Enhanced Chemical Separation by Freestanding CNTâPolyamide

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Enhanced chemical separation by freestanding CNT-polyamide/ imide nanofilm synthesized at the vapor-liquid interface Amirhossein Droudian, Seul Ki Youn, Linda A. Wehner, Roman Michael Wyss, Meng Li, and Hyung Gyu Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02329 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Applied Materials & Interfaces

Enhanced chemical separation by freestanding CNTpolyamide/imide nanofilm synthesized at the vapor-liquid interface Amirhossein Droudian,† Seul Ki Youn,† Linda A. Wehner, Roman M. Wyss, Meng Li, Hyung Gyu Park* Nanoscience for Energy Technology and Sustainability, Department of Mechanical and Process Engineering, Eidgenössische Technische Hochschule (ETH) Zurich, Tannenstrasse 3, Zurich CH-8092, Switzerland †

These authors contributed equally to this work *

E-mail: [email protected]

ABSTRACT In chemical separation, thin membranes exhibit high selectivity, but often require a support at the expense of permeance. Here, we report a pinhole-free polymeric layer synthesized within freestanding carbon nanotube buckypaper through vapor-liquid interfacial polymerization (VLIP). The VLIP process results in thin, smooth and uniform polyamide and imide films. The scaffold reinforces the nanofilm, defines the membrane thickness and introduces an additional transport mechanism. Our membranes exhibit superior gas selectivity and osmotic semipermeability. Plasticization resistance and high permeance in hydrocarbon separation together with a considerable improvement of water-salt permselectivity highlights their potential as new membrane architecture for chemical separation.

Keywords Gas separation, desalination, interfacial polymerization, composite membrane, carbon nanotube scaffold, thin selective layer, plasticization resistance 1 ACS Paragon Plus Environment

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Membrane technology can play a crucial role in efficient chemical separation and process intensification.1 Despite many successful demonstrations, continued research is required for adequate separation performance under practical process conditions.2-4 Challenges in the field of membrane materials today stem largely from meeting all process requirements such as selectivity, permeability, and stability. For example, excellent separation performance of molecular sieving membranes made from metal-organicframework5 or perforated graphene6 is hindered by the scalability issue associated with mechanical stability, narrowing their applications for water desalination and hydrocarbon separation. The most widespread membrane architecture today comprises a dense polymeric selective layer with a robust porous2 or dense-permeable support.7 These membranes can offer a cheap, process-friendly and scalable solution, however, in the frame of an upper-bound limit of the separation performance of the polymer membrane.1, 4 The architecture of the supported membranes results in permeance gain upon decreasing selective layer thickness with added transport resistance associated with the mechanical support. One of the conceivable solutions to gain higher permeance is a self-supporting, thin, selective layer by reinforcement.8 Previous reports include self-supporting membranes of hybrid sol-gel materials,9 selfassembled carbon monolayer,10 surfactant-assisted deposited inorganics,11 layer-by-layer assembled molecules12 and interfacial polymerized layers.7 Polymers in the thickness range of 10-50 nm have achieved great permeance and excellent selectivity in organic solvent nanofiltration7 and gas separation13 very recently. Nevertheless, these ultrathin films contain weaknesses such as poor chemical, mechanical and thermal stabilities, insufficient salt rejection in water desalination,14 and severe plasticization in the separation of condensable gases.15 To obtain the self-supporting selective nanofilms resistant to external stresses and plasticization, we have established a synthesis method based on vapor-liquid interfacial polymerization (VLIP) around a sturdy porous scaffold of carbon nanotube buckypaper (CNT-BP). The synthesized CNT-BP–polymer nanofilms are mechanically robust to allow a freestanding configuration in the membrane based separation process despite their sub-100-nm thickness. Self-limiting feature of VLIP process within the CNT-BP scaffold 2 ACS Paragon Plus Environment

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provides a straightforward way to control the thickness as well as the surface smoothness of the nanofilm, crucial factors for a selective layer. Use of semi-rigid, aromatic polyamide and polyimide as the polymer component gives the nanofilms superiority in the transport and separation, whereas the CNT scaffold grants the anti-plasticization properties. VLIP takes place at the vapor-liquid interface of the monomer in solution added to the porous CNT-BP scaffold, which is saturated with volatile amine vapors (Figure 1A). A vacuum-filtered and isolated, freestanding CNT-BP (Figure 1B) provides a reticular scaffold of which thickness is controlled through the amount of CNT dispersion during the vacuum filtration (Figure S1). 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) is the dissolved monomer selected for its widespread use in gas separation.3 The monomer has aromatic moieties that may interact favorably with the graphitic surface of CNTs to restrain excessive polymer chain movement and swelling.16 Tiny and volatile ethylene diamine (EDA) vapors can penetrate through the buckypaper scaffold and saturate the CNT surface. On the EDAsaturated surface of CNT, 6FDA polymerizes and instantaneously forms polyamide (PA) along the scaffold. PA converts to polyimide (PI) via thermal imidization17 (Figure S2K) monitored by X-ray photoelectron spectroscopy (Figure S3). Polyamide is the gold standard for water desalination, whereas polyimide proved its separation capability in hydrocarbon purification.2-3 We prepared a number of freestanding selective layers polymerized on the CNT-BP scaffold of various thicknesses to characterize their osmotic transport and gas separation properties (Table S1). Embedded fully in a polymer matrix, the freestanding scaffold determines the thickness of the CNT-BP–polymer nanofilms and neither thick excess polymer nor uncoated CNT is found on either side of the scaffold (Figure 1C, D). In contrast, if the polymerization is performed on the conventional porous support such as anodized aluminum oxide (AAO), the thickness control is challenging due to the intrusion of the monomer solution into the support (Figure S6). For CNT-BP-Polymer, insufficient supply of the dissolved monomer could result in non-selective large cavities (Figure S5C). We optimized the volume and concentration of the dissolved monomer to suppress their formation. Transmission electron 3 ACS Paragon Plus Environment


microscopy (TEM) confirms the formation of a pinhole-free polymer on the nanofilm surface (Figure S2I). In agreement with the uniform top-layer, the cross-section scanning electron microscopy (SEM) image shows completely filled CNT-BP scaffold (Figure 1E). π-π interactions between polymer aromatic chains and graphitic CNT surface16, 18 may help to create smooth interfaces. Diffuse halo pattern of the corresponding fast Fourier transformation suggests that the matrix is amorphous polymer (Figure S2I). Atomic force microscopy (AFM) reveals that a reticulate structure of our CNT-BP scaffold is interlocked and can maintain the morphology during the VLIP process. In other words, surface topography of the resultant nanofilm follows that of CNT-BP scaffold with smoothened texture upon polymerization (Figure 1F). VLIP-synthesized polymer exhibits smoother surface compared to a typical polymer thinfilm fabricated through liquid-liquid interfacial polymerization on a porous support (Figure S6), likely resulting from the absence of a diffusion-limited reaction. Due to the reinforcing scaffold, CNT-BP– polymer nanofilms are mechanical robust, freestanding and easy to handle despite their thickness (Figure 1G). The surface smoothness is highly desirable for a selective layer to suppress membrane fouling. Raman mapping of the nanofilm displays homogeneous intensities of the characteristic D- and G-peaks of the embedded single-walled CNTs over a 100-µm-wide mapping area, as well as showing a high G-to-D ratio of ~19 (Figure S2J). In agreement with the AFM characterization result, not only the scaffold configuration but also the quality of CNTs remain intact through the VLIP process. According to a thermogravimetric analysis, a CNT content of the nanofilm exceeds 90 wt % surpassing nearly all the CNT/polymer nanocomposite membranes previously reported (Figure S4C).19-21 The strong tendency of CNTs to aggregate in the polymer matrix and form defects in the composites has been one of the major issues in the development of CNT composite materials even at low loading concentrations.22 Remarkably, our material design and synthetic method – growth of polymer chains along the reticulate CNT scaffold of a great surface area – enables the homogeneous dispersion of CNTs at the record high content in a polymer matrix maximizing the interface area. Our composite membranes withstand the measurement conditions of 6 bar gas in the supported configuration and in the freestanding form 1 bar hydraulic 4 ACS Paragon Plus Environment

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pressure difference. The use of a freestanding CNT-BP scaffold not only reinforce the mechanical strength of a selective layer, but it also provides favorable interface to the polymer chains. The VLIP synthesis route contributes to the precise control of the selective layer thickness, the polymer-CNT interface, and the surface roughness. In particular, the thickness of the selective layer is primarily correlated with the concentration of CNT-BP precursor (Figure S5), providing a straightforward way of tuning the nanofilm thickness at the synthesis step.

Figure 1. Vapor-liquid interfacial polymerization of freestanding CNT-BP–polymer nanofilms: (A) a schematic of the VLIP process; (B) SEM image of reticulate structure of the CNT-BP scaffold before polymerization; (C-E) SEM image of CNT-BP–PA film after polymerization, (C) side view, (D) top view and (E) its cross-section cut via focused ion beam (the dashed lines are symbolic); freestanding samples showed in B-E are transferred onto AAO for an imaging purpose; (F) AFM characterization of CNT-BP and CNT-BP–PA nanofilm, (G) photograph of a freestanding CNT-BP–PA nanofilm in a 5-mm-wide frame.

The porous CNT-BP scaffold can permeate gas molecules easily at permeance values in the order of magnitude of 104 GPU (Figure 2A). It shows nanochannel transport behavior with an inverse-square-root 5 ACS Paragon Plus Environment


molecular mass scaling of the permselectivity and no general size dependency of permeance, with the exception of hydrocarbons (Figure 2A, S8). This behavior can be attributed to preferential adsorption to and diffusion along the CNT exterior surfaces, in good agreement with gas transport characteristics in CNT.23 After polymerization and formation of CNT-BP–PI, the membranes pose features of glassy polymer (Figure 2B). Due to the characteristics of these membranes gas permeance drops compared to CNT-BP by values of 2-4 orders of magnitude with a more pronounced effect for larger molecular sizes (Figure 2B, gray area). The permeability of inorganic species like H2 exhibits pressure dependency (Figure 2C) stemming likely from a dual-sorption mechanism or compression effect of polymeric volume.24 These observations altogether suggest that a dense polymer film is formed over an entire area of the CNT-BP scaffold to turn the dominant transport mechanism of Knudsen diffusion in the CNT-BP to a solutiondiffusion in the glassy polymer of the polyimide. Hydrocarbon gases deviate from this pattern by showing reverse molecular size dependency, or reverse selectivity25 (Figure 2B, orange area), likely caused by preferential interaction between graphitic surface of CNT-BP and penetrant molecules. Indeed, permeability of hydrocarbon gas species is independent of the applied pressure up to 2 bar (Figure 2C). This observation indicates enhanced adsorption onto CNTs and a possible preferential transport pathway at the CNT-polymer interface that exceeds pressurized saturation of the polymer non-equilibrium free volume. Separation factors of the CNT-BP–PI membrane display practically meaningful values for hydrocarbons – 3.7 for C2H4/C2H6 and 6 for CO2/CH4 – and gas mixtures challenging to separate – ~4 for N2/CH4. All these values exceed Knudsen selectivity attributable to transport through selective polymer matrix with little competitive adsorption (Figure 2D). Such excellent separation properties combined with enhanced permeance due to the membrane thickness make our CNT-BP–PI membrane comparable with the stateof-the-art polymeric membranes. In ethylene-ethane separation, these features place the selectivitypermeance pair outside the upper-bound of advanced membranes (Figure 2E). The separation factors and 6 ACS Paragon Plus Environment

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permeances are invariant with duration and pressure of the CNT-BP–PI exposure to hydrocarbons (Figure 2F), demonstrating resistance of our membranes to plasticization and swelling. There has been an effort to restrict polymer chain movements and obtain plasticization resistance by dispersing interactive solid nanoparticles into the polymer matrix.26 The incorporation of the reticulate network of evenly distributed CNTs into the thin selective layer is conceptually akin to the interactive solid nanoparticle dispersion. The plasticization and swelling resistances we obtain from our CNT-BP–PI membranes may enable to address a practically challenging problem of olefin-paraffin separation.

Figure 2. Gas transport and separation performance of fabricated membranes: (A) CNT-BP before polymerization - permselectivity relative to N2 normalized by the inverse-square-root scaling and single gas permeance with respect to gas kinetic diameter; (B-F) CNT-BP-PI after polymerization - (B) single gas permeance with respect to kinetic diameter, measured at 1.3 bar and 26 oC, with illustration of sizeexclusion- (gray) and surface-diffusion-dominant (orange) transport regimes; (C) single gas permeability of H2 and C3H8 with respect to feed pressure; (D) permselectivity, separation factor, and Knudsen selectivity of gas pairs; (E) separation performance of CNT-BP-PI (solid red circle), pure polymers (green triangle), carbon molecular sieves (CMS) (empty cyan circle), and mixed matrix membranes (MMM) (orange square), refer to Figure S9 for references; (F) CO2/CH4 (1.3 bar) mixture separation performance over C3H8 (2 bar) exposure.

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We investigated water transport and salt passage across the CNT-BP–polyamide (PA) membranes with different thicknesses in forward osmosis (FO)-like diffusion test. These membranes are pinhole-free, sturdy and thin enough to demonstrate superior semipermeability for potential applications in water desalination. Both fluxes of water permeation and salt diffusion tend to increase with decreased membrane thickness (Figure 3A). Particularly, large fluxes are obtained from the CNT-BP–PA nanofilms synthesized and tested in a freestanding configuration. To the best of our knowledge, these results are the first reports on the support-free osmotic performance characterization of polymer nanofilm. To evaluate the desalination capability, we calculate the salt rejection from the measured diffusion fluxes (Supporting Information 2.5).27-28 Salt rejection values of the tested membranes lie above 98% and particularly over 99.5% for those in the smallest thickness range (Figure 3A). On the other hand, permselectivity – a thickness-independent indicator of the transport properties – actually increases with water permeability (Figure 3B). In other words, our composite membranes gain water permeability without losing water/salt selectivity. Accordingly, CNT-BP–PA nanofilms clearly differ from the pure polymer desalination membranes, which commonly reveal a trade-off between these two material properties.2,

4

This observation indicates that the effect of CNT incorporation on the

membrane properties is beyond the performance enhancement based on the inverse flux-thickness relation in pure polymers, perhaps related to the change in the internal structure of composite matrix distinct from the pure polymer cases. We hypothesize that our very thin nanofilms have an additional molecular pathway, favorable to water transport yet against the passage of the hydrated salt ions, in addition to the free volume of packed polymer. We rule out any transport pathway through CNT interiors, because HiPco CNTs we employed are not uncapped at their tips. In addition, if a change in polymer properties was the only reason for observed performance in thinner membranes, then according to the trade-off of polymers the selectivity would decrease simultaneously with increasing the water permeability, which disagrees with our observation. Therefore, transport through an alternative transport path with higher selectivity than polymer was facilitated by incorporation of CNT while reducing the membrane thickness. Alteration 8 ACS Paragon Plus Environment

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of structural change in the molecular pathway built along the interface between PA and CNT is considered as the main source of the new membrane properties upon thinner films. Previous studies reported that polymer chains with planar aromatic moieties are prone to parallel arrangement to CNT walls for their strong π-π interaction.16, 18, 29-30 We believe that the interface in our films is tight and shifts the paradigm of non-selective gap existent in conventional composite membranes22 to a selective one. Interfaces formed between polymer chains and CNTs in this mechanism could possibly host water molecules yet exert energy penalty to the passage of hydrated salt ions. In addition to the selective passage through the packed polymer, such interfaces could potentially lead to enhancement in salt rejection.

Figure 3. Water transport and salt rejection across CNT-BP–PA membranes: (A) reverse salt flux and salt rejection (inset) with respect to water flux in an FO-like osmosis characterization using pure water as feed and a NaCl solution (0.3 M) as draw; CNT-BP–PA nanofilms (round) synthesized in the presence (empty) and absence (filled) of a PVDF support and characterized as freestanding samples (cross) or atop a PVDF support (no cross); (B) relationship between water-salt permselectivity and water permeance tested over a wide range of osmotic gradients (0.05M – 0.6M), along with the trade-off for polymeric membranes2, 4 (dashed line); see Figure S7 and Table S1 for further membrane specification and measurement conditions.



Polymerization at the interface of liquid and vapor-saturated CNT scaffold produces smooth, freestanding selective nanofilms for gas separation and water desalination applications. The scaffold does reinforce the nanofilm and control the nanofilm thickness down to 100 nm. Uniformly dispersed, high density CNTpolymer interfaces lead to restriction of plasticization and unique transport pathways for condensable gas species and water. Proper control of the thickness and the polymer-scaffold interface allows the nanofilms to capture selective and enhanced transport properties unseen in the pure polymer-based membranes. The VLIP synthesis concept with a preformed robust scaffold can be further applied to other rigid and selective inorganic materials and polycondensable polymers.

Author Contribution H.G.P. and S.K.Y. conceived the project. A.D. conceived the idea of vapor-liquid interfacial polymerization. A.D. and S.K.Y. carried out research. A.D., S.K.Y. and L.A.W. synthesized the nanofilms. R.M.W. characterized the nanofilm thicknesses via focused ion beam microtome. M.L. obtained XPS data. A.D., S.K.Y. and H.G.P. wrote the manuscript. A.D., S.K.Y. and L.A.W. revised the manuscript.

Acknowledgements This work was carried out at Binnig and Rohrer Nanotechnology Center of ETH Zurich and IBM Zurich. H.G.P. acknowledges Prof. Kamalesh K. Sirkar of New Jersey Institute of Technology, Newark, NJ, USA for his initial inspiration of interfacial polymerization on buckypaper. M.L. thanks Jörg Patscheider of Empa, Switzerland for support in the XPS measurement. A part of this work was financially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry, & Energy (MOTIE) of the Republic of Korea (No. 20168510011420) for which H.G.P. and L.A.W. are grateful.

Supporting Information Materials and Methods 10 ACS Paragon Plus Environment

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Supplementary Text Figures S1 to S9 Table S1 References (1–21)

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Enhanced Chemical Separation by Freestanding CNTâPolyamide

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