macroporous polymer beads and monoliths from pickering simple, double, and triple emulsions

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Macroporous Polymer Beads and Monoliths From Pickering Simple, Double, and Triple Emulsions

Amro K. F. Dyab*

Various polymeric materials based on simple, double, and triple emulsions stabilized solely by dichlorodimethylsilane (DCDMS)-modifi ed amorphous silica nanoparticles of well-controlled surface hydrophobicities or organo-modifi ed Laponite clay nanoparticles are described. Mag-netic and fl uorescent polymer beads having different mor-phologies are fabricated from double-emulsion templates. Polymer foams based on Pickering emulsions are fabricated and functionalized using different materials such as sporopol-lenin and zinc metal particles. Suspension polymerization of both oil phases of a novel Pickering triple emulsion results in the formation of new porous hierarchical structures. Polymer foams and beads based on novel nonaqueous Pickering simple and double emulsions are fabricated.

1. Introduction

The synthesis of multifunctional polymeric microparticles and diverse porous materials has been topics of increasing interest in both academic and industrial fi elds. Several functionalities are able to be simultaneously encapsulated into polymeric microspheres, such as magnetism and fl uo-rescence, [ 1 ] and others. [ 2–6 ] These functional microspheres exhibit an ability to change their physicochemical proper-ties in response to environmental stimuli, including mag-netic or electric fi elds, light, temperature, pH, chemicals, or mechanical stress. Such multifunctional microspheres have found diverse applications in biomedical and biotech-nological fi elds, such as controlled drug delivery, [ 7 ] cellular imaging, [ 8 ] cell recognition, [ 9 ] cancer-specifi c multimodal imaging, [ 10 ] and dual-protein delivery. [ 11 ] The combination

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Dr. A. K. F. DyabChemistry Department, Faculty of Science, Minia University, Minia 61519, Egypt E-mail: [email protected] Dr. A. K. F. DyabSurfactant Research Chair, Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia

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of magnetic and fl uorescent properties into a single micro- or nanocomposite material may greatly enhance the applications of the material in the biomedical and biop-harmaceutical fi elds.

Emulsion templating methods represent an attractive route to highly porous and permeable polymeric materials with a well-defi ned porosity. The process has been known since at least the 1960s [ 12 ] and was developed extensively by workers at Unilever in the 1980s [ 13 ] and in recent years has seen increased interest from both academia and industry. A high-internal-phase emulsion (HIPE) is a concentrated emulsion in which the droplet phase com-prises greater than 70% of the total volume of the emul-sion. The external (non-droplet) phase is converted into a solid polymer and the emulsion droplets are removed yielding (in most cases) a highly interconnected network of micron sized pores of quite well-defi ned diameter. The resulting material is often termed a polymerized HIPE, or poly(HIPE). [ 14 ] In recent years, there has been increasing interest in the use of solid particles as sole emulsifi er to stabilize the internal phase (e.g., droplets) of the HIPEs. However, one important limitation was raised but has been recently overcomed. Kralchevsky et.al. [ 15 ] have theo-retically predicted that particle-stabilized emulsions will phase invert above an internal-phase volume fraction of

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0.5. Colver and Bon [ 16 ] reported that, in practice, emul-sions are stabilized by sub-micrometer microgel particles with volume fractions of the dispersed phase of only 50%. Binks and co-workers [ 17 ] showed that particle-stabilized water-in-oil (w/o) emulsions commonly phase invert between volume fractions of 0.65–0.70. However, Arditty et al. [ 18 ] described oil-in-water (o/w) emulsions with up to 90% internal phase and w/o emulsions having internal phase up to 75% by manual shaking and stabilized by as received hydrophilic silica particles and silanized hydro-phobic silica particles, respectively. [ 18 ] Recently, a suc-cessful preparation of polyHIPEs with closed pores struc-ture stabilized solely by oleic acid-functionalized titania or silica particles has been reported with 90% internal phase volume. [ 19 ] In addition, different polyHIPEs have been prepared using laboratory modifi cation of silica nan-oparticles with different silanizing agents. [ 20 ] Surfactant-stabilized polyHIPE monodisperse porous polymer beads have been prepared via oil-in-water-in-oil (o/w/o) emul-sion-templated sedimentation polymerization. [ 21 ]

The knowledge that fi ne solid powders can stabilize emulsions dates back to the turn of the last century. The credit is usually given to Pickering, [ 22 ] who originally rec-ognized the role of fi nely divided insoluble solid particles in stabilizing emulsions. It is now well established that solid particles of colloidal dimensions can act as excellent emulsifi es alone for both o/w and w/o emulsions. [ 23–26 ] Since the energy of attachment of particles to oil–water interfaces is very high (thousands of kT per particle), such particles are held very strongly in the interface giving rise to extremely stable drops within emulsions. [ 23 ] Multiple emulsions were fi rst described by Seifriz [ 27 ] in 1925, but it is only in the last 20 years that they have been studied in more detail. Common types of multiple emulsions may be either of the water-in-oil-in-water type (w/o/w) or of the (o/w/o). Increasing attention has been devoted to these systems with the aim of taking advantage of their multiple compartment structure. [ 28 ] Multiple emulsions are generally prepared with high concentrations (5–50%) of two emulsifi ers (one lipophilic, one hydrophilic) of oppositely curved interfaces. However, the most common problem associated with multiple emulsions is their inherent thermodynamic instability since the emulsifi ers tend to mix at the interfaces and destabilize giving simple o/w emulsions. [ 29 , 30 ] This is why the use of multiple emul-sions as commercial products is so restricted, although much attention has been paid to their many potential practical applications. [ 28 ]

Extremely stable Pickering simple and multiple emul-sions of the two common types stabilized solely by silica nanoparticles for different liquids have been reported in previous work. [ 25 ] The formulated multiple emulsions were completely stable against coalescence for around 10 years (see optical images of emulsions in Figure S1,

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Supporting Information). The motivation of the current work was to fabricate different polymeric functionalized materials ranging from porous polymer microparticles to porous monoliths using different Pickering aqueous and nonaqueous emulsion templates. We report here straightforward methods for fabrication of polymeric materials including dual function magnetic and fl uores-cent hybrid microparticles with different morphologies, namely the multicore or porous polymer beads (based on double w/o/w emulsions). We show that the struc-ture of the formed microparticles from double emulsion templated can be tailored by the volume fraction of the primary simple emulsion of the parent double emulsion. Generally, the use of fl uorescent quantum dots showed many advantages over the conventional organic dyes, such as, high intensities and improved photostability. In addition, incorporation of quantum dots in polymeric matrix can effectively inhibit the release of quantum dots and avoid their health side effects. Porous materials based on aqueous or novel nonaqueous emulsions stabi-lized solely by hydrophobic silica nanoparticles or organo-modifi ed Laponite clay nanoparticles have been prepared. Selected porous structures have been functionalized by inclusion of natural sporopollenin microcapsules (derived from Lycopodium clavatum ) for possible encapsulations of inorganic or organic materials within the cavities of these pores. Incorporation of zinc metal particles into the polymer can provide an additional functionality that can fi nd potential applications, for example, in catalysis. In addition, we present here a novel hierarchical porous structure based on polymerization of a triple Pickering water-in-oil-in-water-in-oil (w/o/w/o) emulsion of dif-ferent morphologies for the fi rst time.

2. Experimental Section

2.1. Materials

Fumed silica nanoparticles were produced (by Wacker Chemie, Germany) via the introduction of volatile chlorosilanes into an oxyhydrogen fl ame and consist of ultrapure amorphous silicon dioxide. The nanoparticles had a mean primary particle dia meter of approximately 20 nm and were coated to different extents with dichlorodimethylsilane (DCDMS). According to the supplier, particle wettability was characterized in terms of the measured percentage of unreacted SiOH groups remaining on the surface using a base titration method. [ 25 ] This ranged from 100% (most hydrophilic) to 14% (most hydrophobic). The coated silica nano-particles with measured values of percentage SiOH were kindly supplied by Wacker-Chemie (Germany). Although the primary particle diameter was 20 nm, the powder also contained fused aggregates of multiple primary particles and larger agglomer-ates of the fused aggregates. Laponite (RD) clay nanoparticles were provided by (Southern Clay Products Texas, USA). Iron (III) chloride (FeCl 3 , anhydrous, 97%), ethyl alcohol, and acetone were

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Macroporous Polymer Beads and Monoliths From Pickering . . .

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purchased from Fisher Chemicals. Iron (II) chloride tetrahydrate (FeCl 2 .4H 2 O, 98%), zinc powder, and ammonia solution (33 wt%) were purchased from BDH Chemicals. 1,6 hexanediol diacrylate (HDDA, 90%), styrene (99.5%), thermal initiator 2,2 ′ -azobis(2-methylpropionitrile) (AIBN, 98%), toluene, divinylbenzene (DVB), CaCl 2 · 2H 2 O, Cetyltrimethylammonium bromide (CTAB, 99.99%), Glycerine ( > 95.5%), and Rhodamine B (95%) were purchased from Sigma–Aldrich. Formamide ( > 98.5%) was purchased from Merck. The photoinitiator (Irgacure 184) was supplied by Ciba Specialty Chemicals (Switzerland). Lycopodium clavatum pollen powder was purchased from Fagron, UK. HDDA and styrene were dein-hibited by passing through neutral alumina and then washed with 10% NaOH solution three times prior to use. Cadmium tel-luride (CdTe) quantum dots (PlasmaChem GmbH, Germany) with λ max = 560 ± 5 nm, M. W. = 240.01, and theoretically calculated diameter of 3.02 nm was used. The particles of CdTe are coated by thiocarbonic acid and have a negative charge in aqueous solu-tions. We have verifi ed the charge of these nanoparticles using Marvlen Zeta-Sizer and the zeta-potential of dilute sample in pure water was found to be 77.5 ± 2 mV. All the experiments were car-ried out at room temperature unless otherwise stated. Water was passed through a reverse osmosis unit and then a Milli-Q reagent system. The CdTe nanoparticles were supplied as 50 mg powder sample and a stock solution of 10 mg mL − 1 was formulated with pure water. The typical concentration of the CdTe used in most of the formulations was 1 mg mL − 1 . The water-based QD sample was prepared by adding 50 mL of CdTe (10 mg mL − 1 ) into 0.5 mL pure water.

2.2. Modifi cation of Laponite (RD) Nanoparticles

Hydrophobic Laponite nanoparticles were obtained according to the cation-exchange method described elsewhere. In a round fl ask, 20 g of Laponite clay was dispersed in 500 mL distilled water containing 6 g, 65 × 10 − 4 M of CTAB, which caused com-plete cation exchange at room temperature, then the temper-ature was increased to 80 ° C under vigorous stirring for 6–8 h with a condenser. The resulting modifi ed Laponite clay was separated by fi ltration, washed several times with distilled water to remove any free surfactant (checked by AgNO 3 solu-tion), then vacuum dried at 60 ° C for 24 h, and kept in a sealed container until use.

2.3. Preparation of Magnetite Nanoparticles

We have used a classical coprecipitation of Fe 3 + and Fe 2 + with NH 4 OH to prepare the magnetite (Fe 3 O 4 ) nanoparticles. The method involves coprecipitation from Fe 2 + and Fe 3 + aqueous salt solutions by addition of a base. An initial molar ratio of Fe 3 + :Fe 2 + = 2:1 is needed for the production of Fe 3 O 4 . The required amount of FeCl 3 and FeCl 4 · 4H 2 O was dissolved in 40 mL of Milli-Q water. The solution was heated at 80 ° C for 1 h while being stirred. Then, 12 mL of NH 4 OH (33 wt%) was added. The resulting suspension is vigorously stirred for another 1 h at the same temperature and then cooled to room temperature. The precipitated particles were washed fi ve times with water and ethanol, separated by magnetic decantation, and dried in oven at 80 ° C.


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2.4. Preparation of Double w/o/w Emulsions

Table 1 shows recipes of selected emulsion templates used in fabrication of various polymeric systems studied here. Double w/o/w emulsions were prepared using a two-stage process. [ 25 ] Stage 1 involved the preparation of a simple w/o emulsion, in which the volume fraction of water ( φ w ) equals 0.2. It was pre-pared by adding 2.5 cm 3 of water into 10 cm 3 of monomer oil containing a known mass of hydrophobic silica nanoparticles of 50% SiOH. The mixture was then homogenized using a DI-25 basic Yellow-line, IKA (Germany) homogenizer (rotor/stator) with an 18 mm head operating at 13 000 rpm for 2 min. Water phase of the primary w/o emulsion can be loaded, as required, with 1 mg mL − 1 CdTe quantum dots and 1 wt% magnetite Fe 3 O 4 nanoparticles. The oil phase of the primary w/o emulsion con-tains, as required, 2 wt% (based on oil mass) AIBN or Irgacure 184 and 1 wt% DVB (based on oil mass). In stage 2, the primary w/o emulsion, just prepared in stage 1, is re-emulsifi ed into 10 cm 3 of an aqueous phase containing a known mass (expressed in wt% of the continuous phase) of hydrophilic silica nanopar-ticles of 80% SiOH (using the homogenizer at 8000 rpm for 10 s). The volume fraction of the w/o emulsion ( φ w/o ) in the fi nal double emulsion can be varied from 0.1 to 0.6. In some cases, the second step of homogenization was carried out simply by hand shaking for 10 s. The emulsion continuous phase was determined by measurement by observation of what happened when a drop of emulsion was added to a volume of each of the pure liquid phases. The emulsions only dispersed in the liquid when its continuous phase matched the liquid to which it was added.

2.5. Preparation of Polymeric Magnetic and Fluorescent Microparticles

The as-prepared Pickering double w/o/w emulsions of either HDDA or styrene were either photo or thermally polymerized without stirring after bubbling in pure N 2 gas for 5 min. Poly-merization was started by placing the double emulsion in a UV cell under a 12 W UV lamp (UVP, Upland, USA) for 3.5 h. Thermal polymerization was started by heating the system to 75 ° C for 24 h in glass vessels (diameter around 25 mm) to complete the polymerization reaction without stirring. Finally, microparticles were recovered by washing three times with ethanol and then with Milli-Q water to remove any unreacted oil and excess of stabilizers.

Images and videos were recorded by optical and fl uorescence microscopy using (Olympus BX-41 fi tted with DP70 digital camera) and applying the Rhodamine TRITC fi lter set. Images and videos were processed using Corel Paintshop pro X4 software. In some experiments, we applied a permanent magnet close to the sample on the microscope stage while capturing still images or videos to examine the response of the magnetite inside the polymeric microparticles to the external magnetic fi eld. Mean emulsion drop diameters were obtained from the images by measuring a minimum of 20 drops from each slide. Digital images were taken by personal Samsung galaxy smart phone S1. Polymeric materials were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) using the JEOL SEM (JSM-6380 LA). Around 0.5 cm 3 or solid dry

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Table 1. Recipes for selected Pickering emulsion templates used in the given polymeric systems.

System Phase components

Inner (W 1 ) a) Middle (O 1 ) b) Outer (W 2 ) Outermost (O 2 )

polyLIPDE-1 1 mg mL − 1 CdTe, 1 wt% Fe 3 O 4

HDDA, 1 wt% DVB, 3 wt% (50% SiOH) silica, 2 wt% Irgacure 184, φ w = 0.2

2 wt% (80% SiOH) silica, φ w/o = 0.2

–

polyLIPDE-2 1 wt% Rhodamine B, 1 wt% Fe 3 O 4

Styrene, 1 wt% DVB, 3 wt% (50% SiOH) silica, 2 wt% AIBN, φ w = 0.2


–

polyMIPDE-3 1 wt% Rhodamine B, 1 wt% Fe 3 O 4

Styrene, 1 wt% DVB, 3 wt% (50% SiOH) silica, 2 wt% AIBN, φ w = 0.2


–

polyHIPE-4 0.3 M CaCl 2 .2H 2 O Styrene/DVB (50:50) vol%, 20 vol% toluene, 3 wt% (50% SiOH) silica,

2 wt% AIBN, φ w = 0.8

– –

polyHIPE-5/6 0.3 M CaCl 2 .2H 2 O Styrene/DVB (50:50) vol%, 20 vol% toluene, 1.5 (or 3) wt% modifi ed Laponite, 2 wt% AIBN, φ w = 0.8

– –

polyHIPE-7 0.3 M CaCl 2 .2H 2 O Styrene/DVB (50:50) vol%, 20 vol% toluene, 3 wt% (50% SiOH)

silica, 2 wt% zinc powder, 2 wt% AIBN, φ w = 0.8

– –

polyMIPTE-8 1 wt% Rhodamine B, 1 wt% Fe 3 O 4

Styrene, 1 wt% DVB, 3 wt% (50% SiOH) silica,

2 wt% AIBN, φ w = 0.2


Styrene, 2 wt% (50%

SiOH) silica, 2 wt% AIBN φ w/o/w = 0.5

a) (W) refers to the aqueous phase; b) (O) refers to the oil phase.

powder of each sample was fi xed in the SEM sample holder by double-sided carbon black sticker. All samples were Pt coated before examined by SEM. Pose size was measured as an average of around 30 pores in the sample. Brunauer-Emmett-Teller (BET) surface areas were measured using a Micromeritics instrument. Thermogravimetric analysis (TGA) was carried out using Mettler Toledo TGA (Switzerland) under nitrogen atmosphere in the temperature range of 25–600 ° C with heating rate of 10 ° C min − 1 .

2.6. Preparation of Pickering polyMIPE/HIPEs

The HIPEs were prepared by adding the required volume of aqueous phase (containing 0.3 M CaCl 2 · 2H 2 O) or nonaqueous phase (without salts) to (50:50) styrene:DVB mixture (con-taining 2 wt% AIBN initiator and a known mass of the desired solid nano particles (based on monomer oil mass). The mixture was stirred by a standard magnetic stirrer at fi xed speed of 400 rpm for 10 min. Viscous MIPE and HIPE were obtained and a drop test was performed to ensure the oil is the continuous phase. The resulted MIPE or HIPE were transferred into glass vessels and heated in a water bath at 75 ° C for 24 h to produce polyMIPE–HIPEs. The formed monolith was released from vessels by breaking the glass and then purifi ed in acetone using Soxhlet extraction for 3 h and eventually dried at 100 ° C for 24 h. Porous microstructure was studied by SEM.



2.7. Preparation of Pickering Triple w/o/w/o Emulsions

Triple Pickering w/o/w/o emulsions were simply prepared by re-homogenize by hand shaking for 10 s the double w/o/w emulsions described above in the outermost oil (styrene) phase containing 2 wt% AIBN and a known mass of solid nanoparticles. Triple emulsions were polymerized in the same way as described for PolyHIPEs above.

3. Results and Discussion

3.1. Preparation and Characterizations of Pickering Double w/o/w Emulsion Templates

We have used two types of silica nanoparticles differing only in their hydrophobicites to stabilize the double and triple emulsions. Dispersing the hydrophilic silica particles (80% SiOH) in water using ultrasound pro-duced stable bluish dispersions at particle concentra-tions between 1 and 4 wt% with no apparent change in solution viscosity. For partially hydrophobic silica particles (50% SiOH) dispersed in HDDA or styrene, the dispersions were found to have similar viscosity of the monomer oil up to around 1 wt%. Between 2 and 4 wt%,





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Figure 1 . (a) An optical image of HDDA double emulsion 2 months after preparation. The emulsion was stabilized by 3 wt% of 50% SiOH silica in the HDDA and 2 wt% of 80% SiOH silica in the outer water phase. The inner water phase was loaded with 1 wt% Fe 3 O 4 and 1 mg mL − 1 CdTe. (b) Average diameter of HDDA globules in freshly prepared LIPDEs as a function of concentration of both 50% SiOH silica particles in HDDA ( φ w = 0.2), in wt% given, and 80% SiOH silica particles in the outer water phase ( φ w/o = 0.2). The scale bar is 50 μ m.

the dispersions became more viscous and bluish in color. This was attributed to the formation of a three-dimensional network of the silica agglomerates. [ 17 ] As this network forms, the viscosity of the liquid increases greatly. Pickering double emulsion of the type w/o/w was fi rst prepared with HDDA or styrene as a monomer oil phase with 1 wt% DVB as a crosslinker. The inner water phase, having a volume fraction of φ w = 0.2, of the double emulsion was loaded with 1 wt% Fe 3 O 4 mag-netite nanoparticles and 1 mg mL − 1 CdTe quantum dots. The water drops in the primary w/o emulsion were stabilized by 3 wt% of partially hydrophobic 50% SiOH silica nanoparticles (based in oil phase). The volume fraction of the primary w/o emulsion in the fi nal double w/o/w emulsion was φ w/o = 0.2. The oil globules of the w/o/w emulsions was stabilized by 2 wt% hydrophilic 80% SiOH silica nanoparticles based in the outer water phase. We have designated the obtained double emul-sion as a low-internal-phase double emulsion (LIPDE), since we used φ w/o = 0.2. An optical micrograph of the resulted double emulsion 2 months after preparation is presented in Figure 1 a. Oil globules with diameters in the range of 20–50 μ m can be clearly seen entrapping a number inner water drops. The emulsion exhibited creaming without coalescence and remained stable for more than 2 months. The problems associated with sur-factant-stabilized double emulsions are numerous and remain mostly unsolved. The emulsifi er combination as well as the nature of the oil phase and the volume ratio of the different media are basic and essential parameters for double emulsion stability. The resulting instability of the double emulsions is of crucial impor-tance since it governs the storage and the potential use of such systems. In order to study the stability of the double emulsion templates, a set of LIPDEs has been prepared and stabilized by different concentrations of both 50% SiOH silica particles (0.5–4 wt%) in oil and 80% SiOH silica particles (1–4 wt%) in water. The freshly pre-pared double emulsions exhibited creaming, without coalescence, at concentrations of 80% SiOH particles between 1 and 3 wt% for all concentrations of 50% SiOH particles, but were stable to creaming at 4 wt% of 80% SiOH particles above 1 wt% of 50% SiOH particles. This was thought to be due to a combination of a smaller globule size and an increase in the viscosity of the con-tinuous aqueous phase with 80% SiOH particles con-centration, both of which reduced the rate and extent of creaming. We can easily control the size and the number of both the inner water drops and the oil glob-ules of the resulted double emulsions by adjusting the concentration of the silica particles used in stabilizing both water and oil phases. Effects of silica contents on the average globule diameters immediately after prepa-ration are given in Figure 1 b where it can be seen that




an increase in 50% SiOH particle concentration in oil resulted in an increase in oil globule size, whereas an increase in 80% SiOH particles concentration caused a continuous decrease in oil globule size. The oil globule sizes of emulsions ranged from 25 to 65 μ m in diameter. In addition, the oil globule sizes were larger than those of corresponding simple o/w emulsions stabilized by 80% SiOH silica particles alone, indicative of the ability of oil globules to entrap a high number of small water drops and swell as a result. We noticed that no signifi -cant growth of oil globules occurred as a result of ageing (35 μ m after preparation and 37 μ m after 3 months) with no evidence of coalescence of either inner water drops or outer oil globules indicative of an excellent stability of these emulsions and the feasibility to be used as tem-plates for preparing different polymeric materials.

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3.2. Fabrication of Dual-Function and Macroporous Polymer Beads Templated from Double w/o/w Emulsions

The as-prepared LIPDE was photopolymerized for 3.5 h using 2 wt% Irgacure 184 (based on monomer weight) at room temperature. PolyLIPDE-1 multi-aqueous core spher-ical microparticles with size range of 20–50 μ m in diameter were obtained with magnetic and fl uorescent properties as can be seen in the optical and fl orescence images pre-sented in Figure 2 a and b, respectively.

The resulted dual function microparticles showed a response to an external magnetic fi eld generated by using a Neodymium magnet as evident by the recorded videos (see Supporting Information). One can see, from dif-ferent angles, the internal aqueous drops are embedded

Figure 2 . (a) Optical and (b) fl uorescence images of multi-core polymer microparticles (polyLIPDE-1) formed after UV polymeri-zation of a w/o/w double emulsion of HDDA stabilized by 3 wt% of 50% SiOH silica particles in the inner phase, φ w = 0.2, containing 1 mg mL − 1 CdTe quantum dots and 1 wt% Fe 3 O 4 . The oil globules were stabilized by 2 wt% of 80% SiOH silica particles, φ w/o = 0.2. The scale bars are 50 μ m.



inside the polymeric microparticles, indicative of the existence of these drops totally within the polymerized oil globules. We noticed also the fl uorescence behavior of the multicores inside these microparticles, which fur-ther confi rms the integrity of the structure before and after polymerization. In similar way, we have prepared another dual function microparticles (polyLIPDE-2) using styrene as the monomer oil globules, which were poly-merized thermally at 75 ° C using AIBN for 24 h. We have examined the morphology of the formed emulsion and organic–inorganic hybrid microspheres using fl uorescent and the SEM microscopy as shown in Figure 3 a–c. The fi rst notice was that the resulted stable w/o/w emulsion exhibited high loading capacity of the inner water drops compared with that prepared with HDDA. We found that the microparticles (25–65 μ m) were also spherical having multicore compartment and silica aggregates were seen covering signifi cant parts of their surfaces. The inset of Figure 3 c shows a closer look at the surface morphology of one microparticle where the existence of silica aggre-gates that stabilized the outer oil–water interface of the precursor w/o/w double emulsion is evident.

Interestingly, different nonspherical morphologies, namely multi-hollow or porous structure, of the micropar-ticles (polyMIPDE-3) were observed when the precursor w/o/w emulsion was prepared with volume fraction of the primary w/o emulsion of φ w/o = 0.5 as depicted in optical image in Figure 3 d and the SEM images in Figure 3 e–i. It is obvious from Figure 3 d that the fabricated multiple emul-sion exhibited a unique morphology where the internal water drops, entrapped in oil globules, are remarkably swollen and existed in few numbers. These arrangements provided a nonspherical character of the oil globules where a few large water drops can be seen going out of the oil globules. As seen in Figure 3 e–g, each polymer micro-particle contains a number of pores having sizes ranging from 20–40 μ m in diameter, while the microparticles sizes range was 30–90 μ m. Figure 3 g shows an evidence of formation of few other shapes of the polymer particles such as pot and cylindrical-shaped microparticles having single or multiple large pores, respectively. Close exami-nations of the pores of the resulted porous beads revealed that most of these pores were empty and few were fi lled with apparent silica aggregates as can be seen in the SEM image shown in Figure 3 h. In addition, silica aggregates were also seen armoring the surface of polymer beads indicative of the formation of hybrid porous beads as evident from Figure 3 i. We have analyzed the area inside and outside these pores in the polymer beads for iron and silica using EDX as shown in Figure S2 (Supporting Infor-mation). Iron was detected only within the pores while silica was found in larger amount either within the pores or on the bead surfaces, which was expected during the formulation of the parent double emulsions. The TGA





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Figure 3 . Fluorescent, optical, and the SEM images of emulsions and hybrid polymeric microparticles with different morphologies. (a) A fl uorescence image of a double w/o/w emulsion of styrene/DVB sta-bilized by 3 wt% of 50% SiOH silica particles in the oil phase, φ w = 0.2. The inner water phase contains Rhodamin B and 1 wt% Fe 3 O 4 . The outer aqueous phase contains 2 wt% of 80% SiOH silica particles, φ w/o = 0.2. (b,c) SEM images of multicore polymeric spherical micro-particles (polyLIPDE-2) formed from a w/o/w double emulsion of styrene as in “a” after thermal poly merization. (d) An optical image for a double w/o/w emulsion prepared as in “a” but with volume fraction of primary emulsion of φ w/o = 0.5, showing large inner water drops and the nonspherical morphology. The arrows indicate pos-sible colloidosomes formed from the released water drops from inside the multiple emulsion globules. (e–i) The SEM images of the porous microparticles (polyMIPDE-3) formed from polymerization of the precursor double w/o/w emulsion shown in “d.”

Figure 4 . (a) Optical image of spherical drops (indicated by arrows) formed in the double w/o/w emulsion shown in Figure 3 d. (b) The SEM image of one of these drops after polymerization showing the surface morphology, the inset image is a closer look at the aggregated silica on the surface.

analysis of polyMIPDE-3 under inert conditions is shown in Figure S3 (Supporting Information). The TGA revealed that the main weight loss was recorded in temperature




range of 430–510 ° C by the thermal decomposition of pol-ystyrene, indicative of enhancement of thermal stability of the porous microparticles as a result of incorporation of silica nanoparticles. The initial weight loss of 5.8% at 25–130 ° C can be attributed to the evaporation of physi-cally adsorbed water. After the complete decomposition of polystyrene above 510 ° C, the residual weight of 3.57% is considered to be the amount of incorporated silica and magnetite nanoparticles in the polymer microparticles, which is close to the initial content of silica and magnetite nanoparticles loaded into the parent double emulsion (2.3 wt%). The BET surface area of these porous micropar-ticles was 9.6 m 2 g − 1 , which is in the range of some of the reported surfactant-stabilized millimetre-sized porous beads. [ 21 ] In addition, we noticed the existence of a number of spherical drops in the formed emulsion that resemble water colloidosomes, which were indicated by arrows in the optical image shown in Figure 4 a. These drops showed an apparent rough surface morphology compared to that

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of all oil globules. We assume that these possible “water colloidosomes” have been released from inside the swollen oil globules where they already been covered with a dense layers of hydrophobic silica particles used in stabilizing them inside the oil globules. When they came out to the outer water phase they did not merge since they covered with hydrophobic silica and also they became dispersed in water phase containing hydrophilic silica. Figure 4 b shows the SEM image of the surface morphology of one of these drops after polymerization where a dense layer of silica aggregates (the inset image) was seen covering the par-ticle. Further investigations of the structure and possible formation mechanisms of these drops are required.

The mechanism of the formation of such porous mor-phology shown in Figure 3 d–i is not exactly known. How-ever, we performed a series of experiments where we changed the volume fraction of the primary w/o emul-sion (from φ w/o = 0.1–0.6) in the double w/o/w emulsions. It transpired that the double w/o/w emulsions were cata-strophically phase inverted between φ w/o = 0.5 and φ w/o = 0.6 to simple w/o emulsions. This novel phase inversion has not been explored before for Pickering multiple emul-sions. Upon polymerization of the inverted emulsion, formed at φ w/o = 0.6, a solid polymer monolith material was obtained, confi rming that the monomer oil is the continuous phase of the simple emulsion. In this case, one can say that the inversion of the w/o/w emulsion leads to formation of a medium internal phase simple emulsion (MIPE), which upon polymerization gives polyMIPE. For the resulted w/o/w emulsion with φ w/o = 0.5, the effective water volume fraction would be 0.6, which is within the region of the phase inversion process. Based on the early concept of Ostwald [ 31 ] who stated that phase inversion occurs in simple emulsions when the system reaches a critical close packing condition, Brooks and Richmond [ 32 ] argued that, by multiple emulsion formation, the effec-tive volume fraction of dispersed phase required to induce inversion can be obtained by enclosing the continuous phase into drops of the dispersed phase. This occurs as a result of the coalescence of drops of the dispersed phase. The effective volume fraction of dispersed phase (made up of droplets inside globules), thus, increases as long as this inclusion dominates over loss of such enclosed droplets by, say, dissolution back into the continuous phase. Inver-sion is thus governed by the balance between the breakup of bulk liquid/drops and the coalescence of drops. This can shed some light about the mechanism of the forma-tion of such porous microparticles shown in Figure 3 d–i. We hypothesis that this could be as a result of inclusion of part of the continuous water phase into the oil glob-ules, which, in turn, enhances coalescence and eventually lead to swelling of the inner water droplets of the w/o/w emulsion during the start of the phase inversion process as evident from optical image shown in Figure 3 d. It is

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worth mentioning that the number of pores in each single microparticle was few ( ≈ 1–10). In addition, the average size of these porous microparticles was relatively larger, by about 25 μ m, than that prepared with low volume fraction ( φ w/o = 0.2) of the primary emulsion (Figure 2 and Figure 3 a–c). This trend was expected since globule sizes normally increase upon increasing the volume fraction of the inner phase of silica-stabilized multiple emulsions. [ 25 ] Another possible factor that could help increasing the globule sizes is the coalescence of small water drops inside the globules, forming larger few ones. Moreover, based on our previous studies on the release from Pick-ering multiple emulsions, the swelling of water drops can also be driven by changing the osmotic pressure across the middle oil fi lm in a controlled way using simple salt and glucose solutions. [ 25 ] The latter factor is currently under investigation.

3.3. Functionalized Monoliths From Pickering w/o HIPE Emulsion Templates

The fi ndings described above have motivated us to use either the fumed silica or Laponite nanoparticles (Figure 5 ) to prepare porous monolith materials with added func-tionalities from suspension polymerization of Pickering emulsions. Preliminary experiments were conducted where we prepared a w/o emulsion having water volume fraction of φ w = 0.8 using 3 wt% of the very hydrophobic 23% SiOH silica nanoparticles initially dispersed in the oil phase. The monomer oil phase was a mixture of styrene/DVB in (50:50) volume ratio containing 20 vol% toluene (as pores generator) and 2 wt% AIBN (based on total oil mass). The aqueous phase was added dropwise to the oil phase with constant stirring speed at 400 rpm. The resulted emulsion type was w/o, confi rmed by the drop test. The emulsion exhibited fast coalescence, which resulted in a signifi cant release of most of the initial oil volume after 30 min. After polymerization at 75 ° C for 24 h, a mono-lith was obtained with millimeter-sized cells that can be seen by naked eyes as shown in Figure S4a (Supporting Information). The structure of the formed monolith can be explained by the signifi cant destabilization of the resulted w/o emulsion when using very hydrophobic silica parti-cles. However, stable w/o HIPE containing internal water volume fraction of 0.8 was successfully prepared with 3 wt% of 50% SiOH silica nanoparticles initially dispersed in the oil phase. The formed HIPE with 50% SiOH silica did not show sedimentation or coalescence over a period of 48 h indicative of the effectiveness of this type of silica in stabilizing the water drops and the closeness values of porosity to the initial internal volume fraction (80%). After polymerization of the stable HIPE, porous polyHIPE-4 with mostly closed cells was obtained as depicted in Figure 5 a and f and Figure S4 b,c (Supporting Information). Pore


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Figure 5 . (a) SEM image for polyHIPE-4 prepared from Pickering HIPE stabilized by 3 wt% of 50% SiOH silica and φ w = 0.8. (b) polyHIPE-5 prepared with 1.5 wt% organo-modifi ed Laponite and φ w = 0.8. (c) polyHIPE-6 prepared with 3 wt% organo-modifi ed laponite and φ w = 0.8 showing interconnected pores by arrows. (d) polyHIPEs prepared with 3 wt% organo-modifi ed Laponite with 1 wt% sporopollenin and φ w = 0.8. (e) A close look at cells wall, indicating sporopollenin by arrows. (f) A monolith of polyHIPE-4. (g,h) Monoliths of polyHIPEs shown in c and d, respectively.

sizes were found to be in the range of 150–500 μ m in diameter. The cells appeared as fused solid hollow spheres with few cracks on their surfaces as shown in Figure S4b (Supporting Information). Close examination of one of these closed pores revealed that it has a smooth inner sur-face morphology without the appearance of signifi cant amount of silica aggregates. As summarized in Table 2 , BET surface area of the formed monolith was very low, 0.02 m 2 g − 1 , indicative of the formation of mostly closed cell structure of the polymer foam. The polyHIPE-4 sample showed excellent thermal stability as can be seen in the TGA curve in Figure S3 (Supporting Information). The ini-tial weight loss of 6.5% until 200 ° C can be attributed to the




evaporation of physically adsorbed water and the decom-position of the remaining CaCl 2 · 2H 2 O that was initially used as the inner aqueous phase. The main weight loss occurred between 400 and 530 ° C, which can be attributed to the decomposition of the crosslinked polystyrene–DVB polymer. The residual weight of 18.1% can be attributed to the amount of silica particles used in the emulsion (3 wt%) and the amount of remaining weight after the decomposi-tion of CaCl 2 · 2H 2 O (13.2 wt% relative to the weight of the formed polyHIPE-4). The results obtained above suggested that the presence of silica particle reinforced the thermal stability of the polymer foam compared with polyHIPE prepared using magnetite nanoparticles. [ 1 ]

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Table 2. Summary of the properties of some of the fabricated polymeric materials.

System Stabilizer Internal phase volume fraction

Average size of microparticles [ μ m]

Average size of pores [ μ m]

BET surface area [m 2 g − 1 ]

Morphology

polyLIPDE-1 50/80% SiOH silica

Φ w/o (0.2) 20–50 – – multicore/spherical

polyLIPDE-2 50/80% SiOH silica

Φ w/o (0.2) 25–65 – – multicore/spherical

polyMIPDE-3 50% SiOH silica Φ w/o (0.5) 30–90 20–40 9.60 porous non-spherical

microparticles

polyHIPE-4 50% SiOH silica Φ w (0.8) – 150–500 0.02 mostly closed cells

polyHIPE-5 1.5 wt% Laponite

Φ w (0.8) – 200–1000 7.90 large closed cells, brittle

polyHIPE-6 3 wt% Laponite Φ w (0.8) – 50–600 8.45 open-closed-interconnected

cells

polyHIPE-7 silica, zinc Φ w (0.8) – 100–300 – mostly closed cells

polyMIPTE-8 50/80% SiOH silica

Φ w/o/w (0.5) 200–800 3.81 porous microparticles in

microcapsules

It is worth mentioning here that when HIPEs of silica were prepared using a high-shear rotor/stator homog-enizer with 18 mm head working at 13 000 rpm for 2 min, catastrophic phase inversion occurred resulted in the formation of o/w emulsions. This was in agreement with the previous results obtained with the same type of silica nanoparticles. [ 17 ] Therefore, our results suggested that there is a relation between the amount of energy introduced during the homogenization step and the state of phase inversion of the emulsions. For surfactant-stabilized emulsions, it was suggested that the position of the phase inversion (locus) was strongly different for two emulsifi cation routes, namely the direct emulsifi ca-tion and the wash-out routes. [ 33 ] For particle-stabilized emulsions, Ikem et.al. [ 19 ] and Gurevitch and Silverstein [ 20 ] reported that HIPEs can be prepared using a less energetic emulsifi cation process whereas using too high energy input resulted in emulsion phase inversion. However, it was concluded that the presence of mechanical barrier of solid particles prevent the phase inversion. [ 19 ] This sug-gestion seems inadequate as the formation of such a bar-rier is confi rmed in Pickering emulsions prepared either with high shear [ 34 , 35 ] or, as we show in this study, with low-energy homogenization.

We have extended our investigation to prepare poly-HIPEs using organo-modifi ed disk-shaped Laponite nan-oparticles with different concentrations (1.5 and 3 wt%) as shown in Figures 5 b and c with internal water volume fraction of φ w = 0.8. Laponite clay was modifi ed using CTAB



according to the method described elsewhere. [ 36 ] Pick-ering HIPE was initially formed with φ w = 0.8 (Figure 6 a and b) and showed stability against coalescence and sed-imentation for more than 48 h. Upon polymerization, polyHIPE-5 having large closed cells were formed when using 1.5 wt% modifi ed Laponite as stabilizer. Pore sizes were found to be in the range of 200–1000 μ m and the formed monolith was very brittle. Open, closed, and some interconnected cells were observed when the HIPE was stabilized by 3 wt% of the modifi ed Laponite as can be seen in Figure 5 c. The resulted polyHIPE-6 exhibited more mechanical stability (tested by hand) compared to that obtained with 1.5 wt% Laponite. As noted in Table 2 , both polyHIPE-5/6 showed different average pore sizes, but their surface areas were comparable. To test the fea-sibility of Laponite modifi cation on emulsion stability, an attempt was made to prepare w/o HIPE with 3 wt% unmodifi ed Laponite particles, but complete phase sepa-ration has occurred immediately since these particles prefer to be completely wetted by the aqueous phase (Figure 6 c).

In order to widen the applications of the formed poly-HIPEs and increase their functionalities, natural sporopol-lenin empty microcapsules were incorporated in the oil phase of the HIPE stabilized by 3 wt% modifi ed Laponite before polymerization. Sporopollenin has long been rec-ognized as suitable for microencapsulation of a variety of inorganic and organic materials and was used and treated as described by Paunov and co-workers. [ 37 ] Figure 5 d, e, g





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Figure 6 . (a) Optical micrograph for w/o HIPE emulsion stabilized by 3 wt% organo-modifi ed laponite nanoparticles and φ w = 0.8. (b) Digital image of the emulsion shown in “a” after prepara-tion showing its stability. (c) Immediate phase separation in an attempt to prepare w/o emulsion stabilized by 3 wt% of the unmodifi ed Laponite particles.


Macromol. Chem. Phys. 2012,© 2012 WILEY-VCH Verlag G

show the porous structure and the monolith formed by incorporating sporopollenin with the HIPE where many of these microcapsules were seen embedded within the polymer matrix of the resulted PolyHIPE foams, hence adding more sites capable of further functionalization. These porous monoliths can fi nd possible applications in separation, enzyme immobilization, or even in gas storage.

We have explored the possibility of adding a different functionality to the polyHIPE via the incorporation of zinc metal particles in the polymer cells or matrix. This could provide a kind of conductive porous materials and/or a possible use of zinc particles as templates for other applications such as adsorption or catalysis. The w/o HIPE was prepared in a similar way as in polyHIPE-4 shown in Figure 5 a but with the inclusion of 2 wt% of zinc metal particles to the oil phase before the mixing step. The resulted HIPE was viscous and did not exhibit sedimen-tation or coalescence for the time of the experiment. Figure 7 a–d show the obtained zinc functionalized poly-HIPE-7 with pore sizes ranging from 100–300 μ m, which is comparable to that obtained for the same monolith but without zinc particles (polyHIPE-4). As can be seen in Figure 7 a–c, mostly closed cells were obtained with the evidence of the existence of zinc metal particles as aggre-gates inside many cells’ surfaces having sizes in the range of 2–10 μ m. In addition, the gray color of the obtained monolith ensuring the incorporation of zinc particles.

3.4. Monoliths From Pickering Triple w/o/w/o Emulsion Templates

Having prepared polymeric materials ranging from porous microparticles to solid foams, we explored the possibility of including both structures in on single system based also on Pickering emulsions. In this respect, we develop and demonstrate for the fi rst time a method for fabrication of novel porous hierarchical structures based on polymeriza-tion of Pickering w/o/w/o triple emulsions as shown in Figures 8 and 9 . In this method, the emulsifi cation of a Pick-ering double emulsion of the type w/o/w in a second oil phase was successfully achieved, simply by hand shaking. This allowed us to prepare a kind of stable microcapsules dispersed in another microcapsules, which we believe is diffi cult to achieve with surfactant-stabilized emulsions. The double w/o/w emulsion shown in Figure 3 d was reho-mogenized by hand shaking for 10 s in styrene containing 2 wt% of 50% silica nanoparticles and 2 wt% AIBN. In the resulted triple emulsion, the water volume fraction in the primary w/o emulsion was φ w = 0.2, the primary emulsion volume fraction in the double w/o/w emulsion was φ w/o = 0.5 and fi nally the volume fraction of the double emulsion in the triple w/o/w/o emulsion was φ w/o/w = 0.5. An optical micrograph of the resulted triple w/o/w/o emulsion is

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Figure 7 . (a–c) SEM images for polyHIPE-7 prepared from Pickering HIPE stabilized by 3 wt% of 50% SiOH silica nanoparticles and 2 wt% zinc particles, φ w = 0.8. (d) A digital image of the resulted monolith.

Figure 8 . Emulsions and monoliths formed from polymerization of pre-made water-in-styrene-in-water-in-styrene triple Pickering emulsion. (a,b) Optical and digital images for a triple w/o/w/o emulsion before polymerization, showing its original structure. (c) A monolith formed after polymerization. (d) A fl uorescent image for the monolith showing the large cells entrapping a number of macroporous microparticles.

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Figure 9 . (a–f) SEM images of the hierarchical porous polyMIPTE-8 structure formed from polymerization of pre-made water-in-styrene-in-water-in-styrene (w/o/w/o) triple Pickering emulsion stabilized by 3 wt% of 50% SiOH silica nanoparticles in the inner oil phase, and 2 wt% of the same silica type in the outer oil phase. Oil globules was stabilized by 2 wt% of 80% SiOH silica based in second water phase. The fi rst water phase contains Rhodamin B and 1 wt% Fe 3 O 4 . The fi rst oil phase contains 1 wt% DVB and 2 wt% AIBN, both based on monomer mass. The outer second oil phase contains only 2 wt% AIBN in addition to the silica. The volume fraction of the phases was φ w = 0.2, φ w/o = 0.5, and φ w/o/w = 0.5.

shown in Figure 8 a. The size range of the outer water glob-ules was 150–600 μ m and the emulsion exhibited excel-lent stability to coalescence for several weeks. However, the emulsion sedimented within 30 min after prepara-tion, releasing a layer of free oil on top as can be seen in Figure 8 b. This spontaneous sedimentation enhanced the packing of the sedimented emulsion; hence, increased the effective internal volume fraction of the double emul-sion. The resulted monolith, polymerized medium internal phase triple emulsion (polyMIPTE-8), was very hard to cut, without the help of extra tools, indicative of high mechan-ical strength of the product (Figure 8 c). Figure 8 d shows a



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fl uorescent image of one large cell entrapping a number of macroporous particles, indicative of the formation of a complex structure.

Figure 9 a–f shows detailed microstructures of the resulted porous polyMIPTE-8 using SEM. In this unique structure, we realized three distinct interfaces, which may rationalize many of the phenomena observed and the key parameters for excellent stabilization of solid-stabilized emulsions studied here. From the fi rst look at the structure shown in Figure 9 a–e, one can realize the evidence of encapsulation of porous microparticles (30–90 μ m) inside the large (200–800 μ m) water globules

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(cells), which themselves were dispersed in the outer-most oil phase. We can see also few open and mainly closed cells at different depths in the outermost poly mer matrix. Both open and closed cells have a number of small pores having sizes of 10–40 μ m (Figure 9 b and e). The closed cells are similar to colloidosomes where the shell is composed of densely packed silica nanoparti-cles. In addition, this polymerized (frozen) structure is a replica of the parent w/o/w/o emulsion shown in the optical image in Figure 8 a indicative of excellent coa-lescence stability of the parent emulsion even during polymerization at 75 ° C for 24 h. The fi rst interface rep-resents the contact between the fi rst water phase and the fi rst oil phase, containing the hydrophobic silica, in the multi-hollow porous microparticles that formed from polymerization of a double w/o/w emulsion just before the phase inversion (Figure 3 e–i). This interface can be represented as the closed pores formed in the polymer-ized double w/o/w emulsion. We hypothesized that there were no more smaller water drops remained inside that double emulsion as all the smaller drops has merged into larger ones to form these closed large pores. The second interface is the contact between the fi rst oil phase and the second water phase where the double emulsion is encapsulated in large water globules. We can see clearly how the excess of hydrophilic 80% SiOH silica nanopar-ticles, based on the second water phase, form aggregates inside the large water cells that surround the porous microparticles as shown in Figure 9 c and d. This “frozen” structure gave us a good indication about the steric stabi-lization of these microparticles through the formation of 3D network of the stabilizing silica. The third interfaces is formed between the large water globules and the outer-most oil continuous phase, again containing hydrophobic silica, where one can see the rough interface (shell) having average thickness of 1–1.5 μ m, estimated from SEM. This interface separates hydrophilic and hydro-phobic water and oil phases, respectively. Therefore, both types of silica nanoparticles might have been contrib-uted in forming such a thick shell. For Pickering simple o/w emulsions studied by low-temperature fi eld-emis-sion electron microscopy (LTFE–SEM), it was concluded that the thickness of the interface was around 0.5 μ m for oil drops stabilized by similar hydrophilic silica used in our study. [ 34 ] This also supports the results obtained from the ellipsometric study of silica nanoparticles at a planar oil–water interface where it was concluded that a multilayer of silica particles is evident. [ 35 ] Interestingly, the third interface was generated by low energy homog-enization, namely hand shaking for 10 s. We noticed the existence of smaller pores having diameters between 10–40 μ m located among the large cells (Figure 9 f). These pores might have been formed from the excess of small water drops that dispersed and silica-stabilized in the



outermost oil phase during the fi nal homogenization step. It is worth mentioning that the parent w/o/w/o emulsion underwent sedimentation without coalescence leaving around 1/3 of the original oil (outermost phase) in top of the emulsion (Figure 8 b). Therefore, the effective volume fraction of w/o/w in the polymerized triple emul-sion was φ w/o/w = 0.6, that is, MIPTE was actually formed by spontaneous sedimentation. Our results shown above suggested that we can control the fi nal structure of the resulted materials by polymerize the fi rst/second water (oil) phases in the four phases triple emulsions (either w/o/w/o or o/w/o/w) to fabricate a wide range of dif-ferent structures such as microcapsules in microcapsules, microcapsules in large hydrogels, porous materials, and others. Such study is currently underway.

To test our hypotheses, we prepared a triple w/o/w/o emulsion in similar way as described above but lowering both the volume fraction of the primary w/o emulsion in the second water phase to φ w/o = 0.2 and that of the double emulsion ( φ w/o/w = 0.3). Figure 10 a shows a fl u-orescent image of the obtained stable w/o/w/o triple emulsion. The outer water globules showed diameters in the range of 100–400 μ m and encapsulated many spher-ical double w/o/w emulsion globules. It is obvious from this fl uorescent image that the encapsulated double emulsion is spherical and showed remarkable structure differences form that shown in Figure 3 d, where non-spherical structure was evident. Therefore, selecting the desired volume fraction of the primary w/o emulsion in the double emulsion is crucial for the structure of the resulted emulsions as well as for the porous materials. The parent triple emulsion was monitored for several weeks and showed excellent stability against coales-cence. Figure 10 b shows digital images of the obtained monolith after polymerization and it fl oated in pure eth-anol indicative of low density of the monolith and the formation of closed cells.

The microstructure of the triple emulsion after poly-merization was studied by SEM and is shown in Figure 11 a and b. Large closed cells were observed (100–500 μ m) with few spherical microcapsules seen entrapped inside. Smaller pores were also seen in some of the large cells. It is obvious that different morphologies, size range, and number of encapsulated microparticles were achieved by changing the φ w/o of the primary w/o simple emul-sion. It can be noticed from Figure 11 b that the number of microparticles that remained inside the large cells are smaller than that of the triple emulsion prepared with higher φ w/o/w (Figure 9 b), which we expected as depicted in Figure 1 b and from the previous studies on Pickering systems. [ 25 ] These new triple emulsion systems could be used in potential applications either as nonpolymerizable emulsions in their liquid state or as polymerized solid triple emulsions.





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Figure 10 . (a) A fl uorescent micrograph of a freshly prepared triple w/o/w/o water-in-styrene-in-water-in-styrene Pickering emulsion showing its original structure before polymerization. The emulsion was stabilized by by 3 wt% of 50% SiOH silica nano-particles in the inner oil phase and 2 wt% of the same silica in the outer oil phase. Oil globules was stabilized by 2 wt% of 80% SiOH silica based in second water phase. The fi rst water phase contains Rhodamin B and 1 wt% Fe 3 O 4 . The fi rst oil phase contains 1 wt% DVB and 2 wt% AIBN, both based on a monomer mass. The outer second oil phase contains only 2 wt% AIBN. The volume fraction of the phases were, φ w = 0.2, φ w/o = 0.2, and φ w/o/w = 0.3. (b) Dig-ital images for the resulted monolith after polymerization of the triple emulsion showing its low density when put on ethanol.

Figure 11 . (a,b) SEM images of the obtained Pickering polyMIPTE after polymerization of the triple w/o/w/o emulsion represented in Figure 10 a.

3.5. Monoliths from Non-Aqueous Pickering Emulsion Templates

Water-based systems, containing only oil and water phases, are by far the most studied in the literature as well for emulsion, mini-emulsion and dispersion polymeriza-tion processes. In comparison with aqueous emulsions, oil-in-oil (o/o) emulsions also called nonaqueous emul-sions have had relatively scarce attention in literature. [ 38 ] For this type of emulsions, almost exclusively steric sta-bilization is adopted. For such emulsions, it was shown


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that tailor-made block copolymers are by far the most effi cient stabilizers compared with low-molecular-weight surfactants. [ 38 ] Polymerizable nonaqueous emulsions are of interest as nanoreactor systems for the polymerization of water-sensitive monomers or catalysts. Novel Pickering simple and multiple emulsions containing ionic liquids with aqueous or nonaqueous systems have been introduced in previous report. [ 39 ] Moreover, ionic liquids were used as internal phases in nonaqueous open pores polyHIPEs using surfactants as stabilizers. [ 40 ] Unlike all the above studies, which utilize either surfactants or block copoly-mers as stabilizers, we have successfully prepared novel polymerizable simple (o/o) and double (o/o/o) nonaqueous emulsions stabilized solely by solid nanoparticles, which upon polymerization resulted in polyHIPNAE (polymerized high-internal-phase nonaqueous emulsion) or PolyMIP-NADE (polymerized medium-internal-phase nonaqueous double emulsion), respectively. Figure 12 a and b shows an example of polyHIPNAE formed from polymerization of a

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Figure 12 . (a,b) SEM images of Pickering polyHIPNAE formed from polymerization of a pre-made nonaqueous (o/o) formamide-in-styrene emulsion, φ f = 0.8, stabilized by 3 wt% of 50% SiOH silica nanoparticles dispersed initially in styrene. (c,d) SEM images of polyMIPNADE microparticles formed from polymerization of a pre-made formamide-in-styrene-in-formamide double emulsion stabilized by 2 wt% of 50% SiOH silica particles in the styrene phase, φ f = 0.2. The inner formamide phase contains Rhodamin B and 1 wt% Fe 3 O 4 . The outer forma-mide phase contains 2 wt% of the same type of silica particles, φ f/s = 0.4.

pre-made nonaqueous formamide-in-styrene (fa/s) emul-sion with an internal phase volume fraction of φ f = 0.8. The emulsion was stabilized by 3 wt% of 50% SiOH silica nanoparticles, based in styrene. Mostly closed cells with a size range of 50–500 μ m were formed. We noticed that most cells contain silica aggregates inside their internal interfaces as shown in Figure 12 a and b. These signifi -cant silica aggregates did not appear in the corresponding aqueous polyHIPE-4 system shown in Figure 5 a, indicative of the occurrence of most of the silica particles outside the polymer matrix.

In line with the results obtained above, magnetic and fl uorescent polyMIPNADE was also prepared as shown in Figure 12 c and d. The double nonaqueous formamide-in-styrene-in-formamide (fa/s/fa) emulsion was stabi-lized by 2 wt% of 50% SiOH silica particles in the styrene phase, φ f = 0.2 while the styrene globules were stabilized by 2 wt% of the same type of silica particles in outer for-mamide phase, φ f/s = 0.4. We selected this value of volume fraction of the primary fa/s emulsion in the formed fa/s/fa double emulsion since it was catastrophically phase inverted to simple fa/s type when we prepared it with φ f /s = 0.5. Therefore, the nonaqueous fa/s/fa double emulsion was catastrophically phase inverted at a lower value compared to that obtained in the corresponding



aqueous w/o/w emulsion shown in Figure 3 d. As can be clearly seen in Figure 12 c and d, nearly spherical micro-particles having size range of 25–100 μ m, which covered with dense silica aggregates were formed as a result of polymerization of the fa/s/fa double emulsion. Very few microparticles with small pores with sizes of 10–25 μ m were observed as shown in Figure 12 d. This morphology is somewhat similar to that of microparticles obtained after polymerization of w/o/w emulsion as shown in Figure 3 c. These novel nonaqueous simple and double Pickering emulsions can serve as versatile templates for potential applications in synthesis of new types of hybrid organic–inorganic polymer nano- or micropar-ticles and macroporous polyHIPNAE-MIPNADE non-aqueous foams.

4. Conclusion

We have fabricated various materials based on aqueous and nonaqueous stable simple, double, and triple Pickering emulsions systems stabilized solely by DCDMS-modifi ed amorphous silica nanoparticles of well-controlled surface hydrophobicities or organo-modifi ed laponite clay nano-particles. The different types of emulsions and polymer





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materials described here open up several intriguing ave-nues of research. We have prepared dual function mag-netic and fl uorescent hybrid microparticles in different morphologies, namely multi-core and multi-hollow porous microparticles, via Pickering suspension polymerization of double w/o/w emulsions. We have shown that micropar-ticle morphologies can be facilely tailored by changing the volume fraction of the primary simple w/o emulsion in the precursor double w/o/w emulsion. Stable Pickering w/o HIPEs with volume fraction of internal phase of 0.8 were fabricated and stabilized by DCDMS-modifi ed silica or organo-modifi ed disk-shaped laponite clay nanoparticles. Moreover, incorporation of sporopollenin empty micro-capsules or zinc metal particles in PolyHIPEs could provide additional functionalities. Stable Pickering triple w/o/w/o emulsion was facilely prepared for the fi rst time via homogenization of a double emulsion in an outer oil phase by just hand shaking. Pickering suspension polymerization of both oil phases of the triple w/o/w/o emulsion resulted in the formation of novel porous hierarchical structures, which we called microcapsules in microcapsules. These triple emulsions and their polymerized hybrid micro-spheres can be used in potential important applications such as novel drug carriers, as they can allow sequential release of different drugs (hydrophilic and hydrophobic) through their interfaces. In addition, the embedded inor-ganic nanoparticles will give these multiple compart-ments a versatile functionality, such as optical, magnetic, and fl uorescent properties. These materials can be also used as microreactors or fi ne templates for synthesizing advanced materials and therefore adding a new dimension to Pickering multiple emulsions. We presented an example of suspension polymerization of novel Pickering simple nonaqueous o/o and double o/o/o emulsions, which resulted in the formation of polyHIPNAE and polyMEPNAE, respectively. Although we present a proof of concept, this approach could be applied using different types of oil or ionic liquid phases.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements : This project was supported by King Saud University, deanship of scientifi c research, college of science, research center. The author wishes to thank Wacker Chemie (Germany) for kindly providing the silica nanoparticles.

Received: April 3, 2012 ; Revised: June 4, 2012; Published online: ; DOI: 10.1002/macp.201200172

Keywords: hierarchical structures; macroporous polymers; nanoparticles; nonaqueous emulsions; Pickering emulsions




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macroporous polymer beads and monoliths from pickering simple, double, and triple emulsions

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