Porous polymers are of interest for a wide variety of applications including absorption, adsorption, tissue engineering, membranes, controlled release, reaction supports, and shape memory foams. Emulsion templating can be used to generate high-porosity, macroporous polymer monoliths with highly interconnected, micrometer-scale porous structures through polymerization in the external, continuous phase followed by removal of the internal, dispersed phase. Emulsion templating possesses, on one hand, the benefit of being seemly simple. This simplicity, however, belies its inherent versatility and considerable parameter space that enables creative design of innovative new materials in terms of their macromolecular structures, their porous structures, and their properties. As described here, approaching emulsion templating with a specific structure or application in mind can enable a do-it-yourself outlook to imaginatively selecting the most appropriate emulsion type, stabilization strategy, polymerization mechanism, crosslinking strategy, and post-synthesis modification. The research and development of emulsion-templated polymers has been blossoming, as reflected not only in the number of articles published, but also in the number of novel porous polymer materials synthesized and in the number of heretofore unexplored applications investigated. It is the hidden complexity of emulsion templating that enables a continuous stream of pioneering works stemming from breakthrough insights in connected and contiguous scientific fields. This appraisal, highlighting emulsion templating strategies, will serve as a guide for those involved in developing innovative polymers with unique macromolecular and porous structures that engender exceptional properties. Contemplating the future directions of emulsion templating, given the robust nature of its established foundation, suggests that innovative research and development will continue to flourish.

Hierarchically porous structures can enhance the applicative performance of porous polymers. Macroporous polymer monoliths (polyHIPEs) can be generated by templating within high internal phase emulsions (HIPEs) and mesoporous polymers can be generated through block copolymer (BCP) templating. Here, novel hierarchically porous polymer monoliths were generated by emulsion templating BCP-like macromolecular structures consisting of a sacrificial poly(epsilon-caprolactone) (PCL) mid-block and hypercrosslinkable poly(styrene-co-divinyl- benzene) (P(S-co-DVB)) end-blocks synthesized from a PCL macroinitiator using activators generated by electron transfer for atom transfer radical polymerization. The macroporous structures were networks of sub-micrometer channels. Hierarchical porosities were generated by etching the PCL and/or by hypercrosslinking the P(S-co- DVB). Etching removed similar to 40 % of the PCL, generating mesoporosity. Hypercrosslinking generated both micro- porosity and mesoporosity, with a specific surface area of ca. 800 m(2)/g for 50 wt % P(S-co-DVB). Unexpectedly, hypercrosslinking the etched polyHIPE yielded less microporosity and mesoporosity, indicating that the PCL was integral to the mechanism of pore generation. This work serves as the foundation of an approach to generating innovative hierarchical porosities in polyHIPEs that can be pre-designed through the synthesis of BCPs followed by post-synthesis modifications.

PolyHIPEs are macroporous polymers templated within high internal phase emulsions (HIPEs). The ability to tailor the macromolecular and porous structures makes polyHIPEs of interest for three dimensional tissue engineering scaffolds. In this work, polyHIPEs with densities ranging from 0.18 to 0.28 g/cc were synthesized from novel biodegradable poly(epsilon-caprolactone) (PCL) macromers based on methacrylated oligomeric PCL diols of various molecular weights. Different types of internal phases generated porous structures that varied from networks of channels to highly interconnected voids. The crosslinked macromolecular structure limited PCL crystallization, resulting in elastomeric behavior with moduli of around 20 kPa. The HIPEs proved suitable for 3D printing both in air and in an innovative gel-bath. The suitability of the polyHIPEs for tissue engineering applications was indicated by their moduli, by their complete degradation within 4 h in 3 M NaOH, and by mesenchymal stem cells adhering and proliferating. The high level of viability can be attributed to the porosity that enables sufficient nutrient and waste diffusion. These results provide a foundation for designing 3D HIPE inks for printing macroporous tissue engineering scaffolds.

PolyMIPEs are emulsion-templated macroporous polymers synthesized within medium internal phase emulsions (MIPEs), emulsions with dispersed phases between 40 and 74 vol %. Here, biodegradable poly(epsilon-caprolactone) (PCL) polyMIPEs, synthesized using ring-opening polymerization within oil-in-oil MIPEs, were cross-linked with either a novel combination of pentaerythritol (initiator) and glutaraldehyde (star cross-linker) or with bis(epsilon-caprolactone-4-yl). PolyMIPEs from emulsions stabilized using a surfactant, nanoparticles (NPs), or innovative polymer brushes (PBs) with predesigned arms were compared. The polyMIPEs exhibited moduli similar to those of soft tissues, withstood compressive strains up to 70%, and underwent complete hydrolytic degradation. PolyMIPEs from NP-stabilized MIPEs exhibited closed-cell structures with relatively large voids and lower moduli, while polyMIPEs from PB-stabilized MIPEs exhibited significantly smaller voids and higher moduli, revealing an unexpected connection between the void diameters and the mechanical properties. This work lays the foundation for modifying the macromolecular structures, porous structures, and properties of macroporous PCL monoliths through the cross-linking and emulsion-stabilization strategies.

Hierarchically porous polymers are of interest for a variety applications including adsorption, catalysis, scaffolds, energy storage, and controlled release. In this work, two simultaneous, mutually exclusive reactions within the external phases of water-in-oil high internal phase emulsions (HIPEs) were used to generate macroporous semi-interpenetrating polymer network (semi-IPN) polyHIPE monoliths. The styrenic copolymer of vinylbenzyl chloride (VBC) and divinylbenzene (DVB) was synthesized using free radical polymerization. The non-crosslinked poly(urethane urea) (PUU), based on a polycaprolactone (PCL) triol, was synthesized using step-growth polymerization. The open-cell, semi-IPN polyHIPEs had densities of-0.17 g/cm3, corresponding to porosities of-84 %. The semi-IPN polyHIPE with the higher P(VBC-co-DVB) content (75 wt %) exhibited a porous structure typical of polyHIPEs, with an average void diameter of-10 pm. The porous structure of the semi-IPN polyHIPE with the lower P(VBC-co-DVB) content (50 wt %) resembled a network of channels. Hierarchical porosities were generated by either etching the flexible PCL-based non-crosslinked PUU to generate mesoporosity or by hypercrosslinking the stiff styrenic copolymer framework to generate microporosity. Etching the semi-IPN polyHIPEs in 3 M NaOH removed most of the PUU. Hypercrosslinking generated microporosity and specific surface areas of 445 and 176 m2/g for 75 and 50 wt % P(VBC-co-DVB), respectively. In the semi-IPNs, the hypercrosslinking process also generated mesoporosity from the removal of some of the PUU.

Phase change materials (PCMs) based on hydrated salts are promising candidates for energy storage and release applications owing to their relatively high latent heats per volume, their high thermal conductivities, and their nonflammability. The use of these PCMs, however, is limited owing to incongruent melting, significant supercooling during crystallization, and irreversible phase transformations. PolyHIPEs, polymers templated within high internal phase emulsions (HIPEs), were proven to be effective at encapsulating aqueous solutions and organic liquids. Here, calcium chloride hexahydrate (CCHH) was successfully encapsulated within elastomeric, acrylate-based, emulsion-templated polymers by using free-radical polymerization within molten-salt-in-oil HIPEs. The effects of adding a nucleating agent to the internal phase to reduce supercooling and the effects of enhancing HIPE stability through modification of the external phase were investigated. The crosslinking and stabilization modifications made to enhance the original polyHIPE formulation successfully enhanced the encapsulation efficiency. The most promising system, 75% CCHH dispersed as relatively uniform internal phase droplets of 100-300 mu m in diameter, exhibited melting and crystallization heats of similar to 120 J/g, a relatively low degree of supercooling, and negligible transformation to calcium chloride tetrahydrate. This work, demonstrating that molten salt hydrates can be successfully encapsulated within elastomeric, emulsion-templated monoliths, can be used as a blueprint for the encapsulation of other salt hydrates.

Emulsion-templated foams have displayed promise as injectable bone grafts; however, the use of a surfactant as an emulsifier resulted in relatively small pores and impedes cell attachment. Hydroxyapatite nanoparticles were explored as an alternative stabilizer to address these limitations. To this end, hydroxyapatite nanoparticles were first modified with myristic acid to generate the appropriate balance of hydrophobicity to stabilize a water-in-oil emulsion of neopentyl glycol diacrylate and 1,4-butanedithiol. In situ surface modification of the resulting foam with hydroxyapatite was confirmed with elemental mapping and transmission electron microscopy. Nanoparticle-stabilized foams displayed improved human mesenchymal stem cell viability (91 ± 5%) over surfactant-stabilized foams (23 ± 11%). Although the pore size was appropriate for bone grafting applications (115 ± 71 μm), the foams lacked the interconnected architecture necessary for cell infiltration. We hypothesized that a co-stabilization approach with both surfactant and nanoparticles could be used to achieve interconnected pores while maintaining improved cell attachment and larger pore sizes. A range of hydroxyapatite nanoparticle and surfactant concentrations were investigated to determine the effects on microarchitecture and cell behavior. By balancing these interactions, a co-stabilized foam was identified that possessed large, interconnected pores (108 ± 67 μm) and improved cell viability and attachment. The co-stabilized foam was then evaluated as an injectable bone graft including network formation, microscale integration with bone, push out strength, and compressive properties. Overall, this work demonstrated that in situ surface modification with nHA improved cell attachment while retaining desirable bone grafting features and injectability.

Polymers of intrinsic microporosity (PIMs) are linearmacromoleculesfrom rigid, twisted monomers. PolyHIPEs, macroporous polymer monolithstemplated within high internal phase emulsions (HIPEs), are of interestfor adsorption applications. Here, PIM-like linear polymers synthesizedwithin oil-in-oil HIPEs generated powders. However, the addition ofa cross-linking comonomer enabled, and was essential for, the generationof monoliths with densities of similar to 0.08 g/cm(3) and uniform,highly interconnected, open-cell structures. Interestingly, the voiddiameters strongly depended upon the internal phase, similar to 3 mu mfor silicone oil and similar to 52 mu m for hexadecane. The unexpectedlyflexible polyHIPEs exhibited poor thermal stabilities compared toPIM-1, indicating significant macromolecular differences that originatein the emulsion-templated synthesis and in the cross-linking. Whilethe polyHIPEs synthesized at 80 degrees C exhibited relatively highspecific surface areas (similar to 154 m(2)/g), they were farsmaller than that of PIM-1. The polyHIPEs adsorbed up to similar to 28mg/g of a cationic dye but only negligible amounts (similar to 1 mg/g)of an anionic dye. These results demonstrate that hierarchically porousmonoliths that may be of interest for adsorption applications canbe generated through PIM-like syntheses within HIPEs.

Polymers templated within high internal phase emulsions (polyHIPEs) are characterized by a highly interconnected macroporous structure and exhibit remarkable absorption properties. Hyper-cross-linked resins (HCLRs) are micro/mesoporous polymers with high adsorption capacities toward organic molecules. Here, polyHIPEs based on styrene (ST) and divinylbenzene (DVB) and containing unmodified and amino-functionalized HLCRs based on vinylbenzyl chloride (VBC) and DVB are realized using two different approaches, one based on the post-HIPE addition and the other based on the in-HIPE addition of the HCLRs. The amino-functionalization of the HCLRs was used to induce their localization on the polyHIPE's surface. PolyHIPEs containing the amino-functionalized HCLRs exhibited significantly enhanced specific surface areas and sorption capacities for polar volatile organic compounds (VOCs) and CO2, and the addition of HCLRs does not negatively affect the high uptake of organic solvents by the polyHIPEs. The polyHIPEs containing the HCLRs exhibited regenerability above 99% in five sorption cycles. Nitrogen adsorption analysis and sorption tests demonstrated that the polyHIPEs with HCLRs synergistically combine the adsorption of the micro/mesoporous HCLRs with the absorption capacities of the polyHIPEs to generate advanced sorbent systems.

Hierarchically porous polymers combine microporosity, mesoporosity, and macroporosity to enhance pore accessibility and transport. This work describes generating hierarchically porous polymers and carbons derived therefrom by combining emulsion-templated macroporous polymers bearing novel macromolecular structures with simultaneous Friedel-Crafts hyper-cross-linking and porogen removal. The hyper-cross-linking was based on a copolymer of vinylbenzyl chloride and divinylbenzene, while the porogen was based on poly(epsilon-caprolactone) (PCL). The two polymer systems were combined using one-pot syntheses of either simultaneous interpenetrating polymer networks (IPNs) containing a PCL-based poly(urethane urea) (PUU) or a PCL-based semi-IPN. In the semi-IPN, the microporosity was enhanced through hyper-cross-linking and through porogen removal during carbonization. The effects of incorporating a PCL-based PUU depended upon the diisocyanate. Unexpectedly, mesoporosity was generated from hyper-cross-linking an IPN synthesized with an aliphatic diisocyanate. On the other hand, the formation of interconnected networks from hyper-cross-linking an IPN synthesized with an aromatic diisocyanate enhanced both the microporosity and thermal stability.

The ability of beta-cyclodextrin (beta-CD) to form a host-guest complex with relatively hydrophobic molecules such as bisphenol A (BPA), a common wastewater contaminant, has inspired the development of beta-CD-containing polymers for adsorption applications. For the most part, these polymers are powders, which can limit their applicability. Here, low-density (similar to 0.1 g cm(-3)), macroporous monoliths based on unmodified beta-CD were synthesized using emulsion templating. The relatively simple, one-pot, polyurethane synthesis took place within the external phase of oil-in-oil (o/o) high internal phase emulsions (HIPEs) that contained beta-CD and a diisocyanate. The combination of o/o emulsions and a polyurethane reaction enabled the incorporation of relatively high beta-CD contents (up to 63 wt%) and limited the occurrence of the water-isocyanate urea reaction. The resulting open-cell porous structures varied from an average pore size of 5.0 mu m (lower surfactant content) to the typical structure associated with polyHIPEs (higher surfactant content), voids of similar to 50 mu m and interconnecting holes ranging from submicrometer to similar to 2 mu m. The compressive mechanical behaviors of the easily handled monoliths were similar to those of flexible foams, reaching strains of 70% without failure. The BPA adsorption capacities, up to 117.7 mg g(-1), may make these monoliths advantageous for adsorption applications.

Polysaccharides are abundant natural resources that are of interest for various biomedical applications. In addition to being biocompatible and biodegradable, these polymers can undergo a wide range of application-specific chemical modifications. Here, macroporous polymer monoliths based on four different polysaccharides (alginate, pectin, dextran, and chitosan) were synthesized using a relatively simple and straightforward procedure, templating within oil-in-water high internal phase emulsions (HIPEs). Interfacial step-growth polymerization between the components in the external aqueous phase (water, a polysaccharide, a surfactant) and a diisocyanate in the internal organic phase was used to synthesize a set of poly(urethane urea) polyHIPEs. The diisocyanate content, the nature of the polysaccharide, and the nature of the polymerization catalyst significantly impacted the porous structure. Cells growing in the amine-catalyzed polyHIPEs adhered to the walls, spread, and penetrated into the porous structures. This work serves as a blueprint for generating macroporous monoliths from the multitude of available polysaccharides.

PolyHIPEs, highly porous polymers synthesized within high internal phase emulsions (HIPEs), emulsions with over 74% internal phase, are of interest for applications such as absorbents, reaction supports, and tissue engineering scaffolds. Typically, the surfactant contents for HIPE stabilization are relatively high, ranging from 20 to 30 wt% of the external phase, with the monomers usually being the remainder. One drawback of using surfactants for these applications is the potential for leachables, necessitating intensive purification processes for their removal. Pickering HIPEs, HIPEs stabilized using amphiphilic solid nanoparticles that spontaneously migrate to the oil-water interface, can be used as an alternative HIPE stabilization strategy. Although nanoparticles can add surface functionality advantageous for the application, polyHIPEs from Pickering HIPEs often lack the interconnecting holes needed for the high permeability required for such applications. This work describes a successful approach for designing an HIPE stabilization system that is based on a combination of nanoparticles and reactive surfactants and that generates the desired surface functionality, an interconnected porous structure, and a low leachable content. Such an approach can extend the applicative utility of such polyHIPEs by circumventing the need for extensive purification.

Porous carbons are of interest for a wide range of advanced-technology 'green' energy applications including fuel cells, hydrogen storage, supercapacitors and batteries. Functional groups, heteroatoms and a more accessible hierarchical porous structure would be advantageous for many of these applications. This paper describes the generation of carbonaceous monoliths with hierarchically porous structures and nitrogen functionalities by using a one-pot, simultaneous combination of hydrogel synthesis and hydrothermal carbonization (HTC) that involves templating within high internal phase emulsions (HIPEs). A carbon monolith with a density of 0.058 g cm(-3), a highly interconnected, bimodal porous structure and an apparent specific surface area (S-BET) of 101 m(2) g(-1) was produced by carbonizing a HTC monolith based on 2-hydroxyethyl methacrylate (HEMA) at 450 degrees C. S-BET of 1540 m(2) g(-1) was produced through subsequent chemical activation with ZnCl2 at 700 degrees C, but the overall residual mass (R-m) was only 9 wt%. Direct chemical activation of the HTC monolith, on the other hand, generated S-BET of 1250 m(2) g(-1) and an overall R-m of 28 wt%, corresponding to a higher apparent surface area per mass of HTC monolith. Carbon monoliths with N/C ratios of 0.09 and 0.07 were achieved using nitrogen-rich monomers (acrylamide and vinylimidazole, respectively) as compared to the HEMA-based carbon monolith with an N/C ratio of 0.03. This work demonstrates that the hierarchically porous structures and the chemical structures of these highly porous monoliths can be fine-tuned by modifying the HIPE composition and/or the processing conditions. (c) 2021 Society of Industrial Chemistry.

PolyHIPEs, macroporous polymers synthesized within the external phases of high internal phase emulsions (HIPEs), have been developed for a wide range of applications (catalyst supports, tissue engineering, drug de-livery) in which the leaching of traditional surfactants can become a significant liability. HIPEs can also be stabilized with relatively small amounts of amphiphilic solid nanoparticles, as well as with miktoarm stars, poly (divinylbenzene) (PDVB) cores bearing different types of polymer arms. In this paper, two types of stars bearing both poly(ethylene oxide) and polyacrylate arms were used to stabilize HIPEs, non-reactive stars and stars bearing reactive methacrylate groups. The 0.1 wt % of stars successfully used to stabilize HIPEs containing styrene and DVB were not enough, at low DVB contents in the HIPE, to provide the crosslinking support needed to resist the capillary stresses generated during drying. However, with sufficient DVB in the HIPE, star contents as low as 0.05 wt % were able to successfully stabilize a HIPE with 90% internal phase and produce a polyHIPE with a density of 0.06 g/cm(3). As little as 0.07 wt % stars were enough to stabilize HIPEs containing long side-chain (meth)acrylates and similar to 0.7 wt % of a crosslinking polyhedral silsesquioxane (POSS) and yield polyHIPEs with densities of around 0.2 g/cm(3). These polyHIPEs exhibited rapid shape recovery and recovery ratios of 100% when a temporary shape (a 70% compressive strain) was imposed above the melting point of the crystalline phase. Star stabilization combined with POSS crosslinking enable the generation of tunable shape memory polymer foams.

PolyHIPEs are porous, emulsion-templated polymers synthesized within high internal phase emulsions (HIPEs), concentrated emulsions usually containing over 74% of a dispersed internal phase. PolyHIPEs are usually hydrophobic, crosslinked polymers synthesized within water-in-oil (w/o) HIPEs through free radical polymerization (FRP). There have only been a few attempts to use controlled radical polymerizations such as reversible addition–fragmentation chain transfer (RAFT) polymerization for polyHIPE synthesis. Here, the effects of polymerization mechanism, initiator solubility, RAFT agent to initiator ratio, and crosslinking comonomer content upon the porous structure, the static and dynamic mechanical properties, and the swelling of poly(styrene-co-divinylbenzene) polyHIPEs were investigated. The surface-active RAFT agent produced a significant reduction in void size and a significant enhancement in void connectivity. The locus of initiation affected the mechanical behavior, with the moduli from interfacial initiation being significantly higher than those from organic-phase initiation. In addition, using RAFT instead of FRP produced significant changes in the thermal, mechanical, and uptake behaviors that seem to reflect the enhancements in macromolecular mobility and uniformity often associated with controlled polymerization.

Macroporous, emulsion-templated, linear poly(urethane urea) elastomers were synthesized from polyols (poly(ε-caprolactone)s or polycarbonates) and a diisocyanate. Growing cells adhered to the walls, spread, and penetrated into the porous structures.

Cellulose is one of the most abundant resources in nature. Cellulose-based porous monoliths have been fabricated by direct drying of oil-in-water high internal phase emulsions (HIPEs) stabilized using either cellulose derivatives or surface-modified cellulose. The resulting cellulose-based porous polymers, however, were usually fragile, reflecting the lack of crosslinking. Herein, we report a new strategy to fabricate cellulose-based porous materials (polyurethane polyHIPEs, PU polyHIPEs) templated within non-aqueous HIPEs through the covalent crosslinking between isocyanates and unmodified cellulose. The PU polyHIPEs exhibited interconnected macroporous structures and exhibited tunable wettability from hydrophilicity/oleophilicity to hydrophobicity/oleophilicity. The PU polyHIPEs were able to absorb a wide variety of oils, exhibiting relatively large capacities and high absorption rates. We further showed that the PU polyHIPEs were robust, and they did not fail under compressive stress even at strains of 70%. The robustness, large absorption capacities, rapid absorption, and hydrophobicity/oleophilicity make these cellulose-based PU polyHIPEs suitable for oil absorption and/or oil–water separation applications.

A typical description of interpenetrating polymer networks (IPN) can be surprisingly simple, systems that consist of two crosslinked polymer networks that are physically entangled but not chemically linked. That simplistic description, however, successfully encompasses a wide range of synthesis processes and macromolecular architectures that can include “semi-IPN” (IPN-like systems containing only one crosslinked polymer) and interconnected polymer networks (IPN-like systems that also include a limited amount of inter-network chemical links). The macromolecular topologies of these systems combine kinetically entrapped molecular-level mixing with limited phase separation into a continuous range of nanodomain compositions. This perspective-review presents the family of IPN systems, describes the synthesis parameters used to generate a variety of macromolecular topologies, and discusses the damping properties, the ability to process latex IPN, the mechanical robustness of double network hydrogels, and IPN as templates for porous polymers, as well as recent innovations and cutting-edge applications. The wide gamut of macromolecular topological options described herein will serve as a guide to realizing synergistic behaviors by combining polymers in IPN-like structures.