Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of

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Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization Syed Imran Ali,† Johan P. A. Heuts,*,† Brian S. Hawkett,‡ and Alex M. van Herk*,† †

Laboratory of Polymer Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands, and ‡ Key Centre for Polymers and Colloids, School of Chemistry, University of Sydney, Sydney, Australia Received April 9, 2009. Revised Manuscript Received May 20, 2009

Anisotropic polymer-inorganic composite latex particles were synthesized by using a RAFT-based encapsulation approach on cationic gibbsite platelets. By using the RAFT agent dibenzyl trithiocarbonate, a series of amphipatic living random RAFT copolymers with different combinations of acrylic acid and butyl acrylate units were synthesized. These RAFT copolymers were used as living stabilizers for the gibbsite platelets and chain extended to form a polymeric shell by starved feed emulsion polymerization. Cryo-TEM characterization of the resulting composite latexes demonstrates the formation of anisotropic composite latex particles with mostly one platelet per particle. Monomer feed composition, chain length, and hydrophilic-lipophilic balance of the RAFT copolymer were found to be important factors for the overall efficiency of the encapsulation. Good control over platelet orientation and high encapsulation efficiency were achieved via this route.

Introduction Polymer-clay nanocomposites are an important type of materials which offer significantly improved properties as compared to the corresponding pure polymers.1-3 Recently these materials have gained attention in coating technology because of their significantly improved properties such as better hardness, enhanced mechanical strength, excellent gas barrier properties, scratch resistance, and superior optical and thermal properties for a variety of applications.4-6 These properties are very important for coatings and would be desirable in soft film forming latexes for coating applications. Although emulsion/miniemulsion polymerizations in the presence of spherical inorganic particles, such as titanium oxide, carbon black, silica, and some other pigment particles, have *To whom correspondence should be addressed. E-mail: j.p.a.heuts@tue. nl (J.P.A.H) and [email protected] (A.M.v.H). (1) LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J. Polymer-layered silicate nanocomposites: an overview. Appl. Clay Sci. 1999, 15, 11-29. (2) Alexandre, M.; Dubois, P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater. Sci. Eng., R 2000, 28, 1-63. (3) Kazuhisa Yano, A. U.; Okada, A.; Kurauchi, T.; Kamigaito, O. Synthesis and properties of polyimide-clay hybrid. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2493-2498. (4) Majumdar, D.; Blanton, T. N.; Schwark, D. W. Clay-polymer nanocomposite coatings for imaging application. Appl. Clay Sci. 2003, 23, 265-273. (5) Oh, T. K.; Hassan, M.; Beatty, C.; El-Shall, H. The effect of shear forces on the microstructure and mechanical properties of epoxy-clay nanocomposites. J. Appl. Polym. Sci. 2006, 100, 3465-3473. (6) Sugama, T. Polyphenylenesulfied/montomorillonite clay nanocomposite coatings: Their efficacy in protecting steel against corrosion. Mater. Lett. 2006, 60, 2700-2706. (7) Bechthold, N.; Tiarks, F.; Willert, M.; Landfester, K.; Antonietti, M. Miniemulsion polymerization: Applications and new materials. Macromol. Symp. 2000, 151, 549-555. (8) Erdem, B.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. Encapsulation of inorganic particles via miniemulsion polymerization. I. Dispersion of titanium dioxide particles in organic media using OLOA 370 as stabilizer. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4419-4430. (9) Haga, Y.; Watanabe, T.; Yosomiya, R. Encapsulating Polymerization of Titanium-Dioxide. Angew. Makromol. Chem. 1991, 189, 23-34. (10) Yang, Y.; Kong, X. Z.; Kan, C. Y.; Sun, C. G. Encapsulation of calcium carbonate by styrene polymerization. Polym. Adv. Technol. 1999, 10, 54-59.

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resulted in their encapsulation,7-12 the encapsulation of clays appears to be very challenging. Emulsion polymerization in the presence of unmodified clay platelets has resulted almost always in the formation of so-called armored latex particles13 (platelets being located at the particle surface). Several researchers13-16 have used conventional emulsion polymerization in the presence of face-modified clay platelets to prepare polymer-clay hybrid latex particles but again one ends up producing the armored morphology. Thus far, the only true encapsulation of clay platelets was achieved by utilizing edge modification resulting in dumbbell or peanut-shaped particles.17 The disk shape morphology, large aspect ratio, and high surface energy of the clay pose a challenge for their encapsulation attempts.17 It is very difficult to form an inherently lower energy state of a polymer layer around the platelet. There is a need to explore more feasible routes such as controlled polymer growth from the surface of the platelet, using, for instance, controlled radical polymerization techniques.18 Besides the true encapsulation of the single clay platelets, control over their orientation in the final polymeric film is also (11) Tiarks, F.; Landfester, K.; Anonietti, M., Encapsulation of carbon black by miniemulsion polymerization. Macromol. Chem. Phys. 2001, 202, 51-60. (12) Viala, P.; Bourgeat-Lamy, E.; Guyot, A.; Legrand, P.; Lefebvre, D. Pigment encapsulation by Emulsion Polymerisation, redespersible in water. Macromol. Symp. 2002, 187, 651-661. (13) Negrete-Herrera, N.; Putaux, J. L.; David, L.; De Haas, F.; Bourgeat-Lami, E. Polymer/Laponite composite latexes: Particle morphology, film microstructure, and properties. Macromol. Rapid Commun. 2007, 28, 1567-1573. (14) Cauvin, S.; Colver, P. J.; Bon, S. A. F. Pickering stabilized miniemulsion polymerization: Preparation of clay armored latexes. Macromolecules 2005, 38, 7887-7889. (15) Putlitz, B. Z.; Landfester, K.; Fischer, H.; Antonietti, M. The generation of “armored latexes” and hollow inorganic shells made of clay sheets by templating cationic miniemulsions and latexes. Adv. Mater. 2001, 13, 500. (16) Bourgeat-Lami, E. Organic-inorganic nanostructured colloids. J. Nanosci. Nanotechnol. 2002, 2, 1-24. (17) Voorn, D. J.; Ming, W.; van Herk, A. M. Clay platelets encapsulated inside latex particles. Macromolecules 2006, 39, 4654-4656. (18) Samakande, A.; Sanderson, R. D.; Hartmann, P. C. Encapsulated Clay Particles in Polystyrene by RAFT Mediated Miniemulsion Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 7114-7126.

Published on Web 06/17/2009

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Scheme 1. Expected Orientation of the Platelets in the Final Film with Use of Latex Particles of Different Shapes: (a) Spherical Particles (b) Dumbbell or Peanut-Shaped Particles, and (c) Flat Particlesa

a

Control over the particle morphology will enable the control over the orientation of the platelets during film formation.

Scheme 2. Dibenzyl Trithiocarbonate (DBTTC)

micrographs of the resulting composite latexes clearly demonstrate the formation of anisotropic composite latex particles with mostly one platelet per particle and a controllable shell thickness.

Experimental Section of importance for the final coating properties. Encapsulating single platelets with a layer of polymer, producing anisotropic, preferably plate-like flat composite latex particles would be a first step toward this goal. These flat particles are likely to induce anisotropy into the final film (see Scheme 1) potentially improving the barrier properties and the scratch resistance.3 In this paper we explore the effectiveness of a RAFT-based approach for the encapsulation of platelet-like substrates to synthesize anisotropic nanocomposite latex particles. This approach has previously been used for the encapsulation of pigment particles,19 but to the best of our knowledge this is the first report on the use of this approach to encapsulate platelet-like colloidal substrates. Dibenzyltrithiocarbonate (DBTTC, Scheme 2) was chosen as a RAFT agent, as it is easy to synthesize and, like other similar trithiocarbonates, has a reactivity adequate for the polymerization of acrylic acid and acrylates in a living fashion.20,21 Gibbsite platelets (γ-Al(OH)3) were used as a model substrate to demonstrate the effectiveness of this approach. The most important reason to select this model substrate is that in the final composite latex, the gibbsite platelet, being thicker than the natural clay platelet, is much easier to visualize by electron microscopy. Random copolymers of butyl acrylate (BA) and acrylic acid (AA) were synthesized by means of the RAFT process and used as stabilizer for the gibbsite platelets. The adsorbed random RAFT copolymers were then chain extended to form a polymer shell around the gibbsite platelets with a fresh supply of monomer and initiator (Scheme 3). In the remainder of the paper we will show that this approach was successful and the cryo-TEM (19) Nguyen, D.; Zondanos, H. S.; Farrugia, J. M.; Serelis, A. K.; Such, C. H.; Hawkett, B. S. Pigment encapsulation by emulsion polymerization using macroRAFT copolymers. Langmuir 2008, 24, 2140-2150. (20) Freal-Saison, S.; Save, M.; Bui, C.; Charleux, B.; Magnet, S. Emulsifier-free controlled free-radical emulsion polymerization of styrene via RAFT using dibenzyltrithiocarbonate as a chain transfer agent and acrylic acid as an ionogenic comonomer: Batch and spontaneous phase inversion processes. Macromolecules 2006, 39, 8632-8638. (21) Couvreur, L. Dibenzyltrithiocarbonate (DBTTC) perfomances overview of a commercially available RAFT agent. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2005, 46, 219.

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Materials. Monomers butyl acrylate (BA; Aldrich, 99%), methyl methacrylate (MMA; Aldrich, 99%), and acrylic acid (AA; Fluka, 99%) were distilled under reduced pressure prior to use. Initiator N,N0 -azobis(isobutyronitrile) (AIBN; Fluka, g98%) was recrystallized from methanol. The water-soluble azo initiator 4,40 -azobis-4-cyanovaleric acid (V-501, Fluka, 98%) was used as received. Sodium sulfide (Fluka, 99%), benzyl chloride (Aldrich, 99%), carbon disulfide (Fluka, 99%), aluminum tri-sec-butoxide (Aldrich, 97%), aluminum isopropoxide (Fluka, 99+%), and the phase transfer catalyst tetrabutylammonium bromide (Aldrich, 99%) were all used as received. Tetrahydrofuran (THF, Biosolve), ethanol (Biosolve), dioxane (Merck), hydrochloric acid (HCl, Merck, 32%), sodium hydroxide (VWR), chloroform-d (Campro scientific), and dimethyl sulfoxide-d6 (Campro scientific) were all used without any treatment. The synthetic clay gibbsite was synthesized according to the literature protocol.22 Synthesis of Dibenzyl Trithiocarbonate. The RAFT agent dibenzyl trithiocarbonate (DBTTC, Scheme 2) was synthesized by using a slight modification of the literature protocol.20 First, 146.3 g (1.125 mol) of hydrated sodium sulfide, 8.2 g (0.025 mol) of tetrabutylammonium bromide dissolved in 6.7 g (0.37 mol) of water, and 88.4 g (1.15 mol) carbon disulfide were mixed for 1 h at room temperature in 351 g (19.5 mol) of water in a 1-L roundbottomed flask equipped with a magnetic stirrer. The solution turned red as the sodium trithiocarbonate was formed. A 255 g (2 mol) sample of benzyl chloride was then added slowly over a period of 15 min, after which the stirring was continued for 3 h. The temperature was then raised to 70 °C and the flask was heated for an additional 60 min after which another charge of the phase transfer catalyst tetrabutylammonium bromide (5.2 g, 0.016 mol) dissolved in 5.1 g (0.28 mol) of water was added. The solution was stirred overnight without heating. Yellow crystals of DBTTC were obtained from the organic phase by precipitation in 500 mL of cold ethanol. The product was filtered off and washed further with cold ethanol and then dried in a vacuum oven. (22) Wierenga, A. M.; Lenstra, T. A. J.; Philipse, A. P. Aqueous dispersions of colloidal gibbsite platelets: synthesis, characterisation and intrinsic viscosity measurements. Colloids Surf. A 1998, 134, 359-371.

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H NMR (CDCl3): δ 4.68 (CH2), 7.38 (aromatic H). Yield: 75%. Purity: 99%. A commercially available DBTTC (purity ∼95%) was also kindly provided by Arkema and the preliminary results with this agent were similar to those obtained with the agent synthesized in this work. All results reported in this paper were obtained with our own synthesized DBTTC. Synthesis of Random RAFT Copolymers. RAFT copolymers containing different combinations of randomly distributed AA and BA units were synthesized in dioxane with DBTTC as chain transfer agent. Table 1 summarizes the structural compositions and recipes used for their synthesis. As a typical example, a RAFT copolymer containing on average 5 BA units and 10 AA units, abbreviated as BA5-coAA10, was synthesized as follows: 9.67 g (76 mmol) of BA, 10.9 g (151 mmol) of AA, 4.4 g (15 mmol) of DBTTC, and 0.22 g (1.34 mmol) of AIBN were mixed in 25 g of dioxane in a Schlenk flask. The mixture was degassed by 3 freezepump-thaw cycles and then heated and stirred at 70 °C for 5 h. Other RAFT copolymers listed in Table 1 were synthesized in a similar manner. Adsorption Studies. In eight different vials, calculated amounts of RAFT copolymer were transferred from a 10 mM aqueous stock solution and the volume was made up to 10 mL by adding water. Equal volumes of gibbsite dispersion (1 wt %) were then added dropwise into these vials under stirring and the solutions were stirred overnight at room temperature. The pH of the dispersion was around 7. ζ potential and particle size measurements were performed on these samples. Encapsulation Experiments. Hybrid latex particles were synthesized by starved feed emulsion polymerization performed in a 50 mL 3-necked flask equipped with a magnetic stirrer and heating bath. The recipes used for the encapsulation experiments are summarized in Table 2. Briefly, the required amounts of double deionized water (DDI) and RAFT copolymer (from a 1

Table 1. Recipes for RAFT Copolymer Synthesisa RAFT copolymer

[DBTCC] (mM)

[AIBN] (mM)

[acrylic acid] (mM)

15.2 1.3 151.3 BA5-co-AA10 18.8 1.7 187.8 BA2.5-co-AA10 12.7 1.2 126.7 BA7.5-co-AA10 7.7 0.7 38.7 BA5-co-AA5 6.8 0.6 138.0 BA15-co-AA20 8.3 0.8 165.1 BA10-co-AA20 a In dioxane at 70 °C, [DBTTC]/[AIBN] = 11.

[butyl acrylate] (mM) 75.5 47.0 95.2 38.7 102.7 82.6

10 mM stock solution) were transferred into the flask and an equal volume of gibbsite dispersion was then added dropwise under constant stirring at room temperature. The resulting dispersion was then sonicated for 5 min, using a Vibracell tip sonicator (micro tip, at 30% amplitude), after which the required amount of initiator V-501 was added and the dispersion was flushed with argon for 30 min. The reactor was heated to 70 °C with an oil bath followed by the addition of 2.35 g of deoxygenated monomer mixture (MMA/BA, ratios are given in Table 2) at a rate of 0.01 g/min, using a Dosimat autotitrator. After the completion of the monomer addition, the reaction was stirred at 70 °C for another 2 h. At regular intervals during the polymerization process, samples were collected for molecular weight and particle size determination. Characterization. Number average molecular weight and the polydispersity index (PDI) of the RAFT copolymers and the composite latexes were measured by using gel permeation chromatography (GPC), using a Waters GPC equipped with a Waters model 510 pump and a model 410 differential refractometer. A set of two mixed bed columns (Mixed-C, Polymer Laboratories, 30 cm, 40 °C) were used. Tetrahydrofuran was used as the eluent, and the system was calibrated by using narrow molecular weight polystyrene standards (range = 580-7.5  106 g/mol). FTIR spectra were recorded on a Bio-Rad Infrared Excalibur 3000 FTIR spectrometer. 1H NMR spectra were recorded on a Varian 400 MHz spectrometer, using dimethyl sulfoxide-d6 and chloroform-d as solvents. The particle size distribution and ζ potential were determined at 25 °C by dynamic light scattering (DLS), using a Malvern Zetasizer Nano ZS instrument. The ζ potential was calculated from the electrophoretic mobility (μ), using the Smoluchowski relationship: ζ = ημ/ε, with κa .1 (where η is the viscosity, ε is the dielectric constant of the medium, and κ and Table 2. Recipes of Encapsulation Experimentsa entry

latex

RAFT copolymer

V-501 DDI water feedb composition (mg) (g) MMA:BA

6.37 10.2 10:1 L5,10 BA5-co-AA10 9.7 10:1 L2.5,10 BA2.5-co-AA10 8.00 10.6 10:1 L7.5,10 BA7.5-co-AA10 5.50 BA5-co-AA5 8.13 9.7 10:1 L5,5 6.37 10.2 7:3 L5,10 BA5-co-AA10 6.37 10.2 10:0 L5,10 BA5-co-AA10 11.4 10:1 L15,20 BA15-co-AA20 2.9 10.8 10:1 L10,20 BA10-co-AA20 4.32 a For all experiments: 37.5 mg of RAFT copolymer, 2.35 g of monomer (fed at a rate of 0.01 g/min), 0.125 g of gibbsite, and [RAFT]: [V-501] = 1 was used. Polymerization was carried out at around pH 7. b Feed ratios in w/w. 1 2 3 4 5 6 7 8

Scheme 3. Schematic Representation of the Synthesis of Anisotropic Polymer/Gibbsite Nanocomposite Latex Particles by Aqueous Starved Feed Emulsion Polymerization with Use of RAFT Copolymers As Stabilizers

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Ali et al. Scheme 4. Synthesis of Random RAFT Copolymers

Figure 1. FTIR spectra of different RAFT copolymers: (a) BA5co-AA10, (b) BA2.5-co-AA10, (c) BA7.5-co-AA10, and (d) BA5-coAA5. a are the Debye-H€ uckel parameter and the particle radius, respectively). Cryogenic transmission electron microscopy (cryo-TEM) measurements were performed on a FEI Tecnai 20, type Sphera TEM instrument (with a LaB6 filament, operating voltage = 200 kV). The sample vitrification procedure was performed by using an automated vitrification robot (FEI Vitrobot Mark III). A 3 μL sample was applied to a Quantifoil grid (R 2/2, Quantifoil Micro Tools GmbH; freshly glow discharged for 40 s just prior to use) within the environmental chamber of the Vitrobot and the excess liquid was blotted away. The thin film thus formed was shot into melting ethane. The grid containing vitrified film was immediately transferred to a cryoholder (Gatan 626) and observed under low dose conditions at -170 °C.

Results and Discussion Synthesis and Characterization of Random RAFT Copolymers. RAFT copolymers with different combinations of randomly distributed acrylic acid and butyl acrylate units were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization in solution. Random copolymers are chosen because, unlike block copolymers, they cannot easily form micelles, hence minimizing new particle formation. Table 1 lists the structural compositions and the recipes used for the synthesis and a general reaction scheme is depicted in Scheme 4. The reaction was performed under nitrogen in dioxane at 70 °C, using 10526 DOI: 10.1021/la9012697

dibenzyl trithiocarbonate (Scheme 2) as chain transfer agent. The synthesized RAFT copolymers were characterized by FTIR and 1H NMR spectroscopy. Figure 1 shows the FTIR spectra of the RAFT copolymers, where the absorption bands at 1060-1070 cm-1 correspond to the stretching vibration of the CdS double bond and demonstrate the presence of the RAFT moiety in the polymer chain. A sharp band attributed to the hydrogen-bonded carbonyl groups was observed at 1700 cm-1 with a shoulder (assigned to free carbonyl groups) appearing at 1723 cm-1. The characteristic bands of the phenyl ring at 1452 and 1493 cm-1 were also observed. Copolymerization of acrylic acid and butyl acrylate is expected to give random copolymers because of their similar reactivity ratios.23 Dibenzyl trithiocarbonate is a symmetric RAFT agent having two benzyl groups (Scheme 2). NMR experiments show that the chain grows from both ends of the RAFT agent, because the trithiocarbonate functionality is found to be in the middle of the polymer chain (Scheme 4). This can be seen from Figure 2, which shows the 1H NMR spectra of RAFT copolymer BA5-coAA10. From the ratio of 5 for the integral values of the peaks at 7.1 to 7.4 ppm and 4.6 ppm, it can be concluded that the trithiocarbonate moiety is situated in the middle of the chain and the benzyl groups are situated at both ends of the polymer chain; if the chain had only grown in one direction, this ratio would have been 10. Average copolymer compositions were also determined from NMR and the results are summarized in Table 3. Assuming the presence of the residues of a single RAFT agent in every chain, a value for Mn can be estimated. These values are also listed in Table 3 (Mn,NMR) and are compared with those obtained by GPC (Mn,GPC) and the theoretically expected values (Mn,th). The theoretical number average molecular weight of the polymer prepared by the RAFT process can be calculated by using the following equation: ½M0 xm0 þ MRAFT ð1Þ Mn, th ¼ ½RAFT0 where [M]0 and [RAFT]0 are the starting concentrations of monomer and RAFT agent, respectively, x refers to the overall monomer conversion, and m0 and MRAFT are the molecular weights of the monomer and RAFT agent, respectively. The number average molecular weights (Mn) and polydispersity indices (PDI) values were measured by GPC. The Mn,GPC and (23) Brandrup, J.; Immergut, E. H.; Grulke, E. A.; Abe, A.; Bloch, D. R. Polymer Handbook, 4th ed.; Wiley: New York, 2005.

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Figure 2. 1H NMR spectra of RAFT copolymer BA5-co-AA10. Table 3. Characterization of RAFT Copolymers Mn,GPCa (g/mol) RAFT copolymer

Mn,th (g/mol)

Mn

PDI

FAAb

1652 1459 1.2 0.61 BA5-co-AA10 1331 1052 1.2 0.78 BA2.5-co-AA10 1972 1700 1.3 0.54 BA7.5-co-AA10 1292 1412 1.3 0.41 BA5-co-AA5 3654 3575 1.3 0.55 BA15-co-AA20 2432 2632 1.4 0.68 BA10-co-AA20 a Values against PS standards, b Measured by 1H NMR

Mn,NMR (g/mol) 1662 1336 2040 1294 3356 2253

Mn,NMR values (listed in Table 3) are in good agreement with each other. These experimentally determined Mn values (Mn,GPC and Mn,NMR) correspond well with the theoretically calculated Mn values (Mn,th). This, together with the low polydispersity index values, is a strong indication that the RAFT method worked successfully for the synthesis of the RAFT copolymers. Adsorption of RAFT Copolymers on Gibbsite. Adsorption of charged polymers onto oppositely charged surfaces is a process that is mainly electrostatically driven and depends on many factors such as charge density of the polymer, ionic strength, and the resident ion on the surface of the substrate.24-26 Close to the isoelectric point, the net charge on the surface of the (24) Durand-Piana, G.; Lafuma, F.; Audebert, R. Flocculation and adsorption properties of cationic polyelectrolytes toward Na-montmorillonite dilute suspensions. J. Colloid Interface Sci. 1987, 119, 474-480. (25) Loiseau, J.; Ladaviere, C.; Suau, J. M.; Claverie, J. Dispersion of calcite by poly(sodium acrylate) prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization. Polymer 2005, 46, 8565-8572. (26) Schwarz, S.; Bratskaya, S.; Jaeger, W.; Paulke, B. R. Effect of charge density, molecular weight, and hydrophobicity on polycations adsorption and flocculation of polystyrene latices and silica. J. Appl. Polym. Sci. 2006, 101, 34223429.

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Figure 3. Cryo-TEM micrograph of the gibbsite platelets synthesized in this study.

polyion-coated particle is reduced and particles start aggregating because of a reduction in the electrostatic repulsion. When the amount of polymer is higher, more polyions collapse than are needed to neutralize the surface. The resulting dispersion can therefore display an overall charge of the opposite sign to the one the particle originally bears. This interaction mechanism is called the “charge inversion effect”.27,28 (27) Nguyen, T. T.; Shklovskii, B. I. Overcharging of a macroion by an oppositely charged polyelectrolyte. Phys. A (Amsterdam, Neth.) 2001, 293, 324-338. (28) Sennato, S.; Bordi, F.; Cametti, C. Correlated adsorption of polyelectrolytes in the “charge inversion” of colloidal particles. Europhys. Lett. 2004, 68, 296-302.

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Figure 5. (a) Encapsulated gibbsite platelets obtained by using RAFT copolymer BA5-co-AA10 and a feed composition ratio of MMA:BA = 10:1 for encapsulation. (b) A single polymer-encapsulated gibbsite platelet obtained by using RAFT copolymer BA5co-AA5 and a feed composition ratio of MMA:BA = 10:1 for encapsulation at a higher magnification (black dots in the frame are 10 nm gold particles added for calibration).

Figure 4. Effect of RAFT copolymer concentration on z-average diameter (A) and ζ potential (B) of the gibbsite platelets. Used RAFT copolymers: 9, BA5-co-AA10; O, BA2.5-co-AA10; blue triangle, BA7.5-co-AA10; red triangle, BA5-co-AA5.

For the adsorption studies, cationic hexagonal gibbsite platelets (γ-Al(OH)3) were used as substrate. Unlike natural clays, synthetically produced gibbsite platelets have a thickness of around 9 nm and are easy to visualized (see Figure 3). The isoelectric point of the gibbsite platelets is around pH 9, which provides an ideal pH window for the encapsulation via the RAFT route. The charge on the platelets originates from the ionization of the surface aluminol (AlOH) groups into AlOH2+ and AlOat pH values below and above the isoelectric point, respectively.22 Adsorption studies were performed by using different amounts of RAFT copolymers and the evolution of particle diameter and ζ potentials was monitored by means of dynamic light scattering (DLS). Gibbsite platelets were characterized to have a z-average diameter of around 144 nm and a ζ potential of around +50 mV at pH 7. The average particle diameter from electron microscopy was found to be around 135 nm, which is in fair agreement with the DLS result. Figure 4 shows the dependence of the z-average diameter and the ζ potential as a function of RAFT copolymer concentration. For all the RAFT copolymers studied, at low concentrations the size of the particles is close to the size of the original gibbsite platelets. With increasing concentration of RAFT copolymer the z-average diameter of the platelets increases and the ζ potential decreases. This is because with an increase in concentration, more RAFT copolymer is adsorbed onto the gibbsite platelets and the inherent positive charge of the platelet decreases. This reduces the 10528 DOI: 10.1021/la9012697

electrostatic repulsion between the platelets, causing them to start aggregating.28,29 The particle diameter values reach maxima at a concentration of RAFT copolymer around 150 mg/g of clay at which flocculation occurs and the measurement of particle diameter is no longer reliable by DLS. As shown by the ζ potential measurements (Figure 4B), this occurs close to the isoelectric point where the total charge of the adsorbed copolymer nearly counterbalances the surface charge of the platelets and the maximum sized aggregates were obtained.28,30 Further increasing the concentration of RAFT copolymer decreases the z-average diameter and ζ potential until a plateau was reached and anionically stabilized gibbsite platelets were obtained. This is because of the surface charge polarity reversal due to an excess of anionic charges in the adsorbed polymer layer.28,30 No further effect on z-average diameter and ζ potential was observed because no more anionic RAFT copolymers adsorbed on the surface of the anionically stabilized platelets. Preparation and Characterization of Anisotropic Composite Nanoparticles. Table 2 summarizes the recipes used for the encapsulation experiments. Encapsulation reactions were performed by using starved feed emulsion polymerization to avoid formation of monomer droplets in the aqueous phase, which can potentially compete for the RAFT copolymers, reducing the colloidal stability, and can also give rise to gibbsite-free polymer particles; this would result in reduced encapsulation efficiency. The gibbsite platelets were first dispersed in water by using RAFT copolymers as dispersants (Scheme 3). The adsorbed RAFT copolymers were then chain extended to form a polymeric shell around the platelets by the feeding of a monomer mixture comprised of MMA and BA in the presence of a nonoxidizing water-soluble initiator 4,40 -azobis-4-cyanovaleric acid (V-501). The amount of RAFT copolymer was kept at almost double the isoelectric point concentration, because sufficient RAFT copolymer should be present in the aqueous phase to adsorb onto the growing surface during encapsulation19 to provide the required stabilization. Cryo-TEM was used to examine the morphology of the composite latex particles in their wet state. These micrographs reveal that platelet encapsulation is achieved and the resulting composite particles possess a core-shell type (29) Bordi, F.; Cametti, C.; Diociaiuti, M.; Gaudino, D.; Gili, T.; Sennato, S. Complexation of anionic polyelectrolytes with cationic liposomes: evidence of reentrant condensation and lipoplex formation. Langmuir 2004, 20, 5214-5222. (30) Schwarz, S.; Lunkwitz, K.; Kessler, B.; Spiegler, U.; Killmann, E.; Jaeger, W. Adsorption and stability of colloidal silica. Colloids Surf. A 2000, 163, 17-27.

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Article Table 4. Particle Diameters and ζ Potentials of the Composite Latexes

Figure 6. Cryo-TEM micrographs of the encapsulated gibbsite particles obtained by using different RAFT copolymers for the encapsulation: (a) BA5-co-AA10, (b) BA2.5-co-AA10, (c) BA7.5-coAA10, and (d) BA5-co-AA5 (entries 1 to 4, Table 2). Monomer feed composition ratio in all the reactions was MMA:BA=10:1.

morphology in which the platelet is encapsulated inside a polymeric shell (Figure 5a,b). Effect of Hydrophilic-Lipophilic Balance of the RAFT Copolymer. The ratio of BA to AA units in the RAFT copolymer was found to have a considerable influence on the final morphology, encapsulation efficiency, particle diameter, and colloidal stability of the composite latex. Figure 6 shows representative cryo-TEM images of the composite latex particles obtained by using RAFT copolymers BA5-co-AA10, BA2.5-co-AA10, BA7.5-coAA10, and BA5-co-AA5 for encapsulation with a 10:1 mixture of the monomers MMA and BA (entries 1 to 4, Table 2). These micrographs reveal that platelet encapsulation is achieved in all cases and that the resulting morphology, encapsulation efficiency, and latex stability depends on the structure of the RAFT copolymer used. Figure 6a shows the composite latex particles obtained by using RAFT copolymer BA5-co-AA10 for encapsulation (entry 1, Table 2). From the cryo-TEM images, a thickness of around 40-50 nm is estimated for the polymer shell formed on the surface of the platelets. Particles with platelet basal planes oriented parallel and perpendicular to the electron beam can be seen, demonstrating that the composite latex particles are flat indeed. The near-hexagonal shape of the particles demonstrates that the growth of the polymer takes the shape of the substrate, i.e., the original hexagonal gibbsite platelets. In these experiments, good encapsulation efficiency is achieved resulting in almost every platelet encapsulated and a negligible amount of gibbsite-free polymer formed in the aqueous phase. From the DLS measurements, the final z-average particle diameter and the polydispersity index were 209 nm and 0.03, respectively (entry 2, Table 4), as compared to a z-average diameter of around 144 nm and the polydispersity index of about 0.3 for the free gibbsite (entry 1, Table 4). The encapsulation experiment with the more hydrophilic RAFT copolymer BA2.5-co-AA10 resulted in aggregation of the final encapsulated particles (Figure 6b and entry 2, Table 2), which suggests that there is a lack of surface charge for the Langmuir 2009, 25(18), 10523–10533

entry

latex

1 2 3 4 5 6 7 8 9

gibbsite L5,10 L2.5,10 L7.5,10 L5,5 L5,10 L5,10 L15,20 L10,20

feed composition z-average ζ potential MMA:BA (w/w) diameter (nm) PDI (mV) 10:1 10:1 10:1 10:1 10:3 10:0 10:1 10:1

144 209 2230 181 201 223 870 154 170

0.3 0.03 0.6 0.1 0.05 0.2 0.8 0.2 0.2

+50 -52 -49 -46 -48 -55 -50 -51 -50

stabilization. This can be explained on the basis of the hydrophilicity of the RAFT copolymer. More hydrophilic copolymers are more likely to depart from the particle surface and migrate into the aqueous phase. This would leave the particle surface deficient in the much needed stabilizing charge and hence the particles aggregate. The aggregation of the encapsulated particles is also evident from the dynamic light scattering measurements giving an average particle diameter of about 2 μm and a higher polydispersity index of about 0.6 (entry 3, Table 4). Nevertheless, apart from the stability problems, the encapsulation of the platelets is achieved even with this RAFT copolymer (Figure 6b). As compared to BA5-co-AA10, increasing the hydrophobicity of the RAFT copolymer resulted in appreciable growth of polymer particles in the aqueous phase. Figure 6c shows the composite particles obtained by using RAFT copolymer BA7.5co-AA10 (entry 3, Table 2) and it is clear that as a result of the free polymer particles, the thickness of the polymer shell formed on the surface of the platelets is relatively small compared to the one obtained with the RAFT copolymer BA5-co-AA10 as is also evident from a relatively small particle diameter value of 181 nm obtained by DLS (entry 4, Table 4). It is conceivable that the RAFT copolymer with high BA content may contain some amphiphilic species which can self-assemble in the aqueous phase and cause secondary nucleation. Figure 6d shows the composite latex particles obtained by using the RAFT copolymer BA5-co-AA5 (entry 4, Table 2). The absence of any appreciable free polymer particles suggests that most of the polymer is formed on the substrate surface. Substantial numbers of composite particles were also observed containing more than one gibbsite platelet, which might result from the particle aggregation during encapsulation. This can be explained on the basis of the lower charge density of the RAFT copolymer BA5-co-AA5 because of which a larger amount would be needed to neutralize the platelet surface charge and to get the charge inversion.26,31 This would leave the starting concentration of the RAFT copolymer in the water phase lower. As describe earlier, in order to maintain the colloidal stability, sufficient RAFT copolymer should be present in the aqueous phase to adsorb onto the growing surface during encapsulation.19 Because of the insufficient starting concentration of the RAFT copolymer it is likely that during encapsulation its depletion causes the encapsulated particles to aggregate forming larger particles containing more than one platelet. Effect of Chain Length of RAFT Copolymer. The chain length of the RAFT copolymers is expected to be of importance for the success of the encapsulation by the RAFT approach. It can be expected that the longer RAFT copolymers having more (31) Rojas, O. J.; Ernstsson, M.; Neuman, R. D.; Claesson, P. M. Effect of polyelectrolyte charge density on the adsorption and desorption behavior on mica. Langmuir 2002, 18, 1604-1612.

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Figure 7. Cryo-TEM micrographs of encapsulated gibbsite platelets obtained by using the RAFT copolymers (a) BA15-co-AA20 and (b) BA10-co-AA20 for the encapsulation. Monomer feed composition ratio was MMA:BA=10:1 for both reactions.

charged units can neutralize the platelet with a smaller number of chains. This would lead to (a) a smaller number of RAFT groups on the surface and hence less control on polymer growth and (b) to more RAFT copolymers in the aqueous phase, which can potentially give rise to secondary nucleation. Besides, water solubility of the longer copolymers can also impart an adverse effect on the encapsulation process. To study the effect of the chain length, two more random RAFT copolymers BA15-co-AA20 (with the same BA/AA ratio as BA7.5-co-AA10) and BA10-co-AA20 (with the same BA/AA ratio as BA5-co-AA10) were synthesized and used for the encapsulation experiments, using a 10:1 MMA/BA feed (entries 7 and 8, Table 2). Panels a and b of Figure 7 show the cryo-TEM micrographs of the resulting latex particles. In both cases, appreciable growth of free polymer particles in the aqueous phase is observed. This is also reflected in small particle diameters and high polydispersity index values obtained for these two latexes (entries 8 and 9, Table 4). Because of the formation of the free polymer particles in the aqueous phase, with the same amount of monomer, the amount of polymer formed on the surface of the platelets is reduced, resulting in a smaller particle diameter and higher polydispersity index. Formation of free polymer particles in the aqueous phase is an expected result for longer RAFT copolymers. Owing to their limited solubility longer RAFT copolymers can possibly collapse in the form of very small particles and induce secondary nucleation to give rise to the free polymer particle formation. Longer copolymer chains may have self-assembling properties which can also give rise to secondary nucleation. Relatively higher viscosity of the medium in the presence of long-chain RAFT copolymers can also give rise to the polymer particles in the aqueous phase because of a hindered monomer transport. It should be noted that the extent of free polymer particle formation with the copolymer BA15-co-AA20 is much higher than that with the more hydrophilic BA10-co-AA20, which is similar to what was observed for their smaller counterparts BA7.5-co-AA10 and BA5-co-AA10 (Figure 6, parts c and a, respectively). For efficient encapsulation, the RAFT copolymer should be small enough to give a maximum number of (RAFT group containing) chains on the substrate. This will facilitate the rapid transfer of the polymer growth between the polymer chains. However, RAFT copolymers that are too small and possess a smaller number of anchoring charge units are more liable to migrate into the aqueous phase and might cause problems of emulsion stability during the encapsulation reaction. Effect of Monomer Feed Composition. To study the effect of monomer feed composition on the encapsulation, two more feed compositions were explored (i.e., MMA:BA=7:3 and 100% 10530 DOI: 10.1021/la9012697

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Figure 8. Cryo-TEM micrographs of the composite latex samples obtained by the encapsulation attempt of the gibbsite platelets by using two different monomer compositions: (a) mixture of MMA: BA=7:3 and (b) MMA only. RAFT copolymer BA5-co-AA10 was used for the encapsulation.

MMA) with use of RAFT copolymer BA5-co-AA10. A clear effect on the colloidal stability and the morphology of the latex particles was observed as can be seen from Figure 8, panels a and b (entries 5 and 6, Table 2). Control over the platelet orientation was completely lost and the majority of the platelets were located outside the surface of the latex particle in case of a higher butyl acrylate containing feed (MMA/BA = 7:3, Figure 8a). This effect can be attributed to the composition of the encapsulating polymer. The copolymer shell formed with the feed composition containing more BA is likely to be more hydrophobic and with low glass transition temperature (Tg). High interfacial tension and the surface energy of the hydrophobic copolymer cause a minimization of the surface area and drive the more hydrophilic gibbsite platelets toward the polymer water interface. This process is aided by the low glass transition temperature (Tg) of the copolymer, which provides enough mobility at the reaction temperature for the migration of the platelets toward the particle surface. Similar morphology transformations were reported by Bon et al.32 and Armes et al.33 In these studies the migration of hydrophilic silica nanoparticles toward the particle surface was observed for monomer compositions rich in more hydrophobic components such as BA32 and styrene33 and the obtained morphologies were explained by surface thermodynamics. No segregation of the gibbsite platelets was observed for the particles obtained with an all MMA monomer feed (entry 6, Table 2), as is clearly seen in Figure 8b. The surface of the polymer shell on the platelet is uneven and the particles tend to aggregate as a result of insufficient colloidal stability. The aggregation is also evident from the larger particle diameters (870 nm) and higher polydispersity index values (0.8) obtained with DLS (entry 7, Table 4). The uneven polymer growth is presumably caused by aggregation of secondary polymer particles onto the gibssite containing particles and because of the relatively high glass transition temperature of the PMMA the polymer mobility is not high enough for creating a smooth surface. Aggregation of the encapsulated gibssite particles is probably caused by the fact that the increased surface area of the uneven PMMA shell requires more RAFT copolymers for stabilization than are present in the system. (32) Colver, P. J.; Colard, C. A. L.; Bon, S. A. F. Multilayered nanocomposite polymer colloids using emulsion polymerization stabilized by solid particles. J. Am. Chem. Soc. 2008, 130, 16850-16851. (33) Percy, M. J.; Amalvy, J. I.; Barthet, C.; Armes, S. P.; Greaves, S. J.; Watts, J. F.; Wiese, H. Surface characterization of vinyl polymer-silica colloidal nanocomposites using X-ray photoelectron spectroscopy. J. Mater. Chem. 2002, 12, 697-702.

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Particle Size Distributions. To probe the evolution of particle diameter and polydispersity index (PDI) as a function of the amount of monomer added, samples were taken at different percentages of monomer addition and were subjected to DLS measurements. In Figure 9a the obtained particle diameters and polydispersity index (PDI) are shown as a function of the added amount of monomer for the encapsulation reaction with use of the RAFT copolymer BA5-co-AA10 and a monomer feed composition of MMA: BA=10:1 (entry 1, Table 2) . The z-average particle diameter increases during encapsulation indicating the growth of the polymer on the surface of the substrate; the polydispersity index first increases during encapsulation and then drops to a minimum at the final stages of the encapsulation. A possible explanation for this trend is the aqueous phase growth of the RAFT copolymers. During encapsulation some of the excess RAFT copolymer chains present in the aqueous phase may also grow and at some stage become amphiphilic to form free polymer particles causing the PDI to increase. These particles eventually adsorb onto the surface of the growing polymer shell on the substrate hence reducing the final PDI of the latex. The same trend was also observed for the encapsulation reactions involving RAFT copolymers BA7.5-co-AA10 and BA5-co-AA5 under the same conditions. Figure 9b shows the cryo-TEM micrographs at 50% and 100% monomer addition. The free polymer particles are clearly seen at 50% monomer addition, but have virtually disappeared at 100% monomer addition. Figure 10a shows the evolution of particle size and PDI as a function of the amount of monomer added for the encapsulation reaction with use of RAFT copolymer BA5-co-AA10 with a feed monomer composition of MMA:BA=7:3 (entry 5, Table 2). The increase in the particle diameter with the amount of monomer is indicative of the polymer growth on the surface of the platelets. There is a sharp increase in the PDI of the particles at the final stages of the encapsulation reaction (around 75% of monomer added), which as explained earlier might be due to the rearrangement

Article

of the hydrophilic platelets in the more hydrophobic copolymer. Figure 10b shows the evolution of particle size and PDI for the

Figure 9. (a) Evolution of particle diameter (9) and polydispersity index (O) as a function of the amount of monomer added for the encapsulation of gibbsite platelets by using RAFT copolymer BA5co-AA10 and monomer feed composition ratio of MMA:BA = 10:1. (b) Cryo-TEM micrographs of the latex sample at 50% and 100% monomer added.

Figure 10. Evolution of particle size (9) and polydispersity index (PDI) (O) as a function of conversion for the encapsulation of gibbsite platelets by using (a) RAFT copolymer BA5-co-AA10 (monomer feed composition ratio MMA:BA = 7:3), (b) RAFT copolymer BA5-coAA10 (monomer feed MMA only), and (c) RAFT copolymer BA15-co-AA20 (monomer feed composition ratio MMA:BA = 10:1). Langmuir 2009, 25(18), 10523–10533

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Figure 11. Molecular weight evolution during encapsulation by using RAFT copolymer BA5-coAA10 and a feed composition of MMA:BA=10:1 (entry 1, Table 2): (a) molecular weight distributions and (b) Mn (3) and PDI (9). The straight line in panel b corresponds to the theoretical Mn vs conversion obtained by using eq 1.

encapsulation reaction with use of BA5-co-AA10 with a monomer feed comprising MMA only (entry 6, Table 2). A sharp increase in the particle diameter and PDI in the final stage of the encapsulation might be attributed to the aggregation of the particles. Figure 10c shows the evolution of particle size and PDI as a function of the amount of monomer added for the encapsulation reaction with use of the long chain RAFT copolymer BA15-co-AA20 with a feed composition of MMA:BA=10:1 (entry 7, Table 2). The increase in the particle diameter indicates the polymer growth, while the increase in polydispersity index was caused by the formation of a large number of free polymer particles, as also shown by the cryo-TEM micrograph (Figure 7a). ζ Potentials. The measured ζ potentials of the original gibbsite platelets and the composite latex particles are given in Table 4. The ζ potential of the gibbsite platelets was found to be around +50 mV at the working pH of around 7 because of the ionization of the surface AlOH groups to Al(OH)2+. This value changes to around -50 mV after the encapsulation, indicating that the composite latex particles are stabilized by a layer of negatively charged carboxylic acid groups of the RAFT copolymer chains. Molecular Weight Distributions. To probe whether polymerization occurred under RAFT control, the molecular weight growth was followed during the polymerization by using GPC. Because the encapsulation reactions were conducted under 10532 DOI: 10.1021/la9012697

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starved feed conditions, the amount of monomer added into the system can be approximated to be the monomer converted. Samples were taken at different amounts of monomer added into the system and the GPC measurements were carried out on the total polymer present. Parts a and b of Figure 11 show the molecular weight distributions and the evolution of the molecular weight as a function of the added amount of monomer, respectively, for the encapsulation reaction, using RAFT copolymer BA5-co-AA10 and a monomer feed composition MMA:BA= 10:1 (entry 1, Table 2). From the continuous increase of the number average molecular weight with conversion, it is clear that the polymerization happens under RAFT control up to high percentages of monomer conversion. Initially the obtained chain lengths are larger than those predicted by eq 1, but at high conversions they become smaller. A reason for this behavior may lie in the fact that only those RAFT copolymers that are adsorbed onto the Gibbsite will participate in the polymerization process.19 With increasing conversion, more RAFT copolymers will adsorb onto the growing particle from the aqueous phase, thus increasing the number of growing polymer chains and hence decreasing the value for Mn. This effect and the relatively low [RAFT]0/[I]0 ratio also explain the high PDI. Mechanism of Encapsulation. From the above observations, the mechanism of the encapsulation with RAFT copolymer is proposed to be as follows. The amphipatic random RAFT copolymers when dispersed in water can act as stabilizers and adsorb onto the oppositely charged substrate by electrostatic interactions. The resulting dispersion displays an overall charge of opposite sign to the one the substrate particle originally bears. The random nature of the RAFT copolymer chains prevents them from self-assembling in the aqueous phase, and thus reducing the formation of centers for secondary particle nucleation. Once the polymerization is started, the adsorbed RAFT copolymer chains extend by undergoing rapid transfer and incremental growth on the particle surface, resulting in a uniform coating over the entire surface. As the reaction proceeds, some of the RAFT copolymer present in the aqueous phase becomes adsorbed onto the growing surface offered to them by the growth of polymer on the surface of the substrate providing the required stabilization. The RAFT copolymers present in the aqueous phase can also chain extend, become amphiphilic, and selfassemble to form centers for secondary particle nucleation. This would generate a crop of new particles most of which may also eventually adsorb onto the surface of the substrate.

Conclusions We presented a simple RAFT copolymer approach to synthesize anisotropic polymer-inorganic nanocomposite latex particles. In this approach the composite latex particles are stabilized by a layer of negatively charged carboxylic acid groups of the RAFT copolymer chains thus avoiding the need for any external surfactant. The approach was successful to encapsulate the model clay gibbsite and as revealed by the cryo-TEM characterizations, the goal to synthesize anisotropic composite latex particles by encapsulating single platelets inside a latex particle was achieved with excellent control over the platelet orientation. The hydrophilic-lipophilic balance and chain length of the RAFT copolymers and monomer feed composition were found to have a significant effect on the efficiency of the encapsulation reaction. More hydrophobic and long-chain RAFT copolymers give rise to more secondary particle formation. A monomer feed comprising more hydrophobic monomer results in the loss of morphology control during encapsulation, leading to an “armored” morphology. The best encapsulation Langmuir 2009, 25(18), 10523–10533

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results were obtained by using RAFT copolymer BA5-co-AA10 with a monomer feed composition MMA:BA=10:1. The layer thickness can easily be controlled by the amount of monomer fed into the system. In the future the approach will be extended to encapsulate the natural clays such as montmorilonite and laponite to obtain composite latexes. Some preliminary results show that the encapsulation of natural clays is also feasible through this route.

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Acknowledgment. We gratefully acknowledge Dr. Nico Sommerdijk and Paul Bomans of the Soft Matter cryo-TEM Research Unit of Eindhoven University of Technology for helpful discussions about cryo-TEM and Dr. Laurence Couvreur of Arkema for a kind gift of a sample of the RAFT agent dibenzyl trithiocarbonate. S.I.A. is grateful for financial support by the Higher Education Commission, Government of Pakistan, under the HEC-NUFFIC program.

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