7Porous Materials by Templating of Small Liquid DropsHaifei Zhang
7.1Introduction
Templating is one of the most frequently used methods for the preparationof porous materials with pore sizes ranging from nanometers to micrometers.Porous materials, which may be made of ordered pores, different pore shapes, orpores of different sizes (hierarchical pores), have a wide range of applicationscovering nearly every aspects of industrial applications. There are diverse materialsthat can be used as templates, for example, surfactant micelles, block copolymerself-assemblies, inorganic particles, colloids, polymer gels, preformed structures,bubbles, emulsions, and so on. In general, the template materials can include hardtemplates and soft templates. Hard templates are usually removed by calcination orpyrolysis, chemical etching, or washing, while the soft templates can be removedsimply by solvent evaporation or sublimation.
In this chapter, we describe the use of soft templates and, more specifically,small liquid drops as templates for the fabrication of porous materials. To useliquid droplets as templates, the materials need to be solidified around the droplets,and the subsequent removal of droplets by solvent evaporation produces templatedporous materials. Emulsions are two immiscible solvents mixed together withone phase in the form of droplets dispersed in the other phase. In general, asurfactant or stabilizer is required to stabilize the droplets because emulsionsare not thermodynamically stable. The droplets suspended in the emulsionsmay be used as templates for the fabrication of porous structures. To do this,monomers or other reactive molecules are dissolved in the continuous phase.A polymerization or a sol–gel process is carried out in the continuous phase duringwhich the emulsion should be stable, that is, no phase separation and very limitedcoalescence of droplets into larger drops. The reaction in the continuous phaselock in the emulsion structure and the droplets can then be removed to generateemulsion-templated porous structures.
Obviously, the number of droplets in a certain volume of emulsion can affect theporosity of the resulted materials: the higher the number of droplets, the higherthe porosity in the materials. The number of droplets or the volume ratio of the
210 7 Porous Materials by Templating of Small Liquid Drops
droplet phase in the emulsion can be easily tuned by mixing different amounts ofwater and oil phase to make the emulsion. A high internal phase emulsion (HIPE)is an emulsion with the volume ratio of the internal droplet phase >74.05 v/v%.HIPEs have been widely used to prepare highly interconnected porous materials.In addition to the number of droplets, the size of the droplets can directly affectthe size of the pores in the materials. For emulsions that are stable enough duringpolymerization, the size of the droplets can be very close to the size of the pores.However, this is not the case for microemulsion templating. In this chapter,we describe emulsion templating, HIPEs as templates for porous hydrophilicpolymers, and the use of microemulsion templating for porous structures.
The polymerization or sol–gel process in the continuous phase of the emulsionsnormally occurs at elevated temperatures or sometimes at room temperature.However, it is also possible to solidify the emulsions by rapid freezing. Thesubsequent sublimation of the frozen solvent in a freeze-drying process canproduce dry porous materials. This concept and the latest progress in this area havebeen introduced in this chapter.
It has been noticed since long that water from the moist air can condense ona cold surface and the water droplets can pack into ordered hexagonally arrangedpatterns. This phenomenon is also widely known as breath figures (BFs). Onlyrecently was the BFs pattern exploited to prepare ordered porous films, (also called,honeycomb-structured films) via the use of water droplets as templates. It was alsopossible to form three-dimensional (3D) porous structures using this templatingapproach. In Section 7.3, we explain the parameters affecting the formation of BFpatterns, how BF patterns are used as templates for porous films, and differenttypes of porous films/structures that have been prepared in recent years.
7.2Emulsion Templating
Emulsions are heterogeneous mixtures of one phase in the form of dropletssuspended in another immiscible continuous phase. In most cases, water is usedas one of the solvents. An emulsion can be formed by dispersing water droplets inan oil phase, known as a water-in-oil (W/O) emulsion, or dispersing oil dropletsin water, known as an oil-in-water (O/W) emulsion. In the case of the O/Wemulsion, because of the worldwide effort of reducing the use of an organic solventand the sustainable green nature of compressed or supercritical CO2, emulsionsof CO2-in-water (C/W) have also been prepared and used for various applicationsincluding in templating of porous materials [1, 2].
Emulsions can be used as templates for the synthesis of polymer colloids, porouspolymers, or polymer composites. As illustrated in Figure 7.1, when monomers areonly dissolved in the droplet phase or monomer droplets suspended in a continuousphase, the polymerization of the monomers produces polymer colloids. This routeis widely known as emulsion polymerization. When the monomers are dissolved onlyin the continuous phase, the polymerization of the monomers and the subsequent
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Polymer latex
Polymerizedispersed phase
Porous material Composite
Polymerizecontinuous phase
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Polymerizeboth phases
Figure 7.1 Schematic representation of polymerization of an emulsion in the dispersedphase, continuous phase, and both phases for the preparation of colloids, porous materi-als, and composites, respectively. (Reprinted with permission from Ref. [3], copyright 2005Royal Society of Chemistry.)
removal of the droplets phase leads to the formation of porous polymer. Polymercomposites are formed when different types of monomers are present both in thedroplet phase and in the continuous phase and then they are both polymerized [3].
In the case of emulsion templating for porous polymers, W/O emulsions havebeen frequently employed to make porous hydrophobic structures [4–7]. For someapplications, porous structures with a hydrophilic surface are required. In orderto enhance the hydrophilicity, a functionalizable comonomer, such as vinylbenzylchloride, is added to the nonfunctionalizable monomer such as styrene in theemulsions [8, 9]. Silverstein et al. described the production of hydrophilic porouspolymer through hydrolysis of the hydrophobic polymer. The porous hydrophobicpolymer was prepared on the basis of t-butyl acrylate. All the t-butyl acrylate in theformed porous material could be hydrolyzed [10]. In another effort, 4-hydroxymethylphenyl (Wang linker) moieties and tris(hydroxymethyl)aminomethane were immo-bilized onto vinylbenzyl chloride/divinylbenzene (DVB) porous polymer matricesvia displacement of the chlorine in chloromethyl groups [11]. A high loading of OH
212 7 Porous Materials by Templating of Small Liquid Drops
groups per gram of polymer support was achieved. The reactivity of these polymersupports was demonstrated by the immobilization of 4-iodobenzoic acid.
Pine and Imhof pioneered the use of O/W emulsion and nonaqueous emulsionsfor the preparation of ordered macroporous hydrophilic polymer and ceramics[12]. The ordered macroporous materials have potential for application in photoniccrystals [13]. Uniform macroporous silica was made in an O/W emulsion ofuniform iso-octane droplets stabilized by sodium dodecylsulfate (SDS). A suitableaqueous sol was first made by dissolving tetramethoxysilane in diluted aqueoushydrochloric acid. Most of the methanol formed by hydrolysis of the alkoxide wasdistilled off at room temperature under a low vacuum. To make porous hydrophilicpolyacrylamide (PAM), the monomer acrylamide (AM) and cross linker methylenebisacrylamide (MBAM) were dissolved in the aqueous phase of a concentratedmonodisperse emulsion stabilized by SDS. To initiate the polymerization at 60 ◦C,ammonium persulfate (APS) and tetramethylethylenediamine (TMEDA) wereadded into the aqueous phase. As most metal alkoxides are extremely reactive withwater, nonaqueous emulsions were employed to prepare ordered porous ceramics.The oil droplets were stabilized in the highly polar liquid formamide by the triblockcopolymer (PEG) poly(ethylene glycol)-b-poly(propylene glycol)-b-PEG. Stable solsof titanium and zirconium alkoxides in formamide were prepared by modificationwith acetylacetone and partial hydrolysis with water and then used to obtain orderedporous titanium dioxide and zirconium dioxide [12].
The volume ratio of the internal phase to continuous phase can be varied in a widerange, for example, 10–99 v/v%. A HIPE is defined as an emulsion containing adroplet phase with a volume ratio of 74% or greater to the emulsion [4, 7]. The valueof 74% represents the maximum volume ratio of uniform nondeformable sphereswhen packed in a most efficient manner. When a HIPE is formed with a highervolume ratio than 74%, the droplets are polydisperse and may also deform intoa polyhedral. The use of less-concentrated emulsions (e.g., droplet phase volumeratio <60%) as templates produces more closed-cell porous structures. In Pine’sapproach to making porous materials, the emulsions were concentrated leadingto packing of droplets into an ordered assembly [14]. This was essential for theproduction of interconnected ordered porous structures. Indeed, when trying toprepare porous materials by emulsion templating, HIPEs have usually been usedto produce materials with highly interconnected pores [3–7]. In this chapter, wefocus on reviewing the use of O/W or C/W HIPEs for the production of hydrophilicporous polymers and related materials.
7.2.1HIPE Templating for Hydrophilic Polymers and Related Materials
7.2.1.1 O/W HIPEsIt seemed that emulsion-templated porous PAM monoliths were first preparedby Pine et al. [14] Zhang and Cooper developed an oil-in-water-in-oil (O/W/O)sedimentation polymerization to prepare uniform porous polymer beads [15].HIPEs were prepared by emulsifying light mineral oil in an aqueous solution
7.2 Emulsion Templating 213
(a) (b)
(c) (d)
Figure 7.2 SEM images showing theemulsion-templated bead. (a) Bead surface.(b) Sectioned ‘‘half-bead’’ showing the in-ternal pore structure. (c) Porous surface ata higher magnification. (d) Internal porous
structure at a higher magnification. Scalebars, 500 μm for (a and b), 100 μm for(c and d). (Reprinted with permission fromRef. [15], copyright 2002 American ChemicalSociety.)
containing monomers AM and MBAM and initiator APS. The prepared emulsionwas injected into a hot oil medium using a syringe. The emulsion drops sankthrough the hot oil and the polymerization took place during the sedimentationprocess. It was found that the addition of TMEDA into the emulsions was essentialto allow fast polymerization so that the emulsion drops could be polymerized andwould not be aggregating at the bottom of the vessel. After polymerization, thebeads with a diameter of around 2 mm were prepared and dried to produce uniformporous PAM beads. The beads showed a highly interconnected macroporousstructure and pores open to the surface (Figure 7.2). A high pore intrusion volumeof 7.29 cm3 g−1 was achieved for porous PAM. This route could also be used toprepare highly porous poly(N-isopropyl acrylamide (PNIPAM), poly(hydroxyethylmethacrylate (PHEMA), and poly(acrylic acid) (PAA) beads [15].
Recently, an O/W HIPE consisting of acrylic acid (AA), water, and a cross linkerMBAM as the water phase, and toluene as the oil phase was successfully stabilized tosustain thermal initiation of radical polymerization resulting in porous open cellularmonolithic material [16]. HEMA-based hydrogel polyHIPEs were synthesized in theexternal phase of O/W HIPEs. The resulting hydrogel could exhibit superior waterabsorption capabilities [17]. By varying the emulsion composition and preparationcondition, it was possible to tune the void size of the PHEMA polyHIPEs [18].O/W emulsion-templated PNIPAM hydrogels were produced with a pore sizedistribution in the range of 1–40 μm [19]. The porous hydrogels showed very rapid
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swelling/shrinking in accordance with the temperature swing. The fast responsewas attributed to the convection flow of water through the macropores.
Gelatin polyHIPEs were prepared via two different cross-linking procedures:(i) radical polymerization of the methacrylate-functionalized gelatine and (ii) for-mation of isopeptide bridges among the gelatin chains promoted by the enzymemicrobial transglutaminase [20]. The materials were tested for the culture of hep-atocytes. It was found that the enzymatically cross-linked scaffold resulted in lesscytotoxicity and the cultured hepatocytes expressed a better differentiated pheno-type. Barbetta et al. also synthesized porous alginate hydrogels by cross linkingO/W HIPEs and using the solid foaming process [21]. In the polyHIPE synthesis,commercially available alginate was firstly degraded and then dissolved in water.This was to reduce the viscosity of the resulting aqueous solution. Toluene wasemulsified into aqueous alginate solution with Triton X-405 as the surfactant. After-ward, 1-ethyl-3,3-[3-(dimethylamino)propyl]carbodiimide hydrochloride (EDC) andN-hydroxysuccinimide were added to the HIPE. The HIPE was polymerized insidean oven set at 40 ◦C for 24 h [21].
7.2.1.2 C/W HIPEsThe use of C/W emulsion templating not only avoids the use of an organic solventbut also allows the easy removal of the droplet phase simply by depressurizationbecause CO2 reverts to the gaseous state upon depressurization. There is noorganic solvent residual in the prepared porous materials. This may be particularlybeneficial when the porous scaffolds are used for tissue engineering or otherbiological applications. However, to form C/W emulsion, CO2-philic fluorinatedpolymers or Si-containing polymers need to be used as surfactants as commonpolymeric or ionic surfactants are not very effective in stabilizing C/W emulsions.Perfluoropolyether (PFPE) ammonium carboxylate could form C/W emulsionswith kinetic stability [1]. This surfactant, with the addition of poly(vinyl alcohol)(PVA) as a costabilizer, was used to form C/W HIPEs in a high-pressure reactorwith monomer AM and cross linker MBAM dissolved in the continuous aqueousphase. After the polymerization of the C/W emulsions and the removal of CO2 bydepressurization, open-cell porous PAM was produced, which conformed closelyto the interior of the high-pressure reactor [2].
The PFPE surfactant was also used to emulsify CO2 into an aqueous sodiumalginate solution containing complexed calcium ions. CO2 could dissolve in theaqueous phase, thus producing the acidity that released the calcium ions fromtheir chelated form, cross-linking the alginate and forming the hydrogel. Thisprocess was named reactive emulsion templating owing to the dual role of CO2
as a reagent and a template [22]. In another study by Palocci et al., dextran wasmethacrylated via the reaction with 4-(N,N-dimethylamino)pyridine (DMAP) andglycidyl methacrylate (GMA) in dimethyl sulfoxide (DMSO). The reaction schemeis shown in Figure 7.3 [23]. An aqueous solution of DMAP, PFPE, and K2S2O8
was added into a high-pressure reactor, which was then charged with CO2 to formthe emulsions. The polymerization was then carried out at 60 ◦C for 20 h. Afterthe CO2 was vented slowly, highly interconnected porous dextran materials were
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Figure 7.3 (A) Reaction scheme for the synthesis of metacrylated dextran. (B) C/W HIPEformation and curing. A schematic representation of the polymeric network making up thepolyHIPE walls is shown in (a). (Reprinted with permission from Ref. [23], copyright 2007American Chemical Society.)
produced. It was found that the concentration of the surfactant could dramaticallyalter the pore size distribution, while the volume ratio of the CO2 phase only had alimited influence.
However, the fluorinated surfactant PFPE is expensive and nonbiodegradable.Inexpensive hydrocarbon surfactants such as Tween 40 with the costabilizer PVAwere found to be able to form C/W emulsions with monomers AM, HEMA, andhydroxyethyl acrylate (HEA). The polymerization could be carried out at 20 ◦Cwith pressure <70 bar to produce corresponding porous polymers [24]. However,the C/W emulsions made from commercially available hydrocarbon surfactantswere not sufficiently stable, which limited the application of C/W emulsions indifferent systems and in different areas. Because CO2 is a relatively weak solvent,the lack of inexpensive CO2-soluble surfactants has been a barrier to the use ofcompressed or supercritical CO2. Progress has been made on the synthesis ofpoly(ether carbonate) copolymers and sugar acetate as renewable CO2-philes [25].Recently, end-functionalized poly(vinyl acetate) oligomers (OVAc) were used asCO2-philic building blocks to synthesize the CO2-soluble block polymer surfactant.It was found that a C/W emulsion with the volume ratio of CO2 at 97 v/v%and an OVAc-b-PEG-b-OVAc triblock surfactant would be stable for at least 48 h.The stability of the emulsion was further demonstrated by polymerization of theaqueous phase to produce highly porous cross-linked PAM [26, 27].
The OVAc-based triblock polymer was further used to prepare PVA hydrogels viathe C/W emulsion-templating route [28]. The C/W emulsions were prepared usingan aqueous solution of PVA containing an OVAc-based surfactant (2 w/v% to water)and glutaraldehyde (20 w/w% based on PVA). The milky white emulsions wereformed when CO2 was charged into the high-pressure reactor. The polymerizationwas initiated by injecting a catalyst (HCl) via flushing a reservoir under high
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(a) (b)
30 μm100 μm × 1000× 300
Internal
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Figure 7.4 SEM images showing open-cell porous PVA hydrogel produced from C/W emul-sions. (a) Internal and surface pore structures and (b) surface morphology at a higher mag-nification. (Reprinted with permission from Ref. [28], copyright 2007 American ChemicalSociety.)
pressure (120 bar) into the reactor (100 bar). The solidified gel occupied 100%of the reactor volume. The dry materials showed a highly interconnected porousstructure (Figure 7.4). A similar method was also used to prepare blended PVA/PEGand chitosan porous materials [28].
7.2.1.3 Related MaterialsA tetraethyl orthosilicate (TEOS) sol was prepared and added into the aqueous con-tinuous phase, which also contained AM and MBAM. After O/W/O sedimentationpolymerization and removal of the oil droplet phase, porous silica/PAM compositebeads were produced. Calcining the composite beads at 520 ◦C resulted in theformation of hierarchically porous silica beads [29]. These silica beads had a highpore volume of 5.81 cm3 g−1 and a surface area of 420 m2 g−1 with macroporesaround 4.85 μm and mesopores around 10 nm.
When using O/W emulsion templating to prepare porous silica or metal oxide,the limitations include the high reactivity of metal alkoxides to water and theemulsion instability during polymerization at elevated temperatures [12–14, 29].To address these problems, dry porous PAM beads were used as templates toprepare emulsion-templated porous inorganic beads. For example, PAM beads weresoaked in TEOS sol, Al(O-s-Bu)3 sol in acetone, Ti(OiPr)4 sol in isopropanol, andZr(OPr)4-propanol solution. A control on the soaking time and washing procedureproduced interconnected emulsion-templated porous composite beads. RemovingPAM by calcination led to the preparation of hierarchically interconnected poroussilica, titania, alumina, and zirconia beads. A triblock copolymer could be addedinto the sols in order to introduce further mesoporosity in the materials uponcalcination [30]. The PAM beads were also soaked in silica colloidal suspensionsand sodium silicate solution to make silica/PAM composite beads [31]. Calciningthe composite beads resulted in hierarchically porous silica beads. The mechanicalstability of the beads could be enhanced by sintering at 1450 ◦C under argon.
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Hybrid silica/alumina and silica/titania beads were further prepared by immersingthe silica/PAM composite beads in the corresponding precursor solutions [31].
It was found that porous PAM could irreversibly adsorb gold nanoparticles(GNPs) from their aqueous suspension. The loading of GNPs could be controlledby the soaking time and the concentration of GNPs. When a highly concentratedGNPs suspension (0.15 g/l) was used, a high loading of GNPs in porous PAM wasachieved. After the removal of PAM by calcination at 520 ◦C, highly interconnectedmacroporous gold beads with a golden luster were produced. By adjusting theloading of GNPs, it was possible to prepare hollow gold beads with macroporousshells [32]. The color of the GNP/PAM composite beads was red, suggesting thatthe GNPs were not aggregating in the porous PAM. By soaking the GNP/PAMcomposite beads in a TEOS sol followed by calcination, materials containingsite-isolated GNPs on hierarchically porous silica were produced [33].
It is well known that PNIPAM is responsive to temperature. For linear PNIPAM,it is soluble in water below a low critical solution temperature (LCST, around 32 ◦C)and precipitates from the solution above the LCST. For cross-linked PNIPAM,the materials can swell below the LCST and then shrink above the LCST. Theswelling behavior of emulsion-templated cross-linked PNIPAM in response totemperature was demonstrated [19]. In an approach combining emulsion templat-ing and freeze-drying, organic nanoparticles were formed in situ within porousPNIPAM [34]. The organic nanoparticles were trapped in the swollen structurewhen the porous PNIPAM/nanoparticle composite was soaked in water at roomtemperature (18 ◦C). However, the organic nanoparticles could be released intowater in a burst when the temperature was raised above the LCST [34]. Indeed,the emulsion-templated porous PNIPAM acted like a pump by swelling and con-tracting to release the loaded nanoparticles. Grant et al. further demonstrated theuse of porous PNIPAM for multicycle uploading by swelling below the LCST andreleasing of polystyrene (PS) colloids by contracting above the LCST (Figure 7.5)[35]. Porous PNIPAM with a lower cross-linking ratio (1 or 2%) displayed a highercapacity of loading PS colloids in relation to the mass of the polymer. However,the percentage releases of loaded PS colloids were not very different for porousPNIPAM with different cross-linking densities. It should be noted that nearly allthe newly loaded PS colloids could be released from the second cycle [35].
If the emulsion-templated porous polymers contain a high content of carbon,it is possible to carbonize the structure to prepare the corresponding porouscarbon materials. For example, an O/W HIPE was prepared with an aqueousresorcinol–formaldehyde precursor solution. The precursor solution was cured,and fluid elements were extracted from the monolith via solvent exchange. Thesample was then pyrolyzed to create a hierarchical, open-cell foam consisting ofmacropores with mesoporous carbon xerogel walls. The peak mesopore size distri-bution was tuned between 5 and 8 nm. The average diameters of the macroporeswere adjusted between 0.7 and 2.1 μm. Pore volumes up to 5.26 cm3 g−1 andelectrical conductivities as high as 0.34 S/cm were observed for the carbonizedmaterials [36].
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Figure 7.5 Four cycles of loading and release of PS colloids via porous PNIPAM. (a) Massrelease of loaded PS colloids, shown as the mass percentage of loaded PS colloids basedon the mass of PNIPAM. (b) Percentage release of loaded PS colloids. (Reprinted with per-mission from Ref. [35], copyright 2010 American Chemical Society.)
7.2.2Microemulsion Templating
Emulsions with droplet sizes in the nanometer range (typically 20–500 nm) areoften referred to in the literatures as miniemulsions, nanoemulsions, or submicronemulsions [37]. The nanoemulsions are normally prepared via high-energy emul-sification methods such as high-shear stirring, high-pressure homogenization, orultrasonication. Nanoemulsions are also thermodynamically unstable. Nanoemul-sions have been widely used for the preparation of colloids/nanoparticles and drugdelivery, but have been rarely reported for the preparation of porous materials[37–39].
Microemulsions are thermodynamically stable dispersions stabilized by a sur-factant and usually with a cosurfactant. It has been generally accepted that thereare two types of microemulsions: discrete and bicontinuous microemulsions. Thediscrete microemulsions are either oil droplets dispersed in water or water droplets
7.2 Emulsion Templating 219
dispersed in oil. Bicontinuous microemulsions contain comparable amounts ofoil and water with randomly connected oil and water domains. Unless otherwisestated, microemulsions normally refer to discrete microemulsions. The sizes ofmicroemulsion droplets are in the range of 5–50 nm. The droplets of microemul-sions have been successfully used as nanoreactors for the synthesis of metallic andinorganic nanoparticles [40]. Drugs are also encapsulated into the droplets of themicroemulsions as novel drug delivery systems [41]. Although it would be goodto use the microemulsion droplets as templates to build highly porous materials,there have been considerable barriers when trying to replicate microemulsions viapolymerization. The surfactant monolayers surrounding the droplets are not rigidenough to preserve the original shape during polymerization. Furthermore, thestructural changes in a microemulsion are much faster (∼1 μs) than the polymer-ization reaction (∼1 ms per step) [42]. When a microemulsion is used to prepareparticles, the droplet structure changes continuously during the polymerizationbecause the system has enough time to respond to the compositional and volumechanges caused by the consumption of monomer. The resulting particles areusually 5–10 times larger than the templating microemulsion droplets [43].
In order to make a porous structure replicating the templating emulsion, oneroute is the use of polymerizable surfactants. The polymerization of the surfactantsthat stabilize microemulsion droplets may therefore lock the microemulsion struc-ture. This route may be more useful for the preparation of polymeric colloids orcapsules. The other route is via gelation or freezing of the microemulsion droplets.Stubenrauch et al. tried to prepare high-surface-area PNIPAM by gelling polymer-izable microemulsions [42]. Bicontinuous microemulsions were used to preparemicroskeletal frameworks of crystalline calcium phosphate [44] and macroporoussilica by freezing the oil phase [45]. The bicontinuous microemulsions were formedfrom mixtures of alkane oils, water, and surfactant. In the case of preparing poroussilica, didodecyldimethylammonium bromide (DDAB) was used as the surfactant,while tetradecane (melting point 5.5 ◦C) and hexadecane (melting point 18.2 ◦C)were the oil phase. The microemulsions were prepared at room temperature andthen stored at 2 ◦C for up to 18 days by rapidly freezing in liquid nitrogen andtransferring to a refrigerator [45].
Microemulsions usually involved the use of water. Gao et al. prepared bicontin-uous microemulsions using concentrated sugar solutions (equimolar mixture ofsucrose and trehalose in water at the concentration of 70, 75, and 80 w/w%) [46, 47].The oil phase was either DVB or isobutylacrylate typically at the equal mass of thesugars (but could be up to 80 wt%). The sugar-based microemulsions containingDVB were dehydrated with anhydrous calcium sulfate at 60 ◦C in a sealed chamberfilled with DVB vapor. Optically transparent microemulsion glasses were thusformed. The DVB in the microemulsion glasses was polymerized by UV at 45 ◦C.The key to successful dehydration and polymerization of these microemulsionglasses was the use of a temperature-insensitive UV initiator. After polymerization,the sugars were removed by washing with extra amounts of water to producetransparent porous membranes with 25 nm pores, as shown in Figure 7.6 [47].The small-angle scattering spectra of the microemulsion glasses before and after
220 7 Porous Materials by Templating of Small Liquid Drops
a = 0.50 a = 0.60 a = 0.70
Figure 7.6 Cross-sectional SEM of photopolymerized microemulsion glasses following dis-solution of the sugar template with excess water. Samples were prepared from precursorliquid microemulsions with initial DVB oil loadings of α = 0.50, 0.60, and 0.70. (Reprintedwith permission from Ref. [47], copyright 2006 American Chemical Society.)
polymerization showed almost no change in the microstructure. However, whenisobutylacrylate was used as the oil phase and the microemulsion glasses werepolymerized, dissolution of the sugar template with water collapsed the porousstructure. This was assumed to be due to the low glass transition temperature ofpolyisobutylacrylate [46].
Recently, the polycondensation behavior of melamine formaldehyde (MF) resinsunder acidic polymerization conditions was studied within a bicontinuous mi-croemulsion comprising an oil phase (heptane), a water phase (containing areactive resin), and an iso-C13-(EO)7 type nonionic surfactant. By incorporatingalkyl and phenyl groups into the resin, gels were formed consisting of a continuousaggregated nanoparticle phase with a continuous pore network. The macroscalephase separation was successfully suppressed and porosities between 80 and 85%were achieved with 65–400 nm pore sizes [49].
However, it has been very difficult to use discrete microemulsions as templatingstructures to prepare porous materials. A few studies were reported that focused onthe preparation of porous silica. Stucky and coworkers employed microemulsiontemplating to prepare siliceous mesostructured cellular foams (MCFs), where themicroemulsions were formed from trimethylbenzene (TMB), a triblock polymerPluronic 123 (P123), and aqueous siliceous sol [48, 50]. The formed microemulsionswere stirred at 37–40 ◦C for 20 h and then transferred to an autoclave and aged at 100or 120 ◦C for 24 h under static conditions. The precipitates rather than a monolithwere filtered, dried, and then calcined at 500 ◦C for 8 h to produce MCF materials.The sizes of the microemulsion droplets were related to the amount of TMBdissolved in the P123 micelles. The weight ratios of TMB/P123 were controlled.It was found that the cell diameters of the MCF materials increased linearly withthe cube root of the TMB concentration, as shown in Figure 7.7a, where the TMB
7.2 Emulsion Templating 221
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Figure 7.7 (a) Correlation of the cell sizes of MCFs prepared with and without NH4F ad-dition as a function of the cube root of the concentration of TMB. (b) TEM image showingthe pore structure of the MCF material prepared with TMB/P123 = 0.75. (Reprinted withpermission from Ref. [48], copyright 2000 American Chemical Society.)
concentration was expressed as the weight ratio of TMB/P123. Figure 7.7b showsthe mesopore structure of the MCF made from TMP/P123 = 0.75 [48]. The MCFmaterials consisted of uniform spherical cells measuring 24–42 nm in diameter,possessing BET surface areas up to 1000 m2 g−1 and porosities of 80–84%. Thesematerials lacked long-range order but the small size distributions could generatehigher-order scattering peaks. In another study, Pine, Stucky, and coworkers re-ported a generalized method for the synthesis of periodic mesoporous silica by directliquid crystal templating. This method was also extended to microemulsion-typequaternary (surfactant/cosurfactant/water/oil) systems [51]. Amino-functionalizedMCF materials could be synthesized by the cocondensation of TEOS andaminopropyl-triethoxysilane using a similar microemulsion system [52].
7.2.3Freeze-Drying of Emulsions
As mentioned in the previous sections, the monomers in the continuousphase of an emulsion can be polymerized normally by heating to solidify
222 7 Porous Materials by Templating of Small Liquid Drops
the emulsion-templated structure [3–7]. Alternatively, the emulsions may berapidly frozen, for example, in liquid nitrogen to lock in the emulsion structure.The solvents in the continuous phase and the internal droplet phase are thenremoved by freeze-drying to generate emulsion-templated porous materials.Whang et al. [53] have reported the freeze-drying of water/poly(lactic-co-glycolicacid) (PLGA)-methylene chloride emulsions to form porous biodegradable PLGAmaterials. Using the same method, bovine serum albumin (BSA) and horseradishperoxidase solutions were dispersed into PLG-methylene chloride solutions toform emulsions. After freeze-drying, porous materials loaded with protein wereproduced and used as the matrix for the controlled delivery of protein [54, 55].Cameron et al. [56] have prepared porous PLGA and poly(ε-caprolactone) (PCL)materials by freeze-drying W/O emulsions using Span 80 as a surfactant. Theadhesion and proliferation of human bladder stromal cells on the scaffolds wereinvestigated. The surface of porous PLGA scaffolds was further modified withheparin to deliver growth factors for soft tissue engineering [57].
Combination of emulsion templating and freeze-drying to prepare porous ma-terials has several advantages. For example, emulsion stability in general is nota problem because the emulsion structure is locked in by the rapid freezingprocess. The percentage of the droplet phase in the emulsion can be varied overa wide range, thus providing great control on the pore structure and porosityof the resulting porous materials. In one study, cyclohexane was emulsified inaqueous SCMC solutions containing SDS as a surfactant at volume ratios of 0 : 100,20 : 80, 40 : 60, 60 : 40, and 75 : 25. Freeze-drying of the emulsions produced porousmaterials with both emulsion-templated and ice-templated pores (Figure 7.8) [58].These porous polymeric materials could be further used as templates to pre-pare porous zirconia with systematically controlled pore morphology and porevolume.
An emulsion may also be directly frozen and processed to prepare porousmicroparticles. For example, a PCL-xylene solution was emulsified in a PVA–SDSsolution to form an O/W emulsion. The emulsion was directionally frozen andfreeze-dried to produce aligned porous microparticles embedded in an alignedporous matrix (Figure 7.9a) [59]. The aligned porous microparticles could beeasily collected by dissolving the composite material in water and then filtering(Figure 7.9b). The porous PCL particles were further tested as scaffolds to supportthe growth of mouse embryonic stem cells. There was no evidence of toxicityover a total culture period of seven days [59]. By dissolving an organic compoundother than the polymer in the oil droplet phase, this route can also be used tofabricate organic nanoparticles within porous polymers [60, 61]. For example, anorganic dye oil red (OR) was dissolved in the oil droplet phase. After freeze-dryingthe emulsion, OR nanoparticles within highly porous PVA were obtained. Theporous material could be easily dissolved in water to produce a stable aqueousnanosuspension. The average sizes of the OR nanoparticles were around 90 nm.In order to demonstrate the potential application of aqueous organic nanoparticledispersion, a poorly water soluble antibacterial agent, triclosan, was dissolved
7.3 Breath Figures Templating 223
(a) (b)
40 μm
40 μm 20 μm
200 μm 200 μm
(d) (e)
(c)
Figure 7.8 Porous SCMC materials prepared by freeze-drying the emulsions with differentemulsion ratios. From (a) to (e) the volume percentage of oil phase is 0, 20, 40, 60, and75%. The white circle indicates one of the emulsion-templated pores. (Reprinted with per-mission from Ref. [58], copyright Royal Society of Chemistry 2009.)
in the droplets. After freeze-drying the emulsion and dissolving the preparednanocomposite in water, the resulting aqueous triclosan nanoparticle dispersiondisplayed a significantly enhanced biocidal activity compared to that of the triclosansolution in water/ethanol [60].
7.3Breath Figures Templating
In principle, any type of stabilized liquid droplets may be used as templates toprepare porous materials. For example, a gold substrate was prepatterned witha hydrophobic self-assembled monolayer of hexadecanethiol and a hydrophiliclayer of mercaptopropionic acid by microcontact printing. When the substrate wasimmersed in an aqueous colloidal suspension with an organic solvent layer on thetop, organic droplets formed on the hydrophobic part, which was then used astemplates during the deposition process of colloidal spheres to fabricate colloidalcrystal films with ordered voids [62]. In addition to emulsion templating, the mostextensively investigated templating by liquid droplets may be the use of condensedwater droplets, widely known as breath figures.
224 7 Porous Materials by Templating of Small Liquid Drops
(a)
100 μm
50 μm
(b)
Figure 7.9 Aligned porous PCL particles in aligned porous PVA (a) and aligned porousPCL microparticles only (b) [59].
7.3.1Breath Figures
Condensation of water is observed in everyday life and can lead to fogging ofsmooth solid surfaces. The foggy arrays of water droplets on surfaces are termedbreath figures because of their easy formation by blowing to a surface. Dependingon the wetting properties of the surface, the condensed water may form a finepattern of self-assembled droplets. The structure and the kinetics of growth of BFswere investigated by Beysens and Knobler [63]. The parameters that were variedincluded the nature of the substrate, its temperature, the temperature of the gas,the gas flow rate, and the percentage of water saturation. The pattern for water onglass was studied by direct observation and light scattering. When the contact angleθ = 0◦, a uniform layer forms whose thickness grows with time t. For θ = 90◦,droplets are formed. The radius of isolated droplets grows as t0.23 at a constantgas flow rate and relative humid environment, but, as a result of coalescence, theaverage droplet radius grows as t0.75.
Only in 1994, Francois and coworkers discovered that star-shaped PS and poly(p-phenylene)-b-PS form self-assembled honeycomb morphologies when a drop oftheir solution in carbon disulfide is exposed to a flow of moist air [64]. Srinivasaraoet al. further reported the formation of a 3D ordered array of air bubbles in a polymerfilm through a templating mechanism based on thermocapillary convection [65].
7.3 Breath Figures Templating 225
Flow of moist air
Cold surface
Solventevaporation
Water condensation(nucleation)
Water droplets form close packedarray
Array cools and sinks into solution
3D array remains after solventand water evaporate
New close packed arraytemplated by underlying layer
New generation of water droplets(a)
(b)
(c) (g)
(f)
(e)
(d)
Figure 7.10 A scheme showing the formation mechanism of ordered porous films via theBF-templating approach. (Reprinted with permission from Ref. [65], copyright 2001 Ameri-can Association for the Advancement of Science.)
Dilute solutions of PS with a carboxylic acid group in a volatile solvent were cast on aglass slide in the presence of moist air blowing across the surface. Figure 7.10 showsthe formation mechanism of the ordered porous films. Evaporative cooling leadsto the generation of hexagonally packed water droplets in the polymer solution.When samples are generated from a solvent denser than water, such as carbondisulfide (CS2), only a single layer is formed. In contrast, when a solvent withdensity lower than water is used, such as benzene or toluene, the hexagonal arraypropagates through the film and a 3D structure can be formed. The water dropletsare stabilized by the polymer in solution and the thermocapillary flow arrangesthem into ordered 3D arrays. The 3D ordered structures are preserved after theorganic solvent and water evaporate to solidify the films.
226 7 Porous Materials by Templating of Small Liquid Drops
Compared with the solid surface, the interactions between water droplets ona liquid surface are different because the substrate is locally mobile and curved.Knobler and coworkers suggested that the growth of BFs on liquids evolvesthrough three stages [66]. The first stage is for nucleation and growth of dropletsas isolated objects with weak interdroplets interactions. The average diameter Dof droplets is increased with time t following the rule D ∼ ta, a ≈ 1/3. At thesecond stage, maximal surface coverage by water droplets is achieved. The dropletsare separated by a liquid film, giving rise to a short-ranged, hard-sphere-likeinteraction and leading to uniformity in the size of the droplets. The third stage isrepresented by constant surface coverage and coalescence between droplets. Thegrowth rule follows D ∼ ta, a ≈ 1. To form honeycomb-structured porous filmsby BF templating, the hexagonally packed water droplets need to be stabilized bypolymer in organic solutions in order to avoid or reduce the coalescence of thedroplets. The dry porous films are formed by precipitation of the polymer aroundthe water droplet during the evaporation process, followed by complete evaporationof the organic solvent and water [67].
The crucial step of forming porous films is largely influenced by the nature of thesolvated polymer. CS2 works well with conjugated polymers, star-/block PS, andcarboxy-terminated PS, while droplets array can be obtained for a PS solution inchloroform, nitrocellulose in amyl acetate, and fluorinated polymers in Freon-typesolvents. Depending on different systems, benzene, toluene, and xylene may alsogive good BF arrays [68]. Honeycomb-structured porous films with pore sizesranging from 200 nm to 7 μm can be prepared by BF templating. Stenzel et al.reviewed the formation of porous films of this type via BFs with different polymerarchitectures [67]. In another excellent review, Bunz described BF as a dynamictemplating method for polymers and nanomaterials [68]. Interested readers mayrefer to these two reviews for detailed information. Here we focus on the progressin recent years in the areas discussed below.
7.3.2Polymer
7.3.2.1 General PolymersIn early years, PS or block copolymers containing PS blocks were usually involvedin the BF-templating process [67, 68]. It has been since established that it isnot necessary for the PS to be contained in the polymer architect in order tofabricate BF-templated, honeycomb-structured porous films. Through the designand synthesis of polymer molecules, selected use of stabilizer and solvent, andcareful tuning of preparation conditions, a wide range of ordered porous films havebeen prepared.
Conjugated, rodlike polymers were used to prepare films with hexagonallyordered bubble arrays. The films were constructed by evaporative cooling with thesubsequent condensation of water droplets onto dilute solutions of the polymer inCS2 followed by the complete removal of CS2 and water [69]. Porous poly(ethyleneoxide)-b-polyfluorooctylmethacrylate (PEO-b-PFOMA) diblock copolymer films
7.3 Breath Figures Templating 227
were drop cast onto substrates from Freon (1,1,2-trichlorotrifluoroethane) ina humid atmosphere. Films with the best ordered pores were formed withPFOMA-to-PEO ratios of 70 : 2 kDa. The influence of water droplet nucleationon the final pore size and packing order in the polymer films was discussed[70]. Nitroxide-mediated polymerization of styrene was performed in bulkwith 2-methylaminoxyproponic-SG1 (MAMA) as the initiator to synthesize anα-carboxylic polystyrene (PS-COOH), a type of polymer with one chain end ionicfunctionality. The films were simply prepared by spreading out polymer CS2
solutions without additives over various substrates such as flexible poly(vinylchloride) sheets or rigid poly(methyl methacrylate) plates. A technique based onreflected and transmitted light was used to correlate the pore sizes inside and onthe top of the film [71].
The control on morphology change in the fabrication of porous polymer filmswas realized through the synthesis of star polymer with both the size of the2,2-bis(methoxy)propionic acid-based dendron end group and the dendron func-tionality being varied [72].
Films were prepared by casting (20 μl) of the star polymer–benzene solutiononto a glass cover slip. A humidified flow (70% relative humidity (RH) at 25 ◦C) ofair was directed onto these samples at a rate of 3 l/min. Figure 7.11 illustrates themorphology changes as a result of changing the end group of the functionalized starpolymers [72]. What would the structure of the materials be if a polymer solutionwas cast in an organic nonsolvent vapor atmosphere? Xiong et al. cast linear andstar-shaped poly(styrene-b-butadiene) copolymers dissolved in solvents such astoluene, chloroform, and dichloromethane onto the surface of the glass substratein methanol or ethanol vapor atmosphere [73]. It was found that a monotonousmicrosphere pattern could be obtained if the surface tension of the starting polymersolution was 1.5 mN m−1 higher than that of the condensed liquid.
Most of the BF-templated films have been fabricated on flat substrates.Qian and coworkers synthesized core-cross-linked star (CCS) polymer basedon poly(dimethylsiloxane) (PDMS) arms. They demonstrated the formation ofordered honeycomb films on nonflat substrates by casting the solutions of starPDMS in benzene on TEM (transmission electron microscopy) grids with 600,1000, 2000 meshes [74]. It was believed that such structures were formed becauseof the CCS polymer structure and low Tg (approximately –122◦). Different shapesof microparticles (spherical- and doughnut-shaped kaolin particles and irregularsilica particles) were mixed with PDMS solutions, which were then cast on a glasssurface. The honeycomb-structured-film-coated particles were produced becauseof the soft-flowing nature of PDMS (Figure 7.12) [75].
Pyrrole was templated along an amphiphilic block copolymer PS-b-PAA. Sub-sequent oxidation of pyrrole to polypyrrole resulted in the formation of a solublepolypyrrole-containing polymer. The polymer in CS2 was cast on a glass slide andthen processed to prepare honeycomb-structured porous films. Initial fibroblastcell culture studies on these films demonstrated a dependency of cell attachmenton the pore size of scaffolds [76]. In another interesting study, polymer surfaceswith reversibly switchable ordered morphology were produced [77]. The THF
228 7 Porous Materials by Templating of Small Liquid Drops
OO
OO
O
O
OO O
O
O
O
O
O
O
OO
OO
O
O O
OO
OO
O
O
O
O
F3C(F2C)6
F3C(F2C)6
F3C(F2C)6
F3C(F2C)6
F3C(F2C)6
F3C(F2C)6
F3C(F2C)6
F3C(F2C)6
O O
OO
OO
O
OO
O
O
OO
HO
HO
HO
HO
HO
HOHO
HO
= End group
= End group
= End group
Figure 7.11 Schematic representation of morphology changes as a result of changing theend group of the G3-functionalized star polymers [72].
(tetrahydrofuran) solutions of PS and poly(2-vinylpyridine) (P2VP) were cast tomake structured films with P2VP distributed in the holes of PS. When the porousfilms were exposed to different solvent vapors, the surface morphology switchedbetween the honeycomb pattern (for solvents ethanol, chloroform, methyl ethylketone, and dimethylformamide) and the islandlike pattern (CS2, THF, toluene).
7.3.2.2 Proteins RelatedThe patterning of proteins onto surfaces has potential applications in proteinmicroarrays, biosensing, and high-throughput bioanalysis. In general, protein sur-face patterns may be fabricated via microfabrication and lithography techniques.
7.3 Breath Figures Templating 229
(a)
10 μm 10 μm
10 μm 10 μm
100 μm 10 μm
(b)
(d)
(f)
(c)
(e)
Figure 7.12 SEM images of a spherical kaolin particle on a glass surface (a) before and(b) after coating with a PDMS star film with honeycomb morphology, a ‘‘doughnut’’-shapedkaolinparticle on a glass surface (c) before and (d) after coating with the PDMS star film,and silica chromatography particles (e) before and (f) after coating with PDMS honeycombfilm. (Reprinted with permission from Ref. [75], copyright 2007 Royal Society of Chemistry.)
Here, we describe the use of BF-templated films for the preparation of pat-terned proteins. The honeycomb structure PS-b-PAA was first prepared. Thesehoneycomb-structured porous films were subsequently exposed to a mixture of(+)-biotinamidohexanoic acid hydrazide and a coupling agent EDC in an acidicbuffer solution, which resulted in the covalent attachment of biotin. After sev-eral washing steps, the films were immersed in an alkaline buffer solution ofstreptavidin to obtain microwells with patterned protein streptavidin [78].
DNA-based honeycomb films were constructed by the BF-templating processthrough the complexation of DNA with a cationic surfactant ditetradecyldimethy-lammonium (DTDA) [79]. The DNA–DTDA complexes were prepared by replacingsodium counter ions of DNA with the cationic surfactant. The porous films wereprepared by directly casting 20 μl of the DNA–DTDA complex chloroform solutiononto glass substrates under a moist airflow. Carbohydrate–protein interactions are
230 7 Porous Materials by Templating of Small Liquid Drops
critical in many biological processes. The use of carbohydrate microarrays providesan effective tool to explore the interaction. Honeycomb-structured porous filmswere prepared from an amphiphilic block copolymer PS-b-PHEMA by casting itsCS2 solution onto a poly(ethylene terephthalate) substrate. The hydroxyl groupsfrom PHEMA were mostly on the surface of the pores, which provide the sitesto selectively graft 2-(2,3,4,6-tetra-O-acetyl-β-d-glucosyloxy)ethyl methacrylate inthe pores by a surface-initiated atom transfer radical polymerization. The specificrecognition to lectin was performed on these carbohydrate microarrays [80].
7.3.2.3 Modification of Film Casting and Evaporation ProcessIn most cases, the porous films were prepared under a moist airflow over thesolution surface. The use of the air flow may disturb the humidity over a big surface,leading to the difficult control of the quality of the films. Li et al. investigated theinfluence of vacuum on the formation of porous polymer films via BF templating[81]. A low pressure in a vacuum chamber can accelerate the evaporation of thesolvent. It was found that porous films prepared by this method showed goodrepeated production in a large area. The pore sizes could be easily tuned from 5.6to 17.1 μm by changing the vacuum level.
Cai et al. reported the fabrication of porous PS films by Marangoni-flow-induceddroplet arrays [82]. The formation of hexagonal and square arrays of water dropletsis due to the pinning and sliding of water fingers on silicone oxide and siliconsubstrates respectively. This technique could address some limitations of theBF method. For example, by controlling the humidity of the airflow during theMarangoni flow, the pore size and pore distance could be controlled independently.
Usually, the methods used for BF-templated films involve a humid air envi-ronment. As many processes for dealing with polymer films need dry conditionsin a clean room, Park and Kim developed a method to prepare BF patterns byspin coating [83]. To prepare large porous films with uniform thickness, the spincoater was placed inside an acryl box where a hot water beaker was placed tocontrol RH. To avoid a humid environment, a small amount of water was addedto THF polymer solutions. Spin coating of these solutions was performed under adry atmosphere (RH = 30%). The water in the THF solution provided the sourceof condensed water droplets. The pore size became larger with increasing watercontent in THF solution and decreasing rotating speed. Through the interplayof BF pattern formation during a spin-coating process and the self-assembly oftriblock polymer poly(2,3,4,5,6-pentafluorostyrene)-b-PS-b-poly(PEG) methyl ethermethacrylate), hierarchically micro- and nanostructured polymer films were pre-pared in block copolymer/homopolymer (PS) blends [84]. BF-patterned poly(lacticacid) films were prepared and the interpretation of the BF process was attemptedon the basis of the role of kinetics and the influence of interfacial tension [85]. Thefilms were produced by casting the solutions on glass slides and spin coating. Thetime evolution of pore size in terms of solvent vapor pressure, humid flow rate,and polymer concentration was explained via a model based on the Boltzmannsigmoid.
7.3 Breath Figures Templating 231
7.3.3Particles
7.3.3.1 Polymer + NanoparticlesNanoparticles may be incorporated into the polymer solutions to make hierar-chically structured functional arrays by BF templating. This process involves theself-assembly of condensed water droplets and nanoparticles. A 7 wt% chloro-form solution of PS with 1 wt% CdSe nanoparticles (core size: 4 nm) was castin a chamber with 80% RH. During the evaporation cooling process, before thepolymer solution became too viscous, tri-n-octylphosphine oxide–stabilized CdSenanoparticles moved and aggregated to the interface of water droplets and polymersolution. After complete removal of water and chloroform, a uniform layer ofCdSe was formed on the surface-ordered BF-templated pores [86]. A few drops ofsilica nanoparticle suspension in ethanol into PS-chloroform solution were castonto a clean glass substrate. BF-patterned PS films were produced with the silicananoparticles clearly being observed at the interface of BF-templated pores [87].
Magnetic nanoparticles and conducting polymers have potential applications inareas such as battery, electrodisplay, molecular electronics, and nonlinear opticalmaterials. Fe3O4 was prepared by thermal decomposition of Fe(acac)3 in phenylether and was then blended with 4-dodecylbenzenesulfonic acid-doped polyanilinechloroform solution. The honeycomb-structured composite films were preparedand the magnetic properties of the films were investigated with a vibrating sam-ple magnetometer [88]. In another study, purified multiwalled carbon nanotubes(CNTs) were dispersed in a solution of amine-terminated PS in benzene. Theordered macroporous nanocomposite films were fabricated by the BF-templatingmethod. After pyrolysis of the nanocomposites, highly stable CNT scaffold filmswith diverse morphologies were formed (Figure 7.13) [89]. High electrical conduc-tivity and field-emission properties were observed from the CNT films.
7.3.3.2 Nanoparticles OnlyIt is possible to cast nanoparticle suspensions without polymer to produceBF-patterned films. However, the stabilizers of the nanoparticles need to becarefully selected so that a stable nanoparticle suspension can be formed. Thesolvation property and the interaction of the stabilizer with the solvent are veryimportant for the fabrication of BF-patterned films. For example, gold nanorodsstabilized by CTAB were prepared using a modified, seed-mediated growth method.A CTAB-thiol exchange reaction was used with 4-mercaptophenol-THF solution toget phenol-functionalized nanorods. After purification, the phenol-functionalizednanorods were dispersed in dichloromethane and covalently coupled withcarboxybiphenyl-terminated PS. When a carbon-coated grid was dipped in aCH2Cl2 solution and the drop was allowed to dry in air at room temperature, thenanorods spontaneously organized into ring structures (Figure 7.14) [90]. Thediameter of the rings varied from 300 nm to a few microns, and their typical widthwas about 50 nm. The data suggested that the nanorods assembled around water
232 7 Porous Materials by Templating of Small Liquid Drops
(a) (b)
(c) (d)
1 μm 1 μm
1 μm1 μm
Figure 7.13 SEM images of monolayered cellular scaffolds before and after pyrolysis.(a) Plane view of the porous PS/CNT nanocomposite film. (b) Cross-sectional image ofthe PS/CNT nanocomposite film. (c) Plane view of the highly entangled CNT scaffold af-ter pyrolysis of the polymer matrix. (d) 60◦ tilted SEM image of the fractured CNT scaffold.(Reprinted with permission from Ref. [89], copyright 2009 Royal Society of Chemistry.)
droplets that were condensed from the air when highly volatile CH2Cl2 evaporatedand cooled its surface below the dew point.
Highly ordered and strongly fluorescent two-dimensional (2D) arrays of CdSequantum dots (QDs) were obtained on gold substrates by combining micro-contact printing and BF-templating techniques [91]. Trioctylphosphine oxide(TOPO)-capped CdSe QDs with sizes of 3 and 4.5 nm were used. The sizeand shape of the rings could be readily tuned by varying the pattern dimensionsand assembly conditions. Xu et al. synthesized a wide range of nanocrystals witholeic acid or oleylamine as capping ligands, which ensured the satisfactory dis-persion of nanocrystals in nonpolar solvents (cyclohexane or chloroform) [92].Via BF templating and self-assembly of nanocrystals, hierarchically ordered 2Darchitectures were produced, showing structural combinations such as superlatticeislands inside ordered pores, function and composition combinations such asmetal–magnets, metal–fluorescent species, and metal–semiconductors, as well asdimension combinations such as one-dimensional nanorod and zero-dimensionalQDs.
In another interesting study, the BF-templating method was used to preparedodecanethiol-capped GNP macroporous structures with pore diameters from 1.7to 3.5 μm on an air/water interface [93]. A two-step procedure was employed; first byforming a surfactant monolayer on water and then drop casting a GNP dispersionin chloroform onto the surfactant monolayer (Figure 7.15). The rapid evaporation
7.3 Breath Figures Templating 233
(a) (b)
(c) (d)
100 nm 200 nm
300 nm 300 nm
Figure 7.14 TEM images of rings formed by the gold nanorods stabilized bycarboxybiphenyl-terminated PS from a solution in CH2Cl2 [90].
Chloroform
DODMAC
Thin film
Evaporation
Water
Condensed water droplet
Air
Figure 7.15 A representation scheme of the BF-templated film formation at an air/waterinterface [93].
of the solvent reduced the air/liquid interfacial temperature below its dew point,resulting in the condensation of microscopic water droplets on the surface of thesolution. The water droplets self-assembled into ordered arrays under the effects ofthermocapillary forces and Marangoni convection.
234 7 Porous Materials by Templating of Small Liquid Drops
7.3.4Posttreatment of BF-Templated Films
Ordered porous polymer or composite films are formed via solvent evaporationand self-assembly of block copolymers or nanoparticles during the BF-templatingprocess. These films are in general mechanically weak, sensitive to solvent vapor,and may also be chemically unstable. Cross-linking of the polymers or calcina-tion/carbonization of composite structures has been employed to improve thestability of films chemically and mechanically.
7.3.4.1 Cross-linkingBF-templated polymeric films can be prepared with functional groups on thepolymer chains. The films may be cross linked chemically via a selected reaction.For example, porous films were prepared by evaporation of a chloroform solutionof poly(styrene-co-maleic anhydride). The cross linking of the films were achievedby immersion in an ethanol solution of 1,8-diaminooctane solution [94]. Thenon-cross-linked films were hydrophobic with a water contact angle of more than90◦, whereas the cross-linked films became hydrophilic, so that a water drop couldpenetrate into the films. After cross linking, the honeycomb structure was stable toup to 350 ◦C, an increase of more than 150 K as compared to the non-cross-linkedfilms.
Owing to the convenience of photopolymerization, photo-cross-linking reactionshave been employed more often to cross link the films. Photopolymerizablefunctional groups and photoinitiators should be included in the films. Forexample, four-arm star poly(d,l-lactide) (PLA) was synthesized using a PEG-basedmacroinitiator, pentaerythritol ethoxylate. Photoreactive methacrylated end groupswere obtained via functionalization of the hydroxyl-terminated star polymer with2-isocyanatoethyl methacrylate (IEM), which also incorporated an adjacent ure-thane site to enhance the mechanical performance of the structured films throughperipheral, hydrogen-bonding interactions. A photoinitiator, 2,2-dimethyl-2-phenolacetophenone (DMPA), was added to the IEM-functionalized PLA-CH2Cl2 solu-tion. After the preparation of porous films via the BF-templating process, thecross-linking procedure was performed by passing the films through an 1800 WFusion UV system at a wavelength of 360 nm. A short exposure to UV light wasrequired to cross link the films with the honeycomb structure maintained [95].
It has been seen that the BF-templated process to prepare patterned filmsnormally involves the use of polymers rather than small molecules. Kim et al.synthesized small molecules that contain a benzamide dendron and an aliphaticdendron containing two diacetylene groups [34]. Benzamide derivatives wereknown to form interesting self-assembled structures, especially in dendritic forms.The diacetylene groups were expected to cross link upon UV irradiation to formpolymer networks. The honeycomb-structured films were produced by castingthe chloroform solution on a Si wafer in a flow of moist air. The structure wascross linked by exposure to UV irradiation through a photomask, which induced
7.3 Breath Figures Templating 235
selective chemical cross linking in the exposed region. Thus, lithographicallypatterned, cross-linked porous films were produced [96].
7.3.4.2 CarbonizationBF-templated films made of polymer with a high content of carbon may becarbonized to obtain stable porous films. For example, honeycomb-patternedporous films were prepared from hyperbranched poly(phenylene). The preparedfilms were first cross linked by irradiating high-energy UV light. The absorptionpeak at λmax = 318 nm arising from stilbene units was slightly shifted to a lowerwavelength in the course of the photoreaction, which indicated a reduction in theconjugated length. Additionally, the absorption markedly decreased with increasingUV irradiation time, which indicated a loss of conjugated double bonds. Thecarbonization process was carried out by heating to 600 ◦C under a N2 atmosphere.During the thermal treatment, originally yellow honeycomb films turned orange,red, and finally black because of the carbonization [97].
7.3.4.3 CalcinationA calcination process leads to the removal of organic components from the porousfilms at a high temperature and produces inorganic/ceramic porous films. Thefilms can be either made from polymers that contain Si or other metal elementsin the polymer molecules or from mixed solutions of polymer and inorganiccomponents.
Cyclopentadienyl-cobalt-containing poly(p-phenylene ethynylene) were first syn-thesized. This organometallic polymer was co-dissolved with carboxy-terminatedPS in a carbon disulfide/pentane mixture. Ordered pattern porous films were pro-duced via the BF-templating process. Upon calcination at 500 ◦C, the BF-templatedstructure was retained, and the resulting films were insoluble in organic solventsor water [98]. A PDMS-b-PS polymer was used to prepare honeycomb-structuredporous films via the BF-templating process. The prepared films were exposed toUV light (500 W at 254 nm) to cross link the PS blocks. The cross-linked films werethen calcined to produce ordered ceramic porous structures on solid substrates [99].
We have mentioned the preparation of porous films from polymer and nanopar-ticles. If inorganic nanoparticles are used, it is possible to further calcine thefilms to generate porous ceramic structures. Sanchez and coworkers combinedfunctional nanobuilding blocks (e.g., SiO2, TiO2, Co, and CdS) and BF processingto prepare porous materials with hierarchical porosity [100]. The suspensions ofsurfactant-modified nanoparticles in chloroform or THF were directly cast to fab-ricate the porous films. In order to remove the surfactants, the films were calcinedat 300 ◦C for 1 h, then at 400 ◦C for 20 min, and finally at 500 ◦C for 15 min in air.The adjustable size of the nanobuilding block units allowed an easy tuning of theaverage size of the mesoporosity between 2 and 50 nm.
Carboxy-terminated PS was dissolved in chloroform with titanium tetraiso-propoxide in a sealed flask and then subjected to ultrasonic treatment until atransparent solution was obtained. BF-patterned films were produced from the so-lution. A vapor phase hydrothermal method was employed to form titania/polymer
236 7 Porous Materials by Templating of Small Liquid Drops
hybrid films. The polymer component was then removed by means of calcination
to generate titania film with an ordered porous structure [101]. In another study,honeycomb-structured photochromic polymer films were constructed from PAA
coupled with a hydroxyl functional spiropyran [102]. The polymer in the filmscould bind metal ions (e.g., Pd2+) to create hybrid organic–inorganic porous struc-tures. Unique metal microrings could be prepared by reduction of the metal and
calcination of the organic materials.
7.4Conclusions
As one of the soft templating methods, small liquid droplets have been used astemplates for the preparation of porous materials. Emulsions, with small liquid
droplets suspended in a continuous phase, are employed to form porous mate-rials by solidifying the continuous phase around the liquid droplets followed by
the removal of the droplets phase. While microemulsions can be used to pro-duce mesoporous materials, emulsion-templating methods are generally utilized
to obtain macroporous materials with HIPEs for highly interconnected porousstructures. The materials prepared by emulsion templating cover polymer, metal,silica and metal oxides, and polymer/inorganic composites. Various applications
based on these materials have been demonstrated, for example, as scaffolds fortissue engineering, catalysis, and controlled delivery, to name a few.
BFs templating is a technique that uses patterned arrays of condensed waterdroplets as templates mainly for the preparation of ordered porous thin films.
A range of materials aimed for different types of applications have been obtainedwith this templating method.
The advantages associated with liquid droplets templating include the following:
(i) the easy removal of liquid droplets either by washing or solvent evaporation;(ii) reactive molecules being easily incorporated into the liquid droplets to allow
the desired functionalization of pore surfaces; (iii) facile control on the interfacechemistry or noncovalent interaction; and (iv) being applicable for a wide range of
materials.We envisage that liquid droplets templating will be extensively investigated for
the fabrication of porous materials. The research is likely focused on functionalporous materials designed and synthesized for applications such as tissue scaf-fold, drugs/protein delivery, chromatography, and supported catalysis. In addition
to the engineering of pore size, pore morphology, and pore interconnectivity,other desirable targets for the materials depending on the type of applications
may include mechanical stability, trigger sensitivity for responsive materials, sur-face functionality, biocompatibility and biodegradability, and reusability of the
materials.
References 237
Acknowledgment
Discussion with and suggestion from Professor Andy Cooper is highly appreciated.The financial support from the Engineering and Physical Sciences ResearchCouncil in the United Kingdom is acknowledged.
References
1. Lee, C.T., Psathas, P.A., Johnston,K.P., de Grazia, J., and Randolph, T.W.(1999) Langmuir, 15, 6781–6791.
2. Butler, R., Davies, C.M., and Cooper,A.I. (2001) Adv. Mater., 13, 1459–1463.
3. Zhang, H. and Cooper, A.I. (2005) SoftMatter, 1, 107–113.
