Emulsion-templated Porous Polymers Research Articles

Control of pore interconnectivity in emulsion-templated porous polymers

The applications of porous polymers obtained from polymerized high internal phase emulsions (polyHIPEs) depend on their morphology, especially interconnectivity of pores (openness). For example, an open-cell structure is required for filtration and adsorbent materials, whereas a closed-cell structure is ideal for the applications in energy storage, insulation, and encapsulation. However, the remaining challenge is to control the interconnectivity of polyHIPEs, which is influenced by the complex process of pore throat formation. In this work, we demonstrate that the formation of pore throats can be tuned through varying the attractive interdroplet interactions in the HIPE precursor. The attractive interactions among the droplets are manipulated by employing different surfactants and the refractive index (RI)-matching agent. While the depletion attraction and steric repulsion can be indirectly controlled by changing the surfactant, the van der Waals interaction is controlled by varying the amount of RI-matching agent. The scaled viscoelastic moduli of HIPEs with Laplace pressure are used to further confirm the variation of interdroplet interactions. In addition to the interdroplet interactions, we investigate the potential effect of interfacial rheology on the interconnectivity of polyHIPEs.

Photoresponsive Emulsion-Templated Porous Materials via Orthogonal Photoclick Chemistry.

The functionalization of emulsion-templated porous polymers (polyHIPEs) utilizing modern and efficient chemistries is an important avenue for tailoring the properties of these scaffolds for specific and specialized applications. Herein, tetrazole photoclick chemistry is utilized for the efficient functionalization of polyHIPEs synthesized from various monomer systems and polymerization chemistries. Using both radical polymerization and thiol-ene polymerization, polyHIPEs with well-defined, interconnected open-cell morphologies are synthesized with tetrazole concentrations ranging from 0 to 5 w/v %, with the pore diameters ranging from 3 to 24 μm. Analyzed by fluorescence spectroscopy, FTIR spectroscopy, and confocal microscopy, spatially controlled functionalization to generate photopatterned fluorescent polyHIPEs is demonstrated via the reaction with residual acrylate and thiol groups. In addition, the scaffolds can be readily functionalized with external dipolarophiles such as acrylates to incorporate a functionality onto the polyHIPE surface. With many functional tetrazoles also reported in the literature, a PEG-tetrazole is also used to explore the photoinduced functionalization of polyHIPEs possessing tunable ratios of thiol and acrylate groups, and the effect on fluorescence, wettability, and biocompatibility is analyzed. Overall, the reaction is shown to be a broadly applicable tool for polyHIPE functionalization with many avenues for further development toward specific applications.

Fabrication of Macroporous Polymers via Water-in-Water Emulsion-Templating Technique.

Emulsion-templated porous polymers have attracted broad attention due to their great application prospects in many fields. However, scaling up the emulsion-templated technique from the lab to industrial production remains a great challenge, especially for systems involving an oil-in-water (o/w) emulsion template that is used normally for preparing hydrophilic porous polymers. These systems require large amounts of organic solvents to be the internal phase (i.e., major phase) of the emulsion templates, which causes a significant environmental impact and cost. Herein, a water-in-water (w/w) emulsion-templated technique is presented to prepare porous hydrophilic polymers. The w/w emulsion is prepared by mixing a PEG aqueous solution and a dextran aqueous solution with cellulose nanocrystals (CNCs) as a stabilizer. With varying the mass ratio of dextran/PEG in the range of 1/2 to 8/1, a series of dextran-rich-phase-in-PEG-rich-phase (dextran/PEG) emulsions are obtained. Subsequently, monomers, such as acrylamide, acrylic acid, and/or 2-acrylamido-2-methylpropanesulfonic acid, are introduced to the emulsions to fabricate porous hydrophilic polymers. These polymers have an open-cell structure like those of o/w emulsion-templated polymers. The system developed herein is an environmentally friendly, low cost, and universal emulsion-templated method toward porous hydrophilic polymers, which avoids the defects caused by the presence of large amounts of organic solvents in an o/w emulsion-templating method and can be moved from the lab to industrial-scale production.

Nanofibrous, hierarchically porous poly(ether sulfone) xerogels templated from gel emulsions for removing organic vapors and particulate matters

Emulsion-templated method is an emerging technology for the fabrication of porous polymers, but the conventionally chemical crosslinking still restricts the popularization. Based on the physical gelation, emulsion-templated porous polymers (polyHIPEs) can be possible to break the bottleneck. This work reports gel emulsions and corresponding xerogels solidified by solvent-induced crystallization of the poly(ether sulfone) for removing organic vapors and particulate matters. Gel emulsions showed wide adaptability in the dispersed phase fraction (from 5% to 80%) and high stability over 5 months without change in appearance. Emulsion-templated xerogels were obtained from gel emulsions by atmospheric drying, which represents a facile method for the preparation of emulsion-templated porous polymers. The resulting xerogels exhibited controllable external shapes, low shrinkages, emulsion-templated voids, nanofibrous structures, hierarchically interconnecting pores, robust mechanical properties and the reshape ability, which are much more advantageous than conventional polyHIPEs. The xerogels exhibited high adsorption capacities to the organic solvent vapors from air (1.21 mL g−1 for toluene and 1.45 mL g−1 for chloroform), high reusability (without deterioration after five adsorption-desorption cycles) and enhanced adsorption rate imposed by emulsion-templated structures. Moreover, the xerogels captured a mass of particulate matters from air with high durability. The wide adaptability, facile preparation, unique morphology and high capacity and stability to capture solvent vapors and particulate matters enable the current xerogels to be excellent candidates for cleaning air.

Emulsion-Templated Porous Polymers for Efficient Dye Removal.

A high internal phase emulsion (HIPE) method was used to produce adsorbents with an interconnected porous structure. HIPE was prepared using vinyl benzyl chloride (VBC), divinylbenzene (DVB), tert-butyl acrylate, and Span80 as the organic phase and water with K2S2O8 and CaCl2 as the water phase. The polymerization of the organic phase produced highly porous polymers called polyHIPE, carrying two functional groups. As a result of the template method, polyHIPEs have a low surface area. To overcome this drawback, polyHIPE was hyper-cross-linked through VBC to create meso- and micropores, resulting in a higher surface area. Then the polymer surface was tailored with carboxylic acid groups by simple hydrolysis of tert-butyl acrylate. The adsorption performances of the acidic functional hyper-cross-linked polyHIPEs prepared for the various reaction times of 0, 15, and 60 min were compared for methylene blue. The hyper-cross-linked polyHIPEs showed an enhanced adsorption kinetics for methylene blue, and the 15 min hyper-cross-linking reaction increased the rate of methylene blue adsorption significantly. It was proven that the polyHIPE adsorbent can be reused by treating it with an aqueous acidic solution in ethanol.

Nanofibrous, porous monoliths formed from gelating high internal phase emulsions using syndiotactic polystyrene

Gelating high internal phase emulsions (HIPEs) is critical for the preparation of emulsion-templated porous polymers, and the gelation is conventionally realized through chemical crosslinking of monomers or through physical interaction of a HIPE stabilizer. Here, we present a new method to gelate HIPEs and to fabricate emulsion-templated porous monoliths by incorporating a gelating polymer, syndiotactic polystyrene (sPS), into the continuous phase of an HIPE, without using any chemical crosslinker or any gelating stabilizer. The emulsion-templated sPSs exhibited controllable shapes, nanofibers (with average diameters of around 24 nm) on void walls, emulsion-templated macropores, and windows in two different scales (interconnecting pores from emulsion-templating and interconnecting pores among nanofibers), robust mechanical property (without failure even at a high compressive strain of 70%) and large surface areas (up to 325 m2 g−1), facilitating rapid absorption of airborne volatile organic compounds. After absorption, the porous sPSs could be easily regenerated under vacuum. Therefore, the porous monoliths can be excellent candidates for removal of airborne volatile organic compounds.

Branched macromonomers from catalytic chain transfer polymerisation (CCTP) as precursors for emulsion-templated porous polymers

Catalytic chain transfer polymerisation (CCTP) is combined for the first time with emulsion-templating to generate polyHIPE materials where functionality and rigidity can be tightly tailored, broadening their scope of application.

Enhanced Differentiation Potential of Primary Human Endometrial Cells Cultured on 3D Scaffolds.

Novel approaches for culturing primary human cells in vitro are increasingly needed to study cell and tissue physiology and to grow replacement tissue for regenerative medicine. Conventional 2D monolayer cultures of endometrial epithelial and stromal cells fail to replicate the complex 3D architecture of tissue. A fully synthetic scaffold that mimics the microenvironment of the human endometrium can ultimately provide a robust platform for investigating tissue physiology and, hence, take significant steps toward tackling female infertility and IVF failure. In this work, emulsion-templated porous polymers (known as polyHIPEs) were investigated as scaffolds for the culture of primary human endometrial epithelial and stromal cells (HEECs and HESCs). Infiltration of HEECs and HESCs into cell-seeded polyHIPE scaffolds was assessed by histological studies, and phenotype was confirmed by immunostaining. Confocal microscopy revealed that the morphology of HEECs and HESCs is representative of that found in vivo. RNA sequencing was used to investigate transcriptome differences between cells grown on polyHIPE scaffolds and in monolayer cultures. The differentiation status of HEECs and HESCs grown in polyHIPE scaffolds and in monolayer cultures was further evaluated by monitoring the expression of endometrial marker genes. Our observations suggest that a 3D cell culture model that could approximate native human endometrial architecture and function can be developed using tailored polyHIPE scaffolds.

Emulsion-templated porous polymers prepared by thiol-ene and thiol-yne photopolymerisation using multifunctional acrylate and non-acrylate monomers

The chemical and mechanical properties of macroporous polymer substrates play a crucial role in the determination of their end-application. The preparation of highly (macro)porous monolithic polymers (polyHIPEs) by emulsion templating and thiol-ene/yne photopolymerisation, using multifunctional acrylate, allyl ether and alkyne-based monomers with trimethylolpropane tris (3-mercaptopropionate) (TMPTMP), is described in this work. Issues associated with monomer solubility and/or stability of the produced high internal phase emulsions (HIPEs) are tackled. Scanning electron microscopy (SEM) is used to study the morphology and porosity (average void diameters) of the obtained materials. Due to the nature of the photoinitiated thiol-ene reactions, materials obtained from acrylate monomers display residual thiols that are quantified by a colourimetric (Ellman's) assay. Raman spectroscopy is also shown to be a complementary technique to evaluate the residual thiol content. The influence of the monomer functionality on the mechanical properties of the material is explored using compression tests. Significant differences in the surface functionality and mechanical behavior between materials prepared with comonomers able to homopolymerise (acrylates) and those unable to homopolymerise (allyl ethers; alkynes) are demonstrated.

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.

The effect of para-divinyl benzene on styrenic emulsion-templated porous polymers: a chemical Trojan horse

Emulsion-templated porous polymers such as polymerized high internal phase emulsions (polyHIPEs) are an exciting class of porous polymer due to tunable physical and chemical properties. One original use of polyHIPEs was in high-energy laser experiments. However, a density heterogeneity issue has been identified in polyHIPEs during screening for suitability as a laser target. To understand this problem, we studied the physical properties in detail of a well-understood polyHIPE system, styrene-co-divinyl benzene (S-co-DVB). Dynamic mechanical analysis (DMA), scanning electron microscopy, Mercury porosimetry, and time domain nuclear magnetic resonance (TD-NMR) identified that para-DVB, a component of divinyl benzene (DVB80), was an unstable species—effectively behaving as a chemical Trojan horse. TD-NMR spin–lattice and spin–lock relaxation times elucidated more structural detail on the polymer structure for all the polyHIPEs studied, such as long spin–lattice times corresponded to high modulus values. Furthermore, we observed that the Young’s modulus was reduced by 40% in polyHIPEs containing para-DVB as the cross-linker. Understanding the factors affecting polyHIPE physical properties is an important step to further enhance their effectiveness in the many growing applications of polyHIPEs.

Influence of the Macromolecular Surfactant Features and Reactivity on Morphology and Surface Properties of Emulsion-Templated Porous Polymers

This work investigates key parameters of a straightforward macromolecular surfactant-assisted functionalization strategy of porous polymers produced by high internal phase emulsion (HIPE) polymeriz...

Chemical functionalization of emulsion-templated porous polymers by thiol–ene “click” chemistry

Thiol–acrylate polyHIPE materials possess residual thiols, which act as convenient groups for chemical modification by thiol–ene “click” chemistry.

Amine-functionalization of glycidyl methacrylate-containing emulsion-templated porous polymers and immobilization of proteinase K for biocatalysis

Glycidyl methacrylate (GMA) emulsion-templated porous polymers (polyHIPEs) were prepared by thermal and photopolymerisation and derivatised with morpholine, tris(2-aminoethylamine) and a bisamino-PEG homobifunctional molecule. The extent of the functionalization reactions was investigated by a range of qualitative and quantitative techniques (FTIR, CHN analysis, titration, XPS, HR-MAS NMR spectroscopy, ninhydrin assay and Fmoc number determination) and was found to be excellent for small molecule amines (up to 89% conversion) but low for the reaction with PEG (2% conversion). This was ascribed to the high exclusion volume of the PEG chains in solution. Proteinase K (Pro K) was subsequently immobilized covalently onto the GMA polyHIPE material, both directly via reaction with surface epoxy groups and indirectly by activation of the pendent amine groups of PEGylated polyHIPE with glutaraldehyde then reductive amination with the enzyme. The activity of the supported enzymes was determined by a continuous electrochemical assay involving the hydrolysis of N-acetyl-l-tyrosine ethyl ester. The directly immobilized Pro K was found to have an activity of only 3.6 U/g whereas the activity of the enzyme immobilized via the PEG linker was much higher (up to 78 U/g).

Carbon nanotubes in emulsion-templated porous polymers: Polymer nanoparticles, sulfonation, and conductivity

Sulfonated polymers are of interest for ion exchange resins, reaction supports, and membranes for separation, filtration, fuel cells, and electrochemical devices. Sulfonic groups have been introduced into polystyrene (PS) through exposure to sulfuric acid, and carbon nanotubes (CNTs) have been added to polymers to enhance proton conductivity without creating an electronic percolation pathway. PolyHIPEs, emulsion-templated porous polymers with highly interconnected hierarchical open-cell porous structures, are synthesized through polymerization in the external phases of high internal phase emulsions (HIPEs). In this article, the synthesis of PS-based CNT-filled polyHIPEs, their structure, sulfonation, and conductivity are described. Adding CNT dispersions to the HIPEs produced polymer nanoparticle–covered polyHIPEs from polymerization within the water-soluble surfactant micelles in the internal aqueous phase droplets. The CNTs migrated from the HIPE's aqueous phase droplets into the HIPE's organic phase and formed interconnected bundles within the polyHIPE walls, reflecting a reduction in the surfactant's ability to disperse the CNTs. The water adsorption in the hygroscopic sulfonated polyHIPEs increased the conductivity by several orders of magnitude. The conductivity of the sulfonated polyHIPE containing CNTs was more than an order of magnitude greater than that of the sulfonated polyHIPEs with no CNTs. The CNTs act as “bridges,” enhancing the connection between existing conductive pathways. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4369–4377

Reactive thiol-ene emulsion-templated porous polymers incorporating pentafluorophenyl acrylate

Highly porous polymers (polyHIPEs) incorporating activated esters have been prepared by photopolymerisation of high internal phase emulsions (HIPEs) containing pentafluorophenyl acrylate (PFPA) in the monomer phase. The resulting materials have nominal porosity of 80% and a well-defined, interconnected pore morphology with average pore diameters ranging from 30 to 50 μm. PFPA could be added at up to 50 wt% of the monomer phase without destabilising the HIPE noticeably, however analysis of the polyHIPE materials revealed that only around half of this was incorporated into the final porous materials. The pentafluorophenyl groups were shown to be reactive towards a range of amines (tris-(2-aminoethyl)amine, benzylamine and Rhodamine 123), giving near complete loss of fluorine over the period of the reaction. The extent of functionalisation was however determined to be around 50% by elemental analysis, suggesting some loss of fluorine by hydrolysis of activated esters. We believe that this represents a facile method for the preparation of a variety of well-defined functional porous polymers by a post-polymerisation functionalisation route.

Synthesis of Emulsion-Templated Acrylic-Based Porous Polymers: From Brittle to Elastomeric

High internal phase emulsion polymers (PolyHIPEs) are novel materials that have high porosity and interconnected open-cell structure and are used in various applications such as supports for catalytic systems, media for separation of similar molecules, and scaffolds for tissue engineering. In this study, 90% porous acrylic based polyHIPE structures with various cellular structure and mechanical characteristics were developed by using stearyl acrylate (SA), isodecyl acrylate (IDA), isobornyl methacrylate (IBMA), and divinylbenzene (DVB). Elastomeric polyHIPEs were produced from the comonomers of SA and IDA, and had high ability of recovery when the applied stress was removed. IBMA based polyHIPEs were brittle and demonstrated higher Young's modulus and compression strength than that of conventional styrene based polyHIPEs at the same void volume. Therefore, by varying the composition, it became possible to alter the mechanical properties of polyHIPEs from brittle to elastomeric, without changing the interconnected cellular structures.

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 35mm, 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 30vol.% 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.

Preparation of emulsion-templated porous polymers using thiol–ene and thiol–yne chemistry

Highly porous polymeric materials are prepared by thiol–ene and thiol–yne mediated network formation using high internal phase emulsion templates. The efficiency of network formation is between 80 and 90% for all materials while the thiol–yne materials display enhanced strength and toughness due to their higher degree of crosslinking.

Uploading and Temperature-Controlled Release of Polymeric Colloids via Hydrophilic Emulsion-Templated Porous Polymers

Porous cross-linked poly(N-isopropylacrylamide) (PNIPAM) was prepared using high internal phase emulsions as templates. The materials could absorb a large volume of water and swell largely at room temperature. When the aqueous phase was heated above the lower critical solution temperature (LCST) of PNIPAM, the swollen structure could contract and squeeze some of the absorbed liquids out. This capability was utilized in the uploading at room temperature and then release of polystyrene colloids by increasing the temperature above the LCST. The thermoresponsive porous PNIPAM acted like a pump to load and then release the polymer colloids. The multicycles of loading and release were demonstrated to show its efficiency. Importantly, it showed that most of the PS colloids from the second upload onward could be released during the heating cycle.