PolyHIPEs: Recent advances in emulsion-templated porous polymers

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PolyHIPEs: Recent advances in emulsion-templated porous polymers Michael S. Silverstein Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel

a r t i c l e

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Article history: Received 14 February 2013 Received in revised form 24 June 2013 Accepted 1 July 2013 Available online xxx Keywords: Porous polymers Emulsion templating High internal phase emulsions PolyHIPEs Polymer chemistry Emulsion stabilization

a b s t r a c t Porous polymers with well-defined porosities and high specific surface areas in the form of monoliths, films, and beads are being used in a wide range of applications (reaction supports, separation membranes, tissue engineering scaffolds, controlled release matrices, responsive and smart materials) and are being used as templates for porous ceramics and porous carbons. The surge in the research and development of porous polymer systems is a rather recent phenomenon. PolyHIPEs are porous emulsion-templated polymers synthesized within high internal phase emulsions (HIPEs). HIPEs are highly viscous, pastelike emulsions in which the major, “internal” phase, usually defined as constituting more than 74% of the volume, is dispersed within the continuous, minor, “external” phase. This review focuses upon the recent advances in polyHIPEs involving innovations in polymer chemistry, macromolecular structure, multiphase architecture, surface functionalization, and nanoparticle stabilization. The effects of these innovations upon the natures of the resulting polyHIPE-based materials (including bicontinuous polymers, nanocomposites, hybrids, porous ceramics, and porous carbons) and upon the applications involving polyHIPEs are discussed. The advances in polyHIPEs described in this review are now being used to generate new families of porous materials with novel porous architectures and unique properties. © 2013 Elsevier Ltd. All rights reserved.

Abbreviations: AA, acrylic acid; AAm, acrylamide; AGET, activators generated by electron transfer; AN, acrylonitrile; ATRP, atom transfer radical polymerization; BA, butyl acrylate; BDP, boron-dipyrromethene; BDP-S, BDP-styrene; CB, carbon black; CNT, carbon nanotubes; CTAB, cetyltrimethylammonium bromide; DCPD, dicyclopentadiene; DVB, divinylbenzene; EBP, elastin-based side-chain polymers; EGDMA, ethylene glycol dimethacrylate; EHA, 2ethylhexyl acrylate; EOF, electroosmotic flow; FA, furfuryl alcohol; FRP, free radical polymerization; GMA, glycidyl methacrylate; GSH, glutathione; GST, glutathione S-transferase; H, applied field; HgPtMS, 3-mercaptopropyltrimethoxysilane; HIPE, high internal phase emulsion; HPLC, high performance (or pressure) liquid chromatography; IL, ionic liquid; IL/O, ionic-liquid-in-oil; LCST, lower critical solution temperature; LDE, liquid droplet elastomer; M, magnetization; MAA, methacrylic acid; MAP, 4-(N-methylamino)pyridine; MBAm, N,N-methylenebisacrylamide; MG63, a human osteoblast-like cell line; MMA, methyl methacrylate; NiPAAm, N-isopropylacrylamide; NP, nanoparticle; O/O, oil-in-oil; O/W, oil-in-water; O/W/O, oil-in-water-in-oil; PAA, poly(acrylic acid); PAN, polyacrylonitrile; PCL, polycaprolactone; PEG, poly(ethylene glycol); PEGDMA, poly(ethylene glycol) dimethacrylate; PEO, poly(ethylene oxide); PFA, poly(furfuryl alcohol); PGA, polyglutaraldehyde; PGMA, poly(glycidyl methacrylate); PGPR, polyglycerol polyricinoleate; PHEMA, poly(hydroxyethyl methacrylate); PLGA, poly(l-lactide-co-glycolide); PMMA, poly(methyl methacrylate); PNiPAAm, poly(N-isopropylacrylamide); PPO, poly(propylene oxide); PS, polystyrene; PtBA, poly(tert-butyl acrylate); PU, polyurethane; PUU, poly(urethane urea); PVBC, poly(4-vinylbenzyl chloride); RAFT, reversible addition-fragmentation chain transfer polymerization; ROMP, ring opening metathesis polymerization; S, styrene; SEM, scanning electron microscopy; SMO, sorbitan monooleate (Span 80); SMP, shape memory polymer; SS, styrene sulfonate; SSA, specific surface area; SWCNT, single wall CNT; tBA, tert-butyl acrylate; TEM, transmission electron microscopy; TEOS, tetraethoxyorthosilane; THF, tetrahydrofuran; Tm , melting point; UV, ultraviolet; VBA, 1-(azidomethyl)-4-vinylbenzene; VBC, 4-vinylbenzyl chloride; VT, 1-vinyl-1,2,4-triazole; W/O, water-in-oil; W/O/W, water-in-oil-in-water. E-mail address: [email protected] 0079-6700/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.progpolymsci.2013.07.003

Please cite this article in press as: Silverstein MS. PolyHIPEs: Recent advances in emulsion-templated porous polymers. Prog Polym Sci (2013), http://dx.doi.org/10.1016/j.progpolymsci.2013.07.003

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Contents 1.

2.

3.

4. 5.

6.

7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Porous polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. HIPEs and polyHIPEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PolyHIPE chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Polymerization mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Novel monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Bicontinuous incompatible polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HIPE stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Surfactant-stabilized HIPEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Particle-stabilized Pickering HIPEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Particle-stabilized Pickering HIPEs containing surfactant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposite and hybrid polyHIPEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Supports and membranes: chemical reactions, separations, absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Tissue engineering, responsive polymers, controlled release, and water retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porous ceramics and porous carbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Porous ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Porous carbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction 1.1. Porous polymers Porous materials have been shown to be useful for a wide range of applications [1,2]. Porous polymers are a subset of porous materials that take advantage of the ease of processability associated with polymers to generate monoliths, films, and beads, often with well-defined porosities and high specific surface areas (SSAs). Porous polymers are of interest for such applications as microelectronics, biomedical devices, membrane processes, and catalysis as well as for precursors that can be used to synthesize porous ceramics or porous carbon. This paper, a review of recent (2009–2012) work in emulsion-templated porous polymers, will use the pore size classifications adopted by the International Union of Pure and Applied Chemistry: microporous (less than 2 nm), mesoporous (between 2 and 50 nm), and macroporous (greater than 50 nm). The high level of interest in the research and development of porous polymer systems is a recent phenomenon, as seen in Fig. 1 (left-hand y-axis). Fig. 1 presents the results of a relatively restrictive literature search for articles that contain both “porous” and “polymer”. Until 1990 there were only a few tens of articles per year that fulfilled this criterion. In 1991, the number of articles that fulfilled this criterion “jumped” to several hundreds. The number of articles per year that fit this criterion has increased rapidly over the last 20 years, with over 2200 articles in 2012. On one side of the pore size spectrum, there is increasing interest in glassy polymers with inherent microporosity [3]. On the other side of the pore size spectrum, there is increasing interest in macroporous polymers generated using phase separation techniques and in polymer foams with millimeter-size pores [4]. There are several templating methods that can be used for the production of porous polymers, including

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block copolymer templating and colloidal templating. The removal of one of the nanometer-scale phases in microphase separated block copolymers can be used to generate mesoporous polymers [5]. In a typical colloidal templating scenario, a biphasic system is generated and then the continuous phase (or, in the case of bicontinuous systems, one of the co-continuous phases) is polymerized [6]. The colloidal entities serve to create porosity in the final polymeric material and are removed following polymerization. Depending on the nature of the colloidal system employed (emulsions, microemulsions, solid particles, or breath figure droplets), the characteristic pore size can range from a few nanometers to hundreds of micrometers. 1.2. HIPEs and polyHIPEs PolyHIPEs are porous emulsion-templated polymers synthesized within high internal phase emulsions (HIPEs)

Fig. 1. The number of publications per year resulting from a topic search for: (left-hand y-axis) both “porous” and “polymer”; (right-hand y-axis) “polyHIPE”.

Please cite this article in press as: Silverstein MS. PolyHIPEs: Recent advances in emulsion-templated porous polymers. Prog Polym Sci (2013), http://dx.doi.org/10.1016/j.progpolymsci.2013.07.003

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[7–12]. HIPEs are highly viscous, paste-like emulsions in which the major, “internal” phase, usually defined as constituting more than 74% of the volume, is dispersed within the continuous, minor, “external” phase [13–15]. For example, in mayonnaise, the major phase, vegetable oil, is emulsified in the minor phase, vinegar, using the lecithin in egg yolk as the surfactant. Internal phase contents of over 74% can be reached for monodispersed droplets through their deformation into polyhedra [16]. Internal phase contents of over 74% can also be reached through the formation of a polydisperse droplet size distribution. Theoretical analysis has shown that the formation of polyhedra from monodispersed droplets should be favored over the formation of a polydisperse droplet size distribution [16]. Recent work has shown that a small amount of cosurfactant in the internal phase of water-in-oil (W/O) HIPEs reduces the interfacial tension and increases the polygonal nature of the internal phase [17]. It is important to note that only a limited number of emulsifiers are able to stabilize a major phase dispersed within a minor phase. Sorbitan monooleate (SMO or Span 80) is the most commonly used emulsifier for W/O HIPE stabilization. If the internal phase, external phase, or both phases of the HIPE contain monomers, then polymers can be synthesized within the HIPE. A concentrated latex is produced if the monomers are only present in the internal phase. A polyHIPE, based on a continuous polymer phase surrounding the dispersed droplets of the internal phase, is produced if the monomers are only present in the external phase. The formation of a typical W/O HIPE (an aqueous internal phase dispersed within the hydrophobic monomers in the external phase) and the synthesis of a typical polyHIPE are illustrated schematically in Fig. 2. A bicontinuous hydrophilic–hydrophobic system of incompatible polymers can result if monomers are present in both phases. Often the term “medium internal phase emulsion” is used for internal phase volumes of 30–74% and the term “low internal phase emulsion” is used for internal phase volumes of less than 30% [18]. For simplicity, this review will use the

terms HIPE and polyHIPE for all the emulsion-templated systems used to produce porous materials. There are often highly significant differences between the structure of the original HIPE and the structure of the resulting polyHIPE. Droplet coalescence and/or Ostwald ripening can occur during polymerization, especially when the elevated temperatures used for polymerization enhance diffusion and interfacial destabilization. Ruptures, often termed holes, interconnects, or windows, often develop at the thinnest points of the external phase envelope surrounding the internal phase. The widespread formation of such holes transforms the discrete droplets of the HIPE’s internal phase into a continuous interconnected phase in the polyHIPE. Removal of the internal phase, which has now become continuous, yields voids in place of the internal phase droplets and results in a highly interconnected, open-cell, emulsion-templated porous structure. A “typical” polyHIPE porous structure with voids and interconnecting holes is seen in Fig. 3 (scanning electron microscope (SEM) micrograph). This polyHIPE was produced through the copolymerization of styrene (S) and divinylbenzene (DVB) within a W/O HIPE (90% internal phase). This type of porous structure yields materials with low bulk densities and the ability to rapidly absorb large quantities of liquids through capillary action. Recent work on polyHIPEs with tunable void sizes and narrow void size distributions has developed theoretical models to describe the relationships between the void size and the mechanical behavior [19]. Typically, polyHIPEs are synthesized using conventional free radical polymerization (FRP) in surfactant-stabilized W/O HIPEs. However, other polymerization methods (e.g. step-growth, atom transfer radical polymerization (ATRP), ring opening metathesis polymerization (ROMP)), other HIPE stabilization techniques (e.g. the particle-based stabilization in Pickering HIPEs), and other types of HIPEs (e.g. oil-in-water (O/W)) have also been investigated [20]. The types of polyHIPE systems developed include copolymers, interpenetrating polymer networks, biodegradable polymers, bicontinuous polymers, organic–inorganic hybrids,

Fig. 2. Schematic illustration of HIPE formation and polyHIPE synthesis within a W/O HIPE that contains an aqueous internal phase dispersed within a hydrophobic monomer external phase, based on Ref. [6].

Fig. 3. A typical porous polyHIPE structure (SEM). Reproduced with permission from Ref. [69]. Copyright 2002, John Wiley & Sons.

Please cite this article in press as: Silverstein MS. PolyHIPEs: Recent advances in emulsion-templated porous polymers. Prog Polym Sci (2013), http://dx.doi.org/10.1016/j.progpolymsci.2013.07.003

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porous inorganics, and nanocomposites [20]. PolyHIPEs as carriers and absorbents for liquids were originally developed by Unilever [21] and a number of Unilever patents followed, including patents on polyHIPEs as substrates and as templates for porous carbon [21,22]. PolyHIPEs as liquid absorbents and as materials for heat and sound insulation were developed by Procter & Gamble [23]. PolyHIPEs as liquid absorbents were also developed by Kimberley Clark, Dow Chemical, 3M, and Arkema [24–27]. More recently, the patent literature has focused upon polyHIPEs for gas storage applications [28–30]. PolyHIPEs have also been developed for applications such as chemical synthesis, chromatography, ion exchange, separation, sensing, tissue engineering, and controlled drug delivery, to name but a few [20]. While polyHIPEs from W/O HIPEs are the most common, polyHIPEs from O/W HIPEs are becoming more and more common. The range of HIPEs investigated has been expanded to include supercritical-CO2 -in-water HIPEs and non-aqueous oil-in-oil (O/O) HIPEs [31–35]. Recent work has broadened the range of HIPEs available for polyHIPE synthesis further still by synthesizing polyHIPEs within non-aqueous HIPEs having an organic external phase and an ionic liquid (IL) internal phase [36]. The ability to synthesize polyHIPEs within waterfree HIPEs could have a major impact upon polyHIPE development, expanding the types of polymerization reactions that are available for polyHIPE synthesis to include reactions that are sensitive to the presence of water (step-growth polymerizations, anionic, cationic, or metathesis chain-growth polymerizations). In addition, the use of ionic liquids can extend the range of synthesis temperatures, which has been limited by the presence of water, to above 100 ◦ C. In recent work, the external phase (20%) of the IL/O HIPE was lauryl methacrylate and the internal phase (80%) was a 1.15 wt% solution of bis(trifluoromethane)sulfonimide lithium salt in 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide [36]. A typical polyHIPE porous structure with voids and interconnecting holes was achieved once the appropriate surfactant concentration was determined. PolyHIPEs are usually fabricated as monoliths, and monolithic structures are appropriate for many of the applications under consideration. The range of available

polyHIPE shapes and sizes has been extended considerably through the fabrication of beads and membranes. Millimeter-scale polyHIPE beads were fabricated using oilin-water-in-oil (O/W/O) [37,38] and water-in-oil-in-water (W/O/W) [39] sedimentation polymerization or using W/O/W suspension polymerization [40]. PolyHIPE membranes hundreds of micrometers thick were fabricated using a molding technique [41]. Recently, a microfluidic setup was used to fabricate monodisperse polyHIPE spheres and rods whose diameters were around 400 ␮m [42]. Droplets were generated in a co-flow device from a water-in-oil HIPE, thereby creating a W/O/W system. Spherical bead were fabricated through the downstream photopolymerization of monomers within the individual droplets (Fig. 4). Rods were fabricated through the breakup into rod-like segments of the oil stream within an unstable W/O jet (Fig. 5). The relatively high viscosity of the HIPE was integral to the process, preventing the rods from relaxing into spherical droplets. The beads and rods exhibited typical polyHIPE structures whose macropores were significantly larger than those of conventionally synthesized porous polymer beads (polymer phase separation during polymerization in the presence of porogenic solvents). There are several extensive reviews that describe HIPEs and polyHIPEs [6,20,43–46]. One recent review has focused on hybrid and nanocomposite polyHIPEs [47]. Other recent reviews have focused on functional polyHIPEs, describing new monomers, new chemistries, post-synthesis modification, and applications [48,49]. A convenient table summarizing the monomers and the polymerization mechanisms used to synthesize various types of HIPEs has also been published [49]. The recent surge in publications on polyHIPE systems mirrors that for porous polymers, as seen in Fig. 1 (right-hand y-axis), the results of a relatively restrictive literature search for articles that contain “polyHIPE”. This article is a detailed review and survey of recent advances made in polyHIPE synthesis, structure, properties, and application (2009–2012). This review will focus on: the polymerization mechanisms and polymer chemistries that affect the macromolecular structure; the use of particle-stabilization in surfactant-free Pickering emulsions; the synthesis of more complex systems such as bicontinuous polyHIPEs, functionalized polyHIPEs, and composite polyHIPEs; applications involving polyHIPEs;

Fig. 4. Near-spherical polyHIPE beads produced using microfluidics: (a) OM; (b) SEM. Reproduced with permission from Ref. [42]. Copyright 2009, the American Chemical Society.

Please cite this article in press as: Silverstein MS. PolyHIPEs: Recent advances in emulsion-templated porous polymers. Prog Polym Sci (2013), http://dx.doi.org/10.1016/j.progpolymsci.2013.07.003

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Fig. 5. PolyHIPE rods produced using microfluidics: (a) OM; (b) SEM. Reproduced with permission from Ref. [42]. Copyright 2009, the American Chemical Society.

and the use of polyHIPEs as templates for the generation of porous inorganics and porous carbons. 2. PolyHIPE chemistry Much of the recent polyHIPE research and development has focused on expanding the polymerization mechanisms available for polyHIPE synthesis, on novel monomers for polyHIPE synthesis, and on functionalizing polyHIPE surfaces. These developments in the chemical nature of polyHIPEs are usually directed toward specific properties and applications. However, the resulting innovations can be used to generate novel families of innovative polyHIPE systems. 2.1. Polymerization mechanisms Most polyHIPEs are synthesized using conventional FRP, and most of the FRP reactions are thermally initiated. Recent research has focused upon a variety of polymerization-related issues including the locus of initiation for thermally initiated FRP, alternative FRP initiations, ATRP, thiol-ene and thiol-yne reactions, ROMP, and reversible addition-fragmentation chain transfer polymerization (RAFT). The locus of initiation has a significant effect on the polyHIPE’s porous structure [50]. Polyhedral voids were formed for thermal FRP initiation at the interface using a

water-soluble initiator in W/O Pickering emulsions (Fig. 6). The void shapes became “locked-in” at the beginning of interfacially initiated polymerization. Unexpectedly, the stabilizing NPs were “pushed” into the middle of the polyHIPE wall by the preferential diffusion of monomer toward the polymerization front at the interface (Fig. 7). In contrast, spherical voids were formed for thermal FRP initiation within the organic phase using an oil-soluble initiator within practically identical W/O Pickering emulsions. More extensive droplet coalescence and/or Ostwald ripening can occur since the polymerization is not “localized” at the oil–water interface for organic-phase initiation. As expected, the stabilizing NPs were located on the polyHIPE surface (at the HIPE’s oil–water interface) since there was no preferential diffusion of monomer toward the interface. Alternative FRP initiation mechanisms include ultraviolet (UV) photoinitiation and 60 Co ␥-ray initiation [51]. ␥-ray initiation can be carried out at room temperature and there is no residual initiator to be removed [51]. Since these reactions can proceed faster than those from thermal initiation and since there is no exposure to the elevated temperatures that can destabilize HIPEs, the resulting polyHIPEs have smaller voids and narrower polydispersities. One consequence is that the crush strength of the polyHIPE polymerized using 60 Co ␥-ray initiation was greater than that of the polyHIPE polymerized using conventional FRP. This enhancement in the mechanical properties may result from the smaller void size, as has been seen elsewhere [19].

Fig. 6. Polyhedral structures in a polyHIPE synthesized using interfacial FRP initiation within a W/O Pickering HIPE (SEM). Reproduced with permission from Ref. [50]. Copyright 2010, John Wiley & Sons.

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Fig. 7. Cross-section of the polyhedral polyHIPE seen in Fig. 6. The NPs were “pushed” into the middle of the wall through preferential monomer diffusion to the polymerization front at the interface (TEM). Reproduced with permission from Ref. [50]. Copyright 2010, John Wiley & Sons.

Fig. 8. A scheme depicting the NP-based AGET ATRP polymerization mechanism. Reproduced with permission from Ref. [52]. Copyright 2011, the American Chemical Society.

Fig. 9. Cross-section of a polyHIPE synthesized using NP-based ATRP initiation within a W/O Pickering HIPE. The NPs were “locked together” at the oil–water interface at the beginning of the polymerization (TEM). Reproduced with permission from Ref. [52]. Copyright 2011, the American Chemical Society.

Please cite this article in press as: Silverstein MS. PolyHIPEs: Recent advances in emulsion-templated porous polymers. Prog Polym Sci (2013), http://dx.doi.org/10.1016/j.progpolymsci.2013.07.003

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Recently, several different polymerization mechanisms were applied to polyHIPEs for the first time. Activators generated by electron transfer (AGET) ATRP was used to synthesize elastomeric polyacrylate polyHIPEs from both surfactant-stabilized W/O HIPEs and nanoparticlestabilized Pickering W/O HIPEs (Fig. 8) [52]. Two different types of initiators were used in combination with a water-soluble reducing agent for activator generation. For an organic-soluble initiator, the polymerization took place at the interface, where the initiator and the reducing agent can interact. For an initiator on the surface of silane-modified silica nanoparticles (NPs), the initiation took place on the NP surface. AGET ATRP initiation at either the interface or the NP surface “locked-in” the polyhedral shape of the original HIPE at the beginning of the polymerization, as seen for interfacial FRP initiation [50]. For interfacial AGET ATRP initiation, as for interfacial FRP initiation, the stabilizing NPs were pushed into the middle of the polyHIPE wall by preferential monomer diffusion toward the polymerization front at the interface. For NP-based AGET ATRP initiation, the NPs became “locked together” at the oil–water interface (the polyHIPE surface) at the beginning of the polymerization (Fig. 9). AGET ATRP was also used to synthesize HIPE-based liquid droplet elastomers (LDEs), monoliths consisting of an elastomeric framework (15%) that surrounds individually encapsulated micrometer-scale water droplets (85%), as well as a degradable polyHIPE [53]. PolyHIPEs were prepared in W/O HIPEs (80% internal phase) by thiol-ene and thiol-yne mediated network formation through the photopolymerization of a trithiol with either a triacrylate or an aliphatic diyne (Fig. 10) [54]. The efficiency of network formation was between 80 and 90% and the porous structure was typical of polyHIPEs, with voids on the order of tens of micrometers. The thiol-yne materials displayed enhanced strength and toughness due to their higher degree of crosslinking. Thiol-ene chemistry (photopolymerizable trimethylolpropane tris(3mercaptopropionate) with dipentaerythritol penta/hexaacrylate) was used to synthesize biodegradable polyHIPEs that may be suitable for regenerative medicine applications [55]. The resulting typical polyHIPE structures had nominal

Fig. 10. A scheme depicting the monomers used for thiol-ene and thiolyne polyHIPE synthesis. (1) triacrylate; (2) octadiyne; (3) trithiol. Reproduced with permission from Ref. [54]. Copyright 2011, the Royal Society of Chemistry.

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porosities that ranged from 80 to 90%. After 15 weeks at 37 ◦ C in cell culture medium, the biodegradable polyHIPE lost 19% of its mass. When a few mole percent of fluorescein o-acrylate was used as a comonomer, the polyHIPEs exhibited a green-colored fluorescence under UV light. RAFT was used to synthesize polyHIPEs based upon polystyrene (PS) [56]. Controlled radical polymerizations are expected to yield network structures that are more homogeneous than those of conventional FRP. While the FRP- and RAFT-synthesized polyHIPEs from practically identical HIPEs exhibited similar porous microstructures, the moduli and crush strengths of the RAFT-synthesized polyHIPEs were 3-fold those of the FRP-synthesized polyHIPEs. Unlike emulsions, miniemulsions are kinetically stable colloidal systems that are usually stabilized by a surfactant and co-stabilizer. The surfactant is used to suppress droplet coalescence and the co-stabilizer, a smaller molecule soluble in the droplet phase but barely soluble in the dispersing media, is used to suppress Ostwald ripening. In miniemulsions, internal phase droplet sizes of 30–500 nm can be reached through the application of large shear forces, e.g. through ultra-sonication. A W/O miniemulsion template method with internal phase contents down to 40% was used for the RAFT synthesis of porous PS-based monoliths with highly interconnected porous structures [57]. Highly interconnected voids of around 300 nm and interconnecting holes of around 100 nm were achieved for polymers with as little as 47% porosity (Fig. 11). Reducing the porosity from 80% in a conventional FRP-synthesized, emulsiontemplated material to 47% in a RAFT-synthesized, miniemulsion-templated material produced an increase of around 40-fold in modulus and in crush strength. Some alternative polymerization mechanisms, including ring-opening metathesis polymerization (ROMP) and step-growth, have also been developed for polyHIPE synthesis [58–61]. The reaction of 2-nitroresorcinol and cyanuric chloride produced a highly aromatic molecular structure with HCl as a reaction by-product [59]. Poly(furfuryl alcohol) polyHIPEs were synthesized through an acid-catalyzed condensation reaction within an oil-in-alcohol HIPE [62]. The reaction of oligomeric polycaprolactone (PCL) triols and diisocyanates was expected to produce biodegradable polyurethanes (PUs). However, the diisocyanate also reacted with the water which constituted 75% of the HIPE. The reaction of the isocyanate with water formed an amine which reacted immediately with another isocyanate to form urea, releasing carbon dioxide as a by-product (Fig. 12). Therefore, the reaction of PCL-diols and diisocyanates within W/O HIPEs produced poly(urethane urea) (PUU) polyHIPEs. Interestingly, the PUU polyHIPEs were “foamed” by the CO2 -generating urea-forming reaction (Fig. 13) [60]. A more controlled version of PUU polyHIPE synthesis involved first synthesizing the urethane groups by end-capping a biodegradable PCL oligomeric triol with hexane diisocyanate [61]. The resulting biodegradable polyHIPEs, of interest for tissue engineering scaffold applications, exhibited relatively large voids and broad void size distributions. PolyHIPEs based on dicyclopentadiene (DCPD) were synthesized using ROMP (Fig. 14) within

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Fig. 11. A polymer with open-cell porosity synthesized within a W/O miniemulsion with only 40% internal phase (SEM). Reproduced with permission from Ref. [57]. Copyright 2012, the Royal Society of Chemistry.

Fig. 12. A scheme depicting the reaction of isocyanate with water to produce urea and CO2 . Reproduced with permission from Ref. [60]. Copyright 2009, John Wiley & Sons.

surfactant-stabilized W/O HIPEs (80% internal phase) [63,64]. The porous structures were typical of polyHIPEs and the relatively high moduli reflected the aromatic structure of PDCPD. Oxidation of the resulting polyHIPEs generated carbonyl and hydroxyl functionalities on the polyHIPE surface. These groups were further functionalized with hydrazine, producing a significant increase in modulus (from 23 to 130 MPa). In a different approach to

functionalization, thiol groups were incorporated on the polyHIPE surface by adding a thiol-bearing reagent to the HIPE and effecting a thiol-ene reaction. 2.2. Novel monomers While polyHIPEs based on styrene, acrylates, and methacrylates have been investigated in detail, there is

Fig. 13. A PUU polyHIPE foamed by the isocyanate reaction with water that produces urea and CO2 (SEM). Reproduced with permission from Ref. [60]. Copyright 2009, John Wiley & Sons.

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30%. The voids were around 7 ␮m and the interconnecting holes were around 2 ␮m. These highly hygroscopic polyHIPEs contained around 9% water, corresponding to having 50% of the triazole groups linked to water molecules. A typical Young’s modulus was 21 MPa for a polyHIPE with a porosity of 87% and a density of 0.14 g/cm3 . PolyHIPEs with even higher nitrogen contents were based on 1-vinyl-5-amino [1–4] tetrazole and were synthesized in O/W HIPEs similar to those described above [66]. These polyHIPEs exhibited typical polyHIPE porous structures with micrometer-scale voids and a porosity of 88%. They also exhibited relatively high SSAs for polyHIPEs, around 36 m2 /g. 2.3. Functionalization

Fig. 14. A scheme depicting ROMP synthesis of a PDCPD-based polyHIPE. Reproduced with permission from Ref. [64]. Copyright 2009, the Royal Society of Chemistry.

still a long list of monomers that can be used to produce polyHIPEs with unique properties. As mentioned above, ROMP opened the door to the synthesis of polyHIPEs from DCPD, whose cyclic structure yields high-modulus materials [63,64]. PolyHIPEs with high nitrogen contents in their backbones were synthesized using triazole and tetrazole monomers (Fig. 15). PolyHIPEs based on 1-vinyl-1,2,4-triazole (VT) were developed to achieve properties that are of interest to the food industry (e.g. nontoxicity, high hydrophilicity, chemical stability, complexing power, and heat resistance) [65]. Such polyHIPEs are also potential engineering and energetic materials. The external phase of the O/W HIPE was an aqueous solution containing around 50% monomers (VT and a crosslinking comonomer) and the internal phase (between 75 and 90 vol%) was dodecane. The porous structures were characteristic of polyHIPEs, with porosities of between 69 and 87%, in spite of shrinkage that was around

Fig. 15. Schemes depicting nitrogen-rich monomers. (1) 1-vinyl-1,2,4triazole; (2) 1-vinyl-5-amino[1–4]tetrazole.

Many different routes to polyHIPE functionalization have been developed, ranging from chemically etching the surface, to depositing coatings on the surface, to adding polymerizable moieties bearing functional groups for post-synthesis reaction [45,48,49,67,68]. Recent research has focused on the functionalization of reactive groups (carboxylic acid, epoxy) present on polyHIPE surfaces. Polymerization from the surfaces of polyHIPEs was achieved by coating polyHIPEs with polymerization catalysts, by swelling the polyHIPEs in solutions containing FRP initiators, or by adding polymerizable ATRP initiators to the HIPE’s external phase. Carboxylic acid groups on polyHIPE surfaces can be advantageous for post-synthesis functionalization. To this end, polyHIPEs based on acrylic acid (AA) were synthesized within O/W HIPEs, hydrophobic polyHIPEs were coated with PAA, and polyHIPEs based on tert-butyl acrylate (tBA) were synthesized within W/O HIPEs and were then hydrolyzed to PAA (Fig. 16) [68–70]. The as-synthesized PtBA-based polyHIPEs were hydrophobic, did not absorb water, and exhibited a high contact angle with water. However, the hydrolyzed PtBA-based polyHIPEs absorbed water rapidly, to around 800% of the polyHIPE’s mass. While it is difficult to introduce the highly hydrophilic AA into a W/O HIPE’s oil phase, it was possible to introduce methacrylic acid (MAA). PolyHIPE copolymers of styrene, divinylbenzene, and MAA with up to 90% porosity were synthesized in W/O HIPEs [71]. The SSA increased with increasing DVB content, reaching 186 m2 /g. The carboxylic acid groups on these polyHIPEs were functionalized, first with thionyl chloride and then with multifunctional amines, yielding amine loadings of around 2 mmol/g. Glycidyl methacrylate (GMA) was added to the organic external phase of W/O HIPEs to provide epoxy groups that would be available for post-synthesis functionalization. Copolymerization of GMA and DVB in the presence of a porogenic solvent (DVB/GMA/toluene in the ratio 40/10/50) in a W/O HIPE (85% internal phase) produced polyHIPEs with relatively high SSAs and pore volumes (reaching 371 m2 /g and 0.42 cm3 /g, respectively) and relatively low average mesopore diameters (reaching 5.6 nm) [72]. While the overall porous structures were typical of polyHIPEs for various DVB/GMA ratios and for various porogens, the void walls had nodular structures (Fig. 17). Such nodular structures are typically found in polyHIPEs

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Fig. 16. A scheme depicting the hydrolysis of PtBA to PAA. Reproduced with permission from Ref. [68]. Copyright 2009, John Wiley & Sons.

Fig. 17. The nodular wall structure of a polyHIPE from a W/O HIPE with a DVB/GMA/chlorobenzene ratio of 10/40/50 (SEM). Scale bar is 200 nm. Reproduced with permission from Ref. [72]. Copyright 2009, Elseiver Ltd.

synthesized from HIPEs (W/O or O/W) that contains both porogenic solvents and crosslinking comonomers in the external phase [73,74]. Crosslinked microgel particles, formed within the porogenic solvent in the early stages of the polymerization, assemble to form a polyHIPE wall with a nodular structure. The percentage of epoxy groups that underwent hydrolysis during polyHIPE synthesis and purification increased from 25 to 77% with increasing GMA content, limiting the amount of epoxy available for postsynthesis functionalization. It seems that the epoxy groups were less likely to undergo hydrolysis for the smaller pore

sizes (lower GMA contents) since they were less accessible to water. GMA was used to synthesize epoxy-functionalized polyHIPE beads using a microfluidic device [42]. These beads then underwent a two-step ‘click–click’ reaction. The ring opening of the epoxy group with NaN3 , used to introduce azide groups, was followed by the Cu-catalyzed cycloaddition of alkynes to those azide groups (Fig. 18). The reactions proved more efficient for polyHIPE beads than for conventional beads prepared from the same monomers through “polymerization induced phase separation” in the presence of porogenic solvents. The mechanical properties of PS-based polyHIPEs were enhanced by coating them with poly(furfuryl alcohol) (PFA) [75]. The polyHIPEs were first coated in a p-toluenesulfonic acid catalyst solution, then exposed to furfuryl alcohol (FA) in the vapor phase, and finally heated for polymerization. The thickness of the PFA coating reached 74 nm for a 24 h FA vapor exposure. The PFA coating reduced the porosity from 90 to 71%, almost tripled the density, and increased the compressive Young’s modulus by more than 7-fold. For the most part, the stress-strain behavior was similar to that of the original PS-based polyHIPE (a linear region at low strains followed by a stress-plateau-like region), with the yield stress increasing as the PFA thickness increased. The polyHIPE with a relatively thick PFA coating (74 nm) exhibited a rapid decrease in stress following a relatively high yield stress since the coating had shattered and was no longer stress-bearing. PAA coatings were used to enhance the hydrophilicity of polyHIPEs. A HIPE was formed by pre-curing a glycidyl

Fig. 18. A scheme depicting: (a) PolyHIPE synthesis; (b) ‘click–click’ modification. Reproduced with permission from Ref. [42]. Copyright 2009, the American Chemical Society.

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the azlactone-functionalized polyHIPEs as amine scavengers was evaluated using 4-fluorobenzylamine in both a batch process and a flow-through process (the polyHIPE was synthesized within high-performance liquid chromatography (HPLC) columns). 2.4. Bicontinuous incompatible polymers

Fig. 19. A PAA coating on a silica-filled epoxy-based polyHIPE surface (TEM). Reproduced with permission from Ref. [76]. Copyright 2009, Elseiver Ltd.

amino epoxy in the presence of a surfactant before the internal phase (70%), an aqueous suspension of colloidal silica, was added [76]. Further curing took place within the HIPE, yielding an epoxy-containing polyHIPE. The polyHIPE was swollen in an acetone solution of an organic-soluble initiator and then dried, leaving the initiator within the polyHIPE. The polyHIPE was then immersed in a solution of AA (in water or methanol) and polymerization took place at 60 ◦ C under nitrogen. The polyHIPE with a 100 nm thick PAA coating (Fig. 19) absorbed 5 times more water than was absorbed by an uncoated polyHIPE. A polymerizable initiator for ATRP (2-acryloxyethyl-2 bromoisobutyrate) was incorporated into HIPEs without compromising emulsion stability [77]. Photopolymerization produced polyHIPEs with ATRP initiator groups on the surface that were available for grafting reactions. Poly(methyl methacrylate) (PMMA) was grafted from the polyHIPE surface through ATRP. A block copolymer was formed on the polyHIPE surface through ATRP re-initiation, polymerizing PHEMA from the PMMA that was grafted to the polyHIPE. In addition, ATRP was used to produce epoxide-functionalized polyHIPEs by grafting PGMA from the polyHIPE surface. The hydrophilicity of the PGMA-grafted polyHIPE surface was enhanced through epoxy ring opening to produce a glycol. The hydrophobicity of the ring-opened PGMA-grafted polyHIPE surface was enhanced through reaction of the glycol’s hydroxyl groups with 2,3,4,5,6-pentafluorobenzoyl chloride. In further work, the PGMA coating (around 700 nm thick) underwent near quantitative azidation through a Huisgentype ‘click’ reaction [78]. The azide was then reacted with propargyl alcohol (conversions of around 80%) and other alkynes. The biofunctionalization of the azide-bearing polyHIPEs was achieved by reacting with protected amino acids that bore alkyne groups. The subsequent successful deprotection of the amino acids was achieved owing to the hydrolytic stability of the triazole ring. Azlactone-functionalized polyHIPEs were synthesized by free radical copolymerization of DVB and N-(pvinylbenzyl)-4,4-dimethylazlactone [79]. The efficiency of

Typically, polyHIPEs have bicontinuous structures consisting of highly interconnected open-cell porosity within a continuous polymer scaffold. This bicontinuous structure is generated during the polymerization/processing. Holes/ruptures are formed in the walls separating the individual internal phase droplets which thereupon become a continuous phase that can be subsequently removed. There are several ways to leverage this inherent polyHIPE bicontinuity toward generating bicontinuous systems incorporating incompatible polymers. One approach involves the post-polymerization synthesis of a second continuous polymer phase by filling the polyHIPE’s porous structure with a polymer solution or with a monomer that is subsequently polymerized. A second approach to achieving bicontinuous polymer systems involves combining two polymer solutions within a HIPE, followed by casting and drying [32,33]. Bicontinuous, intertwined, hydrophobic–hydrophilic systems of incompatible polymers can also be produced through an in situ one-pot simultaneous synthesis in which monomers are added both to the HIPE’s external phase and to the HIPE’s internal phase [80–83]. Recent research has investigated both hydrogelfilled hydrophobic polymers synthesized in W/O HIPEs and elastomer-filled hydrogels synthesized in O/W HIPEs. Copolymerization between the monomers located in the internal phase and the monomers located in the external phase has been investigated for both W/O and O/W HIPEs. Bicontinuous, hydrogel-filled hydrophobic polyHIPEs were synthesized in W/O HIPEs by combining hydrophobic monomers (S, DVB) in the external organic phase with aqueous solutions of hydrophilic monomers (acrylamide (AAm), N,N-methylenebisacrylamide (MBAm)) in the internal phase (Fig. 20) [84,85]. Both the hydrophilic

Fig. 20. A PAAm-filled PS-based polyHIPE (SEM). Reproduced with permission from Ref. [85]. Copyright 2011, Elseiver Ltd.

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monomer content and the locus of initiation (interface, organic phase, both phases) were found to strongly influence the macromolecular structure, the bicontinuous morphology, and the properties. The unexpected and significant reduction in the moduli of hydrated polyHIPEs with increasing acrylamide contents in the aqueous phase indicated that the simultaneous polymerizations in the external and internal phases were not mutually exclusive. Rather, it seems that acrylamide was incorporated into the molecular structure of the hydrophobic polymer in the external phase [84]. Upon the incorporation of AAm into the macromolecular structure of the polymer in the external phase, the polymer becomes more hydrophilic and hygroscopic. Ultimately, this reduces the stiffness of the hydrated polyHIPEs and enhances their tendency to collapse during drying. Both organic-phase initiation and pre-polymerization in the organic phase were found to reduce the extent of copolymerization with AAm in the external phase, enhancing the PS-like nature of the scaffold, producing typical polyHIPE porous structures, and enhancing the compressive moduli of the hydrated polyHIPEs. The ability to reversibly dry and hydrate these bicontinuous polyHIPEs demonstrated their potential for biomedical and separation applications. The uptake of a water soluble model drug (Eosin Y) was between 150% and 300% of the sample mass and the power-law coefficient describing its release indicated a release process that could be described by diffusion through an assembly of polydisperse spheres [85,86]. Varying the AAm content extended the release from 10 h to more than 10 days while pre-polymerization extended the release to 3 weeks. Bicontinuous polyHIPE systems with hydrogel-filled PS scaffolds were also synthesized using aqueous solutions of either AA or NiPAAm in the internal phase (Fig. 21) [87,88]. The pressure drop for flow through the resulting monoliths was affected by the hydrogel content and by the degree of crosslinking. The flow resistance for the polyHIPEs containing a PNiPAAm hydrogel was influenced by whether

the temperature was above or below PNiPAAm’s lower critical solution temperature (LCST). Inverse size exclusion chromatography was used to determine the pH-dependent nature of the structure in these bicontinuous polyHIPEs. Relatively flexible bicontinuous polyHIPEs consisting of a crosslinked polyacrylate elastomer within a continuous sulfonated hydrogel network were synthesized [89]. Both surfactant-stabilized and polymer-NP-stabilized O/W HIPEs were formed by adding an internal phase consisting of hydrophobic monomers (2-ethylhexyl acrylate (EHA) and DVB) to an external phase consisting of aqueous solutions of hydrophilic monomers (styrene sulfonate (SS) and MBAm). The bicontinuous morphology was strongly dependent upon the type of HIPE stabilization (Fig. 22) and the crosslinked PSS walls contained nanodomains of crosslinked PEHA. Both the modulus and the water absorption were significantly greater for the polyHIPEs from polymer-NP-stabilized HIPEs, which had significantly thicker hydrogel walls. PolyHIPEs were also synthesized within HIPEs where the external phase consisted of an aqueous solution of AAm and a water-soluble initiator (no crosslinking comonomer) and the internal phase consisted of an n-heptane solution of ethylene glycol dimethacrylate (EGDMA) and an oil-soluble initiator [90]. If copolymerization between the internal and external phases were to occur, as was observed within W/O HIPEs [84], then the water-insoluble EGDMA would crosslink the PAAm. Dimensionally stable polyHIPEs with high degrees of cross-linking were obtained, indicating that copolymerization between the phases occurs within these O/W HIPEs. The morphology became more closed-cell-like as the EGDMA content increased and the amount of water that could be absorbed by the hydrogel was reduced. An increase in the closed-cell nature of the porous structure with increasing EGDMA content has also been observed for other polyHIPE systems [91]. 3. HIPE stabilization 3.1. Surfactant-stabilized HIPEs

Fig. 21. A PAA-filled PS-based polyHIPE (SEM). Reproduced with permission from Ref. [87], Copyright 2009, the American Chemical Society.

The number of surfactant systems that are able to successfully stabilize HIPEs, while limited, is increasing. The Bancroft rule states that the liquid in which the surfactant is predominantly dissolved will form the external or continuous phase of an emulsion. Usually, W/O HIPEs are stabilized using nonionic surfactants in the organic external phase. Recently, W/O HIPEs were stabilized using a cationic surfactant, cetyltrimethylammonium bromide (CTAB), which was added to the external organic phase, and polyHIPEs were successfully synthesized [92]. It seems that during HIPE formation there was a limited amount of polymerization. As has been observed for other HIPE systems involving pre-polymerization, the presence of a polymer in the external phase enhances HIPE stability [84]. While the inclusion of some surfactant in the HIPE’s internal phase was found to enhance the formation of polyhedral structures, this can have unexpected consequences for the resulting polyHIPEs [17]. Such phenomena were recently observed by the author for polyHIPEs from W/O HIPEs containing surfactant-stabilized aqueous

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Fig. 22. PEHA-filled PSS-based polyHIPEs from: (a,b) a surfactant-stabilized HIPE (SEM,TEM); (c) a NP-stabilized HIPE (SEM). Reproduced with permission from Ref. [89]. Copyright 2012, the American Chemical Society.

dispersions of either carbon nanotubes (CNT) or carbon black (CB) [93]. Unusual structures consisting of void surfaces covered with polymer NPs were observed for these polyHIPEs (Fig. 23). These structures originated in two different polymerization reactions that occurred simultaneously. Polymerization occurred within the HIPE’s external phase, producing the polyHIPE’s walls. Simultaneously, emulsion polymerization occurred within the individual droplets of the HIPE’s internal phase, producing the polymer NPs. The anionic surfactants used to disperse the CB or the CNTs in water formed micelles within the individual droplets of the internal phase. In conventional emulsion polymerization, monomer diffuses from

micrometer-scale monomer droplets to the nanometerscale micelles. The monomer in the micelles undergoes polymerization in the presence of a water-soluble initiator. In the HIPE, monomer diffused from the continuous phase into the internal phase droplets and underwent emulsion polymerization. The resulting polymer NPs were deposited on the polyHIPE surfaces upon drying. 3.2. Particle-stabilized Pickering HIPEs The amount of surfactant needed for HIPE stabilization can often reach up to 30% of the external phase, reflecting the instability inherent in dispersing the HIPE’s major

Fig. 23. Polymer-NP-covered polyHIPEs from W/O HIPEs containing surfactant stabilized CNTs in the aqueous internal phase (SEM). Reproduced with permission from Ref. [93]. Copyright 2013, John Wiley & Sons.

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phase within the HIPE’s minor phase. The presence of residual surfactants within polyHIPEs has also been shown to affect polyHIPE properties [94,95]. The surfactants are usually removed following polyHIPE synthesis since they are leachable contaminants present in relatively large quantities that could have undesirable effects in applications involving polyHIPEs. In addition, the surfactants used for HIPEs can be relatively expensive and can be difficult and costly to remove. Replacing the surfactants should prove advantageous, especially for the synthesis of polyHIPEs for support, membrane, or bio-related applications. It is clear, however that the polyHIPE’s porous morphology and properties will be strongly influenced by the nature of HIPE stabilization. Pickering emulsions are surfactant-free emulsions that are stabilized by solid amphiphilic particles that preferentially migrate to, and self-assemble at, the oil–water interface, thus hindering droplet coalescence [96]. The breakthrough in synthesizing surfactant-free polyHIPEs was achieved through the formation of Pickering HIPEs [97–99]. HIPEs, often with over 80% internal phase, were successfully stabilized using relatively low loadings (based upon the monomer content) of titania NPs (1%), silica NPs (1–5%), or carbon nanotubes (0.4–1.7%) and polyHIPEs were successfully synthesized from these Pickering HIPEs. The NPs for the formation of W/O Pickering HIPEs were usually dispersed in the organic continuous phase, but W/O Pickering HIPEs were also formed successfully through the dispersion of CNTs in the aqueous internal phase. The amount of research on polyHIPEs synthesized within Pickering HIPEs has been expanding rapidly, reflecting the added value that NP-stabilization can provide. The NPs can act as functional centers, crosslinking centers, or ATRP initiators, and can also be used to effect significant changes to the polyHIPE’s mechanical, electrical, and/or magnetic properties. Recent research on polyHIPEs synthesized within Pickering HIPEs includes the effects of particle type (inorganic, polymer), particle content, surface modification, locus of initiation, particle functionality, and the combination of NP-stabilization with relatively small amounts of surfactant. In addition, in a variation on conventional polyHIPE synthesis, HIPE-gels were synthesized within Pickering HIPEs [100]. PS-based polyHIPEs were synthesized within W/O Pickering HIPEs (up to 88% internal phase) that were stabilized using only 1% titania NPs functionalized with oleic acid (OA) [101]. The polyHIPE void size (between 140 and 1100 ␮m) and the void size distribution both increased with increasing internal phase volume fractions and with decreasing NP contents (Fig. 24). The OA content proved integral to HIPE stabilization and reducing the OA content reduced HIPE stability. Much of the research on polyHIPEs from Pickering HIPEs has focused on modifying the hydrophobic–hydrophilic nature of the NP surfaces to enhance preferential migration to, and assembly at, the oil–water interface. Incorporating functional groups on the NP surface can be used to extend the role of the NPs beyond that of HIPE stabilization. In this manner, the NPs can have two or three different tasks. Such functional groups can be incorporated on the surface of inorganic NPs that do not undergo swelling within the HIPE

or within polymer NPs that do undergo swelling within the HIPE. The incorporation of vinyl groups can produce NPs that can serve as crosslinking centers. The incorporation of initiator groups can produce NPs that can serve as initiation centers. Silane-modification of silica NPs offers myriad possible surface chemistries. The members of the silane family can bear alkoxy groups or chlorines that can react with the hydroxyl groups on the silica surface. The silanes can also bear a large variety of organic groups suitable for surface functionalization. The silane chemistry, silane content, and NP content were shown to have significant effects on the size of the polyhedral, closed-cell-like polyHIPE voids that resulted from interfacial FRP initiation (using a watersoluble initiator in a NP-stabilized W/O Pickering HIPE) [50]. Interestingly, the NPs were located within the polyHIPE’s walls (Fig. 7) rather than at its surface (the HIPE’s oil–water interface), as would be expected. It seems that the NPs were “pushed” into the walls by the preferential diffusion of monomer toward the polymerization front at the oil–water interface. The locus of initiation was shown to have a significant effect on polyHIPE morphology, especially for NP-stabilized Pickering HIPEs [52]. In HIPEs that were compositionally identical (except for the small amount of initiator added) organic-phase FRP initiation (an oil-soluble initiator in W/O Pickering HIPEs) produced larger, more spherical voids (Fig. 25), as compared to the smaller polyhedral voids (Fig. 6) produced by interfacial FRP initiation. The larger spherical voids indicate that more extensive droplet coalescence occurs before the structure becomes “locked-in” at the gel point. For organic-phase initiation, the NPs were located at the surface (the HIPE’s oil–water interface), as seen in Fig. 25. Since initiation occurred simultaneously throughout the entire organic phase, there was no preferential diffusion of monomer toward the interface and the NPs were not “pushed” into the wall (Fig. 7). PolyHIPEs from Pickering HIPEs stabilized using silane-modified silica NPs were also synthesized using AGET ATRP. For AGET ATRP, a water-soluble reducing agent was used with either an organic-phase-based initiator or a NP-based initiator. As seen for FRP, the locus of AGET ATRP initiation had a significant effect on the polyHIPE morphology [52]. AGET ATRP initiation combining an organic-soluble initiator with a water-soluble reducing agent takes place at the W/O HIPE interface. As seen for interfacial FRP initiation (Figs. 6 and 7), interfacial AGET ATRP initiation produces relatively polyhedral voids that were “locked-in” at the beginning of the polymerization and NPs that were “pushed” into the polyHIPE walls by preferential monomer diffusion to the polymerization front at the interface. For NP-based AGET ATRP initiation, the NPs were “locked together” and packed tightly at the interface from the beginning of polymerization (Fig. 9). Most of the polyHIPEs synthesized within Pickering HIPEs are crosslinked via a crosslinking comonomer in the external phase. A crosslinking comonomer is usually essential, preventing the collapse of the polyHIPE during polymerization and drying. However, HIPE-stabilizing silane-modified silica NPs bearing vinyl groups as crosslinking centers can be used to eliminate the need

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Fig. 24. PS-based polyHIPEs from Pickering HIPEs stabilized with titania NPs. Internal volume fraction/oleic acid NP functionalization (SEM): (a) 70%/1%; (b) 80%/1%; (c) 80%/10%. Reproduced with permission from Ref. [101]. Copyright 2010, the American Chemical Society.

for a crosslinking comonomer. The NP crosslinking centers can be used to generate unique network structures, networks that are quite different from those generated through the use of crosslinking comonomers. A combination of NP-stabilization and NP-crosslinking has been used to synthesize polyHIPE systems with unusual properties including LDEs and porous shape memory polymers (SMPs). In LDEs, micrometer-scale water droplets from a W/O HIPE (85% internal phase) are individually encapsulated within a closed-cell elastomeric monolith [53 (a)]. In porous SMPs, compression to strains of around 70% can be “frozen-in” and then the original dimensions can be recovered through thermal cycling of the crosslinked polyHIPE above and below a glass transition temperature (Tg ) or a

melting temperature (Tm ) that is associated with the polymer walls [102]. Polymeric NPs have also been used to stabilize HIPEs for polyHIPE synthesis. In particular, they were successfully used to synthesize PMMA-based polyHIPEs, which are relatively difficult to synthesize [103]. As opposed to inorganic NPs, crosslinked polymer NPs can swell in the HIPE and uncrosslinked polymer NPs can dissolve. Copolymer NPs of S, MMA, and AA were used to stabilize W/O HIPEs (up to 95% internal phase) for a variety of monomers [103–105]. The uncrosslinked polymer NPs (around 9% of the monomer) were added to the aqueous internal phase so that their dissolution upon exposure to the organic phase would be delayed. PS-based or PMMA-based polyHIPEs

Fig. 25. A polyHIPE synthesized using organic-phase FRP initiation within a W/O Pickering HIPE: (a) spherical voids (SEM); (b) NPs on the polyHIPE surface (the oil–water interface) (TEM). Reproduced with permission from Ref. [52]. Copyright 2009, the American Chemical Society.

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Fig. 26. A PMMA-based polyHIPE from a HIPE (84% internal phase) stabilized with non-crosslinked polymer NPs and various amounts of NaCl stabilizing salt (SEM): (a) 0.2 mol/l; (b) 6.1 mol/l. Reproduced with permission from Ref. [103]. Copyright 2009, the Royal Society of Chemistry.

Fig. 27. A scheme depicting HIPE stabilization using non-crosslinked polymer NPs and the effect of stabilizing salt concentrations. Reproduced with permission from Ref. [104]. Copyright 2011, Elseiver Ltd.

synthesized within polymer-NP-stabilized HIPEs had an open-cell structure with smaller voids than those in typical polyHIPEs from Pickering HIPEs (Fig. 26). NP dissolution, whose rate could be modified through the concentration of the stabilizing salt in the internal phase (Fig. 27), could generate surfactant-like macromolecules. Dispersion of the polymer NPs in the organic external phase (3% of the monomer) was also investigated. Poly(urethane urea) (PUU) NPs (around 12% of the external phase) were used to stabilize O/W HIPEs (up to 95% internal phase) whose external phase contained around 35% water-soluble monomers [106]. The polymerization produced open-cell, hydrophilic PAAm-based polyHIPEs, whose voids were smaller than those of typical polyHIPEs from Pickering HIPEs (Fig. 28). In other work, the synthesis of “green” polyHIPEs based on photopolymerized acrylated epoxidized soybean oil was made possible through the use of W/O Pickering HIPEs stabilized with modified bacterial cellulose whiskers (0.5–5% of the monomer) [107]. Extremely small amounts of amphiphilic star polymers, synthesized “arm-first” using ATRP, were able to successfully stabilize W/O HIPEs [108] and were recently used by the author to synthesize polyHIPEs with highly interconnected porous structures that are typical of polyHIPEs [109].

In an interesting twist to the synthesis of polyHIPEs within Pickering HIPEs, W/O Pickering HIPE-gels (up to 85% internal phase) were produced by combining toluene with an aqueous NP dispersion where the NPs (a 2-ureido-4[1H] pyrimidinone functionalized copolymer of MMA and BA)

Fig. 28. A PAAm-based polyHIPE from a PUU-NP stabilized O/W HIPE (80% internal phase) (SEM). Reproduced with permission from Ref. [106]. Copyright 2010, Elseiver Ltd.

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Fig. 29. A scheme depicting the formation of gels using polymer-NP-stabilized O/W HIPEs. (a) confocal images of the as-formed gel (80% internal phase) and (b) of the same gel 30 min later, when preferential toluene evaporation led to the formation of a polyhedral structure. Reproduced with permission from Ref. [100]. Copyright 2012, the Royal Society of Chemistry.

constituted as little as 0.4% of the external phase (Fig. 29, schematic) [100]. The NPs migrated preferentially to, and assembled spontaneously at, the oil–water interface. The strong and highly directional hydrogen bonding between the NPs resulted in the formation of a stabilizing gel network (Fig. 29). Gels were formed from O/W Pickering HIPEs (up to 90% internal phase (hexane)) using aqueous NP dispersions where the NPs (P(S-co-NiPAAm-co-MBAm)) constituted as little as 0.2% of the external phase [110]. Drying the HIPE-gels yielded macroporous materials whose walls consisted of assemblies of NPs that continued to maintain their individual identities (Fig. 30). 3.3. Particle-stabilized Pickering HIPEs containing surfactant While polyHIPEs from Pickering HIPEs usually tend to have closed-cell-like structures, it has proven relatively easy to remove the internal phase through cracks and flaws in the walls. Small amounts of surfactant were used to promote the formation of more open-cell structures

for such polyHIPEs. In surfactant-stabilized HIPEs, a relatively low surfactant content (5% of monomers) is usually not enough to yield the highly interconnected open-cell structure typical of polyHIPEs from surfactant-stabilized HIPEs. However, interestingly enough, polyHIPEs with relatively interconnected open-cell structures were generated through the addition of small amounts of surfactant (5% of monomers) to a Pickering HIPE stabilized with silica NPs (3% of monomers) after all the internal phase was added [111]. PolyHIPEs synthesized from HIPEs with almost identical phase volumes and phase contents, but stabilized using different techniques, were compared in Fig. 31. A highly interconnected open-cell polyHIPE from a surfactant-stabilized HIPE had voids of around 5 ␮m and interconnecting holes of around 1.5 ␮m. A closed-cell-like polyHIPE from a NP-stabilized Pickering HIPE had voids of around 210 ␮m. An open-cell polyHIPE based on a NPstabilized Pickering HIPE to which 5% surfactant was added had voids of around 100 ␮m and interconnecting holes of around 26 ␮m. Thus, the porous structure is similar to

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Fig. 30. A macroporous material prepared from a dried P(S-co-NiPAAmco-MBAm)-NP-stabilized O/W Pickering HIPE (80% internal phase) (SEM). Reproduced with permission from Ref. [110]. Copyright 2010, the American Chemical Society.

those of polyHIPEs from surfactant-stabilized HIPEs while the void size scale is similar to those of polyHIPEs from surfactant-free Pickering HIPEs. The surfactant reduces the interfacial tension, reducing the droplet size, increasing the number of droplets, and reducing the walls thickness. This reduction in wall thickness allows interconnecting holes to be formed during polymerization and/or processing. The differences in the interconnectivities between polyHIPEs from surfactant-stabilized HIPEs and polyHIPEs

from NP-stabilized Pickering HIPEs has profound effects upon their permeabilities, and thus, upon their suitability for various applications. The nitrogen permeability of a polyHIPE from a surfactant-stabilized HIPE was 0.46 D (1 D ≈ 0.99 × 10−12 m2 ) while a compositionally similar polyHIPE from a NP-stabilized Pickering HIPE was relatively impermeable. Interestingly, a compositionally similar polyHIPE from a NP-stabilized Pickering HIPE to which 5% surfactant was added, which has significantly larger interconnected voids, exhibited a nitrogen permeability that was almost five-fold that of the highly interconnected polyHIPE from a surfactant-stabilized HIPE. Doubling the amount of surfactant added following HIPE formation to 10% reduced the maximum void size from 250 to 130 ␮m. Unfortunately, the practical applicability of such polyHIPEs was limited by their relatively poor mechanical properties (low crush strengths). The mechanical properties of such polyHIPEs were improved by replacing some or all of the rigid DVB crosslinking comonomer with the more flexible poly(ethylene glycol) dimethacrylate (PEGDMA) crosslinking comonomer, by reducing the internal phase contents, or by increasing the amount of surfactant added following Pickering HIPE formation [112]. Reasonable combinations of nitrogen permeability and crush strength could be achieved through such modifications. A variation on this type of HIPE formation, simultaneous addition to the NP-containing monomer of both the aqueous phase and the organic-soluble surfactant, produced similar results [113]. In other work along this line, a synergistic effect was found to exist between relatively small amounts of silica NPs (around 3% of the monomers) and relatively small amounts of a poly(l-lactide-co-glycolide) (PLGA) stabilizer

Fig. 31. PS-based polyHIPEs from silica-NP-stabilized W/O Pickering HIPEs (75% internal phase): (a) surfactant-stabilized; (b) NP-stabilized only, no surfactant added; (c) NP-stabilized with 5% surfactant added following HIPE formation (SEM). Reproduced with permission from Ref. [111]. Copyright 2010, John Wiley & Sons.

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(around 4% of the monomers) [114]. The silica NPs alone were limited in the maximum internal phase content that they could stabilize, while the PLGA alone was not able to stabilize HIPEs. The combination of the NPs and the PLGA was able to stabilize HIPEs with up to 90% internal phase. 4. Nanocomposite and hybrid polyHIPEs The combination of polymers and inorganic materials within polyHIPEs are often described as either nanocomposites or hybrids. The term nanocomposites usually refers to the incorporation of nanoscale inorganic particles, while the term hybrids usually refers to molecular or macromolecular integration between the polymer and the inorganic. There is, however, a gray area between the two terms, which will be used somewhat interchangeably herein. The combination of polymers and inorganic materials within polyHIPEs can be achieved by adding nanoscale inorganic particles to the HIPEs before polymerization or by in situ synthesis of nanoscale inorganics within the HIPEs. The formation of nanocomposites is expected to enhance the polyHIPE’s mechanical and thermal properties without producing a significant increase in density. The stabilization of HIPEs using nanoparticles has added a new dimension to the synthesis of nanocomposite polyHIPEs, since the stabilizing particles can perform two independent tasks, HIPE stabilization and polyHIPE property enhancement. The location of the particles in the HIPE, dispersed within the external phase, dispersed within the internal phase, or assembled at the oil–water interface, will affect the properties of the resulting nanocomposite polyHIPE. If there are no chemical bonds between the inorganic moieties and the polymer, then the inorganic moieties act as a filler. If there is a single chemical bond between each inorganic moiety and the polymer, then the inorganic moieties are essentially grafted to the polymer chain. If there are multiple chemical bonds between the inorganic moieties and the polymer, then the inorganic moieties act as crosslinking centers (Fig. 32) [115–117]. Recent research on nanocomposite polyHIPEs includes polyHIPEs that contain clay, CB, CNT, metallic NPs, or metal oxide NPs. Exfoliated organo-modified clays (montmorillonite or bentonite) were added to the comonomers in W/O HIPEs to produce clay nanocomposite polyHIPEs [118,119]. Relatively low clay contents, while not affecting the porous structures, which were typical of polyHIPEs, did produce significant increases in modulus. A comparison was made of polyHIPEs produced from W/O HIPEs in which the clay was dispersed using different routes [120]. Either organomodified clay was dispersed in the organic external phase or unmodified clay was dispersed in the aqueous internal phase. The void size was found to decrease with increasing organo-modified clay content in the external phase, reflecting a reduction in interfacial tension from the organo-modified clay acting as a cosurfactant. In other work, organo-modified clay added to the S, DVB, and acrylonitrile (AN) in the external phase of W/O HIPEs (85% internal phase) produced polyHIPEs with exfoliated clay within the walls (Fig. 33). The addition of clay produced a reduction in void size, a significant increase in crush strength, and a reduction in thermal conductivity

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(to around 0.51 W/(m ◦ C) from around 0.68 W/(m ◦ C) for similar polyHIPEs that did not contain clay) [121,122]. The presence of clay within polyHIPEs was found to enhance the dispersion of colloidal silver on the polyHIPE surface (Fig. 34). This enhancement was attributed to the clay’s affect on the polyHIPE’s surface energy [123]. Elastomeric clay nanocomposite polyHIPEs were synthesized by adding organo-modified clay to an EHA-containing external phase of W/O HIPEs (around 90% internal phase) [124]. Surprisingly, the addition of the clay to polyHIPEs based on elastomeric polymers reduced the modulus and the crush strength. CB, CNTs, titania nanorods, scandium oxide NPs, and tungsten NPs have been added to HIPEs to produce nanocomposite polyHIPEs [125–130]. In addition, various metallic NPs were synthesized in situ, within the HIPEs. Low loadings of silica NPs, around 1%, produced a significant increase in surface roughness [131]. High loadings of silica NPs, up to 60%, significantly enhanced the polyHIPE modulus and produced an increase in nitrogen permeability [132]. The reaction between tetraethoxyorthosilane (TEOS) and/or various functional silanes in the external phases of both W/O and O/W polyHIPEs were used to synthesize hybrid polyHIPEs [67,133–135]. Calcination of such hybrid polyHIPEs was used to produce silica monoliths with highly interconnected porous structures (see Section 6) [115,116]. Composite polyHIPEs with conductivities of around 10−3 S/m (sufficient for anti-static and electromagnetic interference shielding) were produced by adding blockcopolymer-stabilized aqueous dispersions of single wall CNTs (SWCNTs) to styrene [136]. Two routes were used to disperse the SWCNTs in the W/O HIPEs and the resulting polyHIPEs were compared. The SWCNTs were dispersed either in the organic external phase (with or without a surfactant) or in the aqueous internal phase (with surfactant) [137]. Based on the monomer content, it was only possible to add 0.1% SWCNTs to the external phase, while it was possible to add up to 2% SWCNTs to the internal phase. The SWCNTs were found within the polyHIPE wall, even for those dispersed in the aqueous internal phase (Fig. 35). The polyHIPEs from SWCNTs that were added to the external phase were non-conductive, while conductivities of around 10−1 S/m were achieved for polyHIPEs from SWCNTs that were added to the internal phase. The percolation threshold was around 0.1 and 0.2% for internal phases of 84 and 75%, respectively. The relatively high conductivity indicates that a percolation network was formed through the SWCNTs spontaneously migrating to and self-assembling at the oil–water interface in these Pickering HIPEs. Magnetic nanocomposite polyHIPEs that exhibited superparamagnetic behavior contained oleic-acid-capped iron oxide NPs distributed throughout the walls, as seen in Fig. 36 [138]. The NPs were added to the external phase of surfactant-stabilized W/O HIPEs (90% internal phase). Superparamagnetic behavior was also observed for polyHIPEs polymerized using 60 Co ␥-radiation that contained oleic-acid-capped iron oxide NPs [51]. Superparamagnetic polyHIPEs with relatively high magnetic moments were also synthesized within W/O Pickering HIPEs stabilized using oleic-acid-modified iron oxide NPs [139]. Depending

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Fig. 32. A scheme depicting organic–inorganic polyHIPE nanocomposites: blending, grafting, and crosslinking, based on Ref. [116].

Fig. 33. Exfoliated clay within the wall strut of an epoxy-filled polyHIPE (TEM). Reproduced with permission from Ref. [121]. Copyright 2011, John Wiley & Sons.

upon the oleic acid content, the NPs were either located at the polyHIPE surface (the oil–water interface), Fig. 37, or were partially dispersed in the external phase. These polyHIPEs exhibited the closed-cell-like structures that are typical of polyHIPEs from Pickering HIPEs. PolyHIPEs that were based on W/O Pickering HIPEs stabilized using 12acryloxy-9-octadecenoic acid (AOA) modified magnetite NPs and that were polymerized using room temperature ␥-radiation induced initiation also exhibited superparamagnetic characteristics [140]. The advantage of AOA over

oleic acid is that AOA can react with the polymerizing monomer and form chemical bonds between the polymer and the NP surface. In this manner, internal phase contents up to 90% and relatively low void sizes (compared to polyHIPEs based on Pickering HIPEs) could be achieved. Cement composite polyHIPEs were synthesized using a 70% cement slurry as the aqueous phase in W/O HIPEs (80% internal phase) [141]. The resulting materials had an interconnected biphasic structure consisting of an interpenetrating network of set cement and polymer. As

Fig. 34. Silver NPs deposited on a clay-filled polyHIPE (SEM). Reproduced with permission from Ref. [123]. Copyright 2012, Elseiver Ltd.

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Fig. 35. SWCNTs that were dispersed in the aqueous internal phase are revealed within the polyHIPE wall: (a) a fracture cross-section SEM specimen. Reproduced with permission from Ref. [136]. Copyright 2009, the Royal Society of Chemistry; (b) charge contrast on an uncoated SEM specimen. Reproduced with permission from Ref. [137]. Copyright 2009, Elseiver Ltd.

Fig. 36. (a) NPs in the wall of a PS-based surfactant-stabilized polyHIPE with 6.4% magnetic NPs dispersed in the organic external phase (TEM). (b) The magnetization (M) versus applied field (H) at 5 K for 6.4% (open circles) and 10% (solid line) NPs. The inset shows the attraction of a polyHIPE with 6.4% NPs to a magnet. Reproduced with permission from Ref. [138]. Copyright 2011, Elseiver Ltd.

compared to pure set cement, incorporating the polymer into the cement in this manner enhanced the compressive strain to failure and the chemical resistance, but reduced the modulus and the crush strength.

5. Applications PolyHIPEs have been investigated for myriad applications, many of which involve supports for chemical

Fig. 37. NPs on the surface of a PS-based, NP-stabilized polyHIPE with 5% magnetic NPs dispersed in the organic external phase (TEM). Reproduced with permission from Ref. [139]. Copyright 2011, the American Chemical Society.

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reactions, membranes for separations, and absorbents for purification and storage. PolyHIPEs with added-value properties such as biodegradability, thermo-responsiveness, or pH-responsiveness have been developed for use in biomedicine and agriculture. 5.1. Supports and membranes: chemical reactions, separations, absorption Most polyHIPE research has focused on internal phase contents over 74%. Unfortunately, such materials often exhibit relatively low moduli and low crush strengths, making them unsuitable for many practical applications. Raising the internal phase content can produce significant improvements in the mechanical properties (the relative modulus in foams has been related to the relative density squared) [142]. However, an increase in the relative density (wall thickness) can come at the expense of the highly interconnected open-cell structure typical of polyHIPEs. Recent work has investigated the mechanical properties, the gas permeability, and the mercury permeability of emulsion-templated and miniemulsiontemplated porous polymers whose internal phase contents ranged from 84 to 25% [18]. As expected, the modulus and the crush strength increased, while the permeability decreased, with increasing internal phase contents. Surprisingly, the miniemulsion-templated polymer with 25% internal phase produced a polymer with a porosity of 50%, a nitrogen permeability of 26 × 10−15 m2 , and a mercury permeability of 7 × 10−15 m2 . The nitrogen and mercury permeabilities both increased exponentially with porosity. A polyisodecylacrylate-based polyHIPE bearing a porous structure typical of polyHIPEs was synthesized inside a capillary for electrochromatography and used successfully for the separation of alkylbenzenes (including thiourea, benzene, toluene, ethylbenzene, propylbenzene, and butylbenzene) in a solution of acetonitrile and phosphate buffer [143]. The columns indicated a strong electroosmotic flow (EOF) without any additional EOF-generating monomer, probably due to the presence of the ionizable sulfate groups from the water-soluble polymerization initiator. The efficiency of the column was similar to that of conventional polymethacrylate-based monoliths bearing sulfonic acid functionalities. PolyHIPEs that could undergo hypercrosslinking were based upon poly(4-vinylbenzyl chloride) (PVBC) in the external phase of W/O HIPEs (90% internal phase). The SSA increased from 6 to 990 m2 /g and there was no significant change in the macroporous polyHIPE morphology upon hypercrosslinking the PVBC in a dichloroethane solution of FeCl3 [144]. The attachment of 4-(N-methylamino)pyridine (MAP) to a hypercrosslinked PVBC-based polyHIPE was achieved through the presence of residual reactive PVBC chloromethyl functionalities. MAP-bearing polyHIPEs were used as supports for the acylation of methylcyclohexanol in both non-swelling and swelling solvents where the MAP acted as an organocatalyst. Performing the acylation in a non-swelling solvent achieved 100% conversion, outperforming both non-hypercrosslinked polyHIPEs and conventional hypercrosslinked beads.

Fairly rigid PGMA-based polyHIPE membranes with typical polyHIPE porous structures were synthesized in W/O HIPEs (75% internal phase) [145]. Reducing the crosslinking comonomer (EGDMA) content enhanced membrane flexibility but the resulting increase in swelling made the membrane prone to tearing. Copolymerization of GMA with EHA enhanced membrane flexibility, even at relatively high crosslinking contents, but reduced the interconnecting hole size and the SSA. The membranes were chemically modified through reaction of the epoxy group with diethylamine to yield functional supports for ion exchange chromatography. The resulting membranes exhibited a relatively high dynamic binding capacity (45 mg/ml) for the purification of bovine serum albumin. However, the exponential increase in pressure drop with flow rate indicated that the membranes underwent compression, limiting their productivity for industrial purification processes. PolyHIPEs have highly interconnected macroporous structures that can be advantageous for achieving high transport rates to microporous walls for molecular storage applications. PVBC-based polyHIPEs with up to 80 mol% VBC in the monomer mixture (VBC, S, and DVB) were synthesized in W/O HIPEs (75% internal phase) and then hypercrosslinked [146]. Partial hydrolysis of the chloromethyl groups seemed to occur during polymerization, yielding a chlorine content that was smaller than expected. Following hypercrosslinking, there were no remaining chloromethyl groups. The resulting off-white hypercrosslinked polyHIPEs were somewhat more brittle than the original white polyHIPEs. The hypercrosslinked polyHIPEs exhibited bimodal pore-size distributions, extremely high specific surface areas (up to 1210 m2 /g), high micropore volumes (up to 0.68 cm3 /g), and lower surface polarities than activated carbon. Interestingly, their hydrogen storage capacity was 2.02 wt% and their n-butane absorption capacity was 34.2 wt%. Polyglutaraldehyde (PGA) was added to the external phase of W/O HIPEs (90% internal phase) to produce activated polyHIPEs [147]. A commercial lipase from thermomyces lanuginosus was immobilized by treating polyHIPE powders with the enzyme in solution (Fig. 38). The PGA-activated polyHIPE was more efficient in terms of protein loading (11.4 mg/g immobilized), activity, and operational stability. Since the lipase was covalently bonded, the immobilized lipase activity in the PGAactivated polyHIPEs was constant for multiple re-uses. Biodiesel was synthesized by reacting canola oil with methanol in the presence of the lipase-modified polyHIPEs. The highest yield, 97%, was obtained using a 3-step methanol addition to prevent denaturing the lipase. Similar studies were done for the synthesis of biodiesel from sunflower, soybean, and waste cooking oils, with the best biodiesel yield obtained from sunflower oil (97%) [148]. Reactions in polyHIPE beads and powders produced higher yields than reactions in polyHIPE monoliths, reflecting the less efficient mixing in the monoliths. PGMA-based polyHIPEs were synthesized by adding W/O HIPEs (80% internal phase) to a separation column [149]. The HIPEs were stabilized using a triblock copolymer surfactant consisting of poly(ethylene oxide) (PEO)

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Fig. 38. Lipase immobilization on a powdered polyHIPE support (SEM): (a) before; (b) after. Reproduced with permission from Ref. [147]. Copyright 2009, Elseiver Ltd.

Fig. 39. Florescence microscopy photographs of GMA-based W/O HIPEs (80% internal phase) stabilized using various PEO-PPO-PEO contents (based on H2 O content): (a) 2%; (b) 7%. Reproduced with permission from Ref. [149]. Copyright 2009, the Royal Society of Chemistry.

endblocks and a poly(propylene oxide) (PPO) midblock. The amount of PEO-PPO-PEO surfactant had a significant effect on the structure of the HIPE (Fig. 39) and on the resulting porous structure of the polyHIPE (Fig. 40).

Spherical voids were produced at relatively low surfactant contents and lamellar structures were produced at relatively high surfactant contents. The differences in structure were attributed to the concentration-dependent

Fig. 40. PGMA-based polyHIPEs from W/O HIPEs (80% internal phase) stabilized using various PEO-PPO-PEO contents (based on the water content) (SEM): (a) 2%; (b) 7%. Reproduced with permission from Ref. [149]. Copyright 2009, the Royal Society of Chemistry.

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self-assembly of the surfactant into different structures (spherical micelles, wormlike micelles, bicontinuous structures, lamellae). These different types of structures were not observed when using non-block-copolymer surfactants. The resulting polyHIPEs exhibited relatively large surface areas (161 m2 /g) and relatively high permeabilities (6.1 × 10−13 m2 ). The back pressure was linearly proportional to the flow rate (for water or methanol) indicating that the polyHIPEs were not undergoing compression at flow velocities of 3600 cm/h. The epoxy groups were converted to weak anion-exchange functionalities through reaction with a 1:1 solution of diethylamine and tetrahydrofuran (THF) that was pumped through the polyHIPE at 0.2 ml/min. The modified polyHIPEs were then used to separate a standard protein mixture containing lysozyme, bovine serum albumin, ovalbumin, and pepsin. Good separation was obtained without the loss of resolution, with the almost complete separation in about 1 min at 6 ml/min suggesting potential for the efficient downstream processing of biomolecules. In other work, PGMA-based polyHIPEs from W/O HIPEs (85% internal phase) were functionalized with various amines and then investigated for the absorption of metal ions (Ag, Cu, Cr) [150]. The largest amine content achieved was for 1,4-ethylenediamene, and the absorption was most efficient for Ag and was least efficient for Cr. Among the amines, 2-phenylimidazole proved most effective for heavy metal ions. Microreactors have been shown to have significant advantages with respect to cost, safety, throughput, kinetics and scale-up. PolyHIPEs are of interest for microreactors since their relatively low resistance to flow makes them useful for low pressure, continuous flow technologies. A recent review describes the use of polyHIPEs as supports in such microreactors [151]. A broad range of conventional free radical transformations with excellent conversions (hydrogen atom transfer, radical deoxygenation, radical cyclization) in both organic and aqueous media were accessed by functionalizing polyHIPEs through the introduction of thiol groups or through the introduction of tetra-substituted ammonium salts such as CTAB [152]. Hydrophilic polyHIPEs based on PAA, PHEMA, or PGMA and bicontinuous polyHIPEs based on PS in the external phase and PAA in the internal phase were synthesized within HPLC columns [87,88]. All the hydrophilic polyHIPEs had highly interconnected porous structures, with the PGMA- and PHEMA-based polyHIPEs having porous structures that were typical of polyHIPEs from such monomers [73]. The bicontinuous polyHIPEs exhibited hydrogel-filled scaffold structures typical of such systems [84,85]. Aqueous solutions of various pHs could be pumped through the resulting polyHIPE-filled HPLC columns. PolyHIPEs based upon poly(ethylene glycol) (PEG) with biocompatible surfaces were used for the reversible immobilization of thermo-responsive and pH-responsive elastin-based side-chain polymers (EBPs) whose LCSTs are similar to those of elastin-like peptides [153]. The polyHIPEs were synthesized using commercially available PEG-methacrylate and a crosslinking comonomer (MBAm, EGDMA, or PEGDMA) in an O/W HIPE whose external aqueous phase contained up to 50% monomer (in some cases,

dimethylformamide and/or poly(vinyl alcohol) were used to enhance HIPE stability). The PEG-based polyHIPEs were quite elastic, and although they tended to shrink upon drying, they did recover their original shape upon re-wetting. These polyHIPEs were infiltrated with a dilute solution of a relatively high molecular weight thiol-terminated EBP. The high molecular weight EBP reacted with the Michael acceptors on the polyHIPE surfaces (the residual double bonds from the crosslinking comonomers). While an EBP loading of 12.2 ␮mol/g was expected, a loading of only 0.63 ␮mol/g was achieved. Unmodified and EBP-modified polyHIPEs were then soaked in a solution of an esterified, complementary, lower molecular weight EBP that was fluorescently labeled. The pH of the solution was slowly reduced to 1.5 to induce the co-assembly of the two EBPs. The EBP-modified polyHIPE exhibited fluorescence while the unmodified polyHIPE did not. The lower molecular weight EBP could be recovered reversibly from the EBPmodified polyHIPE by disassembly upon soaking in a pH 3.2 buffer solution. Glutathione-functionalized porous dextran for the selective binding of glutathione S-transferase (GST) was synthesized through the polymerization of dextran and glutathione (GSH) in the aqueous external phase of an O/W HIPE (75% internal phase) [154]. The dextran was vinylated by reaction with GMA and the internal phase was a toluene solution of the initiator. The resulting polyHIPEs had over 86% highly interconnected porosities, very thin walls, and enough GSH moieties for GST separation via specific GSH-GST binding. The polyHIPE monoliths were placed in a column and exhibited a relatively high selectivity for GST during the separation of GST from a crude cell lysate. A heterogeneous photocatalyst incorporating a borondipyrromethene (BDP) compound in a PS-based polyHIPE was synthesized within a W/O HIPE (90% internal phase) [155]. The polymerizable BDP-styrene (BDP-S), which fluoresces bright green, was synthesized using a ‘click’ reaction between 1-(azidomethyl)-4-vinylbenzene (VBA), synthesized from VBC, and BDP bearing a terminal alkyne moiety. Copolymerization of S, DVB, and a few percent BDP-S produced a ductile polyHIPE which fluoresces bright green (Fig. 41). The BDP-containing polyHIPE was a highly effective photocatalyst for the aerobic oxidation of thioanisole to methyl phenyl sulfoxide under fluorescent light, with conversions as high as 99%. There was a gradual decrease in conversion for multiple cycles, reaching 90% in the 9th cycle. There was no such reaction in the absence of the BDP-containing polyHIPE, no such reaction in the absence of light, and only 4% conversion in the absence of oxygen. The BDP-containing polyHIPE was used successfully for the aerobic oxidation of other sulfides, but with lower conversions. 5.2. Tissue engineering, responsive polymers, controlled release, and water retention Porous polymers containing biodegradable components are of interest for tissue engineering scaffolds and other biomedical applications. The key parameters in scaffold selection include adequate mechanical properties, a

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Fig. 41. A BDP-S-containing polyHIPE: (a) daylight; (b) excited with an ultraviolet lamp. Reproduced with permission from Ref. [155]. Copyright 2012, the Royal Society of Chemistry.

highly porous and interconnected pore structure, a high surface area to volume ratio, and biodegradability. The surface chemistry should favor the attachment, proliferation, and differentiation of cells. One of the most widespread problems with the conventional methods for producing scaffolds is that the pores are not fully interconnected. The highly interconnected porous structures typical of polyHIPEs are, therefore, of interest for tissue engineering applications. Biodegradable oligomeric PCL was incorporated into PS-based polyHIPEs by adding to the external phases of

W/O HIPEs either a PCL-diol for semi-IPN formation or a divinyl terminated PCL for polyHIPE crosslinking [156]. As much as 50 wt% of a divinyl-terminated PCL oligomer was used to crosslink PS-, PEHA-, and PtBA-based polyHIPEs [157,158]. The viscous and relatively hydrophilic PCL destabilized the HIPEs to the extent that extremely large voids were formed (Fig. 42). Serendipitously, these large voids may be beneficial for tissue engineering applications. The relatively low-modulus PEHA-based polyHIPE with 50% PCL absorbed more than twice its pore volume of water. Both the PEHA- and PtBA-based polyHIPEs

Fig. 42. A PtBA-based polyHIPE containing 50% divinyl-terminated oligomeric PCL: (a,b) porous structure (SEM); (c) cross-section after seeding with mouse skeletal muscle stem cells (OM). Reproduced with permission from Ref. [158]. Copyright 2009, John Wiley & Sons.

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Fig. 43. Twenty-four hour fluorescent live (green)/dead (red) analysis of a polyHIPE based on a HIPE containing 5% PGPR. Reproduced with permission from Ref. [159]. Copyright 2011, the American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

underwent extensive degradation in an accelerated degradation test. The degradation of the PCL seemed to promote the disintegration of the entire macromolecular structure. Spontaneous differentiation of mouse skeletal muscle stem cells and the formation of myotubes with superior cell adhesion, penetration, and growth, produced a relatively continuous tissue for the PtBA-based polyHIPE (Fig. 42). Injectable biodegradable polyHIPEs from W/O HIPEs (75% internal phase) that cure at physiological temperatures (37 ◦ C) were investigated for bone graft applications [159]. These polyHIPEs, based on a biodegradable propylene fumarate dimethacrylate macromonomer, were designed to avoid the use of toxic diluents and the exposure to high cure temperatures. Among the surfactants studied, polyglycerol polyricinoleate (PGPR) was the only one with hydrogen bonding sites on its tail and the only one to yield stable HIPEs. The polyHIPEs exhibited porosities of around 75%, void sizes ranging from 4 to 29 ␮m, an average compressive modulus of 33 MPa, and an average

compressive strength of 5 MPa. A cytocompatibility analysis indicated 95% viability of 3T3 Swiss mouse fibroblasts after 24 h (Fig. 43). Non-biodegradable P(S-co-DVB-co-EHA) polyHIPE supports for in vitro cell culturing were functionalized through exposure to an oxygen plasma during which the surface oxygen content increased from 5.5 to 20.8 at% [160]. Water formed a large contact angle with the untreated polyHIPE, but flowed readily into plasma-treated polyHIPEs. Both thiols and amines were successfully attached to the plasma-treated polyHIPE surfaces. The growth of human osteoblast-like cell line MG63 on the polyHIPE surface was significantly enhanced by plasma treatment (Fig. 44). Responsive polymers change their shape under a stimulus (thermal, chemical, light) and can be cycled from one shape to another. Thermally responsive polymers usually respond near a transition temperature (Tg , Tm , or LCST). Such polymers usually contain chemical or physical crosslinks that limit the extent of change effected by the transition. Porous polymers, foams, and hydrogels can undergo higher deformations during thermally responsive transitions than can fully dense materials. PNiPAAm-based polyHIPEs were synthesized at room temperature within O/W HIPEs (75% internal phase) containing a 1 M aqueous solution of NiPAAm in the external phase. These polyHIPEs were able to absorb large volumes of water and became highly swollen [161]. Below the LCST, the average void size was 10 ␮m and the pore volume was 5.8 cm3 /g. Above the LCST, the swollen structure contracted, squeezing out absorbed liquid, and the average void size was reduced to 4.8 ␮m and the pore volume was reduced to 3.5 cm3 /g. This phenomenon was used to upload PS colloids at room temperature and then to release them at temperatures above the LCST. The thermo-responsive porous PNiPAAm polyHIPE could, therefore, act like a pump (Fig. 45). A burst “step” release profile was observed upon immersing the PS-colloid-loaded polyHIPE in a water bath at 45 ◦ C. The lowest crosslinking level attained the highest loading of PS colloid. For a single loading followed by multiple release cycles, 50% of the PS colloid was released during the first cycle, 25% during the second cycle, and 7% during the third cycle. For multiple cycles of loading and release,

Fig. 44. Osteoblast growth after 7 days on polyHIPE scaffolds (SEM): (a) untreated; (b) oxygen plasma treated. Scale bar: 200 ␮m. Reproduced with permission from Ref. [160]. Copyright 2009, the American Chemical Society.

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Fig. 45. A PS-colloid-loaded PNiPAAm polyHIPE in water: (a) when placed in a fridge at 4 ◦ C; (b) the PNiPAAm contracts and releases the colloid after heating in a 45 ◦ C water bath. Reproduced with permission from Ref. [161]. Copyright 2010, the American Chemical Society.

most of the loaded PS colloid was released from the second loading step onward. Porous SMPs based upon methacrylates and acrylates with long crystallizable side-chains were synthesized within W/O Pickering HIPEs (85% internal phase) stabilized using silica NPs which also served as crosslinking centers [102]. The nature of the polymer backbone affected the nature of the crystalline phase for identical side chains. At room temperature, the porous SMPs maintained the temporary shape (a compressive strain of 70%) imposed by heating above the Tm , compressing, and cooling while holding the strain at 70% (Fig. 46). These SMPs exhibited good recovery upon reheating for four compression-recovery cycles. While the methacrylate-based SMPs exhibited a single-stage recovery, the acrylate-based SMPs, with identical side-chains, exhibited a two-stage recovery that was associated with the existence of two crystalline phases. The recovery behavior was described using Kelvin-Voigt units in series and the dependence of viscosity on temperature was described using a WLF-like relationship. The SMP response in water was enhanced in recent work by

the author through the synthesis of bicontinuous hydrogelcontaining SMP polyHIPEs within W/O HIPEs using the same long side-chain monomers in the external phase and hydrophilic monomers in the internal phase [162]. Porous systems that offer the ability to manipulate the porous structure are of interested for controlled release applications. Bicontinuous hydrogel-filled hydrophobic polyHIPEs were synthesized in W/O HIPEs by adding an aqueous solution of water-soluble monomers (acrylamide) to hydrophobic monomers (styrene, EHA) in the external phase (Fig. 20) [84]. Dry polyHIPEs were immersed in an aqueous solution of a water-soluble dye and loading took place through a combination of rapid capillary action and hydrogel swelling. The dye remained within the polyHIPEs after the solution-filled polyHIPEs were dried. The release time from a stand-alone PAAm hydrogel that underwent the same loading procedure was 10 h. Incorporating the PAAm hydrogel within a polyHIPE’s internal phase extended the release time to 10 days [85]. The diffusion pathway for this bicontinuous system was modeled using the diffusion through an assembly of polydisperse

Fig. 46. A scheme depicting an SMP cycle based on a crosslinked polyHIPE with crystallizable side chains that is given a temporary shape (a compressive strain of 70%) above the Tm , that maintains its temporary shape when cooled, and that recovers its original shape when reheated. Based on Ref. [102]. Copyright 2012, the Royal Society of Chemistry.

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spheres [86]. Pre-polymerization of the HIPE’s external phase reduced the extent of copolymerization between the monomers in the two phases, reduced the capillarity, increased the tortuosity, and extended the release time to 3 weeks. Encapsulation and storage of liquids within discrete, micrometer-scale containers is essential for living organisms. Water-storage cells can keep plants healthy even during a prolonged drought. Vacuoles are specialized water storage systems in which the walls, vesicle membranes, form enclosed compartments containing aqueous solutions. The cortex of cacti and the leaves of many succulents contain specialized cells with flexible, elastic walls. Encapsulating discrete droplets of water within monolithic synthetic polymers could yield novel water-retention systems with advantageous properties for biomedical, agricultural, pharmaceutical, and cosmetic applications. PS-based polyHIPEs with 90% porosity were sulfonated through immersion in sulfuric acid and then exposed to pulsed microwave irradiation [163]. Two types of structures were investigated. A small 20 ␮m void structure was synthesized through slow addition of the internal phase, extended mixing, and a 240 s microwave exposure. This polyHIPE absorbed 9.9 times its mass in water and remained rigid. A large 150 ␮m void structure was synthesized through rapid addition of the internal phase and limited mixing. After a 150 s microwave exposure the polyHIPE became soft, absorbing 18.1 times its mass in water. These polyHIPEs were used as soil additives for water retention to enhance the biomass yield from rye grass growth. The more rigid polyHIPE produced no significant increase in yield compared to the control. For 0.5 wt% of the soft polyHIPE, the dry biomass yield after 21 days of cultivation increased by about 30, 140, and 300% with increasing water stress (normal, semiarid, and arid conditions, respectively) as compared to the control. Tissue-like elastomeric PEHA LDEs monoliths were synthesized within W/O Pickering HIPEs (85% internal phase) that were stabilized using silica NPs that also served as crosslinking centers (there was no crosslinking comonomer) using silane-modified silica NPs that could also serve as ATRP initiators [53 (a)]. The ability to synthesize LDEs was strongly dependent upon the type of emulsion stabilization, the type of crosslinking, and the type of initiation. The water retention in these unique materials was exceedingly high (Fig. 47). As seen for living systems, the presence of encapsulated water enhanced the resistance to compressive deformation and the resistance to ignition upon direct exposure to a flame (Fig. 48). 6. Porous ceramics and porous carbons PolyHIPEs can be used as templates for the synthesis of porous ceramics and porous carbons. The development of porous ceramics and porous carbons using polyHIPEbased templates has flourished with the burgeoning developments in polyHIPEs. These materials are of interest for such applications as supports for chemical reactions, membranes for separations, and absorbents for purification and storage. The conductivity of carbon makes porous carbons of interest for “green” energy-storage applications

Fig. 47. Log–log water retention curves for a typical polyHIPE (PH-2) and for two LDEs. Reproduced with permission from Ref. [53 (a)]. Copyright 2012, the American Chemical Society.

such as supercapacitors, batteries, and fuel cells. PolyHIPEtemplated porous carbons have been generated through the pyrolysis of either polymethacrylonitrile-based or PVBC-based polyHIPEs in an inert atmosphere [164,165]. Several different approaches were developed for the synthesis of polyHIPE-templated porous ceramics, some involving W/O HIPEs and others involving O/W HIPEs. One W/O HIPE approach involved the synthesis of hybrid polyHIPEs with interconnected organic–inorganic networks through two simultaneous reactions, the copolymerization of organic monomers with vinyl-bearing silanes and the hydrolysis-condensation of the silanes [67,133]. Porous silica monoliths were generated through the calcination of the resulting hybrid polyHIPE monoliths. Porous silica monoliths from W/O HIPEs were also generated through the pyrolysis of hybrid polyHIPEs that contained vinylsilsesquioxane, polyhedral oligomeric silsesquioxane, or tetraethoxyorthosilane (TEOS) in the external phase [115–117,134]. One O/W HIPE approach was the formation of porous silica beads through the sedimentation polymerization and crosslinking of water-soluble monomers in the presence of TEOS within the HIPE’s external phase, followed by calcination [37]. Porous silica, alumina, titania, and zirconia beads were produced in an approach that separated the two reactions [38]. First, the synthesis of hydrophilic polyHIPEs through sedimentation polymerization within O/W HIPEs. Second, the immersion of the resulting polyHIPE beads in an inorganic precursor solution, the sol–gel reaction of the inorganic precursor, and calcination. A variation on this approach involved immersing the polyHIPE in a colloidal silica dispersion [166]. Another O/W HIPE approach involved an external phase that contained TEOS but no organic monomers. Sol–gel reactions of aqueous TEOS solutions in the HIPE’s external phase were used to synthesize inorganic monoliths with highly interconnected porous structures [167]. The addition of functional silanes to the external phase of these HIPEs was used to synthesize highly porous

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Fig. 48. Photographs illustrating the differences in flammability between a typical polyHIPE and an LDE. Reproduced with permission from Ref. [53 (a)]. Copyright 2012, the American Chemical Society.

hybrid monoliths where organic moieties were either grafted to the inorganic network or organic–inorganic interconnected networks were formed [135]. Recent research, involving both monoliths and beads, has furthered the approaches described above and has added new approaches, such as the use of silica-based polyHIPEs as templates for the generation of porous carbons. 6.1. Porous ceramics Silica polyHIPEs from O/W HIPEs containing TEOS and silanes in their external phase were decorated with palladium NPs (around 4 wt%) generated in situ through the reduction of palladium acetate (Fig. 49). The catalytic performance of the Pd-decorated porous silica for a MizorokiHeck coupling reaction was then evaluated [168]. The silica polyHIPE, which had a SSA of 725 m2 /g, was functionalized by grafting with either 3-aminopropyltrimethoxysilane or 3-mercaptopropyltrimethoxysilane (HgPtMS), reducing

Fig. 49. Palladium NP decoration of a silica polyHIPE based on TEOS and silanes (TEM). Reproduced with permission from Ref. [168]. Copyright 2009, the American Chemical Society.

the SSA to around 350 m2 /g. The resulting monoliths were used as supports for the catalysis of a coupling reaction between iodobenzene and styrene, producing 96% of the trans isomer. The mercapto group proved less sensitive to deactivation and leaching than the amino group, with the conversion being almost complete during the first 7 cycles. Porous silica beads were also formed through a sol–gel reaction within a HIPE [169]. An aqueous solution of TEOS was pre-reacted for 1 week at −20 ◦ C or for 1 day at room temperature and then an O/W HIPE (80% internal phase) was formed by adding mineral oil. Droplets of the O/W HIPE were added to diethyl amine at room temperature and left overnight to gel. Calcining the beads at 600 ◦ C produced a glassy material composed of fused NPs that retained a highly interconnected porous structure. The resulting spherical millimeter-size beads had typical polyHIPE structures, a 2.5 ␮m void diameter, and a SSA of 140 m2 /g. An interesting twist to the sol–gel synthesis of macroporous silica within a HIPE was the synthesis of ordered mesoporous/macroporous materials by templating the silica within the cubic liquid crystalline phase formed by a non-ionic surfactant in the aqueous phase [170,171]. Using TEOS, bimodal pore size distributions were produced, with macropores of around 10 ␮m and mesopores of around 3 nm, and SSAs greater than 400 m2 /g were achieved [170]. Unfortunately, the cubic order of the surfactant template was not preserved in the silica, possibly owing to the destabilizing effect of the ethanol that was generated by the sol–gel reaction. However, silica with ordered mesopores of around 4 nm, macropores of around 5 ␮m, and SSAs greater than 550 m2 /g could be achieved using tetra(2-hydroxyethyl) orthosilicate instead of TEOS [171]. Evidently, the ethylene glycol generated by the sol–gel reaction did not destabilize the cubic liquid crystalline structure of the surfactant and the order within the template was transferred to the silica. Silica polyHIPEs were used as templates for the synthesis of porous silicon carbide through their impregnation with a pre-ceramic precursor (polycarbosilane), their pyrolysis, and the removal of the silica through etching [172]. The porous silicon carbides with 78% porosity

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Fig. 50. Porous silicon carbide with ␤-SiC crystallites within the dashed circles (TEM). Reproduced with permission from Ref. [172]. Copyright 2010, the Royal Society of Chemistry.

composed of ␤-SiC on the microscopic length scale (Fig. 50), had SSAs of around 110 m2 /g, uniaxial compression moduli of around 55 MPa, bulk heat capacities of around 0.3 J/(g K), and heat conductivities of around 3.6 W/(m K). The modulus, heat capacity, and heat conductivity decreased with increasing porosity. Macroporous alumina monoliths were synthesized by imbibing a pseudo-bohemite-based hydrosol into a PSbased polyHIPE from a W/O HIPE (82% internal phase) [173]. While calcination at 600 ◦ C yielded an 88% weight loss and trace organic residue, no organic residue remained following calcination at 900 ◦ C. The structure of the porous alumina from calcination at 600 ◦ C replicated the polyHIPE structure (10–50 ␮m voids and 1–20 ␮m interconnecting holes), allowing for shrinkage (Fig. 51a,b). The size of the interconnecting holes increased with increasing calcination temperature until, for calcination at 1300 ◦ C, the macroporous structure was similar to those of ceramic or metallic foams (Fig. 51c), except for a significantly smaller macropore size (20–30 ␮m). Calcination at 600 produced ␥-alumina, a SSA of 228 m2 /g, a mesopore diameter of 2.2 nm, a pore volume of 0.19 cm3 /g, and a compressive strength of 0.12 MPa. Calcination at 1300 ◦ C produced sintered ␣-alumina with a significant decrease in surface area (5 m2 /g), a significant decrease in pore volume (0.01 cm3 /g) and a significant increase in compressive strength (3 MPa). 6.2. Porous carbons PAN-based polyHIPEs crosslinked with various DVB contents (from 5 to 17% of the monomers) were pyrolyzed to porous carbon [174]. For the same 88% internal phase content, the polyHIPE porosity decreased (from 90 to 86%) with increasing AN/DVB ratio since the relatively DVBpoor polyHIPEs were not stiff enough to prevent partial collapse during drying. On the other hand, the mass loss

upon pyrolysis decreased with increasing AN/DVB ratio, indicating that DVB crosslinking interferes with the formation of a carbonaceous structure. The porous structures of the pyrolyzed polyHIPEs were quite similar to those of the original polyHIPEs and both were quite different from typical polyHIPE porous structures (Fig. 52). Porous carbons with microporous to macroporous hierarchical pore architectures (based upon hypercrosslinked PVBC-based polyHIPEs) were produced in recent work by the author [175]. Porous carbon with independently tunable mesoporous and macroporous structures were formed through the pyrolysis of resorcinol-formaldehyde polyHIPEs (75% internal phase) [176]. The external phase was an aqueous resorcinol-formaldehyde precursor solution (30–40% organics) and the internal phase was silicon oil. A mesoporous structure was generated within the walls of the macroporous polyHIPE. Interestingly, lower SSAs and higher void sizes and interconnecting hole sizes were produced by using a more viscous silicon oil. The porous structures of the carbons with porosities of around 85% were similar to those of typical polyHIPEs. The mesoporous walls seen in Fig. 53 often result from the incorporation of porogens into the external phase. The mesopore diameters ranged between 5 and 8 nm, the void diameters between 0.7 and 2.1 ␮m, and the interconnecting hole diameters between 0.18 and 0.53 ␮m. Increasing the pyrolysis temperature, from 800 to 1200 ◦ C, enhanced the graphitization and produced an increase in SSA, from 155 to 254 m2 /g, and an increase in conductivity, from 6.6 to 33.7 S/m. Similarly, porous carbon was produced through the pyrolysis of poly(furfuryl alcohol) polyHIPEs [62]. Porous carbon electrodes were synthesized by coating the silica polyHIPEs described previously with a THF solution of phenol-formaldehyde resin, polymerizing the resin, removing the silica using an acid etch, and pyrolyzing in nitrogen [177]. The membranes were modified with glucose oxidase and an osmium redox polymer for biofuel cell applications. The glucose electro-oxidation current was 13-fold greater on the porous carbon electrode generated using a silica polyHIPE template than on commercial glassy carbon for the same enzyme loading. Porous carbon monoliths were synthesized in a similar manner using two different loadings of the pre-polymer in THF, 25 and 80 wt% [178]. The monolith synthesized using the 80% pre-polymer solution exhibited a microporous volume of 0.34 cm3 /g and a specific surface area of 459 m2 /g. The monolith synthesized using the 25% prepolymer solution exhibited almost twice the microporous volume and twice the SSA. These carbon monoliths were infiltrated with a methyl tert-butyl ether solution of LiBH4 , a potential hydrogen storage material. Hydrogen release for bulk LiBH4 occurs at too high a temperature (above 400 ◦ C) for a safe hydrogen storage process. Here, the LiBH4 exhibited crystalline characteristics for the less microporous carbon monolith and was amorphous for the more microporous carbon monolith. The more microporous carbon monolith exhibited a strong minor dehydrogenation peak at 60 ◦ C and a main dehydrogenation peak at 270 ◦ C, as opposed to 370 ◦ C for the less microporous carbon monolith. Unfortunately, attempts to rehydrogenate at 300 ◦ C

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Fig. 51. The porous structure of (SEM): (a) the original PS-based polyHIPE; porous alumina monoliths from the calcination of hydrosol-filled polyHIPE at: (b) 600 ◦ C; (c) 1300 ◦ C. Reproduced with permission from Ref. [173]. Copyright 2009, Springer.

Fig. 52. The porous structure of (SEM): (a) a PAN-based polyHIPE; (b) porous carbon from the pyrolysis of the PAN-based polyHIPE. Reproduced with permission from Ref. [174]. Copyright 2011, Elseiver Ltd.

Fig. 53. Porous carbon with mesoporous walls whose structure is indistinguishable from that of the original polyHIPE (SEM). Reproduced with permission from Ref. [176]. Copyright 2010, the American Chemical Society.

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Fig. 54. A highly ordered hexagonal mesoporous structure (diameter around 2 nm) in the carbon walls that was generated by block copolymer templating within a silica polyHIPE (TEM). Reproduced with permission from Ref. [179]. Copyright 2011, the American Chemical Society.

were unsuccessful and higher temperatures were probably needed (600 ◦ C is needed for bulk LiBH4 ). In an interesting twist on the synthesis procedure described above, a dual template approach was developed wherein silica polyHIPEs were filled with a solution of a phenolic resin and a triblock copolymer surfactant [179]. Evaporation-induced phase separation and selfassembly in the polymer solution was used to form hexagonally ordered multiphase polymer coatings on the silica polyHIPEs. Calcination and silica removal (or vice versa) produced mesopore diameters between 2 and 6 nm (Fig. 54), SSAs between 600 and 900 m2 /g, porosities up to 80%, and conductivities between 200 and 400 S/m. The performance of these materials as electrodes for electrochemical capacitors or for Li ion battery negative electrodes was investigated. The carbon monoliths, dominated by microporosity, exhibited poor specific capacitance since the microporosity limited the electrolyte penetration and limited the fast diffusion of electrolyte ions through the material. The highest electrochemical capacitance was associated with a more mesoporous structure which exhibited a true double layer capacitance behavior and a nominal capacitance of around 20 F/g. When applied as the negative electrodes for Li ion batteries, the capacity delivered during the first discharge was 500–600 (mA h)/g and was, for the most part, irreversible. Upon charging, only part of the intercalated lithium could be extracted, producing reversible capacities of 120–160 (mA h)/g. 7. Conclusions The recent surge in polyHIPE publications and reviews mirrors that for porous polymers in general. This polyHIPE review has focused upon recent advances in synthesis (polymer chemistry, macromolecular structure, porous structure, surface functionalization, and nanoparticle stabilization), upon novel polyHIPE-based systems (bicontinuous polymers, nanocomposites, hybrids, porous ceramics, and porous carbons), and upon polyHIPE applications. The polymerization mechanisms available for

polyHIPE synthesis have been expanded to include FRP through UV or ␥-ray initiation, step-growth polymerization, ATRP, RAFT, and ROMP as well as thiol-ene and thiol-yne polymerizations. The list of monomers available for polyHIPE synthesis is quite long and is growing rapidly. This list now includes cyclic structures, nitrogen-containing structures, GMA (for subsequent functionalization), and brominated monomers (for subsequent ATRP initiation). A large variety of polyHIPEs based on NPstabilized HIPEs have been synthesized and the synergistic use of small amounts of surfactant and the synergistic use of functionalized NPs with multiple tasks have been explored. Bicontinuous polyHIPE systems have been synthesized within both W/O and O/W HIPEs, nanocomposites containing clay, CNT, magnetic NPs, and ceramic NPs have been synthesized, and silica-based polyHIPEs have been synthesized. In addition, polyHIPEs have been used as templates for the production of porous ceramics (silica, alumina) and for the production of porous carbons. The number of potential polyHIPE applications is increasing and includes supports and membranes (chemical reactions, separation, and absorption), tissue engineering, responsive and smart materials, and controlled release materials. The innovative developments in polyHIPEs described herein are presently being used to generate novel families of porous materials with pre-designed porous structures and unique properties. The success achieved in the development of new polyHIPE synthesis chemistries, wall materials, and porous structures has clearly established their potential for numerous applications and this, in turn, is now driving the continuing expansion and intensification of polyHIPE research and development. Acknowledgements The partial support of the Israel Science Foundation, the United States – Israel Binational Science Foundation, the German – Israeli Foundation for Scientific Research and Development, the Russell Berrie Nanotechnology Institute, the Grand Technion Energy Program, and the Technion VPR Fund is gratefully acknowledged.

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Professor Michael S. Silverstein holds the Sherman-Gilbert Chair in Energy at the Department of Materials Science and Engineering, Technion – Israel Institute of Technology, and is the Chairman of the Technion’s Interdepartmental Program in Polymer Engineering. Silverstein has investigated a plethora of novel emulsion-templated porous polymer systems and has recently developed two extraordinary materials, water-dropletfilled elastomers and porous shape memory polymers. He edited the book “Porous Polymers” (published in 2011), organized six “Porous Polymers” symposia in recent years, and is an editor for Polymer International and for the Journal of Polymer Engineering.

Please cite this article in press as: Silverstein MS. PolyHIPEs: Recent advances in emulsion-templated porous polymers. Prog Polym Sci (2013), http://dx.doi.org/10.1016/j.progpolymsci.2013.07.003