Perspective pubs.acs.org/cm
Hierarchically Porous Materials from Block Copolymers Rachel Mika Dorin, Hiroaki Sai, and Ulrich Wiesner* Department of Materials Science and Engineering, Cornell University, 214 Bard Hall, Ithaca, New York 14850-1501, United States S Supporting Information *
ABSTRACT: In this Perspective, we discuss two recent methods for the generation of hierarchical porous polymer structures for applications in areas including separations, catalysis, and bioengineering. In the first method termed selfassembly and non-solvent-induced phase separation, or SNIPS, asymmetric membranes are generated with block copolymer directed mesoporous surface layers and graded meso- to macroporous substructures. In the second method termed spinodal-decomposition induced macro- and meso-phase separation plus extraction by rinsing, or SIM2PLE, hierarchical pores are generated by a combination of spinodal decomposition and microphase separation induced via solvent evaporation in a mixture of a block copolymer and a small molar mass additive. In addition to providing details on structural characteristics of the resulting materials, possible future directions of research and applications are discussed. KEYWORDS: porous polymers, hierarchical structures, self-assembly, block copolymer, phase-separation, SNIPS, SIM2PLE
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INTRODUCTION Block copolymer (BCP) self-assembly provides a functional platform for producing periodic, ordered organic materials with feature sizes ranging from ∼5 to 50 nm. Utilizing BCP selfassembly as a method for fabricating porous materials has garnered significant attention because of its capacity for generating a high density of uniform pores. These structural features are of particular interest to the separations community because of the conventional trade-off between permeability, which largely depends on pore density, and selectivity, typically a function of pore size uniformity. A variety of methods have been adopted to translate BCP self-assembly into functional membranes, including spin-coated thin film membranes,1−5 and bulk casting processes, such as doctor blading of BCP solutions onto porous supports,6 drop-casting onto nonporous substrates,7−9 and BCP melt extrusion.10 Historically, BCP membrane formation has focused on the achievement of equilibrium structures with nanoscopic dimensions that form through careful solvent or thermal annealing processes. While this allows for quantitative understanding of the resulting structures within the framework of equilibrium BCP thermodynamics, it limits the accessible range of morphologies and pore sizes. Specifically, it does not allow the formation of graded, asymmetric hierarchical structures with a directional continuum of pore sizes from small (nanoscale) to large (micrometer-scale), or conventional hierarchical structures with two or more distinct length scales of porosity, both highly desirable in many applications of porous polymers. Polymer materials with continuous (i.e., accessible) hierarchical porosity across multiple length scales ranging from nanometers to micrometers offer the potential for efficient transport of matter through the pores and mechan© XXXX American Chemical Society
ically robust structures while maintaining ease of processability and relatively high surface areas. In light of the expanded applications possible through BCP-derived hierarchical porous materials, in this perspective with emphasis on work by the Wiesner group at Cornell University, U.S., we focus on two recently developed methods for fabricating such materials. First, we review work on a triblock terpolymer system that undergoes both self-assembly and non-solvent-induced phase separation, a process now known as SNIPS, to form hierarchically porous films that can be used as separation membranes. Second, we describe a variety of BCP systems that undergo spinodaldecomposition induced macro- and meso-phase separation plus extraction by rinsing, a process very recently termed as SIM2PLE. In both cases, in addition to describing results of past studies, we provide our perspectives of potential future developments. Both the SNIPS and SIM2PLE methods are facile and scalable routes to fabricating hierarchically porous polymers. Thus, besides fundamental studies to better understand the underlying formation mechanisms and expanding the libraries of materials that can be submitted to these processes, the approaches may also be useful for industrial applications. Terpolymer SNIPS Membranes. Non-solvent-induced phase separation, or phase inversion, pioneered by Loeb and Sourirajan in the 1960s,11 can produce membranes with a dense skin layer above an asymmetric support layer from both organic Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 17, 2013 Revised: September 21, 2013
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Figure 1. Schematic of SNIPS procedure used to make hierarchically porous membrane films. A casting solution containing BCP and organic solvents is formed into a film via doctor blade. The solvents in the film are then partially evaporated causing a concentration gradient perpendicular to the film surface. Finally, the film is plunged into a nonsolvent bath in which the organic solvents in the film exchange rapidly with the nonsolvent thereby freezing the gradient film structure into the solid state of the BCP. Adapted with permission from ref 17. Copyright 2011 American Chemical Society.
and inorganic materials. Phase inversion has been used extensively for decades in the field of membrane science, and publications on the subject have increased exponentially over the past half-century.12 Typical phase inverted membranes appear as a disordered polymer network and display graded porosity that can range from nanometers to micrometers. More recently, the SNIPS process has combined phase inversion with BCP self-assembly to unite graded porosity across multiple length scales with periodic, ordered mesoscopic pores. This process was first demonstrated by Peinemann et al. on the diblock copolymer system poly(styrene)-block-poly(4-vinyl pyridine) (SV) in a tetrahydrofuran (THF) and dimethylformamide (DMF) solvent system.13 Subsequent work on the SNIPS process has focused on improving membrane functionality largely through the use of diblock copolymer systems.14−16 This perspective will focus on the triblock terpolymer poly(isoprene)-block-poly(styrene)-block-poly(4vinyl pyridine) (ISV) in a 1,4-dioxane (DOX) and THF solvent system, which improves mechanical integrity and enhances chemical tunability of the system relative to diblocks.17 A schematic of the SNIPS process used to fabricate ISV membranes is outlined in Figure 1. After dissolving the ISV in a mixture of DOX and THF, the casting solution is drawn across a substrate with a doctor blade. In the second step of the SNIPS process, the film is allowed to evaporate for a specific period of time during which a concentration gradient develops across the thickness of the film. During this time, the ISV at the top surface of the film reaches a sufficiently high concentration and begins to self-assemble in the presence of solvent, forming a skinlike layer. Finally, the film is plunged into the nonsolvent water, causing the ISV to precipitate as the casting solvents exchange with the nonsolvent, both locking in the selfassembled surface structure and forming a phase inverted asymmetric substructure. Numerous parameters can be adjusted within the SNIPS process, among which are polymer concentration, molecular weight, organic solvent system, doctor blade height, doctor blade speed, evaporation time, casting temperature, environmental humidity, and nonsolvent. When these parameters were optimized for an ISV system composed of a 77 kg/mol terpolymer, the resulting membranes contained uniform pores in a top separation layer above an asymmetric support. Figure 2a shows a scanning electron microscope (SEM) image of the top surface, which illustrates the uniform pore size achieved through self-assembly. Interestingly, the separation layer pores are arranged in a 2D square geometry, while the bulk morphology of the 77 kg/mol ISV used for these membranes was hexagonal,17 indicating that the SNIPS procedure captures a nonequilibrium morphology in the final structure. The separation layer is on the order of 100 nm in thickness, as can
Figure 2. SEM images of SNIPS structure. (a) Top surface showing uniform, periodic pores ∼20 nm in diameter. (b) Cross-section near the surface showing ordered pores extending for about 100 nm vertically into the film above a disordered porous network. (c) Crosssection of the film showing the asymmetric porous substructure. (d) Walls of large pores near bottom surface of film exhibiting mesoscale porosity. (a, b) Reprinted with permission from ref 17. Copyright 2011 American Chemical Society.
be observed in the cross-sectional SEM near the surface of the film in Figure 2b. Directly below the separation layer, the spongy asymmetric sublayer, formed through the phase inversion step, can be identified in both Figures 2b and 2c. The pores increase in size from the nanometer range starting at the top surface to the micrometer range at the bottom surface, with a total membrane thickness of ∼50 μm. The gradient asymmetric structure across the thickness of the film is one aspect of the hierarchical morphology of these porous materials. A second aspect can be observed in Figure 2d, which shows a magnified portion of the walls of larger pores near the bottom surface of the membrane. The large pore walls exhibit a mesoscopic porosity, which may be attributed to terpolymer phase separation within the substructure. Transmission electron microscope (TEM) images of thin sections of the sublayer also indicate that phase separation occurs within the sublayer struts (see Supporting Information). Equilibrium BCP structures frequently require a postmodification step to convey porosity onto the material, for example, etching. A unique feature of the SNIPS process is the formation of continuously porous materials in the absence of such a post-treatment. We speculate that the pores in the top surface develop because densification of the terpolymer at this interface during evaporation is incomplete, leaving significant amounts of organic solvents. The volume occupied by the B
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organic solvents is converted into pores in the final membrane structure. This hypothesis is supported by solution small-angle X-ray scattering (SAXS) experiments performed on casting solutions of a 59 kg/mol ISV with a similar architecture to the 77 kg/mol ISV described previously.18 The 59 kg/mol ISV resulted in membrane structures comparable to those shown in Figure 2, see ref 18. As evident from Figure 3, the 59 kg/mol
Figure 4. Performance characteristics of a 77 kg/mol ISV SNIPS membrane. (a) The membrane exhibits pH dependent flux with permeabilities of ∼2 L m−2 h−1 bar−1 at pH 4 and ∼150 L m−2 h−1 bar−1 above pH 5. (b) The molecular weight cut-of curve performed with PEO molecules of different molecular weight in DI water. Adapted with permission from ref 17. Copyright 2011 American Chemical Society.
described at the beginning, as well as track-etched membranes. Pendergast et al. have recently compared the characteristics of a state-of-the-art phase inverted poly(ether sulfone) (PES) ultrafiltration membrane (Biomax 100 kDa MWCO, Millipore Corp.) shown in Figure 5a, a track-etched polycarbonate Figure 3. Solution SAXS of a 59 kg/mol ISV terpolymer at various concentrations in DOX and THF casting solvents. At 16 wt %, the solution exhibits a solution structure consistent with a bcc morphology (see ticks for expected peak positions for a bcc lattice). Reprinted with permission from ref 18. Copyright 2012 American Chemical Society.
ISV dissolved in DOX and THF shows no well-developed longrange order at terpolymer concentrations ranging from 10−14 wt %. Increasing the concentration to 16 wt % terpolymer, however, the solution has transitioned to a well-ordered morphology consistent with a bcc structure. The solution SAXS data matches the picture of self-assembled terpolymer in the presence of significant amounts of organic solvent at the more concentrated air/film interface, and suggests that the surface pores may arise from the solvent volume. Similar results were found in a hexagonal diblock copolymer SV system, which was further investigated by Marques et al.19 The evolution of solution structure in both diblock and triblock systems that result in the desired SNIPS membranes structure substantiate SAXS as a powerful tool for quickly evaluating casting solutions to optimize for key casting parameters such as polymer concentration and casting solvents. The membranes cast from 77 kg/mol ISV were subjected to performance evaluations using a stirred-cell apparatus (Amicon 8010, Millipore Co.). The permeability of the membranes was found to be pH dependent, as shown in Figure 4a. At pH above 5, the permeability was ∼150 L m−2 h−1 bar−1, while below a pH of 5, the permeability quickly dropped to only about 2 L m−2 h−1 bar−1. This dependency is likely due to the poly(4vinyl pyridine) component of the terpolymer coating the pore walls, which has a pKa of ∼4.6.20 The 77 kg/mol ISV membranes were further challenged with dissolved polyethylene oxide (PEO) molecules in deionized (DI) water to evaluate their rejection characteristics, as shown in Figure 4b. As expected, increasing the size of the PEO solute resulted in a corresponding increase in membrane rejection. Based on this curve and diffusion data for PEO, a pore size of ∼16 nm was calculated for the 77 kg/mol ISV membrane. Conventional ultrafiltration membranes used for industrial and lab-scale processes include the phase inverted membranes
Figure 5. Surface SEM images of various ultrafiltration membranes. (a) PES Biomax 100 phase inverted membrane, (b) poly(carbonate) track-etched membrane, and (c) 59 kg/mol ISV triblock terpolymer SNIPS membrane. The scale bar on the right applies to all images. Reprinted from ref 21, Copyright 2013, with permission from Elsevier.
ultrafiltration membrane (Model no. PCT00320030, 30 nm nominal pore size, Sterlitech Inc., Kent, Washington, U.S.A.) shown in Figure 5b, and an optimized 59 kg/mol ISV terpolymer SNIPS membrane, shown in Figure 5c.21 From the top surface SEMs in Figure 5, the ISV terpolymer membrane structure marries the high pore densities and thus high fluxes observed in phase inverted membranes, with uniform pores and thus high resolution separations seen in track-etched membranes. Performance evaluations of the membranes showed that the 59 kg/mol ISV terpolymer membrane displayed a sharp molecular weight cutoff curve reminiscent of the track-etched membrane when challenged with PEO of varying molecular weights and fluxes on the order of 400 L m−2 h−1 bar−1. On the basis of molecular sieving and convection-diffusion affinity model calculations by Pendergast et al., the promising performance of ISV terpolymer SNIPS membranes may be further enhanced by improving pore alignment, potentially through external fields, or by making the sublayer more porous to eliminate blockage of separation layer pores. The use of ISV terpolymer self-assembly in membrane fabrication offers remarkable improvements in membrane C
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block on the surface, as evidenced by TEM cross sections of the membrane and suggested by solubility parameters.17 Smallangle neutron scattering on the ISV casting solutions would provide direct evidence of micelle structure and may further clarify the formation mechanism. While such mechanistic studies are challenging, detailed understanding of the membrane formation mechanism may open the door to successful translation of a larger variety of BCP chemistries as well as mesoscopic morphologies to the SNIPS membrane structure. Many of the studies performed in research laboratories worldwide garner significant interest and attract resources when real-world applicability is demonstrated. SNIPS membranes represent a long-needed advancement in membranes for separation applications used in industries from water treatment to biopharmaceutical processing. As such, the use of SNIPS membranes under conditions relevant to industrial separations would propel this field forward. Both the model protein bovine serum albumin (BSA) and synthetic PEO have been used to show rejection in SNIPS membranes. Most recently, postfunctionalized PS-b-P4VP SNIPS membranes have been used to selectively separate the similarly sized BSA from bovine hemoglobin (BHb) in diffusion experiments, demonstrating utility for biopharmaceutical separations.29 The next step is to perform similar separations under pressures, flow rates, and feed volumes necessary for industrial processes. While specific conditions will depend on the application, biopharmaceutical processes will require pressures of several bar, volumes of ∼1000 L for pilot scale processes, and fluxes on the order of several hundred L m−2 h−1 bar−1. Finally, a largely unexplored aspect of SNIPS membranes is their use in applications outside of separations science. For example, the pH dependent behavior of the ISV terpolymer SNIPS membranes may serve as a chemical valve. The membranes also exhibit a hierarchical structure with pores that span from ∼10 nm up to ∼10 μm. These high surface area materials would be suitable as a catalyst support, and may be useful in the transport of electrons or fuels. Inorganic, conductive SNIPS membranes may be applicable, for example, as fuel cell electrode supports in which the surface is decorated with catalysts and the pores are infiltrated with a proton conductor. Such a construction may improve efficiency by shrinking the distance between active sites and reducing ion transport resistance. The small list of applications outside of separations described here is by no means exhaustive, and the unique structure of SNIPS membranes may be useful well beyond these examples. Developing and understanding SNIPS membranes is an interdisciplinary process that encompasses polymer chemistry, materials science, biology, and much more. This feature requires the collaborative efforts of experts from a variety of fields and perspectives, and may thrive in particular at organizations that can interface such diversity. The field of SNIPS, conceived less than a decade ago, promises enormous potential in the coming years. SIM2PLE Systems. The physics of spinodal decomposition in polymer blends has been well-studied and modeled using equations developed by Cahn and Hillard.30−32 The miscibility of polymer blends can be tuned by changing the temperature, degree of polymerization, or adding a common solvent. These systems can be quenched via a temperature drop or solvent removal, causing the polymer blends to become immiscible. When the quench is shallow leading to a state between binodal
structure and performance over conventional polymer materials. The SNIPS technique potentially expands the applicability of BCP based membranes to large-scale applications. Future research in the field of SNIPS membranes may advance by targeting progress in five areas: (1) expanding the organic chemical library, (2) introducing functional, inorganic components, (3) understanding formation mechanisms in specific systems, (4) demonstrating real-world applicability via separations of relevant molecules, and (5) exploring applications outside of separations. Although progress has been made in the size of the SNIPS chemical BCP library, the total number of blocks used in the SNIPS process is still in the single digits. All BCPs reported for SNIPS membranes thus far have been synthesized via anionic polymerization. This is likely because this relatively specialized technique can achieve low molar mass dispersities (typically
