Review pubs.acs.org/CR
Design and Preparation of Porous Polymers Dingcai Wu,*,† Fei Xu,† Bin Sun,† Ruowen Fu,† Hongkun He,‡ and Krzysztof Matyjaszewski*,‡ †
Materials Science Institute, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, People's Republic of China ‡ Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States
Chem. Rev. 2012.112:3959-4015. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/18/18. For personal use only.
Biographies Acknowledgments Abbreviations References
1. INTRODUCTION Porosity can be viewed as a profound concept that helps us to understand nature and create advanced structures. There are some interesting examples in nature, such as hollow bamboo, honeycomb with hexagonal cells, and alveoli in the lungs (Figure 1a−c). Design and construction of porous architectures that mimic structures found in nature in synthesized materials, down to the micro- and nanoscale range, have long been an important science subject. Porous polymers (see some examples in Figure 1d−f) especially have received an increased level of research interest because of their potential to merge the properties of both porous materials and polymers. First of all, porous polymers can be designed to show the advantages of high surface area and well-defined porosity.1−3 Second, the porous polymers have easy processability. For example, they can be produced in a molded monolithic form4−7 or in thin films,8,9 which generates significant advantages in many practical applications. Moreover, some of them can even be dissolved in a solvent and then processed directly using solventbased techniques without destroying the porosity,10−12 which is almost impossible to imagine for other types of porous materials like activated carbons, zeolites, or porous silicas. Third, the diversity of synthetic routes for polymers facilitates the design and construction of numerous porous polymers capable of incorporating multiple chemical functionalities into the porous framework or at the pore surface.1,13−15 The functional porous polymers can be designed to demonstrate stimuli-responsive characteristics capable of reversibly changing the pore structure16−19 or even switching between the open and closed porous state after exposure to environmental stimulation.20,21 Such unique characteristics are generally unavailable in other porous materials. Last but not least, due to their organic nature, the polymeric frameworks are composed of light elements providing a weight advantage in many applications.22,23 Porous polymers can be used as gas storage and separation materials,5,13,22,28−46 as encapsulation agents for controlled release of drugs, 47−51 as catalysts, 52 as supports for catalysts53−56 and sensors,57,58 as precursors of nanostructured carbon materials,59−68 as supports for biomolecular immobili-
CONTENTS 1. Introduction 2. Direct Templating Methodology 2.1. Direct Templating with Polymers as the Raw Material 2.1.1. Infiltration 2.1.2. Layer-by-Layer Assembly 2.2. Direct Templating with Monomers as the Raw Material 2.2.1. Conventional Polymerization 2.2.2. Electrochemical Polymerization 2.2.3. Controlled/Living Polymerization 3. Block Copolymer Self-Assembly Methodology 3.1. Self-Assembly with Block Copolymers as the Pore Template 3.2. Self-Assembly with Block Copolymers as the Source of the Framework 3.2.1. Self-Assembly with Sacrificial Component 3.2.2. Self-Assembly with Morphology Reconstruction 3.2.3. Self-Assembly with Vesiculation 4. Direct Synthesis Methodology 4.1. Microporous Polymers 4.1.1. Disordered Microporous Polymers 4.1.2. Ordered Microporous Polymers 4.2. Meso- and/or Macroporous Polymers 4.2.1. Radical Polymerization 4.2.2. Polycondensation 4.3. Hierarchical Porous Polymers 5. High Internal Phase Emulsion Polymerization Methodology 6. Interfacial Polymerization Methodology 7. Breath Figures Methodology 8. Other Methods 9. Summary and Perspective Author Information Corresponding Author Notes © 2012 American Chemical Society
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Received: November 22, 2011 Published: May 17, 2012 3959
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Figure 1. Illustration of porosity existing in nature and synthesized materials with a decreasing pore size: (a) bamboo; (b) honeycomb; (c) SEM image of alveolar tissue in mouse lung (Reprinted with permission from ref 24; Copyright 2011 Wiley-VCH); (d) SEM image of an ordered macroporous polymer from direct templating (Reprinted with permission from ref 25; Copyright 2010 American Chemical Society); (e) SEM image of an ordered mesoporous polymer from self-assembly of block copolymers (Reprinted with permission from ref 26; Copyright 2010 American Chemical Society); (f) structural representation of an ordered microporous polymer (Reprinted with permission from ref 27; Copyright 2009 WileyVCH).
zation and cell scaffolds,69,70 as low-dielectric constant materials,71 as photonic band gap materials,72,73 as filtration/ separation membranes,18,74−82 as proton exchange membranes,83 as templates for structure replication,84−93 as masks for nanopatterning or lithography,94−99 as packing materials in chromatography,4,82,100−103 as electrode materials for energy storage,104,105 as antireflection coating,106,107 and for many other applications. Therefore, these high value applications drive the recent emphasis on development of reliable methods for preparation of porous polymers, with designed pore architectures as well as customized framework and pore surface functionalities. In this review, we define porous polymers as polymeric materials containing one or more pores. Porous polymers generally have many pores, but polymers with a single pore, commonly called hollow polymers such as hollow polymeric spheres and tubes, can also be included. Furthermore, according to the IUPAC recommendation,108 we define microporous polymers as polymeric materials with pore size smaller than 2 nm in diameter, mesoporous polymers with pore size in the range of 2−50 nm, and macroporous polymers with pore size larger than 50 nm. Such a definition is necessary because sometimes polymers with pores in the micrometer scale are also called microporous polymers.109−111 There are several important structural characteristics of porous polymers that should be described, including pore geometry, pore size, pore surface functionality, and polymeric framework structure including composition, topology, and functionality112−114 (Figure 2). Pore geometry includes spherical, tubular, and network-type morphologies that can be either disordered or assembled into ordered arrays. Surface area is a very important parameter that is employed to evaluate the pore structure;
Figure 2. Illustration of pore geometry, pore surface, pore size, and framework structure of porous polymers.
generally, pores with a smaller size (e.g., micropores) contribute predominantly to the generation of materials with high surface area. In addition to the physical structure of the pores, the functionalities of the polymer framework and pore surface are also important.30 The framework and pore surface functionalities can be engineered through the use of functional monomers or by postmodification processes.115−119 The ability to control the structure of pores and incorporate desired functionalities into the material has benefited from the great strides being made in the preparation of porous polymers by various synthetic methods, as summarized below. The past several years have witnessed an expansion of various methodologies directed at preparing porous polymers, including direct templating, block copolymer self-assembly, and direct synthesis methodologies. Each of these method3960
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definite control over pore geometry is relatively difficult
preparation of ordered porous films with pore sizes below 100 nm is a challenge
it is especially suitable for synthesizing nanocapsules
the preparation is simple; honeycomb patterned porous polymer films with ordered structure can be obtained
inheritance of confined space from micelle
removal of condensed water droplet templates
surface area is usually low, and mechanic strength is often weak
precise design and control over hierarchical organization needs to be improved simultaneous construction of hierarchical micro-, meso- and macropores is achieved
porosity is very high, typically 74∼95 vol%; macroporosity with a unique void and window structure is 3D highly interconnected
high internal phase emulsion polymerization interfacial polymerization breath figures
microporous polymers direct synthesis
meso-/macroporous polymers hierarchical po- combination of hypercrosslinking rous polywith phase separation mers removal of internal phase
removal of sacrificial component; morphology reconstruction; vesiculation block copolymer self-assembly
linkage of polymerizable monomer building blocks or sometimes hyper-cross-linkable polymers reaction-induced phase separation
relatively precise control over meso-/macroporous network structure can be realized
strict requirements on the monomer structures and synthetic routes are needed; raw materials are expensive in many cases; fine design and control over meso-/macroporous structure is unavailable there is a loss of micropores
major limitations
sacrificial templates are needed, leading to the difficulty in scaling up production and the limited utility of materials; products lack micropores
major advantages
the synthesis is relatively easy to carry out, since the template structure is fixed; precise control over meso-/macropore structure can be realized; and the introduction of smart functionality into the framework can be achieved porous polymers show tailored pore size, well-defined pore architectures and longrange order; their framework and pore surface can demonstrate sophisticated structures and smart or stimuli-responsive properties permanent porosity with high surface area can be achieved; the pore structure can be tailored by designing monomers with targeted structures
pore generation
removal of templates
methodology
2. DIRECT TEMPLATING METHODOLOGY Direct templating is essentially a molding or casting technique for the direct replication of the inverse structure of the preformed templates with stable morphology.120,121 Currently, a large number of porous polymers, including spherical porous polymers (Figure 3a), tubular porous polymers (Figure 3b), and ordered porous polymers (Figure 3c), have been
direct templating
Table 1. Summary of the Pore Generation, Major Advantages and Limitations for the Main Methodologies of the Preparation of Porous Polymers
ologies has its own strengths and limitations, as summarized in Table 1. The direct templating methodology is a simple and versatile approach for preparation of porous polymers.120−124 It involves a casting or molding process, which essentially follows the same design concept using a predesigned mold (i.e., template) to prepare plastic bottles but is scaled down to the nanometer range. The block copolymer self-assembly methodology is very useful for making mesoporous or macroporous polymers, especially materials with long-range order due to the microphase separation of incompatible blocks yielding mesoscale structures.8,125 The direct synthesis methodology can directly generate pores during solution polymerization, followed by removal of the solvent from the pores.126−129 Microporous polymers with extremely high surface area and permanent porosity that persists even in the dry state can be prepared by direct synthesis procedures, for example, polymerization of rigid and contorted monomer building blocks that inhibit space-efficient packing.130−133 Mesoporous and/or macroporous (meso-/macroporous) polymers can be fabricated by reaction-induced phase separation procedures.129,134 In addition to these three types of frequently used methodologies, high internal phase emulsion polymerization,69 interfacial polymerization,135,136 breath figures,137 and some relatively infrequently utilized but important methods have also been developed. Despite these rapid advances, a long-term goal for the preparation of porous polymers remains development of procedures that provide rational design of tailor-made pore structures and incorporation of customized functionalities while retaining sufficient framework stability. Another longstanding challenge is to understand the implications of structural organization even at the molecular level. In addition, from an application point of view, development of simple and scalable procedures for construction of porous polymers with hierarchical structures at the different organization levels is particularly appealing in many cases, since the hierarchical pores are expected to synergistically exhibit the advantages of each class of pores.64,138,139 Another approach is to merge concepts and tools from different methodologies to develop well-defined hierarchical porous structures with innovative properties targeting selected applications.140 In summary, the synthesis of porous polymers has already become and will continue to become a thriving area of research. In this contribution, we present a comprehensive summary of the various efforts that have been made since 2005 focusing on preparation methodologies along with the control over pore structures ranging from micropores to macropores and the orchestration of the functionalities within the framework and at the pore surfaces. This work builds upon extensive prior art summarized in some reviews published before 2005.141−150 Despite significant progress on preparation of porous polymers, no single method exceeds all the others, so this review will summarize the advantages and limitations of each method in detail and clarify the present challenges and the ongoing efforts to overcome the limitations.
the synthesis is often hard to scale up when using expensive and labsynthesized amphiphilic block copolymers; there is a lack of micropores
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Figure 3. Schematic illustration of fabrication of (a) individual spherical porous polymers from solid spherical nanoparticle templates, (b) tubular porous polymers from tubular porous templates, such as AAO, and (c) ordered macroporous polymers from colloidal crystal templates.
capillary actions that can lead to pore deformation, or even collapse, after template removal.120 Last but not least, the polymeric walls should provide the ability to incorporate designable functionalities selected for targeted applications. This can be accomplished through construction of “built-in” responsive frameworks or the presence of surface functional groups. Considering the fact that the raw materials used for preparation of the polymeric walls include the premade polymers as well as monomers, we categorize the direct templating procedures into two groups: (a) templating with polymers as the raw material and (b) templating with monomers as the raw material. For the former situation, the polymers can be infiltrated into the voids inside the template or coated onto its surfaces. The interplay between the polymers and the templates is generally based on noncovalent interactions. When monomers are used as the raw materials, in addition to noncovalent interactions that assist in infiltration into the voids of template or coating onto the template surface prior to polymerization, covalent interactions (e.g., surface grafting polymerization techniques) used to grow the polymeric framework in situ from the template surface are also employed. The polymerization methods can be divided into three classes: conventional polymerization, electrochemical polymerization, and controlled/living polymerization, as summarized in Figure 5.
successfully prepared by direct templating, as summarized in Table 2, where the template, polymeric framework, pore size, and Brunauer−Emmett−Teller (BET) surface area are provided. The direct templating methodology is different from the indirect templating procedures, in which the templates are not preformed prior to use, such as use of porogenic solvents,128,129,151 gas-forming templates,152,153 micelle templates,154−156 and sacrificial segments in self-assembled block copolymers.8,125 Generally speaking, direct templating procedures include three main steps: (1) infiltration or adsorption of the raw materials required for the preparation of the polymeric framework onto the surfaces or into interstitial voids of the templates, (2) in situ polymerization or solidification of the distributed raw materials, and (3) removal of the templates. Frequently used raw materials include premade polymers and liquid and gaseous monomers, in addition to a few particular occasions where prepolymers are used.157 Templates include silica nanomaterials (e.g., silica nanoparticle and mesoporous silica), anodic aluminum oxide (AAO), and polymer nanomaterials. There are several requirements for a successful direct templating method for preparation of porous polymers. First, the surface properties of the selected templates should be compatible with the raw materials selected for synthesis of the polymeric framework, which will lead to a faithful replication of the template. Thus in many cases, the surface of templates need to be modified to render them compatible with the raw materials or to introduce reaction sites for growing polymers. Second, the templates should have well-defined structures, including controllable morphologies, which allow one to tune the predetermined porous structures of polymer replicas, just by a rational choice of templates. Third, the templates should be easily removed after templating. For example, the most commonly used template is silica since it is very easily removed by wet chemical etching with HF or NaOH. Figure 4 outlines the template removal conditions employed for various templates. Fourth, irrespective of whether the polymeric frameworks are linear or cross-linked, they should be robust enough to withstand the high interfacial energy and strong
2.1. Direct Templating with Polymers as the Raw Material
2.1.1. Infiltration. Generally, polymer solutions or polymer melts are infiltrated into the voids of the templates, and the polymer is solidified within the cavities. The most frequently used templates include AAO and mesoporous silicas. The final structure of the replicated porous polymer is controlled by two major factors: the properties of the templates, such as surface chemistry and pore size, and the properties of the infiltrating fluids, such as molecular weight, polymer chain conformation such as coiled or extended form, concentration of solutions, and viscosity. Generally there is no need for surface modification when infiltrating AAO templates, because infiltrated fluids with low 3962
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Abbreviations: PMAA, poly(methacrylic acid); PCL, polycaprolactone; PS, polystyrene; MF, acid-decomposable melamine formaldehyde particles; PANI, polyaniline; PPY, polypyrrole; CS, chitosan; PAA, poly(acrylic acid); PAH, poly(allylamine hydrochloride); PDMA, poly(2-(dimethylamino)ethyl methacrylate); PDVB, polydivinylbenzene; PFS, polyfluorostyrene; PEGDMA, poly(ethyleneglycol dimethacrylate); PMBAAM, poly(N,N′-methylenebisacrylamide); PNIPAM, poly(N-isopropylacrylamide); P(MBAAM-co-MAA), poly(N,N′-methylene bisacrylamide-co-methacrylic acid); PVAn-g-PANI, Poly(4-vinylaniline-g-polyaniline); PSS, poly(styrenesulfonate); PVPON, poly(vinylpyrrolidone); PLL, poly(L-lysine)hydrobromide; PGA, poly(L-glutamic acid); PC, polycarbonate; MO, methyl orange; PLGA, poly(lactide-co-glycolide); PLLA, poly(L-lactide); PEI, polyethyleneimine; PEG, poly(ethylene glycol); PS-b-P2VP, polystyrene-b-poly-2-vinylpyridine; PEDOT, poly(3,4-ethylenedioxythiophene); PPV, poly-(p-phenylene-vinylene); P(4VP-co-EGDMA), poly(4-vinylpyridine-co-ethylene glycol dimethacrylate); P3HT, (poly(3-hexylthiophene); PS-b-PBD, polystyrene-b-polybutadiene; PVDF, poly(vinylidene fluoride); PAN, poly(acrylonitrile); ABS, acrylonitrile-butadiene-styrene terpolymer; P(Man-alt-VAc), poly(maleic anhydride-alt-vinyl acetate); poly(St/NaSS), poly(styrene/sodium pstyrene sulfonate); PF resins, phenol-formaldehyde resins; PMMA, poly(methyl methacrylate); PAM, polyacrylamide; PETPTA, poly(ethoxylated trimethylolpropane triacrylate); PDMS, poly(dimethyl siloxane); PE, polyethylene; PPMS, poly(p-methylstyrene); P(HEMA-co-EGDMA), poly(2-hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate); PBI, poly(benzimidazole).
up to 1010 m2g−1 2−85 nm PS, PDVB, PAA+PAH, PBI, PAA, P(HEMA-co-EGDMA) mesoporous silica, silica particles, P4VP others
3nm−150 nm ordered porous
nanotubular membranes of AAO, PC and TiO2; nanofibers of Na2SO4, ZnO, V2O5, FeCl3-MO, PMMA, PLGA, PLLA, PLLA/PLGA and PS colloidal crystals of silica, PS, PMMA, P(Man-alt-VAc) and poly(St/NaSS); KIT-6; SBA-15 tubular porous polymer
surface energy can readily spread over the high surface energy inner walls of AAO to form a thin film.143,247,248 The outside diameter of resulting polymer nanotubes is determined by the pore diameter of AAO, and the tube length is essentially dependent on the membrane thickness of the AAO. In addition to providing hollow polymer nanotubes, solid polymer nanopillars (nanofibers or nanorods) are also fabricated through AAO templating.143,249 Significant efforts have been devoted to explaining the mechanisms for formation of polymer nanotubes and nanopillars. For example, a detailed investigation has shown that it is possible to fabricate various onedimensional (1D) polymer nanostructures ranging from nanotubes to nanofibers by different infiltration methods, for example, methods based on wetting, methods based on vacuum, and methods based on spin.200 In another example, a partial to complete wetting transition exists during the wetting of AAO membranes with PS melts when the annealing temperature is increased above a critical temperature, which is determined by the polymer molecular weight.202 Therefore, in order to avoid the formation of nonporous 1D polymer nanostructures like nanofibers, the preparation conditions should be carefully optimized during the preparation of polymer nanotubes by AAO templating. Novel polymeric nanotubes, with a longitudinal composition gradient, have also been prepared by face-to-face wetting of AAO with different polymeric solutions (Figure 6), in contrast to face-to-face wetting with polymeric melts, where tubular components predominantly consist of the pure components that were separated by sharp interfaces.194 When mesoporous silica is utilized as a template to fabricate porous polymers, the mesopores should be large enough to allow access and subsequent accommodation of macromolecules. Sometimes the silica walls need to be functionalized to improve adsorption of the selected macromolecule, for instance, amine surface functionalization allows efficient infiltration of poly(acrylic acid) (PAA).238 At the same time, rational tuning of chain conformation can improve macromolecule infiltration. For example, at low pH or high ionic strength, PAA chains exhibit a coiled conformation and can infiltrate the nanopores, while at high pH or low ionic strength they adopt an extended chain conformation, resulting in spatial exclusion from the nanopores, see Figure 7.238 While the polymer infiltration of templates has been successfully used to fabricate many types of well-defined porous polymer materials with the help of templates, it does not allow precise control of the pore wall thickness and is essentially ineffective when individual solid particles are used as templates. One approach to overcome this limitation is layerby-layer (LbL) assembly deposition. LbL assembly deposition can be applied either on the surface of nonporous templates or on the pore walls of porous templates.250 Pore wall thickness can be manipulated at the nanometer scale by controlling the number of deposited layers, as discussed in the following section. 2.1.2. Layer-by-Layer Assembly. LbL assembly, which was introduced in 1991,251 is one of the more powerful approaches for direct templating with polymers as the raw material.250 LbL assembly allows polymers to be deposited onto the surface of templates by the alternate adsorption of polymers that incorporate oppositely charged moieties. Various porous structures can be obtained with the LbL assembly because of its tolerance for templates with different structures including solid spherical nanoparticles, nanowires, AAO, and mesoporous
a
14, 25, 70, 72, 73, 75, 115, 118, 157, 215−237, 257 55, 238−246 up to 1600 m2g−1
50, 66, 105, 178−214
template
spherical nanoparticles of silica, PMAA, PCL, PS, MF, MnCO3, Au, silica-coating Fe3O4, AgBr and AgCl spherical porous polymer
100∼300 nm
pore size
10 nm ∼ several μm
polymeric framework
PMAA, PS, PANI, PPy+CS, PPY, CS, PCL, PAA, PAH, PDMA, PDVB, PFS-b-PDVB, PEGDMA, PMBAAM, PNIPAM, P(MBAAM-co-MAA), Poly(norbornene ester-b-norbornene), PVAn-g-PANI, PSS+PAH, PVPON+PMAA, PLL+PGA P(NIPAM-co-MBAAM), PEI, PEG, PS, polyphenylene, PMMA, PS-b-P2VP, PPY, Polyfluorene, PHEMA, PEDOT, PPV, PEI+PAA, PAH+PSS, P(4VP-co-EGDMA), P3HT, PS-b-PBD, PVDF, PAN, ABS, substituted poly(norbornenes) PANI, PPY, PF resins, poly(ionic liquid)s, PMMA, PEGDMA, PAM, PS, PETPTA, PDMS, PE, PPMS, P(MAA-co-EGDMA), P(HEMA-co-EGDMA), P(AM-co-AA), PAN, PDVB
BET surface area
refs
16, 17, 47−49, 51, 83, 116, 158−177
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porous polymer
Table 2. Summary of Template, Polymeric Framework, Pore Size, and BET Surface Area for Various Types of Porous Polymers Prepared by Direct Templating Methodologya
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Figure 4. Summary of removal conditions of various templates. Inner ring and outer ring denote templates and their corresponding removal conditions, respectively. The full name of abbreviations can be found in Table 2.
Figure 6. Schematic illustration of face-to-face wetting with polymer solutions. Reprinted with permission from ref 194. Copyright 2008 American Chemical Society.
(allylamine hydrochloride) (PAH),239,240,242,243 anionic PAA vs cationic polyethylenimine (PEI),209 anionic PSS vs cationic PAH, 2 0 8 poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) star polymers vs PAA star polymers,252 and poly(ethylene glycol) (PEG)-based polyurethanes bearing negatively charged lithium sulfonate vs positively charged quaternary ammonium bromide moieties.184 Interlayer crosslinking of the polyelectrolytes with additional cross-linkers209,239,242,243 or heating239,240 can further strengthen the multilayer polymeric wall. Figure 8 illustrates an example of biocompatible polyelectrolyte nanotubes fabricated by utilizing the LbL assembly of a positively charged PEI and a negatively charged PAA on the surface of Na2SO4 nanowires, which are water-soluble and thus avoid any harsh template removal treatments.209 Moreover, as noted above, the stability of the
Figure 5. Summary of major techniques for the direct templating methodology with polymers and monomers as the raw material.
silica.250 Moreover, LbL assembly is an environmentally benign and low cost technique that exhibits a unique ability to judiciously manipulate the polymeric shell thickness for porous polymers at the nanometer scale, just by controlling the number of layers deposited on the template.250 As noted above, generally electrostatic interactions serve as the driving force for the LbL assembly of oppositely charged polyelectrolytes, for example, anionic PAA vs cationic poly3964
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Figure 7. Schematic representation of two extreme cases influencing polymer infiltration: (a) low pH or high ionic strength (resulting in infiltration of coiled PAA molecules in the nanopores) and (b) high pH or low ionic strength (resulting in exclusion of extended PAA molecules from the nanopores). Reprinted with permission from ref 238. Copyright 2007 American Chemical Society.
Figure 8. Nanotube synthesis processes using water-dissolvable Na2SO4 nanowire templates. PE represents polyelectrolyte, that is, PEI-PAA. Reprinted with permission from ref 209. Copyright 2006 American Chemical Society.
multilayer wall can be improved by interlayer chemical crosslinking, in this case with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride as the cross-linker.209,243 With the development of LbL techniques, some novel assembly driving forces based upon nonionic interactions between the uncharged polymers, such as van der Waals interactions,168,179 hydrogen-bonding interactions,253 hybridization of cDNA strands,254 and covalent-bonding interactions,162,185,255 have also been utilized. These new forces driving interlayer assembly provide several advantages compared with the more frequently used electrostatic interaction. For instance, the procedures extend the range of polymer source from polyelectrolytes to uncharged macromolecules, thereby avoiding the need for a second polyelectrolyte component in the polymeric framework. The procedures also allow one to increase the stability of the assembled polymeric wall by constructing covalent bonding interactions. Figure 9 shows a process for the preparation of hollow poly(methyl methacrylate) (PMMA) capsules by a doublestranded helical assembly formed through van der Waals interactions between isotactic and syndiotactic PMMA units.168 Figure 10 illustrates the preparation of responsive polymer capsules based on the covalent LbL assembly of alkynefunctionalized PAA (PAA-Alk) and azide-functionalized PAA (PAA-Az) on the surface of PEI-coated silica particle.174 Each deposited layer was stabilized by covalent click chemistry between contacting deposited layers. Moreover, the clicked multilayers have been demonstrated to serve as a versatile platform for further reactions and functionalization.174
Figure 9. Fabrication process of it-/st-PMMA stereocomplex hollow capsules. Reprinted with permission from ref 168. Copyright 2006 Wiley-VCH.
To summarize, the direct templating with polymers as the raw materials has become a viable tool to design and fabricate various types of porous polymers based upon either polymer infiltration or LbL techniques on a variety of templates. However, some drawbacks remain; for example, templating methods become increasingly difficult when the pore size of template becomes much smaller, as in the case of small mesopores and micropores, due to the molecular dimensions of polymeric raw materials. In addition, the application of LbL assembly to very small nanoparticles is challenging. 2.2. Direct Templating with Monomers as the Raw Material
Direct templating with monomers, or prepolymers in some rare cases, as the raw material, has some advantages over that with polymers as the raw material. First, since monomers, the precursors of all polymers, have much smaller molecular size than polymers, they can easily infiltrate into extremely small pores present in the templates and replicate the template materials at a much higher degree. Second, monomer templating provides the means to expand the diversity of 3965
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Figure 10. Covalent LbL assembly of PAA capsules by click chemistry with alkyne (PAA-Alk) and azide (PAA-Az) modified PAA: (a) template particle; (b) PAA-Az electrostatically adsorbed to the surface; (c) PAA-Alk “clicked” onto the PAA-Az layer; (d) steps b and c repeated until the desired number of layers is achieved; (e) removal of the sacrificial core to form click capsules. Reprinted with permission from ref 174. Copyright 2007 American Chemical Society.
2.2.1.1. Templating Strategies. 2.2.1.1.1. Infiltration and Subsequent Polymerization inside Porous Templates. In this procedure, monomers are infiltrated into the voids present in the selected templates, followed by in situ polymerization. The utilized templates include colloidal crystals, AAO, and mesoporous silica. Many types of porous polymers, including ordered macroporous or mesoporous polymers and tubular polymers, can be prepared after removal of these templates. The infiltrated monomers are generally in three forms: liquid monomers, solutions of monomers in solvents, and gaseous monomers. The first two procedures frequently use fluid infiltration, but the use of gaseous infiltration of monomers requires that the monomers themselves should be easy to gasify or be capable of gasification within special equipment. There are two general rules that should be taken into consideration for a successful preparation of porous polymers by infiltration of monomers. One is to ensure efficient infiltration and avoid incomplete or excess filling, and the other is to control the polymerization conditions to achieve an efficient replication of the template in the resulting polymer framework by taking into account cross-linking degree, reactant concentrations, and ratios of monomer to template, etc. In most cases, the infiltration of monomer liquids or solutions into the voids of the selected template is performed directly, and additional steps are not required. For example, monomer fluids containing initiators and other components are dropped onto the surface of the colloidal crystals, and after allowing sufficient time for infiltration, excess monomers are removed. Recently, an improved infiltration process has been developed using a so-called “capillary-attraction-induced method” and is illustrated in Figure 11.218 In this procedure, monomers are dropped onto the upper edge of the PS opal template on a tilted glass slide and then fall downward slowly along the glass slide to infiltrate the colloidal crystal until the template becomes transparent. Subsequently, the infiltrated monomers are polymerized, and then the PS template is removed to form a porous polymeric thin film. This method can lower infiltration defects and effectively avoid the collapse of an opal template.218 Sometimes, negative pressure or surface modification of the pore wall of the template is needed for
compositions, and hence properties, of the pore frameworks because the choice of monomers is broader. Additionally, functional groups can be easily introduced to the pore surface by copolymerization of a functional monomer or through postpolymerization functionalization chemistries. Third, the polymeric nanostructures can be deliberately designed and controlled by utilization of various polymerization techniques. For example, after template surface functionalization the shell thickness can be tuned, at the molecular level, by the degree of polymerization in a “grafting from” surface-initiated atom transfer radical polymerization (SI-ATRP).256 Currently, a diverse variety of porous polymers with different nanostructures and functionalities have been successfully fabricated based upon polymerization of various monomers on the surface of nanostructured templates or in the internal pores of porous templates. The polymerization methodologies utilized in these procedures can be summarized into three main classes: (i) conventional polymerization; (ii) electrochemical polymerization; (iii) controlled/living polymerization. The following discussion will be subdivided into three parts according to the polymerization methodology employed to form porous polymers. 2.2.1. Conventional Polymerization. Conventional polymerization of monomers in the presence of templates is frequently used to prepare porous polymers because it is a simple procedure that can be applied to a variety of available monomers. Standard free radical polymerization is the most common method, while polycondensation, oxidative polymerization, and others are also used. The utilized templates range from solid nanoparticles to porous materials that allow preparation of various polymeric porous structures. The most frequently used templating strategies can be classified into three groups: (i) infiltration and subsequent polymerization inside the porous templates, (ii) polymerization on the surface of nonporous templates, and (iii) in situ formation of templates and subsequent polymerization. The various templating strategies employed with conventional polymerization processes are discussed in the following sections, and then the review will focus on procedures used to control the pore structure within the porous polymers. 3966
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Figure 11. A schematic illustration of the preparation of inverse-opal hydrogels by using a capillary-attraction-induced method. Reprinted with permission from ref 218. Copyright 2007 Wiley-VCH.
efficient infiltration.180,257 For example, ordered mesoporous silica KIT-6 is functionalized via silylation to generate a hydrophobic surface before infiltration with a solution of divinylbenzene (DVB) monomer.257 The infiltrated monomer liquids or solutions are generally polymerized via radical polymerization, 55 , 23 4, 2 41 ,2 57 , 25 8 oxidative polymerization,72,180,215 polycondensation,157,217 coordination polymerization,224−227 and others.182 Despite the frequent use of the infiltration of monomer liquids or solutions, there remains a need to develop more efficient infiltration methods that result in production of well-defined porous polymers. Infiltration of gaseous monomers has some advantages over that of monomer liquids or solutions: the use of an organic solvent can be avoided, which greatly decreases the filling defects due to the removal of the solvent; vapor infiltration is a more rapid procedure in many cases, compared with liquid infiltration; products with uniform pores and finely controlled structure can be achieved. Generally, initiators or catalysts are first supported on the surface of the templates, and then the monomer vapors are infiltrated followed by polymerization. An example is the fabrication of 3D ordered macroporous PPY by vapor-phase infiltration of pyrrole into the FeCl3 embedded colloidal crystals with subsequent oxidative polymerization, as shown in the scheme in Figure.12.228 Similarly, 3D ordered macroporous polyethylene has been successfully prepared via coordination polymerization after infiltration of gaseous ethylene into a catalyst functionalized silica template (Figure 13).223 Initiated chemical vapor deposition (iCVD) is a similar technique and is illustrated in Figure 14.190 Radicals resulting from thermal decomposition of initiators attack the vinyl bonds
Figure 13. Schematic diagram for fabricating 3D ordered macroporous polyethylene by gaseous infiltration. Reprinted from ref 223, Copyright 2008, with permission from Elsevier.
Figure 14. Schematic of the deposition reactor with the AAO template and the initiated chemical vapor deposition process. During deposition, radicals attack the monomer molecules adsorbed in the pores of the template, initiating the polymerization. Reprinted with permission from ref 190. Copyright 2010 The Royal Society of Chemistry.
of monomers and cross-linkers adsorbed in the pores of the AAO template, initiating the polymerization resulting in polymer deposition. This specific iCVD polymerization leads to the formation of tubes rather than rods within the pores of AAO, and it has the advantage over liquid phase infiltration in terms of controlling the wall thickness of the nanotubes.190 To
Figure 12. Fabrication of polypyrrole inverse opal by vapor-phase oxidative polymerization of pyrrole with a template of poly(St/NaSS) colloid crystal. Reprinted with permission from ref 228. Copyright 2007 American Chemical Society. 3967
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Figure 15. Preparation of hollow polymer microspheres via distillation−precipitation polymerization: (a) PDVB and PEGDMA hollow spheres from silica templates (Reprinted from ref 172, Copyright 2007, with permission from Elsevier); (b) PDVB hollow spheres from PMMA templates (Reprinted from ref 163, Copyright 2007, with permission from Elsevier).
the vinyl groups on the surface of the modified templates such as 3-(methacryloxy)propyltrimethoxysilane-modified silica nanoparticle (see Figure 15a)172 and pyridyl-functionalized poly(methacrylic acid) nanoparticles (Figure 15b),163 which can be utilized to capture the oligomeric radicals of monomers via a chain growth procedure that has been named distillation− precipitation polymerization, which is summarized in Figure 15. 2.2.1.1.3. In Situ Formation of Templates and Subsequent Polymerization. As demonstrated above, the majority of templates for preparation of porous polymers are introduced to the procedure prior to infiltration or surface modification for the subsequent polymerization. However, in an alternate procedure, the template can be formed in situ in the presence of monomers followed by polymerization. This procedure allows the infiltration process or surface modification of preformed templates to be bypassed. The procedure has been applied to the use of colloidal crystals as the template. The in situ assembly of colloidal crystal templates in the presence of monomers, without the need for an infiltration step, is realized by a roll-to-roll compatible, up-scalable doctor blade coating technology,25,221 as shown in Figure 16.25 The immobilized doctor blade applies a unidirectional shear force that aligns a suspension of silica microsphere and ethoxylated trimethylolpropane triacrylate monomer to form highly ordered 3D colloidal crystal−polymer nanocomposites via photopolymerization in a single step, which leads to formation of ordered macroporous polymers after silica removal. Another interesting example is the use of AgCl nanoparticle templates for the preparation of hollow PPY.159 The solid AgCl nanoparticles are formed during the initial stage of the polymerization of pyrrole by the interaction between Ag+ and Cl−, which then adsorb chitosan present in the solution to induce the coating of the nanoparticles with PPY layers.159 This procedure then requires removal of the AgCl to form hollow PPY.159
sum up, the development of new infiltration methods has resulted in a considerable expansion in precise replication of template materials, thereby providing well-defined porous polymers. 2.2.1.1.2. Polymerization on the Surface of Nonporous Templates. This approach to porous polymers utilizes individual solid nanomaterials as templates but generally requires surface modification of the template in order to make the polymerization closely conform to the surface of the templates for efficient replication. A composite structure consisting of a template core and a polymeric shell is initially formed, and after removal of the core, the resulting porous polymer consists of individual pores rather than continuous porous networks. The primary issue for the successful replication of a well-defined porous structure is to ensure a strong interaction between the templates and the monomers and resulting polymer product. For example, introduction of functional groups with electric charges onto the surface of the templates can foster the adsorption of monomers via electrostatic interactions prior to polymerization on the surface of templates, for instance, surface-sulfonated PS particle enhances the adsorption of aniline.165,259 An interesting example is the formation of hollow PANI spheres with double-shelled structures that were prepared using hollow PS spheres as the template. Both the exterior and interior shell surface of these hollow PS spheres were subjected to sulfonation to efficiently adsorb aniline.259 Hydrophobic interactions are useful for directional coating of the template,17,160 for instance, localizing the polymerization of NIPAM and cross-linker methylene bisacrylamide on the surface of PCL nanoparticles, resulting in formation of hollow PNIPAM spheres.17 On the other hand, utilization of covalent interactions between the modified templates and the monomers is also a good strategy.16,163,172 Examples include 3968
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Figure 16. Schematic illustration of the experimental setup for assembling large-area colloidal crystal−polymer nanocomposites by using a simple doctor blade coating technique. Reprinted with permission from ref 25. Copyright 2010 American Chemical Society. Figure 17. Protocol for the preparation of photonic molecularly imprinted polymer films: (A) SiO2 colloidal crystals on glass substrate; (B) infiltration of complex solution into colloidal template followed by photopolymerization; (C) photonic molecularly imprinted polymer film after the removal of SiO2 nanoparticles and dopa template molecules; (D) complex of monomer and template molecule; (E) imprinted molecules within the polymer matrix; (F) imprinted cavities with complementary shape and binding sites to the template molecule. Reprinted with permission from ref 14. Copyright 2006 Wiley-VCH.
The templates used in the above approaches require additional removal steps. However, an interesting reactive self-degraded template that is formed in situ has been successfully developed to prepare PPY nanotubes in a single step without any additional processing required for template removal.66,213 The product can be carbonized into carbon nanotubes that are mostly amorphous in structure,66 which are different from typical carbon nanotubes exhibiting highly ordered graphitic structure.260−265 This procedure utilizes a fibrillar complex of FeCl3 with methyl orange that is instantaneously formed from the added reactants and, after introduction of pyrrole, results in the direct growth of PPY on its surface, during which time the complex template itself automatically degrades owing to the reduction of Fe3+.66,213 2.2.1.2. Pore Structure. The strategies described above allowed preparation of porous polymers with various types of pore structures, including, ordered macroporous structures, 14,70,72,73,75,218,220,266 ordered mesoporous structures,157,257,267,268 tubular pores,180,182,190,205,207,214 and individual spherical pores.16,17,48,163,164,172,173,269 The pore structure can be tuned deliberately by selection of the type and size of templates, monomer/template ratio, cross-linking density of polymeric framework, and so on.244−246 Highly ordered macroporous polymers are fabricated using colloidal crystal templates.120,123,144,270,271 Monodisperse colloidal crystals, which usually have diameter of >100 nm, including silica, PS, PMMA, and some copolymer spheres, are the most frequently used templates. In most cases, it remains a challenge to assemble much smaller spheres into highly ordered crystalline arrays. Therefore, porous polymers with highly ordered structure have pores that often lie in the macropore range. The ordered macroporous structure also depends on the packing pattern of the colloidal templates, which can be tuned by the interactions between templates and substrate.220,228 There is an opportunity to introduce a second template within the crystal voids,14,217 which opens up an avenue for shaping the framework porosity by judicious design of a hierarchical porous structure. Figure 17 illustrates the possibility to imprint photonic reverse opals by simultaneously infiltrating monomer MAA, cross-linker EGDMA, and secondary pore “imprinting molecules”, L-dopa, prior to polymerization.14 The L-dopa not only creates a secondary
pore for control of the skeletal structure at a molecular length scale but also allows the resulting photonic crystal PMAA based hydrogel films to detect the binding of specific enantiomeric analytes and specific stimulant assays. Such a hierarchical porous structure can facilitate the diffusion of specific targeted molecules and enable the polymers to self-report sensitively. In addition to the use of imprinting molecules for secondary templating, simultaneous infiltration of self-assembled supramolecular complexes composed of phenolic resins and block copolymer templates have also been used to introduce ordered mesopores into the walls of ordered macroporous polymers.217 Currently, fabrication of hierarchical porous carbons and silicas using such multitemplating techniques, with or without an infiltration step, has received increasing attention.217,272−275 Therefore, it is believed that more and more novel procedures for the design of hierarchical porous polymers that can be applied to a broader field will be developed. When AAO is used as a template, chemical polymerization can occur inside its tubular walls, leading to formation of a tubular pore structure. An example, leading to hyperbranched polyphenylene nanotubes, is to infiltrate solution of monomers that can be subjected to subsequent Diels−Alder reactions into the AAO.182 The formed porous polymeric structures can then be transformed into carbon nanotubes.182 However, care must be taken since there is a chance of producing nanorods or other solid structures instead of tubes; for example, the oxidative polymerization of 3,4-ethylenedioxythiophene formed polymeric nanorods and nanotubes.180 The size of pores formed in polymers templated with colloidal crystals and AAO is generally large, leading to a very small surface area. However, porous polymers with much higher surface area, up to 1000 m2 g−1, and smaller mesopores (e.g., down to 3.7 nm257) can be achieved with mesoporous 3969
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illustrates the procedure used to prepare mesoporous poly(2hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate) with pressed pellets of fumed silica as the template.245 In the case of high to intermediate cross-linking density, permanent porosity is detectable with the pore radius dictated by the template; however in the case of intermediate to low crosslinking density, no permanent porosity can be detected due to closure or collapse of the pores.245 A pore collapse occurred when the DVB cross-linker content was ∼20 wt %, but with increasing amounts of cross-linker, the surface area increases and the pore size decreases.246 At the same time, the pore size of silica template also plays an important role. For instance, when targeting an ordered mesoporous PDVB using ordered mesoporous silica KIT-6 as the template, the ordered cubic Ia3̅d mesoporous structure can only be retained when the pore diameter of the template exceeds 5 nm.257 In summary, conventional polymerization procedures are the most popular route for the preparation of porous polymers by direct templating with various monomers as the raw material. The process is relatively simple to carry out. Sometimes the template can be formed in situ prior to polymerization, and additional removal steps can be avoided when using selfdegrading templates. However, electrochemical polymerization is a better choice than conventional polymerization when targeting conducting polymers, because precise control over the structural quality is often deficient when using conventional polymerization procedures. 2.2.2. Electrochemical Polymerization. Electrochemical polymerization is most frequently used for fabrication of porous materials consisting of conducting polymers, such as PANI, PPY, poly(3-hexylthiophene) (P3HT), poly(carbazole), and PEDOT. These porous conducting polymers are generally shaped into ordered macroporous structures or tubular structures by using either colloidal crystals115,216,222 or AAO,105,181,183,191 but track-etched porous polycarbonate membranes,204 TiO2 nanotube arrays,211 ZnO nanowire arrays210 and other one-dimensional particles50,203 have also been used as templates. Figure 19 provides a scheme illustrating the procedure employed for the preparation of ionic-liquiddoped PANI inverse opals via electrochemical polymerization within the interstitial voids of colloidal crystals deposited on a gold substrate.222 Electrochemical polymerization offers several advantages compared with conventional polymerization. First, it provides a greater degree of control over the film thickness, the number of deposited layers, and the structures of porous polymers such
silicas as the template. Indeed, the preparation of wellcontrolled ordered mesoporous polymers can be expected with templates with highly ordered pore structures and uniform pore size.157,257 However, in such cases, the preparation of highly cross-linked stable polymeric frameworks is required to avoid the collapse of the small mesopores as a consequence of large surface energies and capillary pressures generated during the drying procedures.120 Consequently cross-linking density is a key parameter that affects the stability of the pore structures during drying. Generally, highly cross-linkable frameworks are used, such as PDVB55,234,241,257 or phenol−formaldehyde resins.157 Figure 18
Figure 18. Summary of the porosity analysis in the dry and swollen state of poly(2-hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate) hydrogels: (a) in the case of high to intermediate cross-linking density, permanent porosity is detectable with the pore radius dictated by the used template; (b) in the case of intermediate to low crosslinking density, no permanent porosity can be detected due to pore closure or collapse; (c) in the case of high cross-linking density, the pores are filled with water, but no significant swelling of the pore wall takes place (δ denotes the nonfreezing interfacial layer); (d) materials of intermediate cross-linking density show well-defined water domains; additionally, the pore walls are also swollen to some extent; (e) materials of low cross-linking density feature a biphasic structure; however, they possess an ill-defined interface between the swollen polymeric walls and the pure water domains. Reprinted with permission from ref 245. Copyright 2010 American Chemical Society.
Figure 19. Procedure for the fabrication of ionic-liquid-doped PANI inverse opals. Reprinted with permission from ref 222. Copyright 2009 WileyVCH. 3970
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as pore wall thickness,210 size of pores, and pore morphology characteristics, (e.g., open vs closed,166,216 top-closed vs mushroom-like210 topology of the topmost layer). Second, it provides a greater ability to control the polymerization rate and achieves a much more compact polymer structure, leading to the improved quality of the resulting inverse opal films.216 Third, the as-constructed porous nanostructures contact more tightly with conducting substrates, which is convenient for constructing an electrical device.276 Fourth, it provides the ability to selectively choose polymerization sites, for example, as shown in Figure 20, by site-specific deposition of PPY in the space between the tube walls instead of inside the tubes in the case of using TiO2 nanotube array as the template.211 Figure 21. Voltage changes during the electropolymerization process for preparing PANI inverse opals by a galvanostatic method at a current density of 0.05 mA cm−2. The inset sketches show the stage of the formed PANI inside the PS template at each point as indicated by the arrows. SEM images of the corresponding PANI inverse films obtained by stopping the polymerization at these points are also shown. Reprinted with permission from ref 216. Copyright 2005 American Chemical Society.
Figure 20. Formation of self-organized PPY nanopore arrays: (a) electropolymerization of pyrrole at the bottom of nanotubular titania frameworkpolymer is incorporated in the space between the tube walls; (b) extended polymerization leading to surface deposition; (c) selective dissolution of TiO2 by treatment in 5 vol % hydrofluoric acid. Reprinted with permission from ref 211. Copyright 2010 The Royal Society of Chemistry. Figure 22. Potential dependence of the tubular portions as a function of applied potential at given monomer concentrations in the electrochemical template synthesis of PEDOT nanostructures. The nanotubular structures become relatively insensitive to monomer concentrations at very low oxidation potentials in the yellow boxed region. Reprinted with permission from ref 276. Copyright 2008 American Chemical Society.
The ultimate structure of this type of porous conducting polymer can be controlled by various experimental parameters, such as the potential scan rate, polymerization time, applied potential, monomer concentration, base electrode shape, pulse current approach, electrolyte concentration, template thickness, stirring, and temperature.105,181,183,216,276 A typical example is the structural control over PANI inverse opals formed with a PS colloidal crystal template simply by changing the scan rate and the electrochemical polymerization time.216 Slowing the scan rate facilitates a much more compact structure without blocking the pores above the PANI growth front, leading to a high structural quality in the inverse opals. However, too high a scan rate could lead to the collapse of the 3D ordered structures after template removal because of very loose packing of the rapidly formed PANI chains. Moreover, by controlling the polymerization time, one can exactly tailor the film thickness and the open/closed characteristic of the topmost layer of the inverse opal, as illustrated in Figure 21.216 Another typical example is the structural control over PEDOT nanotubes prepared with an AAO template through the applied potential and monomer concentration.181 The effect of potential on the tubular portions (see R in Figure 22) of PEDOT nanotubes is complicated but can be revealed by separately considering two different oxidation potentials divided at 1.4 V.276 At high oxidation potentials (≥1.4 V), the growth mechanism can be explained based on diffusion and
reaction kinetics, as shown in the scheme in Figure 22. In the case of high potential and low monomer concentration (e.g., 1.8 V, 10 mM EDOT), the rate of polymerization is very high, which leads to insufficient monomer available for uniform filling of the pores. Since the reaction initiates along the electrode surface at the bottom of the pore, the first formed PEDOT chains are continuously deposited along the pore wall, leading to formation of PEDOT nanotubes (Figure 23). In contrast, for relatively low potentials and high monomer concentrations (e.g., 1.4 V, 100 mM EDOT), the polymerization rate is low, and the monomers have enough time to diffuse to the bottom of the pore and polymerize on the entire electrode surface without any notable deposition preference, leading to the formation of PEDOT nanowires (Figure 23).181,276 On the other hand, at very low oxidation potentials (
