doetschman, D. C.; Shutle, J. T. Nucleophilic Chemistry of X-type Faujasite Zeolites with 2-Chloroethyl Ethyl Sulfide (CEES), a Simulant of Common Mustard Gas. Microporous Mesporous Mater. 2009, 124, 232. (18) Hudiono, Y. C.; Carlisle, T. K.; LaFrate, A. L.; Gin, D. L.; Noble, R. D. Novel Mixed Matrix Membranes Based on Polymerizable RoomTemperature Ionic Liquids and SAPO-34 Particles to Improve CO2 Separation. J. Membr. Sci. 2011, 370, 141. (19) Lee, J. W.; Shin, J. Y.; Chun, Y. S.; Jang, H. B.; Song, C. E.; Lee, S.-G. Toward Understanding the Origin of Positive Effects of Ionic Liquids on Catalysis: Formation of More Reactive Catalysts and Stabilization of Reactive Intermediates and Transition States in Ionic Liquids. Acc. Chem. Res. 2010, 43, 985.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the U.S. Defense Threat Reduction Agency (HDTRA1-08-1-0028) and the U.S. Army Research Office (AB07CBT010) for funding this research.
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REFERENCES
(1) In Strategies to Protect the Health of Deployed U.S. Forces: Force Protection and Decontamination; Wartell, M. A., Kleinman, M. T., Huey, B. H., Duffy, L. M., Eds.; National Academy Press: Washington, DC, 1999. (2) For a recent review of chemical and biological warfare agent protective garment materials and needs, see Schreuder-Gibson, H. L.; Truong, Q.; Walker, J. E.; Owens, J. R.; Wander, J. D.; Jones, W. E., Jr. Chemical and Biological Protection and Detection in Fabrics for Protective Clothing. Mater. Res. Soc. Bull. 2003, 28, 574. (3) For recent reviews of CWAs and their chemistry, see (a) Yang, Y. C.; Baker, J. A.; Ward, J. R. Decontamination of Chemical Warfare Agents. Chem. Rev. 1992, 92, 1729. (b) Yang, Y. C. Chemical Reactions for Neutralizing Chemical Warfare Agents. Chem. Ind. 1995, 334. (c) Talmage, S. S.; Watson, A. P.; Hauschild, V.; Munro, N. B.; King, J. Chemical Warfare Agent Degradation and Decontamination. Curr. Org. Chem. 2007, 11, 28. (4) For a recent review of sulfur mustard, see: Kehe, K.; Balszuweit, F.; Steinritz, D.; Theirmann, H. Molecular Toxicology of Sulfur Mustard-Induced Cutaneous Inflammation and Blistering. Toxicology 2009, 263, 12. (5) Nagano, H. In Exxon Butyl Rubber Compounding and Applications; Exxon Chemical, Japan Polymers Technical Center: Yokohama, Japan, 2001; pp 1−9. (6) Abbott, N. J. In Coatings Technology Handbook, 2nd ed.; Satas, D., Tracton, A. A., Eds.; Marcel Dekker: New York, 2001; pp 819−823. (7) Wagner, G. W.; Bartrum, P. W.; Koper, O.; Klabunde, K. J. Reactions of VX, GD, and HD with Nanosize MgO. J. Phys. Chem. B 1999, 103, 325. (8) Gall, R. D.; Hill, C. L.; Walker, J. E. Selective Oxidation of Thioether Mustard (HD) Analogs by tert-Butylhydroperoxide Catalyzed by H5PV2Mo10O40 Supported on Porous Carbon Materials. J. Catal. 1996, 159, 47. (9) Schreuder, H. L.; Gibson, P.; Senecal, K.; Sennett, M.; Walker, J.; Yeomans, W.; Ziegler, D.; Tsai, P. T. Protective Textile Materials Based on Electrospun Nanofibers. J. Adv. Mater. 2002, 34, 44. (10) Hammond, P. S.; Forester, J. S.; Lieske, C. N.; Durst, H. D. Hydrolysis of Toxic Organophosphorus Compounds by o-Iodosobenzoic Acid and Its Derivatives. J. Am. Chem. Soc. 1989, 111, 7860. (11) Rivin, D.; Meermeier, G.; Schneider, N. S.; Vishnyakov, A.; Neimark, A. V. Simultaneous Transport of Water and Organic Molecules Through Polyelectrolyte Membranes. J. Phys. Chem. B 2004, 108, 8900. (12) Lu, X.; Nguyen, V.; Zhou, M.; Zeng, X.; Jin, J.; Elliott, B. J.; Gin, D. L. Crosslinked Bicontinuous Cubic Lyotropic Liquid-Crystal/ButylRubber Composites: Highly Selective, Breathable Barrier Materials for Chemical Agent Protection. Adv. Mater. 2006, 18, 3294. (13) Lu, X.; Nguyen, V.; Zeng, X.; Elliott, B. J.; Gin, D. L. Selective Rejection of a Water-Soluble Nerve Agent Simulant Using a Nanoporous Lyotropic Liquid Crystal−Butyl Rubber Vapor Barrier Material: Evidence for a Molecular Size-Discrimination Mechanism. J. Membr. Sci. 2008, 318, 397. (14) LaFrate, A. L.; Gin, D. L.; Noble, R. D. High Water Vapor Flux Membranes Based on Novel Diol−Imidazolium Polymers. Ind. Eng. Chem. Res. 2010, 49, 11914. 7456
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