Research Article www.acsami.org
Self-Decontaminating Fibrous Materials Reactive toward Chemical Threats Lev Bromberg,† Xiao Su,† Vladimir Martis,‡ Yunfei Zhang,† and T. Alan Hatton*,† †
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Surface Measurement Systems, Ltd., Unit 5, Wharfside, Rosemont Road, Alperton, London HA0 4PE, United Kingdom
‡
S Supporting Information *
ABSTRACT: Polymers that possess highly nucleophilic pyrrolidinopyridine (Pyr) and primary amino (vinylamine, VAm) groups were prepared by free-radical copolymerization of N,N-diallylpyridin-4-amine (DAAP) and N-vinylformamide (NVF) followed by acidic hydrolysis of NVF into VAm. The resulting poly(DAAPco-VAm-co-NVF) copolymers were water-soluble and reacted with waterdispersible polyurethane possessing a high content of unreacted isocyanate groups. Spray-coating of the nylon−cotton (NYCO), rayon, and poly(p-phenylene terephthalamide) (Kevlar 119) fibers pretreated with phosphoric acid resulted in covalent bonding of the polyurethane with the hydroxyl groups on the fiber surface. A second spray-coating of aqueous solutions of poly(DAAP-co-VAm-co-NVF) on the polyurethane-coated fiber enabled formation of urea linkages between unreacted isocyanate groups of the polyurethane layer and the amino groups of poly(DAAP-co-VAm-co-NVF). Fibers with poly(DAAP-co-VAm-co-NVF) attached were compared with fibers modified by adsorption of water-insoluble poly(butadiene-co-pyrrolidinopyridine) (polyBPP) in terms of the stability against polymer leaching in aqueous washing applications. While the fibers modified by attachment of poly(DAAP-co-VAm-co-NVF) exhibited negligible polymer leaching, over 65% of adsorbed polyBPP detached and leached from the fibers within 7 days. Rayon fibers modified by poly(DAAP-co-VAm-co-NVF) were tested for sorption of dimethyl methylphosphonate (DMMP) in the presence of moisture using dynamic vapor sorption technique. Capability of the fibers modified with poly(DAAP-co-VAm-co-NVF) to facilitate hydrolysis of the sorbed DMMP in the presence of moisture was uncovered. The self-decontaminating property of the modified fibers against chemical threats was tested using a CWA simulant diisopropylfluorophosphate (DFP) in aqueous media at pH 8.7. Fibers modified with poly(DAAPco-VAm-co-NVF) facilitated hydrolysis of DFP with the half-lives up to an order of magnitude shorter than that of the unmodified fibers. The presented polymers and method of multilayer coating can lead to a development of self-decontaminating textiles and other materials. KEYWORDS: self-decontaminating material, cellulosic and polyamide fibers, pyrrolidinopyridine, isocyanate, organophosphate hydrolysis
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low toxicity by itself, and perform in all weather conditions.10 Incorporation of reactive functionality into an SDM requires careful design that counterbalances mechanical robustness of the material with reactivity that calls for the initial sorption of the chemical threat, the presence of sufficient local concentration of water as a reaction medium,20 and the appearance of the reaction products capable of poisoning the reactive and catalytic sites. Traditional adsorbents, while effective at removing chemical agents, do not possess sufficient reactive properties to neutralize the adsorbed toxic agents and thus become a threat from the off-gassing toxins and/or sorbed vapors. Polymeric decontaminants are advantageous over reactive particle dispersions21−23 in terms of the ease of the polymer incorporation into engineered surfaces via conven-
INTRODUCTION Surface modification that enables a material’s resistance toward contamination through reactivity or resistance to a threat results in self-decontaminating material (SDM). The present work focuses on SDM resistant to chemical rather than biological threats. A great number of SDM studies have been dedicated to combating biothreats.1−4 Reactive SDM capable of degrading chemical threats such as chemical warfare agents (CWAs), hazardous industrial chemicals, and pesticides include polymeric coatings, titania suspensions, heterogeneous catalysts, metal−organic frameworks and metal/metal oxide particles, quaternary ammonium salts, polyoxometalates, Nhalamines, light-activated compounds, and liquid formulations containing peroxides, enzymes, and various nucleophiles.5−19 These materials are often tailored to degrade organophosphate esters via hydrolysis and sulfur mustards via dehydrochlorination and analogous reactions.10,18 An SDM agent should act rapidly, produce byproducts of low toxicity, possess relatively © 2016 American Chemical Society
Received: May 4, 2016 Accepted: June 16, 2016 Published: June 16, 2016 17555
DOI: 10.1021/acsami.6b05241 ACS Appl. Mater. Interfaces 2016, 8, 17555−17564
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tional techniques such as solution impregnation, grafting, and cross-linking as well as stability of the resulting SDM and lack of the polymer leaching. We have developed efficient polymeric reactive decontaminants that sorb water and react with neurotoxic and mustard CWAs and their simulants to yield less toxic products.17,18,24 High contents of very reactive groups such as amidoximate, oximate, dialkylaminopyridine, and bromosuccinimide, among others, in these polymers present a challenge for the polymer binding with the protective clothing layers, membranes, and other engineered SDMs where these polymers are intended for application. In the present work, we aimed at covalent attachment, while maintaining reactivity and catalytic properties, of the most nucleophilic yet hydrophilic polymers onto the surfaces of fibrous materials that are common in nonwoven and woven textiles: cotton, rayon (regenerated cellulose), and polyamides. The latter were represented by nylon-6,6 (blended with cotton fiber) and poly(p-phenylene terephthalamide) (Kevlar 119). Cellulosic materials possess numerous −OH groups on their surfaces that can be readily modified by isocyanates via formation of amide (urethane) bonds. Coating and lamination of textiles with cellulosic and other hydroxyl-containing surfaces by isocyanate adhesives that form polyurethanes is welldeveloped.25 However, covalent attachment of polymeric hypernucleophilic catalysts to the engineered surfaces has not been reported, even in paint formulations. Thus far, such catalysts have been simply impregnated into fabrics.26 Recently, we developed polymers wherein highly nucleophilic alkylaminopyridine and oximate groups were utilized to modify watersoluble polyamines, significantly augmenting the ability of the polyamines to catalyze hydrolysis of CWA at a range of pH 0.97 in all cases, Figure 12). The DFP hydrolysis in the presence of fibers at pD 8.7 was also a first-order reaction (Figure 13). In the kinetic experiments, the only phosphorus-containing product of the DFP hydrolysis was diisopropylphosphate, and pertaining 31P NMR spectra of the products are described in detail in our previous works.23,49 During the reaction, the
As is seen in Figure 9, both original rayon fibers and surfacemodified fibers exhibited sigmoidal DMMP sorption isotherms and sorbed 1−1.2 wt % DMMA at saturated DMMP vapor pressure. However, the desorption isotherms showed up to 0.4 wt % hysteresis for the original fibers and up to 0.9 wt % hysteresis with modified fibers. The modified fibers retained ∼0.3 wt % DMMP at PDMMP = 0, indicating that an irreversible chemical reaction occurred. Original fibers released all sorbed DMMP, suggesting reversible physisorption. To confirm reactivity of the modified fibers with DMMP, we conducted extraction of the products from the fiber surface (Figure 10). The fibers had been exposed to 100% humidity prior to the DMMP vapor sorption as described in the Experimental Section. DMMP extracted from the unmodified fibers exhibited its original signal at 33 ppm, whereas the organophosphate 17560
DOI: 10.1021/acsami.6b05241 ACS Appl. Mater. Interfaces 2016, 8, 17555−17564
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parameters are collected in Table 1. It appears that poly(DAAP-co-VAm-co-NVF) facilitated complete hydrolysis of DFP within less than 20 min, and k″N for batch 2 that contained 40 mol % Pyr groups was an order of magnitude higher than that of its unmodified polyamine analogues, poly(vinyl amine) and polyallylamine,18 and higher than that with electrospun nanofibers composed of α-nucleophilic polyacrylamidoxime.49 The rate constants observed for poly(DAAP-co-VAm-co-NVF) were comparable to those for the supernucleophilic copolymers such as polyBPP and αnucleophilic 4-pyridinealdoxime-modified polyvinylamine (PVAm-Ox).18 However, the latter polymers cannot be covalently attached to the engineered surfaces either because of the lack of appropriate chemical groups (polyBPP) or the fact that the nucleophilic oxime groups would react with isocyanate at the same or even higher rate than the primary amine, thus eliminating the catalytic capabilities of the PVAmOx polymer. Hence, it appears that the poly(DAAP-co-VAm-coNVF) copolymers were extremely well-suited for the fiber modification. Modification of the fibers by nucleophilic polymers accelerated the hydrolysis in all cases, proving the selfdecontaminating capability of the polymer-modified materials. However, the apparent rates of the nucleophilic hydrolysis, k″N, were considerably smaller with the modified fibers than in the solutions or suspensions of the corresponding polymers (Table 1). This observation can be rationalized by the use of nominal Ccat measured as a total concentration of the Pyr groups in the heterogeneous fiber suspension in the buffer solution. However, not all of the Pyr groups of the polymer adsorbed or attached to the fiber surfaces can be accessible for DFP immediately after the reaction commencement, leading to the overestimation of the effective Ccat and underestimation of k″N. To enhance the accessibility of the Pyr groups in NYCO fibers covalently modified with poly(DAAP-co-VAm-co-NVF), we conducted an additional series of measurements wherein the modified fibers were allowed to equilibrate at room temperature in the 50 mM AMPD/D2O buffer at pH 8.7 for 48 h prior to the addition of DFP and hydrolysis reaction commencement. NYCO fibers modified with DDN and poly(DAAP-co-VAm-co-NVF), batches 1 and 2, yielded τ1/2 and k″N of 85 min and 7.1 M−1 min−1 and 81 min and 7.5 M−1 min−1, respectively, which corresponded well with the same parameters obtained for the poly(DAAP-co-VAm-co-NVF) solutions (Table 1) and confirmed the accessibility of the catalytic groups on the modified surfaces of the modified fibers to the water-soluble substrate, DFP. Furthermore, we conducted a series of measurements wherein the NYCO fibers modified with DDN and poly(DAAP-co-VAm-co-NVF), batch 1, that had been exposed to an aqueous solution containing 0.5 wt % Na2CO3 and 0.5 wt % Triton X-100 surfactant at room temperature for 7 days were tested for their ability to hydrolyze DFP. The washing procedure did not alter the fibers’ ability to hydrolyze the organophosphate (washed fibers, Table 1), further confirming the stability of the polymer attachment.
Figure 12. DFP hydrolysis catalyzed by poly(DAAP-co-VAm-co-NVF) solutions and polyBPP suspension in 50 mM AMPD aqueous buffer. DFP, 5 mM; effective polymer concentration in D2O, 10 mM; pD 8.7.
Figure 13. Kinetics of DFP hydrolysis catalyzed by NYCO fibers (control) and NYCO fibers modified by polyBPP and poly(DAAP-coVAm-co-NVF) (batch 1 and 2) polymers. Reaction medium: 50 mM AMPD D2O buffer, pD 8.7; DFP, 5 mM. NYCO fibers containing attached poly(DAAP-co-VAm-co-NVF) contained ∼0.15 and 0.3 mmol/g Pyr catalytic groups when treated with batches 1 and 2 of this copolymer, respectively. Effective Pyr group content in the polyBPP-treated fibers was ∼0.4 mmol/g. Effective Pyr concentration in D2O was set by varying total weight of fibers added such that an effective 1 mM Pyr concentration in water ensued.
nucleophilic Pyr attacks DFP and cleaves the P−F bond (Figure 14).
Figure 14. Nucleophilic hydrolysis of diisopropyl fluorophosphate (DFP) by a pyrrolidinopyridine catalyst in the presence of water.
The hydrolysis reaction was also first-order with respect to the initial Pyr concentration in water (Ccat). It was of interest to compare the rates of DFP hydrolysis by the poly(DAAP-coVAm-co-NVF) polymers introduced in the present work to other nucleophilic polymers reported previously.18 The observed DFP hydrolysis rate constant measured in our experiments, kobs = k0 + k″NCcat, is the sum of the spontaneous and nucleophilic rate constants; Ccat is the total effective concentration of the catalytic (Pyr) groups in aqueous solutions or suspensions, which was set at 10 mM (Figure 12) and 1 mM (Figure 13). The measured τ 1/2 = ln(2)/k obs and k″ N
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CONCLUDING REMARKS Utilization of polyurethanes in the coatings, paints, adhesives, sealants, elastomers, and thermoplastics meets the needs of multiple markets. Polyurethanes, particularly those used in automotive or construction paints, are capable of absorbing chemical agents. However, polyurethanes can be made resistant to chemical threats including CWA. Thus far, efforts have 17561
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Table 1. Half-Life (τ1/2), Observed Rate Constant (kobs), and Second-Order Rate Constant of Diisopropylfluorophosphate (DFP) Nucleophilic Hydrolysis (k″N) in 50 mM Aqueous 2-Amino-2-methyl-1,3-propanediol (AMPD) Buffer at pH 8.7 and 25 °C polymer or fiber species
τ1/2 (min)
k″N = (kobs − kcontrol)/Ccat (M−1min−1)a
control (AMPD buffer only) polyBPPb poly(DAAP-co-VAm-co-NVF), batch 1c poly(DAAP-co-VAm-co-NVF), batch 2d NYCO fibers in buffer (control) NYCO fibers mod. polyBPP NYCO fibers mod. poly(DAAP-co-VAm-co-NVF), batch 1 NYCO fibers mod. poly(DAAP-co-VAm-co-NVF), batch 2 NYCO fibers mod. poly(DAAP-co-VAm-co-NVF), batch 1, washed fibers. rayon fibers in buffer (control) rayon fibers, mod. polyBPP rayon fibers mod poly(DAAP-co-VAm-co-NVF), batch 1 rayon fibers mod poly(DAAP-co-VAm-co-NVF), batch 2 Kevlar fibers in buffer (control) Kevlar fibers, mod. polyBPP Kevlar fibers, mod. poly(DAAP-co-VAm-co-NVF), batch 1 Kevlar fibers, mod. poly(DAAP-co-VAm-co-NVF), batch 2
640 12 15 9 670 315 170 154 150 690 196 330 137 630 240 170 130
n/a 5.6 4.5 7.3 n/a 1.2 3.1 3.5 3.5 n/a 2.5 1.1 4.0 n/a 1.8 3.0 4.2
a Initial polymer concentrations (Ccat) in poly(DAAP-co-VAm-co-NVF) solutions and polyBPP suspensions are 10 mM (on the monomer basis); in modified fiber suspensions, Ccat is taken as effective Pyr concentration set at 1 mM. Initial DFP concentrations are 5 mM throughout. bAverage MW of the monomeric unit is 214 g/mol. cAverage MW of the monomeric unit is 71 g/mol. dAverage MW of the monomeric unit is 101 g/mol.
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ACKNOWLEDGMENTS This work was supported by the Defense Threat Reduction Agency.
focused to create polyurethanes resistant to chemical threats via incorporation of hydrophobic and impermeable components or by postsynthetic modification of painted or coated surfaces by methods such as chemical vapor deposition.42 In the present work, we undertook a different approach that included creating an amphiphilic polyurethane layer that served as a link between the surface of fibrous materials and a hydrophilic and supernucleophilic polymer that was specifically designed to be covalently bound to the polyurethane yet maintain accessible catalytic groups on the fiber surface. It appears that the newly designed poly(DAAP-co-VAm-co-NVF) copolymers attached to the fibers resulted in efficient self-decontaminating fibrous materials capable of hydrolyzing chemical threats in the presence of water. The method of deposition of the catalytic layers presented in this work is straightforward and has a potential for implementation in industry.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05241. Details of synthesis of DAAP and terpolymer, 1H NMR spectra of PVAm and poly(DAAP-co-VAm-co-NVF), typical kinetics of mass change of rayon fibers upon vapor sorption, FTIR spectrum of Demodur DN, SEM images of rayon and NYCO fibers, coated and uncoated, SEM images of Kevlar 119 fibers, and FTIR spectra of Kevlar fibers (original, treated with H3PO4, and coated with polymers) (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 17562
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