Hierarchical polymerized high internal phase emulsions synthesized from surfactant-stabilized emulsion templates

Synthesis of PEG-Polycycloether Block Copolymers: Poloxamer Mimics Containing a Rigid Helical Block

Poloxamers are amphiphilic block copolymers consisting of poly(ethylene glycol) (PEG) and poly(propylene glycol) segments. Their self-assembly and interfacial properties are tied to the relative hydrophilicity and hydrophobicity of each block and can therefore be adjusted by changing block lengths. Here, a series of PEG-polycycloether block copolymers is synthesized that have the same structure as a poloxamer, but they encompass a rigid polycyclic backbone as the hydrophobic block. A variety of polymer structures are synthesized, for example diblock or triblock architectures, with/without olefinic units, atactic or isotactic backbone, and different block lengths. Due to their amphiphilicity, self-assembly into spherical aggregates (diameters ranging from 64 to 132 nm) at low concentrations (critical aggregation concentration as low as 0.04 mg mL-1) is observed in water. Low surface tensions (as low as 26.7 mN m-1) are observed as well as the formation of stable high internal phase emulsions (HIPEs) irrespective of the oil/water ratio. This contrasts with the properties of the commonly used poloxamers P188 or P407 and illustrates the significance of the rigid polycycloether block. These new colloidal properties offer new prospects for applications in emulsion formulations for biomedicine, cosmetics, and the food industry.

Polydimethylsiloxane Polymerized Emulsions for Acoustic Materials Prepared Using Reactive Triblock Copolymer Surfactants

Porous polymers have interesting acoustic properties including wave dampening and acoustic impedance matching and may be used in numerous acoustic applications, e.g., waveguiding or acoustic cloaking. These materials can be prepared by the inclusion of gas-filled voids, or pores, within an elastic polymer network; therefore, porous polymers that have controlled porosity values and a wide range of possible mechanical properties are needed, as these are key factors that impact the sound-dampening properties. Here, the synthesis of acoustic materials with varying porosities and mechanical properties that could be controlled independent of the pore morphology using emulsion-templated polymerizations is described. Polydimethylsiloxane-based ABA triblock copolymer surfactants were prepared using reversible addition-fragmentation chain transfer polymerizations to control the emulsion template and act as an additional cross-linker in the polymerization. Acoustic materials prepared with reactive surfactants possessed a storage modulus of ∼300 kPa at a total porosity of 71% compared to materials prepared using analogous nonreactive surfactants that possessed storage modulus values of ∼150 kPa at similar porosities. These materials display very low longitudinal sound speeds of ∼35 m/s at ultrasonic frequencies, making them excellent candidates in the preparation of acoustic devices such as metasurfaces or lenses.

Charge Density Overcomes Steric Hindrance of Ferrocene Surfactant in Switchable Oil-in-Dispersion Emulsions

A ferrocene surfactant can be switched between single and double head form (FcN⁺ C₁₂ /Fc⁺ N⁺ C₁₂ ) triggered by redox reaction. FcN⁺ C₁₂ can neither stabilize an O/W emulsion alone nor an oil-in-dispersion emulsion in combination with alumina nanoparticles due to the steric hindrance of the ferrocene group. However, such steric hindrance can be overcome by increasing the charge density in Fc⁺ N⁺ C₁₂ , so that oil-in-dispersion emulsions can be co-stabilized by Fc⁺ N⁺ C₁₂ and alumina nanoparticles at very low concentrations (1×10^-7  M (≈50 ppb) and 0.001 wt %, respectively). Not only can reversible formation/destabilization of oil-in-dispersion emulsions be achieved by redox reaction, but also reversible transformation between oil-in-dispersion emulsions and Pickering emulsions can be obtained through reversing the charge of alumina particles by adjusting the pH. The results provide a new protocol for the design of surfactants for stabilization of smart oil-in-dispersion emulsions.

Surfactant-free oil-in-oil emulsion-templating of polyimide aerogel foams

A surfactant-free oil-in-oil emulsion-templating method is presented for fabrication of monolithic polyimide aerogel foams using monomer systems that produce fast sol–gel transition. An aerogel foam is a high porosity (∼90%) material with coexisting meso- and macropores inherent to aerogels with externally introduced micrometer size open cells (macrovoids) that are reminiscent of foams. The macrovoids are introduced in polyimide sol using surfactant-free emulsion-templating of droplets of an immiscible liquid that are stabilized against coalescence by fast sol–gel transition. Three immiscible liquids – cyclohexane, n-heptane, and silicone oil – are considered in this work for surfactant-free emulsion-templating. The aerogel foam monoliths, recovered by supercritical drying, exhibit smaller size macrovoids when n-heptane and cyclohexane are used as emulsion-templating liquid, while the overall porosity and the bulk density show weak dependence on the emulsion-templating liquid.

The role of excess attractive particles in the elasticity of high internal phase Pickering emulsions

A high internal phase emulsion (HIPE), which has a volume fraction of dispersed phase of over 74%, shows a solid-like property because of concentrated polyhedral droplets. Although many studies have proposed theoretical and empirical models to explain the rheological properties of HIPEs, most of them are only limited to the emulsions stabilized by surfactants. In the case of high internal phase Pickering emulsions (HIPPEs), much greater values of elastic modulus have been reported, compared to those of surfactant-stabilized HIPEs, but so far, there have been no clear explanations for this. In this study, we investigate how colloidal particles attribute to the significantly high elasticity of HIPPEs, specifically considering two different contributions, namely, interfacial rheological properties and bulk rheological properties. Our results reveal that the flocculated structures of colloidal particles that possess a significant elasticity can be interconnected between dispersed droplets. Furthermore, this elastic structure is a crucial factor in the high elasticity of HIPPEs, which is also supported by a simple theoretical model.

Bioinspired organohydrogels with heterostructures: Fabrications, performances, and applications

Since emerging in 1960, the artificial hydrogels have garnered enormous attentions in scientific community due to their high level of similarities to biological soft tissues in both structures and properties. With the proceeding of research, the concern of hydrogels is gradually shifted from fundamental investigation to abundant functionalization. In contrast to the natural soft tissues, the current artificial hydrogels still possess relatively simple structures and unsatisfactory environmental adaptability, extremely limiting their practical applications in complex environments. Enlightened by the prominent adaptability of biological organisms, the binary cooperative complementary principle is utilized to develop bioinspired organohydrogels by combining two components with opposite but cooperative physiochemical features. The present review provides the advanced progresses of bioinspired organohydrogels with sophisticated heterogeneous networks and desirably environmental adaptabilities. We clearly summarize the synthesizing strategies in regard to both corresponding mechanisms and typical examples, including macroscopic organohydrogels, organohydrogels with binary solvent, organohydrogels with heteronetworks, and emulsion-based organohydrogels. Meanwhile, the intriguing features of the reported organohydrogels, such as temperature resistance, switchable mechanics, adaptive wettability, and opposite components compatibility, are also clearly highlighted with a short overview of their promising applications. Ultimately, the current challenges and perspectives on the future development of bioinspired organohydrogels are also discussed.

Polydimethylsiloxane nanocomposite macroporous films prepared via Pickering high internal phase emulsions as effective dielectrics for enhancing the performance of triboelectric nanogenerators

Polydimethylsiloxane (PDMS) nanocomposite (NC) macroporous films were prepared by a Pickering high internal phase emulsion (HIPE) templating technique and used as effective dielectrics for enhancing the performance of triboelectric-nanogenerators (TENGs). HIPEs were formulated using commercial PDMS and water as the continuous and dispersed phase, respectively. The formation and solidification of PDMS-based HIPEs were possible through stabilization with silver-nanoparticles (Ag-Nps) and surfactant (Span 20) mixtures. The resulting PDMS-NC-polyHIPE films presented an interconnected 3D macroporous structure with Ag-Nps on their porous surface. The addition of different amounts of Ag-Nps (0, 4, 20, 28, 36 wt%) in HIPE formulations allowed modification of the pore size, total pore volume and dielectric properties of the tribo-materials. Results revealed that both the porosity and dielectric properties of these materials play an important role in enhancing the output performance of TENGs. Thus, the best TENG based on the PDMS-NC-polyHIPE film was achieved with 20 wt% of Ag-Nps, with voltage, current and power values of 4.88 V, 0.433 μA and 2.1 μW, respectively, which gives over 3.28-fold power enhancement compared with the reference TENG (based on a PDMS film without porosity or Ag-Nps). Therefore, the preparation of tribo-materials through a Pickering HIPE templating technique provides a novel, effective and easy way for the improvement of the TENG's performance.

Recent advances in separation applications of polymerized high internal phase emulsions

Polymerized high internal phase emulsions as highly porous adsorption materials have received increasing attention and wide applications in separation science in recent years due to their remarkable merits such as highly interconnected porosity, high permeability, good thermal and chemical stability, and tailorable chemistry. In this review, we attempt to introduce some strategies to utilize polymerized high internal phase emulsions for separation science, and highlight the recent advances made in the applications of polymerized high internal phase emulsions for diverse separation of small organic molecules, carbon dioxide, metal ions, proteins, and other interesting targets. Potential challenges and future perspectives for polymerized high internal phase emulsion research in the field of separation science are also speculated at the end of this review.

Tailoring the morphology of poly(high internal phase emulsions) synthesized by using deep eutectic solvents

High internal phase emulsions (HIPEs) are among complex biphasic fluids that expand on the traditional emulsion compositions, for instance, for the preparation of macroporous polymers by emulsion templating. The use of deep eutectic solvents (DESs) as the nonaqueous internal phase of HIPEs allows expanding the conditions at which polymerizations are typically carried out in aqueous HIPEs. Herein, the properties of polystyrene macroporous polymers were studied by polymerizing DES-in-oil HIPEs using choline chloride-based DESs as the internal phase. The effect of DESs’ composition – with amide, alcohol, and carboxylic acid as hydrogen bond donors – and the homogenization method used for the HIPE preparation – vortexing versus high-speed homogenizer – was studied. The stability and droplet size of HIPE precursor, as well as the macroporous structure and the mechanical properties of the synthesized polyHIPEs, are discussed.

Preparation of porous adsorbent via Pickering emulsion template for water treatment: A review

Porous materials as emerging potential adsorbents have received much more attention because they are capable of capturing various pollutants with fast adsorption rate, high adsorption capacity, good selectivity and excellent reusability. In order to prepare porous materials with decent porous structure, Pickering emulsion template method has been proved to be one of the most effective technologies to create pore structure. This paper reviewed comprehensively the latest research progress on the preparation of porous materials from various Pickering emulsions and their applications in the decontamination of pollutants (e.g., heavy metal ions, organic pollutants) and in the oil/water separation. It was expected that the summaries and discussions in this review will provide insights into the design and fabrication of new efficient porous adsorbents, and also give us a better understanding of the subject.

Emulsion-templated synthetic polypeptide scaffolds prepared by ring-opening polymerization of N-carboxyanhydrides

Ring-opening polymerization of N-carboxyanhydrides was performed in oil-in-oil high internal phase emulsion to obtain well-defined macroporous synthetic polypeptides.

Preparation of fluoropolymer materials with different porous morphologies by an emulsion template method using supercritical carbon dioxide as a medium

The choice of a suitable surfactant is key to the formation of a stable water-in-CO2 (W/C) or CO2-in-water (C/W) emulsion. It is even more critical in stabilization of the emulsion containing carbon dioxide (CO2). In this study, the successful preparation of W/C emulsion was achieved by using the amphiphilic block polymer poly(ethylene glycol) methyl ether-b-poly(trifluoroethyl methacrylate) (mPEG45-b-(TFEMA) n ) as a surfactant, in which CO2 was used as a solvent for the fluoromonomer, trifluoroethyl methacrylate (TFEMA). In the case of the W/C emulsion, CO2 and TFEMA were used as the continuous phase and water as the internal phase of the emulsion system. It has been found that in the length of the block polymer mPEG45-b-(TFEMA) n , the fluorine-containing chain end has a significant effect on the morphology of the polymer and the type of emulsion formed. The morphology of the polymer was observed by scanning electron microscopy which confirmed the type of emulsion formed. With the fluorine-containing end segment, the morphology of the polymer changes from a small hollow sphere in a large hollow sphere to a hollow spherical to a porous structure. Correspondingly, it could be concluded that the type of emulsion could go through the process from water-in-CO2-in-water-in-CO2 (W/C/W/C) emulsion to water-in-CO2-in-water (W/C/W) emulsion to water-in-CO2 (W/C) emulsion. Also, suitable co-surfactants were identified in this study. Investigations were also attempted to check the effect of the amount of surfactant, cross-linker and water/CO2 ratio on the type of emulsion formed as well as the morphology of the resultant polymer.

Development of a Highly Sensitive, Broad-Range Hierarchically Structured Reduced Graphene Oxide/PolyHIPE Foam for Pressure Sensing

Highly sensitive pressure sensors are usually made from soft materials that allow large deformations to be obtained when very small pressures are applied. Unfortunately, this current paradigm limits the ability to create sensors capable of high sensitivities and broad dynamic ranges as these materials are prone to saturation responses when attempting to obtain measurements involving high pressures. In this paper, we detail a piezoresistive pressure sensor that is capable of high sensitivity over a pressure range spanning from 0.6 Pa (a mosquito touching a surface) to 200 kPa (an elephant standing on the surface). The sensor's ability to cover such a broad dynamic range is made possible by the fairly hard foam used in its construction as this material is capable of propagating strain in a highly effective manner due to its hierarchical porous structure. The material was fabricated by using high-internal-phase emulsion (HIPE) as a template to generate a highly porous material consisting of small pores packed between larger ones whose inner walls are lined with reduced graphene oxide. The developed foam exhibits very fast response times (less than 15.4 ms) and excellent cyclic stability (at least 10,000 cycles). Furthermore, it is capable of responding to the entire tactile pressure range, and it can be formatted as pixelated arrays, which makes it highly suitable for integration into wearable electronic devices. Such arrays were built and used to identify and render the shape of objects with different geometries, including a sphere, a triangle, a square, and two nearly identical rods differing only by 0.4 mm in diameter.

Macroporous monoliths with pH-induced switchable wettability for recyclable oil separation and recovery

HYPOTHESIS

The effective separation and recovery of oils from water is important for the protections of ecosystems and the environment. Polymeric porous monoliths have been demonstrated as attractive absorbents for oil/water separation. However, the recyclability was mainly realized by squeezing, combustion, or centrifugation, which may restrict in elastic materials, destroy the adsorbates or need special apparatus. Thus it is desirable to developing monoliths with controllable oil absorption and desorption.

EXPERIMENTS

A series of "smart" monoliths with pH-induced switchable wettability were fabricated by high internal phase emulsion (HIPE) polymerization and epoxide ring-opening for the incorporation of amine groups. The resultant monoliths and their wettabilities were examined using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), nitrogen adsorption/desorption and contact angle analysis, respectively. The oil separation efficiency and recyclability were evaluated.

FINDINGS

The monoliths with macroporous structure can undergo switchable wettability under reversible pH stimulation. As an absorbent, the monoliths not only separated and recovered organic solvents and oils (including crude oil) from aqueous mixtures through a reversible and recyclable absorption and desorption process upon alternating the pH between 7.0 and 1.0, but also continuously expulsed oils from water surfaces in a continuous manner with the aid of external driving pressures. Moreover, the monoliths also allowed the effective separation of surfactant-free and surfactant-stabilized oil-in-water emulsions with high separation efficiency.

Synthesis of Emulsion-Templated Magnetic Porous Hydrogel Beads and Their Application for Catalyst of Fenton Reaction

The pristine Fe₃O₄ nanoparticle (FeNP) is supposed to be a good catalyst of Fenton processes which have shown significant potential for water purification. Herein the magnetic macroporous hydrogel beads, having an open-cell structure, were synthesized by sedimentation polymerization of pristine FeNP stabilized oil-in-water high internal phase emulsions. The effects of the FeNP amount, internal phase fraction, and the costabilizer Tween85 concentration on the structure, such as interconnecting degree, void size, and its distribution of both the surface and inner of the beads, were investigated. With a methyl orange (MO) aqueous solution passing through a chromatography column that was filled with the FeNPs loaded hydrogel beads, the efficiency of these hydrogel beads as catalyst for Fenton reaction to decompose MO in water was tested. The MO was decomposed quickly at the first hour, followed by decomposed gradually in a further 5 h, and the decomposition rate of MO could be up to 99.6% at the end of the test. Moreover, MO decomposition rate remained over 98.2% in six batches which were run in the same beads filled column. The results showed that these FeNPs loaded porous hydrogel beads were reusable and highly efficient supporter for catalysis of Fenton reaction for decomposing organic pollutants in water.

Modeling compressive behavior of open-cell polymerized high internal phase emulsions: effects of density and morphology

The compressive behavior of poly(HIPE) foams was studied using the developed micromechanics based computational model. The model allowed identifying the morphological parameters governing the foam compressive behavior. These parameters comprise: (i) foam density, (ii) Sauter mean diameter of voids calculated from the morphological analysis of the polydispersed microstructure of poly(HIPE), and (iii) polymer/strut characteristic size identified as the height of the curvilinear triangular cross-section. The model prediction compared closely with the experiments and considered both the linear and plateau regions of the compressive poly(HIPE) behavior. The computational model allows the prediction of structure-property relationships for poly(HIPE) foams with various relative densities and open cell microstructure using the input parameters obtained from the morphology characterization of the poly(HIPE). The simulations provide a pathway for understanding how tuning the manufacturing process can enable the optimal foam morphology for targeted mechanical properties.

Highly porous and chemical resistive P(TFEMA–DVB) monolith with tunable morphology for rapid oil/water separation

A facile preparation for a series of porous poly(2,2,2-trifluoroethylmethacrylate–divinylbenzene) P(TFEMA–DVB) foams is discussed in this paper. The foams have adjustable morphology utilizing a suitable commercial surfactant, Hypermer B246, as stabilizer, and were compared with traditional organic surfactants or macromolecular block-polymers. Combining the porous properties and advantages of fluorine atoms, this type of fluoropolymer exhibited superb chemical stability and hydrophobicity performances with high porosity. These porous fluoro-monoliths preserved their regular porous structure without any degradation after immersion into strong acidic or basic solution for three days, hence demonstrating an excellent potential to deal with environmental pollution caused by oil spillages in severe environments. The tunable morphology (open and closed pores) and pore sizes were achieved by investigating various parameters like surfactant concentration, amount of external crosslinker, and aqueous phase volume. Droplet sizes of HIPEs were characterized using an optical microscope under different experimental conditions. The influence of pore structure and surface properties of polyHIPE on water contact angle and oil adsorption capacity was also explored. The results indicated that the porous material has an excellent oleophilicity and hydrophobicity, with water contact angles (WCA) up to 146.4°. Additionally, the results presented a noticeable adsorption with a very fast rate towards organic oils from either a water surface or bottom with adsorption saturation achieved in about 120 s. The prepared polyHIPEs showed a good recycling ability; even after 10 adsorption–centrifugation experiments, the adsorption capacity was still more than 85%.

A facile preparation for a series of porous poly(2,2,2-trifluoroethylmethacrylate–divinylbenzene) P(TFEMA–DVB) foams is discussed in this paper.

Simultaneous synthesis and chemical functionalization of emulsion-templated porous polymers using nitroxide-terminated macromolecular surfactants

The design of functional 3D macroporous monoliths has become a necessity for a wide range of applications.

Deep-Eutectic Solvents as MWCNT Delivery Vehicles in the Synthesis of Functional Poly(HIPE) Nanocomposites for Applications as Selective Sorbents

We report an alternative green strategy based on deep-eutectic solvents (DES) to deliver multiwalled carbon nanotubes (MWCNTs) for a bottom-up approach that allows for the selective interfacial functionalization of nonaqueous poly(high internal phase emulsions), poly(HIPEs). The formation and polymerization of methacrylic and styrenic HIPEs were possible through stabilization with nitrogen doped carbon nanotube (CN_X) and surfactant mixtures using a urea-choline chloride DES as a delivering phase. Subtle changes in CN_X concentration (less than 0.2 wt % to the internal phase) produced important changes in the macroporous monolith functionalization, which in turn led to increased monolith hydrophobicity and pore openness. These materials displayed great oleophilicity with water contact angles as high as 140° making them apt for biodiesel, diesel, and gasoline fuel sorption applications. Overall, styrene divinylbenzene (StDvB) based poly(HIPEs) showed hydrophobicity and fuel sorption capacities as high as 4.8 (g/g). Pore hierarchy, namely pore openness, regulated sorption capacity, and sorption times where greater openness resulted in faster sorption and increased sorption capacity. Monoliths were subject to 20 sorption-desorption cycles demonstrating recyclability and stable sorption capacity. Finally, CN_X/surfactant hybrids made it possible to reduce surfactant requirements for successful HIPE formation and stabilization during polymerization. All poly(HIPEs) retained acceptable conversion as a function of CN_X loading nearing 90% or better with thermal stability as high as 283 °C.

Interconnected Porous Polymers with Tunable Pore Throat Size Prepared via Pickering High Internal Phase Emulsions

Interconnected macroporous polymers were prepared by copolymerizing methyl acrylate (MA) via Pickering high internal phase emulsion (HIPE) templates with modified silica particles. The pore structure of the obtained polymer foams was observed by field-emission scanning electron microscopy (FE-SEM). Gas permeability was characterized to evaluate the interconnectivity of macroporous polymers. The polymerization shrinkage of continuous phase tends to form open pores while the solid particles surrounding the droplets act as barriers to produce closed pores. These two conflicting factors are crucial in determining the interconnectivity of macroporous polymers. Thus, poly-Pickering HIPEs with high permeability and well-defined pore structure can be achieved by tuning the MA content, the internal phase fraction, and the content of modified silica particles.

Facile synthesis of hierarchical pore foam catalysts with Brønsted–Lewis acid sites for the one-pot conversion of cellulose to 5-hydroxymethylfurfural

The Whole process of synthesis of HPFCs by Pickering high internal phase emulsions and application in conversion of cellulose to HMF.

Soft porous silicone rubbers as key elements for the realization of acoustic metamaterials

In this work, macroporous materials made of polydimethylsiloxane, a soft silicone rubber, are prepared using UV polymerization with an emulsion-templating procedure. The porosity of the final materials can be precisely controlled by adjusting the volume of the dispersed phase. We show that the porous structure of the materials is the template of the droplets of the initial emulsions. Mechanical tests show that the materials Young's moduli decrease with the porosity of the materials. Acoustic measurements indicate that, in such a porous elastomeric matrix, the sound speed also decreases dramatically as soon as the porosity increases to attain values of as low as 80 m/s. The results are compared to earlier ones on silica aerogels and are interpreted within the framework of a simple theoretical approach. We show that the very low sound speed value is a consequence of the low value of the polymer shear modulus. This explains why such porous soft silicone rubbers are so efficient at playing the role of slow-soft resonators in acoustic metamaterials. Moreover, the fast rate of polymerization of such UV-curable fluid allows for a facile shaping of the final material as beads or rods in microfluidic devices.1.

Hierarchically porous polystyrene membranes fabricated via a CO2-expanded liquid selective swelling and in situ hyper-cross-linking method

This paper presents a novel, etching-free and versatile preparation methodology, using commercially available polystyrene and a CO₂-expanded liquid selective swelling process combined with a hyper-cross-linking reaction.

Emulsion-templated macroporous polymer/polymer composites with switchable stiffness

Emulsion templates containing monomers in both emulsion phases were used to manufacture polystyrene-co-divinylbenzene based polymerized high internal phase emulsions (polyHIPEs) which have been reinforced by poly(methacrylic acid) (polyMAA) and poly(dimethyl aminoethyl methacrylate) (polyDMAEMA). The morphology of the hydrogel-filled polyHIPEs is affected by the hydrogels synthesized in the aqueous emulsion phase. The pore structure of polyMAA-filled polyHIPEs is highly interconnected indicating the formation of a methacrylic acid-co-styrene copolymer at the oil/water interface of the emulsion templates during synthesis. However, polyDMAEMA-filled polyHIPEs are predominately closed celled and the pore walls are covered by grafted hydrogel. The ability of the hydrogel-filled polyHIPEs to absorb water decreased with increasing crosslinking density of the hydrogels. The dry hydrogel reinforced the polyHIPE scaffolds possessed higher elastic moduli and crush strengths than the control polyHIPEs. The reinforcing ability of the dry hydrogels was further enhanced by increasing their degree of crosslinking. However, the reinforcement could be “switched off” simply by hydrating the hydrogels. The switchable mechanical properties of the hydrogel-filled polyHIPEs could potentially be utilized in smart humidity sensor technology.