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Multiscale Porous Interconnected Nanocolander Network with Tunable Transport Properties Young Hun Kim, Hyo Kang, Sungmin Park, A Reum Park, Yong Man Lee, Do Kyung Rhee, Sungsoo Han, Hyuk Chang, Du Yeol Ryu,* and Pil J. Yoo*
Efficient material transport in a chemical system has been considered to be one of the most important aspects when designing high-performance catalysts or membranes.[1,2] To impart enhanced transport characteristics, various attempts have been made in terms of material selection and structural manipulation.[3–6] Among these, hierarchically designed multiscale architectures have received much attention due to their intricate structures and their similarity to naturally formed complex materials.[1] The strategy is to prepare individual functioning elements in such a way that both enhanced transport characteristics and maximized surface activity can be attained concurrently,[7–9] the former through larger pathways and the latter from finer pathways. When these well-designed unit elements are extended and accumulated into a three-dimensionally (3D) integrated system on a bulk scale, however, the total activity or performance can be seriously undermined, despite the use of a multiscale architecture for individual functioning elements. At the core of the problem is non-parallel utilization of functional sites and consecutive spatial dependency of the activity inside the 3D structure, which is related to individual addressability. To resolve this limitation, which is inevitable in practical scale-up processes, simultaneous and independent addressability needs to be established uniformly to each of the unit elements even inside the 3D structure. One plausible solution is to embed additional macro-level pathways within the 3D architecture so as to ensure the structural interconnectivity and facilitate transport characteristics even on a bulk-scale. However, embedding complexly shaped macro-level interconnecting pathways within a nanoarchitectured frame is both technologically Y. H. Kim, A R. Park, Y. M. Lee, D. K. Rhee, Prof. P. J. Yoo School of Chemical Engineering SKKU Advanced Institute of Nanotechnology (SAINT) Sungkyunkwan University Suwon 440-746, Republic of Korea E-mail: [email protected] Dr. H. Kang, Dr. S. Han, Dr. H. Chang Advanced Materials Research Centre Samsung Advanced Institute of Technology (SAIT) Yongin 446-712, Republic of Korea S. Park, Prof. D. Y. Ryu Department of Chemical and Biomolecular Engineering Yonsei University Seoul 120-749, Republic of Korea E-mail: [email protected]
DOI: 10.1002/adma.201402436
Adv. Mater. 2014, DOI: 10.1002/adma.201402436
challenging and not cost-effective, typically for large-scale applications. Therefore, it would be desirable to employ a spontaneous structuring method based on self-assembly, obviating the need for extra-processes relying on sophisticated facilities. To accomplish this task, here we present a design strategy in the form of nanocolander networks, wherein a multiscale design is employed to create a macroporous template via colloidal self-assembly and to build into the template mesoscale functional elements based on block copolymer (BCP) microphase separation (Figure 1a). The primary template carrying an inverse-opal (IO) structure is constructed from the self-assembled opal structure of colloidal particles. The microphase-separated block copolymer creates well-defined nanosieves with uniform mesopores inside the IO-structured template, enabling the successful realization of intricate multiscale porous architecture. The empty chamber with nanosieves at the chamber windows (internecking pores) of the IO frame is reminiscent of a colander in nanoscale. Thus, it is termed a nanocolander; successively ordered arrays of nanocolanders are referred to as nanocolander networks. An intriguing phenomenon that occurs during nanocolander network formation is that, as schematically presented in Figure 1b, the procedure of embedding the BCP nanosieves can induce the spontaneous creation of internal macro-conduits which can function as macro-level interconnecting pathways. If some of the internecking pores of the IO template remain open, this would generate free pathways of macroconduits for the transporting medium (permeate) to flow through, or bypasses. Due to the facilitated infiltration of the permeate through the macro-level interconnecting pathways of the internal conduits, the active separation sites of the isoporous block copolymer nanosieves are spread throughout the internal space of the nanocolander network. This results in enhanced transport characteristics when compared with conventional two-dimensional membranes. To create the nanocolander network, colloidal particles (polystyrene, diameter ≈ 600 nm) were first self-assembled into an opal structure (Figure 1a). A UV-curable prepolymer, poly(urethane acrylate) (PUA), was then allowed to permeate through the void space of the opal phase and subsequently cured by UV-irradiation. A free-standing film of the IO structure was obtained after removing the internal colloidal particles using an organic solvent, which yielded a primary macroporous template. Unlike the inorganic precursors that are typically used for IO structures, which yield lots of macroscale defects, the PUA prepolymer has a relatively low shrinkage rate (less than 3%) upon crosslinking, which ensures adequate dimensional
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Figure 1. a) Schematic of the generation of the nanocolander network using sequential combination of macroporous IO templates and mesoporous BCP nanosieves. b) Schematic of embedding macroscale interconnecting pathways inside the nanocolander network to provide enhanced transport characteristics.
stability for large scale applications.[10] The size of internecking pores inside the IO structure can be tuned according to the formulation of PUA prepolymer (see Figure S1 in the Supporting Information, Section S1). The IO structured template was then coated with a BCP solution of polystyrene-block-poly(methyl methacrylate) (PSb-PMMA; 86–88 kg/mol and a PMMA volume fraction of 20–28%, see Figure S2 in the Supporting Information, Section S2) to form BCP films at the necking pores by solvent evaporation. The PUA material constituting the IO structure has a low surface energy (ca. 23 dyne/cm)[11] such that dewetting occurs on the PUA surface, which eventually drives BCP to be localized at the necking pores (Figure 2a). Since the BCP dewetting occurs both in the neighbored IO chambers at the necking pore, the resulting suspended BCP films are expected to be stably mounted at the pores, much as a pulley holds the rope. Solvent evaporation was followed by vacuum-annealing at 170 °C for 24 h, which led to microphase separation between the PS and PMMA blocks. This directed self-assembly of BCP thin films only at the necking pores with air/polymer interfaces
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induced the formation of PMMA cylinders which were oriented normal to the air surface at both sides, as on neutral substrates.[12] Finally, selective removal of the PMMA minor phase by UV exposure and subsequent solvent treatment with acetic acid resulted in the formation of nanosieves at the necking pores (the detailed experimental procedure is available in the Supporting Information, Section S3 and the sequential change of the structural order from IO to BCP-coated IO is characterized with reflectance spectra in Supporting Information, Section S4). This interconnected nanocolander network structure can be readily formed over a large area (2.5 cm × 5 cm) as shown in Figure 2b. The SEM image in Figure 2c shows that while most of the macropores were successfully covered with BCP nanosieves, some pores remained open. This could be associated with the evaporation pathways of the solvent during coating and the subsequent drying of the BCP solution. The perpendicular orientation of the cylinders in the BCP film at the pores was assured by a combination of careful material selection, the formation of the suspended film, and confinement effects. In general, the microdomain
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Adv. Mater. 2014, DOI: 10.1002/adma.201402436
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COMMUNICATION Figure 2. a) Dewetting-induced selective formation of BCP nanosieves at the necking pores of the IO frame. The scanning electron microscopy image shows the emergence of boundaries of the BCP nanosieves after 10 min of annealing at 170 °C. b) Cross-sectional view of a 12 µm-thick membrane. The inset shows a large-scale demonstration of the interconnected nanocolander network as a free-standing film. c) Highly magnified image of the nanocolander network.
orientation of PS-b-PMMA films is significantly influenced by interfacial interactions, particularly for cylinder-forming BCPs.[13] Hence, a neutral condition has been employed for developing the perpendicular orientation of BCP microdomains, in which the formation of the parallel orientation is suppressed due to the balanced interfacial interactions on the substrate. In BCP nanosieves, however, there exists no underlying substrate. Rather, films were suspended at the necking pores, and the symmetric interfaces present on both sides of the BCP films created a confinement effect at the air/polymer interfaces. Because the surface energies of the two different block types were nearly equal (PS: 29.9 dyne/ cm and PMMA: 30.0 dyne/cm at 170 °C), the PMMA cylinders were oriented perpendicular to the film surface.[14] In addition, any IO-structured PUA walls in contact with the BCP thin films turned out to be neutral (or weakly selective) toward the two blocks. This was evaluated by homopolymer dewetting experiments on PUA surface (see the Supporting Information, Section S5). Along with energetics, the BCP thin films were subjected to the geometric constraint of the finite-sized necking pores that formed rim boundaries with the PUA surface.
Adv. Mater. 2014, DOI: 10.1002/adma.201402436
The condition under which the IO chambers remain empty after repetitive coatings with the BCP solution and subsequent drying has been found to hinge on the film thickness at the necking pores. When the BCP film was relatively thin (15–20 nm), and preferential localization at the IO necking pores was facilitated by BCP dewetting on the PUA surface, consecutive formation of nanosieves embedded with the vertical nanopores was promoted (Figure 3a). When the film thickness was increased slightly (20–40 nm) and the spontaneous BCP dewetting on the PUA surface was suppressed to a certain extent, BCP completely filled the internal domain of the hollow chambers of the IO structure, leading to the formation of nanodome structures that still retained the vertical nanopores. However, when the thickness of the BCP film was further increased (>40 nm), parallel PMMA cylinders rather than perpendicular cylinders were generated due to the mismatch between the intrinsic period (Lo) of the BCP and the total film thickness. This yielded a corrugated structure once the PMMA phase was removed. Since the BCP coating inside the IO frame was carried out with highly diluted BCP solution (2 wt% in toluene), the deposition of BCP films was gradually proceeded with increasing the coating number, thereby yielding fairly uniform formation of
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Figure 3. a) Morphology control of the BCP thin films inside the hollow chamber of the IO frame according to variations in film thickness. Internal structures ranged from nanosieves (left), to nanodomes (middle) and corrugated structures containing both perpendicular and parallel cylinders (right). The scale bar represents 200 nm. b) Plot of the measured BCP domain period with respect to the size of the nanosieves. c) Pseudo-concentric characteristic of the various shapes of the BCP nanosieves. The scale bar represents 100 nm.
the BCP microstructures throughout the cross-section of IO matrix (Supporting Information, Section S6). A question still remaining with regard to the perpendicular orientation is that deformation and grain boundary defects can affect the orientation. The BCP nanosieves contained the periodic microdomains of perpendicular cylinders irrespective of their sizes or shapes (Figure 3b,c). Basically, the structural characteristics of microphase separation in BCP thin films are determined by the energetics at the interfaces associated with the imposed geometric constraints.[15,16] As a result, the measured average period (L ) of BCP under spatial confinement is clearly distinguishable from the original period observed in the bulk phase.[17] It follows that the L for free-standing but laterally confined BCP thin films must be commensurate with the finite size of the nanosieves (D, average diameter). This is accomplished either by stretching or contraction of the chains of each block. As shown in Figure 3b, the L values are observed to be greater than that of the bulk period (Lo = 42.3 nm), indicating the dominance of chain stretching in the generation of BCP nanosieves. In particular, the tendency of chain stretching is pronounced for the smaller nanosieves while the confinement effect dissipates with increasing distance from the center. Since both block chains have an equal selectivity to the PUA walls with the perpendicular orientation of PMMA cylinders in the PS matrix, the extension in L could be a consequence of lateral BCP chain stretching due to entropic effects.[15] As a result, whether the BCP nanosieves were circular or deformedelliptical, they consistently maintained a pseudo-concentric characteristic (Figure 3c), indicative of the fact that effective geometric commensuration takes place concentrically in nanosieves under highly spatially confined condition.[18]
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Before we move on to test the interconnected nanocolander network as a membrane with capabilities of size-selective separation and high permeation flux, it is necessary to explain the creation mechanism of macro-conduits that can deliver the feed solution in near-parallel to a number of BCP nanosieves placed in the extended region of a 3D nanocolander network structure. As shown in Figure 2c, some pores remain open after repetitive coatings and drying of the BCP solution although the majority of pores are covered with the BCP nanosieves. The open pores must have been created by escaping solvent vapor upon drying, thereby providing free pathways for permeate flow. However, it should be noted that these macropores are not interconnected throughout the entire cross-section of the nanocolander network membrane. This is the case because BCP film condensation at the necking pores and the solvent drying process occur simultaneously; this condition drives the random opening of macropores in localized regions, thereby generating limited interconnectivity between the opened macropores. As a result, any bypassing flow would inevitably encounter another nanosieve in one of the following chambers of the interconnected nanocolander network. Therefore, excellent separation efficiency can be achieved without significantly compromising the permeation flux. The nanocolander network membranes are more amenable to multiple feed distributions within the 3D-structure, effectively increasing the active area of separation compared with 2D screening membranes (Figure 4a). The fraction occupied by empty macropores and/or the fraction of bypasses in the nanocolander network can be controlled by varying the number of BCP solution coatings. As shown in Figure 4b, this fraction decreases with an increasing number of BCP coatings. The fraction of macropores becomes greater
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COMMUNICATION Figure 4. a) Comparative schematic illustrating the extended active area for the feed filtration and subsequent enhancement of the transport characteristics through internal macro-conduits in the 3D nanocolander network. b) Plot of the ratio of macroporous bypasses with respect to the number of BCP solution coating and the corresponding variations in BCP morphology. The concentration of the BCP solution was fixed at 2 wt% in toluene. The values of the bypass fraction were empirically determined by SEM image analysis for the outermost surface of the structured films. c) Size-selective separation performance of the nanocolander networks with varying particulate size of Au nanoparticles (5 to 30 nm). The pore size of the BCP nanosieves was fixed at 18 nm in diameter (corresponding to the volume fraction of PMMA at 28%). The dotted line represents the estimated separated efficiency based on Zeman and Wales equation for filtration with 18 nm-sized pores. d) Plot of pure water flux through membranes with respect to the operating pressure with varying numbers of BCP solution coatings.
than 20% when the structural form of the film is a nanosieve. As the coating number is further increased, a structural change gradually occurs from nanosieves to nanodomes and corrugated structures. Since the perpendicular ordering of BCP is retained for morphologies of nanosieves or nanodomes, the number of BCP coatings used in this study was varied between 7 and 10 times. This produced changes in the bypass fraction ranging from 34 to 10%, accordingly. The macroconduit-laden nanocolander network was tested for membrane performance with Au nanoparticle solutions. The pore size of the BCP nanosieve was fixed at 18 nm with a relatively narrow pore size distribution. Au nanoparticle solutions containing particles with diameters of 5, 10, 20, or 30 nm were filtered through the assembled membranes and the permeate solutions were characterized by their UV–vis absorption spectra. The separation efficiencies of filtration are summarized in Figure 4c. The results show that complete separation is achieved, when the nanoparticle size is larger than the pore size of the nanosieves and the number of BCP solution coatings is greater than 8. This result strongly supports the fact that the generated macro-conduits comprising the interconnected macropores are successfully localized inside the nanocolander network without forming bulk-scale defects and/or cracks that penetrate the membrane, and do not interfere with the separation activity of the BCP nanosieves. However, when the coating number was decreased to 7, only ca. 80% of the 30 nm-sized particles were separated. This indicates a failure in the bypass localization and the undesirable formation of bulk-scale defects. It is also noteworthy that even for nanoparticles smaller than the pore size of
Adv. Mater. 2014, DOI: 10.1002/adma.201402436
the nanosieves, considerably high separation efficiency could be obtained. This result matches well the relationship explained by the Zeman and Wales equation.[19] An SEM image of the filtered nanoparticles (30 nm in diameter) inside a nanosieve-embedded hollow chamber is shown in the inset of Figure 4c. Finally, the transport performance of the nanocolander networks was evaluated by measuring the pure water permeability. The results for the permeation flux of deionized water are shown in Figure 4d. Water permeability (L m−2 h−1 bar−1) was obtained from the slope of each line. As shown in the Figure 4b, when the number of BCP solution coating was 10, the BCP film morphology predominantly consisted of nanodome structures. More importantly, excessive coatings suppressed the formation of internal macro-conduits (bypass fraction ≈ 10%), resulting in a poor permeability of 53 L m−2 h−1 bar−1 (LMH/bar). When this ratio was increased to ca. 18% by decreasing the number of solution coatings to 9, the permeability increased to 632 LMH/bar, despite an identical BCP morphology (nanodome structure). An order of magnitude difference in the pure water permeability confirms that the formation of macro-conduit bypasses enhances the transporting capability of the nanocolander networks. When the bypass fraction was increased further to 27% by decreasing the number of solution coatings to 8, the permeability increased up to 755 LMH/bar. This is greater than the permeation flux reported for BCP-based two-dimensional membranes by a factor of three.[20–22] Considering the overall thickness of the membranes (ca. 12 µm) along with the numerously repeated 3D structure of the nanocolander networks, the attained values of the permeability are remarkably high and confirm the efficacy of our
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design strategy. Therefore, the presence of bypasses and the tunability of the bypass fraction assure outstanding separation efficiency with highly improved permeability for filtration applications. Though not shown in Figure 4d, the mechanical stability of the nanocolander networks was also confirmed by the linear characteristic of water permeability with increasing pressure up to 5 bar (> 3700 LMH for the number of BCP coatings of 8), which is limited only by the maximum operation pressure of the syringe holder. The demonstration of the nanocolander network presented here, which is both highly permeable and extremely selective, signifies the advent of a new generation of porous structures in which very thin mesoporous functional layers are spatially distributed for active utilization throughout the entire realm of a macroporous template frame. This fabrication design highlights the systematic manipulation of the interfacial compatibility and wetting characteristics of polymeric interfaces in building a mechanically stable, mesoporous structure at desired locations within macroporous main frame. Together, with the insight gained from the block copolymer microphase separation under highly confined and freely suspended conditions, the strategies outlined here for generating highly organized, multiscale porous structures can serve as a platform technology for the systematic control of permeability and selectivity of porous media.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by research grants of NRF (2013R1A1A2059130, 2014R1A2A2A01004364), Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7055540), Basic Science Research Program (2010-0027955), and APCPI ERC (R11-2007-05000000) funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea. Received: June 1, 2014 Revised: August 24, 2014 Published online:
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[1] D. R. Rolison, J. W. Long, J. C. Lytle, A. E. Fischer, C. P. Rhodes, T. M. McEvoy, M. E. Bourg, A. M. Lubers, Chem. Soc. Rev. 2009, 38, 226. [2] J.-E. Gu, S. Lee, C. M. Stafford, J. S. Lee, W. Choi, B.-Y. Kim, K.-Y. Baek, E. P. Chan, J. Y. Chung, J. Bang, J.-H. Lee, Adv. Mater. 2013, 25, 4778. [3] X. Li, X. Liu,Y. Ma , M. Li, J. Zhao, H. Xin, L. Zhang, Y. Yang, C. Li, Q. Yang, Adv. Mater. 2012, 24, 1424. [4] Y. Yu, C.-H. Chen, Y. Shi, Adv. Mater. 2007, 19, 993. [5] Y. M. Lee, B. Jung, Y. H. Kim, A. R. Park, S. Han, W.-S. Choe, P. J. Yoo, Adv. Mater. 2014, 26, 3899. [6] P. V. Braun, Chem. Mater. 2014, 26, 277. [7] W. Fan, M. A. Snyder, S. Kumar, P.-S. Lee, W. C. Yoo, A. V. McCormick, R. Lee Penn, A. Stein, M. Tsapatsis, Nat. Mater. 2008, 7, 984. [8] Y. Ren, Z. Ma, R. E. Morris, Z. Liu, F. Jiao, S. Dai, P. G. Bruce, Nat. Commun. 2013, 4, 2015. [9] Y. Wang, A. D. Price, F. Caruso, J. Mater. Chem. 2009, 19, 6451. [10] S. J. Yeo, H. Kang, Y. H. Kim, S. Han, P. J. Yoo, ACS Appl. Mater. Interfaces 2012, 4, 2107. [11] P. J. Yoo, S.-J. Choi, J. H. Kim, D. Suh, S. J. Baek, T. W. Kim, H. H. Lee, Chem. Mater. 2004, 16, 5000. [12] D. Y. Ryu, S. Ham, E. Kim, U. Jeong, C. J. Hawker, T. P. Russell, Macromolecules 2009, 42, 4902. [13] S. Ham, C. Shin, E. Kim, D. Y. Ryu, U. Jeong, T. P. Russell, C. J. Hawker, Macromolecules 2008, 41, 6431. [14] G. J. Kellogg, D. G. Walton, A. M. Mayes, P. Lambooy, T. P. Russell, P. D. Gallagher, S. K. Satija, Phys. Rev. Lett. 1996, 76, 2503. [15] A.-C. Shi, B. Li, Soft Matter 2013, 9, 1398. [16] J. A. Lee, S. T. Ha, H. K. Choi, D. O. Shin, S. O. Kim, S. H. Im, O. O. Park, Small 2011, 7, 2581. [17] J. Yu, C. Geng, Y. Zeng, Q. Yan, X. Wang, D. Shen, ACS Macro Lett. 2012, 1, 62. [18] P. Chen, H. Liang, A.-C. Shi, Macromolecules 2008, 41, 8938. [19] L. J. Zeman, A. L. Zydney, Microfiltration and Ultrafiltration: Principles and Applications, Marcel Dekker, New York 1996. [20] P. Zavala-Rivera, K. Channon, V. Nguyen, E. Sivaniah, D. Kabra, R. H. Friend, S. K. Nataraj, S. A. Al-Muhtaseb, A. Hexemer, M. E. Calvo, H. Miguez, Nat. Mater. 2012, 11, 53. [21] K.-V. Peinemann, V. Abetz, P. F. W. Simon, Nat. Mater. 2007, 6, 992. [22] S. Y. Yang, J. Park, J. Yoon, M. Ree, S. K. Jang, J. K. Kim, Adv. Funct. Mater. 2008, 18, 1371.
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Adv. Mater. 2014, DOI: 10.1002/adma.201402436
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Multiscale Porous Interconnected Nanocolander Network with Tunable Transport Properties Young Hun Kim, Hyo Kang, Sungmin Park, A Reum Park, Yong Man Lee, Do Kyung Rhee, Sungsoo Han, Hyuk Chang, Du Yeol Ryu,* and Pil J. Yoo*
Efficient material transport in a chemical system has been considered to be one of the most important aspects when designing high-performance catalysts or membranes.[1,2] To impart enhanced transport characteristics, various attempts have been made in terms of material selection and structural manipulation.[3–6] Among these, hierarchically designed multiscale architectures have received much attention due to their intricate structures and their similarity to naturally formed complex materials.[1] The strategy is to prepare individual functioning elements in such a way that both enhanced transport characteristics and maximized surface activity can be attained concurrently,[7–9] the former through larger pathways and the latter from finer pathways. When these well-designed unit elements are extended and accumulated into a three-dimensionally (3D) integrated system on a bulk scale, however, the total activity or performance can be seriously undermined, despite the use of a multiscale architecture for individual functioning elements. At the core of the problem is non-parallel utilization of functional sites and consecutive spatial dependency of the activity inside the 3D structure, which is related to individual addressability. To resolve this limitation, which is inevitable in practical scale-up processes, simultaneous and independent addressability needs to be established uniformly to each of the unit elements even inside the 3D structure. One plausible solution is to embed additional macro-level pathways within the 3D architecture so as to ensure the structural interconnectivity and facilitate transport characteristics even on a bulk-scale. However, embedding complexly shaped macro-level interconnecting pathways within a nanoarchitectured frame is both technologically Y. H. Kim, A R. Park, Y. M. Lee, D. K. Rhee, Prof. P. J. Yoo School of Chemical Engineering SKKU Advanced Institute of Nanotechnology (SAINT) Sungkyunkwan University Suwon 440-746, Republic of Korea E-mail: [email protected] Dr. H. Kang, Dr. S. Han, Dr. H. Chang Advanced Materials Research Centre Samsung Advanced Institute of Technology (SAIT) Yongin 446-712, Republic of Korea S. Park, Prof. D. Y. Ryu Department of Chemical and Biomolecular Engineering Yonsei University Seoul 120-749, Republic of Korea E-mail: [email protected]
DOI: 10.1002/adma.201402436
Adv. Mater. 2014, DOI: 10.1002/adma.201402436
challenging and not cost-effective, typically for large-scale applications. Therefore, it would be desirable to employ a spontaneous structuring method based on self-assembly, obviating the need for extra-processes relying on sophisticated facilities. To accomplish this task, here we present a design strategy in the form of nanocolander networks, wherein a multiscale design is employed to create a macroporous template via colloidal self-assembly and to build into the template mesoscale functional elements based on block copolymer (BCP) microphase separation (Figure 1a). The primary template carrying an inverse-opal (IO) structure is constructed from the self-assembled opal structure of colloidal particles. The microphase-separated block copolymer creates well-defined nanosieves with uniform mesopores inside the IO-structured template, enabling the successful realization of intricate multiscale porous architecture. The empty chamber with nanosieves at the chamber windows (internecking pores) of the IO frame is reminiscent of a colander in nanoscale. Thus, it is termed a nanocolander; successively ordered arrays of nanocolanders are referred to as nanocolander networks. An intriguing phenomenon that occurs during nanocolander network formation is that, as schematically presented in Figure 1b, the procedure of embedding the BCP nanosieves can induce the spontaneous creation of internal macro-conduits which can function as macro-level interconnecting pathways. If some of the internecking pores of the IO template remain open, this would generate free pathways of macroconduits for the transporting medium (permeate) to flow through, or bypasses. Due to the facilitated infiltration of the permeate through the macro-level interconnecting pathways of the internal conduits, the active separation sites of the isoporous block copolymer nanosieves are spread throughout the internal space of the nanocolander network. This results in enhanced transport characteristics when compared with conventional two-dimensional membranes. To create the nanocolander network, colloidal particles (polystyrene, diameter ≈ 600 nm) were first self-assembled into an opal structure (Figure 1a). A UV-curable prepolymer, poly(urethane acrylate) (PUA), was then allowed to permeate through the void space of the opal phase and subsequently cured by UV-irradiation. A free-standing film of the IO structure was obtained after removing the internal colloidal particles using an organic solvent, which yielded a primary macroporous template. Unlike the inorganic precursors that are typically used for IO structures, which yield lots of macroscale defects, the PUA prepolymer has a relatively low shrinkage rate (less than 3%) upon crosslinking, which ensures adequate dimensional
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Figure 1. a) Schematic of the generation of the nanocolander network using sequential combination of macroporous IO templates and mesoporous BCP nanosieves. b) Schematic of embedding macroscale interconnecting pathways inside the nanocolander network to provide enhanced transport characteristics.
stability for large scale applications.[10] The size of internecking pores inside the IO structure can be tuned according to the formulation of PUA prepolymer (see Figure S1 in the Supporting Information, Section S1). The IO structured template was then coated with a BCP solution of polystyrene-block-poly(methyl methacrylate) (PSb-PMMA; 86–88 kg/mol and a PMMA volume fraction of 20–28%, see Figure S2 in the Supporting Information, Section S2) to form BCP films at the necking pores by solvent evaporation. The PUA material constituting the IO structure has a low surface energy (ca. 23 dyne/cm)[11] such that dewetting occurs on the PUA surface, which eventually drives BCP to be localized at the necking pores (Figure 2a). Since the BCP dewetting occurs both in the neighbored IO chambers at the necking pore, the resulting suspended BCP films are expected to be stably mounted at the pores, much as a pulley holds the rope. Solvent evaporation was followed by vacuum-annealing at 170 °C for 24 h, which led to microphase separation between the PS and PMMA blocks. This directed self-assembly of BCP thin films only at the necking pores with air/polymer interfaces
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induced the formation of PMMA cylinders which were oriented normal to the air surface at both sides, as on neutral substrates.[12] Finally, selective removal of the PMMA minor phase by UV exposure and subsequent solvent treatment with acetic acid resulted in the formation of nanosieves at the necking pores (the detailed experimental procedure is available in the Supporting Information, Section S3 and the sequential change of the structural order from IO to BCP-coated IO is characterized with reflectance spectra in Supporting Information, Section S4). This interconnected nanocolander network structure can be readily formed over a large area (2.5 cm × 5 cm) as shown in Figure 2b. The SEM image in Figure 2c shows that while most of the macropores were successfully covered with BCP nanosieves, some pores remained open. This could be associated with the evaporation pathways of the solvent during coating and the subsequent drying of the BCP solution. The perpendicular orientation of the cylinders in the BCP film at the pores was assured by a combination of careful material selection, the formation of the suspended film, and confinement effects. In general, the microdomain
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COMMUNICATION Figure 2. a) Dewetting-induced selective formation of BCP nanosieves at the necking pores of the IO frame. The scanning electron microscopy image shows the emergence of boundaries of the BCP nanosieves after 10 min of annealing at 170 °C. b) Cross-sectional view of a 12 µm-thick membrane. The inset shows a large-scale demonstration of the interconnected nanocolander network as a free-standing film. c) Highly magnified image of the nanocolander network.
orientation of PS-b-PMMA films is significantly influenced by interfacial interactions, particularly for cylinder-forming BCPs.[13] Hence, a neutral condition has been employed for developing the perpendicular orientation of BCP microdomains, in which the formation of the parallel orientation is suppressed due to the balanced interfacial interactions on the substrate. In BCP nanosieves, however, there exists no underlying substrate. Rather, films were suspended at the necking pores, and the symmetric interfaces present on both sides of the BCP films created a confinement effect at the air/polymer interfaces. Because the surface energies of the two different block types were nearly equal (PS: 29.9 dyne/ cm and PMMA: 30.0 dyne/cm at 170 °C), the PMMA cylinders were oriented perpendicular to the film surface.[14] In addition, any IO-structured PUA walls in contact with the BCP thin films turned out to be neutral (or weakly selective) toward the two blocks. This was evaluated by homopolymer dewetting experiments on PUA surface (see the Supporting Information, Section S5). Along with energetics, the BCP thin films were subjected to the geometric constraint of the finite-sized necking pores that formed rim boundaries with the PUA surface.
Adv. Mater. 2014, DOI: 10.1002/adma.201402436
The condition under which the IO chambers remain empty after repetitive coatings with the BCP solution and subsequent drying has been found to hinge on the film thickness at the necking pores. When the BCP film was relatively thin (15–20 nm), and preferential localization at the IO necking pores was facilitated by BCP dewetting on the PUA surface, consecutive formation of nanosieves embedded with the vertical nanopores was promoted (Figure 3a). When the film thickness was increased slightly (20–40 nm) and the spontaneous BCP dewetting on the PUA surface was suppressed to a certain extent, BCP completely filled the internal domain of the hollow chambers of the IO structure, leading to the formation of nanodome structures that still retained the vertical nanopores. However, when the thickness of the BCP film was further increased (>40 nm), parallel PMMA cylinders rather than perpendicular cylinders were generated due to the mismatch between the intrinsic period (Lo) of the BCP and the total film thickness. This yielded a corrugated structure once the PMMA phase was removed. Since the BCP coating inside the IO frame was carried out with highly diluted BCP solution (2 wt% in toluene), the deposition of BCP films was gradually proceeded with increasing the coating number, thereby yielding fairly uniform formation of
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Figure 3. a) Morphology control of the BCP thin films inside the hollow chamber of the IO frame according to variations in film thickness. Internal structures ranged from nanosieves (left), to nanodomes (middle) and corrugated structures containing both perpendicular and parallel cylinders (right). The scale bar represents 200 nm. b) Plot of the measured BCP domain period with respect to the size of the nanosieves. c) Pseudo-concentric characteristic of the various shapes of the BCP nanosieves. The scale bar represents 100 nm.
the BCP microstructures throughout the cross-section of IO matrix (Supporting Information, Section S6). A question still remaining with regard to the perpendicular orientation is that deformation and grain boundary defects can affect the orientation. The BCP nanosieves contained the periodic microdomains of perpendicular cylinders irrespective of their sizes or shapes (Figure 3b,c). Basically, the structural characteristics of microphase separation in BCP thin films are determined by the energetics at the interfaces associated with the imposed geometric constraints.[15,16] As a result, the measured average period (L ) of BCP under spatial confinement is clearly distinguishable from the original period observed in the bulk phase.[17] It follows that the L for free-standing but laterally confined BCP thin films must be commensurate with the finite size of the nanosieves (D, average diameter). This is accomplished either by stretching or contraction of the chains of each block. As shown in Figure 3b, the L values are observed to be greater than that of the bulk period (Lo = 42.3 nm), indicating the dominance of chain stretching in the generation of BCP nanosieves. In particular, the tendency of chain stretching is pronounced for the smaller nanosieves while the confinement effect dissipates with increasing distance from the center. Since both block chains have an equal selectivity to the PUA walls with the perpendicular orientation of PMMA cylinders in the PS matrix, the extension in L could be a consequence of lateral BCP chain stretching due to entropic effects.[15] As a result, whether the BCP nanosieves were circular or deformedelliptical, they consistently maintained a pseudo-concentric characteristic (Figure 3c), indicative of the fact that effective geometric commensuration takes place concentrically in nanosieves under highly spatially confined condition.[18]
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Before we move on to test the interconnected nanocolander network as a membrane with capabilities of size-selective separation and high permeation flux, it is necessary to explain the creation mechanism of macro-conduits that can deliver the feed solution in near-parallel to a number of BCP nanosieves placed in the extended region of a 3D nanocolander network structure. As shown in Figure 2c, some pores remain open after repetitive coatings and drying of the BCP solution although the majority of pores are covered with the BCP nanosieves. The open pores must have been created by escaping solvent vapor upon drying, thereby providing free pathways for permeate flow. However, it should be noted that these macropores are not interconnected throughout the entire cross-section of the nanocolander network membrane. This is the case because BCP film condensation at the necking pores and the solvent drying process occur simultaneously; this condition drives the random opening of macropores in localized regions, thereby generating limited interconnectivity between the opened macropores. As a result, any bypassing flow would inevitably encounter another nanosieve in one of the following chambers of the interconnected nanocolander network. Therefore, excellent separation efficiency can be achieved without significantly compromising the permeation flux. The nanocolander network membranes are more amenable to multiple feed distributions within the 3D-structure, effectively increasing the active area of separation compared with 2D screening membranes (Figure 4a). The fraction occupied by empty macropores and/or the fraction of bypasses in the nanocolander network can be controlled by varying the number of BCP solution coatings. As shown in Figure 4b, this fraction decreases with an increasing number of BCP coatings. The fraction of macropores becomes greater
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Adv. Mater. 2014, DOI: 10.1002/adma.201402436
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COMMUNICATION Figure 4. a) Comparative schematic illustrating the extended active area for the feed filtration and subsequent enhancement of the transport characteristics through internal macro-conduits in the 3D nanocolander network. b) Plot of the ratio of macroporous bypasses with respect to the number of BCP solution coating and the corresponding variations in BCP morphology. The concentration of the BCP solution was fixed at 2 wt% in toluene. The values of the bypass fraction were empirically determined by SEM image analysis for the outermost surface of the structured films. c) Size-selective separation performance of the nanocolander networks with varying particulate size of Au nanoparticles (5 to 30 nm). The pore size of the BCP nanosieves was fixed at 18 nm in diameter (corresponding to the volume fraction of PMMA at 28%). The dotted line represents the estimated separated efficiency based on Zeman and Wales equation for filtration with 18 nm-sized pores. d) Plot of pure water flux through membranes with respect to the operating pressure with varying numbers of BCP solution coatings.
than 20% when the structural form of the film is a nanosieve. As the coating number is further increased, a structural change gradually occurs from nanosieves to nanodomes and corrugated structures. Since the perpendicular ordering of BCP is retained for morphologies of nanosieves or nanodomes, the number of BCP coatings used in this study was varied between 7 and 10 times. This produced changes in the bypass fraction ranging from 34 to 10%, accordingly. The macroconduit-laden nanocolander network was tested for membrane performance with Au nanoparticle solutions. The pore size of the BCP nanosieve was fixed at 18 nm with a relatively narrow pore size distribution. Au nanoparticle solutions containing particles with diameters of 5, 10, 20, or 30 nm were filtered through the assembled membranes and the permeate solutions were characterized by their UV–vis absorption spectra. The separation efficiencies of filtration are summarized in Figure 4c. The results show that complete separation is achieved, when the nanoparticle size is larger than the pore size of the nanosieves and the number of BCP solution coatings is greater than 8. This result strongly supports the fact that the generated macro-conduits comprising the interconnected macropores are successfully localized inside the nanocolander network without forming bulk-scale defects and/or cracks that penetrate the membrane, and do not interfere with the separation activity of the BCP nanosieves. However, when the coating number was decreased to 7, only ca. 80% of the 30 nm-sized particles were separated. This indicates a failure in the bypass localization and the undesirable formation of bulk-scale defects. It is also noteworthy that even for nanoparticles smaller than the pore size of
Adv. Mater. 2014, DOI: 10.1002/adma.201402436
the nanosieves, considerably high separation efficiency could be obtained. This result matches well the relationship explained by the Zeman and Wales equation.[19] An SEM image of the filtered nanoparticles (30 nm in diameter) inside a nanosieve-embedded hollow chamber is shown in the inset of Figure 4c. Finally, the transport performance of the nanocolander networks was evaluated by measuring the pure water permeability. The results for the permeation flux of deionized water are shown in Figure 4d. Water permeability (L m−2 h−1 bar−1) was obtained from the slope of each line. As shown in the Figure 4b, when the number of BCP solution coating was 10, the BCP film morphology predominantly consisted of nanodome structures. More importantly, excessive coatings suppressed the formation of internal macro-conduits (bypass fraction ≈ 10%), resulting in a poor permeability of 53 L m−2 h−1 bar−1 (LMH/bar). When this ratio was increased to ca. 18% by decreasing the number of solution coatings to 9, the permeability increased to 632 LMH/bar, despite an identical BCP morphology (nanodome structure). An order of magnitude difference in the pure water permeability confirms that the formation of macro-conduit bypasses enhances the transporting capability of the nanocolander networks. When the bypass fraction was increased further to 27% by decreasing the number of solution coatings to 8, the permeability increased up to 755 LMH/bar. This is greater than the permeation flux reported for BCP-based two-dimensional membranes by a factor of three.[20–22] Considering the overall thickness of the membranes (ca. 12 µm) along with the numerously repeated 3D structure of the nanocolander networks, the attained values of the permeability are remarkably high and confirm the efficacy of our
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design strategy. Therefore, the presence of bypasses and the tunability of the bypass fraction assure outstanding separation efficiency with highly improved permeability for filtration applications. Though not shown in Figure 4d, the mechanical stability of the nanocolander networks was also confirmed by the linear characteristic of water permeability with increasing pressure up to 5 bar (> 3700 LMH for the number of BCP coatings of 8), which is limited only by the maximum operation pressure of the syringe holder. The demonstration of the nanocolander network presented here, which is both highly permeable and extremely selective, signifies the advent of a new generation of porous structures in which very thin mesoporous functional layers are spatially distributed for active utilization throughout the entire realm of a macroporous template frame. This fabrication design highlights the systematic manipulation of the interfacial compatibility and wetting characteristics of polymeric interfaces in building a mechanically stable, mesoporous structure at desired locations within macroporous main frame. Together, with the insight gained from the block copolymer microphase separation under highly confined and freely suspended conditions, the strategies outlined here for generating highly organized, multiscale porous structures can serve as a platform technology for the systematic control of permeability and selectivity of porous media.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by research grants of NRF (2013R1A1A2059130, 2014R1A2A2A01004364), Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7055540), Basic Science Research Program (2010-0027955), and APCPI ERC (R11-2007-05000000) funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea. Received: June 1, 2014 Revised: August 24, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201402436
