Emulsion Templating - American Chemical Society

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Perspective Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Emulsion Templating: Porous Polymers and Beyond Tao Zhang,†,‡ Rajashekharayya A. Sanguramath,† Sima Israel,† and Michael S. Silverstein*,† †

Department of Materials Science and Engineering, Technion−Israel Institute of Technology, Haifa 32000, Israel College of Textile and Clothing Engineering, Soochow University, Suzhou 215123, China

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ABSTRACT: Emulsion templating presently extends far beyond the original hydrophobic porous polymers that were synthesized within surfactant-stabilized water-in-oil high internal phase emulsions (HIPEs) by using free radical polymerization. This Perspective presents the extraordinary versatility of emulsion templating that has emerged with the growing numbers of HIPE systems, HIPE stabilization strategies, monomers, polymerization chemistries, multicomponent materials, and surface functionalities. Emulsion templating now goes far beyond “porous polymers” by encompassing the encapsulation of aqueous solutions, ionic melts, and organic liquids as well as by encompassing porous carbons and porous inorganics. Herein, we present comprehensive pictures of the state-of-the-art, of the prospective large-scale and niche applications, of the advantages and challenges for industrial scale-up, and of the crucial directions that should be pursued in future work. We demonstrate that it is emulsion templating’s considerable and versatile parameter space that offers opportunities for pioneering work, breakthrough innovations, scientific/engineering achievements, and industrial adoption.

1. INTRODUCTION A variety of approaches can be used to generate porous polymers: macromolecular design (porous frameworks, rigid structures with inherent microporosity), porogen incorporation, phase inversion, and templating.1−4 The available templating techniques include block copolymer (BCP) templating, solid particle templating, breath figure templating, and, the focus of this Perspective, emulsion templating.5,6 This Perspective provides an in-depth look at the current state of “emulsion templating”, noting the conceptual breakthroughs that led to the evolution of emulsion templating from a process that had a limited scope to a process that has a continually expanding scope. The original application of emulsion templating was to synthesize hydrophobic cross-linked porous polymer monoliths known as “polyHIPEs”. A polyHIPE was usually synthesized through the free radical polymerization of monomers in the external phase of a water-in-oil (w/o) high internal phase emulsion (HIPE), a highly viscous, paste-like emulsion containing an internal phase content >74% (the maximum packing fraction of monodisperse, undeformable spheres).5,7,8 Such high internal phase contents in emulsions can be reached by deforming monodispersed droplets into polyhedra or by forming a polydisperse distribution of droplet sizes.9 The emulsion’s two-phase structure serves as a template for the structure of the resulting porous polymer monolith, hence the terms “emulsion templating” and “polyHIPE” (polymerized HIPE, PH). The developments in emulsion templating, described in a number of recent review articles, have revealed a world of possibilities.10−19 Recent work on emulsion templating has gone far beyond the original PH systems and © XXXX American Chemical Society

now includes a variety of novel emulsion systems, porous polymers generated by using internal phase contents as low as 25%, an extensively expanded polymerization chemistry toolbox, a wide variety of hydrogel PHs (HG-PHs), noncross-linked PHs, PH beads and fibers, porous carbons, porous inorganics, and encapsulation systems. The objective of this Perspective is to generate an understanding of where emulsion templating stood originally, where emulsion templating stands now, and where emulsion templating could, given its great potential, arrive in the future. This Perspective will provide a broad overview of the recent advances in emulsion templating, with the individual emulsion-templated systems described in general terms, and additional details available in the cited articles. The first “emulsion” + “polymer” association that usually comes to mind is “emulsion polymerization”. Typically, emulsion polymerization takes place in oil-in-water (o/w) emulsions where hydrophobic monomers form the dispersed “internal” phase, usually between 10 and 30% of the emulsion. The monomers are emulsified within water (the continuous “external” phase) by using a water-soluble surfactant and polymerized by using a water-soluble radical initiator. A simplified description has the polymerization beginning with monomer molecules that have been solubilized within surfactant micelles through initiation at the oil−water interface. The polymerization continues within monomer-swollen polymer nanoparticles (NPs) whose final diameters typically range Received: December 3, 2018 Revised: June 20, 2019

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DOI: 10.1021/acs.macromol.8b02576 Macromolecules XXXX, XXX, XXX−XXX

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windows). This terminology has been adopted to prevent confusion when discussing PHs with microporous walls for adsorption applications or when discussing PHs containing biological cells for tissue engineering applications. The International Union of Pure and Applied Chemistry classifications for porosity will be used: microporous (50 nm). This Perspective will identify the key challenges associated with emulsion templating and suggest how future research can be used to address these challenges and to expand the applicability of emulsion templating to other material systems and other applications. Recent emulsion templating research involves emulsion chemistry, polymer chemistry, polymer engineering, and even mechanics (microfluidics is now being used to generate PHs with tunable void sizes and narrow void size distributions to aid in the development of theoretical models to describe the relationships between void size and mechanical behavior).23 Since emulsions are thermodynamically unstable, they will, eventually, undergo phase separation. In HIPEs, the major phase is dispersed within the minor phase, and therefore, the tendency toward phase separation, or phase inversion, is even greater. The advances made in emulsion stabilization have been critical for achieving higher internal phase contents, for forming a wide variety of emulsion types (beyond w/o), for extensively expanding the monomer and polymer chemistry toolbox, and for enabling the functionalization of PH surfaces. It is, therefore, appropriate to begin this Perspective by describing the advances and challenges in emulsion stabilization and formation before discussing the advances and challenges in polymer chemistry, postsynthesis functionalization, material systems “beyond polymers”, applications, and prognostications.

from tens to hundreds of nanometers. Emulsion polymerization can also be used to synthesize hydrophilic polymer NPs using “inverse” w/o emulsions. The second “emulsion” + “polymer” association that usually comes to mind is microencapsulation, where hydrophobic liquids are encapsulated within micrometer-scale particles. Microencapsulation typically takes place within o/w emulsions by using interfacial step-growth polymerizations that involve isocyanates in the internal phase. There are several important differences between typical emulsion templating systems and the typical emulsion polymerization/microencapsulation systems described above. Emulsion templating typically takes place within w/o emulsions, while emulsion polymerization and microencapsulation typically take place within o/w emulsions. HIPEs are usually defined as having internal phase contents >74% (although internal phase content limits of 70% and 64%, the maximum for random close packing, have also been suggested),18,20 while emulsion polymerization and microencapsulation typically use significantly smaller internal phase contents. The reactions in emulsion templating typically occur within the external phase, while the reactions in emulsion polymerization typically occur in monomers solubilized within surfactant micelles (and later in monomer-swollen polymer NPs), and the reactions in microencapsulation typically occur at the oil−water interface. Emulsion templating typically generates monoliths, while emulsion polymerization and microencapsulation typically generate particles. There are often important differences between the structure of the HIPE, which consists of dispersed, individual droplets, and the structure of the resulting PH. For example, droplet coalescence and/or Ostwald ripening can occur during polymerization, especially when the elevated temperatures used for polymerization enhance diffusion and interfacial destabilization. An integral, and defining, difference between the structures of PHs and the original HIPEs are the ruptures in the polymer walls that develop at the thinnest points of the external phase envelopes that surround 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 PH. Typically, the macroporous structures in PHs are generated through the removal of the internal phase, leaving air-filled “voids” in place of the evacuated droplets. The voids, whose diameters can range from a few micrometers to hundreds of micrometers, form an open-cell structure, connected by the holes in the polymer walls. Open-cell, interconnected, porous structures have also been obtained at significantly lower internal phase volume fractions, since the formation of the interconnecting holes is dependent upon a large number of synthesis parameters. Often, “medium internal phase emulsion” (MIPE) is used to describe internal phase volume fractions of 30−74%, and “low internal phase emulsion” (LIPE) is used to describe internal phase volume fractions of 50%.78 Porogens, such as solvents, have been used to influence the void interconnectivity.79−81 The effects of porogenic solvents on the PH’s internal structure were studied for Span-80stabilized w/o HIPEs. PHs with relatively low specific surface areas (SSAs) (312 m2/g) were obtained by using relatively polar solvents, since they enabled the transport of water through the continuous phase. PHs with higher SSAs (554 m2/ g) were obtained by increasing the miscibility of the solvent in the evolving polymer networks. Soxhlet extraction was found to have a significant effect on the SSA and on the mechanical properties.81 After extraction with isopropanol, the SSA of a PH based on divinylbenzene (DVB) was doubled, although the mechanical properties degraded for extractions longer than 12 h. The dispersed phase content also affects the void interconnectivity. Closed-cell structures were fabricated from HIPEs with 75% internal phase stabilized by using a BCP, while open-cell structures were fabricated by increasing the internal phase content to 83%. Similarly, the void interconnectivity could be enhanced by increasing the amount of BCP surfactant.82 The void interconnectivity in PHs from Pickering HIPEs is usually clear-cut. The droplets in Pickering HIPEs are usually significantly larger than those in surfactant-stabilized HIPEs, and thus, the monomer-polymer film at the closest point of contact between two adjacent droplets is significantly thicker. This increase in polymer wall thickness impedes the formation of interconnecting holes. The stabilizing particles are usually strongly adsorbed at the oil−water interface, preventing droplet coalescence and leading to the formation of closed-cell structures. This strong adsorption prevents the particle content, the internal phase content, and the presence of porogens from having significant effects on HIPE stabilization and on void interconnectivity. Open-cell PHs from Pickering HIPEs have been produced under specific conditions. Strong interaction between the monomer and the particulate stabilizer can lead to the formation of interconnecting holes.36,83 PHs with interconnected structures were fabricated from lignin-stabilized o/w HIPEs containing a melamine−formaldehyde prepolymer in the external phase as a monomer, with the interconnectivity increasing with increasing prepolymer content. The interaction between the prepolymer and the lignin drew the particles from the oil−water interface into the walls, leading to the formation of interconnected structures. The addition of surfactants to Pickering HIPEs was found to significantly reduce the average droplet diameter, and thus the wall thickness, leading to the formation of interconnecting holes.84 Modifying the particle amphiphilicity can also be used to produce interconnectivity. Highly interconnected, hydrophobic PHs could be synthesized within w/o HIPEs when the stabilizing SiO2 NPs were modified by using n-octadecyltrimethoxysilane,52 and highly interconnected HG-PHs were synthesized within o/w HIPEs when the stabilizing graphene oxide NPs were modified by using cetyltrimethylammonium bromide.46 Stabilization with two different types of particles, where one of the particles is a porogen that can be removed from the polymer wall following polymerization, can be used to directly produce interconnectivity. Closed-cell PHs were formed from a HIPE that was stabilized by both charged polystyrene particles and inorganic silica particles, and interconnected structures were then realized F


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4.1. Free Radical Polymerization. Typically, PHs are synthesized via thermally initiated FRP, a relatively simple reaction that is easy to perform, convenient, applicable to a wide range of vinyl monomers, and performed under relatively mild conditions (a requirement for many biology-medical-related applications). The relatively rapid decomposition of an initiator (2,2′-azobis(isobutyronitrile)) in the presence of a catalyst (cetyltrimethylammonium bromide) was used to enable the polymerization of styrene at room temperature89 and of MMA at 40 °C,90 temperatures considerably lower than those typically used for thermal initiation. In addition, several redox initiation systems (benzoyl peroxide with trimethylaniline,91 ammonium persulfate with tetramethylethylene diamine,92 and ascorbic acid/iron(II) sulfate heptahydrate/hydrogen peroxide93) were adopted to achieve faster curing at moderate temperatures. The two main reaction parameters that, essentially, determine the macromolecular structure, the porous structure, and the properties of FRP-synthesized PHs, namely, the cross-linking strategy and the locus of initiation (schematically described in Figure 3), are described in the following

a number of PH-based thermal energy storage-release systems have been described recently, including hydrophobic structures through organic-phase initiation in w/o HIPEs,70 hydrophilic structures through interfacial initiation in o/w HIPEs,87 and amphiphilic structures through interfacial step-growth polymerization in o/w HIPEs.12,88 The challenge of future work is to intelligently manipulate the interfacial stabilization strategy for the a priori design of the PH’s internal phase continuity.

4. POLYMERIZATION CHEMISTRY Precise control of the PH’s macromolecular structure is of paramount importance for practical applications. Myriad polymerization methods were adopted to rationally design the PH’s macromolecular structures in an attempt to harness optimal properties. Conventional FRP is, by far, the most widely applied polymerization mechanism used to synthesize PHs. FRP, however, is limited to monomers with reactive double bonds (typically styrenics and (meth)acrylates), is limited in the homogeneity of the resultant polymer networks and is limited in the available reaction conditions. There are a variety of controlled radical polymerizations (CRPs) available, such as ATRP (and various versions of ATRP such as reverse ATRP and AGET) and reversible addition−fragmentation chain-transfer (RAFT) polymerization. CRPs have been used as a complementary and/or alternative method to overcome some of the limitations associated with FRP, for example, to enhance network homogeneity. In addition to radical polymerizations, the polymerization reactions used for PH synthesis include step-growth polymerization, Diels−Alder polymerization, ROMP, and the thiol−ene/thiol−yne click reactions. A schematic summary, listing the most common polymerization mechanisms used for PH synthesis, the most commonly associated monomeric systems, and the mechanism’s relative popularity, is shown in Figure 2.

Figure 3. Schematic illustration showing the possible effects of the locus of initiation on the porous morphology. External-phase initiation tends to produce open-cell structures, while interfacial initiation tends to produce closed-cell structures (taken after Figure 3 of ref 103).

sections. A “flow chart” describing, via micrographs, the effects of the surfactant content,78 the locus of initiation,35,94−97 and various other factors12,98,99 on the formation of “truly closedcell” or “quasi-closed-cell” structures is presented in Figure 4. 4.2. Cross-Linking Strategy. Given the extensive scope for PH applicability in various fields, a great deal of recent research has been focused on fine-tuning the mechanical properties. The various strategies explored include increasing the PH density,21 introducing physical reinforcement using polymeric particles

Figure 2. Schematic summary listing the most common polymerization mechanisms used for PH synthesis with the most commonly associated monomeric systems. The counterclockwise arrow indicates the mechanism’s relative popularity. G


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Figure 4. “Flow chart” showing, via micrographs, the various experimental conditions that can be used to produce closed-cell structures: (a) w/o HIPEs; (b) o/w HIPEs. (A1) Low surfactant contents (3−5% with respect to the monomers); (A2) interfacial initiation using BCP surfactants; (A3) interfacial initiation producing polyhedral voids; (B1) low-viscosity HIPEs; (B2) interfacial initiation using macro-RAFT agents as surfactants; (B3) external-phase initiation producing spherical voids; (C1) PHs with high degrees of cross-linking; (C2) interfacial initiation using microfluidics for HIPE formation; (D1) interfacial step-growth polymerization; (D2) interfacial initiation using macro-RAFT agents. (A1) Reproduced with permission from ref 78. (A2) Reproduced with permission from ref 35. (A3, B3) Reproduced with permission from ref 97. Copyright 2010 John Wiley and Sons. (B1) Reproduced with permission from ref 98. (B2) Reproduced with permission from ref 95. Copyright 2015 Springer-Verlag. (C1) Reproduced with permission from ref 99. Copyright 2008 Royal Society of Chemistry. (C2) Reproduced with permission from ref 94. (D2) Reproduced with permission from ref 96. Copyright 2016 Royal Society of Chemistry. (D1) Reproduced with permission from ref 12. Copyright 2017 Elsevier.

linked, with the cross-linking providing the mechanical stability needed to maintain the monolithic structure and prevent collapse during polymerization, drying, and purification. Cross-

and inorganic particles (SiO2, CNTs, TiO2, nanoclay, and magnetic NPs),11,50,100 using rigid monomer structures, and modifying the cross-linking strategy. Almost all PHs are crossH


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closed-cell structures for all concentrations of amphiphilic BCP surfactants (Figure 4, A2).35 The mechanisms through which the locus of initiation can influence the PH morphology and properties were revealed in a recent investigation of PHs from w/o HIPEs. The HIPE was prepared by using a microfluidic lab-on-a-chip to produce monodisperse droplets, resulting in highly ordered structures.94 Highly ordered, open-cell structures were produced by using organic-phase initiation. Highly ordered, closed-cell structures with polyhedral voids and relatively thick walls were produced by using interfacial initiation at various surfactant concentrations (5−30 wt %), dispersed phase contents, and polymerization temperatures (50−70 °C) (Figure 4, C2). The droplet shapes in HIPEs can range from polyhedral, for monodisperse droplets, to spherical, for a broad distribution of droplet sizes. Polymerization within Pickering HIPEs often produces closed-cell PHs, irrespective of the type of initiation (Figure 4, A3 and B3).11 The locus of initiation can, however, have a significant effect on the void morphology, with externalphase initiation producing spherical voids and interfacial initiation producing polyhedral voids. For interfacial initiation in a w/o HIPE stabilized with silane-modified silica NPs, the gel point was reached relatively rapidly, “locking-in” the original polyhedral droplets before extensive droplet coalescence and/ or Ostwald ripening could occur. On the other hand, for external phase initiation in the same HIPE, the PH had larger, spherical voids, indicating that more extensive droplet coalescence and/or Ostwald ripening occurred before the gel point could “lock in” the droplet shape.44,97 In addition, for interfacial initiation, the stabilizing NPs were found within the void walls instead of on the void surfaces. The monomer depletion at the interface generated by the polymerization, and the resulting diffusion of monomer toward the growing polymer chains, seems to drive the NPs into the external phase. The locus of initiation might also affect the fine structure of the PH in the case of polymerizations in w/o emulsions that produce polymers that are significantly more hydrophobic than their monomers.108 In such a system, initiation within the external phase yields a relatively rapid polymerization and the formation of an interconnecting hole structure. Interfacial initiation, however, yields a relative slow polymerization that allows phase inversion and the formation of an o/w emulsion to occur, producing a granular structure. 4.3.3. NP-Based Initiation. Inorganic or polymeric NPs bearing chlorine or bromine on their surfaces can serve as initiation centers for AGET ATRP, whether dispersed within the organic phase in surfactant-stabilized HIPEs or acting as the HIPE stabilizer at the oil−water interface in Pickering HIPEs.11,44,97,109 The porous structures of the PHs from surfactant-stabilized HIPEs were highly interconnected, whether using molecular-based or NP-based AGET ATRP initiators that were dispersed in the organic phase. For PHs from Pickering HIPEs, on the other hand, NP-based AGET ATRP initiation produced a polymer with a high glass transition temperature (Tg), spherical voids, and NPs on the void surfaces, similar to the results from FRP using external-phase initiation but unlike the results from external-phase AGET ATRP initiation using an organic-soluble initiator. 4.3.4. Photoinitiation. PHs have predominantly been synthesized by using thermal initiation. Thermal initiation, however, is not suitable for thermally sensitive or highly reactive monomers, for HIPEs that are relatively unstable, for applications involving biocatalysis, biosensors, and biomedi-

linking has a significant effect on the mechanical behavior (e.g., the modulus, the fracture strength, and the fracture strain). While PHs based on styrene and cross-linked with DVB are rigid and brittle, making them unsuitable for applications requiring structural flexibility and robustness, using a more flexible cross-linker, such as polyethylene glycol dimethacrylate (PEGDMA) or polyurethane diacrylate, generates PHs that are significantly more robust.101,102 4.3. Locus of Initiation. FRP initiation can take place either within the external phase (using an external-phasesoluble initiator) or at the HIPE’s oil−water interface (using an internal-phase-soluble initiator), and the locus of initiation can have a profound effect upon the macromolecular structure and upon the porous structure (as illustrated in Figure 3). For surfactant-stabilized HIPEs, the macromolecular structures of the resulting PHs tend to be strongly dependent upon the locus of initiation, while the porous structures tend to be less strongly dependent. For Pickering HIPEs, the locus of initiation can have a determining impact upon the porous structure and, hence, on the properties of the resulting PHs.11,103 While there is no definitive set of clear guiding principles governing the effects of the locus of initiation upon the porous morphology, the “flow-chart” guide in Figure 4 provides a useful starting point. 4.3.1. External-Phase Initiation. An open-cell structure that is typical of PHs is almost always the outcome of polymerizations that are initiated within the external phase (Figure 3). The interconnecting holes that form at the gel point “lock-in” the wall structure,104 with the size and number of such holes determining the interconnectivity. The interconnectivity can be enhanced by using monomers that undergo a higher degree of volume shrinkage during polymerization (methyl acrylate and styrene exhibit 22 and 14% shrinkage, respectively),105 by increasing the volume fraction of the internal phase, and/or by selecting a HIPE stabilization strategy that will reduce the droplet size and, therefore, the wall thickness. In many cases, the interconnectivity, the mechanical strength, the crystallinity (for crystallizable monomers), and the polymer bicontinuity (in bicontinuous PHs from HIPEs with monomers in the internal phase) are strongly dependent upon the locus of initiation.106,107 4.3.2. Interfacial Initiation. For interfacial initiation, with the initiator added to the internal phase in Figure 3, polymeric films rapidly form around the internal phase droplets. Since the polymeric film around the droplet can already be inviolable when the volume shrinkage within the walls becomes significant, there is a tendency for more closed-cell structures to be formed. PHs often have “quasi-closed-cell” structures, structures which appear closed cell in the SEM, but from which the internal phase can be removed easily, indicating the presence of an evacuation pathway. The polymer wall thickness can be manipulated by adjusting the volume fraction of the internal phase and by judiciously selecting the stabilization strategy. A seminal investigation of interfacial initiation and its effect upon the porous morphology demonstrated that the concentration of conventional surfactants with respect to the monomers was the determining factor (a closed-cell structure resulting from a low surfactant content is shown in Figure 4, A1).78 In general, the droplet size decreases, and the number of droplets increases, with increasing surfactant concentration, reducing the thickness of the polymer film surrounding the droplets to such an extent that it can be easily ruptured. On the other hand, interfacial initiation was demonstrated to produce I


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Macromolecules cine, and for additive manufacturing techniques.110−112 Photoinitiation, on the other hand, is especially advantageous, offering rapid reactions at room temperature for a wide range of polymerization chemistries. Photoinitiation was successfully applied to enable the thiol−ene and thiol−yne “click” chemistries and used to generate PHs for biomedical applications with biodegradable ester groups in the polymer backbones.113−115 Open-cell PHs are usually required for biorelated applications, and therefore, the locus of initiation is usually in the external phase. Open-cell, acrylate-based PHs for biocatalysis were synthesized by using photoinitiation since UV-initiated systems are easily applied to (meth)acrylate monomers.110 The key criteria for tissue engineering scaffolds usually include an appropriate void size (usually >50 μm) and (bio)degradability. Degradability was achieved via thiol−ene “click” photopolymerization of multifunctional thiols and acrylates, yielding macromolecular networks with ester-based degradable cross-linking.113,114 Porous polyester PH membranes were also prepared by using photoinitiated thiol−ene polymerizations.116 Photopolymerization is only efficient when the sample sizes are small and relatively transparent. Light transmittance is hampered in larger samples owing to their opacity, resulting in incomplete polymerization. The refractive indices of the two phases can, however, be tuned to match, producing isorefractive HIPEs with enhanced transparency. Isorefractive w/o HIPEs were obtained by adding glycerol to the aqueous phase to match the refractive index of the organic phase.117 Relatively large PHs could then be produced using photoinitiation, taking full advantage of the transparency of the isorefractive HIPEs. 4.4. Controlled Radical Polymerization. RAFT and ATRP, two of the most important CRP techniques, have been adopted for PH synthesis. CRP enables the synthesis of more homogeneous network structures and more complex macromolecular architectures. 4.4.1. RAFT. RAFT, a versatile polymerization technique, is routinely used to prepare well-defined polymers with wide ranging functionalities, as described in a recent “Perspective”.118 The application of RAFT to PH synthesis has been used to enhance control over the porous structure, to improve the mechanical properties, and to tailor the functional groups on the void surfaces.95,96,119,120 The macromolecular heterogeneity from conventional FRP has been associated with a Young’s modulus that is less than that predicted from mechanics.121 CRP is expected to enhance macromolecular homogeneity and produce more uniform network structures.119 PHs with controlled porosities and enhanced mechanical properties were prepared through an astute selection of a RAFT agent and its concentration.119 Amphiphilic macroRAFT chain transfer agents, comprised of BCPs with precisely controlled block ratios, were employed as the sole stabilizer for the preparation of PHs within both o/w and w/o HIPEs.95,96 The concentration of the macro-RAFT agent, the pH, and the locus of initiation all affected the porous structures of the resulting PHs (Figure 4, D2).96 The functional groups of the macro-RAFT agent surfactant, which were located on the void surfaces, were then utilized for subsequent modifications. In spite of its advantages, RAFT has not been applied widely for PH synthesis. Further studies on the design and use of effective chain transfer agents for PH synthesis would be extremely valuable. 4.4.2. ATRP. There are only a few reports of PH synthesis using AGET ATRP.44,109,122 Various aspects of the PH

synthesis, the nature of the HIPE stabilization (surfactant or NPs), the locus of initiation (interfacial, external-phase, or NPbased), and the design of the catalyst system were investigated.44 The locus of initiation seemed to have little effect upon the porous structures of the PHs synthesized within surfactant-stabilized HIPEs. Typical PH open-cell structures were obtained whether the initiator was dissolved in the external phase or located on the surfaces of NPs dispersed in the external phase. The locus of initiation, however, had a significant effect on the PHs synthesized within Pickering HIPEs. The lower Tg (−20 °C) for interfacial initiation (compared to 5 °C for NP-based initiation) indicated the formation of a polymer network that was enriched in the more surface active monomer (2-ethyhexyl acrylate (EHA), as compared to DVB, the cross-linking comonomer). The nature of the ATRP catalyst system also plays an important role in PH synthesis and affects the resulting properties. Fully degradable PHs were prepared by ATRP copolymerization of 2-ethylhexyl methacrylate (EHMA) and bis(2-methacryloyloxyethyl) disulfide, a degradable cross-linker, within a w/o HIPE.122 Interestingly, full degradability was achieved by using a relatively hydrophobic ATRP catalyst system which was uniformly distributed in the external phase. A nondegradable PH with a lower Young’s modulus, on the other hand, was produced by using a less hydrophobic ATRP catalyst system that was preferentially located near the interface. Therefore, more uniform cross-linked networks and higher Young’s moduli can be produced by distributing the catalyst system throughout the external phase.122 For ATRP polymerizations within Pickering HIPEs, closedcell structures with relatively large, polyhedral voids (around 120−210 μm) were produced by using interfacial initiation, while smaller, relatively spherical voids (75−120 μm) were produced by using NP-based initiation.44 The locus of initiation also affected the location of the stabilizing NPs within the PH walls, with the NPs driven into the polymer wall for interfacial initiation and “anchored” to the oil−water interface for NPbased initiation. In spite of the clear advantages of ATRP, its application to PH synthesis has been relatively small (although immobilizing ATRP initiators on the void surfaces has been used for postsynthesis modification).123−125 Adapting ATRP for PH synthesis, expanding the inventory of catalyst systems suitable for the various HIPE environments, should be the focus of future work on the development of PH polymerization chemistries. 4.5. Step-Growth Polymerization. 4.5.1. PUU. The synthesis of polyurethanes, a step-growth polymerization between polyols and diisocyanates, is complicated by the inevitable reaction of the diisocyanates with the dispersed aqueous phase in w/o HIPEs, producing urea groups and generating CO2. The parallel urethane and urea reactions within HIPEs yield PUU PHs. PUU PHs were fabricated by using a biodegradable poly(ε-caprolactone) (PCL) triol and a diisocyanate, with the urea content varied by changing the hydroxyl-to-isocyanate molar ratio.126 Control of the urethane to urea ratio was achieved by using an “end-capping” reaction, reacting the polyol with excess diisocyanate before HIPE formation. Following HIPE formation, the “end-capped” polyol reacted with water to form a PUU PH with a urethane-to-urea ratio of 2:1.126 The PUU PH scaffolds for soft tissue regeneration based on “end-capped” PCL diols and triols were elastomeric, biodegradable, injectable, and curable at physiological temperature (37 °C), with the porous structures J


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addition reactions in HIPEs, furan derivatives, such as polycaprolactone functionalized with pendent furan moieties or poly(styrene-co-furfuryl methacrylate) copolymer particles, along with a difunctional 1,6-bis(maleimide) hexane crosslinking dienophile, were used. Here again, as seen for so many of these polymerization chemistries, there are only isolated “proof of concept” instances of Diels−Alder reactions, leaving the field open for development. 4.6.3. Ring-Opening Polymerization (ROP) and ROMP. ROMP was used to synthesize PHs based on strained cycloalkene monomers such as norbornene (NB) (and its derivatives), dicyclopentadiene (DCPD), and cyclooctene.134−137 The resulting open-cell PHs exhibited outstanding mechanical properties, with moduli as high as 159 MPa for the PHs based on DCPD.135 The mechanical properties could be tuned either by changing the porosity (the internal volume fraction) or by adding non-cross-linking monomers such as NB.136,138 P(DCPD-co-NB) PHs used as separator membranes for lithium-ion batteries exhibited resilience (after 100 charge/ discharge cycles) that was similar to that of commercial polyolefin-based separators.139 Eventually, the PDCPD underwent oxidative embrittlement which degraded the mechanical properties.139 In other work, PHs with functionalized surfaces were produced by covalently incorporating a surfactant into polycyclooctene networks.137 The efficiency of the ROMP reaction in a HIPE depends upon the nature of the ROMP catalyst, the temperature, and the duration of the reaction. ROP of cyclic monomers within HIPEs has met with limited success. Pentadecanolide, a lactone monomer, was polymerized in a w/o HIPE by using a water-soluble enzyme catalyst, Lipase TL.140 While the monomer was in the external phase, the ROP occurred at the interface since the catalyst was located in the internal phase, with both the rate and the degree of polymerization increasing with increasing catalyst concentration. 4.7. Renewable Resource Monomers. Emulsion-templated polymers can also be generated from renewable resource (usually plant-based) monomers. Polymers based on tannin, templated within o/w HIPEs and cross-linked by using an acidcatalyzed polycondensation reaction, with either a furfuryl alcohol resin141 or hexamethylenetetramine,142 formed highly stable cross-linked polymer networks which were subsequently carbonized to generate porous carbons. The PHs had densities between 0.46 and 0.47 g/cm3, Young’s moduli ranging from 50 to 110 MPa, and compressive strengths ranging from 8 to 11 MPa. Emulsion-templated polymers were produced from kraft black liquor (an aqueous solution of lignin residues that is a pulp and paper waste product) mixed with epichlorohydrin (the cross-linker) in the external phase and either 1,2-dichloroethane or castor oil in the internal phase.143−145 Macroporous carbons with high SSAs, indicating micropores and mesopores, were subsequently generated through carbonization. In addition, porous NC-PHs that were based on a renewable resource monomer (acrylated epoxidized soybean oil) were templated within w/o Pickering emulsions stabilized by using renewable resource nanofibrils (hydrophobized bacterial cellulose).146 The hydrothermal carbonization (HTC) of biomass-based HIPEs has been used to generate macroporous carbonaceous monoliths from the cellulosic fraction of biomass and the bark of fruit trees147,148 and even from glucose.12 Interfacial step-growth polymerization within o/w HIPEs was used to synthesize PUU PHs for tissue engineering and encapsulation applications. The highly porous monoliths were

and the mechanical properties of the PHs depending upon the triol-to-diol ratio and upon the catalyst concentration.127 4.5.2. PDMS. Polydimethylsiloxane (PDMS) PHs were synthesized by using a hydrosilylation reaction between the Si−H groups of a siloxane oligomer, polymethylhydrosiloxane, and the vinyl groups of the vinyl-containing cross-linking monomers.128 In general, stable, open-cell monoliths were obtained for Si−H/vinyl ratios between 0.8 and 1.3. Recently, closed-cell elastomeric monoliths based on PDMS were prepared by using an aqueous solution of NaHCO3 (a foaming agent) as the internal phase.71 The PHs were inflated under reduced pressures and could be expanded to 30 times their original size. Open-cell PDMS-based PUU PHs were generated by using the reaction between PDMS diols and a diisocyanate.12 4.5.3. Urea−Formaldehyde. The reaction of urea with formaldehyde produces urea−formaldehyde polymer networks. Highly interconnected, open-cell, urea−formaldehyde PHs were synthesized within Pickering HIPEs stabilized by using lignin particles and were used as adsorbents for the recovery of toxic phenolic compounds.129 The PHs exhibited ample functional groups (hydroxyl and carbonyl) on their surfaces and were successful in binding phenolic compounds for four adsorption−desorption cycles. 4.6. Other Polymerizations. 4.6.1. Click Chemistry. Thiol−ene “click” chemistry has been applied (typically using equimolar amounts of −SH groups and CC groups) to prepare functional PHs. The ambient temperature photopolymerization of trithiols and triacrylates has been used to produce highly cross-linked PHs.115 Higher order acrylates, such as pentaacrylates and hexaacrylates, were added to enhance the mechanical properties by increasing the crosslink density,114,116 producing moduli that were nearly 100 times greater than those of the PHs based on triacrylates alone. The resulting network structures were (bio)degradable, since they contain ester groups in their macromolecular backbones, and were successfully employed as tissue engineering scaffolds for human keratinocytes and murine fibroblasts.113,114 Thiol−ene “click” chemistry and photopolymerization were advantageous for preparing thin, ester-containing membranes via doctor blading, since the relatively rapid polymerization “locks in” the open-cell porous structure.116 Interestingly, some acrylate FRP also occurs during the thiol−ene photopolymerization, producing −SH-functionalized surfaces that can be used for postsynthesis modifications.130 There has only been limited work on PH synthesis using thiol−yne “click” polymerizations. Such PHs exhibited higher cross-link densities and, hence, higher moduli than similar PHs synthesized by using the thiol−ene reaction.115 Recently, PHs were synthesized within o/w HIPEs by using a copper(I)catalyzed 1,3-dipolar cycloaddition of difunctional azides and trifunctional terminal alkynes.131 The resulting open-cell PHs exhibited a strong affinity for copper(II) ions, with adsorption capacities as high as 52 mg/g. 4.6.2. Diels−Alder. In its simplest form, the Diels−Alder reaction forms cyclohexene adducts from a [4 + 2] cycloaddition between an electron-rich diene (such as furan derivatives) and an electron-deficient dienophile (such as maleic acid derivatives). Diels−Alder reactions, widely used to construct complex organic structures under moderate reaction conditions, were used to prepare conventional, open-cell PHs within surfactant-stabilized HIPEs132 and closed-cell PHs within Pickering HIPEs.133 To enable Diels−Alder cycloK


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Figure 5. Representative examples of the wide variety of PH shapes that are accessible (in some instances the original images were cropped and scale bars were added to enhance the figure). Beads generated using (A1) double emulsions, (A2) microfluidics, and (A3) frozen polymerization or sedimentation polymerization. Rods and fibers generated by using (B1) electrospinning, (B2) microfluidics, and (B3) 3D printing. Monoliths generated by using (C1) double emulsions, (C2) microfluidics, and (C3) casting. Complex shapes generated by using (D1) 3D printing. (A1) Reproduced with permission from ref 157. (A2) Reproduced with permission from ref 160. Copyright 2015 Royal Society of Chemistry. (A3) Reproduced with permission from ref 158. (B1) Reproduced with permission from ref 159. Copyright 2016 Elsevier. (B2, C2) Reproduced with permission from ref 161. Copyright 2016 Royal Society of Chemistry. (B3, D1) Reproduced with permission from ref 111. Copyright 2013 Wiley. (C1) Reproduced with permission from ref 156. Copyright 2016 Elsevier. (C3) Reproduced with permission from ref 163. Copyright 2011 Elsevier.

and films with ionic conductivities ranging from 4.0 to 9.0 mS/ cm that can be used as separator membranes for lithium ion batteries were produced by using a lithium salt in a lowviscosity, room-temperature ionic liquid as the internal phase.25,26 PHs based on (meth)acrylic and styrenic monomers in which 95% of the internal phase could be recovered by washing were produced by using deep eutectic solvents based on urea and choline chloride as the internal phase.27,150 In addition, it was only possible to produce PHs functionalized with hydroxyapatite NPs that migrated to the interface when the HIPE contained a deep eutectic solvent as the internal phase (the NPs did not migrate to the interface in w/o HIPEs).151 4.9. Future Directions. Vis-à-vis radical polymerization, it is imperative to develop a guiding framework that describes the influence of the locus of initiation over various aspects of the resulting PHs. In-depth studies that cover a range of HIPE systems, stabilization strategies, monomers, polymerization mechanisms, and experimental variables would prove highly beneficial for the rational design of PHs with tailored properties for specific applications through radical polymerization. The three main areas that are ripe for breakthroughs via the polymerization chemistry are described below.

based on a variety of renewable resource polymers including aqueous solutions of polysaccharides (e.g., sodium alginate, chitosan, dextran, and pectin) and/or polyphenols (e.g., tannic acid) in the external phase and solutions of isocyanates in the internal phase.12 In addition, the internal organic phase (e.g., paraffinic phase change materials for thermal energy storage and release, solvents, and oils) could be encapsulated by using specific combinations of polyphenols and polysaccharides (Figure 4, D1).12,88 4.8. Nonaqueous HIPEs. There have only been a handful of reports on the synthesis of PHs within nonaqueous HIPEs. Nonaqueous HIPEs can be particularly advantageous when the conditions of the desired polymerization mechanism include high temperatures, low pressures, and/or the absence of water. PHs were synthesized within HIPEs whose external phase consisted of a maleimide-terminated poly(aryl ether sulfone) and various comonomers that were dissolved in various polar organic solvents.149 Since the water miscibility of the polar organic solvents precluded the use of an aqueous internal phase, nonpolar petroleum ether was used instead. Nonaqueous HIPEs can also be advantageous when the properties of the internal phase are of integral importance to effective applicative performance. Recently, PHs have been developed by using ionic liquids25,26 or deep eutectic solvents27,28,150,151 as the HIPE’s internal phase. Highly interconnected PH monoliths L


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polymerization in a HIPE that has been dispersed as droplets within a continuous phase. Hydrophobic PH beads have been obtained from water-in-oil-in-water (w/o/w) double emulsions (dispersing a w/o HIPE in water) that was stabilized by using a single surfactant.157 The average diameters of such PH beads varied from several micrometers to hundreds of micrometers (Figure 5, A1), with the size determined by such factors as the pH of the water and the polarity of the organic phase. Double emulsions can also be used to generate PH monoliths. For example, a water-soluble monomer can be added to a water-intoluene-in-water HIPE and then polymerized (Figure 5, C1).156 5.2. Beads: Frozen Polymerization and Sedimentation Polymerization. PH beads have also been fabricated through frozen polymerization and sedimentation polymerization (Figure 5, A3). The size of the PH beads in both processes was determined by the diameter of the needle through which the HIPE droplets were extruded. Frozen polymerization involved using liquid nitrogen to freeze droplets of an o/w HIPE with an aqueous NiPAAm solution in the external phase and then polymerizing the NiPAAm at −20 °C with UV irradiation.31 The polymerization occurred within the external phase of the HIPE droplet, while the frozen state prevented the droplets from sticking together. Interestingly, the formation of ice crystals within the continuous phase could be used as a template for the formation of aligned wall structures using directional cooling.31 Sedimentation polymerization, on the other hand, was conducted by dripping o/w HIPE droplets containing an aqueous solution of acrylamide and N,N′methylenebis(acrylamide) (MBAAm) into a column containing mineral oil at a temperature that can initiate polymerization (60−90 °C).158 Since the polymerization begins at the surface of the HIPE droplets, the polymer at the surface prevents the polymer beads from sticking together at the bottom of the column. Both frozen polymerization and sedimentation polymerization generate PH beads that are usually a few millimeters in diameter. 5.3. Fibers: Electrospinning. Electrospinning, a powerful technique for producing nanometer-scale fibers, requires a viscous, polar, viscoelastic solution that maintains a relatively constant viscosity during the process. Polymer-based HIPEs can satisfy these requirements, and fibrous, bicontinuous polymers have been fabricated by electrospinning w/o HIPEs consisting of aqueous poly(vinyl alcohol) (PVA) solutions dispersed within PCL-in-toluene solutions.159 Highly porous bicontinuous polymer fibers could then be generated by simply allowing the solvents to evaporate (Figure 5, B1), without the need for polymerization. Increasing the dispersed phase fraction, decreasing the PCL content, and decreasing the PVA content all reduced the wall thickness and the fiber diameter. Highly porous, viscoelastic PCL fibers were obtained by the subsequent dissolution of the PVA in water. HIPEs based on oligomeric monomers, which usually have relatively high viscosities suitable for electrospinning, can be combined with rapid UV-initiated polymerization to produce PH fibers. 5.4. Beads, Rods, Fibers, and Monoliths: Microfluidics. Microfluidics has proven advantageous for generating homogeneous emulsions for the preparation of PHs. Recent endeavors have expanded this technique to predetermine the PH monolith structure and produce beads, rods, and fibers (Figure 5, A2 to C2).160 Monoliths with interconnected porous structures were then formed from the assembly of beads within the microfluidic channels.160 Porous polymer fibers were generated by using microfluidic wet spinning of a HIPE stabilized using a pH-

(1) Stereospecific PHs. PHs with highly interconnecting porous structures are suitable materials for separation applications.152 The achiral nature of PHs, however, makes them unsuitable for the stereospecific separation of organic compounds. PHs formed from enantiopure polymers and crystalline PHs formed from chiral monomers would be of great interest, but innovative chemistries will be needed to enable the synthesis of such materials.153 (2) Self-healing PHs. The irreversible covalent cross-linking in most PHs renders them unsuitable for applications that require self-healing. Supramolecular self-healing systems are often based on reversible associations, which, in PHs, remain relatively unexplored. Introducing reversible associations (hydrogen bonding, metal−ligand coordination, a host−guest system, or ionic interactions) into the macromolecular structure can be used to generate PHs for applications where self-healing would be advantageous. Judicious selection of the monomers and of the polymerization reactions can be used to generate PHs that combine both reversible cross-linking (for self-healing) and irreversible cross-linking (for mechanical stability). (3) Synergistic PHs combining individual networks. IPNs are two individual, physically intertwined, chemically unconnected networks that can be synthesized simultaneously or sequentially. Simultaneous IPNs can be synthesized through the formation of two independent networks by using two mutually exclusive polymerization mechanisms. Sequential IPNs can be synthesized through formation of the second network through polymerization within a monomer-swollen first network. Semi-IPNs are similar in concept, but only one of the polymers is crosslinked. Interconnected networks (ICNs), on the other hand, are two networks that are, to a certain extent, chemically interconnected. IPN-PHs and ICN-PHs are expected to exhibit synergistic properties that result from the influences of the networks upon each other. There have only been a few reports that describe the integration of different polymerization mechanisms to synthesize PHs that combine two independent polymer networks.154,155

5. POLYHIPE SHAPE Polymerization within the external, continuous phase of a HIPE is usually used to produce a monolithic PH. The external structure or shape of the PH can influence its applicative performance, and therefore, research on controlling the monolith shape (e.g., producing beads, rods, or fibers) is attracting considerable attention from both academia and industry. The PH shape can be controlled by formulating a more complex emulsion structure (e.g., a double emulsion), by controlling the polymerization environment (e.g., sedimentation polymerization (SP) or frozen polymerization (FP)), or by applying novel techniques (e.g., electrospinning, microfluidics, or 3D printing). PH beads, fibers, and monoliths with complicated architectures have been fabricated by using such methods. 5.1. Beads: Double Emulsions. PH beads may be fabricated by forming HIPE-based double emulsions, and a variety of double emulsions have already been formulated.156 The beads are synthesized through the external phase M


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Figure 6. Common functionalizations of PHs based upon (a) styrene, (b) VBC, and (c) (meth)acrylates (taken after Scheme 1 of ref 13).

sensitive branched copolymer and functionalized clay.161 Microfluidics have also been used to produce highly ordered HIPEs for the production of PH monoliths with well-controlled porosities and pore sizes.162 5.5. 3D Printing. 3D printing has become a powerful tool for manufacturing objects with complicated structures. PHs with complicated architectures have been fabricated by using HIPEs as inks, with both rigid and flexible tubular structures being realized.111 The porous morphologies and mechanical properties of the structures produced by 3D printing are similar to those of the corresponding PH monoliths. 3D printing enables PHs to be produced with complicated architectures and structures that are controlled independently on different length scales (Figure 5, B3 and D1). 3D printing is an emerging disruptive technology that will prove to be a powerful tool for accessing complicated PH shapes. Given the wide variety of PH porous structures, macromolecular structures, and surface functionalities, 3D printing will enable access to new levels of structure−functionality complexity. 5.6. Membranes. PH membranes can be generated quite easily by casting thin HIPE films and then polymerizing. PH membranes for protein purification, with thicknesses that ranged between 285 and 565 μm, were generated from HIPEs containing glycidyl methacrylate (GMA), EGDMA, and EHMA (Figure 5, C3).163

steps following PH synthesis. Recently, a great deal of progress has been made in enhancing the suitability of PHs for various applications through such postsynthesis functionalizations, either by changing the nature of the surface or by changing the nature of the PH itself. This section focuses on the functionalizations that can be effected following PH synthesis. The approaches to postsynthesis PH functionalization include changing the surface chemistry using surface-specific modifications,13 generating mesoporosity and/or microporosity (etching, hyper-cross-linking, surface activation, and porogen removal), or using the PH as a template for the generation of porous carbons and/or porous inorganics (as described in section 7).154,164,165 Although significant progress has been made in developing postsynthesis PH functionalization processes, there remain important challenges to the practicality of their adoption. The number of synthesis steps required to obtain a desired functionality can often be daunting and must be reduced. In addition, postsynthesis functionalization can often deleteriously affect the properties of the original PH, especially the mechanical properties (in many cases the PH becomes extremely brittle). Therefore, postsynthesis functionalization should strive to minimize the degradation in PH properties. 6.1. Surface Modification and Surface Enrichment. The nature of a PH surface can be modified by removing unreacted components following PH synthesis, producing changes in hydrophilicity−hydrophobicity, or by removing a porogen, producing changes in surface roughness.81,166 Simple, one-step, radical surface functionalizations for displaying −Br, −Cl, −OH, or −NH2 groups can be used as well as more

6. POSTSYNTHESIS FUNCTIONALIZATION The most common approach to enhancing PH functionality involves introducing functional components (usually monomers) into the HIPE and applying further chemistry/processing N


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6.2. Hyper-Cross-Linking. High SSAs are integral to a variety of porous polymer applications (supports for heterogeneous catalysts, stationary phases for chromatography, adsorbents) for which the highly interconnected, highly porous PH structures would be advantageous. Unfortunately, the SSAs of typical PHs, determined by using nitrogen adsorption, are only on the order of 10 m2/g (the SSAs of swollen PHs, determined by using inverse steric exclusion chromatography, were somewhat higher).186 Two main methods have been used to enhance the PH SSAs, introducing a porogen into the HIPE and/or postsynthesis hyper-cross-linking via Friedel−Crafts reactions. Porogenic solvents have been used to achieve SSAs of up to 570 m2/g,81 while hyper-cross-linking has achieved SSAs of up to 1600 m2/g.12 The PHs used to attain relatively high SSAs were usually based on styrenic monomers (styrene, VBC, and DVB), with the VBC-based PHs attaining higher SSAs than the styrene-based PHs.187 Unfortunately, hyper-cross-linking usually produces a significant deterioration in the mechanical behavior. Preventing this deterioration while achieving high SSAs would be advantageous for most PH applications. For PHs based on styrene and DVB that were hyper-cross-linked by using an external cross-linker, reducing the DVB content enhanced the swelling during hyper-cross-linking and enhanced the SSA at the expense of the mechanical robustness.184,188 PHs based on styrenics have also been used as precursors for the generation of porous carbons (see section 7), and interestingly, a large part of the increase in SSA produced by hyper-crosslinking the PH is transferred to the porous carbons derived therefrom.189 Hyper-cross-linking styrene-based and DVB-based polymers requires an external cross-linker, as described in Figure 7a, with the most common being formaldehyde dimethyl acetal and some unexplored possibilities being 4,4′-bis(chloromethyl)-

complex, multistep processes for displaying novel functional groups. Most of the PH functionalization processes that have been developed were focused on PHs based on (meth)acrylates, styrene, or vinylbenzyl chloride (VBC), as seen in Figure 6, a scheme illustrating a number of commonly accessible surface functionalization routes.13 The hydrolysis of acrylate-based PHs has often been used to provide the foundation for postsynthesis functionalization.167−169 PHs based on tert-butyl acrylate were hydrolyzed to yield carboxyl groups,167,168 while other acrylate-based PHs underwent postsynthesis functionalizations using a variety of methods including click reactions and functional coatings.40,82,170 Functionalization with amine groups has often been used as the foundation for further surface modifications, especially with acrylate-based PHs, and was used to enhance the adsorption of CO2, dyes, and metal ions.171,172 PHs based on hydroxyethyl methacrylate (HEMA) were functionalized with amines using two steps: (1) a better leaving group than that found in HEMA was generated through either halodehydroxylation (Cl, Br) or tosylation; (2) the generated leaving group was functionalized with an amine nucleophile. PHs based on GMA were easily functionalized with amine groups (e.g., 1,4ethylenediamine) in a one-step modification. Similarly, different types of amine functionalities were added to a variety of acrylate-based PHs.172 Hydrophobic acrylate monomers (nBA, GMA, and trimethylolpropane triacrylate) are, therefore, commonly used in PHs intended for subsequent functionalization.173 PHs containing aryl esters were functionalized by their reaction with NaOH, H2NC(CH2OH)3, or N(CH2CH2NH2)3, as shown in Figure 6.174 The postsynthesis functionalizations of styrene-based PHs have included increasing the hydrophilicity to enhance water absorption,166 coating with gold films,175 reacting with Bu4NNO3/(CF3CO)2O to generate nitrogen oxide groups,176 and chloromethylation using Friedel−Crafts alkylation to produce an anionic functionality for amination using a pyridine.177 PHs with sulfur-based surface functionalities were produced by using sulfonated monomers such as sulfonated styrene, 4,4′-diazidostilbene-2,2′-disulfonic acid disodium salt· 4H2O, or 2-acrylamido-2-methylpropanesulfonic acid in o/w HIPEs or by using postsynthesis treatments of styrene-based PHs that were synthesized within w/o HIPEs.13,50,131,178−181 The PH’s interaction with the functionalization medium is critical for success. Cylindrical, hydrophobic, styrene-based PHs immersed in a hydrophilic sulfonation reagent (concentrated sulfuric acid) produced nonuniform sulfonation across the cross section, with high levels on the surface and low levels at the center, while a hydrophobic sulfonation reagent (lauroyl sulfate) produced a more uniform sulfonation.13 PHs bearing chloromethyl groups, such as those in VBC, are excellent candidates for postsynthesis functionalization. The chloromethyl groups can be used as the key to anionic functionalities, hyper-cross-linking, amination (e.g., with morpholine, hexamethylenetetramine, ethylenediamine, piperazine, and aminopiperidine), and azidation.13,177,182−185 A large number of recently developed PH macromolecular chemistries have been described in the preceding sections. The multitude of possible postsynthesis functionalizations for these novel PH macromolecular chemistries has yet to be developed. Given that such functionalizations are expected to extensively broaden PH applicability, it is assumed that intensive efforts in that direction are currently under way.

Figure 7. Schematic illustration of postsynthesis cross-linking: (a) hyper-cross-linking of styrenic PHs; (b) metal coordination crosslinking of HG-PHs containing anionic groups. O


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Figure 8. Porous structures: (a, c, e, h, j) before carbonization; (b, d, f, g, i, k) following carbonization. The PH was based on: (a, b) Styrene and DVB. Reproduced with permission from ref 188. Copyright 2017 Elsevier. (c, d) AN and DVB. Reproduced with permission from ref 189. Copyright 2016 Elsevier. (e, f) HEMA, MBAAm, glucose, and borax. Reproduced with permission from ref 12. Copyright 2017 Elsevier. (g) Tannin. Reproduced with permission from ref 202. Copyright 2013 Elsevier. (h, i) Kraft black liquor. Reproduced with permission from ref 143. Copyright 2016 Elsevier. (j, k) A chain-extended, urea-based deep eutectic. Reproduced with permission from ref 205. Copyright 2017 Royal Society of Chemistry.

1,1′-biphenyl and α,α′-dichloro-p-xylene.190 The methylene chloride hyper-cross-linking of VBC-based PHs can be accomplished directly, with no need for an external crosslinker, by using an FeCl3-catalyzed Friedel−Crafts reaction, as described in Figure 7a. An additional advantage of using VBC is that any methylene chloride groups that did not undergo hypercross-linking can be used for further functionalization.184 The parameters of the hyper-cross-linking reaction, especially the nature of the external cross-linker, affect both the SSA and the mechanical properties.184,191 An elegant approach to PH hypercross-linking that does not use a Friedel−Crafts reaction, taking advantage of the unreacted vinyl groups in a DVB-based PH, produced SSAs of up to 356 m2/g.192 While most postsynthesis cross-linking has been effected in hydrophobic polymers, as described above, metal coordination has been used to effect additional cross-linking in HG-PHs. Cyclical shape memory behavior was achieved in a HG-PH containing anionic groups through “double cross-linking”, the fixation of a temporary shape by using the metal coordination cross-links that formed upon immersion of the HG-PH in an FeCl3 solution, as described in Figure 7b.193 The original shape was recovered by removing the metal coordination cross-links through light-induced reduction in a citric acid solution. The metal coordination cross-linking produced a significant increase in the modulus of the swollen HG-PH, demonstrating that postsynthesis functionalization can also be used to enhance the mechanical behavior.

encapsulate individual liquid droplets, usually through a reaction at the interface, as described in the previous sections, and to produce NC-PHs, hybrid PHs, and porous inorganics. Porous carbons, often with hierarchical porosities, have been produced through carbonization of the PHs described in the preceding sections or through direct HTC of biomasscontaining HIPEs.12,184 NC-PHs were formed through the inclusion of inorganic NPs in the HIPE (e.g., Fe2O3 for magnetic properties or CNTs for mechanical or conductive properties), and hybrid PHs were formed by adding reactive organometallics to the external phase.11 Porous inorganics can be produced through the calcination of hybrid PHs, through the calcination of NC-PHs or through a direct synthesis based on organometallics in the external phase.11 Often, the motivation behind adding inorganic components to PHs and the formation of nanocomposites and hybrids is to produce an enhancement in the thermal and/or mechanical properties.178,194 7.1. Porous Carbons. Porous carbons with hierarchical porous structures, generated though PH carbonization, have been evaluated for a variety of applications (catalyst supports, sorption, and gas storage).189,195,196 Porous carbons have been generated directly, following PH synthesis, or generated following the introduction of micropores through hyper-crosslinking. PH carbonization usually involves extensive reductions in mass and volume as well as a reduction in SSA for high-SSA PHs. Surprisingly, the hierarchical porous morphologies of the carbon monoliths are often strikingly similar to those of the original PHs, in spite of the reductions in mass and volume that can reach up to 90% (for example, Figure 8a,b). Porous carbons with hierarchical porosities and SSAs up to 553 m2/g were derived from macroporous PHs into which microporosity was introduced through hyper-cross-linking.184 Incorporating controlled mesoporosities into PHs and into the carbons produced by PH carbonization is not straightforward. The SSA of a porous carbon derived from a hyper-cross-linked

7. BEYOND POLYMERS While most emulsion-templating research and development is focused on porous polymer systems, emulsion templating also enables access to a wide variety of alternative systems that are either “polymers” but not “porous” or “porous” but not typical “polymers”; hence, the title of this Perspective is “Porous Polymers and Beyond”. Emulsion templating has been used to P


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ingly, the urea-based PH exhibited an extremely high residual mass (87%), a relatively high residual volume (54%), a relatively small reduction in void size, and an increase in the interconnecting hole density (Figure 8j,k).205 Porous carbons were also produced by using “hard templating”, coating emulsion-templated silicas (“Si(HIPE)s”, described in section 7.2.3) with a phenol−formaldehyde resin, polymerizing the resin, removing the silica template with a strong acid, and carbonizing the resulting porous polymer,206,207 yielding carbon electrodes with an SSA of 459 m2/g (biofuel cell, battery, electrochemical capacitor, or hydrogen storage applications) and mechanically robust porous carbon monoliths with an electrical conductivity of 20 S/cm (Figure 9a,b).208 A disadvantage of using Si(HIPE)s as

PH based on acrylonitrile (AN) and DVB was almost an order of magnitude higher than that of the porous carbon derived from the same PH that did not undergo hyper-cross-linking, with carbonization producing an 89% reduction in mass and a large reduction in void size (from 386 to 146 μm, Figure 8c,d).189 Carbons that were generated directly from DVB-based PHs also had relatively high SSAs (505 m2/g).184,188 HTC is an exothermic dehydration and decarboxylation process for the thermochemical conversion of biomass into solid, coal-like products that takes place at relativity low temperatures (130−250 °C) and high pressures.197 HTC-based monoliths, produced from HIPEs containing saccharide derivatives, were carbonized to yield SSAs of up to 730 m2/g, meso- and micropores, and electrical conductivities of up to 3 S/cm without the resorcinol/formaldehyde precursors and DVB-based polymers typically used to generate porous carbons from PHs.147 HTC-based PHs were produced by using a onepot reaction within o/w HIPEs that combined HG-PH synthesis (based on HEMA and MBAAm) with biomass HTC (based on glucose and borax).12 The porous structure of the HTC-based PH (Figure 8e) was bimodal, and its carbonization (450 °C) produced a porous carbon with a similar porous structure (Figure 8f), a density of 0.058 g/cm3, and an SSA of 101 m2/g (which was increased to 1540 m2/g on chemical activation with ZnCl2). Chemical activation, which usually involves depositing an etchant (e.g., KOH or ZnCl2) on the carbon surface and heating to elevated temperatures, has been used to produce significant enhancements in the SSAs of carbonized PHs (e.g., from 521 to 1400 m2/g).198 KOH activation of a DVB-based PH synthesized in a HIPE containing toluene as a porogen increased the SSA from 711 to 2189 m2/g.199 Unfortunately, the extremely low residual mass that results from the carbonization process is reduced even further during the activation process. These extremely low residual masses is one of the central challenges to producing hierarchically porous carbons from PHs, a challenge common to all the polymerbased processes for generating porous carbons. A variety of other monomers and polymerization chemistries were used to synthesize PH templates for the generation of porous carbons. Porous carbons with relatively high SSAs (723 m2/g) were generated from PHs based on tannin (Figure 8g), a water-soluble, renewable polyphenol present in plants.200−202 Porous carbons were also produced through the carbonization of furfuryl alcohol-based PHs that were synthesized within oilin-alcohol HIPEs.203 Biobased “carboHIPEs” with relatively high SSAs, 1112 m2/g, and highly interconnected open-cell structures were synthesized from kraft black liquor-based PHs (Figure 8h,i).143 Although modifying the surface chemistry of PH-derived carbons could prove highly advantageous for a variety of applications, there has not been a great deal of work on postcarbonization surface functionalization.199 Oxygen and nitrogen surface functionalities in PH-derived carbons have often been obtained by using oxygen-containing monomers (such as sucrose) or nitrogen-containing monomers (such as AN).204 AN-based PHs were hyper-cross-linked to introduce microporosity and were then carbonized, producing SSAs of up to 417 m2/g with 0.75 at % nitrogen and 6.7 at % oxygen.12,189 Nitrogen- and oxygen-doped porous carbons with microporosity, mesoporosity, and macroporosity and with SSAs reaching 812 m2/g were generated within a HIPE whose external phase contained a urea-based deep eutectic. Interest-

Figure 9. Emulsion-templated porous inorganics: (a, b) TEOS-based Si(HIPE) template and the carbon derived therefrom. Reproduced with permission from ref 208. Copyright 2009 John Wiley & Sons. (c, d) Carbosilane-based PH and the SiC(HIPE) derived therefrom. Reproduced with permission from ref 220. Copyright 2011 Royal Society of Chemistry. (e, f) NC-PH (MMA, EGDMA, and Al2O3 particles) and the microporous alumina derived therefrom. Reproduced with permission from ref 218. Copyright 2016 Elsevier.

templates is the strong acids needed to remove the silica. Carbons with extremely high SSAs (2345 m2/g) were generated within emulsion-templated silicon carbides (“SiC(HIPE)s”, described in section 7.2.3) within a HIPE that contained both a liquid polycarbosilane and DVB in the external phase.209 Following DVB polymerization, the PH was heated in argon to 700 °C, underwent chlorination to remove the silicon at 700 °C, and then underwent reduction in hydrogen at 600 °C to remove the chlorine residue.209 7.2. Nanocomposites, Hybrids, and Inorganics. 7.2.1. Nanocomposite PHs. Adding NPs to the external Q


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inorganic beads (SiO2, Al2O3, TiO2, and ZrO2) could be generated though the calcination of the resulting hybrid PH beads.165 7.2.3. Porous Inorganics. Porous silica monoliths have been produced by heating silicon-containing hybrid PHs to 600 °C, first in nitrogen and then in air. Similar to the generation of carbon monoliths from polymer PHs, the porous structures of the resulting silica monoliths were quite similar to those of the original PHs in spite of the significant reductions in mass and volume.154 Porous inorganic monoliths were also produced through the calcination of composite PHs and NC-PHs. Alumina monoliths, whose porous structures were quite similar to those of the original composite PHs, were generated from MMA-based composites containing alumina powder in which the polymerization was followed by calcination in air at 1400 °C (Figure 9c,d).218 A great deal of work has been invested in producing emulsion-templated inorganic porous monoliths directly derived from organosilane-based HIPEs containing no organic monomers. Much of this work was based on TEOS within o/w HIPEs, and the resulting monoliths were termed “Si(HIPE)s”.72,214,219 Si(HIPE)s with more complex, hierarchical porous structures (microporosity, mesoporosity, and bimodal macroporosity) were produced by combining TEOS with various organosilanes.219 In addition, porous silicon carbides were synthesized by adding polycarbosilane to a Si(HIPE) followed by thermal treatment and the removal of the silica template by etching with hydrofluoric acid (Figure 9e,f).209,220 7.3. Far Beyond Polymers. A great deal of research and development has been invested in applying emulsion templating to a variety of material systems, going beyond the organic polymers synthesized in the first days of emulsion templating. The “beyond polymers” porous materials achieved include families of carbons, nanocomposites, hybrids, and inorganics. Recent efforts have also focused on producing porous carbons and porous inorganics directly, without first synthesizing a polymer-based PH. There is still a great deal to be done, including enhancing control of the carbon structure and its nitrogen content, broadening the list of porous inorganics that can be produced, and, most importantly, controlling the mass loss, the volume loss, and the resulting porous structure upon carbonation/calcination. Presently, there are many groups that are continuing to work toward these ends with promising results.

phase of a surfactant-stabilized HIPE has been used to enhance the mechanical behavior and/or to add a functionality, such as imparting a response to the application of a magnetic field. Silane-modified silica NPs that also serve as inorganic crosslinking centers significantly enhanced the mechanical behavior of EGDMA-based PHs, with a 5-fold increase in modulus and a 6-fold increase in crush strength.194,210 Inorganic NP loadings (up to 10 wt % of the monomers) have been readily incorporated into HIPEs, with higher loadings becoming more difficult owing to the increase in HIPE viscosity and the decrease in HIPE stability.194 It is possible, however, to achieve significantly higher inorganic contents (up to 60 wt % of the monomers) by adding the inorganic component as a monomer and then synthesizing hybrid PHs. NC-PHs with functional NPs located on the void surfaces were produced from Pickering HIPEs stabilized by using amphiphilic, inorganic NPs possessing properties of interest. Silica NPs, whose hydrophilic surfaces were modified to render them amphiphilic, are commonly used to stabilize Pickering HIPEs.97 While the NC-PHs from Pickering HIPEs tend to possess closed-cell structures,97,211 there has been some success in producing NC-PHs with open-cell structures by modifying the nature of the NP surfaces, by modifying the nature of the monomers, and by postsynthesis etching.85,105 7.2.2. Organic−Inorganic Hybrid PHs. Highly porous, emulsion-templated, organic−inorganic hybrids have been synthesized by using a large variety of routes. Often, the inorganic precursors, usually organometallics, were added to the HIPEs as monomers that could react with each other and/ or could copolymerize with the organic monomers. The inorganic precursors can form a hybrid interpenetrating network structure with the developing polymer network and/ or can form inorganic NPs, depending upon the reaction conditions.212 Interconnected network structures can be formed if the organometallic monomer is able to react with the organic monomer (e.g., it bears (meth)acrylate groups). While the silicon−oxygen systems (based on silica, silsesquioxanes, or silanes) have been investigated in greater detail, both TiO2 and iron oxide systems have also been investigated. In addition, inorganic components have been added to HIPEs as nonreactive additives and have been used to modify PH surfaces through postsynthesis functionalization. Organotrialkoxysilanes bearing double bonds (such as methacryloxypropyltrimethoxysilane (MPtMS) and vinyltrimethoxysilane) were incorporated into PHs that were based on styrene or EHA to enhance the high-temperature mechanical behavior, with the modulus at 250 °C increasing with increasing MPtMS content.154 Organosilanes that cannot react with organic monomers during polymerization, such as tetraethyl orthosilicate (TEOS), are commonly used to synthesize hybrid PHs.212−214 Similarly, a variety of reactive and nonreactive organometallic monomers based on silsesquioxanes or polyhedral oligomeric silsesquioxanes were used to synthesize hybrid PHs.215−217 Hybrid PHs have been synthesized by combining organic monomers (acrylates), MPtMS, and TEOS, with FRP followed by postsynthesis TEOS polycondensation under either acidic or basic conditions.214 Hybrid PHs have also been synthesized by using purely postsynthesis modifications, with no organometallic components in the original HIPE. In one approach, an EHAbased PH was immersed in an inorganic precursor solution (prehydrolyzed MPtMS), and in another, HG-PH beads were immersed in organometallic solutions such that porous

8. APPLICATIONS PH research and development is very often application driven. The challenges associated with industrial adaptation and commercial viability are, therefore, of great significance. These challenges not only include those commonly encountered with one-phase polymerization processes but also include the challenges that arise from the presence of the internal phase and, often, the presence of significant quantities of surfactant. The previous sections described the current state-of-the-art in emulsion stabilization, polymerization chemistry, postsynthesis functionalization, and systems “beyond polymers”. Conventional emulsions are, inherently, thermodynamically unstable. The challenges in developing novel PH systems are complicated even further by the extremely high internal phase contents and the associated reduction in stability. The challenges in developing novel polymerization chemistries in the HIPE’s external phase are greater than those commonly encountered with one-phase systems: the reaction components R


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zwitterionic HG-PHs exhibited an increase in absorption with increasing salt concentration (an anti-polyelectrolyte effect) and a minimum in absorption at pH 7.231 In addition, a polymer with a lower critical solution temperature (LCST) has been used to produce temperature-sensitive HG-PHs for delivery applications.232 In general, the advantages of HG-PHs include an enhanced sensitivity to changes in the aqueous environment, an accelerated rate of absorption, and an amplified magnitude of absorption. 8.1.2. Aqueous Solutions: Extraction/Purification. Absorption and adsorption applications usually involve aqueous systems. The hydrophobic polymers and surface-modified hydrophobic polymers investigated were usually based on styrene-like monomers (e.g., styrene, DVB, and VBC) and (meth)acrylates. The aqueous-system applications have included the extraction of caffeine,233 the extraction of cytokinins from plant extracts,234 the extraction of trace tetracycline antibiotics,235 and the capture of cis-diol-containing flavone.236 The adsorption of dyes and the adsorption of ions, usually metal ions, from aqueous solutions have undergone widespread investigation. PH adsorptions on the orders of tens to hundreds of milligrams per gram sorbent have been found for dyes such as Para Red and Sudan,237 Congo Red,172 Methylene Blue,32,46,231 and Turquoise Blue X-GB.238 Similar PH adsorbances were found for ions such as nickel,178 chromium,168,177 cadmium,239,240 copper,46,240 lead,239 and NO3−.241 The advantages of PHs for such applications included a mass transfer coefficient similar to those of packed beds (∼17600 m2/m3), but with a much lower pressure drop,233 and high column capacities with good reproducibility, stability, and durability (∼200 repetitions without measurable loss of performance).237 8.1.3. Water−Oil Mixtures. The PHs used for the sorption of solvents/oils from water−oil mixtures were usually based on styrenics and (meth)acrylates. The resulting sorptions were usually of similar orders of magnitude, with any differences originating in the nature of the polymer, the porosity, and the nature of the sorbate. PHs have exhibited up to ∼25 mL/g for various mixtures of solvents with water242 and up to ∼17 mL/g for various mixtures of oil with water.243 In an investigation that included both solvents and oils, the solvent sorption was up to ∼35 mL/g, which is around twice the sorption of oil.72,244 In many cases, the polymer was plasticized during sorption, and the sorbate could be recovered by squeezing, enabling the sorbent to be reused for multiple cycles without losing its sorption capacity. Magnetic PHs are highly advantageous for such applications since the sorbent can be collected by using a magnet. Various strategies have been used to produce magnetic PHs including the addition of carbonyl iron powders to the HIPE245 and the synthesis of HG-PHs in Pickering HIPEs that were stabilized with Fe3O4 NPs.239,240 8.1.4. Gas Adsorption and Permeability. Gas adsorption and storage in PHs have been investigated, with the interest in PHs for CO2 adsorption continuing to increase.246−248 The ability to adsorb CO2 was enhanced through chemical modification of the surfaces (polyethylenimine modification yielded ∼285 mg/g),249 through the generation of microporosity in the polymer itself, or through the incorporation of high SSA additives (a 794 m2/g metal−organic framework (MOF) yielded ∼108 mg/g).164 HG-PHs that incorporated up to 92 wt % MOF exhibited SSAs as high as 980 m2/g as well as relatively high water vapor uptakes for heat transformation applications, reaching 420 mg/g (compared to 260 mg/g for the PH with no MOF).250 The nitrogen permeabilities of

can be partially soluble in, or can react with, the internal phase, the reaction can be inhibited by the internal phase, the reaction temperature can be limited by the nature of the internal phase, and the reaction products/byproducts can induce HIPE destabilization. This section, which focuses on the applicative side of the cutting-edge materials described in the previous sections, presents the results of proof-of-concept evaluations and tries to discern which applications seem to be the “ripest” for industrial scale-up and commercialization. A great deal of research and development is being invested in applying PHs as “flow-through” supports, column packing, and membranes.221,222 The advantages of PHs include the ability to polymerize inside a column, the variety of surface modifications that can be applied with relative ease, and the relatively low back-pressures. Adsorption, absorption, and catalysis were the most commonly evaluated PH applications, followed by PH scaffolds for tissue engineering and controlled release. Interestingly, both hydrophobic PHs and HG-PHs are of interest for such applications. In addition, there has been some recent interest in PHs for heat and sound insulation applications.223 A general sense of the wide variety of applicative possibilities, as well as some limited proof-ofconcept results, will be described here in an attempt to inspire the further adaptation of PH systems. Detailed information on the nature of the PH, the polymer chemistries, the surface modifications, and the evaluation conditions can be found in the cited articles. 8.1. Adsorption and Absorption. Originally, the development of hydrophobic PHs was driven by absorbent-related applications, especially the absorption of bodily fluids,224−228 and, later, the absorption of contaminants from water.166,229 Recent work has broadened the applicability of PHs to include organic fluids on the absorbate side, HG-PHs on the absorbent side, and broad-spectrum absorbents.41 The hierarchical porous structure that enables flow-through with relatively low backpressures, the ability to design the surface chemistry for specific applications in catalysis or adsorption, and the evaluation of PHs for a wide variety of such processes provide a clear indication that these applications are on the threshold of wider industrial adoption and commercialization, waiting for a specific combination of materials, processing, and applications that will make PHs the most practical and economically viable solution. 8.1.1. Aqueous Solutions: Absorption. Hydrophobic PHs, surface-modified hydrophobic PHs, and HG-PHs continue to be investigated intensively for the absorption of aqueous solutions. For hydrophobic PHs, whose polymers do not absorb water, the variables include the surface modification, the crosslinking, the porosity of the dry PH, the presence of additives such as salts, and the mechanical properties of the polymer that can enable osmotic-pressure-driven volumetric expansion. For HG-PHs, whose polymers absorb water, the variables additionally include the ionic nature of the polymer and the interaction of the polymer with the solution. For HG-PHs, the absorption of water usually increases with a decrease in the cross-linking and with a decrease in the solution’s salt concentration and, for ionic polymers, with attaining the appropriate pH.230 The relatively large absorptions of aqueous solutions by HG-PHs were ascribed to a hydrogel-swelling-driven void expansion mechanism, which makes HG-PHs advantageous systems for applications involving the absorption of aqueous solutions.230 Water absorptions as high as 977 g/g and artificial urine absorptions as high as 130 g/g have been reported for HG-PHs with strongly binding sulfonic acid groups.32 Interestingly, S


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highly interconnected, high-porosity, open-cell structures, PHs synthesized within HIPEs stabilized by using very low surfactant contents or within Pickering HIPEs stabilizing by using NPs have often exhibited closed-cell structures structures that do not exhibit an obvious interconnected structure in SEM micrographs. Usually, these PH structures are not “truly closed-cell” since the internal phase can be removed easily. Elastomeric PHs that were “truly closed-cell” were recently generated within w/o Pickering HIPEs,109 and most recently, “truly closed-cell” PHs were generated within w/o surfactant-stabilized HIPEs.12 The elastomeric “truly closedcell” PHs exhibited extraordinary water retention, a resistance to compressive deformation similar to that seen in vegetable matter, and self-extinguishing fire retardation. The ability of the PHs to store fertilizer (concentrated aqueous solutions and inorganic salt hydrate melts) for controlled-release and to encapsulate phase-change materials (inorganic salt hydrate melts) for thermal energy storage and release has also been evaluated.12 In addition, “truly closed-cell” PHs for the encapsulation of organic liquids and melts have recently been synthesized by using interfacial step-growth polymerization within o/w HIPEs and evaluated for thermal energy storage and release (the encapsulation of paraffinic phase-change materials).12,88 8.4. Tissue Engineering Scaffolds. There are quite a number of biomedical applications for which macroporous structures are advantageous: 3D cell culture, tissue engineering, and controlled release. The commercial use of PHs for applications in the biomedical field, however, must take into account the far more stringent tolerances for material impurities (for example, unreacted monomers, catalysts, initiators, solvents, internal phase components, surfactants, and NPs). On the other hand, biomedical applications are, in many ways, “niche” applications where the benefits derived can far outweigh the material, synthesis, purification, and processing cost outlays. While the contents of an aqueous internal phase can be removed relatively easily through washing and drying, complete removal of the residual, unbound external phase components without having a deleterious effect on the porous structure can be more challenging. In addition, biocompatibility is required, and biodegradability is usually preferred, for applications involving implantation. Therefore, the materials and their possible degradation products must all be nontoxic. The use of PHs for tissue engineering was first mentioned in 1993, but the PHs investigated did not support the growth of an endothelial-based cell line.257 Some 8 years later, however, the evaluation of PHs as tissue engineering scaffolds became widespread.98,167,258−268 Prominent in that work was the development of 3D cell culture scaffolds based on copolymers of styrene and DVB,265,269 research that produced a commercial product, Alvetex by ReproCELL (previously Reinnervate). Interestingly, Alvetex is now used as reference system during the development of more exotic polymer chemistries for the synthesis of PHs with enhanced biodegradability.114 The PH chemistries currently being explored for tissue engineering applications include polyesters (especially polyurethanes based on PCL),12,91,103,126,270−275 gelatin grafted with PNiPAAm (for temperature response),58 polymethacrylate hydrogels,73 polyacrylates,276 and polyacrylamide.277,278 Recent work includes fully biodegradable and biocompatible PCLbased PHs which supported growth of fibroblast cells (L929) over 7 days and demonstrated that the degradation products were nontoxic to the cells up to a concentration of 0.1 mg/

various PHs, ranging from 0.01 to 1.1 D, depended upon the average diameter of the interconnecting holes, upon the content and location of any NPs, and upon the surface chemistry.46,86 8.2. Flow-Through Reactions and Catalysis. Various PHs were evaluated for their performance as flow-through reaction and catalyst supports. For catalyst supports, the nature of the catalyst, the incorporation of the catalyst onto the PH surface, and the availability of the catalyst were critical for success. A MOF-containing PH membrane with 130 m2/g was successfully used for the Friedel−Crafts alkylation of pxylene.251 Hydrophobic PHs were used as stationary phases for thin layer chromatography, exhibiting reasonable resolution for various separations,252 Pt-NP catalyst supports for reduction reactions,75 supports for an organic photocatalyst for oxidation reactions,182 supports for fluorinating agents,183 and coatings for open-tubular capillary electrochromatography separation columns.253 Pd NPs located on PH supports, used for heterogeneous Suzuki−Miyaura carbon−carbon coupling reactions between iodobenzene and benzeneboronic acid, exhibited high catalytic activity, yields close to 100%, and good recyclability over four reaction cycles.59 NIR-responsive polypyrrole NP “nanoheater” coatings that were able to heat PHs from 20 to 180 °C within 10 s were used as a bacteria filter for water sterilization and exhibited a maximum efficiency of 94% and a bacteria inactivation rate of 95%.82 PH catalysts with acid−base sites on hydrophobic surfaces were generated within Pickering HIPEs stabilized using aminofunctionalized NPs that were subsequently sulfonated.254 The acid−base sites were used for the one-pot conversion of cellulose from various sources to 5-hydroxymethylfurfural, exhibiting yields of up to 41%. PH catalysts with Lewis− Brønsted double acid sites, generated through ion-exchange fixation of Cr3+ and/or H+ on the surface, were used for the one-pot conversions of carbohydrates to 5-hydroxymethylfurfural.181 PolyHIPEs decorated with catalytic gold NPs were used to effectively catalyze the reduction of 4nitrophenol to 4-aminophenol with conversions of up to 99%, with turnover frequencies as high as 13.5 1/h, and with only a small decreases in activity when recycled six times.175 8.3. Release and Encapsulation. Absorption/adsorption applications often involve the need for sorbate removal to enable sorbent recycling and reuse. PH applications that are primarily concerned with controlled release usually involve incorporating the substance to be released within the voids. It is possible, in some cases, to incorporate the substance to be released within the HIPE’s internal phase, such that the release system can be generated in an advantageous one-pot synthesis. In other cases, the HIPE’s internal phase must be removed and the substance to be released must be loaded. The release behavior of a water-soluble dye within a bicontinuous hydrogelfilled PH indicated that the tortuous porous structure had a significant effect on the release behavior.107,255 The uptake and release behaviors of PH microbioreactors in the soil, used to produce a synthetic rhizosphere by loading a bacterial broth into the HIPE’s internal phase, have also been investigated.256 The ability to encapsulate and store water within discrete, micrometer-scale containers is essential for living organisms. The encapsulation of liquids in such discrete, micrometer-scale containers is also essential in the pharmaceutical and cosmetics industries. Developing methods whereby discrete droplets could be encapsulated within monolithic synthetic polymers could yield novel materials systems with advantageous properties for storage and release. While PHs usually have T


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Macromolecules mL.113 In addition, HG-PHs investigated for regenerative medicine applications exhibited cytocompatibility (97% NIH3T3 cell viability after 5 days) and, in cell proliferation studies, 55−88% cell viability after 7 days.73 Other HG-PHs exhibited good biocompatibility and good cell proliferation 2 or 3 days after seeding human embryonic lung diploid fibroblast cells or H9c2 cardiac muscle cells.24 There are several exciting new directions being pursued in the development of PHs for tissue engineering (in addition to the advanced polymer chemistries described above). One direction is the inclusion of bioactive NPs such as hydroxyapatite within the PHs, whether through their use as stabilizing NPs in Pickering HIPEs such that they would be found on the void surfaces or simply through their use as a filler in NC-PHs.58,270,271,273,276 The ability to add bioactive NPs to the scaffold, some on the surface and others within a biodegradable wall, will enable fine-tuning of the scaffold’s biological functionality. PCL-based PHs have been synthesized from Pickering HIPEs stabilized with poly(L-lactic acid)modified hydroxyapatite NPs.273 The mechanical properties and bioactivity increased with increasing hydroxyapatite content, the ibuprofen within the walls exhibited a sustained release profile, and the in vitro cell culture assays demonstrated biocompatibility, with the cells adhering, spreading, and proliferating. The growing body of work on the use of HIPEs in 3D printing (additive manufacturing) will enable the generation of additional size scales in the hierarchical porous structures as well as the ability to generate the complex shapes that are vital for tissue engineering applications involving specific organs.112,276,279 Perhaps the most interesting, promising, and challenging of the biomedical applications under development is the generation of injectable HIPEs that react to form PHs in the body. All the HIPE components must be nontoxic, the reaction must occur at physiological temperatures following injection (and not before), and the reaction byproducts must be nontoxic. PHs based on propylene fumarate dimethacrylate as a biodegradable cross-linking monomer were synthesized within w/o HIPEs.103,271,275 An alternative injectable system, based on a polyurethane reaction, was not as attractive since it could raise toxicity concerns.127 The practicability of the injection system was enhanced by using a double-barrel syringe which mixed two w/o HIPEs, each of which, when alone under storage conditions, does not polymerize.91 The gelatin-based HG-PHs investigated for tissue engineering applications were also injectable, even when cell laden.58 Potentially, commercial tissue engineering applications for PHs hold great promise. 8.5. Shape Memory Materials. Shape memory polymers are used in numerous applications and are of interest for a variety of biomedical applications, especially for implants that will only require a minimal incision for the insertion of a temporary shape that can then be deployed to a larger, permanent shape.280 Thermally activated shape memory polymers usually contain a reversible transition element responsible for maintaining the temporary shape (usually imposed through deformation) and a restoration element responsible for recovering the original shape. The reversible transition element can be based upon a Tg or upon a melting point (Tm), and the restoration mechanism element can be physical/chemical cross-linking or a transition temperature that is higher than that of the reversible transition element. The rapidly growing library of macromolecular structures that are available for PHs, with the associated thermal

transitions and reversible bonding scenarios, provides fertile ground for the design of novel shape memory PH systems. Shape memory PHs were generated using (meth)acrylates with long, crystallizable, aliphatic side chains.281,282 The temporary shape (a compressive strain of 70%) was imposed above the side-chain Tm and fixed (with fixity ratios of 100%) by cooling beneath the Tm. The original shape, restored by the elastomeric network upon heating above the Tm, exhibited recovery ratios of around 90%. While the recovery rates of these shape memory PHs in air were relatively slow, reflecting the thermally insulating nature of the PHs, shape memory PHs that recovered rapidly when immersed in hot water were generated through the synthesis of bicontinuous, hydrogel-filled, PHs.283 Shape memory HG-PHs were also generated by using double crosslinking in which a temporary shape can be imposed and “locked in” by using a reversible metal coordination cross-linking mechanism, and the original shape can be recovered by “removing” the metal coordination cross-linking.193 8.6. Membranes and Beads. One of the applicative advantages of PHs is the ability to produce monolithic shapes with relative ease. Techniques such as doctor blading can be used to produce PH membranes, enhancing the adaptability of PHs for membrane-related applications.116 The ability to rapidly, reproducibly, and controllably generate PH beads, rods, and fibers by using relatively simple technologies (as described in section 5) enables a wide range of additional applications. PH beads for wastewater treatment with photocatalytic TiO2 NPs embedded on the void surfaces were synthesized within an o/w/o emulsion. The PH beads in water (0.46 wt %) exhibited high photocatalytic efficiency, with up to 99.4% of the methyl orange in a 20 ppm solution degraded after 2.5 h of UV exposure with no significant decrease in photocatalytic performance during nine cycles.284 8.7. Porous Carbons and Porous Inorganics. The generation of porous carbons and porous inorganics from PHs has been described in section 7. Most of the applications for the carbons are adsorption-related where the advantageous high SSAs were produced through the generation of microporosity and/or mesoporosity. PH precursors yield macroporous carbons whose hierarchical porosities enable rapid transport. The PHs for carbon generation are usually based on styrene, DVB, and/or VBC, and the high SSAs can be generated through carbonization, hyper-cross-linking,184 porogen removal, and/or activation. Other important parameters for the resulting porous carbons include the surface chemistry, where nitrogen-rich carbons are often advantageous,285 and conductivity. While most of the PH precursors were synthesized within w/o HIPEs, recent work has also shown that PHs from o/w HIPEs can be used for the generation of porous carbons, either through HTC12 or through the use of urea-based deep eutectics.205 SSAs over 1000 m2/g have been achieved143,198,209 as well as conductivities over 100 S/m.143,188 Porous carbons based on PHs were investigated for dye adsorption,205 for heptane vapor and water vapor adsorption,286 for oxygen reduction reactions,201 for methanol electro-oxidation,189 and for supercapacitor electrode applications.179 Porous, emulsiontemplated Si(HIPE)s have recently been used as hosts for bacterial colonization in the development of potential biotechnological applications.287 8.8. Other Applications. A variety of approaches have been used to produce porous, conductive, PH-based systems for a variety of applications. The approaches include incorporating CNTs (reaching over 10−1 S/m when combined U


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Macromolecules with sulfonation),50 coating with conductive polymers (reaching over 10−3 S/m for polypyrrole),288 or generating conductive porous carbons. Porous, elastomeric, and pressure-sensitive conducting polyurethane-based PHs were produced by dispersing multiwalled CNTs in both phases such that they were within the walls and on the void surfaces.289 A compressive strain of around 40% enhanced the conductive pathway on the void surfaces, producing a reduction in resistance (from 500 MΩ) of 2 orders of magnitude and demonstrating that such systems could be used as pressure sensors. PH membranes were evaluated as separators in lithium-ion batteries and performed similarly to commercial polyolefin-based separators while exhibiting advantageous thermal and dimensional stabilities.139 The effectiveness of PHs as micromixers was evaluated by using two parallel reactions and demonstrated that the degree of micromixing increased with the reduction in the size of the interconnecting holes.290

emulsion templating.311−314 Similar to emulsion templating, these methods use surfactants and/or NPs to stabilize twophase fluid systems that can be used as templates for the synthesis of porous polymers. Complex HIPE stabilization systems are becoming increasingly prevalent and include a fluorinated BCP,315 gemini surfactants (two connected monomeric surfactants),316 surfactant mixtures that are tuned using a phase diagram,317 and monomeric BCP surfactants.41,42 Macromolecular surfactants were used for a CO2-in-ionic liquid HIPE containing vinylbearing ionic liquid monomers and cross-linkers,318 and a hyperbranched hydrolyzable polyethoxysiloxane surfactant was used to produce highly porous silica monoliths from w/o HIPEs.319 Recent approaches to reducing the high surfactant contents that are a predominant challenge to industrial adoption of emulsion templating include limiting the HIPE’s ability to phase separate, combining freezing and UV polymerization,320 and, more commonly, using Pickering HIPEs where the presence of NPs can be leveraged to provide additional functionalities. The NP focus of recent work includes MOFs,321−325 proteins,326,327 gelatin,328 melamine-based microporous polymer NPs,329 and nanocellulose.330 In addition, emulsion templating within gel emulsions is becoming increasingly common.331−333 The many unresolved issues in understanding the relationships between the porous structures and the mechanical behaviors continue to receive prominent attention. A recent in-depth investigation that combined the synthesis of PHs, the use of a computational model for the porous structures, the use of a micromechanics model, and a comparison of the computational results with the experimental results indicated that the voids have a greater influence on the overall mechanical behavior than the interconnecting holes.334 A comparison of emulsion templating and porogen templating, effected using pairs of methacrylate-based porous monoliths with identical compositions and identical porosities, demonstrated that emulsion templating produced a higher modulus and a higher compressive strength.335 Recent work has reiterated that the mechanical properties can be enhanced by moving from low molecular weight cross-linking comonomers to high molecular weight cross-linking comonomers42,336 and that adding a relatively small amount of NPs can produce a significant reduction in void size and a significant increase in modulus.337 The relationships between the PH’s acoustic properties and its mechanical properties is also beginning to be explored.338 There are a number of topics that evince a continuing, persistent interest, sometimes with an unexpected twist. The oxidation of DCPD-based PHs, which degrades its excellent mechanical properties,339 was used to impart advantageous functionality for the simultaneous sequestration and decontamination of toxic chemicals and for a self-decontaminating air filter for chemical warfare agents.340,341 The pursuit of hierarchical porosities continues with the development of alternative routes for incorporating microporosity and mesoporosity into the macroporous PHs through hyper-crosslinking.342,343 Postsynthesis modifications that have recently been developed to enhance the PH surface functionality include atomic layer deposition344 and sol−gel deposition345 of inorganics. The applicative importance of controlling monolith shape has driven a great deal of development that tracks the developments in microfluidics and in 3D printing: producing highly porous beads using microfluidics346 or a combination of

9. PERSPECTIVES AND PROGNOSTICATIONS Porous polymers can be produced through a number of approaches, many of which are technologically mature and are commonly used in industrial production. Macromolecular design is used to produce microporous powders, self-assembly is used to produce mesoporous films, and solution-separationfoaming technologies are used to produce macroporous systems. Emulsion templating, an extremely versatile technology for the production of a wide variety of highly interconnected, highly porous monolithic systems, seems to be located on the cusp of wider industrial adoption. In this Perspective, we described the conceptual developments in emulsion templating, the unique niche occupied by emulsion templating, and the resulting materials. The advantages of emulsion templating for a wide range of applications are becoming more and more pronounced, as demonstrated by the advances in emulsion templating described herein. We have described the manifold advances that conclusively demonstrate the extraordinary versatility inherent in emulsion templating, versatility that exists in the type of HIPE, in the HIPE stabilization strategy, in the polymerization chemistry, in the macromolecular structure, in the porous structure, in the processability, in the access to complex systems,291−294 in the postsynthesis functionalization,295−297 and in the adaptability to applications. This built-in, wide-ranging versatilitythe essential key to understanding emulsion templating’s prospects for development and its potential for applicationwas not always the case. Emulsion templating began with simple monomers and with rudimentary systems, FRP within w/o emulsions.224,298−302 It is only recently that in-depth studies have revealed the effects of the parameters listed above can have on the ability to generate innovative materials.78,303−307 9.1. State-of-the-Art. While the versatility of emulsion templating has expanded to a great extent, the most recent research and development indicates that the field has not yet reached full “maturity” and that there is still some way to go. This is especially true in light of the ever-increasing developments in emulsion types and in emulsion stabilization strategies. Emulsion templating increasingly includes interfacial reactions between monomers in the different phases12,308,309 and porous polymers have been templated within emulsions with relatively low internal phase contents.310 In addition, the generation of porous polymers using foamed emulsions and using bubble templating are beginning to be explored as alternatives to V


Perspective

Macromolecules microfluidics and self-emulsification,347 3D printing of wound dressings348 and of tissue engineering scaffolds,349,350 and hard templating.351 The strong application-focused nature of much of this research is a sign of the growing maturity in the field. The most prominent applications continue to be those concerned with water purity, with one type focusing on the adsorption of ionic or organic contaminants from aqueous solutions and the other type focusing on the absorption of oil from oil−water mixtures (oil spill “clean-up”). Most of the PHs synthesized for adsorption applications were based on styrene and/or DVB, and the adsorptive functionality for inorganics352−357 and organics358−364 was added by using comonomers, grafting, and/ or functionalized NPs. The PHs developed for oil absorption included both hydrophobic systems (usually based on styrene and/or DVB)365−368 and hydrophilic systems.231,369 The great interest in the high-porosity, highly interconnected, macroporous PHs for flow-through applications continues unabated, and the influences of the molecular hindrance, mass transport, and molecular activity were recently described.370 The recently developed applications for the PH flow-through systems include the covalent binding of horseradish peroxidase,371 Fenton reaction catalysts for the decomposition of methyl orange,372 the chromatographic removal of large molecular virus particles,373 CO2 adsorption,374,375 and the removal of phenol from cigarette smoke.376 The recently developed non-flow-through catalysis applications include the catalytic reduction of 4-nitrophenol to 4-aminophenol,377 the catalytic transformation of cellulose into 5hydroxymethylfurfural,378 and biocatalytic waste stream upcycling.379 Encapsulation and release PH applications also continue to be of interest and include the encapsulation of thermal energy storage-release phase change materials308,380,381 and the release of the antibacterial drug enrofloxacin382 and cinnamon oil (which also exhibits antibacterial activity).383 The most recent work on porous carbons (with PHs based on styrene and/or DVB continuing to be the leading precursor) has emphasized hierarchical porosity, nitrogen enrichment, stability during carbonization, and ultracapacitor applications. These works include chemical activation,384 enhancing conductivity,385 enhancing the stability during carbonization, and enhancing the specific capacitance.386 Emulsion templating also continues to be used for the generation of macroporous silica, with recent work emphasizing the generation of mesoporosity.387,388 9.2. Are We There Yet? The HIPE systems available have expanded far beyond w/o and o/w and now include nonaqueous HIPEs that can be based on deep eutectic solvents or ionic liquids. Such HIPEs will prove highly advantageous for accessing polymer synthesis mechanisms that are watersensitive or that take place at relatively high temperatures and for accessing novel internal phase functionalities and surface functionalities. The ability to stabilize HIPEs has moved far beyond the original handful of surfactants. BCPs have proved incredibly versatile for the stabilization of both w/o and o/w HIPEs, often with the added bonus of surface functionality, and a wide variety of amphiphilic NPs (inorganic, organic, spheres, rods, and flakes) bearing various functionalities have been used to form surfactant-free Pickering HIPEs. Even for the relatively simplistic FRP-based polymerizations, the significance of the locus of initiation, external phase or interface, was notable. The ever-increasing number of available polymerization chemistries for PH synthesis is inspiring. Each

pioneering step in advancing the development of a polymer chemistry, usually undertaken for a specific monomer, enables access to a world of monomers and promotes further research and development. Controlling the porosity, the void size, the void size distribution, and the degree of interconnectivity has been one of the main goals of PH research and development. One recent, unexpected application of emulsion-templated polymers that will prove important for a wide variety of applications is the encapsulation of the individual micrometerscale internal phase droplets within “truly closed-cell” structures. The inherent shape versatility of emulsion templating means that both 2D and 3D structures, both of great interest for a number of applications, are easily fabricated. One of the great advantages of PHs is the ease of molding monolithic shapes and the ease of scaling-up the process. The recently developed ability to produce controlled, monodisperse, porous structures through microfluidics will prove advantageous for numerous applications. The surfaces of PHs are becoming easier and easier to functionalize through prepolymerization and/or postpolymerization approaches. In addition, beyond polymers, emulsion templating enables the generation of a wide variety of porous systems including carbons, hybrids, nanocomposites, and inorganics. Originally, strong industrial interest (Unilever, Lever Brothers, Procter and Gamble) drove the development of emulsion-templated polymers for absorption and liquid carrier applications and, later, for insulation applications. The flowthrough capabilities of emulsion-templated polymers have enabled application as column packing, as supports, and as membranes for a variety of chemical engineering applications. The absorption of liquids has recently come to the forefront again, with HG-PHs absorbing hundreds of times their weights in aqueous liquids, hydrophobic PHs absorbing organic liquids, and hydrophobic−hydrophilic PHs exhibiting broad-spectrum absorptions. Adsorption applications are of increasing interest, as ionic HG-PHs and porous carbons derived from PHs have joined a variety of hydrophobic PHs in offering high adsorption capacities. Biomedical and agricultural applications, where degradability and stimulus-response behavior are of interest, continue to stimulate intensive research and development. 9.3. Challenges and Opportunities. The greatest challenge in the field of emulsion-templated polymer systems seems to be moving the technology from the lab to industrialscale production. Batch emulsion templating involves mixing the components and heating, so that scaling up would seem to be relatively easy. However, emulsion templating can only become industrially attractive for a broad range of applications if the technologies needed to reduce costs, to reduce any negative environmental impact, and to enhance the “green” aspects (materials and purification processes) are applied. Some technologies for the continuous processing of PHs may presently be in their infancy, but they are developing rapidly and have exhibited industrial potential. Various processes for the continuous production of PH beads and fibers are already in place, and the continuous extrusion of PHs has also been demonstrated. The encapsulation systems will be the simplest target for industrial scale-up since the internal phase need not be removed. Similarly, the PH precursors for porous carbons and inorganics will be relatively simple to scale up since the carbonization/calcination processes can also remove the internal phase. In general, the materials templated using w/o W


Perspective

Macromolecules HIPEs will be relatively easy to scale up since removing (and even recycling) the water is relatively straightforward with many industrial precedents. The most difficult systems to scale up will most likely be those involving o/w HIPEs. On one hand, while low boiling point organic internal phases may be relatively easy to remove, the process may involve a relatively high environmental impact and cost. On the other hand, “greener” (renewable resource) and higher boiling point organic internal phases may be more difficult to remove. The ever-increasing advancements in the versatility of emulsion templating and in the availability of PHs with unique sets of desirable properties ensure that the industrial interest in emulsion-templated materials will continue to intensify for both large-scale applications and niche applications. The prospects for specific emulsion-templated materials are difficult to predict, given the large number of parameters involved in HIPE stabilization, PH synthesis, and postsynthesis processing. Yet, it is emulsion templating’s large and versatile parameter space that offers opportunities for pioneering work and breakthrough innovations. Given the rapid pace at which novel emulsiontemplated systems have been introduced over the past few years, it seems certain that there exists an extremely bright future for scientific/engineering achievements and for industrial adoption.

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Rajashekharayya A. Sanguramath received an MSc in Inorganic Chemistry from Karnatak University (2006) and worked with Prof. C. N. R. Rao as a Research Assistant (2006−2008) at JNCASR, India. He obtained a PhD from the School of Chemistry at the University of Bristol in 2012, working with Dr. Christopher A. Russell, and was a postdoctoral researcher at the Hebrew University of Jerusalem, working with Prof. Roy Shenhar from then until 2017. He has been a researcher in Prof. Silverstein’s group at the Technion since July 2017. His current research is focused on the preparation of emulsion templated polymers using renewable resources and biodegradable polymers for a variety of applications.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.S.S.). ORCID

Tao Zhang: 0000-0001-9032-9982 Michael S. Silverstein: 0000-0002-9377-4608 Author Contributions

T.Z., R.A.S., and S.I. contributed equally to this work. Notes

The authors declare no competing financial interest. Biographies Sima Israel received her BSc in Chemical Engineering from the Technion−Israel Institute of Technology in 2007. She is currently pursuing her PhD at the Department of Materials Science and Engineering, Technion. Her research focuses on the synthesis of hyper-cross-linked, emulsion-templated polymer monoliths with hierarchical porosities, and the porous carbons generated therefrom, for adsorption applications.

Tao Zhang received a B.Eng. degree (2007) and a M.Eng. degree (2010) in Polymer Science and Engineering at Hefei University of Technology and earned his Ph.D. from Deakin University (2016) under the guidance of Prof. Qipeng Guo. After a postdoc with Prof. Michael S. Silverstein, he joined Soochow University, College of Textile and Clothing Engineering, in early 2018. His research is focused on high internal phase emulsion stabilization and emulsiontemplated polymers. X


Perspective

Macromolecules

(16) Brun, N.; Ungureanu, S.; Deleuze, H.; Backov, R. Hybrid foams, colloids and beyond: From design to applications. Chem. Soc. Rev. 2011, 40 (2), 771−788. (17) Kimmins, S. D.; Cameron, N. R. Functional Porous Polymers by Emulsion Templating: Recent Advances. Adv. Funct. Mater. 2011, 21 (2), 211−225. (18) Pulko, I.; Krajnc, P. Porous Polymer Monoliths by Emulsion Templating. In Encyclopedia of Polymer Science and Technology; John Wiley & Sons, Inc.: New York, 2017. (19) Pulko, I.; Krajnc, P. High Internal Phase Emulsion Templating − A Path To Hierarchically Porous Functional Polymers. Macromol. Rapid Commun. 2012, 33 (20), 1731−1746. (20) Koler, A.; Paljevac, M.; Cmager, N.; Iskra, J.; Kolar, M.; Krajnc, P. Poly(4-vinylpyridine) polyHIPEs as catalysts for cycloaddition click reaction. Polymer 2017, 126, 402−407. (21) Manley, S. S.; Graeber, N.; Grof, Z.; Menner, A.; Hewitt, G. F.; Stepanek, F.; Bismarck, A. New insights into the relationship between internal phase level of emulsion templates and gas-liquid permeability of interconnected macroporous polymers. Soft Matter 2009, 5 (23), 4780−4787. (22) Kim, K.; Bismarck, A. Science Babel: Does the lack of a common terminology in the field of emulsion templating hinder progress? eXPRESS Polym. Lett. 2010, 4 (4), 196. (23) Ceglia, G.; Mahéo, L.; Viot, P.; Bernard, D.; Chirazi, A.; Ly, I.; Mondain-Monval, O.; Schmitt, V. Formulation and mechanical properties of emulsion-based model polymer foams. Eur. Phys. J. E 2012, 35 (4), 1−11. (24) Luo, W.; Zhang, S.; Li, P.; Xu, R.; Zhang, Y.; Liang, L.; Wood, C. D.; Lu, Q.; Tan, B. Surfactant-free CO2-in-water emulsion-templated poly (vinyl alcohol) (PVA) hydrogels. Polymer 2015, 61, 183−191. (25) Kot, E.; Shirshova, N.; Bismarck, A.; Steinke, J. H. G. Nonaqueous high internal phase emulsion templates for synthesis of macroporous polymers in situ filled with cyclic carbonate electrolytes. RSC Adv. 2014, 4 (22), 11512−11519. (26) Shirshova, N.; Johansson, P.; Marczewski, M. J.; Kot, E.; Ensling, D.; Bismarck, A.; Steinke, J. H. G. Polymerised high internal phase ionic liquid-in-oil emulsions as potential separators for lithium ion batteries. J. Mater. Chem. A 2013, 1 (34), 9612−9619. (27) Perez-Garcia, M. G.; Carranza, A.; Puig, J. E.; Pojman, J. A.; del Monte, F.; Luna-Barcenas, G.; Mota-Morales, J. D. Porous monoliths synthesized via polymerization of styrene and divinyl benzene in nonaqueous deep-eutectic solvent-based HIPEs. RSC Adv. 2015, 5 (30), 23255−23260. (28) Carranza, A.; Song, K.; Soltero-Martínez, J. F. A.; Wu, Q.; Pojman, J. A.; Mota-Morales, J. D. On the stability and chemorheology of a urea choline chloride deep-eutectic solvent as an internal phase in acrylic high internal phase emulsions. RSC Adv. 2016, 6 (85), 81694− 81702. (29) Williams, J. M. High Internal Phase Water-in-Oil Emulsions: Influence of Surfactants and Cosurfactants on Emulsion Stability and Foam Quality. Langmuir 1991, 7 (7), 1370−1377. (30) Fu, X.; Wang, P.; Miao, Q.; Liu, K.; Liu, H.; Liu, J.; Fang, Y. Polymerizable organo-gelator-stabilized gel-emulsions toward the preparation of compressible porous polymeric monoliths. J. Mater. Chem. A 2016, 4 (39), 15215−15223. (31) Lu, Z.; Zhu, Y.; Zhang, S.; Chen, J. Aligned Porous Beads Prepared by Frozen Polymerization of Emulsion-Templates Involving Tiny Emulsifier. MATEC Web Conf. 2016, 67, No. 03045. (32) Kovačič, S.; Silverstein, M. S. Superabsorbent, high porosity, PAMPS-based hydrogels through emulsion templating. Macromol. Rapid Commun. 2016, 37 (22), 1814−1819. (33) Zhang, T.; Xu, Z.; Wu, Y.; Guo, Q. Assembled Block Copolymer Stabilized High Internal Phase Emulsion Hydrogels for Enhancing Oil Safety. Ind. Eng. Chem. Res. 2016, 55 (16), 4499−4505. (34) Zhang, T.; Xu, Z.; Cai, Z.; Guo, Q. Phase inversion of ionomerstabilized emulsions to form high internal phase emulsions (HIPEs). Phys. Chem. Chem. Phys. 2015, 17 (24), 16033−16039.

Michael S. Silverstein holds the Sherman−Gilbert Chair in Energy at the Department of Materials Science and Engineering, Technion− Israel Institute of Technology, which he joined in 1989. He is the Chairman of the Technion’s Interdepartmental Program in Polymer Engineering and an Honorary Fellow of the Israel Polymers and Plastics Society. He is presently investigating a plethora of novel emulsion-templated polymers and has developed an array of extraordinary materials including porous superabsorbent and adsorbent hydrogels, liquid-droplet-filled monoliths for storage and release, porous shape memory polymers, and carbons with hierarchical porosities. He received an Honours B.A.Sc. in Engineering Science from the University of Toronto in 1983, received a D.Sc. from the Technion in 1988, working with Prof. Moshe Narkis, and spent a year as a Postdoctoral Research Associate at Case Western Reserve University, working with Professors Eric Baer and Anne Hiltner.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the partial support of the Israel Science Foundation (294/12 and 519/16), the Israel Ministry of Science (880011), the United States−Israel Binational Science Foundation (2012074), and the German− Israeli Foundation for Scientific Research and Development (1134-16.5/2011). T. Zhang was partially supported at the Technion by an Israel Council for Higher Education Fellowship.

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REFERENCES

(1) Wu, D.; Xu, F.; Sun, B.; Fu, R.; He, H.; Matyjaszewski, K. Design and Preparation of Porous Polymers. Chem. Rev. 2012, 112 (7), 3959− 4015. (2) Gokmen, M. T.; Du Prez, F. E. Porous polymer particlesA comprehensive guide to synthesis, characterization, functionalization and applications. Prog. Polym. Sci. 2012, 37 (3), 365−405. (3) Silverstein, M. S.; Cameron, N. R.; Hillmyer, M. A. Porous Polymers; John Wiley & Sons: New York, 2011. (4) Qiu, S.; Ben, T. Porous Polymers: Design, Synthesis and Applications; Royal Society of Chemistry: Cambridge, UK, 2016. (5) Cameron, N. R.; Krajnc, P.; Silverstein, M. S. Colloidal Templating. In Porous Polymers; Silverstein, M. S., Cameron, N. R., Hillmyer, M. A., Eds.; Wiley: New York, 2011; pp 119−172. (6) Todd, E. M.; Hillmyer, M. A. Porous Polymers from SelfAssembled Structures. In Porous Polymers; Silverstein, M. S., Cameron, N. R., Hillmyer, M. A., Eds.; John Wiley & Sons: New York, 2011; pp 31−78. (7) Cameron, N. R.; Sherrington, D. C. High internal phase emulsions (HIPEs) - Structure, properties and use in polymer preparation. Adv. Polym. Sci. 1996, 126, 163−214. (8) Silverstein, M. S.; Cameron, N. R. PolyHIPEs  Porous Polymers from High Internal Phase Emulsions. In Encyclopedia of Polymer Science and Technology; John Wiley & Sons, Inc.: New York, 2010. (9) Lissant, K. J. Geometry of Emulsions. J. Soc. Cosmet. Chem. 1970, 21 (3), 141−154. (10) Silverstein, M. S. Emulsion-templated porous polymers: A retrospective perspective. Polymer 2014, 55 (1), 304−320. (11) Silverstein, M. S. PolyHIPEs: Recent advances in emulsiontemplated porous polymers. Prog. Polym. Sci. 2014, 39 (1), 199−234. (12) Silverstein, M. S. Emulsion-templated polymers: Contemporary contemplations. Polymer 2017, 126, 261−282. (13) Cameron, N. R. High internal phase emulsion templating as a route to well-defined porous polymers. Polymer 2005, 46 (5), 1439− 1449. (14) Zhang, H.; Cooper, A. I. Synthesis and applications of emulsiontemplated porous materials. Soft Matter 2005, 1 (2), 107−113. (15) Ruckenstein, E. Concentrated emulsion polymerization. Adv. Polym. Sci. 1997, 127, 1−58. Y


Perspective

Macromolecules (35) Viswanathan, P.; Johnson, D. W.; Hurley, C.; Cameron, N. R.; Battaglia, G. 3D Surface Functionalization of Emulsion-Templated Polymeric Foams. Macromolecules 2014, 47 (20), 7091−7098. (36) Zhang, T.; Xu, Z.; Guo, Q. Closed-cell and open-cell porous polymers from ionomer-stabilized high internal phase emulsions. Polym. Chem. 2016, 7 (48), 7469−7476. (37) Kovačič, S.; Preishuber-Pflügl, F.; Pahovnik, D.; Ž agar, E.; Slugovc, C. Covalent incorporation of the surfactant into high internal phase emulsion templated polymeric foams. Chem. Commun. 2015, 51, 7725−7728. (38) Mathieu, K.; Jérôme, C.; Debuigne, A. Influence of the Macromolecular Surfactant Features and Reactivity on Morphology and Surface Properties of Emulsion-Templated Porous Polymers. Macromolecules 2015, 48 (18), 6489−6498. (39) Khodabandeh, A.; Dario Arrua, R.; Desire, C. T.; Rodemann, T.; Bon, S. A. F.; Thickett, S. C.; Hilder, E. F. Preparation of inverse polymerized high internal phase emulsions using an amphiphilic macro-RAFT agent as sole stabilizer. Polym. Chem. 2016, 7 (9), 1803− 1812. (40) Mathieu, K.; De Winter, J.; Jérô me, C.; Debuigne, A. Simultaneous synthesis and chemical functionalization of emulsiontemplated porous polymers using nitroxide-terminated macromolecular surfactants. Polym. Chem. 2017, 8 (11), 1850−1861. (41) Zhang, T.; Silverstein, M. S. Microphase-separated macroporous polymers from an emulsion-templated reactive triblock copolymer. Macromolecules 2018, 51, 3828−3835. (42) Zhang, T.; Silverstein, M. S. Robust, highly porous hydrogels templated within emulsions stabilized using a reactive, crosslinking triblock copolymer. Polymer 2019, 168, 146−154. (43) Menner, A.; Ikem, V.; Salgueiro, M.; Shaffer, M. S. P.; Bismarck, A. High internal phase emulsion templates solely stabilised by functionalised titania nanoparticles. Chem. Commun. 2007, 41, 4274−4276. (44) Gurevitch, I.; Silverstein, M. S. Nanoparticle-Based and Organic-Phase-Based AGET ATRP PolyHIPE Synthesis within Pickering HIPEs and Surfactant-Stabilized HIPEs. Macromolecules 2011, 44 (9), 3398−3409. (45) Kim, K.; Kim, S.; Ryu, J.; Jeon, J.; Jang, S. G.; Kim, H.; Gweon, D. G.; Im, W. B.; Han, Y.; Kim, H.; Choi, S. Q. Processable high internal phase Pickering emulsions using depletion attraction. Nat. Commun. 2017, 8, 14305. (46) Yi, W.; Wu, H.; Wang, H.; Du, Q. Interconnectivity of macroporous hydrogels prepared via graphene oxide-stabilized pickering high internal phase emulsions. Langmuir 2016, 32 (4), 982−990. (47) Hu, Y.; Huang, J.; Zhang, Q.; Yang, Y.; Ma, S.; Wang, C. Functional nanoparticle-decorated graphene oxide sheets as stabilizers for Pickering high internal phase emulsions and graphene oxide based foam monoliths. RSC Adv. 2015, 5 (125), 103394−103402. (48) Wong, L. L. C.; Barg, S.; Menner, A.; do Vale Pereira, P.; Eda, G.; Chowalla, M.; Saiz, E.; Bismarck, A. Macroporous polymer nanocomposites synthesised from high internal phase emulsion templates stabilised by reduced graphene oxide. Polymer 2014, 55 (1), 395−402. (49) Hermant, M. C.; Verhulst, M.; Kyrylyuk, A. V.; Klumperman, B.; Koning, C. E. The incorporation of single-walled carbon nanotubes into polymerized high internal phase emulsions to create conductive foams with a low percolation threshold. Compos. Sci. Technol. 2009, 69 (5), 656−662. (50) Cohen, N.; Samoocha, D. C.; David, D.; Silverstein, M. S. Carbon nanotubes in emulsion-templated porous polymers: Polymer nanoparticles, sulfonation, and conductivity. J. Polym. Sci., Part A: Polym. Chem. 2013, 51 (20), 4369−4377. (51) Ikem, V. O.; Menner, A.; Bismarck, A. High internal phase emulsions stabilized solely by functionalized silica particles. Angew. Chem., Int. Ed. 2008, 47 (43), 8277−8279. (52) Zheng, X.; Zhang, Y.; Wang, H.; Du, Q. Interconnected macroporous polymers synthesized from silica particle stabilized high internal phase emulsions. Macromolecules 2014, 47 (19), 6847−6855.

(53) Vilchez, A.; Rodriguez-Abreu, C.; Esquena, J.; Menner, A.; Bismarck, A. Macroporous polymers obtained in highly concentrated emulsions stabilized solely with magnetic nanoparticles. Langmuir 2011, 27 (21), 13342−52. (54) Li, Z.; Ming, T.; Wang, J.; Ngai, T. High internal phase emulsions stabilized solely by microgel particles. Angew. Chem., Int. Ed. 2009, 48 (45), 8490−8493. (55) Zhu, Y.; Zhang, R.; Zhang, S.; Chu, Y.; Chen, J. Macroporous polymers with aligned microporous walls from Pickering high internal phase emulsions. Langmuir 2016, 32 (24), 6083−6088. (56) Li, Z.; Xiao, M.; Wang, J.; Ngai, T. Pure protein scaffolds from pickering high internal phase emulsion template. Macromol. Rapid Commun. 2013, 34 (2), 169−174. (57) Capron, I.; Cathala, B. Surfactant-Free High Internal Phase Emulsions Stabilized by Cellulose Nanocrystals. Biomacromolecules 2013, 14 (2), 291−6. (58) Oh, B. H. L.; Bismarck, A.; Chan-Park, M. B. Injectable, Interconnected, High-Porosity Macroporous Biocompatible Gelatin Scaffolds Made by Surfactant-Free Emulsion Templating. Macromol. Rapid Commun. 2015, 36 (4), 364−372. (59) Yi, F.; Xu, F.; Gao, Y.; Li, H.; Chen, D. Macrocellular polymer foams from water in oil high internal phase emulsion stabilized solely by polymer Janus nanoparticles: preparation and their application as support for Pd catalyst. RSC Adv. 2015, 5 (50), 40227−40235. (60) Li, T.; Liu, H.; Zeng, L.; Yang, S.; Li, Z.; Zhang, J.; Zhou, X. Macroporous magnetic poly(styrene−divinylbenzene) nanocomposites prepared via magnetite nanoparticles-stabilized high internal phase emulsions. J. Mater. Chem. 2011, 21 (34), 12865. (61) Zhao, Z.; Liu, Y.; Zhang, K.; Zhuo, S.; Fang, R.; Zhang, J.; Jiang, L.; Liu, M. Biphasic synergistic gel materials with switchable mechanics and self-healing capacity. Angew. Chem., Int. Ed. 2017, 56 (43), 13464−13469. (62) Zhu, Y.; Zhang, S.; Chen, J.; Hu, C. P. High internal phase emulsions prepared with poly(urethane urea) aqueous nanodispersion at different temperatures. J. Polym. Sci., Part A: Polym. Chem. 2010, 48 (19), 4356−4360. (63) Chen, Y.; Ballard, N.; Gayet, F.; Bon, S. A. F. High internal phase emulsion gels (HIPE-gels) from polymer dispersions reinforced with quadruple hydrogen bond functionality. Chem. Commun. 2012, 48 (8), 1117−1119. (64) Chen, Y.; Ballard, N.; Bon, S. A. F. Moldable high internal phase emulsion hydrogel objects from non-covalently crosslinked poly(Nisopropylacrylamide) nanogel dispersions. Chem. Commun. 2013, 49 (15), 1524−1526. (65) Chen, Q.; Cao, X.; Liu, H.; Zhou, W.; Qin, L.; An, Z. pHresponsive high internal phase emulsions stabilized by core crosslinked star (CCS) polymers. Polym. Chem. 2013, 4 (15), 4092−4102. (66) Chen, Q.; Deng, X.; An, Z. pH-induced inversion of water-in-oil emulsions to oil-in-water high internal phase emulsions (HIPEs) using core cross-linked star (CCS) polymer as interfacial stabilizer. Macromol. Rapid Commun. 2014, 35 (12), 1148−52. (67) Miao, R.; Peng, J.; Fang, Y. Molecular Gels as Intermediates in the Synthesis of Porous Materials and Fluorescent Films: Concepts and Applications. Langmuir 2017, 33 (40), 10419−10428. (68) Chen, X.; Liu, K.; He, P.; Zhang, H.; Fang, Y. Preparation of Novel W/O Gel-Emulsions and Their Application in the Preparation of Low-Density Materials. Langmuir 2012, 28 (25), 9275−9281. (69) Zhang, T.; Guo, Q. High internal phase emulsion (HIPE) organogels prepared from charge-driven assembled polymer organogels. Chem. Commun. 2013, 49 (100), 11803−11805. (70) Puupponen, S.; Mikkola, V.; Ala-Nissila, T.; Seppälä, A. Novel microstructured polyol−polystyrene composites for seasonal heat storage. Appl. Energy 2016, 172, 96−106. (71) Tebboth, M.; Jiang, Q.; Kogelbauer, A.; Bismarck, A. Inflatable Elastomeric Macroporous Polymers Synthesized from Medium Internal Phase Emulsion Templates. ACS Appl. Mater. Interfaces 2015, 7 (34), 19243−50. (72) Zhang, T.; Guo, Q. Continuous preparation of polyHIPE monoliths from ionomer-stabilized high internal phase emulsions Z


Perspective

Macromolecules (HIPEs) for efficient recovery of spilled oils. Chem. Eng. J. 2017, 307, 812−819. (73) Nalawade, A. C.; Ghorpade, R. V.; Shadbar, S.; Qureshi, M. S.; Chavan, N. N.; Khan, A. A.; Ponrathnam, S. Inverse high internal phase emulsion polymerization (i-HIPE) of GMMA, HEMA and GDMA for the preparation of superporous hydrogels as a tissue engineering scaffold. J. Mater. Chem. B 2016, 4 (3), 450−460. (74) Grant, N. C.; Cooper, A. I.; Zhang, H. Uploading and temperature-controlled release of polymeric colloids via hydrophilic emulsion-templated porous polymers. ACS Appl. Mater. Interfaces 2010, 2 (5), 1400−1406. (75) Wan, Y.; Feng, Y.; Wan, D.; Jin, M. Polyamino amphiphile mediated support of platinum nanoparticles on polyHIPE as an over 1500-time recyclable catalyst. RSC Adv. 2016, 6 (110), 109253− 109258. (76) Kovacic, S.; Kren, H.; Krajnc, P.; Koller, S.; Slugovc, C. The use of an emulsion templated microcellular poly(dicyclopentadiene-conorbornene) membrane as a separator in lithium-ion batteries. Macromol. Rapid Commun. 2013, 34 (7), 581−7. (77) Wong, L. L.; Ikem, V. O.; Menner, A.; Bismarck, A. Macroporous polymers with hierarchical pore structure from emulsion templates stabilised by both particles and surfactants. Macromol. Rapid Commun. 2011, 32 (19), 1563−1568. (78) Williams, J. M.; Wrobleski, D. A. Spatial-Distribution of the Phases in Water-in-Oil Emulsions - Open and Closed Microcellular Foams from Cross-Linked Polystyrene. Langmuir 1988, 4 (3), 656− 662. (79) Cameron, N. R.; Barbetta, A. The influence of porogen type on the porosity,surface area and morphology of poly(divinylbenzene) PolyHIPE foams. J. Mater. Chem. 2000, 10, 2466−2471. (80) Barbetta, A.; Cameron, N. R. Morphology and surface area of emulsion-derived (polyHIPE) solid foams prepared with oil-phase soluble porogenic solvents: span 80 as surfactant. Macromolecules 2004, 37 (9), 3188−3201. (81) Pakeyangkoon, P.; Magaraphan, R.; Malakul, P.; Nithitanakul, M. Effect of Soxhlet Extraction and Surfactant System on Morphology and Properties of Poly(DVB)PolyHIPE. Macromol. Symp. 2008, 264 (1), 149−156. (82) Huang, X.; Yang, Y.; Shi, J.; Ngo, H. T.; Shen, C.; Du, W.; Wang, Y. High-Internal-Phase Emulsion Tailoring Polymer Amphiphilicity towards an Efficient NIR-Sensitive Bacteria Filter. Small 2015, 11 (37), 4876−4883. (83) Yang, Y.; Wei, Z.; Wang, C.; Tong, Z. Lignin-based Pickering HIPEs for macroporous foams and their enhanced adsorption of copper(II) ions. Chem. Commun. 2013, 49 (64), 7144−7146. (84) Ikem, V. O.; Menner, A.; Horozov, T. S.; Bismarck, A. Highly Permeable Macroporous Polymers Synthesized from Pickering Medium and High Internal Phase Emulsion Templates. Adv. Mater. 2010, 22 (32), 3588−92. (85) Tu, S.; Zhu, C.; Zhang, L.; Wang, H.; Du, Q. Pore Structure of Macroporous Polymers Using Polystyrene/Silica Composite Particles as Pickering Stabilizers. Langmuir 2016, 32 (49), 13159−13166. (86) Tebboth, M.; Kogelbauer, A.; Bismarck, A. Highly permeable macroporous polymers via controlled agitation of emulsion templates. Chem. Eng. Sci. 2015, 137, 786−795. (87) Gui, H.; Zhang, T.; Guo, Q. Closed-cell, emulsion-templated hydrogels for latent heat storage applications. Polym. Chem. 2018, 9, 3970. (88) Weinstock, L.; Sanguramath, R. A.; Silverstein, M. S. Encapsulating an organic phase change material within emulsiontemplated poly(urethane urea)s. Polym. Chem. 2019, 10 (12), 1498− 1507. (89) Zhang, T.; Xu, G.; Li, Z.-F.; Regev, O.; Maddumaarachchi, M.; Blum, F. D. PS/CTAB/silica composites from room temperature polymerization of high internal phase emulsion gels. J. Colloid Interface Sci. 2015, 451, 161−169. (90) Zhang, T.; Xu, G.; Regev, O.; Blum, F. D. Low-temperature polymerization of methyl methacrylate emulsion gels through surfactant catalysis. J. Colloid Interface Sci. 2016, 461, 128−135.

(91) Moglia, R. S.; Whitely, M.; Dhavalikar, P.; Robinson, J.; Pearce, H.; Brooks, M.; Stuebben, M.; Cordner, N.; Cosgriff-Hernandez, E. Injectable Polymerized High Internal Phase Emulsions with Rapid in Situ Curing. Biomacromolecules 2014, 15 (8), 2870−2878. (92) Butler, R.; Hopkinson, I.; Cooper, A. I. Synthesis of porous emulsion-templated polymers using high internal phase CO2-in-water emulsions. J. Am. Chem. Soc. 2003, 125 (47), 14473−14481. (93) Wu, R.; Menner, A.; Bismarck, A. Macroporous polymers made from medium internal phase emulsion templates: Effect of emulsion formulation on the pore structure of polyMIPEs. Polymer 2013, 54 (21), 5511−5517. (94) Quell, A.; de Bergolis, B.; Drenckhan, W.; Stubenrauch, C. How the Locus of Initiation Influences the Morphology and the Pore Connectivity of a Monodisperse Polymer Foam. Macromolecules 2016, 49 (14), 5059−5067. (95) Luo, Y.; Wang, A.-N.; Gao, X. One-pot interfacial polymerization to prepare PolyHIPEs with functional surface. Colloid Polym. Sci. 2015, 293 (6), 1767−1779. (96) Khodabandeh, A.; Dario Arrua, R.; Desire, C. T.; Rodemann, T.; Bon, S. A. F.; Thickett, S. C.; Hilder, E. F. Preparation of inverse polymerized high internal phase emulsions using an amphiphilic macro-RAFT agent as sole stabilizer. Polym. Chem. 2016, 7 (9), 1803− 1812. (97) Gurevitch, I.; Silverstein, M. S. Polymerized pickering HIPEs: Effects of synthesis parameters on porous structure. J. Polym. Sci., Part A: Polym. Chem. 2010, 48 (7), 1516−1525. (98) Christenson, E. M.; Soofi, W.; Holm, J. L.; Cameron, N. R.; Mikos, A. G. Biodegradable Fumarate-Based PolyHIPEs as Tissue Engineering Scaffolds. Biomacromolecules 2007, 8 (12), 3806−3814. (99) Livshin, S.; Silverstein, M. S. Cross-linker flexibility in porous crystalline polymers synthesized from long side-chain monomers through emulsion templating. Soft Matter 2008, 4 (8), 1630−1638. (100) Ikem, V. O.; Menner, A.; Bismarck, A. High-Porosity Macroporous Polymers Sythesized from Titania-Particle-Stabilized Medium and High Internal Phase Emulsions. Langmuir 2010, 26 (11), 8836−8841. (101) Wong, L. L. C.; Ikem, V. O.; Menner, A.; Bismarck, A. Macroporous Polymers with Hierarchical Pore Structure from Emulsion Templates Stabilised by Both Particles and Surfactants. Macromol. Rapid Commun. 2011, 32 (19), 1563−1568. (102) Jiang, Q.; Menner, A.; Bismarck, A. Robust macroporous polymers: Using polyurethane diacrylate as property defining crosslinker. Polymer 2016, 97, 598−603. (103) Robinson, J. L.; Moglia, R. S.; Stuebben, M. C.; McEnery, M. A. P.; Cosgriff-Hernandez, E. Achieving Interconnected Pore Architecture in Injectable PolyHIPEs for Bone Tissue Engineering. Tissue Eng., Part A 2014, 20 (5−6), 1103−1112. (104) Cameron, N. R.; Sherrington, D. C.; Albiston, L.; Gregory, D. P. Study of the formation of the open-cellular morphology of poly(styrene/divinylbenzene) polyHIPE materials by cryo-SEM. Colloid Polym. Sci. 1996, 274 (6), 592−595. (105) Xu, H.; Zheng, X.; Huang, Y.; Wang, H.; Du, Q. Interconnected Porous Polymers with Tunable Pore Throat Size Prepared via Pickering High Internal Phase Emulsions. Langmuir 2016, 32 (1), 38−45. (106) Livshin, S.; Silverstein, M. S. Crystallinity and Cross-Linking in Porous Polymers Synthesized from Long Side Chain Monomers through Emulsion Templating. Macromolecules 2008, 41 (11), 3930− 3938. (107) Gitli, T.; Silverstein, M. S. Bicontinuous hydrogel-hydrophobic polymer systems through emulsion templated simultaneous polymerizations. Soft Matter 2008, 4 (12), 2475−2485. (108) Quell, A.; Sottmann, T.; Stubenrauch, C. Diving into the Finestructure of Macroporous Polymer Foams Synthesized via Emulsion Templating: A Phase Diagram Study. Langmuir 2017, 33 (2), 537−542. (109) Gurevitch, I.; Silverstein, M. S. One-Pot Synthesis of Elastomeric Monoliths Filled with Individually Encapsulated Liquid Droplets. Macromolecules 2012, 45 (16), 6450−6456. AA


Perspective

Macromolecules (110) Pierre, S. J.; Thies, J. C.; Dureault, A.; Cameron, N. R.; van Hest, J. C. M.; Carette, N.; Michon, T.; Weberskirch, R. Covalent Enzyme Immobilization onto Photopolymerized Highly Porous Monoliths. Adv. Mater. 2006, 18 (14), 1822−1826. (111) Johnson, D. W.; Sherborne, C.; Didsbury, M. P.; Pateman, C.; Cameron, N. R.; Claeyssens, F. Macrostructuring of emulsiontemplated porous polymers by 3D laser patterning. Adv. Mater. 2013, 25 (23), 3178−3181. (112) Sušec, M.; Ligon, S. C.; Stampfl, J.; Liska, R.; Krajnc, P. Hierarchically Porous Materials from Layer-by-Layer Photopolymerization of High Internal Phase Emulsions. Macromol. Rapid Commun. 2013, 34 (11), 938−943. (113) Johnson, D. W.; Langford, C. R.; Didsbury, M. P.; Lipp, B.; Przyborski, S. A.; Cameron, N. R. Fully biodegradable and biocompatible emulsion templated polymer scaffolds by thiol-acrylate polymerization of polycaprolactone macromonomers. Polym. Chem. 2015, 6 (41), 7256−7263. (114) Caldwell, S.; Johnson, D. W.; Didsbury, M. P.; Murray, B. A.; Wu, J. J.; Przyborski, S. A.; Cameron, N. R. Degradable emulsiontemplated scaffolds for tissue engineering from thiol-ene photopolymerisation. Soft Matter 2012, 8 (40), 10344−10351. (115) Lovelady, E.; Kimmins, S. D.; Wu, J.; Cameron, N. R. Preparation of emulsion-templated porous polymers using thiol-ene and thiol-yne chemistry. Polym. Chem. 2011, 2 (3), 559−562. (116) Sušec, M.; Paljevac, M.; Kotek, J.; Krajnc, P. Microcellular open porous polyester membranes from thiol-ene polymerisations of high internal phase emulsions. Des. Monomers Polym. 2016, 19 (6), 577−583. (117) Zhang, T.; Guo, Q. Isorefractive high internal phase emulsion organogels for light induced reactions. Chem. Commun. 2016, 52 (24), 4561−4564. (118) Perrier, S. 50th Anniversary Perspective: RAFT PolymerizationA User Guide. Macromolecules 2017, 50 (19), 7433−7447. (119) Luo, Y.; Wang, A.-N.; Gao, X. Pushing the mechanical strength of PolyHIPEs up to the theoretical limit through living radical polymerization. Soft Matter 2012, 8 (6), 1824−1830. (120) Streifel, B. C.; Parker, J. F.; Giles, S. L.; Williams, S. J.; Duncan, J. H.; Wynne, J. H. Porosity control in high internal phase emulsion templated polyelectrolytes via ionic crosslinking. J. Polym. Sci., Part A: Polym. Chem. 2016, 54 (16), 2486−2492. (121) Gibson, L. J.; Ashby, M. F. Cellular Solids Structure and Properties, 2nd ed.; Cambridge University Press: Cambridge, 1999. (122) Lamson, M.; Epshtein-Assor, Y.; Silverstein, M. S.; Matyjaszewski, K. Synthesis of degradable polyHIPEs by AGET ATRP. Polymer 2013, 54 (17), 4480−4485. (123) Audouin, F.; Larragy, R.; Fox, M.; O’Connor, B.; Heise, A. Protein Immobilization onto Poly(acrylic acid) Functional Macroporous PolyHIPE Obtained by Surface-Initiated ARGET ATRP. Biomacromolecules 2012, 13 (11), 3787−3794. (124) Sun, F. M.; Xu, W. F.; Chen, M. S.; Zhang, F. A. Polymerization of MMA in polyHIPEs by RATRP. J. Polym. Res. 2014, 21 (12), 1−8. (125) Chen, M.; Sun, F.; Xu, W.; Zhou, L.; Zhang, F. Polymerization of MMA by RATRP in the confined space of polyHIPEs with varied internal phase contents. J. Mater. Sci. 2016, 51 (11), 5113−5121. (126) David, D.; Silverstein, M. S. Porous polyurethanes synthesized within high internal phase emulsions. J. Polym. Sci., Part A: Polym. Chem. 2009, 47 (21), 5806−5814. (127) Moglia, R. S.; Robinson, J. L.; Muschenborn, A. D.; Touchet, T. J.; Maitland, D. J.; Cosgriff-Hernandez, E. Injectable polyMIPE scaffolds for soft tissue regeneration. Polymer 2014, 55 (1), 426−434. (128) Grosse, M.-T.; Lamotte, M.; Birot, M.; Deleuze, H. Preparation of microcellular polysiloxane monoliths. J. Polym. Sci., Part A: Polym. Chem. 2008, 46 (1), 21−32. (129) Pan, J.; Luo, J.; Cao, J.; Liu, J.; Huang, W.; Zhang, W.; Yang, L. Competitive adsorption of three phenolic compounds to hydrophilic urea-formaldehyde macroporous foams derived from lignin-based Pickering HIPEs template. RSC Adv. 2016, 6 (96), 93894−93904.

(130) Langford, C. R.; Johnson, D. W.; Cameron, N. R. Chemical functionalization of emulsion-templated porous polymers by thiol− ene “click” chemistry. Polym. Chem. 2014, 5 (21), 6200−6206. (131) Zhang, H.; Zhu, Y.; Chen, J.; Zhang, S. Preparation of polyHIPE via CuAAC “click” chemistry and its application as a highly efficient adsorbent of Cu(II) ions. J. Polym. Sci., Part A: Polym. Chem. 2017, 55 (13), 2129−2135. (132) Xiao, C.; Zhang, S.; Chen, J. Synthesis of Emulsion-templated Porous Polycaprolactone. MATEC Web Conf. 2016, 67, No. 06108. (133) Xiao, C.; Zhu, Y.; Chen, J.; Zhang, S. Synthesis of emulsiontemplated macroporous materials via Diels-Alder polymerization. Polymer 2017, 110, 74−79. (134) Deleuze, H.; Faivre, R.; Herroguez, V. Preparation and functionalisation of emulsion-derived microcellular polymeric foams (polyHIPEs) by ring-opening metathesis polymerisation (ROMP). Chem. Commun. 2002, 23, 2822−2823. (135) Kovacic, S.; Krajnc, P.; Slugovc, C. Inherently reactive polyHIPE material from dicyclopentadiene. Chem. Commun. 2010, 46 (40), 7504−7506. (136) Mert, E. H.; Slugovc, C.; Krajnc, P. Tailoring the mechanical and thermal properties of dicyclopentadiene polyHIPEs with the use of a comonomer. eXPRESS Polym. Lett. 2015, 9 (4), 344−353. (137) Kovacic, S.; Preishuber-Pflugl, F.; Pahovnik, D.; Zagar, E.; Slugovc, C. Covalent incorporation of the surfactant into high internal phase emulsion templated polymeric foams. Chem. Commun. 2015, 51 (36), 7725−7728. (138) Kovacic, S.; Jerabek, K.; Krajnc, P.; Slugovc, C. Ring opening metathesis polymerisation of emulsion templated dicyclopentadiene giving open porous materials with excellent mechanical properties. Polym. Chem. 2012, 3 (2), 325−328. (139) Kovačič, S.; Kren, H.; Krajnc, P.; Koller, S.; Slugovc, C. The Use of an Emulsion Templated Microcellular Poly(dicyclopentadieneco-norbornene) Membrane as a Separator in Lithium-Ion Batteries. Macromol. Rapid Commun. 2013, 34 (7), 581−587. (140) Sharma, E.; Samanta, A.; Pal, J.; Bahga, S. S.; Nandan, B.; Srivastava, R. K. High Internal Phase Emulsion Ring-Opening Polymerization of Pentadecanolide: Strategy to Obtain Porous Scaffolds in a Single Step. Macromol. Chem. Phys. 2016, 217 (15), 1752−1758. (141) Szczurek, A.; Fierro, V.; Thébault, M.; Pizzi, A.; Celzard, A. Structure and properties of poly(furfuryl alcohol)-tannin polyHIPEs. Eur. Polym. J. 2016, 78, 195−212. (142) Szczurek, A.; Martinez de Yuso, A.; Fierro, V.; Pizzi, A.; Celzard, A. Tannin-based monoliths from emulsion-templating. Mater. Eng. 2015, 79 (0), 115−126. (143) Foulet, A.; Birot, M.; Backov, R.; Sonnemann, G.; Deleuze, H. Preparation of hierarchical porous carbonaceous foams from Kraft black liquor. Materials Today Communications 2016, 7, 108−116. (144) Foulet, A.; Birot, M.; Sonnemann, G.; Deleuze, H. The potential of Kraft black liquor to produce bio-based emulsiontemplated porous materials. React. Funct. Polym. 2015, 90, 15−20. (145) Forgacz, C.; Birot, M.; Deleuze, H. Synthesis of macroporous monolithic materials from a waste renewable source. J. Appl. Polym. Sci. 2015, 132 (1), 41215. (146) Blaker, J. J.; Lee, K. Y.; Li, X. X.; Menner, A.; Bismarck, A. Renewable nanocomposite polymer foams synthesized from Pickering emulsion templates. Green Chem. 2009, 11 (9), 1321−1326. (147) Brun, N.; Edembe, L.; Gounel, S.; Mano, N.; Titirici, M. M. Emulsion-Templated Macroporous Carbons Synthesized By Hydrothermal Carbonization and their Application for the Enzymatic Oxidation of Glucose. ChemSusChem 2013, 6 (4), 701−710. (148) Flexer, V.; Donose, B. C.; Lefebvre, C.; Pozo, G.; Boone, M. N.; Van Hoorebeke, L.; Baccour, M.; Bonnet, L.; Calas-Etienne, S.; Galarneau, A.; Titirici, M. M.; Brun, N. Microcellular Electrode Material for Microbial Bioelectrochemical Systems Synthesized by Hydrothermal Carbonization of Biomass Derived Precursors. ACS Sustainable Chem. Eng. 2016, 4 (5), 2508−2516. AB


Perspective

Macromolecules (149) Cameron, N. R.; Sherrington, D. C. Synthesis and characterization of poly(aryl ether sulfone) PolyHIPE materials. Macromolecules 1997, 30 (19), 5860−5869. (150) Carranza, A.; Pojman, J. A.; Mota-Morales, J. D. Deep-eutectic solvents as a support in the nonaqueous synthesis of macroporous poly(HIPEs). RSC Adv. 2014, 4 (78), 41584−41587. (151) Carranza, A.; Romero-Perez, D.; Almanza-Reyes, H.; Bogdanchikova, N.; Juarez-Moreno, K.; Pojman, J. A.; Velasquillo, C.; Mota-Morales, J. D. Nonaqueous Synthesis of Macroporous Nanocomposites Using High Internal Phase Emulsion Stabilized by Nanohydroxyapatite. Adv. Mater. Interfaces 2017, 4 (16), 1700094. (152) Khodabandeh, A.; Arrua, R. D.; Mansour, F. R.; Thickett, S. C.; Hilder, E. F. PEO-based brush-type amphiphilic macro-RAFT agents and their assembled polyHIPE monolithic structures for applications in separation science. Sci. Rep. 2017, 7 (1), 7847. (153) Liang, J.; Wu, Y.; Deng, X.; Deng, J. Optically Active Porous Materials Constructed by Chirally Helical Substituted Polyacetylene through a High Internal Phase Emulsion Approach and the Application in Enantioselective Crystallization. ACS Macro Lett. 2015, 4 (10), 1179−1183. (154) Silverstein, M. S.; Tai, H.; Sergienko, A.; Lumelsky, Y.; Pavlovsky, S. PolyHIPE: IPNs, hybrids, nanoscale porosity, silica monoliths and ICP-based sensors. Polymer 2005, 46 (17), 6682−6694. (155) Lepine, O.; Birot, M.; Deleuze, H. Preparation of macrocellular PU-PS interpenetrating networks. Polymer 2005, 46 (23), 9653−9663. (156) Lei, L.; Zhang, Q.; Shi, S.; Zhu, S. High internal phase emulsion with double emulsion morphology and their templated porous polymer systems. J. Colloid Interface Sci. 2016, 483, 232−240. (157) Li, Z.; Liu, H.; Zeng, L.; Liu, H.; Yang, S.; Wang, Y. Preparation of high internal water-phase double emulsions stabilized by a single anionic surfactant for fabricating interconnecting porous polymer microspheres. Langmuir 2014, 30 (41), 12154−63. (158) Zhang, H.; Cooper, A. I. Synthesis of monodisperse emulsiontemplated polymer beads by oil-in-water-in-oil (O/W/O) sedimentation polymerization. Chem. Mater. 2002, 14, 4017−4020. (159) Samanta, A.; Nandan, B.; Srivastava, R. K. Morphology of electrospun fibers derived from high internal phase emulsions. J. Colloid Interface Sci. 2016, 471, 29−36. (160) Lafleur, J. P.; Senkbeil, S.; Novotny, J.; Nys, G.; Bogelund, N.; Rand, K. D.; Foret, F.; Kutter, J. P. Rapid and simple preparation of thiol-ene emulsion-templated monoliths and their application as enzymatic microreactors. Lab Chip 2015, 15 (10), 2162−72. (161) Bell, R. V.; Parkins, C. C.; Young, R. A.; Preuss, C. M.; Stevens, M. M.; Bon, S. A. F. Assembly of emulsion droplets into fibers by microfluidic wet spinning. J. Mater. Chem. A 2016, 4 (3), 813−818. (162) Costantini, M.; Colosi, C.; Guzowski, J.; Barbetta, A.; Jaroszewicz, J.; Świȩszkowski, W.; Dentini, M.; Garstecki, P. Highly ordered and tunable polyHIPEs by using microfluidics. J. Mater. Chem. B 2014, 2 (16), 2290−2300. (163) Pulko, I.; Smrekar, V.; Podgornik, A.; Krajnc, P. Emulsion templated open porous membranes for protein purification. Journal of Chromatography A 2011, 1218 (17), 2396−2401. (164) Mazaj, M.; Logar, N. Z.; Zagar, E.; Kovacic, S. A facile strategy towards a highly accessible and hydrostable MOF-phase within hybrid polyHIPEs through in situ metal-oxide recrystallization. J. Mater. Chem. A 2017, 5 (5), 1967−1971. (165) Zhang, H.; Hardy, G. C.; Khimyak, Y. Z.; Rosseinsky, M. J.; Cooper, A. I. Synthesis of hierarchically porous silica and metal oxide beads using emulsion-templated polymer scaffolds. Chem. Mater. 2004, 16 (22), 4245−4256. (166) Sergienko, A. Y.; Tai, H. W.; Narkis, M.; Silverstein, M. S. Polymerized high internal-phase emulsions: Properties and interaction with water. J. Appl. Polym. Sci. 2002, 84 (11), 2018−2027. (167) Lumelsky, Y.; Lalush-Michael, I.; Levenberg, S.; Silverstein, M. S. A degradable, porous, emulsion-templated polyacrylate. J. Polym. Sci., Part A: Polym. Chem. 2009, 47 (24), 7043−7053. (168) Li, C.; Du, Z.; Zou, W.; Li, H.; Zhang, C. Fabrication of copper coated polymer foam and their application for hexavalent chromium removal. React. Funct. Polym. 2015, 88, 24−30.

(169) Livshin, S.; Silverstein, M. S. Enhancing hydrophilicity in a hydrophobic porous emulsion-templated polyacrylate. J. Polym. Sci., Part A: Polym. Chem. 2009, 47 (18), 4840−4845. (170) Feng, Y.; Zhang, X.; Jin, M.; Wan, D. Dendritic amphiphiledecorated polyHIPE as a highly efficient and well recyclable scavenger of micropollutants in water: Topological effect. J. Polym. Sci., Part A: Polym. Chem. 2017, 55 (8), 1294−1302. (171) Majer, J.; Paljevac, M.; Ž agar, E.; Kovačič, S.; Krajnc, P. Functionalization of 2-hydroxyethyl methacrylate-based polyHIPEs: Effect of the leaving group. React. Funct. Polym. 2016, 109, 99−103. (172) San, N.; Mert, E. H.; Kaya, D.; Cira, F. Adsorption characteristics, isotherm and kinetics of a novel polyHIPE/pullulan composite for removing Congo Red dye. Fresenius Environ. Bull. 2016, 25 (9), 3635−3645. (173) Brusotti, G.; Calleri, E.; Milanese, C.; Catenacci, L.; Marrubini, G.; Sorrenti, M.; Girella, A.; Massolini, G.; Tripodo, G. Rational design of functionalized polyacrylate-based high internal phase emulsion materials for analytical and biomedical uses. Polym. Chem. 2016, 7 (48), 7436−7445. (174) Krajnc, P.; Stefanec, D.; Brown, J. F.; Cameron, N. R. Aryl acrylate based high-internal-phase emulsions as precursors for reactive monolithic polymer supports. J. Polym. Sci., Part A: Polym. Chem. 2005, 43 (2), 296−303. (175) Ye, Y.; Jin, M.; Wan, D. One-pot synthesis of porous monolithsupported gold nanoparticles as an effective recyclable catalyst. J. Mater. Chem. A 2015, 3 (25), 13519−13525. (176) Cameron, N. R.; Sherrington, D. C.; Ando, I.; Kurosu, H. Chemical modification of monolithic poly(styrene-divinylbenzene) PolyHIPE(R) materials. J. Mater. Chem. 1996, 6 (5), 719−726. (177) Barlık, N.; Keskinler, B.; Kocakerim, M. M.; Akay, G. Surface modification of monolithic PolyHIPE Polymers for anionic functionality and their ion exchange behavior. J. Appl. Polym. Sci. 2015, 132 (29), 42286. (178) Moghbeli, M. R.; Khajeh, A.; Alikhani, M. Nanosilica reinforced ion-exchange polyHIPE type membrane for removal of nickel ions: Preparation, characterization and adsorption studies. Chem. Eng. J. 2017, 309, 552−562. (179) Patil, U. M.; Ghorpade, R. V.; Nam, M. S.; Nalawade, A. C.; Lee, S.; Han, H.; Jun, S. C. PolyHIPE Derived Freestanding 3D Carbon Foam for Cobalt Hydroxide Nanorods Based High Performance Supercapacitor. Sci. Rep. 2016, 6, 35490. (180) Kovačič, S.; Silverstein, M. S. Superabsorbent, High Porosity, PAMPS-Based Hydrogels through Emulsion Templating. Macromol. Rapid Commun. 2016, 37 (22), 1814−1819. (181) Pan, J.; Mao, Y.; Gao, H.; Xiong, Q.; Qiu, F.; Zhang, T.; Niu, X. Fabrication of hydrophobic polymer foams with double acid sites on surface of macropore for conversion of carbohydrate. Carbohydr. Polym. 2016, 143, 212−222. (182) Li, W.; Li, L.; Cui, G.; Bai, Y.; Xiao, X.; Li, Y.; Yan, L. HIPE Polymerization Materials Functionalized with Iodic-BODIPY on the Surface as Porous Heterogeneous Visible-Light Photocatalysts. Chem. Asian J. 2017, 12 (4), 392−396. (183) Kawada, K.; Okano, K.; Iskra, J.; Krajnc, P.; Cahard, D. SelectfluorTM on a PolyHIPE Material as Regenerative and Reusable Polymer-Supported Electrophilic Fluorinating Agent. Adv. Synth. Catal. 2017, 359 (4), 584−589. (184) Israel, S.; Gurevitch, I.; Silverstein, M. S. Carbons with a hierarchical porous structure through the pyrolysis of hypercrosslinked emulsion-templated polymers. Polymer 2015, 72, 453−463. (185) Saiwan, C.; Muchan, P.; deMontigny, D.; Tontiwachwutikul, P. New Poly(Vinylbenzylchloride/Divinylbenzene) Adsorbent for Carbon Dioxide Adsorption. II. Effect of Amine Functionalization. Energy Procedia 2014, 63, 2317−2322. (186) Jerabek, K.; Pulko, I.; Soukupova, K.; Stefanec, D.; Krajnc, P. Porogenic Solvents Influence on Morphology of 4-Vinylbenzyl Chloride Based PolyHIPEs. Macromolecules 2008, 41 (10), 3543− 3546. (187) Schwab, M. G.; Senkovska, I.; Rose, M.; Klein, N.; Koch, M.; Pahnke, J.; Jonschker, G.; Schmitz, B.; Hirscher, M.; Kaskel, S. High AC


Perspective

Macromolecules surface area polyHIPEs with hierarchical pore system. Soft Matter 2009, 5 (5), 1055−1059. (188) Woodward, R. T.; Jobbe-Duval, A.; Marchesini, S.; Anthony, D. B.; Petit, C.; Bismarck, A. Hypercrosslinked polyHIPEs as precursors to designable, hierarchically porous carbon foams. Polymer 2017, 115, 146−153. (189) Yi, F.; Gao, Y.; Li, H.; Yi, L.; Chen, D.; Lu, S. Nitrogen- and oxygen-codoped porous carbonaceous foam templated from high internal emulsion as PtRu catalyst support for direct methanol fuel cell. Electrochim. Acta 2016, 211, 768−776. (190) Maya, F.; Svec, F. A new approach to the preparation of large surface area poly(styrene-co-divinylbenzene) monoliths via knitting of loose chains using external crosslinkers and application of these monolithic columns for separation of small molecules. Polymer 2014, 55 (1), 340−346. (191) Pulko, I.; Wall, J.; Krajnc, P.; Cameron, N. R. Ultra-High Surface Area Functional Porous Polymers by Emulsion Templating and Hypercrosslinking: Efficient Nucleophilic Catalyst Supports. Chem. - Eur. J. 2010, 16 (8), 2350−2354. (192) Sevšek, U.; Brus, J.; Jeřabek, K.; Krajnc, P. Post polymerisation hypercrosslinking of styrene/divinylbenzene poly(HIPE)s: Creating micropores within macroporous polymer. Polymer 2014, 55 (1), 410− 415. (193) Zhang, T.; Silverstein, M. S. Doubly-crosslinked, emulsiontemplated hydrogels through reversible metal coordination. Polymer 2017, 126, 386−394. (194) Haibach, K.; Menner, A.; Powell, R.; Bismarck, A. Tailoring mechanical properties of highly porous polymer foams: Silica particle reinforced polymer foams via emulsion templating. Polymer 2006, 47 (13), 4513−4519. (195) Zeng, M.; Wang, Y.; Liu, Q.; Yuan, X.; Feng, R.; Yang, Z.; Qi, C. N-doped mesoporous carbons supported palladium catalysts prepared from chitosan/silica/palladium gel beads. Int. J. Biol. Macromol. 2016, 89, 449−455. (196) Huang, K.; Chai, S.-H.; Mayes, R. T.; Tan, S.; Jones, C. W.; Dai, S. Significantly increasing porosity of mesoporous carbon by NaNH2 activation for enhanced CO2 adsorption. Microporous Mesoporous Mater. 2016, 230, 100−108. (197) Titirici, M.-M.; White, R. J.; Falco, C.; Sevilla, M. Black perspectives for a green future: hydrothermal carbons for environment protection and energy storage. Energy Environ. Sci. 2012, 5 (5), 6796− 6822. (198) Woodward, R.; De Luca, F.; Roberts, A.; Bismarck, A. HighSurface-Area, Emulsion-Templated Carbon Foams by Activation of polyHIPEs Derived from Pickering Emulsions. Materials 2016, 9 (9), 776. (199) Hu, W.; Xie, F.; Li, Y.; Wu, Z.; Tian, K.; Wang, M.; Pan, L.; Li, L. Hierarchically Porous Carbon Derived from PolyHIPE for Supercapacitor and Deionization Applications. Langmuir 2017, 33 (46), 13364−13375. (200) Szczurek, A.; Fierro, V.; Pizzi, A.; Celzard, A. Emulsiontemplated porous carbon monoliths derived from tannins. Carbon 2014, 74, 352−362. (201) Seredych, M.; Szczurek, A.; Fierro, V.; Celzard, A.; Bandosz, T. J. Electrochemical Reduction of Oxygen on Hydrophobic Ultramicroporous PolyHIPE Carbon. ACS Catal. 2016, 6 (8), 5618−5628. (202) Szczurek, A.; Fierro, V.; Pizzi, A.; Celzard, A. Mayonnaise, whipped cream and meringue, a new carbon cuisine. Carbon 2013, 58 (0), 245−248. (203) Vilchez, S.; Perez-Carrillo, L. A.; Miras, J.; Solans, C.; Esquena, J. Oil-in-alcohol highly concentrated emulsions as templates for the preparation of macroporous materials. Langmuir 2012, 28 (20), 7614− 21. (204) Fu, R. -w.; Li, Z. -h.; Liang, Y.-R.; Li, F.; Xu, F.; Wu, D.-c. Hierarchical porous carbons: design, preparation, and performance in energy storage. New Carbon Materials 2011, 26 (3), 171−179. (205) Kapilov-Buchman, K.; Portal, L.; Zhang, Y.; Fechler, N.; Antonietti, M.; Silverstein, M. S. Hierarchically porous carbons from

an emulsion-templated, urea-based deep eutectic. J. Mater. Chem. A 2017, 5 (31), 16376−16385. (206) Flexer, V.; Brun, N.; Backov, R.; Mano, N. Designing highly efficient enzyme-based carbonaceous foams electrodes for biofuel cells. Energy Environ. Sci. 2010, 3 (9), 1302−1306. (207) Brun, N.; Janot, R.; Sanchez, C.; Deleuze, H.; Gervais, C.; Morcrette, M.; Backov, R. Preparation of LiBH4@carbon micromacrocellular foams: tuning hydrogen release through varying microporosity. Energy Environ. Sci. 2010, 3 (6), 824−830. (208) Brun, N.; Prabaharan, S. R. S.; Morcrette, M.; Sanchez, C.; Pécastaings, G.; Derré, A.; Soum, A.; Deleuze, H.; Birot, M.; Backov, R. Hard Macrocellular Silica Si(HIPE) Foams Templating Micro/ Macroporous Carbonaceous Monoliths: Applications as Lithium Ion Battery Negative Electrodes and Electrochemical Capacitors. Adv. Funct. Mater. 2009, 19 (19), 3136−3145. (209) Oschatz, M.; Borchardt, L.; Thommes, M.; Cychosz, K. A.; Senkovska, I.; Klein, N.; Frind, R.; Leistner, M.; Presser, V.; Gogotsi, Y.; Kaskel, S. Carbide-Derived Carbon Monoliths with Hierarchical Pore Architectures. Angew. Chem., Int. Ed. 2012, 51 (30), 7577−7580. (210) Wu, R.; Menner, A.; Bismarck, A. Tough interconnected polymerized medium and high internal phase emulsions reinforced by silica particles. J. Polym. Sci., Part A: Polym. Chem. 2010, 48 (9), 1979− 1989. (211) Toledo, L.; Urbano, B. F. Poly(2-hydroxyethyl methacrylate)based porous hydrogel: Influence of surfactant and SiO2 nanoparticles on the morphology, swelling and thermal properties. Eur. Polym. J. 2016, 81, 316−326. (212) Normatov, J.; Silverstein, M. S. Porous interpenetrating network hybrids synthesized within high internal phase emulsions. Polymer 2007, 48 (22), 6648−6655. (213) Pakeyangkoon, P.; Magaraphan, R.; Malakul, P.; Nithitanakul, M. Polymeric foam via polymerized high internal phase emulsion filled with organo-modified bentonite. J. Appl. Polym. Sci. 2009, 114 (5), 3041−3048. (214) Audouin, F.; Heise, A. Synthesis of polymer−silica hybrid polyHIPEs by double in situ polymerization of concentrated water in oil emulsion. Polymer 2014, 55 (1), 403−409. (215) Normatov, J.; Silverstein, M. S. Highly porous elastomersilsesquioxane nanocomposites synthesized within high internal phase emulsions. J. Polym. Sci., Part A: Polym. Chem. 2008, 46 (7), 2357− 2366. (216) Normatov, J.; Silverstein, M. S. Interconnected SilsesquioxaneOrganic Networks in Porous Nanocomposites Synthesized within High Internal Phase Emulsions. Chem. Mater. 2008, 20 (4), 1571− 1577. (217) Normatov, J.; Silverstein, M. S. Silsesquioxane-Cross-Linked Porous Nanocomposites Synthesized within High Internal Phase Emulsions. Macromolecules 2007, 40 (23), 8329−8335. (218) Turnšek, M.; Krajnc, P.; Liska, R.; Koch, T. Macroporous alumina with cellular interconnected morphology from emulsion templated polymer composite precursors. J. Eur. Ceram. Soc. 2016, 36 (4), 1045−1051. (219) Ungureanu, S.; Birot, M.; Laurent, G.; Deleuze, H.; Babot, O.; Julian-Lopez, B.; Achard, M. F.; Popa, M. I.; Sanchez, C.; Backov, R. One-Pot Syntheses of the First Series of Emulsion Based Hierarchical Hybrid Organic-Inorganic Open-Cell Monoliths Possessing Tunable Functionality (Organo-Si(HIPE) Series). Chem. Mater. 2007, 19 (23), 5786−5796. (220) Ungureanu, S.; Sigaud, G.; Vignoles, G. L.; Lorrette, C.; Birot, M.; Derre, A.; Babot, O.; Deleuze, H.; Soum, A.; Pecastaings, G.; Backov, R. Tough silicon carbide macro/mesocellular crack-free monolithic foams. J. Mater. Chem. 2011, 21 (38), 14732−14740. (221) Kovacic, S.; Krajnc, P. Macroporous monolithic poly(4vinylbenzyl chloride) columns for organic synthesis facilitation by in situ polymerization of high internal phase emulsions. J. Polym. Sci., Part A: Polym. Chem. 2009, 47 (23), 6726−6734. (222) Choudhury, S.; Connolly, D.; White, B. Supermacroporous polyHIPE and cryogel monolithic materials as stationary phases in separation science: a review. Anal. Methods 2015, 7 (17), 6967−6982. AD


Perspective

Macromolecules (223) LaVon, G. D.; Young, G. A.; Taylor, G. W. High efficiency absorbent articles for incontinence management. US 5147345, 1992. (224) Barby, D.; Haq, Z. Low density porous cross-linked polymeric materials and their preparation and use as carriers for included liquids. 60138, 1982/03/09, 1982. (225) Ko, Y. C.; Lindsay, J. D. Absorbent articles comprising supercritical fluid treated HIPE, I-HIPE foams and other foams. 7053131 B2, 2002/12/03, 2006. (226) Malone, B. A.; Mork, S. W. Continuous process for polymerizing, curing and drying high internal phase emulsions. US 6299808, 2001. (227) Soo, P. P.; Clear, S. C.; Sura, R. K.; Parthasarathy, R. V. High internal phase emulsion foams containing polyelectrolytes. US 0204510 A1, 2004. (228) Lepizzera, S.; Pith, T. Water-in-oil emulsions with high internal phase content and their application to the manufacture of absorbing composite foams with porous structure including a superabsorbent acrylic polymer. FR2876290A1, 2006. (229) Sergienko, A. Y.; Tai, H. W.; Narkis, M.; Silverstein, M. S. Polymerized high internal phase emulsions containing a porogen: Specific surface area and sorption. J. Appl. Polym. Sci. 2004, 94 (5), 2233−2239. (230) Ovadia, M.; Silverstein, M. S. High porosity, responsive hydrogel copolymers from emulsion templating. Polym. Int. 2016, 65 (3), 280−289. (231) Zhang, T.; Silverstein, M. S. Highly porous, emulsiontemplated, zwitterionic hydrogels: amplified and accelerated uptakes with enhanced environmental sensitivity. Polym. Chem. 2018, 9 (25), 3479−3487. (232) Zhang, H.; Cooper, A. I. Thermoresponsive Particle Pumps: Activated Release of Organic Nanoparticles from Open-Cell Macroporous Polymers. Adv. Mater. 2007, 19 (18), 2439−2444. (233) Tebboth, M.; Kogelbauer, A.; Bismarck, A. Liquid−Liquid Extraction within Emulsion Templated Macroporous Polymers. Ind. Eng. Chem. Res. 2015, 54 (29), 7284−7291. (234) Du, F.; Sun, L.; Zhen, X.; Nie, H.; Zheng, Y.; Ruan, G.; Li, J. High-internal-phase-emulsion polymeric monolith coupled with liquid chromatography−electrospray tandem mass spectrometry for enrichment and sensitive detection of trace cytokinins in plant samples. Anal. Bioanal. Chem. 2015, 407 (20), 6071−6079. (235) Du, F.; Zheng, X.; Sun, L.; Qin, Q.; Guo, L.; Ruan, G. Development and validation of polymerized high internal phase emulsion monoliths coupled with HPLC and fluorescence detection for the determination of trace tetracycline antibiotics in environmental water samples. J. Sep. Sci. 2015, 38 (21), 3774−3780. (236) Pan, J.; Liu, J.; Ma, Y.; Huang, X.; Niu, X.; Zhang, T.; Chen, X.; Qiu, F. Wulff-type boronic acids suspended hierarchical porous polymeric monolith for the specific capture of cis-diol-containing flavone under neutral condition. Chem. Eng. J. 2017, 317, 317−330. (237) Du, F.; Zheng, X.; Deng, J.; Zou, J.; Zeng, Q.; Li, J.; Ruan, G. High Internal Phase Emulsion Polymeric Monolith Extraction Coupling with High-Performance Liquid Chromatography for the Determination of Para Red and Sudan Dyes in Chilli Samples. Food Analytical Methods 2017, 10 (6), 2018−2026. (238) Hu, R.; Wan, D.; Jin, M. Molecular nanocapsule-decorated porous monolith: preparation and elimination of cationic dyes from water. RSC Adv. 2016, 6 (60), 55682−55688. (239) Zhu, Y.; Zheng, Y.; Wang, F.; Wang, A. Fabrication of magnetic macroporous chitosan-g-poly (acrylic acid) hydrogel for removal of Cd2+ and Pb2+. Int. J. Biol. Macromol. 2016, 93, 483−492. (240) Zhu, Y.; Zheng, Y.; Zong, L.; Wang, F.; Wang, A. Fabrication of magnetic hydroxypropyl cellulose-g-poly(acrylic acid) porous spheres via Pickering high internal phase emulsion for removal of Cu2+ and Cd2+. Carbohydr. Polym. 2016, 149, 242−250. (241) Barlık, N.; Keskinler, B.; Kocakerim, M. M.; Akay, G. Functionalized PolyHIPE polymer monoliths as an anion-exchange media for removal of nitrate ions from aqueous solutions. Desalin. Water Treat. 2016, 57 (55), 26440−26447.

(242) Yu, S.; Tan, H.; Wang, J.; Liu, X.; Zhou, K. High Porosity Supermacroporous Polystyrene Materials with Excellent Oil−Water Separation and Gas Permeability Properties. ACS Appl. Mater. Interfaces 2015, 7 (12), 6745−6753. (243) Zhang, N.; Zhong, S.; Chen, T.; Zhou, Y.; Jiang, W. Emulsionderived hierarchically porous polystyrene solid foam for oil removal from aqueous environment. RSC Adv. 2017, 7 (37), 22946−22953. (244) Chen, X.; Liu, L.; Liu, K.; Miao, Q.; Lu, Y.; Fang, Y. Compressible porous hybrid monoliths: preparation via a low molecular mass gelators-based gel-emulsion approach and exceptional performances. J. Mater. Chem. A 2015, 3 (48), 24322−24332. (245) Zhang, N.; Jiang, W.; Wang, T.; Gu, J.; Zhong, S.; Zhou, S.; Xie, T.; Fu, J. Facile Preparation of Magnetic Poly(styrenedivinylbenzene) Foam and Its Application as an Oil Absorbent. Ind. Eng. Chem. Res. 2015, 54 (44), 11033−11039. (246) Cooper, A. I.; Bray, C. L.; Su, F.; Carter, B. O.; Wang, W.; Adams, D. J. Clathrates for gas storage in the presence of emulsiontemplated porous support. WO 068912, 2009. (247) Akay, G. Ammonia production by integrated intensified processes. WO 025767, 2012. (248) Jonschker, G.; Koch, M.; Pahnke, J.; Schwab, M. Monolithic polymer materials for gas storage. WO 095153, 2009. (249) Wang, Q.; Liu, Y.; Chen, J.; Du, Z.; Mi, J. Control of Uniform and Interconnected Macroporous Structure in PolyHIPE for Enhanced CO2 Adsorption/Desorption Kinetics. Environ. Sci. Technol. 2016, 50 (14), 7879−7888. (250) Wickenheisser, M.; Paul, T.; Janiak, C. Prospects of monolithic MIL-MOF@poly(NIPAM)HIPE composites as water sorption materials. Microporous Mesoporous Mater. 2016, 220, 258−269. (251) Kovačič, S.; Mazaj, M.; Ješelnik, M.; Pahovnik, D.; Ž agar, E.; Slugovc, C.; Logar, N. Z. Synthesis and Catalytic Performance of Hierarchically Porous MIL-100(Fe)@polyHIPE Hybrid Membranes. Macromol. Rapid Commun. 2015, 36 (17), 1605−1611. (252) Yin, D.; Guan, Y.; Gu, H.; Jia, Y.; Zhang, Q. Polymerized high internal phase emulsion monolithic material: a novel stationary phase of thin layer chromatography. RSC Adv. 2017, 7 (12), 7303−7309. (253) Choudhury, S.; Connolly, D.; White, B. Application of polymeric high-internal-phase-emulsion-coated stationary-phase columns in open-tubular capillary electrochromatography. J. Appl. Polym. Sci. 2016, 133 (48), 44237. (254) Gao, H.; Pan, J.; Han, D.; Zhang, Y.; Shi, W.; Zeng, J.; Peng, Y.; Yan, Y. Facile synthesis of microcellular foam catalysts with adjustable hierarchical porous structure, acid-base strength and wettability for biomass energy conversion. J. Mater. Chem. A 2015, 3 (25), 13507− 13518. (255) Gitli, T.; Silverstein, M. S. Emulsion templated bicontinuous hydrophobic-hydrophilic polymers: Loading and release. Polymer 2011, 52 (1), 107−115. (256) Akay, G.; Burke, D. R. Agro-Process Intensification through Synthetic Rhizosphere Media for Nitrogen Fixation and Yield Enhancement in Plants. Am. J. Agric. Biol. Sci. 2012, 7 (2), 150−172. (257) Schrimpf, G.; Friedl, P. Growth of Human Vascular Endothelial-Cells on Various Types of Microcarriers. Cytotechnology 1993, 13 (3), 203−211. (258) Busby, W.; Cameron, N. R.; Jahoda, C. A. B. Emulsion-derived foams (PolyHIPEs) containing poly(ε-caprolactone) as matrixes for tissue engineering. Biomacromolecules 2001, 2 (1), 154−164. (259) Busby, W.; Cameron, N. R.; Jahoda, C. A. B. Tissue engineering matrixes by emulsion templating. Polym. Int. 2002, 51 (10), 871−881. (260) Bokhari, M.; Birch, M.; Akay, G. Polyhipe polymer: A novel scaffold for in vitro bone tissue engineering. In Tissue Engineering, Stem Cells and Gene Therapies; 2003; Vol. 534, pp 247−254. (261) Akay, G.; Birch, M. A.; Bokhari, M. A. Microcellular polyHIPE polymer supports osteoblast growth and bone formation in vitro. Biomaterials 2004, 25 (18), 3991−4000. (262) Hayman, M. W.; Smith, K. H.; Cameron, N. R.; Przyborski, S. A. Enhanced neurite outgrowth by human neurons grown on solid AE


Perspective

Macromolecules three-dimensional scaffolds. Biochem. Biophys. Res. Commun. 2004, 314 (2), 483−488. (263) Barbetta, A.; Dentini, M.; Zannoni, E. M.; DeStefano, M. E. Tailoring the Porosity and Morphology of Gelatin-Methacrylate PolyHIPE Scaffolds for Tissue Engineering Applications. Langmuir 2005, 21 (26), 12333−12341. (264) Bokhari, M. A.; Akay, G.; Zhang, S. G.; Birch, M. A. Enhancement of osteoblast growth and differentiation in vitro on a peptide hydrogel - polyHIPE polymer hybrid material. Biomaterials 2005, 26 (25), 5198−5208. (265) Hayman, M. W.; Smith, K. H.; Cameron, N. R.; Przyborski, S. A. Growth of human stem cell-derived neurons on solid threedimensional polymers. J. Biochem. Biophys. Methods 2005, 62 (3), 231−240. (266) Barbetta, A.; Massimi, M.; Conti Devirgiliis, L.; Dentini, M. Enzymatic Cross-Linking versus Radical Polymerization in the Preparation of Gelatin PolyHIPEs and Their Performance as Scaffolds in the Culture of Hepatocytes. Biomacromolecules 2006, 7 (11), 3059− 3068. (267) Bokhari, M.; Carnachan, R. J.; Cameron, N. R.; Przyborski, S. A. Novel cell culture device enabling three-dimensional cell growth and improved cell function. Biochem. Biophys. Res. Commun. 2007, 354 (4), 1095−1100. (268) Lumelsky, Y.; Zoldan, J.; Levenberg, S.; Silverstein, M. S. Porous Polycaprolactone-Polystyrene Semi-interpenetrating Polymer Networks Synthesized within High Internal Phase Emulsions. Macromolecules 2008, 41 (4), 1469−1474. (269) Bokhari, M.; Carnachan, R. J.; Przyborski, S. A.; Cameron, N. R. Emulsion-templated porous polymers as scaffolds for three dimensional cell culture: effect of synthesis parameters on scaffold formation and homogeneity. J. Mater. Chem. 2007, 17 (38), 4088− 4094. (270) Hu, Y.; Gu, X.; Yang, Y.; Huang, J.; Hu, M.; Chen, W.; Tong, Z.; Wang, C. Facile Fabrication of Poly(l-lactic Acid)-Grafted Hydroxyapatite/Poly(lactic-co-glycolic Acid) Scaffolds by Pickering High Internal Phase Emulsion Templates. ACS Appl. Mater. Interfaces 2014, 6 (19), 17166−17175. (271) Moglia, R. S.; Holm, J. L.; Sears, N. A.; Wilson, C. J.; Harrison, D. M.; Cosgriff-Hernandez, E. Injectable PolyHIPEs as High-Porosity Bone Grafts. Biomacromolecules 2011, 12 (10), 3621−3628. (272) Hu, Y.; Han, W.; Chen, Y.; Zou, R.; Ouyang, Y.; Zhou, W.; Yang, Z.; Wang, C. One-Pot Fabrication of Poly(ε-Caprolactone)Incorporated Bovine Serum Albumin/Calcium Alginate/Hydroxyapatite Nanocomposite Scaffolds by High Internal Phase Emulsion Templates. Macromol. Mater. Eng. 2017, 302 (4), 1600367. (273) Hu, Y.; Gao, H.; Du, Z.; Liu, Y.; Yang, Y.; Wang, C. Pickering high internal phase emulsion-based hydroxyapatite-poly(ε-caprolactone) nanocomposite scaffolds. J. Mater. Chem. B 2015, 3 (18), 3848− 3857. (274) Changotade, S.; Radu Bostan, G.; Consalus, A.; Poirier, F.; Peltzer, J.; Lataillade, J. J.; Lutomski, D.; Rohman, G. Preliminary In Vitro Assessment of Stem Cell Compatibility with Cross-Linked Poly(ε-caprolactone urethane) Scaffolds Designed through High Internal Phase Emulsions. Stem Cells Int. 2015, 2015, 283796. (275) Robinson, J. L.; McEnery, M. A. P.; Pearce, H.; Whitely, M. E.; Munoz-Pinto, D. J.; Hahn, M. S.; Li, H.; Sears, N. A.; CosgriffHernandez, E. Osteoinductive PolyHIPE Foams as Injectable Bone Grafts. Tissue Eng., Part A 2016, 22 (5−6), 403−414. (276) Wang, A.-j.; Paterson, T.; Owen, R.; Sherborne, C.; Dugan, J.; Li, J.-m.; Claeyssens, F. Photocurable high internal phase emulsions (HIPEs) containing hydroxyapatite for additive manufacture of tissue engineering scaffolds with multi-scale porosity. Mater. Sci. Eng., C 2016, 67, 51−58. (277) Liu, S.; Jin, M.; Chen, Y.; Gao, H.; Shi, X.; Cheng, W.; Ren, L.; Wang, Y. High internal phase emulsions stabilised by supramolecular cellulose nanocrystals and their application as cell-adhesive macroporous hydrogel monoliths. J. Mater. Chem. B 2017, 5 (14), 2671− 2678.

(278) Tan, H.; Wei, J.; Sun, G.; Mu, C.; Lin, W.; Ngai, T. Interconnected macroporous 3D scaffolds templated from gelatin nanoparticle-stabilized high internal phase emulsions for biomedical applications. Soft Matter 2017, 13 (21), 3871−3878. (279) Sears, N. A.; Dhavalikar, P. S.; Cosgriff-Hernandez, E. M. Emulsion Inks for 3D Printing of High Porosity Materials. Macromol. Rapid Commun. 2016, 37 (16), 1369−1374. (280) Behl, M.; Lendlein, A. Shape-memory polymers. Mater. Today 2007, 10 (4), 20−28. (281) Gurevitch, I.; Silverstein, M. S. Shape memory polymer foams from emulsion templating. Soft Matter 2012, 8 (40), 10378−10387. (282) Warwar, C.; Silverstein, M. S. Ball of string. Mater. Today 2013, 16 (7−8), 297−298. (283) Warwar Damouny, C.; Silverstein, M. S. Hydrogel-filled, semicrystalline, nanoparticle-crosslinked, porous polymers from emulsion templating: Structure, properties, and shape memory. Polymer 2016, 82, 262−273. (284) Zhu, Y.; Hua, Y.; Zhang, S.; Wang, Y.; Chen, J. Open-cell macroporous bead: a novel polymeric support for heterogeneous photocatalytic reactions. J. Polym. Res. 2015, 22 (4), 1−6. (285) Cohen, N.; Silverstein, M. S. Synthesis of emulsion-templated porous polyacrylonitrile and its pyrolysis to porous carbon monoliths. Polymer 2011, 52 (2), 282−287. (286) Schlüter, F.; Wilhelm, M.; Rezwan, K. Surfactant assisted syntheses of monolithic hybrid ceramics with hierarchical porosity. J. Eur. Ceram. Soc. 2015, 35 (11), 2963−2972. (287) Depardieu, M.; Viaud, M.; Buguin, A.; Livage, J.; Sanchez, C.; Backov, R. A multiscale study of bacterial proliferation modes within novel E. coli@Si(HIPE) hybrid macrocellular living foams. J. Mater. Chem. B 2016, 4 (13), 2290−2303. (288) Silverstein, M. S.; Tai, H. W.; Sergienko, A.; Lumelsky, Y. L.; Pavlovsky, S. PolyHIPE: IPNs, hybrids, nanoscale porosity, silica monoliths and ICP-based sensors. Polymer 2005, 46 (17), 6682−6694. (289) Zhang, X.; Du, Z.; Zou, W.; Li, H.; Zhang, C.; Li, S.; Guo, W. A porous elastomeric polyurethane monolith synthesized by concentrated emulsion templating and its pressure-sensitive conductive property. RSC Adv. 2015, 5 (81), 65890−65896. (290) Tebboth, M.; Kogelbauer, A.; Bismarck, A. Effectiveness of Emulsion-Templated Macroporous Polymer Micromixers Characterized by the Bourne Reaction. Ind. Eng. Chem. Res. 2015, 54 (22), 5974−5981. (291) Park, J. S.; Ruckenstein, E. Selective permeation through hydrophobic-hydrophilic membranes. J. Appl. Polym. Sci. 1989, 38 (3), 453−461. (292) Ruckenstein, E.; Park, J. S. The separation of water-ethanol mixtures by pervaporation through hydrophilic - hydrophobic composite membranes. J. Appl. Polym. Sci. 1990, 40 (1−2), 213−220. (293) Ruckenstein, E.; Chen, H. H. Preparation of a WaterPermselective Composite Membrane by the Concentrated Emulsion Method - Its Swelling and Permselectivity Characteristics. J. Appl. Polym. Sci. 1991, 42 (9), 2429−2434. (294) Ruckenstein, E.; Chen, H. H. Composite Membranes Prepared by Concentrated Emulsion Polymerization and Their Use for Pervaporation Separation of Water-Acetic Acid Mixtures. J. Membr. Sci. 1992, 66 (2−3), 205−210. (295) Ruckenstein, E.; Hong, L. Binding Catalytic Sites to the Surface of Porous Polymers and Some Catalytic Applications. Chem. Mater. 1992, 4 (1), 122−127. (296) Schoo, H. F. M.; Challa, G.; Rowatt, B.; Sherrington, D. C. Immobilization of Flavin on Highly Porous Polymeric Disks - 3 Routes to a Catalytically Active Membrane. React. Polym. 1992, 16 (2), 125− 136. (297) Ruckenstein, E.; Park, J. S. New method for the preparation of thick conducting polymer composites. J. Appl. Polym. Sci. 1991, 42 (4), 925−934. (298) Bartl, H.; von Bonin, W. ber die Polymerisation in umgekehrter Emulsion. Makromol. Chem. 1962, 57 (1), 74−95. (299) Bartl, H.; von Bonin, W. ber die polymerisation in umgekehrter emulsion. II. Makromol. Chem. 1963, 66 (1), 151−156. AF


Perspective

Macromolecules (300) von Bonin, W.; Bartl, H. Polymeric materials. US Patent 3,255,127, June 7, 1966. (301) Lissant, K. J. High internal phase ratio emulsion polymers. US Patent 3,988,508, 1976. (302) Haq, Z.; Barby, D. Low density porous cross-linked polymeric materials and their preparation and use as carriers for included liquids. US 4522953, 1985. (303) Williams, J. M. Toroidal Microstructures from Water-in-Oil Emulsions. Langmuir 1988, 4 (1), 44−49. (304) Hainey, P.; Huxham, I. M.; Rowatt, B.; Sherrington, D. C.; Tetley, L. Synthesis and Ultrastructural Studies of Styrene Divinylbenzene Polyhipe Polymers. Macromolecules 1991, 24 (1), 117−121. (305) Williams, J. M. High Internal Phase Water-in-Oil Emulsions Influence of Surfactants and Cosurfactants on Emulsion Stability and Foam Quality. Langmuir 1991, 7 (7), 1370−1377. (306) Williams, J. M.; Gray, A. J.; Wilkerson, M. H. Emulsion Stability and Rigid Foams from Styrene or Divinylbenzene Water-inOil Emulsions. Langmuir 1990, 6 (2), 437−444. (307) Ruckenstein, E.; Sun, F. M. Concentrated Emulsion Pathway to Novel Composite Polymeric Membranes and Their Use in Pervaporation. Ind. Eng. Chem. Res. 1995, 34 (10), 3581−3589. (308) Weinstock, L.; Sanguramath, R.; Silverstein, M. S. Encapsulating an organic phase change material within emulsion-templated poly(urethane urea)s. Polym. Chem. 2019, 10, 1498. (309) Olejarka, J.; Ła̧cz, A.; Olejniczak, Z.; Hasik, M. Non-porous and porous materials prepared by cross-linking of polyhydromethylsiloxane with silazane compounds. Eur. Polym. J. 2018, 99, 150−164. (310) Desire, C. T.; Khodabandeh, A.; Schiller, T. L.; Wilson, R.; Arrua, R. D.; Bon, S. A. F.; Hilder, E. F. Preparation of highly interconnected hydrophilic polymers from emulsion templates with improved mechanical properties. Eur. Polym. J. 2018, 102, 56−67. (311) Stubenrauch, C.; Menner, A.; Bismarck, A.; Drenckhan, W. Emulsion and Foam TemplatingPromising Routes to Tailor-Made Porous Polymers. Angew. Chem., Int. Ed. 2018, 57 (32), 10024−10032. (312) Andrieux, S.; Drenckhan, W.; Stubenrauch, C. Generation of Solid Foams with Controlled Polydispersity Using Microfluidics. Langmuir 2018, 34 (4), 1581−1590. (313) Andrieux, S.; Quell, A.; Stubenrauch, C.; Drenckhan, W. Liquid foam templating − A route to tailor-made polymer foams. Adv. Colloid Interface Sci. 2018, 256, 276−290. (314) Tan, H.; Tu, S.; Zhao, Y.; Wang, H.; Du, Q. A simple and environment-friendly approach for synthesizing macroporous polymers from aqueous foams. J. Colloid Interface Sci. 2018, 509, 209−218. (315) Azhar, U.; Zong, C.; Wan, X.; Xu, A.; Yabin, Z.; Liu, J.; Zhang, S.; Geng, B. Methyl Methacrylate HIPE Solely Stabilized by Fluorinated Di-block Copolymer for Fabrication of Highly Porous and Interconnected Polymer Monoliths. Chem. - Eur. J. 2018, 24 (45), 11619−11626. (316) Zhao, Y.; Zhao, Z.; Zhang, J.; Wei, M.; Xiao, L.; Hou, L. Distinctive Performance of Gemini Surfactant in the Preparation of Hierarchically Porous Carbons via High-Internal-Phase Emulsion Template. Langmuir 2018, 34 (40), 12100−12108. (317) Roucher, A.; Emo, M.; Vibert, F.; Stébé, M.-J.; Schmitt, V.; Jonas, F.; Backov, R.; Blin, J.-L. Investigation of mixed ionic/nonionic building blocks for the dual templating of macro-mesoporous silica. J. Colloid Interface Sci. 2019, 533, 385−400. (318) Mathieu, K.; Jerome, C.; Debuigne, A. Macroporous poly(ionic liquid)/ionic liquid gels via CO2-based emulsion-templating polymerization. Polym. Chem. 2018, 9 (4), 428−437. (319) Tu, S.; Zhao, Y.; Tan, H.; Yu, H.; Zhu, X.; Wang, H. Ultralight Silica Foams with a Hierarchical Pore Structure via a Surfactant-Free High Internal Phase Emulsion Process. Langmuir 2018, 34 (35), 10381−10388. (320) Fan, X.; Zhang, S.; Zhu, Y.; Chen, J. Macroporous polymers prepared via frozen UV polymerization of the emulsion-templates stabilized by a low amount of surfactant. RSC Adv. 2018, 8 (18), 10141−10147.

(321) Dong, Y.; Cao, L.; Li, J.; Yang, Y.; Wang, J. Facile preparation of UiO-66 /PAM monoliths via CO2-in-water HIPEs and their applications. RSC Adv. 2018, 8 (56), 32358−32367. (322) Yang, Y.; Cao, L.; Li, J.; Dong, Y.; Wang, J. High-Performance Composite Monolith Synthesized via HKUST-1 Stabilized HIPEs and Its Adsorptive Properties. Macromol. Mater. Eng. 2018, 303 (11), 1800426. (323) Yang, Z.; Cao, L.; Li, J.; Lin, J.; Wang, J. Facile synthesis of CuBDC/Poly(N-methylol acrylamide) HIPE monoliths via CO2-inwater Emulsion stabilized by metal-organic framework. Polymer 2018, 153, 17−23. (324) Wang, J.; Zhu, H.; Li, B.-G.; Zhu, S. Interconnected Porous Monolith Prepared via UiO-66 Stabilized Pickering High Internal Phase Emulsion Template. Chem. - Eur. J. 2018, 24 (61), 16426− 16431. (325) Jin, P.; Tan, W.; Huo, J.; Liu, T.; Liang, Y.; Wang, S.; Bradshaw, D. Hierarchically porous MOF/polymer composites via interfacial nanoassembly and emulsion polymerization. J. Mater. Chem. A 2018, 6 (41), 20473−20479. (326) Jiao, B.; Shi, A.; Wang, Q.; Binks, B. P. High-Internal-Phase Pickering Emulsions Stabilized Solely by Peanut-Protein-Isolate Microgel Particles with Multiple Potential Applications. Angew. Chem., Int. Ed. 2018, 57 (30), 9274−9278. (327) Xu, Y.-T.; Liu, T.-X.; Tang, C.-H. Novel pickering high internal phase emulsion gels stabilized solely by soy β-conglycinin. Food Hydrocolloids 2019, 88, 21−30. (328) Tan, H.; Tu, Z.; Jia, H.; Gou, X.; Ngai, T. Hierarchical Porous Protein Scaffold Templated from High Internal Phase Emulsion Costabilized by Gelatin and Gelatin Nanoparticles. Langmuir 2018, 34 (16), 4820−4829. (329) Lee, J.; Chang, J. Y. Pickering Emulsion Stabilized by Microporous Organic Polymer Particles for the Fabrication of a Hierarchically Porous Monolith. Langmuir 2018, 34 (39), 11843− 11849. (330) Lu, P.; Guo, M.; Yang, Y.; Wu, M. Nanocellulose Stabilized Pickering Emulsion Templating for Thermosetting AESO Nanocomposite Foams. Polymers 2018, 10 (10), 1111. (331) Hori, K.; Sano, M.; Suzuki, M.; Hanabusa, K. Preparation of porous polymer materials using water-in-oil gel emulsions as templates. Polym. Int. 2018, 67 (7), 909−916. (332) Liu, J.; Li, M.; Wang, P.; Liu, K.; Fang, Y. Gel-emulsion templated polymeric monoliths for efficient removal of particulate matters. Chem. Eng. J. 2018, 339, 14−21. (333) Teo, N.; Gu, Z.; Jana, S. C. Polyimide-based aerogel foams, via emulsion-templating. Polymer 2018, 157, 95−102. (334) Kravchenko, O. G.; Gedler, G.; Kravchenko, S. G.; Feke, D. L.; Manas-Zloczower, I. Modeling compressive behavior of open-cell polymerized high internal phase emulsions: effects of density and morphology. Soft Matter 2018, 14 (9), 1637−1646. (335) Paljevac, M.; Kotek, J.; Jeřabek, K.; Krajnc, P. Influence of Topology of Highly Porous Methacrylate Polymers on their Mechanical Properties. Macromol. Mater. Eng. 2018, 303 (2), 1700337. (336) Rezanavaz, R.; Fee, C. J.; Dimartino, S. GMA-based emulsiontemplated solid foams: Influence of co-crosslinker on morphology and mechanical properties. J. Appl. Polym. Sci. 2018, 135 (21), 46295. (337) Yüce, E.; Parın, F. N.; Krajnc, P.; Hande Mert, H.; Hilal Mert, E. Influence of titania on the morphological and mechanical properties of 1,3-butanediol dimethacrylate based polyHIPE composites. React. Funct. Polym. 2018, 130, 8−15. (338) Kovalenko, A.; Brunet, T.; Mondain-Monval, O. Mechanical and acoustic properties of macroporous acrylate materials near glass transition. Polymer 2018, 148, 239−246. (339) Yüce, E.; Krajnc, P.; Mert, H. H.; Mert, E. H. Influence of nanoparticles and antioxidants on mechanical properties of titania/ polydicyclopentadiene polyHIPEs: A statistical approach. J. Appl. Polym. Sci. 2019, 136 (7), 46913. (340) Balow, R. B.; Giles, S. L.; McGann, C. L.; Daniels, G. C.; Lundin, J. G.; Pehrsson, P. E.; Wynne, J. H. Rapid Decontamination of AG


Perspective

Macromolecules Chemical Warfare Agent Simulant with Thermally Activated Porous Polymer Foams. Ind. Eng. Chem. Res. 2018, 57 (25), 8630−8634. (341) McGann, C. L.; Daniels, G. C.; Giles, S. L.; Balow, R. B.; Miranda-Zayas, J. L.; Lundin, J. G.; Wynne, J. H. Air Activated SelfDecontaminating Polydicyclopentadiene PolyHIPE Foams for Rapid Decontamination of Chemical Warfare Agents. Macromol. Rapid Commun. 2018, 39 (12), 1800194. (342) Mezhoud, S.; Paljevac, M.; Koler, A.; Le Droumaguet, B.; Grande, D.; Krajnc, P. Novel hypercrosslinking approach toward high surface area functional 2-hydroxyethyl methacrylate-based polyHIPEs. React. Funct. Polym. 2018, 132, 51−59. (343) Majer, J.; Ž agar, E.; Krajnc, P.; Kovačič, S. In situ hyper-crosslinking of glycidyl methacrylate−based polyHIPEs through the amineenriched high internal phase emulsions. Colloid Polym. Sci. 2019, 297 (2), 239−247. (344) Fitzsimmons, P.; Elsner, F.; Paguio, R.; Nikroo, A.; Thomas, C.; Baker, K.; Huang, H.; Schoff, M.; Kaczala, D.; Reynolds, H.; Felker, S.; Farrell, M.; Watson, B. J. Zinc Oxide−Coated Poly(HIPE) Annular Liners to Advance Laser Indirect Drive Inertial Confinement Fusion. Fusion Sci. Technol. 2018, 73 (2), 210−218. (345) Yu, Y.; Meng, X.; Zeng, N.; Dan, Y.; Jiang, L. Covalent immobilization of TiO2 within macroporous polymer monolith as a facilely recyclable photocatalyst for water decontamination. Colloid Polym. Sci. 2018, 296 (9), 1419−1429. (346) Costantini, M.; Guzowski, J.; Ż uk, P.ł J.; Mozetic, P.; De Panfilis, S.; Jaroszewicz, J.; Heljak, M.; Massimi, M.; Pierron, M.; Trombetta, M.; Dentini, M.; Świȩszkowski, W.; Rainer, A.; Garstecki, P.; Barbetta, A., Electric Field Assisted Microfluidic Platform for Generation of Tailorable Porous Microbeads as Cell Carriers for Tissue Engineering. Adv. Funct. Mater. 2018, 28 (20).1870133 (347) Feng, X.; Wang, X.; Zhang, D.; Feng, F.; Yao, L.; Ma, G. Onestep Preparation of Monodisperse Multifunctional Macroporous Particles through a Spontaneous Physical Process. Small 2018, 14 (10), 1703570. (348) Streifel, B. C.; Lundin, J. G.; Sanders, A. M.; Gold, K. A.; Wilems, T. S.; Williams, S. J.; Cosgriff-Hernandez, E.; Wynne, J. H. Hemostatic and Absorbent PolyHIPE−Kaolin Composites for 3D Printable Wound Dressing Materials. Macromol. Biosci. 2018, 18 (5), 1700414. (349) Whitely, M.; Cereceres, S.; Dhavalikar, P.; Salhadar, K.; Wilems, T.; Smith, B.; Mikos, A.; Cosgriff-Hernandez, E. Improved in situ seeding of 3D printed scaffolds using cell-releasing hydrogels. Biomaterials 2018, 185, 194−204. (350) Sherborne, C.; Owen, R.; Reilly, G. C.; Claeyssens, F. Lightbased additive manufacturing of PolyHIPEs: Controlling the surface porosity for 3D cell culture applications. Mater. Des. 2018, 156, 494− 503. (351) Paljevac, M.; Gradišnik, L.; Lipovšek, S.; Maver, U.; Kotek, J.; Krajnc, P. Multiple-Level Porous Polymer Monoliths with Interconnected Cellular Topology Prepared by Combining Hard Sphere and Emulsion Templating for Use in Bone Tissue Engineering. Macromol. Biosci. 2018, 18 (2), 1700306. (352) Chen, X.; Yuan, W.; Jiang, M.; Xie, X. Surface glycopolymermodified functional macroporous polyHIPE obtained by ATRP for the removal of boron in water. New J. Chem. 2018, 42 (3), 2104−2112. (353) Huang, W.; Liu, S.; Liu, J.; Zhang, W.; Pan, J. 2-Methylol-12crown-4 ether immobilized PolyHIPEs toward recovery of lithium(i). New J. Chem. 2018, 42 (20), 16814−16822. (354) Pribyl, J. G.; Taylor-Pashow, K. M. L.; Shehee, T. C.; Benicewicz, B. C. High-Capacity Poly(4-vinylpyridine) Grafted PolyHIPE Foams for Efficient Plutonium Separation and Purification. ACS Omega 2018, 3 (7), 8181−8189. (355) Zhu, Y. F.; Zhang, H. F.; Ye, X. S.; Wang, A. Q. W/O HIPEs templated polyaniline/polystyrene porous adsorbent for Sr2+ uptake from aqueous solution. Desalination and Water Treatment 2018, 132, 99−108. (356) Kim, Y.; Kim, I.; Lee, T. S.; Lee, E.; Lee, K. J. Porous hydrogel containing Prussian blue nanoparticles for effective cesium ion adsorption in aqueous media. J. Ind. Eng. Chem. 2018, 60, 465−474.

(357) Legan, M.; Trochimczuk, A. W. Bis-imidazolium type ionexchange resin with poly(HIPE) structure for the selective separation of anions from aqueous solutions. Sep. Sci. Technol. 2018, 53 (8), 1163−1177. (358) Huang, Y.; Ruan, G.; Ruan, Y.; Zhang, W.; Li, X.; Du, F.; Hu, C.; Li, J. Hypercrosslinked porous polymers hybridized with graphene oxide for water treatment: dye adsorption and degradation. RSC Adv. 2018, 8 (24), 13417−13422. (359) Mert, H. H.; Tekay, E.; Nugay, N.; Nugay, T.; Ş en, S. Adsorptive polyHIPE composites based on biosorbent immobilized nanoclay: Effects of immobilization techniques. Polym. Eng. Sci. 2018, 58 (8), 1229−1240. (360) Huang, Y.; Zhang, W.; Ruan, G.; Li, X.; Cong, Y.; Du, F.; Li, J. Reduced Graphene Oxide-Hybridized Polymeric High-Internal Phase Emulsions for Highly Efficient Removal of Polycyclic Aromatic Hydrocarbons from Water Matrix. Langmuir 2018, 34 (12), 3661− 3668. (361) Liu, J.; Pan, J.; Ma, Y.; Liu, S.; Qiu, F.; Yan, Y. A versatile strategy to fabricate dual-imprinted porous adsorbent for efficient treatment co-contamination of λ-cyhalothrin and copper(II). Chem. Eng. J. 2018, 332, 517−527. (362) Li, Y.; Gong, C.; Zhang, T.; Feng, X.; Zhou, X.; Li, C. Preparation of PolyHIPE beads and the application in bio-degradation of sulfate containing wastewater. React. Funct. Polym. 2018, 131, 142− 149. (363) Lu, T.; Zhu, Y.; Qi, Y.; Wang, W.; Wang, A. Magnetic chitosan−based adsorbent prepared via Pickering high internal phase emulsion for high-efficient removal of antibiotics. Int. J. Biol. Macromol. 2018, 106, 870−877. (364) Kovačič, S.; Drašinac, N.; Pintar, A.; Ž agar, E. Highly Porous Cationic Polyelectrolytes via Oil-in-Water Concentrated Emulsions: Synthesis and Adsorption Kinetic Study. Langmuir 2018, 34 (35), 10353−10362. (365) Mahadik, D. B.; Lee, K.-Y.; Ghorpade, R. V.; Park, H.-H. Superhydrophobic and Compressible Silica-polyHIPE Covalently Bonded Porous Networks via Emulsion Templating for Oil Spill Cleanup and Recovery. Sci. Rep. 2018, 8 (1), 16783. (366) Wan, X.; Azhar, U.; Wang, Y.; Chen, J.; Xu, A.; Zhang, S.; Geng, B. Highly porous and chemical resistive P(TFEMA−DVB) monolith with tunable morphology for rapid oil/water separation. RSC Adv. 2018, 8 (15), 8355−8364. (367) Wu, P.; Zhang, S.; Yang, H.; Zhu, Y.; Chen, J. Preparation of emulsion-templated fluorinated polymers and their application in oil/ water separation. J. Polym. Sci., Part A: Polym. Chem. 2018, 56 (14), 1508−1515. (368) Zhang, N.; Zhou, Y.; Zhang, Y.; Jiang, W.; Wang, T.; Fu, J. Dual-templating synthesis of compressible and superhydrophobic spongy polystyrene for oil capture. Chem. Eng. J. 2018, 354, 245−253. (369) Guo, Z.; Gu, H.; Chen, Q.; He, Z.; Xu, W.; Zhang, J.; Liu, Y.; Xiong, L.; Zheng, L.; Feng, Y. Macroporous monoliths with pHinduced switchable wettability for recyclable oil separation and recovery. J. Colloid Interface Sci. 2019, 534, 183−194. (370) Roucher, A.; Depardieu, M.; Pekin, D.; Morvan, M.; Backov, R. Inorganic, Hybridized and Living Macrocellular Foams: “Out of the Box” Heterogeneous Catalysis. Chem. Rec. 2018, 18 (7−8), 776−787. (371) Tripodo, G.; Marrubini, G.; Corti, M.; Brusotti, G.; Milanese, C.; Sorrenti, M.; Catenacci, L.; Massolini, G.; Calleri, E. Acrylate-based poly-high internal phase emulsions for effective enzyme immobilization and activity retention: from computationally-assisted synthesis to pharmaceutical applications. Polym. Chem. 2018, 9 (1), 87−97. (372) Zhang, S.; Fan, X.; Zhang, F.; Zhu, Y.; Chen, J. Synthesis of Emulsion-Templated Magnetic Porous Hydrogel Beads and Their Application for Catalyst of Fenton Reaction. Langmuir 2018, 34 (12), 3669−3677. (373) Gu, H.; Liu, Y.; Yin, D.; Cai, L.; Zhang, B.; Zhang, Q. HeparinImmobilized Polymeric Monolithic Column with Submicron Skeletons and Well-Defined Macropores for Highly Efficient Purification of Enterovirus 71. Macromol. Mater. Eng. 2018, 303 (12), 1800411. AH


Perspective

Macromolecules (374) Irani, M.; Jacobson, A. T.; Gasem, K. A. M.; Fan, M. Facilely synthesized porous polymer as support of poly(ethyleneimine) for effective CO2 capture. Energy 2018, 157, 1−9. (375) Wang, Q.; Ma, H.; Chen, J.; Du, Z.; Mi, J. Facile Synthesis of Polyethylenimine and Nano-TiO2 Particles Functionalized PolyHIPE Beads for CO2 Capture. Polym. Sci., Ser. B 2018, 60 (3), 380−386. (376) Yang, S.; Wang, Y.; Jia, Y.; Sun, X.; Sun, P.; Qin, Y.; Li, R.; Liu, H.; Nie, C. Tailoring the morphology and epoxy group content of glycidyl methacrylate-based polyHIPE monoliths via radiationinduced polymerization at room temperature. Colloid Polym. Sci. 2018, 296 (6), 1005−1016. (377) Yuan, W.; Chen, X.; Xu, Y.; Yan, C.; Liu, Y.; Lian, W.; Zhou, Y.; Li, Z. Preparation and recyclable catalysis performance of functional macroporous polyHIPE immobilized with gold nanoparticles on its surface. RSC Adv. 2018, 8 (11), 5912−5919. (378) Zhao, J.; Zhang, Y.; Wang, K.; Yan, C.; Da, Z.; Li, C.; Yan, Y. Development of Hierarchical Porous MOF-Based Catalyst of UiO66(Hf) and Its Application for 5-Hydroxymethylfurfural Production from Cellulose. ChemistrySelect 2018, 3 (41), 11476−11485. (379) Andler, S. M.; Goddard, J. M. Stabilization of Lipase in Polymerized High Internal Phase Emulsions. J. Agric. Food Chem. 2018, 66 (14), 3619−3623. (380) Gui, H.; Zhang, T.; Guo, Q. Closed-cell, emulsion-templated hydrogels for latent heat storage applications. Polym. Chem. 2018, 9 (29), 3970−3973. (381) Balderrama, J. A. M.; Dourges, M.-A.; Magueresse, A.; Maheo, L.; Deleuze, H.; Glouannec, P. Emulsion-templated pullulan monoliths as phase change materials encapsulating matrices. Mater. Today Commun. 2018, 17, 466−473. (382) Hu, Y.; Liu, S.; Li, X.; Yuan, T.; Zou, X.; He, Y.; Dong, X.; Zhou, W.; Yang, Z. Facile preparation of biocompatible poly(l-lactic acid)-modified halloysite nanotubes/poly(ε-caprolactone) porous scaffolds by solvent evaporation of Pickering emulsion templates. J. Mater. Sci. 2018, 53 (20), 14774−14788. (383) Wang, J.; Li, Y.; Gao, Y.; Xie, Z.; Zhou, M.; He, Y.; Wu, H.; Zhou, W.; Dong, X.; Yang, Z.; Hu, Y. Cinnamon oil-loaded composite emulsion hydrogels with antibacterial activity prepared using concentrated emulsion templates. Ind. Crops Prod. 2018, 112, 281− 289. (384) Wu, Z.; Hu, W.; Huang, T.; Lan, P.; Tian, K.; Xie, F.; Li, L. Hierarchically porous carbons with controlled structures for efficient microwave absorption. J. Mater. Chem. C 2018, 6 (32), 8839−8845. (385) Woodward, R. T.; Markoulidis, F.; De Luca, F.; Anthony, D. B.; Malko, D.; McDonald, T. O.; Shaffer, M. S. P.; Bismarck, A. Carbon foams from emulsion-templated reduced graphene oxide polymer composites: electrodes for supercapacitor devices. J. Mater. Chem. A 2018, 6 (4), 1840−1849. (386) Deshmukh, A. B.; Nalawade, A. C.; Karbhal, I.; Qureshi, M. S.; Shelke, M. V. Electrochemical capacitive energy storage in PolyHIPE derived nitrogen enriched hierarchical porous carbon nanosheets. Carbon 2018, 128, 287−295. (387) Roucher, A.; Bentaleb, A.; Laurichesse, E.; Dourges, M.-A.; Emo, M.; Schmitt, V.; Blin, J.-L.; Backov, R. First Macro-Mesocellular Silica SBA-15-Si(HIPE) Monoliths: Conditions for Obtaining SelfStanding Materials. Chem. Mater. 2018, 30 (3), 864−873. (388) Roucher, A.; Roussarie, E.; Gauvin, R. M.; Rouhana, J.; Gounel, S.; Stines-Chaumeil, C.; Mano, N.; Backov, R. Bilirubin oxidase-based silica macrocellular robust catalyst for on line dyes degradation. Enzyme Microb. Technol. 2019, 120, 77−83.

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