Responsive Core−Shell Latex Particles as ... - ACS Publications

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Responsive Core-Shell Latex Particles as Colloidosome Microcapsule Membranes Qingchun Yuan,*,† Olivier J. Cayre,† Syuji Fujii,‡,§ Steven P. Armes,‡ Richard A. Williams,† and Simon Biggs*,† † Institute of Particle Science and Engineering, School of Processing, Environment and Materials Engineering, University of Leeds, Leeds LS2 9JT, U.K., and ‡Department of Chemistry, Dainton Building, The University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K. § Current address: Department of Applied Chemistry, Osaka Institute of Technology 5-16-1, Ohmiya, Asahi-ku, Osaka 535-8585, Japan

Received August 23, 2010. Revised Manuscript Received October 8, 2010 Responsive core-shell latex particles are used to prepare colloidosome microcapsules using thermal annealing and internal cross linking of the shell, allowing the production of the microcapsules at high concentrations. The core-shell particles are composed of a polystyrene core and a shell of poly[2-(dimethylamino)ethyl methacrylate]-b-poly[methyl methacrylate] (PDMA-b-PMMA) chains adsorbed onto the core surface, providing steric stabilization. The PDMA component of the adsorbed polymer shell confers thermally responsive and pH-responsive characteristics to the latex particle, and it also provides glass transitions at temperatures lower than those of the core and reactive amine groups. These features facilitate the formation of stable Pickering emulsion droplets and the immobilization of the latex particle monolayer on these droplets to form colloidosome microcapsules. The immobilization is achieved through thermal annealing or cross linking of the shell under mild conditions feasible for large-scale economic production. We demonstrate here that it is possible to anneal the particle monolayer on the emulsion drop surface at 75-86 °C by using the lower glass-transition temperature of the shell compared to that of the polystyrene cores (∼108 °C). The colloidosome microcapsules that are formed have a rigid membrane basically composed of a densely packed monolayer of particles. Chemical cross linking has also been successfully achieved by confining a cross linker within the disperse droplet. This approach leads to the formation of single-layered stimulus-responsive soft colloidosome membranes and provides the advantage of working at very high emulsion concentrations because interdroplet cross linking is thus avoided. The porosity and mechanical strength of the microcapsules are also discussed here in terms of the observed structure of the latex particle monolayers forming the capsule membrane.

1. Introduction Colloidosome microcapsules of selective permeability have the potential for use as controlled delivery systems.1 Colloidosomes are normally prepared using emulsion droplets as templates. The colloid particles effectively act as stabilizers by adsorbing at the oil/water interface during emulsification and self-organizing into a regular array2 to form a continuous membrane. In such particlestabilized emulsions, also called Pickering emulsions, the adsorption energy of the particle is normally a few thousand times that of small surfactant molecules so that the colloidal particle tends to be permanently adsorbed. These particles can then be immobilized on the surface using a variety of approaches, forming permanent membrane shells. The particle array can provide adjustable porosity through the use of different sizes and shapes of colloid particles,1,3-8 and the method of immobilization also allows for the manipulation of the porosity and permeability of the membrane shell. *Corresponding authors. E-mail: [email protected]; [email protected] (1) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Weitz, D. A. Science 2002, 298, 1006–1009. (2) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (3) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374–2384. (4) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2385–2391. (5) Velev, O. D.; Nagayama, K. Langmuir 1997, 13, 1856–1859. (6) Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092–8093. (7) Liu, G.; Liu, S.; Dong, X.; Yang, F.; Sun, D. J. Colloid Interface Sci. 2010, 345, 302–306. (8) Liu, H.; Wang, C.; Gao, Q.; Liu, X.; Tong, Z. Int. J. Pharm. 2008, 351, 104– 112.

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In colloidosome preparation, the self-assembly of colloidal particles at the oil/water interface and immobilization are crucial. These processes are largely determined by the properties of the colloidal particle. For effective self-assembly at an interface, the colloidal particles need to be preferentially adsorbed at the interface in a manner similar to that for a surfactant. The thermodynamics of such emulsion systems has been extensively studied over the past decade. It has been shown that optimal particle adsorption is achieved when the contact angle between the three phases approached 90°.9 Another important factor that has been less investigated is the adsorption/wetting kinetics of particles at the oil/water interface. It has recently been discussed with respect to their effect on droplet formation in a precisely controlled processing environment such as membrane emulsification.10 The wettability can be controlled by careful formulation through the judicious selection of colloidal particle types, modification of the particle surface, or chemical additions to the oil and aqueous phases.1,3,10-12 Surface modification of particles, both inorganic and organic, has been extensively explored to increase their suitability for (9) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622–8631. (10) Yuan, Q.; Cayre, O.; Manga, M.; Williams, R. A.; Biggs, S. Soft Matter 2010, 6, 1580–1588. (11) Cayre, O. J.; Noble, P. F.; Paunov, V. N. J. Mater. Chem. 2004, 14, 3351– 3355. (12) Liu, H.; Wang, C.; Gao, Q.; Liu, X.; Tong, Z. Acta Biomater. 2010, 6, 275– 281. Duan, H.; Wang, D.; Sobal, N. S.; Giersig, M.; Kurth, D.; G.; Mohwald, H. Nano Lett. 2005, 5, 949–952.

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interfacial self-assembly or colloidal stabilization .9,13,14 Such modifications are normally achieved by surface adsorption or grafting of some chemically active species or by controlled oxidation (e.g., carbon and PS colloid particles) to alter the electrostatic charge type or potential of the surface and provide particle surfaces designed for the immobilization reaction that follows. In the pioneering work of Velev et al.,3 PS latex particles (1 μm) having appropriate positive (amidine) or negative (sulfate) surface charge were assembled on the surfaces of octanol droplets. Such surface-modified latex particles could generate only limited stabilization, and octanol droplets would aggregate as a coalesced oil layer within minutes. Casein (phosphoprotein) was added to block/bridge the particles on the droplet surface and extended the lifetime of the droplets. In addition to requiring the formation of a stable emulsion system, colloidosome preparation also requires some method for linking or fusing the particles at the interface, providing a robust shell. Dinsmore et al.,1 in their seminal paper, stabilized droplets of vegetable oil and toluene (in a 50:50 mixture) in water using PS latex particles. These particles were then immobilized at the interface either by heating to fuse the particles, by the use of an electrolyte to coagulate them, or by the use of a polyelectrolyte to bridge-flocculate them. Heating the system to around the glasstransition temperature of PS (105 °C) allows the latex particles to fuse, but in this work, to avoid boiling the aqueous phase at >100 °C, a large portion of glycerol was added to the system. This obviously makes the processing more complicated. Cayre et al.11 used aliphatic amine polystyrene latex particles to stabilize droplets of agarose aqueous solution in sunflower oil and, after some degree of particle fusing within the monolayer, subsequently fixed the PS particles using glutaraldehyde to cross link the amine groups present on the particles from the continuous phase. Inorganic particles such as platelike Mg/Al-layered doublehydroxide nanoparticles,7 porous CaCO3 microparticles,8 silica, and magnetic Fe2O3 nanoparticles12,15 have also been used in the formation of colloidosomes from either water-in-oil or oilin-water Pickering emulsions. In such cases, locking of the particles at the droplet surface was achieved either by polymer chain bridging or electrostatic coagulation. Similar methods have also been employed to yield colloidosome microstructures from solid-stabilized air bubbles.16,17 For example, yeast cells coated with an ionic polyelectrolyte were successfully demonstrated to stabilize air bubbles. The film of yeast cells at the interface was subsequently permanently fixed through the adsorption of an oppositely charged polyelectrolyte layer. In contrast to dispersing the particles in the continuous phase, colloidosomes can also be prepared by dispersing colloid particles in the disperse phase in the presence of other surface-active agents. Croll et al.18 prepared poly(divinylbenzene-alt-maleic anhydride) colloidal spheres for the preparation of colloidosomes using this approach. The spheres were suspended in an oil disperse phase and emulsified into aqueous solutions of poly(vinyl alcohol) as a dispersant because they did not possess the desired wettability with respect to the aqueous phase and thus could not play the stabilization role on their own. The spheres were immobilized through an interfacial cross-linking reaction of polyethylenimines, (13) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622–8631. (14) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions: Academic Press: London, 1983. (15) Cayre, O. J.; Biggs, S. J. Mater. Chem. 2009, 19, 2724–2728. (16) Subramaniam, A. B.; Gregory, D.; Petkov, J.; Stone, H. A. Phys. Chem. Chem. Phys. 2007, 9, 6476–6481. (17) Brandy, M. L.; Cayre, O. J.; Fakhrullin, R. F.; Velev, O. D.; Paunov, V. N. Soft Mater. 2010, 6, 3494–3498. (18) Croll, L. M.; St€over, H. D. H. Langmuir 2003, 19, 5918–5922. Croll, L. M.; St€over, H. D. H. Pure Appl. Chem. 2004, 76, 1365–1374.

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added in the continuous phase, with the anhydride functional group on the particle surface. Laib et al.19 used polystyrene-co-butylacrylate latex, with low glass-transition temperatures in the range of 30-42 °C, in a water-in-oil system. The particles were suspended in an aqueous phase and then dispersed in an oil medium containing surfactant or electrolyte (e.g., Span 80) to stabilize the aqueous droplets. Colloidosomes were prepared through heat annealing at the glass-transition temperatures of the bulk particle. Colloidosome preparation procedures often include either the removal of the oil core or the transfer of the capsules to a different continuous phase. Such processing steps are often difficult, can result in very low yields,1,3,11 and are likely to be suitable only for academic research. To manufacture colloidosomes on a large scale, it is desirable that (1) the particle that is used can provide good stabilization as the sole stabilizer and facilitate the formation of size-stable droplets in a relatively simple processing environment and (2) particle immobilization can be carried out at higher concentrations to generate a mechanically strong colloidosome membrane to endure any subsequent stages of production, storage, and use at high yields. This has typically provided significant challenges for many of the systems reported to date, as reviewed above. Therefore, there is a significant requirement to develop strategies for the formulation and manufacture of colloidosomes that are suitable for large-scale economic production. In the past few years, we have examined a number of approaches to address these challenges. We established two routes using sterically stabilized PS latex nanoparticles as a stabilizer, which have thermally responsive and pH-responsive features and have demonstrated excellent adjustable stabilization as sole emulsifiers either for oil-in-water or water-in-oil emulsions.20,21 The thermal transition and reactive amine groups of the polymeric chains adsorbed are utilized for thermal annealing and chemical cross linking from within the emulsion droplet to lock the latex particle monolayers, thus forming colloidosome microcapsules. The utilization of the polymeric chain adsorbed on the colloidal particles rather than the PS core itself provided a flexible route to formulating size-stable emulsions and immobilizing the nanoparticles at a high concentration under mild operational conditions. This work has been patented as Microcapsules and Methods.22 Following this patent, Thompson and Armes23 successfully prepared similar colloidosome structures by using a poly(glycerol monomethacrylate)-stabilized PS latex and tolylene 2,4-diisocyanate-teminated poly(propylene glycol) as a cross linker for covalently cross linking emulsion droplets. In this article, we extend the work reported in these previous publications,22,23 presenting an extensive study of the effect of thermal treatment temperatures and cross-linking density on the structure of the microcapsule membranes obtained. Both of the mechanisms presented here show great potential for controlling capsule pore size and strength. Additionally, the polymer shell on the particle surface is responsive to pH. This can be utilized to construct microcapsules capable of releasing encapsulating components in response to external stimuli. These characteristics are currently under investigation in our laboratories and will be reported later. (19) Laib, S.; Routh, A. F. J. Colloid Interface Sci. 2008, 317, 121–129. (20) Amalvy, J. I.; Unali, G.-F.; Li, Y. T.; Granger-Bevan, S.; Armes, S. P.; Binks, B. P.; Rodrigues, J. A.; Whitby, C. P. Langmuir 2004, 20, 4345–4354. (21) Read, E. S.; Fujii, S.; Amalvy, J. I.; Randall, D. P.; Armes, S. P. Langmuir 2004, 20, 7422–7429. Read, E. S.; Fujii, S.; Amalvy, J. I.; Randall, D. P.; Armes, S. P. Langmuir 2005, 21, 1662–1662. (22) Biggs, S.; Williams, R. A.; Cayre, O. Yuan, Q. Microcapsules and Methods. WO/2009/037482, PCT/GB2008/003197. (23) Thompson, K. L.; Armes, S. P. Chem. Commun. 2010, 46, 5274–5276.

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Figure 1. (a) SEM image of polystyrene latex particles adsorbed with PDMA-b-PMMA. (b) Chemical structure of the PDMA-bPMMA diblock copolymer.

2. Experimental Section 2.1. Materials. The PS latex particles used here have poly[2-(dimethylamino)ethyl methacrylate]-b-poly[methyl methacrylate] (PDMA-b-PMMA) polymer chains adsorbed on the surface as a steric stabilizer. The PDMA-b-PMMA diblock copolymer was prepared by group-transfer polymerization.24 The diblock copolymer contained 79 mol % DMA as determined by 1 H NMR spectroscopy and has a Mn of 35.8K and a Mw/Mn of 1.18 versus poly(methyl methacrylate) standards measured by gel permeation chromatography analysis. The copolymer was used to prepare sterically stabilized polystyrene latex via dispersion polymerization in a 7:3 methanol/water mixture using an azobisisobutyronitrile initiator as described previously.20 Purification was achieved using ultrafiltration with periodic replacement of the serum. The surface tension of the supernatant after purification was 70.0 mN/m, confirming that the final latex was not contaminated with nonadsorbed block copolymer stabilizer. An aliquot of the purified latex was dissolved in CDCl3 and analyzed by 1H NMR spectroscopy. The PDMA concentration was determined as approximately 4 wt % on the basis of the PS content. A scanning electron microscopy image of the particles is given in Figure 1a, confirming the essentially spherical nature of the latex particles. The mean diameter of the latex particles measured from the SEM image is in the range of 250-260 nm. Dynamic light scattering gave an average hydrodynamic diameter of 345 nm for the latex in the dispersion. The PDMA-b-PMMA (Figure 1b) diblock copolymer stabilizer confers both pH-responsive and thermally-responsive behavior to the latex colloids. The PDMA chains form a brushlike layer on the particle surface. This polymer is very water-soluble at a low pH because of the protonation of its tertiary amine groups but becomes increasingly hydrophobic in more alkaline media. Thermally responsive behavior is also observed via the appearance of a cloud point (lower critical solution temperatures) at approximately 35 °C when the pH value is above 8.25 Similar materials have been previously shown to allow the formation of both oil-in-water and water-in-oil emulsions by controlling the solution pH and temperature.21,26 1,2-Bis(2-iodoethyloxy)ethane (BIEE, Aldrich, 99%) was used as a cross-linking agent to react with the amine groups on the PDMA-b-PMMA polymer chains on the particle core surface. In this work, oil-in-water emulsion droplets were used as templates for the preparation of the colloidosomes. The oils used included a medium liquid white oil (mineral oil, CAS no. 804247-5) and n-dodecane (Fluka, g98.0% purity). Aqueous solutions of 0.1 M NaOH and HCl were used to adjust the pH of both the aqueous latex particle dispersion and the oil-in-water emulsions. (24) Baines, F. L.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 3416–3420. (25) B€ut€un, V.; Armes, S. P.; Billingham, N. C. Polymer 2001, 42, 5993–6008. (26) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S. Angew. Chem., Int. Ed. 2005, 44, 4795–4798.

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Figure 2. DSC analysis of a sample of the dry responsive PS latex particles. Three phase-change events are recorded and attributed to the glass transition of the PS core (1) and changes occurring in the PDMA-b-PMMA layer absorbed on the core (2, 3).

2.2. Emulsification. The emulsification was conducted using a miniature rotor-stator homogenizer (PolyTron PT2100, Kinematica AG). To prepare each emulsion, 3.0 mL of the latex dispersion containing 2 wt % of the latex was adjusted to pH 8 to 9 and mixed with a known volume of oil (1.5 to 3.0 mL). This mixture was then homogenized for 2 min at either 22 000 rpm for the n-dodecane system or at 30 000 rpm for the mineral oil systems. The resulting emulsions were examined using optical microscopy for the droplet appearance and size and using lowangle laser light scattering for the average size and size distribution of the oil droplets. The particle-stabilized emulsions were then further treated to lock the latex particles in place either by thermal annealing or chemical cross linking. 2.3. Thermal Annealing. A 2.0 mL emulsion was diluted to 20 mL using deionized water and then heated to a known temperature ((75-105) ( 2 °C) in a water bath under gentle stirring for a certain number of minutes. The emulsion was then quenched by cooling rapidly in flowing tap water. Part of the sample was washed with water/isopropanol solutions to remove the oil from the capsule cores and was subsequently redispersed in water. Such prepared samples were then dried and analyzed using SEM. 2.4. Chemical Cross Linking. Colloidosome-confined cross linking, also referred to as internal cross linking, was used to fix the particle monolayers in place. The BIEE cross linker has only limited water solubility. Prior to emulsification, a known amount of BIEE cross linker was dissolved in the oil phase. The resulting emulsions were very stable and were stored at room temperature for several days to allow the cross-linking reaction to reach completion. The microcapsules obtained were treated with water/ethanol mixtures (gradually increasing the concentration of ethanol) to remove the oil core. The obtained colloidosome microcapsules were then redispersed in water and sampled for SEM analysis. 2.5. Characterization. Optical microscopes (a Nikon Eclipse ME600 and an Olympus BX51) were used for emulsion droplet and microcapsule examination. Images were recorded using a digital camera and processed using commercial software packages. Prior to optical microscopy analysis, each sample was spread as a thin layer on a standard glass slide. The microcapsules were also studied using scanning electron microscopy (LEO 1530 FEGSEM). Diluted samples were spread on aluminum stubs and were coated with platinum to reduce sample charging. The stage distance and the accelerating voltage used were ∼3 mm and 3 kV, respectively. Langmuir 2010, 26(23), 18408–18414

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Figure 3. Optical microscopic images of microcapsules obtained from heat treatment at (a) 92 °C for 5 min and (b) 86 °C for 5 min. Droplet size distributions were characterized by low-angle laser light scattering (Malvern Mastersizer 2000 in a Hydro-S dispersion cell). The scattering pattern was deconvoluted using Mie theory. The thermal behavior of the latex particle was analyzed by differential scanning calorimetry (DSC, a Perkin-Elmer DMA 7e). The analysis was carried out at a heating rate of 10 °C/min from room temperature up to 120 °C. The sample used was prepared by directly evaporating the water dispersant and then dried in a vacuum oven at room temperature overnight.

3. Results and Discussion 3.1. Thermal Analysis of the Core-Shell Latex Particles. The thermal behavior of the core-shell latex nanoparticles was analyzed using DSC (Figure 2) and showed three endothermic glass-transition peaks. The peak at 107-108 °C was attributed to the glass transition of the PS core. The other two occurred at temperatures of ∼90 and ∼75 °C. These peaks could be attributed to the thermal transition of the PDMA-b-PMMA polymer chains adsorbed on the PS core. The distinct different thermal transition between the shell and the core provided an opportunity to fuse the particle in place through the thermal transition action of the shell at a much lower temperature than the glass transition of the PS core itself. We tested this hypothesis here. 3.2. Thermal Annealing. As noted earlier, the oil-in-water emulsion droplets stabilized by the latex nanoparticle were first prepared and subsequently used as templates for the preparation of colloidosome microcapsules by locking the particle at the oil/ water interface using two methods. The first method involves annealing the particle through heat treatment. An emulsion prepared using 1.5 mL of mineral oil and 3 mL of 2 wt % of the latex suspension was divided into smaller aliquots for subsequent heat treatment. Initial annealing attempts were made at the PS glass-transition temperature. A sample was dispersed in a glycerol aqueous solution to raise the system boiling point and then heated to ∼105 °C for various times (up to 30 min). Optical microscope observations revealed few individual microcapsules. Instead, we observed large, irregular aggregates. Taking account of the thermal analysis of the latex particles (Figure 2), we attempted instead to hold a dilute sample at 92 °C for 10 min. In this case, optical micrographs showed numerous individual spherical capsules, but once again, a significant number of aggregates/agglomerates were also formed at the same time. To limit the aggregate/agglomerate formation, the heat treatment was then carried out for a shorter time and in one case at a lower temperature: at 92 and 86 °C for 5 min, respectively. The optical microscope images for these experiments are shown in Figure 3. Langmuir 2010, 26(23), 18408–18414

The sample held at 92 °C for 5 min (Figure 3a) had more individual capsules and fewer large agglomerates than the sample held at the same temperature for 10 min. When the heating temperature was lowered to 86 °C (Figure 3b), the polymeric agglomerates disappeared and only individual microcapsules were seen. The colloidosome microcapsules had consistent diameters with those recorded for the template emulsion droplets. The colloidosome microcapsules obtained were further examined using SEM. Figure 4 shows images of the microcapsules directly dried on an SEM stub. It can be seen that after being subjected to high vacuum during sample coating, the encapsulated mineral oil had largely evaporated, leaving hollow capsule shells (Figure 4a). Some of these capsules showed evidence of a large “bursting” hole (Figure 4b) as the oil was forced through the sealed particulate membrane under vacuum. This suggests significant strength for the microcapsule membrane, which may indicate that these systems could provide relatively robust microcapsules. A sample of these heat-treated colloidosome microcapsules was washed with mixed solutions of water and isopropanol to remove the oil prior to sampling for SEM investigation. Complete removal of the nonconductive oil in these samples allowed us to observe the microcapsule membranes in more detail. Figure 5 shows optical and SEM images of such a treated sample. The colloidosomes in the optical image (Figure 5a) were suspended in an isopropanol/water mixture (roughly 50:50). They noticeably survived the alcohol treatment and clearly appeared as intact entities in either spherical or random shapes, with the latter assimilated to a characteristic appearance of Pickering emulsion droplets. In the SEM images, the colloidosome microcapsules appear as mostly a full-shell cage with some evidence of cage breakage on a few microcapsules (Figure 5b,c). Solvent evaporation under vacuum was thought to be responsible for the broken membranes because the microcapsules dispersed in the different water/isopropanol suspensions were clearly observed as intact via optical microscopy. The images in Figure 5d,e show that the latex particles on the microcapsules have apparently adsorbed to the oil/water interface in a closely packed monolayer with very few imperfections. The close packing gives rise to a dense continuous shell. On these images, one can distinguish additional latex particles sparsely adhered to the continuous shell. These are likely to originate from the free latex particles present in the emulsion disperse phase (i.e., not adsorbed on the oil droplet surface). During the thermal treatment, the steric shell of the particles shrinks and softens. Those that efficiently collided with the microcapsules adhere to the external side of the membrane. In principle, the shell thickness can be tailored by controlling the population of the free latex DOI: 10.1021/la1033564

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Figure 4. (a) SEM image of the colloidosome microcapsules prepared from a mineral oil-in-water emulsion and thermal annealing at 86 °C for 5 min and (b) an individual microcapsule showing a hole generated by the fast extraction of the oil phase under vacuum.

Figure 5. Microscopy images of the colloidosome microcapsules (the same sample as shown in Figure 4) after washing off the oil core and redispersion in water (a, optical) and drying and coating under vacuum (b-f, SEM). (c) An individual colloidosome and (d, e) highermagnification images from outside and (f ) inside the capsules.

particles in the emulsion system. In this study, there are only limited free latex particles in the dilute system so that the shell initially formed from a continuous single layer of particles, with some additional particles adhered to the single-layer film in 18412 DOI: 10.1021/la1033564

places. The image of the shell interior (Figure 5f) provides more detail of the single-layer particulate membrane. The information disclosed in Figures 4 and 5 proves our hypothesis that the thermal annealing of the steric polymer chain Langmuir 2010, 26(23), 18408–18414

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in the coronal region of the particle, rather than the PS core (Figure 2), can be used to control the immobilization of the nanoparticles at the oil/water interface. The glass transition of the adsorbed polymer shell rather than that of the PS cores allows the annealing to be successfully carried out at temperatures significantly lower than that of the core. Such core-shell particle structures rather than only the core particle itself as used in previous work1,19 provide an extended scenario for the design of complex microcapsules using a wide range of nanoparticles

Figure 6. Size distributions of a fresh emulsion (9) of dodecane and their corresponding cross-linked sample (gray dots) as prepared at pH 8 to 9 (a) and after acidification to pH 4 (b).

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and for economic, environmentally friendly production on a large scale. 3.3. Internal Cross Linking. In the second preparation method tested here, an oil-soluble cross-linking agent, BIEE, was used to react with the amine groups on the PDMA chains of the steric stabilizer, with the aim of immobilizing the latex nanoparticles on the surface of the emulsion droplet templates. A given amount of cross linker was dissolved in the dispersed oil phase prior to emulsification. It was anticipated that only the latex particles assembled on the oil droplet surface would react with BIEE to form a single layer of particles as the colloidosome microcapsule shell. As a result of this protocol, it is not necessary either to separate any excess nonadsorbed latex particles from the oil droplets or to dilute the emulsion to avoid cross linking between the colloidosome microcapsules or even among the free latex particles remaining in the continuous phase. Hence, this method should be able to produce microcapsules at high concentrations, which is a key advantage for the large-scale cost-effective production of such microcapsules.22,23 The emulsions used for the preparation of the microcapsules using the internal cross-linking method were prepared from 2 mL of n-dodecane and 3 mL of an aqueous phase. Figure 6a gives the size distribution of a freshly made emulsion and the corresponding colloidosome microcapsules after the completion of the crosslinking reaction. After cross linking, the sample retains a similar mean size and size distribution in the range of approximately 20-200 μm in comparison with the fresh emulsion. The responsive latex particles adsorbed on the oil droplets may change their hydrophobicity to some extent through protonating the amine groups of the steric PDMA chains when being acidified. They may detach from the oil-water interface if they are not fixed in place21 and significantly decrease the droplet stability. This feature was first used to examine the fixation of the latex nanoparticles by studying the effect of pH changes on the size distribution of the fresh emulsion and the cross-linked sample. In the case of the non-cross-linked emulsion sample, the results show that at pH 4 a large number of free latex particles are detected at sizes of around 200 nm along with a significant number of droplets larger than 200 μm (Figure 6b) (i.e., the droplets in the fresh emulsion were seen to destabilize). In contrast, the acidification of the cross-linked sample did not cause any significant changes in the mean size or size distribution. This suggests that the cross-linking reaction was successful and that we have obtained robust capsules. A sample of the capsules was then treated using solutions of ethanol/water mixtures to remove the oil core slowly. The ethanol concentration was increased gradually with each successive washing step, and eventually pure ethanol was used. The colloidosomes were finally redispersed in pure water. Figure 7 shows optical and

Figure 7. Colloidosome cages (the oil inside has been removed) prepared from dodecane-templated droplets. (a) Optical microscopy image. (b) SEM image. Langmuir 2010, 26(23), 18408–18414

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Figure 8. SEM images of internally cross-linked microcapsule membranes at varied cross-link agent (BIEE) amounts: (a) 2, (b) 3, (c) 5, and (d) 6 mg.

SEM images of dried capsules. Clearly, the capsules that formed are robust enough to survive the drying, but unlike the thermally annealed examples, they are relatively soft and collapse when dried. The effect of the cross linking was further examined by altering the amount of BIEE used. Figure 8a-d shows a series of SEM images of the colloidosome microcapsules prepared with 2, 3, 5, and 6 mg of BIEE in 2 mL of dodecane, respectively. These images again show that the colloidosome membranes are composed of a single layer of latex particles with good continuity and flexibility. The latex particles are closely packed at the interface, and apparently the membrane that formed has an increasingly annealed appearance as the BIEE concentration increases. Clearly, such a wall structure is extremely porous, but the micrographs showing a large number of capsules (Figure 7) suggest the cross links are sufficiently robust to hold the membrane together. The changes in cross-linker concentration will apparently affect the number of connections between the steric stabilizer chains, and this evidently determines the pore size and permeability of the wall. Given that the steric stabilizers are themselves both pH- and temperature-sensitive, one may postulate that a structure of the type seen in Figure 8a-c would allow the wall to expand and collapse reversibly, although the expansion/contraction of the PDMA chains may be affected by the cross-linking procedure to some extent. Additional work is currently underway in our laboratories to explore this effect. Confining the cross linker within the disperse phase to react with polymer chains on colloidal particles allows cross linking to be conducted at high volume concentrations of the droplet templates with nearly 100% yield. It is thought that the permeability of the colloidosome cage can be tailored by controlling the size of the interstices between the nanoparticles through the degree of cross linking. The degree of cross linking varies with cross-linker concentration and the population density of PDMA-b-PMMA copolymer chains connected to the surface of the PS latex particles.

4. Conclusions Colloidosome microcapsules have been prepared using sterically stabilized PS latex particles through emulsion droplet

18414 DOI: 10.1021/la1033564

templates via two robust methods. The sterically stabilized latex particle has a shell formed by the adsorption of thermally responsive and pH-responsive polymer chains of PDMA-bPMMA. It has been demonstrated that the thermal transition and reactive properties of the shell can be used to immobilize the monolayer particle in place on the droplet surface either by thermal annealing of the shell or by chemical cross linking of the polymer chains from within the emulsion droplets. The PDMA-b-PMMA-coated PS latex particles demonstrated three glass-transition temperatures, of which two that were substantially lower than that of the PS core were attributed to the adsorbed polymer chain. These lower glass-transition temperatures have been successfully used in the thermal annealing of the latex particles to adjacent ones, forming robust microcapsule membranes from the adsorbed particle monolayer. The use of the glass transition of the steric shell instead of that of the particle core itself provides a wide range of synthesis options for the preparation of such microcapsules. Among other applications, this method can potentially be used as a tool to reduce problems associated with encapsulating thermally sensitive ingredients. Amine-reactive cross-linking agent BIEE was used to demonstrate the potential of chemical cross linking from within the emulsion droplet in preparing colloidosome microcapsules. Emulsions containing up to 50 vol % oil have been successfully encapsulated using this method without any dilution. The microcapsules prepared largely maintained the size distribution of the original emulsion and showed little agglomeration. Controlled variation of the porosity and mechanical strength of the colloidosomes by varying the concentration of cross linker in the dispersed phase was also demonstrated. The routes described here provide important scenarios for the manufacture of functional colloidosome microcapsules and for the encapsulation and controlled delivery of sensitive actives on a large scale. Acknowledgment. We thank Mr. I. Francis for his assistance with the preparation of the colloidosome samples tested here.

Langmuir 2010, 26(23), 18408–18414