PolyHIPEs: Recent advances in emulsion-templated porous polymers

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

Emulsion-templated porous carbon monoliths derived from tannins

Tannin-based, carbonised, polymerised High Internal Phase Emulsions (polyHIPEs) are described in detail for the first time. Such highly porous materials were prepared by emulsion-templating, using an aqueous phase made of tannin and a low amount of hexamine dissolved in water, sunflower oil, and ethoxylated castor oil. After hardening of the tannin-based resin, the oil was leached out and the resultant monoliths were pyrolysed. The porous structure of carbon polyHIPEs prepared with an initial oil fraction ranging from 43 to 80 vol.% has been investigated, as well as their mechanical and thermal properties. We show that the most homogeneous materials, having the smallest pores, the narrowest pore size distributions and also the highest mechanical properties, are those made with an initial oil fraction around 70 vol.%. Such value is close to random and hexagonal close packing fractions: 64% and 74%, respectively.     

Synthesis of polymer–silica hybrid polyHIPEs by double in situ polymerization of concentrated water in oil emulsion

Highly porous monoliths from polymerized high internal phase emulsions (polyHIPEs) are proposed for a number of applications including liquid absorbents, biocatalysis, tissue engineering, bioseparation etc. To overcome some limitations of fully organic polyHIPEs hybrid organic–inorganic templated porous polymers are particular attractive. Here we present a straightforward protocol towards hybrid polyHIPEs by combining fast photo polymerization of the organic HIPE with in-situ polycondensation of tetraethylorthosilicate (TEOS). While under acidic conditions TEOS polymerization leads to a more bicontinous structure, true double-layered morphologies were obtained under basic conditions as evident from SEM imaging, mercury intrusion porosimetry and nitrogen physisorption measurements. Moreover, chemical amine functionalization of the silica network surface of the hybrid polyHIPEs was demonstrated using a silane coupling agent and subsequent visualization by reaction with fluorescein isothiocyanate (FITC).     

Synthesis and characterization of porous glycidylmethacrylate–divinylbenzene monoliths using the high internal phase emulsion approach

Highly porous monoliths were synthesised using glycidyl methacrylate (GMA) and divinylbenzene in the presence of a porogen, and emulsion templating as the preparation technique (polyHIPEs). Two surfactants were used as the emulsion stabiliser. It turned out that the choice of the surfactant was essential for the successful synthesis of polyHIPEs containing large amount of the functional monomer GMA, and characterised by a well-defined morphology. Sorbitan monooleate (SPAN 80) showed poor performances in stabilising emulsions with a content of GMA > 40% v/v. On the contrary, polyglycerol of a fatty acid (PGE 080/D) exhibited superior performances allowing to stabilise emulsion with a GMA content up to 80% v/v. The ensuing polyHIPEs presented a well-defined morphology and surface areas very close to those of the corresponding resins of the same composition. This proved that emulsion destabilising phenomenon such as Ostwald ripening was effectively inhibited by PGE.An important part of the research work was dedicated to investigate whether the epoxy group retained its integrity during synthesis and purification. It turned out that the percentage of the exposed epoxy groups that underwent hydrolysis depended on the size of the pores. Pores below a threshold size were not wetted by water and the epoxy groups present inside them were left unchanged. On the contrary, the epoxy groups within pores characterised by a diameter above this threshold underwent hydrolysis.     

Preparation of highly porous ZrB2/ZrC/SiC composite monoliths using liquid precursors via direct drying process

We developed a simple liquid precursor method for the syntheses of porous ZrB₂/ZrC/SiC composite monoliths. Furfuryl alcohol (FA), zirconium n-butoxide, tetraethyl orthosilicate and boric acid are used as the raw materials. By combining the polymerization of FA and gelation of inorganic sols, porous hybrid monoliths are prepared by direct drying the wet gels. The inorganic and organic polymers possibly form interpenetrated network which provides the robustness for the wet gel to withstand the severe changes during dessication. When heat-treated at 1600 °C, hybrid gels are converted into porous ZrB₂/ZrC/SiC monoliths. The microstructure of the ZrB₂/ZrC/SiC monoliths can be easily tailored by controlling the synthesis conditions. The porosities of the ZrB₂/ZrC/SiC monoliths can be tuned around 74.3–81.6%, while the average pore diameters can be tuned ranging from 1.0 to 8.5 μm with pretty narrow distribution. The compressive strengths of such highly porous ceramics are in the range of 1.2–1.9 MPa.     

Crystallizable diluent-templated polyacrylonitrile foams for macroporous carbon monoliths

Polyacrylonitrile (PAN) foams with different pore structures were prepared for the fabrication of macroporous carbon monoliths. The foams were prepared through thermally induced phase separation (TIPS) method using dimethyl sulfone (DMSO2) as a crystallizable diluent. Honeycomb-like porous foam is obtained from PAN/DMSO2 mixture containing about 5 wt.% PAN, and those with channel-like pores are resulted from the mixtures with 10–40 wt.% PAN. However, they only have few mesopores and the porosity is as low as 30–47% for the foams prepared from those mixtures containing 50–60 wt.% PAN. Real-time observation with polarized optical microscopy reveals that the channel-like structure stems from the spherulitic orientation of DMSO2 crystals in the polymer matrix. Taking into account this morphology, DMSO2 crystals are capable of acting as in situ formed templates, which subsequently enable to shape the final pore structure of PAN foams. Macroporous carbon monoliths with honeycomb- or channel-like pores were constructed from PAN foams by oxidative stabilization and carbonization. Their graphitic structure and specific surface areas were analyzed by wide-angle X-ray diffraction and Brunauer–Emmett–Teller measurement. This TIPS method using crystallizable diluent provides a new route to control the porous structure of PAN foams for carbon materials.     

Photopolymerised methacrylate-based emulsion-templated porous polymers

Highly porous and interconnected methacrylate-based porous materials were prepared by photopolymerisation of the continuous phase of high internal phase emulsion (HIPE) templates. The rapid cure afforded by photopolymerisation effectively ‘locks’ the emulsion morphology prior to emulsion destabilisation, in comparison to thermally-initiated HIPEs of similar compositions. Contrary to expectation, it was observed that fully cured photopolymerised polyHIPEs could be prepared with a thickness of up to 35 mm, despite the severe opacity of the parent emulsions. This is attributed to a photofrontal polymerisation process, where radicals generated on the surface propagate rapidly through the bulk of the emulsion. Homogeneous, well-defined polyHIPE materials of up to 95% nominal porosity were obtained by photopolymerisation of HIPEs containing up to 30 vol.% glycidyl methacrylate (GMA) in the monomer phase (the remaining monomers and crosslinker are acrylates). Surprisingly, poly(ethylene glycol) methacrylate (PEG-MA), a nonionic monomer that is miscible with both emulsion phases, could be added to such HIPEs after preparation. On polymerisation, hydrophilic, water-wettable porous materials were obtained. Finally, it was also demonstrated that all-methacrylate HIPEs could be prepared and cured to yield GMA-containing polyHIPEs. These findings demonstrate the versatility of photopolymerisation for the preparation of emulsion templated porous polymers.     

Emulsion-templated porous polymers: A retrospective perspective

PolyHIPEs are porous emulsion-templated polymers synthesized within high internal phase emulsions (HIPEs). HIPEs are highly viscous, paste-like emulsions in which the major, “internal” phase, usually defined as constituting more than 74% of the volume, is dispersed as discrete droplets within the continuous, minor, “external” phase. The surge in polyHIPE research and development parallels that of porous polymers in general and reflects the increasing number of potential applications (reaction supports, separation membranes, tissue engineering scaffolds, controlled release matrices, responsive and smart materials, and templates for porous ceramics and porous carbons). This review focuses upon the research and development in polyHIPEs through the prism of the work in our laboratory. The review includes an overview of the developments in polymerization chemistry, in the types of monomers, in the types of stabilization, in the generation of more complex polyHIPE-based systems (e.g. interpenetrating polymer networks, hybrids, bicontinuous polymers), and in unusual materials systems such as water-retaining polyHIPEs and shape-memory polyHIPEs.     

Dendritic polychelatogens: Synthesis,characterization, and metal ion binding properties

Rigid monoliths were prepared from concentrated oil in water (O/W) emulsions by a base-catalyzed polycondensation reaction of 2-nitroresorcinol with cyanuric chloride. Mercury intrusion/extrusion porosimetry confirmed that the obtained monoliths were porous with an open porosity. However, scanning electron microscopy showed that the structure of these materials was different from that of emulsion-derived materials previously described in the literature [polymerized high internal phase emulsions (polyHIPEs)]. In comparison with polystyrene/divinylbenzene-based polyHIPEs obtained by radical polymerization, these materials exhibited a higher skeletal density, and thermogravimetric analysis and differential scanning calorimetry analysis indicated that they were more thermally stable. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Electrically conductive porous alumina/graphite composite synthesized by starch consolidation with reductive sintering

This paper reports a facile and environment-friendly process to synthesize electrically conductive porous alumina/graphite composites by starch consolidation technique followed by reductive sintering. Green ceramic composites were consolidated with different starches and sintered at different temperatures in an argon atmosphere. Electrical measurements, carbon contents and Raman analyses of carbon structures determined an optimal sintering temperature of 1700 °C, which lead to a uniform formation of conductive graphitic networks at an optimal concentration of about 3.8 vol% in the porous composites. These carbon networks resulted into porous composites having high electrical conductivities measured in the range from 3 to 7 S/cm, which depended on the starch types and their porous properties. Correspondingly, the bulk porosities of the sintered composites were measured from 42 to 46%, with rounded micropores having diameters ranging from 14 to 39 μm. These porous properties of the sintered composites offer promising applications for conductive membrane and porous electrode.     

Hierarchically porous monoliths of carbon and metal oxides with ordered mesopores

Hierarchically porous carbon and metal oxide materials offer great benefits in separations, catalysis and renewable energy. We have here used hierarchically porous silica monoliths with ordered mesopores as hard templates to produce nanocast carbon, Co₃O₄, and NiO monoliths with similar structures. Besides providing the materials with more well-defined physicochemical properties, the ordered mesopore structure also offers an excellent model system for investigating the nanocasting process in detail. The mesopores of the silica monoliths were first infiltrated with furfuryl alcohol or metal nitrate precursor solutions, which subsequently could be thermally converted to carbon or the corresponding metal oxides. After the silica scaffolds have been removed by etching in base solutions, the resulting replica monoliths display macroscopic morphology and macropore structure similar to the original silica template. However, while the carbon and Co₃O₄ materials both display a well-organized nanowire structure, giving rise to high surface area and narrow pore size distribution, the NiO monoliths exhibit a significantly lower surface area and less well-defined mesopore structure implying that only part of the silica mesopores has been replicated. We believe this apparent difference between the two metal oxides is a consequence of differences in mass transport.     

Multiwalled carbon nanotube cryogels with aligned and non-aligned porous structures

Multiwalled carbon nanotube (MWCNT) cryogels were fabricated with aligned and non-aligned porous structures. The MWCNT cryogels contained a major fraction of MWCNTs with a minor fraction of silk fibroin as the structure binder. The morphology of the porous structures was controlled using a sol-gel process of silk fibroin to form the network structures. Microchannel structures were formed by ice-templating. The MWCNT cryogels contained mesopores and formed as monoliths. The MWCNT cryogels with aligned porous structures showed better thermal stability and electrical conductivity than the MWCNT cryogels with non-aligned porous structures due to the advantageous MWCNT interconnections. The morphology of the porous structures was examined by field emission scanning electron microscopy and transmission electron microscopy. The structure-property relationships of the MWCNT cryogels and the performance of the MWCNT cryogels as electrodes were investigated.     

A comparative study of porous scaffolds with cubic and spherical macropores

Two kinds of polyester porous scaffolds having cubic and spherical macropores were fabricated, and a comparative study of their morphologies and mechanical properties were made in this paper. Poly(d,l-lactic-co-glycolic acid) (PLGA) scaffolds were prepared by room temperature compression molding and particulate leaching method based on cubic NaCl particles and paraffin spheres with a similar size range of 355-450 μm and a series of porosities (77-97%). Scanning electronic microscopy demonstrated that the spherical pore scaffolds exhibited better pore interconnectivity than the cubic pore ones. In compressive tests of both kinds of scaffolds, striking yield peaks were found at relatively low porosities, but just non-linear flexure behavior was observed at high porosities. The power-law relationships of compressive modulus and compressive strength versus porosity were confirmed in both foams. Comparison of the underlying scaling exponents reveals that the scaffolds with spherical pores are, at high porosities, with better compressive properties to a certain degree in contrast to those with cubic pores.     

Hierarchically porous sulfur-containing activated carbon monoliths via ice-templating and one-step pyrolysis

Hierarchically porous sulfur-containing activated carbons were prepared by ice-templating from an aqueous sodium poly(4-styrenesulfonate) (Na-PSS) solution followed by a single 800 °C pyrolysis step. This thermal treatment induced crosslinking, with the in-situ generation of Na₂SO₄ (activating agent), before carbonization and activation. The thermal treatment also resulted in the formation of sulfur salts, which could be converted to elemental sulfur upon a simple HCl acid wash. The sulfur content in the monoliths measured by microanalysis could be increased from 17.07 wt. % to 39.74 wt. % by incorporating additional Na₂SO₄ into the monoliths prior to pyrolysis. The sulfur was uniformly dispersed within the micropores of the carbon, and could be selectively removed by degassing (heating under vacuum) at different temperatures, revealing specific surface areas of up to 1051 m² g⁻¹. The materials were characterized by various techniques and were also evaluated for their potential as cathode materials for the lithium–sulfur battery. This work may open up new and facile routes to prepare sulfur-containing porous carbons for applications where its presence is beneficial.     

Magnetically-separable hierarchically porous carbon monoliths with partially graphitized structures as excellent adsorbents for dyes

Magnetically-separable hierarchically porous carbon monoliths with partially graphitized structures were synthesized through confinement self-assembly in polyurethane (PU) foam associated with a direct carbonization process from triblock copolymer F127, phenolic resol and ferric nitrate. It was observed that the magnetic Fe nanoparticles were embedded in the walls of graphitic porous carbon matrix, and the resulting materials exhibited hierarchically porous structure with macropores of 100–450 μm, mesopore size of 4.8 nm, BET surface area of 723 m²/g, pore volume of 0.46 cm³/g, and saturation magnetization of 3.1 emu/g. Using methylene blue as model dye pollutant in water, the carbon monolith materials showed high adsorption capacity of 190 mg/g, exhibiting excellent adsorption characteristics desirable for the application in adsorption of dyes and easy separation under an external magnetic field.     

Synthesis of open porous emulsion-templated monoliths using cetyltrimethylammonium bromide

In recent years, open porous materials (polyHIPEs) have attracted more and more attention because of their specific properties and applications in biological tissue scaffolds, catalysis supports and ion-exchange resin. However, the surfactants used in this type material were limited to nonionic surfactants or the mix of nonionic surfactant and ionic surfactant. In this work, firstly well-defined polyHIPEs were synthesized by W/O emulsions with ionic surfactant alone (e.g., CTAB). Furthermore, the polyHIPEs with much higher pore volume (14.7 cm³ g⁻¹), uniform pore diameter and cell size were obtained by this method. Both the median pore diameter and average cell size of the polyHIPEs rose with increase of DVB concentration and/or water fraction.     

Carbothermal conversion of self-supporting organic/inorganic interpenetrating networks to porous metal boride monoliths

In this work, we present a general sol-gel protocol for the synthesis of highly porous monolithic transition metal borides via carbothermal conversion of the organic/inorganic interpenetrating networks (IPNs). The formation of organic/inorganic IPNs is clearly demonstrated by simple oxidation and boiling water treatment. A series of transition metal boride porous monoliths, including CrB₂, ZrB₂, TiB₂, Cr₃C₂/CrB, and ZrB₂/ZrC with porosities ranging from 70% to 85% and pore sizes ranging from 0.5 to 35 μm, have been prepared. In each case, a porous hybrid monolith is obtained by drying the wet gel under ambient pressure. It is believed that the formation of organic/inorganic IPNs strengthens the gel network, so that it can withstand the severe changes during desiccation to give out a monolithic xerogel. Samples are characterized by TG-DSC, XRD, SEM, EDS, TEM, BET, and MIP, and the ceramic monoliths are shown to be well defined and rather homogeneous.     

Porous polymers prepared via high internal phase emulsion polymerization for reversible CO2 capture

A series of porous polymers with different pore volumes, pore sizes, and crosslinking densities were synthesized by high internal phase emulsion (HIPE) polymerization. The crosslinked polymerized HIPEs (polyHIPEs) were formed by the copolymerization of 4-vinylbenzyl chloride and divinylbenzene using water droplets in conventional or Pickering HIPEs as the templates. These porous materials were further modified by quaternization and ion exchange to introduce quaternary ammonium hydroxide groups. The resulting polyHIPEs were utilized as sorbents for reversible CO₂ capture from air using the humidity swing. The effect of pore structure on the CO₂ adsorption and desorption processes was studied. The polyHIPEs containing large pores and interconnected porous structures showed improved swing sizes and faster adsorption/desorption kinetics of CO₂ compared to a commercial Excellion membrane with similar functional groups.     

One-pot hydrothermal synthesis of nitrogen-doped hierarchically porous carbon monoliths for supercapacitors

The nitrogen-doped hierarchically porous carbon monoliths (N-HPCMs) were successfully synthesized by using dicyandiamide (DCDA) as nitrogen source, phenolic resol as carbon precursor and mixed triblock copolymers as templates via a one-pot hydrothermal approach. The obtained carbon monoliths possess tunable mesopore size (4.3–11.4 nm), large surface area (552–660 m²/g), and high nitrogen content (up to 12.1 wt%). Ascribed to the nitrogen-doped frameworks and hierarchical porosity, N-HPCMs exhibit good electrochemical performance as the supercapacitor electrode with specific capacitance of 268.9 F/g (in 6 M KOH) at a current density of 1 A/g, and a 4.1 % loss of the specific capacitance after 5,000 charge–discharge cycles, indicating a long-term cycling stability. Such unique features make N-HPCMs promising electrode materials for high performance supercapacitors.     

Emulsion templated bicontinuous hydrophobic-hydrophilic polymers: Loading and release

Water absorption is often poor in hydrophobic polyHIPEs, porous polymers synthesized within high internal phase emulsions (HIPEs). This paper describes bicontinuous polyHIPEs, the simultaneous polymerization of hydrophobic monomers (external phase) and hydrophilic monomers (internal phase). Integrating hydrogels within polyHIPEs extended the release of water-soluble dye from 10 h to more than 10 days. PolyHIPE capillary action promoted the rapid distribution of water throughout the hydrogel. The diffusion pathway in this bicontinuous system was similar to diffusion through an assembly of polydisperse spheres. The copolymerization of the hydrophilic monomers with the monomers in the external phase enhanced the hydrophilicity of the scaffold, reduced the modulus of the hydrated polyHIPE, and reduced the tortuosity of the diffusion path. Pre-polymerization of the external phase reduced the extent of copolymerization, enhanced the modulus, sealed the interconnecting holes, reduced the capillary action, increased the tortuosity, and extended the release time to 3 weeks.