Preparation of Freestanding Conjugated Microporous Polymer

Article pubs.acs.org/cm

Preparation of Freestanding Conjugated Microporous Polymer Nanomembranes for Gas Separation Peter Lindemann,† Manuel Tsotsalas,*,† Sergey Shishatskiy,‡ Volker Abetz,‡,§ Peter Krolla-Sidenstein,† Carlos Azucena,† Laure Monnereau,∥ André Beyer,⊥ Armin Gölzhaü ser,⊥ Veronica Mugnaini,† Hartmut Gliemann,† Stefan Bras̈ e,∥,# and Christof Wöll† †

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany Institute of Polymer Research, Helmholtz-Zentrum Geesthacht, 21502 Geesthacht, Germany § Institute of Physical Chemistry, University of Hamburg, 20146 Hamburg, Germany ∥ Institute for Organic Chemistry (IOC), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany ⊥ Faculty of Physics, Bielefeld University, 33615 Bielefeld, Germany # Institute of Toxicology and Genetics (ITG), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany ‡

S Supporting Information *

ABSTRACT: Conjugated microporous polymers (CMPs) have attracted much interest due to their intrinsic porosity, outstanding stability, and high variability. However, the processing of these materials for membrane application has been limited due to their insoluble nature when synthesized as bulk material. Here we report the synthesis of freestanding CMP-nanomembranes via layer-by-layer growth of a “click” based conjugated microporous polymer on a sacrificial substrate. After dissolution of the substrate the CMPnanomembrane can be transferred to porous substrates and continuously cover holes of up to 50 μm diameter. The CMPnanomembranes appear defect-free as inferred from high selectivity values obtained from gas permeation experiments and from electrochemical investigation in the presence of ferrocene. The presented synthesis method represents a versatile strategy to incorporate CMP materials in functional devices for membrane separation, catalysis, or organic electronics.

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are predicted to be ideal separation membranes with many advantages over bulk membranes.12,13 However, the inert nature of most CMP materials causes severe, intrinsic challenges in their processing to yield large scale membranes. Indeed, only branched “soluble conjugated microporous polymers” (SCMPs)14 and linear conjugated polymers of intrinsic microporosity (C-PIMs)15 can be processed from solution. Usually CMPs are, in contrast to most polymers, not soluble in organic solvents,1 and as a result, common processing techniques to fabricate polymer films from a solution such as spin coating cannot be applied. Considering the recent success in using layer-by-layer or quasi-epitaxial approaches for the fabrication of thin MOFlayers (SURMOFs)16,17 we fabricated CMP thin films via the same approach.18 A crucial element of our strategy has been the use of sacrificial substrates19 in order to obtain free-standing CMP-membranes. The approach described here allows

onjugated microporous polymers (CMP) are a class of microporous solids which have recently attracted wide interest due to their large surface areas, low densities, and the possibility to incorporate different kinds of functional groups in a modular fashion.1 In contrast to related metal organic frameworks (MOF)2−5 or covalent organic frameworks (COF)6,7 which are formed through reversible reactions, CMPs are formed through high yielding irreversible reactions of rigid building blocks. The resulting CMP materials are amorphous and at the same time often show narrow pore size distribution.8 The exceptional thermal and chemical stability goes well beyond that of MOFs and COFs and makes this class of porous materials particularly appealing for practical applications such as gas storage, catalysis, and molecular separation.9,10 Among the numerous synthetic routes used in the past, click reaction chemistry has played a special role as a result of its ease of operation. The high purity and readily accessible products of click chemistry are particularly attractive to produce CMP materials.11 Among the large variety of CMP applications, two-dimensional nanomembranes with a thickness below 10 nm exhibiting tunable pore sizes that can act as molecular sieves have a particularly large potential, since they © XXXX American Chemical Society

Received: October 24, 2014 Revised: November 14, 2014

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dx.doi.org/10.1021/cm503924h | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

for molecular structures), respectively.26 The SAMs were prepared on a Si-wafer coated with 5 nm titanium and 100 nm gold. When using the alkyne terminated SAMs, we exposed them to the azide containing building block in the presence of copper(I) catalyst under an inert atmosphere. Afterward, the substrate was rinsed thoroughly and exposed to the alkyne containing building block, rinsed again, and exposed to the azide terminated linker. This process can be repeated several times to achieve the desired film thickness. When starting with the azide terminated SAM, the first layer was the alkyne containing building block; afterward the reaction proceeds with the sequential alternation of the monomeric linkers as described above. The homogeneous thin CMP-films were characterized using infrared reflection absorption spectroscopy (IRRAS) (see Supporting Information Figure S2). The IRRA spectra revealed the formation of the CMP-film as well as the presence of unreacted azide and alkyne functions and physisorbed solvent molecules remaining in the pores, which is in accordance with IR spectra of the bulk materials reported by the groups of Cooper and Nguyen.22,24 To follow the growth process and to determine the morphology using atomic force microscopy (AFM) we prepared a laterally patterned SAM of 11-azidoundecanthiol on gold substrate prepared by means of microcontact printing.27,28 The AFM images shown in Figure 2 were obtained for CMP samples after 4 and 8 growth cycles and show homogeneous CMP thin layers with a height of about 1 nm per growth cycle.

fabrication of CMP-membranes with thicknesses as low as a few nanometers, containing hierarchical composition gradients. In addition, this scheme is favorable (or opened) for further internal and external surface functionalization. This high variability in the design of CMP structures combined with their excellent thermal stability and chemical inertness makes them ideal candidates for highly demanding applications such as nanometer thin membranes in gas separation, catalysis, and nanofiltration. To demonstrate the potential of our CMPnanomembranes for molecular sieving type gas separation, we determined the gas permeances for a set of eight gases having different kinetic diameters which included permanent, noble gases, CO2, and hydrocarbons. The fabrication of porous materials via a layer-by-layer approach relies on the self-terminating character of the growth of the individual layers.20,21 With regard to CMP synthesis, this requirement imposes certain constraints on the choice of the CMP-forming monomers. Application of the layer-by-layer process to CMPs made from a single monomer is therefore not possible. An obvious strategy is to use two different types of monomers, each having multiple copies of the same functional group. The functional groups exposed by the two different monomers should be complementary to each other and be able to react in high yielding reactions with no side products, which is characteristic for the click chemistry that we focused upon in the current study. The molecular components chosen for the present study (tetrakis(4-azidophenyl)-methane, terminated with azide functional groups, and tetrakis(4-ethynylphenyl)methane, terminated with alkyne functional groups) are shown in Figure 1.

Figure 2. Atomic force microscopy (AFM) topographic images of CMP thin layers grown on gold substrates with laterally patterned SAM of 11-azidoundecanthiol after 4 (top) and 8 (bottom) growth cycles. The graphs on the right show the height profiles along the red lines in the AFM images on the left.

Figure 1. Molecular building blocks of the CMP system and schematic representation of their layer-by-layer synthesis on functionalized surfaces.

Both monomers had been successfully used in previous work to form CMP bulk material, 11,22−24 but due to the aforementioned technical problems with the conventional synthesis procedure, the obtained bulk material was not suited for membrane fabrication. We overcome this problem by using an appropriately functionalized substrate. A crucial step for the layer-by-layer process to be applied here20 is indeed the choice of the template substrate functionalization. In the present case, to drive the quasi-epitaxial growth process we chose both a selfassembled monolayer (SAM)25 exposing an azide or an alkyne moiety using 11-azidoundecanthiol and 11-thioacetyl-undecane acid-propargyl amide (see Supporting Information Figure S1

To create freestanding CMP-nanomembranes we used a surface-anchored metal−organic framework (SURMOF) as the starting sacrificial substrate.29 SURMOFs are crystalline and highly oriented30 and have a very low defect density.31 As is the case of bulk MOF materials, SURMOFs can be prepared from a large variety of functionalized organic linkers. Here, we used linkers functionalized with two azide side groups, which served as the starting point for the CMP synthesis. The CMP-MOF system was characterized by IRRAS (see Supporting Information Figure S3). B



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electroactive species and ionic liquid as supporting electrolyte.34 While for a bare gold substrate the reversible reduction and oxidation of ferrocene can be recorded, for the gold substrate coated with the CMP-nanomembrane almost no current is measured. This is clear evidence that the CMP-nanomembrane acts as a blocking barrier between the conductive gold and the ferrocene dissolved in the solution (see Supporting Information Figure S10 for cyclic voltammograms of bare gold before and after transfer of the CMP-nanomembrane). For gas permeation experiments a polydimethylsiloxane/ polyacrylonitrile (PDMS/PAN) thin film composite (TFC) membrane35,36 was used as a support for the CMP-nanomembranes to enhance the mechanical stability and minimize roughness-induced strain. Supporting Information Figure S11 shows a membrane on PDMS/PAN after measurement. The PDMS/PAN was shown to be very suitable as a support for nanomembranes in gas permeation experiments.12 The corresponding gas transport properties of bare PAN, PDMS/ PAN, and CMP-covered PDMS−PAN were determined with He, H2, CO2, Ar, O2, N2, CH4, and C2H6. For these measurements, four PAN/PDMS/CMP TFC membranes of different batches were prepared according to the aforementioned procedure. The permeances of the individual gases of the reference and CMP TFC membrane samples are shown in the Supporting Information (Figures S12−S17). As it can be seen in Supporting Information Figures S12−S15 the gas permeances of the PAN/PDMS/CMPmembranes were significantly slower than the ones of PAN (Supporting Information Figure S16) and PAN/PDMS (Supporting Information Figure S17) membranes alone, indicating that the permeance measured for PAN/PDMS/ CMP was mostly determined by the CMP nanomembrane on top of the PDMS. Figure 4a shows the permeances of each individual layer in the TFC membrane. The information can be extracted if one considers the TFC membrane as a stack of three individual layers working as resistance in series.37 We performed this conversion based on a previous publication12 (for details see Supporting Information). In a comparison of

From previous studies on the stability of SURMOFs we found that solutions of ethylenediaminetetraacetic acid (EDTA) in water/ethanol mixtures readily dissolve the prepared SURMOFs.29 To detach the prepared CMP thin layers we therefore immersed the sample in EDTA solution. After a few minutes of immersion, the detachment of the CMPmembrane was evident by eye (see Supporting Information Figure S4 for optical microscopy image). The freely floating membrane can be transferred easily to a TEM grid for characterization. Figure 3 shows the detachment process as well as a scanning electron microscope (SEM) image of the freely floating CMP-membrane after its transfer to a TEM grid.

Figure 3. Top: Schematic representation of the detachment process. Bottom: Scanning electron microscope (SEM) images of the freely floating CMP-membrane after its transfer to a TEM grid.

To gain more control over the transfer process and to avoid defects and rupture of the membrane, the nanomembranes were transferred via a method introduced for carbon nanomembranes (CNM).32,33 In this process, the thin CMP layers were first covered by a polymeric transfer medium (poly(methyl methacrylate), PMMA) via spin coating. The detachment of the membrane is then achieved by dissolution of the gold layer in aqueous solution of potassium iodide and iodine. After transfer of the PMMA/CMP-membrane to the desired substrate, the protecting PMMA layer was dissolved in acetone to obtain the free CMP-membrane (see Supporting Information Figure S5 for a scheme of the transfer process and Supporting Information Figure S6 for helium ion microscopy (HIM) images of freestanding CMP-nanomembranes on TEM grids).33 Using this method, CMP thin layers grown on gold or mica substrates both functionalized with SAMs or SURMOFs could be used for the transfer process. IRRA spectra of CMPmembranes transferred using the described methodology to a gold wafer and of CMP thin layers grown directly on gold substrates show no differences (see Supporting Information Figure S7). AFM measurement of the transferred CMPnanomembrane on a gold substrate revealed a height of about 10 nm (see Supporting Information Figures S8 and S9). To investigate the absence of defects in the CMP-nanomembranes after transfer to the gold substrate we performed cyclic voltammetry investigations in the presence of ferrocene as

Figure 4. (a) Gas permeances of eight gases for the individual layers of PAN (■), PDMS (▲), and CMP (◆) membrane; (b) ideal selectivity of the CMP-membrane for H2 versus gas x. C



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the ideal gas selectivity of H2 versus the other tested gases (obtained as ratio of the single component permeances), the selectivity pattern of the CMP-membrane (gray) was significantly different from the ones of the PAN/PDMS (light gray) and PAN (dark gray) membranes (Figure 4b). The CMP-membrane showed a clear molecular sieving effect favorable in terms of gas transport through the membrane for the gases with smaller kinetic diameter (He and H2 and, to a lesser extent, CO2). In contrast, the PAN membrane showed Knudsen-like gas flow with higher permeances for gases with smaller molecular weight (as seen clearly in Supporting Information Figure S18 where the H2 selectivity patterns are plotted vs the molecular weight of the gases). The PDMS membrane, on the other hand, shows the highest permeances for the heaviest CO2 and largest C2H6 which is an indication of the solubility controlled gas permeation characteristic for all rubbery polymers.38 The gas permeation characteristics of the CMP-nanomembranes can be clearly distinguished from the support membranes hence clearly demonstrating that the CMP-nanomembrane acts as a defect-free selective layer in the TFC membrane. The ideal gas selectivities obtained as proportion between permeances of two gases measured individually are similar to glassy polymers with stiff polymer backbones acting like a molecular sieve.39−43 However, the selectivity profile is not strictly depending on the kinetic diameter of the gas molecules (e.g., argon having a higher permeance than oxygen). The reason might be that in such thin membranes the rigidity is not as pronounced as in bulk CMP materials, and this could lead to structural changes or surface phenomena as also seen for ultrathin membranes made of polymers with intrinsic microporosity (PIM-1).44 Considering the only 8 nm thickness we obtain a permeability of ∼4 Barrer for H2 which is lower than expected for such rigid and porous materials,8 and it is well beneath the upper bound given by Robeson45 regarding the selectivity of ∼36 for H2/N2. The reason for such a low hydrogen permeability coefficient can be bad interconnection of pores formed during the CMP synthesis, the assumption to be clarified in further planned experiments with monomers of various structures. Nevertheless, by considering the permeability of O2 (∼0.7 Barrer) and O2/N2 selectivity of ∼6 we are close to literature values of other glassy polymer membranes.46 In conclusion our results demonstrate the successful fabrication of freestanding virtually defect free CMP-nanomembranes via layer-by-layer synthesis on sacrificial substrates. This process to fabricate CMP-nanomembranes represents a versatile strategy to create chemically functionalized and even laterally structured nanometer-thin membranes with tunable properties. Their defect free nature was confirmed by electrochemical investigations where the nanomembranes show blocking behavior with respect to ferrocene. In gas permeation experiments the prepared nanomembranes have confirmed that they are in defect-free state by high selectivity values for gases with small kinetic diameter over gases with larger kinetic diameters. In addition the presented method allows further synthesis and studies of CMP-membranes having different chemical compositions and thicknesses, to fine-tune the selectivity and permeance for gas and liquid separation. Remarkably the possibility to transfer CMP-nanomembranes to virtually any substrate via the presented processing method paves the way for the use of such materials in various other applications, such as catalysis, sensing, or organic electronics.

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ASSOCIATED CONTENT

S Supporting Information *

Further information for experimental procedures, IR-spectra, AFM and helium microscopy images, cyclic voltammetry, and permeance measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

* M. Tsotsalas. E-mail: [email protected]. Funding

P.L. acknowledges financial support from the Baden-Württemberg Stiftung. V.M. acknowledges the European Union for granting the Marie Curie fellowship MOLSURMOF (FP7PEOPLE-2011-IEF, No 301110). Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS We thank Samuel Bergdolt and Jens Schönwald for their excellent technical assistance, as well as Georg Albert for the discussions and preparation of the gold on mica substrates.

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ABBREVIATIONS CMP, conjugated microporous polymer; MOF, metal organic framework; COF, covalent organic framework; SAM, selfassembled monolayer; IRRAS, infrared reflection absorption spectroscopy; AFM, atomic force microscopy; SURMOF, surface-anchored metal−organic framework; EDTA, etylenediaminetetraacetic acid; SEM, scanning electron microscope; CNM, carbon nanomembranes; PMMA, poly(methyl methacrylate); HIM, helium ion microscopy; PDMS, polydimethylsiloxane; PAN, polyacrylnitrile; TFC, thin film composite; PIM, polymers with intrinsic microporosity

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Preparation of Freestanding Conjugated Microporous Polymer

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