Ultraselective Pebax membranes enabled by ... - ACS Publications

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

Applications of Polymer, Composite, and Coating Materials

Ultraselective Pebax membranes enabled by templated microphase separation Yatao Zhang, Yijia Shen, Jingwei Hou, Yiming Zhang, Winny Fam, Jindun Liu, Thomas D. Bennett, and Vicki Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03787 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Ultraselective Pebax membranes enabled by templated microphase separation Yatao Zhang,a Yijia Shen,a Jingwei Hou,*b, c Yiming Zhang,a Winny Fam,b Jindun Liu,a Thomas Douglas Bennettc and Vicki Chenb a

School of Chemical Engineering and Energy, Zhengzhou University, 450001, P. R. China

b

UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering,

University of New South Wales, Sydney, Australia c

Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK

KEYWORDS Gas Separation Membrane; Pebax; Halloysite Nanotubes; Microphase Separation; Nanocomposite Membrane

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

ABSTRACT

Block copolymer materials have been considered as promising candidates to fabricate gas separation membranes. This microphase separation affects the polymer chain packing density and molecular separation efficiency. Here we demonstrate a method to template microphase separation within a thin composite Pebax membrane, through the controllable self-assembly of one-dimensional halloysite nanotubes (HNTs) within the thin film via solution casting technique. Crystallisation of the polyamide component is induced at the HNT surface, guiding subsequent crystal growth around the tubular structure. The resultant composite membrane possesses an ultrahigh selectivity (up to 290) for the CO2/N2 gas pair, together with a moderate CO2 permeability (80.4 barrer), being the highest selectivity recorded for Pebax-based membranes, and it easily surpasses the Robeson upper bound. The templated microphase separation concept is further demonstrated with the nanocomposite hollow fiber gas separation membranes, showing its effectiveness of promoting gas selectivity.


2

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction The properties of block copolymers (BCPs), constructed by two or more immiscible macromolecular sections, are governed by the functionality of their constituent sections, and include good processability and chemical and thermal stability.1,2 Exertion of control over the self- assembly of these randomly arranged components to form long-range ordering microstructures, i.e. microphase separation, remains a significant challenge however. Whilst the fabrication of various devices e.g. field effect transistors, bio patterned media and cellular interface with BCP has been well demonstrated,3–7 methodologies to accurately control their microphase separation would enable the creation of more complex nanoscale structures for catalysis, sensing, semiconducting and photonic devices.8,9 BCPs have been extensively investigated for gas separation membranes. Among different polymer materials, polyether(PE)-block-polyamide (PA) copolymer (Pebax®, a trade name of Arkema) has been considered as one of the most important candidates,10 due to its ease of processing and satisfactory gas separation performance, making it attractive for large-scale application. Conventionally, it was considered that gas permeation took place via the more amorphous PE ‘soft’ phase, with the ‘glassy’ PA phase remaining impermeable.11 However, increasing evidence has shown that the nanoscale arrangement between soft and hard phase structures plays an important role to determine the Pebax membrane performance.12,13 For example, small angle X-ray scattering (SAXS) has been used to study the relationship between microphase separation and gas separation behaviors for thick Pebax membranes.14 The shape and orientation of the microphases have been correlated to the gas transport within the film, with imperfect microphase separation compromising the membrane permeability and selectivity simultaneously.


3


Page 4 of 26

Even though the controlled microphase separation has been applied to produce desired porous structures for water treatment membranes,15,16 understanding and subsequent regulating the microphase separation have been challenging for all but a few gas separation membrane systems. This is because the microphase separation within non-porous gas separation membrane involves nanoscale polymer chain arrangement and thus can be more complicated: it is susceptible to the interactions for the boundary region, thickness of the overall layer, and the interface effect. All these aspects can further affect the separation performance of the resultant membranes. During the membrane preparation process, the top layer of the polymer was initially in its relaxed free polymer phase, and then gradually transferred to a more random polymeric structure. As a result, the faster sol-to-gel transition would lead to more random polymeric structure, and potential defects or imperfection on the surface.17 For the thin composite membranes, the effect of microphase separation can be more dominant in regards to the membrane performance, while simultaneously even more difficult to regulate. Considering the large interfacial area, the addition of nanofillers may perform as templates for controlled microphase separation,18 but this has not been fully explored for gas separation membranes. Herein, we demonstrated a technique to control the microphase separation within a composite Pebax-based flat sheet membrane. 1D halloysite nanotubes (HNTs) [Al2(OH)4Si2O5], which are naturally abundant nanoclays with a unique 1D structure,19,20 were modified and then blended into the Pebax casting solution as microphase separation templates to guide the polymer crystallization. The 1D nanotubes have been extensively investigated as nanofiller to promote the antifouling, antibacterial and water flux for the water treatment membranes.21–23 In addition, the self-alignment of such nanomaterials can form a selective layer to separate organic dye molecules from salty water. Due to the hydrophilic nature of the material, the membrane


4

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exhibited good anti-fouling behaviour against the organic dyes.24 Their interactions with Pebax upon casting into membrane fabrication were investigated, finding HNT alignment on membrane surface can be controlled via entropy driven self-assembly, enabling to link HNT spatial distribution and membrane performance. This guided microphase separation concept was finally validated with hollow fiber composite membranes, which are of more practical importance. 2. Experimental Section 2.1 Materials For the fabrication of flat sheet composite membranes, polysulfone ultrafiltration (PSf, MWCO=50000, Beijing Wodun Technology Development Co., Ltd) was used. For the fabrication of hollow fiber composite membranes, polyvinylidene fluoride (PVDF, Beijing OriginWater Technology Co., Ltd. (China)) porous membranes were applied. The hollow fiber membranes had an outer diameter of 1.0 mm, an inner diameter of 0.5 mm, and an average pore size of around 50 nm. For the gutter layer materials, polydimethylsiloxane (PDMS, average Mw=115,000) was purchased from J&K Scientific Ltd. (China) and poly [1-(trimethylsilyl) prop-1-yne] (PTMSP) was obtained from Gelest, Inc., PA, USA. Dibutyltin dilaurate (DBD, 97.5 wt %), and tetraethyl orthosilicate (TEOS, 98 wt %), both from J&K Scientific Ltd. (China), were applied for PDMS gutter layer coating. Halloysite nanotubes (HNTs) were kindly provided by Henan Xianghu Environmental Protection Technology Co., Ltd. Poly (sodium-p-styrene sulfonate) (PSS) were obtained from J&K Scientific Ltd. Commercial block copolymer Pebax 1657 was purchased from Arkema Inc. All the chemicals were of the highest purity and used without further purification. 2.2 PSS modification of HNTs


5


Page 6 of 26

PSS (2 g) was initially dispersed in deionized water (100 ml) and magnetically stirred for 30 min to obtain a homogenous suspension. Then, 2g HNTs were gradually added to. the above solution under continuous agitation. The solution was then left under constant stirring for 48 h at ambient condition and then centrifuged at 6000 rpm for 10 min. The collected HNTs were washed with water for 3-4 times, and finally dried in vacuum oven under room temperature. 2.3 Preparation of flat sheet composite membrane Briefly, the schematic diagram of fabricating flat sheet composite membranes is demonstrated in Scheme 1. For the gutter layer, the coating solution contained 2 wt % PDMS, 1 wt % TEOS and 1 wt% DBD in n-heptane. For the Pebax-based selective layer, a certain amount of the modified HNTs (0.05, 0.1, 0.15 and 0.2 wt % towards the casting solution) were dispersed in H2O/ethanol (30/70 by weight) under sonication. Subsequently, 3 wt % of Pebax was added to the solution, stirred at 80 °C under reflux for 2 h to obtain homogeneous Pebax coating solutions. For the preparation of the flat sheet composite membranes, the PSf substrate membrane was firstly thoroughly rinsed with sodium dodecyl sulfate (SDS) solution to remove the preservatives, and then the membrane was mounted in a stainless-steel membrane module. 0.25 mL PDMS solution (containing 2 wt % PDMS in n-heptane) was dropped on top of the PSf support membrane (ca. 28.3 cm2) evenly and then allowed the gradual evaporation of the solvent under ambient environment.25 Subsequently, the membrane was heated to 50 °C and kept for 12 h for the cross-linking reaction. Then the selective layer coating was achieved using the similar evaporation coating technique with the Pebax coating solution.


6

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Schematic diagram of the flat sheet membrane fabrication process. (a) Drop coating of the PDMS gutter layer. (b) Drop coating of the Pebax/HNTs selective layer. (c) Structure of the resultant composite flat sheet membrane.

2.4 Hollow fiber composite membrane fabrication In this work, 0.1 wt % HNTs were dispersed in 3 wt% Pebax coating solution for hollow fiber composite membrane fabrication. Dip coating technique was applied to obtain a thin Pebaxbased selective layer on top of a four-cycle-coated PTMSP gutter layer. More detailed membrane fabrication and testing conditions can be found in our previous publication.12 In this work, the hollow fiber membrane modules were tested under 0.1 MPa pressure. 2.5 Gas permeation experiments The custom-built gas separation membrane testing apparatus were applied for the test of both flat sheet membranes and hollow fibers. For the flat-sheet membrane module, the effective


7


Page 8 of 26

membrane area was 19.6 cm2 and the temperature control was achieved with a thermostatic cabinet. The flow rate of the permeate during the testing process was recorded by a soap-film flow meter. To minimize the experimental error, each test was carried out for at least 30 min. In this work, pure CO2 and N2 gas were used as the feed gas. For the humid feed, gas was bubbled through a humidifier. For the hollow fiber membranes, they were fabricated with polyvinylidene fluoride (PVDF) hollow fibre porous supports. The supporting membranes were firstly coated with four cycles of 3 wt % poly[1-(trimethylsilyl)-1-propyne] (PTMSP) in hexane, as the PDMS can’t effectively cover the relatively large supporting membrane pores. The composite membranes were then mounted in a quarter inch stainless steel membrane module with an effective membrane area of 17 cm2. The gas permeation tests were conducted under 25 ºC, with a feed pressure of 1 bar. More information about the experimental setup can be found in our earlier publication.26,27 2.6 Characterization Transmission electron microscope (TEM, FEI TECNAI G2) was applied to investigate the morphologies of HNTs. The acceleration voltage was 200 kV and the samples were dispersed on a copper mesh. X-ray diffraction (XRD) analysis of HNTs and modified HNTs was conducted on a PAN Analytical X’Pert Pro (PANalytical, The Netherlands). Copper Kα was used as the radiation source, and the 2θ range was between 5˚ and 80˚ with a step size of 0.02˚. Gas adsorption isothermal was investigated with an ASAP 2420 from Micromeritics Instrument Corporation. The membrane surface and cross-sectional morphology was investigated with scanning electron microscopy (SEM, JEOL Model JSM-6700F). The cross-sectional sample was prepared by fracturing the membrane after soaking in liquid nitrogen.


8

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Atomic force microscopy (AFM) was further performed to investigate the surface morphologies and roughness of the prepared membranes with different HNTs loadings by using Dimension FastScan (Bruker, American).12 Attenuated total reflection-Fourier transform infrared spectra (ATR-FTIR) were used to study the surface chemical groups of the HNTs. X-ray photoelectron spectroscopy (XPS, KRATOS, Japan) was performed to further study the information of elements on the membrane surface. In order to better characterize the interaction between HNTs and Pebax, freestanding Pebax/HNTs films were applied for thermal and phase image analysis. For the thermal analysis, the films were fully dried and then placed in an aluminum pan. The testing temperature was -100 °C (using liquid nitrogen) to 250 °C at a rate of 10 °C/min under a purge gas of N2 at 10 mL/min. The calculation procedure of the crystallinity degree for both the soft and hard phases can be found in our previous publication.12 To study the microphase separation of the HNTs-Pebax composites, the phase images were taken with the scanning probe microscopy equipped with a silicon nitride probe (Bruker Dimension Icon SPM). The testing was under a peak force in tapping mode.12 3 Results and discussion HNTs are formed by rolling flat-sheet kaolinite, producing tubes with SiO2 functionalized outer surfaces, and inner cylinder surfaces of rich Al2O3 concentration. The architecture gives rise to inter-layer channels of 7 Å, and a lumen tube of ~10 diameter in the center of the structure (Figure 1a).28,29 Whilst HNTs have shown promising potential for water purification membranes,30 this has not been demonstrated for gas separation membranes due to unselective gas transport through the relatively wide lumen tube. However, the undesirable effect can be


9


Page 10 of 26

effectively suppressed via horizontal alignment of HNTs, where the non-selective lumen tube would not significantly affect the general gas transport behavior through the whole membrane. Ideally, alignment of the rigid HNTs during fabrication can be achieved via entropy-driven phase transitions, but this process needs to be carried out under equilibrium conditions where entropy effects dominate.24 Therefore, to improve the colloidal stability of the HNTs within the membrane casting solution, poly(styrene sulfonate) (PSS) modification was conducted.31 Based on our previous research, the PSS-modified HNTs are stable as a suspension in an aqueous environment, even after storage for over 1 week, no sedimentation was observed.24 In this work, the prepared membrane casting solution with the modified HNTs can maintain its colloidal stability after 3 days storage at ambient condition (results not shown).

Figure 1. (a) Schematic of the HNT and crystal structure of hydrated halloysite. TEM image of (b) pristine HNTs and (c) PSS-modified HNTs. (d) XRD patterns of pristine HNTs and PSSmodified HNTs. (e) Experimental adsorption isotherms for CO2 and N2 adsorption of HNTs and PSS-HNTs at 298 K. (f) Nitrogen adsorption-desorption isotherms of HNTs and PSS modified HNTs at 77 K.


10

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As shown in Figure 1b-c, both pristine and PSS-modified HNTs exhibited lengths of ca. 4001000 nm, an internal diameter of 10 nm and an external diameter of 60-80 nm – forming cylindrical sharp, hollow, open-ended structures. PSS modification was confirmed by FTIR (Figure S1), and had a negligible effect on structure (Figure 1d), with the two major diffraction peaks in the powder X-ray diffraction (PXRD) patterns at 2θ=12.2° (d001 of 7.23Å) and 2θ=24.7° (d002 of 3.6Å) largely invariant. The observed interlayer spacing of 7.23Å is in accordance with the reported HNT structure.32 Figure 1e shows the adsorption of CO2 and N2 at 298 K for the HNTs and PSS-modified explained by the preferable dipole-quadrupole interaction between CO2 and hydroxyl groups on the kaolinite layers. For the adsorption-desorption isotherms of nitrogen at 77K (Figure 1f), both pristine HNTs and PSS-HNTs exhibit type II isotherms with H3 hysteresis loops.33 The narrow hysteresis loops at a relative pressure higher than P/P0=0.7 is caused by capillary condensation within the interlayer channels (Figure S2). The PSS modification only slightly reduced the HNTs BET surface area and BJH pore volume, confirming a very thin modification layer (Table S1). For the BJH pore size distribution profiles (Figure S3), the broad peak at about 10 nm is the lumen diameter of halloysite nanotube, corresponding well with TEM results (Figure 1b, 1c), and remaining largely unchanged upon PSS modification.24 PSS modified HNTs (unless otherwise stated) were used for subsequent membrane fabrication. To fabricate the flat sheet HNT/Pebax composite membrane, the casting solution was dropcoated onto polysulfone (PSf) porous supports (Scheme 1). To minimize the penetration of the casting solution into the porous substrates, the PSf membrane was first coated with a layer of a


11


Page 12 of 26

highly gas permeable polydimethylsiloxane (PDMS) gutter layer, of ca 400 nm thickness (scanning electron microscope (SEM) images in Figure. S4a). After the subsequent coating of the HNT/Pebax composite layer (containing 0.1 wt% HNTs) the top surface image (Figure 2a-b) indicates horizontally assembled HNTs adopting locally parallel alignments. This leads to a high packing density of HNTs at the composite membrane surface region. Considering the density of the pure Pebax membrane is 1.13 g/cm3,27 the addition of 0.1 wt % of HNTs (2.53 g/cm3) in the 3 wt % Pebax solution should lead to a final volume ratio of ~1:29.8 (HNTs:Pebax) within the selective layer. However, based on the SEM images (Figure 2c), the presence of modified HNTs within the bulk Pebax is limited, indicating the higher concentration of HNTs in the top surface region. During the gradual solvent evaporation process, the HNTs-solvent interface is replaced by HNTs-Pebax solid-solid interface, which has a higher surface energy, causing migration of the HNTs towards the membrane surface, to replace the solid-liquid interface with solid-gas interface, in order to minimize this overall surface energy.34 During the surface aggregation process, the rigid nanotubes still possess free local translational and rotational movement, and the HNTs gradually change to the nematic phase. Even though the alignment of the nanomaterials may look thermal dynamically unfavorable, as the alignment of HNTs can lead to entropy loss. The effect can be compensated by the increased translation and rotational entropy: the aligned HNTs have higher translational and rotational freedom degree compared to the randomly oriented HNTs. Similar entropy-driven phase transition has been reported during the liquid-crystal alignment process (Figure 2d).35,36 More detailed mechanism of the HNTs alignment during the self-assembly process can be found in our earlier publication.24 In addition, the evaporation-induced self-assembly can lead to an unwanted “coffee-ring” effect,


12

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where during the evaporation the suspended particles in solution tend to aggregate at the liquid edge region due to the capillary flow of the curved liquid surface, leading to an uneven coating via evaporation. However, the high aspect ratio of the major-axis/minor-axis (α) for HNTs suppress the “coffee-ring” effect via the shape-dependent capillary interaction, which also facilitates the dense packing of HNTs.37,38 It should be noted that the thin nanocomposite membrane still preserves good flexibility, indicating a good composition between the nanofillers and Pebax matrix.


13


Page 14 of 26

Figure 2. (a) Top-layered SEM of PSf membrane. (b) Top-layer and (c) cross-sectional morphologies of a composite membrane containing 0.1 wt % modified HNTs. (d) Schematic diagram of the Pebax/HNTs composite membrane formation process. PDMS is omitted for clarity. (e-g) FT-IR results of the membrane samples with different HNTs loadings.

The presence of HNTs is also confirmed by XPS tests (Figure S5-6, and Table S2). For the membrane FTIR spectra (Figure 2e-g), the peak at 3300 cm-1 is assigned N-H in Pebax matrix, and the peak at 1640 cm-1 is originated from the hydrogen-bonded C=O in Pebax. The incorporation of HNTs can lead to peak shifts for the chemical groups in the glassy PA sections


14

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of Pebax, confirming the emergence of hydrogen bond between HNTs and Pebax matrix.27 This can rigidify the polymer chains and increase the crystallinity of the Pebax membrane (Table S3).39 SEM imaging of membranes fabricated with different HNTs concentrations are presented in Figure S4. Higher concentrations within the casting solution lead to the increased membrane surface roughness (Table S4). At the highest HNTs loading, homogeneous coatings became difficult to achieve and aggregation is observed, which disrupts the alignment of HNTs during the evaporation process and reduces the packing density at the surface (Figure S4f). Representative AFM height and adhesion images of the Pebax membrane surface clearly show a nanostructure microphase-separation (Figure 3a), with the PA glassy segment (rod-like structure) embedded in the rubbery PEO segment with random orientation. In comparison, for the Pebax/HNTs membrane (Figure 3b), the PA phase near the HNTs surface is more organized, largely vertical to the HNTs long axis with the criss-cross arrangement. The driving force for the dynamic microphase separation is ruled by relatively weak intermolecular interactions, and such interaction is usually at the same order of magnitude as the entropy effects. For the pure Pebax layer, since the nucleation is limited, the formed PA lamellae and spherulites can grow to form a large rod-like structure with low packing density. Therefore, it is difficult to obtain a microdefect free film in a macroscopic area due to the limited driving force and mobility of the selfassembling moieties.40 The probable existence of defects also explains the deviation of the Pebax membrane performance as a thin (50 µm).27 In comparison, for the HNTs-Pebax composite system, the strong interaction between HNTs and PA can overcome the considerable surface energy at the two immiscible polymer blocks.41 Phase separation first occurs at the surface of HNTs, due to the preferred interaction between PA sections and HNTs, with subsequent crystal growth around the HNTs tubular structure.


15


Page 16 of 26

Therefore, the glassy blocks are frozen into relatively small domains, resulting in higher aspect ratio, high molecular organization and packing density, which provides effective control against nanoscale defects thus can potentially lead to better gas selectivity.42 It should be noted that the elastic strain energy associated with the parallel arrangement of the microphase regions can be significant, therefore is difficult to accommodate within in a thin film even at the HNTs surface region.43

Figure 3. Microphase separation and gas separation performance of the Pebax/HNTs membranes. AFM height and adhesion images of (a) neat Pebax 1657 and (b) Pebax/0.1wt%HNTs membranes. Gas separation performance for membranes with (c) PSSHNTs and (d) pristine HNTs. The testing pressure is 0.3 MPa. (e) Schematic diagram of the templated microphase separation, with glassy PA in a blue rod shape and rubbery PE omitted for clarity; and the gas transport mechanism of the organized and randomly arranged Pebax/HNTs layers. The CO2/N2 gas separation performance of the prepared membrane was examined using driedstate pure gas measurements. The CO2 permeance of the Pebax membrane is 38.8 GPU


16

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(permeability of 165 Barrer) with a moderate CO2/N2 selectivity of 44. This result is in agreement with the inherent membrane performance for Pebax material.27 After the addition of HNTs (Figure 3c), the membrane exhibits a dramatically increased selectivity for CO2/N2 of up to 245 while the CO2 permeance remained at around 10 GPU (permeability of approximately 7090 Barrer) and relatively unchanged for membranes with different HNTs loadings. The highest selectivity is observed with 0.1 wt % HNTs, where the horizontally aligned HNTs at the surface region ensures densely packed glassy PA chains, which further leads to the ultraselective gas transport.44 With further increased HNTs loading, the membrane can experience a loss of selectivity, possibly due to the loss of uniform arrangement of the HNTs structure at the high loading conditions: the randomly oriented HNTs allow less selective gas transport via the lumen tube and the interspaces between HNTs. To further explore this, the pristine HNTs were added to Pebax matrix for flat sheet composite membrane fabrication, with the randomly oriented pristine HNTs within the Pebax layer (Figure S7), the gas selectivity is lower than the membrane with PSS-HNTs (Figure 3d-e). Still, the composite membrane with pristine HNTs has a significantly higher selectivity over the pure Pebax membranes, proving the effectiveness of the templated microphase separation with HNTs. After the membrane testing with dry gases, we applied the same batch of membranes for the separation of humid feed gas (Figure 4a). The consistent performance suggests the membranes have good integrity even at the highest testing pressure. Compared with dry conditions, the gas permeance is slightly higher accompanied with a slightly lower selectivity, which can be attributed to the swelling of Pebax polymeric matrix. The loss of selectivity with the increase of operational pressure (Figure 4a and Figure S8) has also been observed with our previous work on


17


Page 18 of 26

the Pebax/ZIF-8 composite membranes.27 The long-term stability test of the membrane (Figure S9) suggests a good operational stability for the membrane.


18

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. a) Comparison of 0.1 wt % HNTs-Pebax/PDMS/PSf membrane at the dry condition and humid condition (with the relative humidity of 80% at 25 ºC). (b) Comparison of membrane performance with other Pebax-based membranes. Schematic diagram of the hollow fiber membrane (c) fabrication process and (d) composite structure. (e) Gas separation performance of the hollow fiber composite membrane with different selective layer. The nanocomposite hollow fiber contains 0.1 wt % modified HNTs. (f) Surface SEM images of the membrane with a pure Pebax selective layer. (g-h) Surface and cross-sectional SEM images of the membrane with Pebax/HNTs selective layer (covered with an extra layer of Pebax protective layer).


19


Page 20 of 26

The membrane performance in this work is compared with other Pebax-based MMMs from literature (Figure 4b).45–47 It is clearly indicated that all MMMs in this work has transcended the upper-bound line, and the selectivity is the highest among all the reported Pebax-based membranes. It should be noted the composite membranes in this work outperform pure PA membranes in both permeance and selectivity, possibly due to the low polymer chain packing density due to the highly glassy nature of PA chains.48,49 To further investigate the versatility of this templated microphase separation concept, a series of hollow fiber composite membranes with the Pebax/HNTs selective layer was fabricated, and the membrane also exhibits a significant increase of selectivity (up to 125 for CO2/N2, permeance 77 GPU, Figure 4c-h). Considering the much less controlled segregation and alignment of HNTs during the hollow fiber composite membrane fabrication, there is still great potential to further optimize the Pebax/HNTs hollow fiber composite membranes for industrial applications.

Conclusions

In summary, Pebax/HNTs composite membranes were successfully prepared on both flat sheet and hollow fiber supports. During the membrane fabrication process, the HNTs acted as crystallization templates for microphase separations, leading to more packed PA sections near the HNTs surface, and subsequently more selective gas transport through the membranes. Our findings also underline the intrinsic relationship of the spatial arrangement of the 1D nanofiller to the membrane performance, and highlighted the opportunities that microphase separation templates can provide in membrane defect control. Through this work, we expect that the controlled microphase separation concept can be expanded for other membrane applications. This work also leads to unsolved


20

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


questions that merit further study, for example the microphase separation mechanism within ultrathin nanocomposite membranes, and the effects controllable microphase separation on membrane separation efficiency. In addition, the evaporation-induced selfassembly membrane fabrication technique opens opportunities for further optimization, e.g., making a thinner composite layer by regulating the evaporation conditions such as temperature, concentration, viscosity, humidity and so on.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI The Supporting Information contains FTIR, gas adsorption and BET surface area of the HNTs; SEM, XPS, DSC, AFM characterization results; and the gas permeation results of the composite membranes.

AUTHOR INFORMATION Corresponding Author *Dr. Jingwei Hou, [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here.


21


Page 22 of 26

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. U1704139 and 21376225), Program for Science & Technology Innovation Talents in Universities of Henan Province (16HASTIT004), Key Science and Technology Program of Henan Province (182102310013), and Training Plan for Young Backbone Teachers in Universities of Henan Province (2017GGJS002). This work was also supported by UNSW Faculty of Engineering Silverstar Award. TDB would like to thank the Royal Society for a University Research Fellowship and their support.

REFERENCES (1)

(2)

(3) (4)

(5)

(6)

(7) (8)

Ji, X.; Dong, S.; Wei, P.; Xia, D.; Huang, F. A Novel Diblock Copolymer with a Supramolecular Polymer Block and a Traditional Polymer Block: Preparation, Controllable Self-Assembly in Water, and Application in Controlled Release. Adv. Mater. 2013, 25 (40), 5725–5729. Warren, N. J.; Armes, S. P. Polymerization-Induced Self-Assembly of Block Copolymer Nano-Objects via RAFT Aqueous Dispersion Polymerization. J. Am. Chem. Soc. 2014, 136 (29), 10174–10185. Stoykovich, M. P. Directed Assembly of Block Copolymer Blends into Nonregular Device-Oriented Structures. Science 2005, 308 (5727), 1442–1446. Xiao, S.; Yang, X.; Steiner, P.; Hsu, Y.; Lee, K.; Wago, K.; Kuo, D. Servo-Integrated Patterned Media by Hybrid Directed Self-Assembly. ACS Nano 2014, 8 (11), 11854– 11859. Guo, L.; Wang, Y. Monolithic Membranes with Designable Pore Geometries and Sizes via Retarded Evaporation of Block Copolymer Supramolecules. Macromolecules 2015, 48 (23), 8471–8479. Son, J. G.; Son, M.; Moon, K.-J.; Lee, B. H.; Myoung, J.-M.; Strano, M. S.; Ham, M.-H.; Ross, C. A. Sub-10 Nm Graphene Nanoribbon Array Field-Effect Transistors Fabricated by Block Copolymer Lithography. Adv. Mater. 2013, 25 (34), 4723–4728. Wang, Y.; Li, F. An Emerging Pore-Making Strategy: Confined Swelling-Induced Pore Generation in Block Copolymer Materials. Adv. Mater. 2011, 23 (19), 2134–2148. Ross, C. A.; Berggren, K. K.; Cheng, J. Y.; Jung, Y. S.; Chang, J.-B. Three-Dimensional Nanofabrication by Block Copolymer Self-Assembly. Adv. Mater. 2014, 26 (25), 4386– 4396.


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9)

(10)

(11)

(12)

(13)

(14)

(15) (16)

(17)

(18) (19)

(20)

(21)

(22)

(23)

Bita, I.; Yang, J. K. W.; Jung, Y. S.; Ross, C. A.; Thomas, E. L.; Berggren, K. K. Graphoepitaxy of Self-Assembled Block Copolymers on Two-Dimensional Periodic Patterned Templates. Science 2008, 321 (5891), 939–943. Car, A.; Stropnik, C.; Yave, W.; Peinemann, K.-V. Tailor-Made Polymeric Membranes Based on Segmented Block Copolymers for CO2 Separation. Adv. Funct. Mater. 2008, 18 (18), 2815–2823. Yoshino, M.; Ito, K.; Kita, H.; Okamoto, K.-I. Effects of Hard-Segment Polymers on CO2/N2 Gas-Separation Properties of Poly(Ethylene Oxide)-Segmented Copolymers. J. Polym. Sci. Part B Polym. Phys. 2000, 38 (13), 1707–1715. Wang, Y.; Li, H.; Dong, G.; Scholes, C.; Chen, V. Effect of Fabrication and Operation Conditions on CO2 Separation Performance of PEO–PA Block Copolymer Membranes. Ind. Eng. Chem. Res. 2015, 54 (29), 7273–7283. Fam, W.; Mansouri, J.; Li, H.; Chen, V. Improving CO2 Separation Performance of Thin Film Composite Hollow Fiber with Pebax®1657/Ionic Liquid Gel Membranes. J. Membr. Sci. 2017, 537, 54–68. Barbi, V.; Funari, S. S.; Gehrke, R.; Scharnagl, N.; Stribeck, N. SAXS and the Gas Transport in Polyether- b Lock -Polyamide Copolymer Membranes. Macromolecules 2003, 36 (3), 749–758. Seo, M.; Hillmyer, M. A. Reticulated Nanoporous Polymers by Controlled Polymerization-Induced Microphase Separation. Science 2012, 336 (6087), 1422–1425. Dorin, R. M.; Phillip, W. A.; Sai, H.; Werner, J.; Elimelech, M.; Wiesner, U. Designing Block Copolymer Architectures for Targeted Membrane Performance. Polymer 2014, 55 (1), 347–353. Saba, S. A.; Mousavi, M. P. S.; Bühlmann, P.; Hillmyer, M. A. Hierarchically Porous Polymer Monoliths by Combining Controlled Macro- and Microphase Separation. J. Am. Chem. Soc. 2015, 137 (28), 8896–8899. Spatz, J. P.; Sheiko, S.; Möller, M. Substrate-Induced Lateral Micro-Phase Separation of a Diblock Copolymer. Adv. Mater. 1996, 8 (6), 513–517. Hashemifard, S. A.; Ismail, A. F.; Matsuura, T. Effects of Montmorillonite Nano-Clay Fillers on PEI Mixed Matrix Membrane for CO2 Removal. Chem. Eng. J. 2011, 170 (1), 316–325. Xiang, L.; Pan, Y.; Zeng, G.; Jiang, J.; Chen, J.; Wang, C. Preparation of Poly(EtherBlock-Amide)/Attapulgite Mixed Matrix Membranes for CO2/N2 Separation. J. Membr. Sci. 2016, 500, 66–75. Hebbar, R. S.; Isloor, A. M.; Ananda, K.; Ismail, A. F. Fabrication of Polydopamine Functionalized Halloysite Nanotube/Polyetherimide Membranes for Heavy Metal Removal. J. Mater. Chem. A 2016, 4 (3), 764–774. Chen, Y.; Zhang, Y.; Zhang, H.; Liu, J.; Song, C. Biofouling Control of Halloysite Nanotubes-Decorated Polyethersulfone Ultrafiltration Membrane Modified with ChitosanSilver Nanoparticles. Chem. Eng. J. 7, 228, 12–20. Zhang, J.; Zhang, Y.; Chen, Y.; Du, L.; Zhang, B.; Zhang, H.; Liu, J.; Wang, K. Preparation and Characterization of Novel Polyethersulfone Hybrid Ultrafiltration Membranes Bending with Modified Halloysite Nanotubes Loaded with Silver Nanoparticles. Ind. Eng. Chem. Res. 2012, 51 (7), 3081–3090.


23


Page 24 of 26

(24) Qin, L.; Zhao, Y.; Liu, J.; Hou, J.; Zhang, Y.; Wang, J.; Zhu, J.; Zhang, B.; Lvov, Y.; Van der Bruggen, B. Oriented Clay Nanotube Membrane Assembled on Microporous Polymeric Substrates. ACS Appl. Mater. Interfaces 2016, 8 (50), 34914–34923. (25) Shen, Y.; Wang, H.; Zhang, X.; Zhang, Y. MoS2 Nanosheets Functionalized Composite Mixed Matrix Membrane for Enhanced CO2 Capture via Surface Drop-Coating Method. ACS Appl. Mater. Interfaces 2016, 8 (35), 23371–23378. (26) Hou, J.; Sutrisna, P. D.; Zhang, Y.; Chen, V. Formation of Ultrathin, Continuous Metal– Organic Framework Membranes on Flexible Polymer Substrates. Angew. Chem. Int. Ed. 2016, 55 (12), 3947–3951. (27) Sutrisna, P. D.; Hou, J.; Li, H.; Zhang, Y.; Chen, V. Improved Operational Stability of Pebax-Based Gas Separation Membranes with ZIF-8: A Comparative Study of Flat Sheet and Composite Hollow Fibre Membranes. J. Membr. Sci. 2017, 524, 266–279. (28) Lvov, Y.; Wang, W.; Zhang, L.; Fakhrullin, R. Halloysite Clay Nanotubes for Loading and Sustained Release of Functional Compounds. Adv. Mater. 2016, 28 (6), 1227–1250. (29) Hofmann, U. On the Chemistry of Clay. Angew. Chem. Int. Ed. Engl. 1968, 7 (9), 681– 692. (30) Zhao, Q.; Liu, C.; Liu, J.; Zhang, Y. Development of a Novel Polyethersulfone Ultrafiltration Membrane with Antibacterial Activity and High Flux Containing Halloysite Nanotubes Loaded with Lysozyme. RSC Adv. 2015, 5 (48), 38646–38653. (31) Zhao, Y.; Cavallaro, G.; Lvov, Y. Orientation of Charged Clay Nanotubes in Evaporating Droplet Meniscus. J. Colloid Interface Sci. 2015, 440, 68–77. (32) Liu, M.; Jia, Z.; Jia, D.; Zhou, C. Recent Advance in Research on Halloysite NanotubesPolymer Nanocomposite. Prog. Polym. Sci. 8, 39 (8), 1498–1525. (33) Wang, Q.; Zhang, J.; Wang, A. Alkali Activation of Halloysite for Adsorption and Release of Ofloxacin. Appl. Surf. Sci. 2013, 287, 54–61. (34) Hou, J.; Dong, G.; Ye, Y.; Chen, V. Laccase Immobilization on Titania Nanoparticles and Titania-Functionalized Membranes. J. Membr. Sci. 2014, 452, 229–240. (35) Gupta, S.; Zhang, Q.; Emrick, T.; Balazs, A. C.; Russell, T. P. Entropy-Driven Segregation of Nanoparticles to Cracks in Multilayered Composite Polymer Structures. Nat. Mater. 2006, 5 (3), 229–233. (36) Frenkel, D. Order through Entropy. Nat. Mater. 2015, 14 (1), 9–12. (37) Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the Coffee-Ring Effect by Shape-Dependent Capillary Interactions. Nature 2011, 476 (7360), 308–311. (38) Luo, D.; Yan, C.; Wang, T. Interparticle Forces Underlying Nanoparticle Self-Assemblies. Small 2015, 11 (45), 5984–6008. (39) Shen, J.; Liu, G.; Huang, K.; Li, Q.; Guan, K.; Li, Y.; Jin, W. UiO-66-Polyether Block Amide Mixed Matrix Membranes for CO2 Separation. J. Membr. Sci. 2016, 513, 155–165. (40) Whitesides, G. M. Self-Assembly at All Scales. Science 2002, 295 (5564), 2418–2421. (41) Yave, W.; Car, A.; Funari, S. S.; Nunes, S. P.; Peinemann, K.-V. CO2 -Philic Polymer Membrane with Extremely High Separation Performance. Macromolecules 2010, 43 (1), 326–333. (42) McLean, R. S.; Sauer, B. B. Tapping-Mode AFM Studies Using Phase Detection for Resolution of Nanophases in Segmented Polyurethanes and Other Block Copolymers. Macromolecules 1997, 30 (26), 8314–8317.


24

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Sohn, B. H.; Yun, S. H. Perpendicular Lamellae Induced at the Interface of Neutral SelfAssembled Monolayers in Thin Diblock Copolymer Films. Polymer 2002, 43 (8), 2507– 2512. (44) Shen, J.; Liu, G.; Huang, K.; Jin, W.; Lee, K.-R.; Xu, N. Membranes with Fast and Selective Gas-Transport Channels of Laminar Graphene Oxide for Efficient CO2 Capture. Angew. Chem. Int. Ed. 2015, 54 (2), 578–582. (45) Nafisi, V.; Hägg, M.-B. Development of Dual Layer of ZIF-8/PEBAX-2533 Mixed Matrix Membrane for CO2 Capture. J. Membr. Sci. 2014, 459, 244–255. (46) Wang, S.; Liu, Y.; Huang, S.; Wu, H.; Li, Y.; Tian, Z.; Jiang, Z. Pebax–PEG–MWCNT Hybrid Membranes with Enhanced CO2 Capture Properties. J. Membr. Sci. 2014, 460, 62– 70. (47) Zarshenas, K.; Raisi, A.; Aroujalian, A. Mixed Matrix Membrane of Nano-Zeolite NaX/Poly (Ether-Block-Amide) for Gas Separation Applications. J. Membr. Sci. 2016, 510, 270–283. (48) Albo, J.; Wang, J.; Tsuru, T. Gas Transport Properties of Interfacially Polymerized Polyamide Composite Membranes under Different Pre-Treatments and Temperatures. J. Membr. Sci. 2014, 449, 109–118. (49) Zarshenas, K.; Raisi, A.; Aroujalian, A. Surface Modification of Polyamide Composite Membranes by Corona Air Plasma for Gas Separation Applications. RSC Adv. 2015, 5 (25), 19760–19772.


25


Page 26 of 26

Table of contents


26

Ultraselective Pebax membranes enabled by ... - ACS Publications

Recommend Documents