Nov 4, 2009 - factant-free emulsion polymerization adapted from the syntheses reported ... a 250 mL two-neck reactor fitted with a nitrogen bubbling inlet and.
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Macroporous Polymer from Core-Shell Particle-Stabilized Pickering Emulsions Zifu Li and To Ngai* Department of Chemistry, The Chinese University of Hong Kong, Shatin. N.T., Hong Kong Received September 19, 2009. Revised Manuscript Received October 15, 2009 Poly(styrene-co-N-isopropylacrylamide) (PS-co-PNIPAM) core-shell particles were synthesized and used as particulate emulsifiers in the preparation of particle-stabilized (Pickering) emulsions. Highly concentrated oil-in-water emulsions with an internal phase up to 80 vol % can be produced using PS-co-PNIPAM core-shell particles along as the emulsifiers in emulsions. The core-shell particles are adsorbed at the liquid interface, acting as a barrier against oil droplet coalescence. In addition, it is likely that excess particles simultaneously form a gel in the continuous phase to trap oil droplets in the gel matrix, in turn inhibiting creaming and phase inversion. Evaporation in air of such a core-shell particle-stabilized emulsion directly leads to porous membranes in the absence of chemical reactions. The pore walls of the final structures are densely packed with layers of the core-shell particles. This provides great flexibility to prepare functionalized porous materials for opening up new applications.
Introduction Porous polymers with controlled microstructures and chemical compositions have received increasing attention. They can be used in a number of applications such as monoliths for separations in analytical chemistry1,2 and are increasingly applied in biorelated applications such as cell culturing, tissue engineering, and bioseparation.3-5 For these applications, the macroporous polymers should ideally be easy to prepare with controllable pore morphology and surface functionality. A class of macroporous materials which has drawn significant attention in that respect is polymers obtained from high internal phase emulsions (HIPEs). HIPEs are often defined as very concentrated emulsions where the volume fraction of the internal phase is larger than 0.74.6 Like ordinary dilute emulsions, HIPEs can be obtained in either normal oil-in-water (o/w) or its inverse water-in-oil (w/o) forms. The preparation of such highly concentrated emulsions requires careful selection of surfactant, which must be soluble only in the continuous phase in order to prevent emulsion inversion at high internal phase volume fractions.2 If the continuous phase consisting of one or more monomeric species in the HIPE is polymerized and the internal droplet phase removed, a highly porous polymeric material remains, known as a polyHIPE.7 Although HIPEs have long been used as templates for the production of porous polymers, styrene-based polyHIPEs remain the most widely studied system.7-9 Generally, these polyHIPEs are prepared by first addition of droplets of an aqueous phase into the nondispersed organic phase (containing monomer, styrene, and cross-linker, divinylbenzene) to form a stable w/o HIPE. Polymerization of the monomeric continuous phase and subsequent *To whom correspondence should be addressed. (1) Cameron, N. R.; Barbetta, A. J. J. Mater. Chem. 2000, 10, 2466. (2) Cameron, N. R. In Monolithic Materials: Preparation, Properties and Applications; Svec, F., Tennikova, T. B., Deyl, Z. Eds.; Elsevier: Amsterdam, 2003. (3) Akay, G.; Birchand, M. A.; Bokhari, M. A. Biomaterials 2004, 25, 3991. (4) Christenson, E. M.; Soofi, W.; Holm, J. L.; Cameron, N. R.; Mikos, A. G. Biomacromolecules 2007, 8, 3806. (5) Choi, S. W.; Xie, J. W.; Xia, Y. N. Adv. Mater. 2009, 21, 2997. (6) Cameron, N. R. Polymer 2005, 46, 1439. (7) Zhang, H. F.; Cooper, A. I. Soft Matter 2005, 1, 107. (8) Williams, J. M. Langmuir 1991, 7, 1370. (9) Carnachan, R. J.; Bokhari, M.; Przyborski, S. A.; Cameron, N. R. Soft Matter 2006, 2, 608.
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removal of the dispersed water leads to a highly porous crosslinked polymer material. Other hydrophobic monomers that are used to prepare porous polymers also from w/o HIPEs include 2-ethylhexyl acrylate (EHA), 2-ethylhexyl methacrylate, and butyl acrylate (BA).7,10 However, it is noteworthy that, for the preparation of these porous polymers, the conventional HIPEs are commonly stabilized against coalescence by large amounts (5-50 vol %) of small molecule surfactants.8,11,12 From application points of view, the sheer quantity of surfactants required to stabilize HIPEs both limits properties and is very expensive. In addition to surfactants, the internal droplet phase of emulsions has also been stabilized by colloidal particles adsorbed at the liquid interfaces, known as particle-stabilized (Pickering) emulsions.13-16 One interesting advantage of the extension of colloidal particles as stabilizers to HIPEs is that these particles can be irreversibly adsorbed at the interface of emulsions because of their high energy of attachment, which makes the final emulsions extremely stable with shelf life stabilities of months or even years.15 On the other hand, production of porous materials from particle-stabilized HIPE templating may lead to other benefits; for example, the pore walls of the final materials will be packed with a layer of particles which may contain functional groups and lead to a variety of further applications.16,17 Macroporous polymers with pore sizes in the range of 100-400 μm have been prepared by polymerizing the continuous phase of HIPEs stabilized with carbon nanotubes,16,18 silica,19,20 and titania nanoparticles,21 (10) Cameron, N. R.; Sherrington, D. C. J. Mater. Chem. 1997, 7, 2209. (11) Haibach, K.; Menner, A.; Powell, R.; Bismarck, A. Polymer 2006, 47, 4513. (12) Barbetta, A.; Cameron, N. R. Macromolecules 2004, 37, 3202. (13) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (14) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (15) Aveyard, R.; Binks, B. P.; Clint, J. H. Adv. Colloid Interface Sci. 2003, 100-102, 503. (16) Zhang, S. M.; Chen, J. D. Chem. Commun. 2009, 2217. (17) Hermant, M. C.; Klumperman, B.; Koning, C. E. Chem. Commun. 2009, 2738. (18) Menner, A.; Verdejo, R.; Shaffer, M.; Bismarck, A. Langmuir 2007, 23, 2398. (19) Binks, B. P. Adv. Mater. 2002, 14, 1824. (20) Ikem, V. O.; Menner, A.; Bismarck, A. Angew. Chem., Int. Ed. 2008, 47, 8277. (21) Menner, A.; Ikem, V.; Salgueiro, M.; Shaffer, M. S. P.; Bismarck, A. Chem. Commun. 2007, 4274.
Published on Web 11/04/2009
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followed by evaporation of the dispersed phase. Note that some of the previous reports on particle-stabilized HIPEs deal with emulsions whose volume fractions of the dispersed phase are less than 0.70.18,22,23 Experimentally, Binks and co-workers24,25 have indicated that particle-stabilized emulsions phase-invert between volume fractions 0.65 and 0.70, which means the majority phase becomes the continuous phase. Moreover, in the production of porous materials by polymerization, the continuous phase requires additional strict control over the reaction chemistry, which can be challenging. We recently developed a simple and effective method to prepare macroporous polymers by templating emulsions stabilized with poly(N-isopropylacrylamide) (PNIPAM)-based microgel particles.26 In that case, stable emulsions with an internal phase up to 90 vol % can be stabilized by soft microgels alone, that is, in the absence of surfactants. Here, we extend this approach for the fabrication of macroporous polymers from emulsions stabilized by poly(styrene-co-N-isopropylacrylamide) (PS-co-PNIPAM) core-shell particles. The method relies on the adsorption of particles at the oil-water liquid interface for the formation of stable highly concentrated emulsions that are subsequently used as templates for the preparation of porous polymers. More importantly, excess particles can simultaneously form a gel in the continuous phase, which allows drying of the particle-stabilized emulsions directly into macroporous polymers without involving of any chemical reactions.
Experimental Section Materials. N-Isopropylacrylamide (NIPAM, Sigma Aldrich) was recrystallized from a toluene/n-hexane mixture. N,N’Methylenebis(acrylamide) (MBAA, Fluka), methacrylic acid (MAA, Sigma Aldrich), potassium persulfate (KPS, Merck), styrene (Sigma), and hexane were used as received without any purification. The fluorescent dye methacryloxyethylthiocarbamoyl rhodamine B (MRB, Polysciences, Inc.) and pyrene (Aldrich) were used as received. Deionized water was used in all the experiments. Particle Preparation. Poly(styrene-co-N-isopropylacrylamide) (PS-co-NIPAM) particles were synthesized using a one-pot, surfactant-free emulsion polymerization adapted from the syntheses reported by the Hellweg group.27 Typically, 1.0363 g of NIPAM, 8.4660 g of styrene, 0.0342 g of MBAA, 0.0434 g of MAA, and 0.0009 g of MRB were dissolved into 140 mL of deionized water in a 250 mL two-neck reactor fitted with a nitrogen bubbling inlet and outlet and a reflux condenser. Then the solution mixture was adjusted to pH 10 with sodium hydroxide solution. After stirring the solution for 40 min at 25 °C under nitrogen bubbling, the polymerization was initiated by adding 0.1130 g of KPS dissolved in 10 mL of deionized water. The reaction mixture was kept at 70 °C for 9 h. Thereafter, the final solution was cooled slowly to room temperature under continued stirring. The resultant particles were cleaned by centrifugation three times at 10 000 rpm for about 1 h at room temperature to remove any unreacted monomers or oligomeric species. After that, the particles were freeze-dried and redispersed in deionized water. Note that the final PS-co-NIPAM particles were labeled by copolymerization with 0.01 wt % MRB for easier identification with confocal microscopy. Particle-Stabilized Emulsion Preparation. Particle-stabilized emulsions were prepared by mixing the particle dispersions (22) (23) (24) (25) (26) 8490. (27) 4330.
Menner, A.; Powell, R.; Bismarck, A. Macromolecules 2006, 39, 2034. Menner, A.; Powell, R.; Bismarck, A. Soft Matter 2006, 2, 337. Binks, B. P.; Lumdson, S. O. Langmuir 2000, 16, 2539. Binks, B. P.; Rodrigues, J. A. Angew. Chem., Int. Ed. 2005, 44, 441. Li, Z. F.; Ming, T.; Wang, J. F.; Ngai, T. Angew. Chem., Int. Ed. 2009, 48, Hellweg, T.; Dewhurst, C. D.; Eimer, W.; Kratz, K. Langmuir 2004, 20,
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Figure 1. Typical hydrodynamic radius distribution f(Rh) of synthesized PS-co-PNIPAM particles in an aqueous dispersion as determined by dynamic laser light scattering. Inset shows the TEM image of the synthesized PS-co-PNIPAM particles after drying. with the proper amount of oils and then homogenizing the mixture using an Ultra Turrax T25 homogenizer (1 cm head) operating at 13 500 rpm for 30 s. The total emulsion volume was kept at 5 mL. Stock solutions of PS-co-NIPAM particles at the desired solid content were prepared by dilution. In some cases, a dye molecule, pyrene, was added to the oil phase to check the type of the formed emulsions. All the emulsions were stable for at least 3 months. After emulsification, PS-co-PNIPAM particle-stabilized emulsions were casted on glass and dried in air at room temperature for 24 h to remove oil and water. Physical Measurements. The size of the PS-co-NIPAM particles were measured using laser light scattering (LLS) at an angle of 20°. The apparatus used for LLS measurements was a modified commercial light-scattering spectrometer equipped with an ALV-5000 multi-τ digital time correlator and a He-Ne laser (output power = 22 mW at λ0 = 632 nm). The measurable angular range is 15-155°. In dynamic laser light scattering (DLS), the intensity-intensity time correlation function G(2)(τ) in the self-beating mode was measured in the scattering angle range 17.5-150°. The Laplace inversion of G(2)(τ) can lead to a line-width distribution G(Γ), which can be further converted to a translational diffusive coefficient distribution G(D) by Γ = Dq2 or a hydrodynamic radius distribution f(Rh) by use of the Stokes-Einstein equation, Rh = kBT/ 6πηD, where η, kB, and T are the solvent viscosity, the Boltzmann constant, and the absolute temperature, respectively.28,29 For TEM observation, the PS-co-NIPAM particles were deposited on carbon-film-coated copper grids and dried at room temperature for 24 h before imaging on a FEI CM120 microscope operating at 120 kV. The confocal microscopy images of the particle-stabilized emulsions were taken on a Nikon Eclipse Ti inverted microscope (Nikon, Japan). Lasers with wavelengths of 543 and 408 nm were used to excite the fluorescent PS-co-NIPAM particles and pyrene molecules, respectively. An oil immersion objective (60, NA = 1.49) was used to view the samples. The particle-stabilized emulsions were placed on the cover slides, and a series of x/y layers were scanned. The temperature was kept at 25 °C. For SEM observation, the prepared particle-stabilized emulsions were dried at room temperature for 24 h and then coated with Au before imaging on a FEI Quanta 400 FEG microscope operating at 10 kV. Rheological tests were performed with a TA Instruments (AR1000) apparatus to obtain the storage modulus (G0 ) and loss modulus (G00 ). G0 and G00 were measured by varying the shear stress from 0.99 to 600 Pa. The frequency was kept at 1 Hz, and temperature was fixed at 21 °C. (28) Chu, B. Laser light scattering: basic principles and practice; Academic Press: Boston, 1991. (29) Berne, B. J.; Pecora, R. Dynamic light scattering: with applications to chemistry, biology, and physics; Dover Publications: New York, 2000.
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Figure 2. (a) Appearance of the formed highly concentrated emulsion after shearing a mixture containing 4 mL of hexane oil with 1 mL of PS-co-PNIPAM dispersion (5 wt % particles). (b, c) Confocal images of the prepared emulsion stabilized by PS-co-PNIPAM particles excited by lasers with wavelengths (b) 543 and (c) 408 nm.
Figure 3. Schematic showing the spontaneous formation of a gel emulsion stabilized by PS-co-PNIPAM core-shell particles.
Results and Discussion PS-co-PNIPAM core-shell particles were prepared using a one-pot, surfactant-free emulsion polymerization adapted from the syntheses reported by the Hellweg group.27 For imaging by confocal microscopy, the particles were also labeled by copolymerization with fluorescent dye, MRB. The hydrodynamic radius (Rh) of the prepared particles was around 166 nm as determined by dynamic LLS (Figure 1). The morphology of the dried PS-coPNIPAM particles was also imaged by transmission electron microscopy (TEM). The inset picture of Figure 1 confirms that the prepared particles are spherical and with PS as the core and PNIPAM as the shell, which are consistent with the previous reports.27,30 It is well-known that, with heating, PNIPAM undergoes a reversible discontinuous phase transition in water, switching from hydrophilic to hydrophobic.31 However, no significant shrinkage of the particles with increasing temperature can be observed for our prepared core-shell particles. It can be attributed to the fact that the synthesized PS-co-PNIPAM core-shell particles used in this study are composed of a high content of styrene monomers, about 88.4% of the total mass. It has been shown by Hellweg et al.27 that, with copolymerization of the styrene content greater than 75%, the final core-shell particles showed no thermoresponsive behavior. Therefore, the addition of NIPAM to the synthesis only adds soft, hydrogel-like qualities (30) McGrath, J. G.; Bock, R. D.; Cathcart, M. J.; Lyon, L. A. Chem. Mater. 2007, 19, 1584. (31) Wu, C. Polymer 1998, 39, 4609.
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Figure 4. Stress dependence of storage and loss moduli (G0 and G00 ) of the emulsion stabilized by PS-co-PNIPAM particles at room temperature. Applied frequency was f = 1 Hz.
that provide particle flexibility but not thermoresponsiveness. This distinctive characteristic appears to be a significant factor in the preparation of emulsions with a gelled continuous phase preventing the inversion of emulsions at high internal phase volume fractions as discussed below. A particle-stabilized o/w emulsion was formed by mechanically shearing a mixture containing the nonpolar oil (4 mL), hexane, and an aqueous dispersion of PS-co-PNIPAM particles (1 mL, 5 wt % fluorescent particles) for 30s with Ultra Turrax T25 homogenizer (10 mm head) operating at 13 500 rpm. The appearance of the emulsion is shown in Figure 2a. Note that the amount of the dispersed phase is as high as 80 vol %, suggesting that the final emulsion is a HIPE. The emulsion stabilized by PS-coPNIPAM particles was very stable, and the relative phase volume did not change for more than 3 months. The confocal image (Figure 2b) indicates that the remarkable stability of the emulsion attributes to the strong adsorption of fluorescent PS-co-PNIPAM particles at the interface acting as a barrier against oil droplet coalescence. Additionally, the pyrene-loaded emulsion (Figure 2c) confirms the presence of polydisperse hexane oil droplets, whose size varies from around several micrometers to tens of micrometers. On the other hand, it is interesting to note that no flow of emulsion was observed even though the vial was inverted Langmuir 2010, 26(7), 5088–5092
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Figure 5. Microstructures of macroporous materials prepared from a hexane-in-water emulsion containing 80% internal phase volume of oil and 5 wt % PS-co-PNIPAM particles in the initial aqueous dispersion.
(Figure 2a), indicating that the formed emulsion is a typical “gel emulsion”. Generally, the gel emulsions are produced using a twostep process, in which oil is first dispersed in the continuous phase that is then gelled to trap the droplets in the gel matrix.32,33 The preparation commonly combines with a sol-gel process. Hence, the single step approach presented here that does not involve any chemical reactions would be easily extended to a wide variety of systems. It is worthy to mention that, for mechanical shearing of only the pure PS-co-PNIPAM particles with concentration of 5 wt %, no gel can be observed. The shearing of particles with a low internal phase volume of oil (less than 60%) leads to conventional emulsions with creaming. Therefore, we anticipated that the formation of excellent stability of the gel emulsion was not only due to adsorbed interfacial particles, but the excess particles in the continuous phase also played an important role. Because of the high internal phase of the oil that provides a barrier effect, the excess particles, which could not move into the oil phase, were confined in the continuous phase. We conjecture that the application of shearing forced the excess PS-co-PNIPAM particles with the extended PNIAPM chains to spend more time in proximity to some neighboring particles and provided the opportunity for interparticle association, leading to a physical gel in the continuous phase to trap the oil droplets in the gel matrix, as schematically shown in Figure 3. For pure PNIPAM microgel particles, Howe et al. recently indicated that, under shearing stress, the chains of microgels easily interacted with neighboring chains from other microgels, resulting in entanglement and aggregation.34 Horozov et al. have also demonstrated a similar approach to increase the stability of emulsions. By increasing the electrolyte concentration and a proper adjustment of the particle concentration of the particle-stabilized emulsions, a viscoelastic three-dimensional network of interconnected particles can be formed in the continues phase to reduce the emulsion creaming and coalescence.35 Figure 4 shows the dynamic mechanical analysis of the emulsion stabilized by PS-co-PNIPAM particles. It can be seen that, in the whole range of rheological measurement, the storage modulus (G0 ) is larger than the loss modulus (G00 ). This indicates that the elastic response dominates due to the formation of the physical gel network, particularly when the shear stress is less than 30 Pa. It corresponds well to the picture shown in Figure 2a that the (32) Perrin, P.; Lafuma, F. J. Colloid Interface Sci. 1998, 197, 317. (33) Dickinson, E. Colloids Surf., A. 2006, 288, 3. (34) Howe, A. M.; Desrousseaux, S.; Lunel, L. S.; Tavacoli, J.; Yow, H. N.; Routh, A. F. Adv. Colloid Interface Sci. 2009, 147-148, 124. (35) Horozov, T. S.; Binks, B. P.; Gottschalk-Gaudig, T. Phys. Chem. Chem. Phys. 2007, 9, 6398.
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formed emulsion was accompanied by a gelled continuous phase so that no flow of emulsion was observed even though the vial was inverted for more than 30 s. However, once the shear stress was increased to surpass 30 Pa, both G0 and G00 decreased significantly, indicating that high shear stress could break the gel emulsions and finally change the solidlike emulsions to liquidlike emulsions. After emulsification, PS-co-PNIPAM particle-stabilized emulsion with the internal phase of 80 vol % was dried in air to remove oil and water. The corresponding SEM images are depicted in Figure 5. Note that no chemical reactions are involved before the oil removal. The evaporation in air directly leads to macroporous materials. The average cavity sizes in the porous structures are comparable to the oil droplet diameters of the precursor emulsions, indicating that these cavities result from a loss of the oil component. More importantly, the high-magnification image (Figure 5b) shows that the pore walls and surfaces of the resulting membranes are densely decorated with the PS-co-PNIPAM particles, which add a great flexibility to functionalize or enhance the surface roughness of the porous materials for a variety of applications in the future. So far, the functionalization or modification of porous materials has been often achieved by the direct incorporation of second monomers or additives in the precursor emulsion, which in some cases is followed by a postmodification step.6,36 This approach, however, has strict limitations, since the stability of the HIPEs is a delicate hydrophobic/hydrophilic balance; that is, for every change of monomer composition, the process conditions have to be optimized. In particular, hydrophilic functional monomers are very difficult to incorporate as they destabilize the emulsion. Undoubtedly, particle-stabilized emulsion templating provides a facile methodology to prepare functional pore materials because any additional properties from the particles can be directly imparted to the final composite materials.
Conclusions In summary, using the core-shell PS-co-PNIPAM particles as particulate emulsifiers, we are able to prepare a highly concentrated emulsion with an internal phase up to 80 vol %. The stability of such A concentrated emulsion can be attributed not only to the strong adsorption of particles at the interface acting as a barrier against oil droplet coalescence, but also to the excess particles simultaneously forming a gel in the continuous phase to separate the droplets in the gel matrix. The creaming of the (36) Pierre, S. J.; Thies, J. C.; Dureault, A.; Cameron, N. R.; van Hest, J. C. M.; Carette, N.; Michon, T.; Weberskirch, R. Adv. Mater. 2006, 18, 1822.
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emulsion thus can be “switched off”, and the resulting emulsions have a longer shelf life. In addition, upon drying in air, the particle-stabilized emulsions can directly lead to porous structures without involving any chemical reactions. The pore walls of the final structures are densely packed with layers of core-shell particles. This adds a great flexibility to tune the surface properties for specific applications in the future. We believe that the porous materials derived from such particle-stabilized emulsions should have a great potential for applications in chemical and biological separations.
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Acknowledgment. We would like to thank Mr. T. Ming from the Department of Physics, the Chinese University of Hong Kong, for help with the SEM and TEM pictures and Mr. F. Hong and Mr. L. Y. Xie from the Department of Chemical Physics, University of Science and Technology of China, for the help of rheological testing. The financial support of this work by the Hong Kong Special Administration Region (HKSAR) General Research Fund (CUHK402707, 2160324) and the Direct Grant for Research 2007/08 of the Chinese University of Hong Kong (CUHK 2060338) is gratefully acknowledged.
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