Langmuir 2004, 20, 11329-11335
11329
Synthesis of Polystyrene/Poly[2-(Dimethylamino)ethyl Methacrylate-stat-Ethylene Glycol Dimethacrylate] Core-Shell Latex Particles by Seeded Emulsion Polymerization and Their Application as Stimulus-Responsive Particulate Emulsifiers for Oil-in-Water Emulsions S. Fujii, D. P. Randall, and S. P. Armes* Department of Chemistry, School of Life Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ, U.K. Received June 21, 2004. In Final Form: October 1, 2004 Surfactant-stabilized polystyrene (PS) latex particles with a mean hydrodynamic diameter of 155 nm were prepared by aqueous emulsion polymerization using 2,2′-azobis(2-amidinopropane) hydrochloride as a cationic radical initiator. Seeded aqueous emulsion copolymerizations of 2-(dimethylamino)ethyl methacrylate (DMA) and ethylene glycol dimethacrylate (EGDMA) were conducted in the presence of these PS particles to produce two batches of colloidally stable core-shell latex particles, in which the shell comprised a cross-linked P(DMA-stat-EGDMA) overlayer. Both the PS and PS/P(DMA-stat-EGDMA) latexes were characterized in terms of their particle size, morphology, and composition using dynamic light scattering, electron microscopy, and FT-IR spectroscopy, respectively. Using the PS/P(DMA-stat-EGDMA) latex particles as a pH-responsive particulate (‘Pickering’-type) emulsifier, polydisperse n-dodecane-in-water emulsions were prepared at pH 8 that could be partially broken (demulsified) on lowering the solution pH to 3. These emulsions were characterized in terms of their emulsion type, mean droplet diameter, and morphology using electrical conductivity and Mastersizer measurements, optical microscopy, and scanning electron microscopy (using critical point drying for sample preparation).
Introduction Recently there have been numerous studies on polymers that are responsive to external stimuli such as temperature,1 electric fields,2 and pH3 for various applications ranging from drug delivery to diagnostics and from separations to robotics.1-3 Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermo-responsive watersoluble polymer that exhibits an unusually sharp phase transition at 32 °C.4 Kawaguchi et al. reported the synthesis of PNIPAM microgel and showed that these thermo-sensitive particles were suitable carriers for biomedically relevant biomacromolecules.5 Makino et al. prepared ‘core-shell’ particles by seeded emulsion copolymerization comprising a cross-linked shell of N-isopropylacrylamide (NIPAM) and N,N′-methylenebisacrylamide and a core of submicrometer-sized poly(styreneNIPAM) copolymer particles. These composite particles were also demonstrated to be thermo-responsive.6 In 1995 Okubo and Ahmad reported7 that aqueous emulsion copolymerization of 2-(dimethylamino)ethyl * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) See, for example: (a) Tanaka, T. Polymer 1979, 20, 1404. (b) Hoffman, A. S. J. Controlled Release 1987, 6, 297. (2) See, for example: (a) Kwon, I. K.; Bae, Y. H.; Kim, S. W. Nature 1991, 354, 291. (b) Kishi, R.; Hara, M.; Sawahata, K.; Osada, Y. In Polymer Gels; DeRossi, D., et al., Eds.; Plenum: New York, 1991; p 205. (3) See, for example: (a) Siegel, R. A.; Firestone, B. A. Macromolecules 1988, 21, 3254. (b) Peppas, N. A.; Buri, P. A. J. Controlled Release 1985, 2, 257. (4) Heskins, M.; Guillet, J. E. J. Macromol Sci. Chem. 1968, A2, 1441. (5) Kawaguchi, H.; Fujimoto, K.; Mizuhara, Y. Colloid Polym. Sci. 1992, 270, 53. (6) Makino, K.; Yamamoto, S.; Fujimoto K.; Kawaguchi, H.; Ohshima, H. J. Colloid Interface Sci. 1994, 166, 251. (7) Okubo, M.; Ahmad, H. Colloid Polym. Sci. 1995, 273, 817.
methacrylate (DMA) with ethylene glycol dimethacrylate (EGDMA) in the presence of PS seed particles led to submicrometer-sized temperature-sensitive polystyrene/ poly[2-(dimethylamino)ethyl methacrylate-stat-ethylene glycol dimethacrylate] [PS/P(DMA-stat-EGDMA)] ‘coreshell’ latex particles. Moreover, it was shown that the adsorption and desorption of proteins onto these latexes could be controlled by adjusting the solution temperature either above or below the cloud point of the cross-linked thermo-responsive shell layer.8-11 Recently Amalvy et al.12 reported that submicrometersized PS latex prepared by dispersion polymerization using a PDMA-poly(methyl methacrylate) diblock copolymer stabilizer could be used as a particulate emulsifier to prepare ‘Pickering’-type13 oil-in-water emulsions at pH 8. However, stable emulsions were not obtained using the same particles at pH 2-3. This was attributed to protonation of the PDMA chains, which no longer wet the oil interface in their cationic polyelectrolyte form. Thus it is clear that, under appropriate conditions, PDMA can act as a pH-sensitive polymer, as well as a thermo-responsive polymer. In the present study, seeded emulsion copolymerization was used to produce PS/P(DMA-stat-EGDMA) core-shell (8) Okubo, M.; Ahmad, H. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 3146. (9) Okubo, M.; Ahmad, H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 883. (10) Okubo, M.; Ahmad, H. Colloids Surf., A 1999, 153, 429. (11) Ahmad, H.; Okubo, M.; Kamatari, Y. O.; Minami, H. Colloid Polym. Sci. 2002, 280, 310. (12) (a) Amalvy, J. I.; Armes, S. P.; Binks, B. P.; Rodrigues, J. A.; Unali, G.-F. Chem. Commun. 2003, 15, 1826; (b) Amalvy, J. I.; Unali, G.-F.; Li, Y.; Granger-Bevan, S.; Armes, S. P.; Binks, B. P.; Rodrigues, J. A.; Whitby, C. P. Langmuir 2004, 20, 4345. (13) (a) Ramsden, W. Proc. R. Soc. 1903, 72, 156. (b) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001.
10.1021/la048473x CCC: $27.50 © 2004 American Chemical Society Published on Web 11/19/2004
11330
Langmuir, Vol. 20, No. 26, 2004
Fujii et al.
Figure 1. Schematic representation of the pH-modulated emulsification of n-dodecane using core-shell latex particles [the core is polystyrene and the shell is a cross-linked copolymer of 2-(dimethylamino)ethyl methacrylate and ethylene glycol dimethacrylate as a particulate emulsifier]. At low pH, the shell is protonated and acquires cationic microgel character and no emulsion is obtained because of the poor affinity between the cationic shell and the oil phase. At high pH, the shell is neutral, which allows the particles to absorb at the oil-water interface and hence promote emulsion formation. This emulsion could be partially demulsified on lowering the solution pH from pH 8-9 to pH 2-3 with mild mechanical agitation.
latex particles similar to those described by Okubo and Ahmad.7 The application of these core-shell latexes as pH-responsive particulate emulsifiers for the preparation of oil-in-water emulsions was investigated (Figure 1). In principle, such core-shell latexes are much easier to prepare than the diblock copolymer-stabilized PS latexes described by Amalvy et al. This is because the PDMAbased diblock copolymer stabilizer was synthesized by group transfer polymerization, which requires rigorously anhydrous conditions and a relatively expensive silyl ketene acetal initiator.14 Moreover, the core-shell latex synthesis used in the present study is conducted in purely aqueous media, whereas the dispersion polymerization technique used by Amalvy et al. required a methanol cosolvent. Furthermore, the surface concentration of PDMA chains on the core-shell latexes should be relatively high, and a cross-linked shell should be more robust than a merely physically adsorbed diblock copolymer stabilizer. In summary, the synthesis of pH-responsive latex-based emulsifiers on an industrial scale is much more likely to be based on aqueous emulsion polymerization than on the protocol described by Amalvy et al.
Table 1. Formulations Used for the Syntheses of PS/ P(DMA-stat-EGDMA) Core-Shell Latex Particles by Seeded Emulsion Polymerizationa with AIBA Initiator in the Presence of Tween 80 Emulsifierd total shell content of composite particles ingredients PS emulsionb Tween 80 DMA EGDMAc AIBA water
(g) (g) (g) (mg) (g) (g)
10 wt%
20 wt%
9.34 0.04 0.095 5 0.18 44.7
9.34 0.04 0.212 11 0.18 44.7
a 60 °C, 8 h, N , 250 rpm (a magnetic stir bar). b Solid content, 2 9.56 wt%; PS seed particles, 0.893 g. c EGDMA was added to prevent the production of water-soluble PDMA homopolymer and its content is 5 wt% based on DMA. d Abbreviations: PS ) polystyrene; DMA ) 2-(dimethylamino)ethyl methacrylate; EGDMA ) ethylene glycol dimethacrylate; AIBA ) 2,2′-azobis(2-amidino-propane) hydrochloride; Tween 80 ) poly(oxyethylene) sorbitan monooleate.
Materials. Styrene, DMA, and EGDMA (Aldrich) were each treated in turn with basic alumina in order to remove inhibitor and were stored at -20 °C before use. Polyoxyethylene sorbitan monooleate (Tween 80), ethanol, and n-dodecane were used as received. 2,2′-Azobis(2-amidinopropane) hydrochloride (AIBA) was recrystallized from water prior to use. Doubly distilled deionized water was used in all the polymerizations. Ultra-clean grade carbon dioxide (purity > 99.999%) was purchased from BOC Edwards. Synthesis of PS Latex by Emulsion Polymerization. Emulsion polymerizations of styrene were performed in batch mode at 60 °C. A typical synthetic procedure was as follows: Tween 80 (53 mg) and styrene (10.65 g) were added in turn to an aqueous medium in a three-necked 100 mL flask fitted with a reflux condenser and a magnetic stirrer. This reaction mixture was vigorously stirred at 60 °C and was degassed using a nitrogen purge. The polymerization commenced after injection of an aqueous solution of AIBA initiator (43 mg of AIBA in 5 mL of water) into the reaction vessel and was allowed to proceed for 24 h with continuous stirring at 250 rpm under a nitrogen atmosphere. Synthesis of PS/P(DMA-stat-EGDM) Latex by Seeded Emulsion Polymerization. Emulsion copolymerization of DMA and EGDMA was performed in the presence of PS seed particles in batch mode at 60 °C under the conditions presented in Table 1. The synthetic protocol was as follows: PS latex (0.893 g; solids content 9.6 wt%) was added to a solution of Tween 80 (40 mg; optional), DMA, and EGDMA in doubly distilled water (total
water volume, 43 mL) at room temperature, and the solution was stirred at 250 rpm for 18 h to allow the PS seed particles to become swollen with DMA and EGDMA. The polymerization was initiated at 60 °C by injecting an aqueous solution of AIBA (0.18 g of AIBA in 7 mL of water) into this stirred solution and allowed to proceed for 8 h under a nitrogen atmosphere. Latex Purification. Serum replacement15 (ultrafiltration) was used to eliminate excess stabilizer (and trace initiator) in order to purify the PS seed particles. Ultrafiltration was performed using a Molecular/Por stirred Spectrum cell (400 mL) by replacing the serum with water; this serum was periodically collected for monitoring (see below). The PS/P(DMA-statEGDMA) particles were purified by centrifugation-redispersion cycles, with each successive supernatant being decanted and replaced with pure water. The extent of purification of each latex was assessed by measuring the surface tension of either the serum or the supernatant. Purification was continued until this surface tension was close to that of water at 22 °C (70-72 mN m-1). Latex Characterization. Dynamic Light Scattering (DLS) Studies. Measurements were made at 20 °C using a Brookhaven Instruments Corp. BI-200SM goniometer equipped with a BI9000AT digital correlator using a solid-state laser (125 mW, λ ) 532 nm) at a fixed scattering angle of 90°. Intensity-average particle diameters (Dz) and polydispersities were calculated by cumulants analysis of the experimental correlation function using the Stokes-Einstein equation for dilute, noninteracting monodisperse spheres. Transmission Electron Microscopy (TEM). TEM studies were carried out using a Hitachi-7100 TEM with an axially mounted Gatan Ultrascan 1000 CCD camera operating at 100 kV. Samples were prepared by drying one drop of latex onto a carbon-filmcoated copper grid.
(14) Webster, O. W. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2855.
(15) Ahmed, S. M.; El-Aasser, M. S.; Micale, F. J.; Poehlein, G. W.; Vanderhoff, J. W. Org. Coat. Plast. Chem. 1980, 43, 120.
Experimental Section.
Synthesis and Applications of Core-Shell Latex Particles
Langmuir, Vol. 20, No. 26, 2004 11331
Figure 2. TEM images of: (a) PS seed latex particles, (b) 20 wt% shell PS/P(DMA-stat-EGDMA) composite particles produced by emulsion and seeded emulsion polymerizations, respectively. Fourier Transform Infrared (FT-IR) Spectroscopy. FT-IR spectroscopy studies were carried out on samples dispersed in KBr disks using a Nicolet Magna 550 IR spectrometer Series II (typically 100 scans per spectrum were accumulated at 4 cm-1 resolution). Emulsion Synthesis. Stock solutions of the latexes (1 wt% solids content) were prepared by serial dilution. The solution pH of 5 mL aliquots of the diluted latexes was adjusted as required by adding a few drops of concentrated HCl or NaOH solution, using a pH meter to monitor the solution pH. The latex dispersions were then homogenized at 20 °C with 5 mL of n-dodecane for 2 min using an IKA Ultra-Turrax T-18 homogenizer with a 10 mm dispersing tool operating at 12 000 rpm. We elected to use n-dodecane as a ‘model’ oil in these experiments. This compound was chosen because it is highly nonpolar, nonvolatile, and relatively inexpensive. Emulsion stabilities after standing for 24 h at room temperature (around 20 °C) were assessed by visual inspection. In some cases, emulsion stabilities were assessed more accurately using graduated vessels by monitoring the movement of the oilemulsion and emulsion-water interfaces with time. Emulsion Characterization. Conductivity Measurements. The conductivity of the emulsions immediately after preparation was measured using a digital conductivity meter (Hanna model Primo 5). The emulsions were classified according to their conductivities. A high conductivity indicated an oil-in-water emulsion and a low (immeasurable) conductivity (
