Polymer Mixed Matrix

Letter www.acsami.org

Transparent Metal−Organic Framework/Polymer Mixed Matrix Membranes as Water Vapor Barriers Youn Jue Bae,†,‡,§ Eun Seon Cho,†,§ Fen Qiu,§ Daniel T. Sun,⊥ Teresa E. Williams,§,|| Jeffrey J. Urban,*,§ and Wendy L. Queen*,⊥ ‡

Department of Chemistry and ||Graduate Group in Applied Science & Technology, University of California, Berkeley, California 94720, United States § The Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL) CH 1051 Sion, Switzerland S Supporting Information *

ABSTRACT: Preventing the permeation of reactive molecules into electronic devices or photovoltaic modules is of great importance to ensure their life span and reliability. This work is focused on the formation of highly functioning barrier films based on nanocrystals (NCs) of a water-scavenging metal−organic framework (MOF) and a hydrophobic cyclic olefin copolymer (COC) to overcome the current limitations. Water vapor transmission rates (WVTR) of the films reveal a 10-fold enhancement in the WVTR compared to the substrate while maintaining outstanding transparency over most of the visible and solar spectrum, a necessary condition for integration with optoelectronic devices. KEYWORDS: metal−organic framework, postsynthetic modification, water vapor transmission rate, mixed-matrix membrane gas barrier, encapsulant

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large-scale fabrication approaches. Herein we present a new and facile methodology to produce highly efficient barrier films that are cost-effective, mechanically stable, transparent films. These films are composed of a hydrophobic cyclic olefin copolymer (COC), which displays excellent light transmission and decent intrinsic barrier properties, incorporated with specially designed metal−organic framework (MOF) NCs, optimized for water scavenging. To date, there are only a few examples of MOF encapsulated barriers10 and to the best of our knowledge, none of those have addressed advanced packaging challenges, such as PV applications. In these applications, multiple modalities (such as light transmissivity and water impermeability) must simultaneously be increased, which introduces additional complexity. Thus, this report opens up a new avenue for multifunctional designer MOF-based membranes. This COCMOF mixed matrix membrane was prepared using a doctor blade technique which is a widely used in industry; the asprepared membrane films result in 10.5-fold enhanced water vapor blocking, compared to that of the bare PET (poly(ethylene terephthalate)) substrate, while also retaining 94% visible light transmission.

ontrolling the permeation of gas molecules into materials is of great importance in applications ranging from gas separation and storage and protective coatings to packaging science.1−3 In particular, preventing the ingress of reactive species such as water vapor is critical in wide-ranging packaging applications, such as preservation of food quality or coatings used to ensure the life span and reliability of electronic devices and photovoltaic (PV) cells.4−6 One challenge for fabricating barrier coatings for electronic coatings and photovoltaic modules, in particular, is simultaneously achieving high light transmittance and mechanical stability, all while maintaining limited permeability of the reactive species into the device. There have been several inorganic thin films proposed such as silicone oxides and metal oxides that satisfy some of the aforementioned criteria, however, their implementation in large scale manufacturing processes is limited by their poor mechanical flexibility and complicated and expensive methods required for deposition.7,8 Mixed-matrix membranes (MMMs) based on polymer matrices are cost-effective and offer easy processability, enabling large-scale production while also introducing desirable properties such as high mechanical flexibility and excellent adhesion under thermal stress.9 Previous studies have shown that highly efficient barrier films can be produced with the incorporation of water-scavenging nanocrystals (NCs) into polymers.5 However, the overall performance was still not exceptional and limited by lack of © XXXX American Chemical Society

Received: January 31, 2016 Accepted: April 13, 2016

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DOI: 10.1021/acsami.6b01299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Illustrative cartoon of a mixed-matrix membrane film comprised of the COC polymer matrix and the water scavenging MOF, LEZr6O4(OH)4(fumarate)6; the chemical structure of COC polymer is depicted in the bottom-left and the crystal structure of Zr6O4(OH)4(fumarate)6 is presented on the right, displaying a hydrogen−bonding network between water molecules and OH groups on the surface. The SEM images of assynthesized MOF NCs (b) 25 ± 5 nm size and (c) 300 ± 45 nm (the insets present the histogram of each size distribution.), and the images of the ligand-exchanged NCs with size of (d) 25 ± 5 nm and (e) 300 ± 45 nm. The inset shows the picture of MOF NCs dispersed in cyclohexane before (left) and after (right) ligand exchange.

MOFs are an emerging class of porous, crystalline materials that have quickly garnered great attention in the chemical sciences because of their unprecedented internal surface areas, chemical tunability, and strong, selectively binding of a large number of small guest molecules.11−13 As such, MOFs, which are constructed by metal-ions or metal ion clusters that are interlinked by organic ligands, are under intense investigation for a variety of applications coupled to gas separations and storage, catalysis, and drug delivery.11,14−16 Through judicious selection of the ligand and metal, which control pore size/shape and MOF−guest interactions, their molecular uptake properties, such as selectivity between gases, can be tuned.12 The assembly of predesigned building blocks is a powerful tool for the development of new solid materials because it yields architectures not only with regular, well-defined crystalline structures, but also with tailored functionalities on their internal surface to achieve specified function. In addition to their internal structure, the chemical functionality on the exterior of the MOF crystallites can be varied, offering an easy methodology to interface these materials with chemically dissimilar systems.17,18 In these ways MOFs, relative to their all-inorganic counterparts, offer unrivaled design opportunities to optimize efficiencies for a variety of environmentally relevant applications. The MOF selected for incorporation into the membrane films is Zr6O4(OH)4(fumarate)6,19−21 (alternatively known as MOF-80122) a framework that consists of 6-connected Zr6

secondary building units that are interlinked by ditopic fumarate ligands. The Zr(IV) ions, found in a squareantiprismatic geometry, are bridged by μ3-O, μ3-OH, and carboxylate groups.15 Along with the presence of surface appended hydroxyl groups, the MOF contains two crystallographically distinct tetrahedral cavities (size of 5.6 and 4.8 Å) and one octahedral cavity (size of 7.4 Å). The hydroxyl functionalization and the size and shape of the aforementioned cavities create a prime environment for water adsorption, even at low pressures.22 For example, previous work has used a combination of neutron and X-ray diffraction to unveil the presence of multiple hydrogen bonding interactions between the surface appended hydroxyl groups and guest water molecules (Figure 1a), demonstrating that these structural features lend to the frameworks excellent water adsorption behavior.22 Given this, we report a new synthetic method to incorporate hydrophilic Zr6O4(OH)4(fumarate)6 NCs into a hydrophobic polymer. Highly transparent mixed-matrix membranes based on these water-scavenging MOF NCs and a hydrophobic polymer are exploited as advanced barrier layers. To understand the effect of MOF NC size on the water scavenging and light transmission properties, we developed a synthesis to yield both small and large MOF NC varying by ∼1 order of magnitude in size. In gas separation membranes and barriers, NC size is known to impact the transport properties of films, further, the NC themselves may have altered gas sorption behavior as the surface to volume ratio is changed.23 In aqueous B


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modification of MOFs, which utilized more synthetically complicated moieties such as polymers, silica, and lipids and report significant changes in gas adsorption properties,17,18 the use of oleic acid simplified our synthetic protocol for film production all while preserving the desired water scavenging characteristics of the MOF. Because oleic acid has a higher pKa (9.85), it is expected to bind more strongly to the zirconium cluster than fumaric acid, which has a significantly lower pKa (3.03). Furthermore, the large oleic acid molecule is hindered from diffusing into Zr6O4(OH)4(fumarate)6, which has pore sizes of 4.8, 5.6, and 7.4 Å.22 Given this, the as-prepared NCs could simply be soaked in the solutions of oleic acid and ethanol for 16 h at 50 °C to achieve postsynthetic ligand exchange. After the ligand exchange, the MOF can be readily dispersed in hydrophobic solvents, like cyclohexane, and remains suspended for 48 h (Figure 1d, e), a behavior contrary to that of the hydrophilic as-prepared MOF. The ligand exchange was verified qualitatively using Fourier Transform Infrared Spectroscopy (FT-IR) (Figure 2a) and quantitatively by elemental analysis (Table S1). The FT-IR spectrum reveals two new bands related to the surface appended oleic acid, at 2856 and 2928 cm−1, which we have assigned as the CH2 asymmetric and CH2 symmetric stretch, respectively;28 no evidence of these peaks were presented in the as-synthesized materials (Figure 2a).29 The ratio between fumaric acid and oleic acid was shown to be 6:1 for 25 nm-MOF and 18:1 for 300 nm-MOF based on combustion analysis (More experimental details are described in the Table S1). After the successful ligand exchange, the integrity of the framework was preserved as shown by the powder X-ray pattern (PXRD) presented in Figure 2b. Further, the 25 nm MOF particles expectedly exhibit a slight line broadening. To discern whether Zr6O4(OH)4(fumarate)6 MOFs with hydrophobic surface appended ligands still exhibit excellent water adsorption properties, vapor analysis was carried out on the MOF crystals themselves before and after the ligand exchange (LE) process. Noticeably, both sizes of MOF NCs still show significant water adsorption at low relative pressure despite decreases in surface area (Table 1). It should be noted that the water adsorption behavior of the as-prepared NCs in this study are in line with bulk powder samples of MOF-801 previously reported by Yaghi and co-workers.22 With the exception of the surface modified 25 nm NCs, all MOF samples readily reach 60% of their water adsorption capacity at low-relative pressures around, 0.15. This low-pressure region is of paramount importance for applications related to vapor barriers. A major hurdle to the preparation of the mixed-matrix membrane films consisting of two very distinct materials, hydrophilic and hydrophobic, is the related to the lack of chemical compatibility between the two components leading to phase segregation. Utilizing the strategically modified LEZr6O4(OH)4(fumarate)6 NCs, the mixed-matrix membranes were prepared to study the effect of water scavenging MOF NCs on the hydrophobic cyclic olefin copolymer (COC) (TOPAS Advanced Polymers), which can be used to effectively block the permeation of water molecules. Using a doctor blading technique, applied to polymer-containing solutions on a PET substrate, three kinds of films were prepared: (i) one without MOF as a control, (ii) one with 25 nm MOF NCs and COC (denoted as COC-25 nm LE-MOF (1)), and (iii) one with 300 nm LE-MOF NCs and COC (denoted as COC-300 nm LE-MOF (1)) (Table 2). Without the surface modification with oleic acid, the MOF NCs tend to aggregate and induce the

solution at room temperature, 25 and 300 nm Zr6O4(OH)4(fumarate)6 NCs were grown; the corresponding SEM images and observed size distributions are shown in Figure 1b, c. The weak coordination bonding observed in MOF chemistry additionally allows for facile functionalization of the external surface of the otherwise hydrophilic Zr6O4(OH)4(fumarate)6 NCs with hydrophobic molecules; this functionalization is key to promoting miscibility of the NCs in the hydrophobic polymeric media. Once incorporated, the MOF-polymer membrane film ultimately increases the impedance of the transport of water vapor through the selfassembled barrier films (Figure 1a), a property that results from a synergistic effect between the two chemically dissimilar systems. For Zr6O4(OH)4(fumarate)6 NC synthesis, the choice of reactants and their concentrations, the amount and identity of modulator, and the identity of solvent, are very important parameters to fine-tune the size and shape of the NCs.20,24 Zr6O4(OH)4(fumarate)6 synthesized in DMF at 120 °C forms randomly shaped, intergrown crystals with a wide range of crystallite sizes (Figure S1).22 On the other hand, the previous st u d y h a s s h o w n t h a t r e l a t i v e l y u n i f o r m s i z e d Zr6O4(OH)4(fumarate)6 NCs form in water; however, this approach requires a week of reaction time because of the slow reactivity of the starting material, ZrCl4.19 Given the limited literature available on the control of MOF shape and size distribution, the use of environmentally harmful and expensive solvents, and slow reaction rates, we have focused on developing a new synthetic route, and in this study report that combining ZrOCl2·8H2O, fumaric acid, and water can lead to formation of a uniform distribution of NCs in 16 h. Additionally, the size and distribution of Zr6O4(OH)4(fumarate)6 NCs can be tailored by changing the amount of modulator and solvent (Figure S2). Surface modulation entails the introduction of a monodentate ligand, such as formic acid, that has functionality that is similar to the multidentate ligand involved in framework formation. The modulator readily mediates the reaction, as it competes with the linkers for coordination to the metal, hindering framework assembly. This methodology, first introduced by Roland and co-workers,25 plays an important role in controlling the size of MOF crystallites.26 At low modulator concentrations, this reaction is in a burst nucleation regime and a very uniform distribution of NCs result. On the other hand, increasing the amount of modulator results in much larger crystals with a wide size distribution, a direct result of significant changes leading to temporal heterogeneity in the nucleation and growth rates. Hence, to control the distribution, the concentration was lowered. According to the classical case of Ostwald ripening, lower concentrations of the reactants, in the presence of a sizeheterogeneous population of crystals, can lead to ripening and growth of larger crystals; however, here we observe that lower concentrations of reactants lead to smaller NCs, consistent with burst nucleation.27 Understanding the role of modulator and the effect of water, we were able to synthesize 25 ± 5 nm sized Zr6O4(OH)4(fumarate)6 NCs using 30 equiv. of formic acid modulator, whereas 300 ± 45 nm NCs were formed with 100 equiv. of modulator, both with low reactant concentrations (Figure 1). To disperse the hydrophilic Zr6O4(OH)4(fumarate)6 NCs into the hydrophobic polymer, it was necessary to functionalize the external surface of the MOF with a hydrophobic molecule such as oleic acid. Unlike previous postsynthetic surface C


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Table 2. WVTR Values of the Films Collected at 40 °C and 100% Relative Humidity thickness (μm) neat PET COC COC-25 nm MOF COC-300 nm MOF (1) COC-300 nm MOF (2)

125 3.71 2.36 2.26 2.26

± ± ± ±

0.46 0.50 0.44 0.40

WVTR of [film+ PET] (g/(100 in2 day))

WVTR of [only composite film] (g mm/(m2 day))

0.365 0.345 0.340 0.314 0.307

0.707 0.362 0.182 0.079 0.067

Figure 2. (a) FT-IR spectra, (b) PXRD pattern, and (c) water adsorption isotherm of 25 and 300 nm Zr6O4(OH)4(fumarate)6 MOF NCs (collected at room temperature) before and after ligand exchange. LE-Zr6O4(OH)4(fumarate)6 refers to the ligand-exchanged MOF NCs. Figure 3. (a) UV−vis spectra with photographs demonstrating an excellent broad spectral transparency of the as-prepared membrane films, (b) SEM top-down image of COC + 300 nm LE-MOF NCs and the inset shows the zoomed in image, (c) Cross-sectional image of COC + 300 nm LE-MOF NCs.

Table 1. BET Surface Area of Zr6O4(OH)4(fumarate)6 before and after Ligand Exchange Zr6O4(OH)4(fumarate)6

surface area (m2/g)

25 nm MOF NCs 25 nm LE-MOF NCs 300 nm MOF NCs 300 nm LE-MOF NCs

1035 510 1068 647

that the MOF NCs are well-distributed without aggregation or phase segregation, an observation that is consistent with SEM images of the film and proving that the oleic acid capping contributed to enhancing the NC compatibility with the hydrophobic COC polymers. Furthermore, the transparency of the membrane films was well preserved even after exposure to humidity (at room temperature) for 3 months and then subsequently to 85 °C and 85% relative humidity for 3 days (Figure S4). To estimate the adsorbed water amount under that condition, we performed TGA measurements using the high humidity and temperature treated film where 3% of weight loss

formation of voids in the membrane film, severely deteriorating the film properties. By adjusting the amount of COC polymer and surface-modified MOF in the solvent, LE-MOF NCs were evenly dispersed throughout the membranes resulting in the successful fabrication of transparent barrier films with two different sizes of NCs as shown in Figure 3. The excellent transparency of the films (>94% in a visible range) indicates D


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process. This surface functionalization can offer much chemical versatility in the future that is dictated by the choice of the surface-appended ligand. From this chemistry, we have successfully fabricated highly efficient films with limited water vapor transmission through the creation of a synergistic effect between a hydrophobic COC polymer and a water adsorbing LE-Zr6O4(OH)4(fumarate)6 MOF. Although the polymer inhibits water diffusion, the MOF scavenges the water molecules that permeate into the film all while maintaining high light transmission. Moreover, MOF incorporation enhances the films ability to resist water permeation, a proof of concept that opens up new avenues for the exploration of MOF-based membranes in various industrially relevant applications. Given the chemical tunability of MOFs and polymers, and the simplicity of the surface chemistry necessary to marry these systems, we envision this work can pave the way for the facile preparation of many mixed-matrix materials as gas barriers and membranes.

was observed, presumably corresponding to the scavenged water amount (Figure S5). In addition, the treated membrane was reactivated and upon TGA measurement there was less than 1% of weight loss. Hence, it was also shown that the treated membrane film could be reactivated under vacuum conditions, implying the potential reusability. For a more quantitative water vapor blocking capability, water vapor transmission rate (WVTR) measurements were performed at 40 °C and 100% relative humidity by Mocon Inc., (USA) to examine the water vapor blocking properties of the films; the results are presented in Table 2. The majority of previous studies have not accounted for other variables such as how the thickness of the film and the presence of the substrate affect the WVTR results. Since water vapor blocking is greatly enhanced due to the aforementioned issues−the film thickness and the substrate−it is critical to characterize the properties of each component in a multilayer structure. In this work the WVTR value of the PET substrate was first determined so that it could be decoupled from the total system to truly evaluate the ability of the as-prepared films to block water vapor30 and then the WVTR of the films were further normalized based on the thickness. (All details are described in the Experimental Details in the Supporting Information). All three mixed-matrix membrane films incorporating MOF NCs showed an enhancement in their ability to block water vapor, compared to the neat PET substrate. The polymer film without MOF displayed a 2fold enhancement in the WVTR, compared to the neat PET, attributed to its hydrophobicity, whereas the MOF-containing membrane films exhibited even better performance; these results imply that implanting MOF NCs into the polymer matrix have an effect on scavenging the few permeated water molecules. In this regard, the film infused with 300 nm MOF NCs has shown a notable enhancement in water blocking, reducing the WVTR by a factor of 9 compared to the neat PET, whereas the film infused with 25 nm sized MOF NCs only decreased the WVTR by a factor of 4. It can be inferred that the difference comes from the larger internal surface area of the modified 300 nm MOF NCs, which contribute to scavenging a larger amount of permeated water molecules (Table 1 and Figure S2), a result that is further supported by water adsorption isotherms and BET surface area analysis (Figure S3, Table 1). To examine the effect of MOF concentration on the WVTR and overall film optimization, we doubled the loading of the 300 nm MOF NCs (COC-300 nm MOF (2)). From this, an additional 18% enhancement in the films ability to block the permeation of water vapor was observed; this corresponds to an overall 10.5 time improvement relative to the neat PET substrate while still maintaining an excellent light transparency (Figure 3a). Noticeably, all prepared films in our study exhibited superior barrier characteristics, compared to a conventional EVA (ethylene vinyl acetate) film, which is commonly used in the PV industry as a barrier layerthese EVA films show a comparatively poor transmission rate of 15.5 g mm/(m2 day) of WVTR value at 38 °C and 100% relative humidity. In conclusion, we report a new COC/MOF NC mixedmatrix membrane film for enhanced water protective barrier layers. Water scavenging Zr6O4(OH)4(fumarate)6 MOF NCs were successfully prepared via a new method for the quick and well-controlled synthesis in aqueous media at room temperature. We show that the surfaces of the otherwise hydrophilic MOF NCs can be tailored for their dispersion into a chemically dissimilar hydrophobic polymer via a facile ligand exchange

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01299. Details of NC synthesis, film preparation, and sample characterization, additional figures and tables, including SEM images of MOF NCs, adsorption isotherms, UV/vis spectra, TGA, and elemental analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: wendy.queen@epfl.ch. Author Contributions †

Y.J.B. and E.S.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Work at the Molecular Foundry and the ALS was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. W. Q. acknowledges support from the Swiss National Science Foundation under grant number PYAPP2_160581. Y. B. acknowledges support provided by the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001015. The formation of the mixed-matrix membranes is based upon work supported by the Department of Energy (DOE) through the Bay Area Photovoltaic Consortium (BAPVC) under Award Number DE-EE0004946 and also in part under the US-India Partnership to Advance Clean Energy-Research (PACE-R) for the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, under Subcontract DE-AC3608GO28308 to the National Renewable Energy Laboratory, Golden, Colorado) and the Government of India, through the Department of Science and Technology under Subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22nd Nov. 2012. The E


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authors greatly thank Dupont Teijin Films for providing the PET substrate. The authors thank Christine Beavers of the Advanced Light Source for her assistance in obtaining powder x-ray diffraction data on BL 12.2.2. D.S. acknowledges support by Laboratory Directed Research and Development (LDRD) funding from Berkeley Lab, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract DEAC02-05CH11231.

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