Letter Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6820−6824
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MOFwich: Sandwiched Metal−Organic Framework-Containing Mixed Matrix Composites for Chemical Warfare Agent Removal Gregory W. Peterson,*,†,‡ Annie X. Lu,∥ Morgan G. Hall,† Matthew A. Browe,† Trenton Tovar,† and Thomas H. Epps, III*,‡,§ †
Edgewood Chemical Biological Center, 8198 Blackhawk Road, Building 3549, Aberdeen Proving Ground, Maryland 21010, United States ‡ Department of Materials Science and Engineering and §Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ∥ Defense Threat Reduction Agency, 8228 Scully Road, Aberdeen Proving Ground, Maryland 21010, United States S Supporting Information *
ABSTRACT: This work describes a new strategy for fabricating mixed matrix composites containing layered metal−organic framework (MOF)/polymer films as functional barriers for chemical warfare agent protection. Through the use of mechanically robust polymers as the top and bottom encasing layers, a high-MOF-loading, highperformance-core layer can be sandwiched within. We term this multifunctional composite “MOFwich”. We found that the use of elastomeric encasing layers enabled core layer reformation after breakage, an important feature for composites and membranes alike. The incorporation of MOFs into the core layer led to enhanced removal of chemical warfare agents while simultaneously promoting moisture vapor transport through the composite, showcasing the promise of these composites for protection applications. KEYWORDS: mixed matrix composite, multilayers, metal−organic framework, chemical warfare agent, separations, block copolymer, elastomer
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Traditional MMMs are generally fabricated using one of two techniques: (1) a filler particle (e.g., carbon, zeolite, MOF, etc.) is mixed with a polymer and cast as a free-standing film, or (2) a polymer or inorganic phase is used to grow the MOF as a secondary layer (i.e., templated growth).16 The former method permits the use of a broad range of MOFs and polymer substrates, as well as provides the ability to scale up MOFs before membrane formation. The latter allows less versatility for the type of MOF used, but generally results in more conformal membranes. Most recent MMM research incorporating MOFs has focused on carbon dioxide (or other gas) separations and sequestration10,13,17,18 with potential applicability to other areas.19,20 To date, one application that has not been investigated using MMMs and similar composites is the reactive removal of chemical warfare agents (CWAs). Recently, Cohen and co-workers utilized the underlying concept of MMMs to produce composites with significantly higher MOF loadings than traditional examples. These mixed matrix composites (MMCs) incorporated MOFs into singlelayer freestanding films based on poly(vinylidene fluoride), PVDF, as well as styrene/butadiene (SBS, SBR) copoly-
ver the past decade, metal−organic frameworks (MOFs) have become a leading class of porous materials for applications in separations,1 air purification,2 catalysis,3 and sensing.4 The ability to tune functionality and pore structure of MOFs enables unprecedented control at the nanoscale,5 which has translated to exciting new properties at the macroscale.6 The development of successful MOF-based technologies depends on not only scaling-related issues, but also on the ability to incorporate these highly active assemblies into industry-relevant engineered constructs such as granules and membranes.7,8 One example of a mature technology, which is most similar to our composites that are described below, is mixed matrix membranes (MMMs). MMMs have previously been prepared using zeolites,9 metal oxides,10 and carbons11 as active fillers for applications in filtration and separations. MMMs also have been fabricated containing MOFs and have demonstrated unique reactive capabilities.7,12,13 Generally speaking, however, MMMs have been limited to low MOF mass loadings, as higher loadings result in catastrophic defects and brittleness.7,14 Furthermore, MMMs containing MOFs have been limited to single component systems13 and therefore lack versatility and multifunctionality. The combination of tailorable MOFs with tunable polymers, and especially block copolymers, enables the optimization of composite materials for specific applications.15 © 2018 American Chemical Society
Received: December 20, 2017 Accepted: February 5, 2018 Published: February 5, 2018 6820
DOI: 10.1021/acsami.7b19365 ACS Appl. Mater. Interfaces 2018, 10, 6820−6824
Letter
ACS Applied Materials & Interfaces mers.14,20 In these cases, the main thrust was to investigate a new form factor for MOFs with a focus on high mass loadings. Although the strategy was able to incorporate MOFs, the composites generally became fragile at appreciable MOF loadings. Ideally, films should be developed that can accommodate high MOF loadings while maintaining structural stability. To overcome the above limitation, herein we present a method and several archetypes for preparing multifunctional composites using a high MOF-loaded, otherwise brittle, “active layer” sandwiched between low MOF-loaded flexible polymer “encasing layers” to stabilize the composite. Our method facilitates the tuning of functionality through the incorporation of multiple polymers, MOFs, and MOF loadings, with the novelty showcased through the ability to tune each parameter to meet specific application requirements. Such requirements include permeation and reaction rates, adsorption capacity, and moisture vapor transport rate (MVTR). As shown in Figure 1, a MOF/polymer film was cast using a draw-down coating method.20 Before the film completely dried, a second layer was drawn across the first layer. The solvent locally swelled or dissolved polymer chains, fusing the layers together, and the resulting layered structure could not be mechanically separated. This approach was repeated any number of times using a broad range of compositions to create multilayered MMCs. For added robustness of composites, we focused primarily on rubbery (elastomeric) encasing polymers. Fabricated composites were characterized using techniques including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD) and nitrogen adsorption. The performance of MMCs as barriers against CWAs and simulants was evaluated through permeation experiments, and the reactivity of various composites was monitored using 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. MVTR measurements were conducted to determine the permeability of each membrane to moisture. Composites also were characterized for other applications, such as sensors, using gas adsorption isotherms and electrical impedance spectroscopy (EIS). Experimental details are provided in the Supporting Information. Our multilayer approach was first demonstrated using a polystyrene-block-polyisoprene-block-polystyrene (SIS) copolymer incorporating either HKUST-1 or UiO-66-NH2 MOFs in the encasing or active layers, with the middle (active layer) containing a higher MOF loading. SIS is a well-known elastomer that provides an excellent combination of strength and elasticity, and is readily processable. The two MOFs were chosen as they represent two of the most studied materials in the literature and also provide significant protection against toxic chemicals such as CWAs,21 ammonia,22 chlorine,23 and nitrogen dioxide.24 Furthermore, HKUST-1 and UiO-66-NH2 are produced in sufficient quantities such that structure-activityprocessing relationships can be readily studied at scales larger than typically available for most MOFs synthesized in subgram quantities. These two initial systems are designated as SIS/ UiO-66-NH2(20%) + SIS/HKUST-1(67%) + SIS/UiO-66NH2(20%), referred to as MOFwich 1 (M1), and SIS/HKUST1(20%) + SIS/UiO-66-NH2(67%) + SIS/HKUST-1(20%) (M2). The details of these composites as well as a much wider array of configurations, chemistries, and constructs, are described in Table S1. The percentages represent the wt % MOF within each layer of the MOFwich. SEM and EDS images
confirmed the delineation of each layer (Figure 2A, C, and Figure S1) in the MMC. The low-MOF-loaded outer layers formed protective borders around the high-MOF-loaded layers, which otherwise cracked without the encasing layer. To exhibit the versatility of the method, a two-layer approach also was used to create a MOFwich consisting of high MOFloaded SEBS. Like SIS, SEBS is easily processed, viscoelastic, and strong. The resulting composite, SEBS/HKUST-1(50%) + SEBS/UiO-66-NH2(50%), is referred to as M3. M3 exhibited a well-defined two-layer structure as shown in Figures 2B, D. Nitrogen uptake data are shown in Figure S2, and surface area calculations are shown in Table S2. Many of the MOFwiches had considerable surface area unlike other composites with relatively low MOF percentages;14 however, all layered MMCs had significantly lower surface area in comparison to the pristine MOFs. The relatively low surface area is typical of MMCs, for which nitrogen is unable to penetrate deeply into the composites unless high MOF loadings are used.14 We note that our MOFwiches are not true MMMs as surface defects may increase nitrogen access to the MOF and govern permeation behavior; yet, it is possible to seal an active layer with high MOF loading with a low (
