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 encapsulationapproach on cationic gibbsite platelets. By using theRAFTagent dibenzyl trithiocarbonate, a series of amphipatic livingrandom RAFT copolymers with different combinations of acrylic acid and butyl acrylate units were synthesized. TheseRAFT copolymers were used as living stabilizers for the gibbsite platelets and chain extended to form a polymeric shellby starved feed emulsion polymerization. Cryo-TEM characterization of the resulting composite latexes demonstratesthe formation of anisotropic composite latex particles withmostly one platelet per particle.Monomer feed composition,chain length, and hydrophilic-lipophilic balance of the RAFT copolymer were found to be important factors for theoverall efficiency of the encapsulation. Good control over platelet orientation and high encapsulation efficiency wereachieved via this route.
Introduction
Polymer-clay nanocomposites are an important type of ma-terials which offer significantly improved properties as comparedto the corresponding pure polymers.1-3 Recently these materialshave gained attention in coating technology because of theirsignificantly improved properties such as better hardness,enhanced mechanical strength, excellent gas barrier properties,scratch resistance, and superior optical and thermal properties fora variety of applications.4-6 These properties are very importantfor coatings andwould bedesirable in soft film forming latexes forcoating applications.
Although emulsion/miniemulsion polymerizations in the pre-sence of spherical inorganic particles, such as titanium oxide,carbon black, silica, and some other pigment particles, have
resulted in their encapsulation,7-12 the encapsulation of claysappears to be very challenging. Emulsion polymerization in thepresence of unmodified clay platelets has resulted almost alwaysin the formation of so-called armored latex particles13 (plateletsbeing located at the particle surface). Several researchers13-16
have used conventional emulsion polymerization in the presenceof face-modified clay platelets to prepare polymer-clay hybridlatex particles but again one ends up producing the armoredmorphology. Thus far, the only true encapsulation of clayplatelets was achieved by utilizing edge modification resulting indumbbell or peanut-shaped particles.17
The disk shape morphology, large aspect ratio, and highsurface energy of the clay pose a challenge for their encapsulationattempts.17 It is very difficult to form an inherently lower energystate of a polymer layer around the platelet. There is a need toexplore more feasible routes such as controlled polymer growthfrom the surface of the platelet, using, for instance, controlledradical polymerization techniques.18
Besides the true encapsulation of the single clay platelets,control over their orientation in the final polymeric film is also
*To whom correspondence should be addressed. E-mail: [email protected] (J.P.A.H) and [email protected] (A.M.v.H).(1) LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J. Polymer-layered silicate nano-
composites: an overview. Appl. Clay Sci. 1999, 15, 11-29.(2) Alexandre, M.; Dubois, P. Polymer-layered silicate nanocomposites: pre-
paration, 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 nanocompo-
site 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 newmaterials. 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 titaniumdioxide 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.
(11) Tiarks, F.; Landfester, K.; Anonietti,M., Encapsulation of carbon black byminiemulsion polymerization. Macromol. Chem. Phys. 2001, 202, 51-60.
(14) Cauvin, S.; Colver, P. J.; Bon, S. A. F. Pickering stabilized miniemulsionpolymerization: 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 templatingcationic miniemulsions and latexes. Adv. Mater. 2001, 13, 500.
(16) Bourgeat-Lami, E. Organic-inorganic nanostructured colloids. J. Nanosci.Nanotechnol. 2002, 2, 1-24.
(18) Samakande, A.; Sanderson, R. D.; Hartmann, P. C. Encapsulated ClayParticles in Polystyrene by RAFT Mediated Miniemulsion Polymerization. J.Polym. Sci., Part A: Polym. Chem. 2008, 46, 7114-7126.
of importance for the final coating properties. Encapsulatingsingle platelets with a layer of polymer, producing anisotropic,preferably plate-like flat composite latex particles would be a firststep toward this goal. These flat particles are likely to induceanisotropy into the final film (see Scheme1) potentially improvingthe barrier properties and the scratch resistance.3
In this paper we explore the effectiveness of a RAFT-basedapproach for the encapsulation of platelet-like substrates tosynthesize anisotropic nanocomposite latex particles. This app-roach has previously been used for the encapsulation of pigmentparticles,19 but to the best of our knowledge this is the first reporton the use of this approach to encapsulate platelet-like colloidalsubstrates. Dibenzyltrithiocarbonate (DBTTC, Scheme 2) waschosen as a RAFT agent, as it is easy to synthesize and, like othersimilar trithiocarbonates, has a reactivity adequate for the poly-merization of acrylic acid and acrylates in a living fashion.20,21
Gibbsite platelets (γ-Al(OH)3) were used as a model substrate todemonstrate the effectiveness of this approach. The most impor-tant reason to select this model substrate is that in the finalcomposite latex, the gibbsite platelet, being thicker than thenatural clay platelet, is much easier to visualize by electronmicroscopy. Random copolymers of butyl acrylate (BA) andacrylic acid (AA)were synthesized bymeans of theRAFTprocessand used as stabilizer for the gibbsite platelets. The adsorbedrandom RAFT copolymers were then chain extended to form apolymer shell around the gibbsite platelets with a fresh supply ofmonomer and initiator (Scheme 3). In the remainder of the paperwe will show that this approachwas successful and the cryo-TEM
micrographs of the resulting composite latexes clearly demon-strate the formation of anisotropic composite latex particles withmostly one platelet per particle and a controllable shell thickness.
Experimental Section
Materials. Monomers butyl acrylate (BA; Aldrich, 99%),methyl methacrylate (MMA; Aldrich, 99%), and acrylic acid(AA; Fluka, 99%) were distilled under reduced pressure prior touse. InitiatorN,N0-azobis(isobutyronitrile) (AIBN; Fluka,g98%)was recrystallized frommethanol. The water-soluble azo initiator4,40-azobis-4-cyanovaleric acid (V-501, Fluka, 98%) was used asreceived. Sodium sulfide (Fluka, 99%), benzyl chloride (Aldrich,99%), carbon disulfide (Fluka, 99%), aluminum tri-sec-butoxide(Aldrich, 97%), aluminum isopropoxide (Fluka, 99+%), andthe phase transfer catalyst tetrabutylammonium bromide(Aldrich, 99%) were all used as received. Tetrahydrofuran(THF, Biosolve), ethanol (Biosolve), dioxane (Merck), hydro-chloric acid (HCl, Merck, 32%), sodium hydroxide (VWR),chloroform-d (Campro scientific), and dimethyl sulfoxide-d6(Campro scientific) were all used without any treatment. Thesynthetic clay gibbsite was synthesized according to the literatureprotocol.22
Synthesis of Dibenzyl Trithiocarbonate. The RAFT agentdibenzyl trithiocarbonate (DBTTC, Scheme 2) was synthesizedby 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) ofwater, and 88.4 g (1.15mol) carbon disulfide weremixed for 1 h atroom temperature in 351 g (19.5 mol) of water in a 1-L round-bottomed flask equipped with a magnetic stirrer. The solutionturned red as the sodium trithiocarbonate was formed. A 255 g(2 mol) sample of benzyl chloride was then added slowly over aperiod of 15 min, after which the stirring was continued for 3 h.The temperaturewas then raised to 70 �Cand the flaskwas heatedfor an additional 60 min after which another charge of the phasetransfer catalyst tetrabutylammonium bromide (5.2 g, 0.016 mol)dissolved in 5.1 g (0.28mol) of water was added. The solutionwasstirred overnight without heating. Yellow crystals of DBTTCwere obtained from the organic phase by precipitation in 500 mLof cold ethanol. The product was filtered off and washed furtherwith cold ethanol and then dried in a vacuum oven.
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
aControl over the particle morphology will enable the control over the orientation of the platelets during film formation.
Scheme 2. Dibenzyl Trithiocarbonate (DBTTC)
(19) Nguyen, D.; Zondanos, H. S.; Farrugia, J. M.; Serelis, A. K.; Such, C. H.;Hawkett, B. S. Pigment encapsulation by emulsion polymerization using macro-RAFT 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 usingdibenzyltrithiocarbonate as a chain transfer agent and acrylic acid as an ionogeniccomonomer: Batch and spontaneous phase inversion processes. Macromolecules2006, 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.
(22) Wierenga, A. M.; Lenstra, T. A. J.; Philipse, A. P. Aqueous dispersions ofcolloidal gibbsite platelets: synthesis, characterisation and intrinsic viscositymeasurements. Colloids Surf. A 1998, 134, 359-371.
A commercially available DBTTC (purity ∼95%) was alsokindly provided by Arkema and the preliminary results with thisagent were similar to those obtained with the agent synthesized inthiswork.All results reported in this paperwere obtainedwith ourown synthesized DBTTC.
Synthesis of Random RAFT Copolymers. RAFT copoly-mers containing different combinations of randomly distributedAA and BA units were synthesized in dioxane with DBTTC aschain transfer agent. Table 1 summarizes the structural composi-tions and recipes used for their synthesis.
As a typical example, a RAFT copolymer containing onaverage 5 BA units and 10 AA units, abbreviated as BA5-co-AA10, 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, and0.22 g (1.34 mmol) of AIBN were mixed in 25 g of dioxane ina Schlenk flask. The mixture was degassed by 3 freeze-pump-thaw cycles and then heated and stirred at 70 �C for 5 h.Other RAFT copolymers listed in Table 1 were synthesized in asimilar manner.
Adsorption Studies. In eight different vials, calculatedamounts of RAFT copolymer were transferred from a 10 mMaqueous stock solution and the volume was made up to 10mL byadding water. Equal volumes of gibbsite dispersion (1 wt%)werethen added dropwise into these vials under stirring and thesolutions were stirred overnight at room temperature. The pHof the dispersion was around 7. ζ potential and particle sizemeasurements were performed on these samples.
Encapsulation Experiments. Hybrid latex particles weresynthesized by starved feed emulsion polymerization performedin a 50 mL 3-necked flask equipped with a magnetic stirrer andheating bath. The recipes used for the encapsulation experimentsare summarized in Table 2. Briefly, the required amounts ofdouble deionized water (DDI) and RAFT copolymer (from a
10mMstock solution)were transferred into the flask andanequalvolume of gibbsite dispersion was then added dropwise underconstant stirring at room temperature. The resulting dispersionwas then sonicated for 5 min, using a Vibracell tip sonicator(micro tip, at 30%amplitude), after which the required amount ofinitiator V-501 was added and the dispersion was flushed withargon for 30 min. The reactor was heated to 70 �C with an oilbath followed by the addition of 2.35 g of deoxygenated mono-mer mixture (MMA/BA, ratios are given in Table 2) at a rateof 0.01 g/min, using a Dosimat autotitrator. After the completionof the monomer addition, the reaction was stirred at 70 �C foranother 2 h. At regular intervals during the polymerizationprocess, samples were collected for molecular weight and particlesize determination.
Characterization.Number average molecular weight and thepolydispersity index (PDI) of the RAFT copolymers and thecomposite latexes were measured by using gel permeation chro-matography (GPC), using aWaters GPC equippedwith aWatersmodel 510 pump and amodel 410 differential refractometer. A setof twomixed bed columns (Mixed-C, PolymerLaboratories, 30 cm,40 �C) were used. Tetrahydrofuran was used as the eluent, andthe system was calibrated by using narrow molecular weightpolystyrene standards (range = 580-7.5 � 106 g/mol). FTIRspectra were recorded on a Bio-Rad Infrared Excalibur 3000FTIR spectrometer. 1H NMR spectra were recorded on a Varian400 MHz spectrometer, using dimethyl sulfoxide-d6 and chloro-form-d as solvents. The particle size distribution and ζ potentialwere determined at 25 �C by dynamic light scattering (DLS),using a Malvern Zetasizer Nano ZS instrument. The ζ potentialwas calculated from the electrophoretic mobility (μ), using theSmoluchowski relationship: ζ= ημ/ε, with κa.1 (where η is theviscosity, ε is the dielectric constant of the medium, and κ and
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
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.bFeed ratios in w/w.
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, operatingvoltage=200 kV). The sample vitrification procedure was per-formed by using an automated vitrification robot (FEI VitrobotMark III). A 3 μL sample was applied to a Quantifoil grid (R 2/2,Quantifoil Micro Tools GmbH; freshly glow discharged for 40 sjust prior to use) within the environmental chamber of theVitrobot and the excess liquid was blotted away. The thin filmthus formed was shot into melting ethane. The grid containingvitrified film was immediately transferred to a cryoholder (Gatan626) and observed under low dose conditions at -170 �C.
Results and Discussion
Synthesis and Characterization of Random RAFT Copo-
lymers. RAFT copolymers with different combinations of ran-domly distributed acrylic acid and butyl acrylate units weresynthesized by reversible addition-fragmentation chain transfer(RAFT) polymerization in solution. Random copolymers arechosen because, unlike block copolymers, they cannot easily formmicelles, hence minimizing new particle formation. Table 1 liststhe structural compositions and the recipes used for the synthesisand a general reaction scheme is depicted in Scheme 4. Thereaction was performed under nitrogen in dioxane at 70 �C, using
dibenzyl trithiocarbonate (Scheme 2) as chain transfer agent. Thesynthesized RAFT copolymers were characterized by FTIRand 1H NMR spectroscopy.
Figure 1 shows the FTIR spectra of the RAFT copolymers,where the absorption bands at 1060-1070 cm-1 correspond tothe stretching vibration of theCdS double bond and demonstratethe presence of the RAFT moiety in the polymer chain. A sharpband attributed to the hydrogen-bonded carbonyl groups wasobserved at 1700 cm-1 with a shoulder (assigned to free carbonylgroups) appearing at 1723 cm-1. The characteristic bands of thephenyl ring at 1452 and 1493 cm-1 were also observed.
Copolymerization of acrylic acid and butyl acrylate is expectedto give random copolymers because of their similar reactivityratios.23 Dibenzyl trithiocarbonate is a symmetric RAFT agenthaving two benzyl groups (Scheme 2). NMR experiments showthat the chain grows from both ends of the RAFT agent, becausethe trithiocarbonate functionality is found to be in the middle ofthe polymer chain (Scheme 4). This can be seen from Figure 2,which shows the 1H NMR spectra of RAFT copolymer BA5-co-AA10. From the ratio of 5 for the integral values of the peaks at 7.1to 7.4 ppm and 4.6 ppm, it can be concluded that the trithiocar-bonate moiety is situated in the middle of the chain and thebenzyl groups are situated at both ends of the polymer chain; ifthe chain had only grown in one direction, this ratio would havebeen 10.
Average copolymer compositions were also determined fromNMR and the results are summarized in Table 3. Assuming thepresence of the residues of a single RAFT agent in every chain, avalue for Mn can be estimated. These values are also listed inTable 3 (Mn,NMR) and are compared with those obtained byGPC(Mn,GPC) and the theoretically expected values (Mn,th).
The theoretical number average molecular weight of the poly-mer prepared by theRAFTprocess can be calculated by using thefollowing equation:
Mn, th ¼ ½M�0½RAFT�0
xm0 þMRAFT ð1Þ
where [M]0 and [RAFT]0 are the starting concentrations ofmonomer and RAFT agent, respectively, x refers to the overallmonomer conversion, and m0 and MRAFT are the molecularweights of the monomer and RAFT agent, respectively. Thenumber average molecular weights (Mn) and polydispersityindices (PDI) values were measured by GPC. The Mn,GPC and
Scheme 4. Synthesis of Random RAFT Copolymers
Figure 1. FTIR spectra of different RAFT copolymers: (a) BA5-co-AA10, (b) BA2.5-co-AA10, (c) BA7.5-co-AA10, and (d) BA5-co-AA5.
(23) Brandrup, J.; Immergut, E. H.; Grulke, E. A.; Abe, A.; Bloch, D. R.Polymer Handbook, 4th ed.; Wiley: New York, 2005.
Mn,NMRvalues (listed inTable 3) are in good agreement with eachother. These experimentally determined Mn values (Mn,GPC andMn,NMR) correspond well with the theoretically calculated Mn
values (Mn,th). This, together with the low polydispersity indexvalues, is a strong indication that the RAFT method workedsuccessfully for the synthesis of the RAFT copolymers.Adsorption of RAFTCopolymers on Gibbsite.Adsorption
of charged polymers onto oppositely charged surfaces is a processthat is mainly electrostatically driven and depends on manyfactors such as charge density of the polymer, ionic strength,and the resident ion on the surface of the substrate.24-26 Closeto the isoelectric point, the net charge on the surface of the
polyion-coated particle is reduced and particles start aggregatingbecause of a reduction in the electrostatic repulsion. When theamount of polymer is higher, more polyions collapse than areneeded to neutralize the surface. The resulting dispersion cantherefore display an overall charge of the opposite sign to the onethe particle originally bears. This interaction mechanism is calledthe “charge inversion effect”.27,28
Figure 2. 1H NMR spectra of RAFT copolymer BA5-co-AA10.
Figure 3. Cryo-TEM micrograph of the gibbsite platelets synthe-sized in this study.
(24) Durand-Piana, G.; Lafuma, F.; Audebert, R. Flocculation and adsorptionproperties of cationic polyelectrolytes toward Na-montmorillonite dilute suspen-sions. J. Colloid Interface Sci. 1987, 119, 474-480.(25) Loiseau, J.; Ladavi�ere, C.; Suau, J. M.; Claverie, J. Dispersion of calcite by
poly(sodium acrylate) prepared by reversible addition-fragmentation chain trans-fer (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 andflocculation of polystyrene latices and silica. J. Appl. Polym. Sci. 2006, 101, 3422-3429.
(27) Nguyen, T. T.; Shklovskii, B. I. Overcharging of a macroion by anoppositely charged polyelectrolyte. Phys. A (Amsterdam, Neth.) 2001, 293,324-338.
(28) Sennato, S.; Bordi, F.; Cametti, C. Correlated adsorption of polyelectro-lytes in the “charge inversion” of colloidal particles. Europhys. Lett. 2004, 68,296-302.
For the adsorption studies, cationic hexagonal gibbsite plate-lets (γ-Al(OH)3) were used as substrate. Unlike natural clays,synthetically produced gibbsite platelets have a thickness ofaround 9 nm and are easy to visualized (see Figure 3). Theisoelectric point of the gibbsite platelets is around pH 9, whichprovides an ideal pHwindow for the encapsulation via the RAFTroute. The charge on the platelets originates from the ionizationof the surface aluminol (AlOH) groups into AlOH2
+ and AlO-
at pH values below and above the isoelectric point, respec-tively.22
Adsorption studies were performed by using different amountsofRAFT copolymers and the evolution of particle diameter and ζpotentials was monitored by means of dynamic light scattering(DLS). Gibbsite platelets were characterized to have a z-averagediameter of around 144 nm and a ζ potential of around+50 mVat pH 7. The average particle diameter from electron microscopywas found to be around 135 nm, which is in fair agreement withthe DLS result.
Figure 4 shows the dependence of the z-average diameter andthe ζ potential as a function of RAFT copolymer concentration.For all the RAFT copolymers studied, at low concentrations thesize of the particles is close to the size of the original gibbsiteplatelets. With increasing concentration of RAFT copolymer thez-average diameter of the platelets increases and the ζ potentialdecreases. This is because with an increase in concentration, moreRAFT copolymer is adsorbed onto the gibbsite platelets and theinherent positive charge of the platelet decreases. This reduces the
electrostatic repulsion between the platelets, causing them to startaggregating.28,29 The particle diameter values reach maxima at aconcentration of RAFT copolymer around 150 mg/g of clay atwhich flocculation occurs and the measurement of particlediameter is no longer reliable byDLS.As shownby the ζ potentialmeasurements (Figure 4B), this occurs close to the isoelectricpoint where the total charge of the adsorbed copolymer nearlycounterbalances the surface charge of the platelets and themaximum sized aggregates were obtained.28,30 Further increasingthe concentration of RAFT copolymer decreases the z-averagediameter and ζ potential until a plateau was reached andanionically stabilized gibbsite platelets were obtained. This isbecause of the surface charge polarity reversal due to an excess ofanionic charges in the adsorbed polymer layer.28,30 No furthereffect on z-average diameter and ζ potential was observedbecauseno more anionic RAFT copolymers adsorbed on the surface ofthe anionically stabilized platelets.Preparation and Characterization of Anisotropic Compo-
site Nanoparticles. Table 2 summarizes the recipes used for theencapsulation experiments. Encapsulation reactions were per-formed by using starved feed emulsion polymerization to avoidformation of monomer droplets in the aqueous phase, which canpotentially compete for the RAFT copolymers, reducing thecolloidal stability, and can also give rise to gibbsite-free polymerparticles; this would result in reduced encapsulation efficiency.The gibbsite platelets were first dispersed inwater by usingRAFTcopolymers as dispersants (Scheme 3). The adsorbed RAFTcopolymers were then chain extended to form a polymericshell around the platelets by the feeding of a monomer mix-ture comprised of MMA and BA in the presence of a nono-xidizing water-soluble initiator 4,40-azobis-4-cyanovaleric acid(V-501).
The amount of RAFT copolymer was kept at almost doublethe isoelectric point concentration, because sufficient RAFTcopolymer should be present in the aqueous phase to adsorbonto the growing surface during encapsulation19 to provide therequired stabilization. Cryo-TEM was used to examine themorphology of the composite latex particles in their wet state.These micrographs reveal that platelet encapsulation is achievedand the resulting composite particles possess a core-shell type
Figure 4. Effect of RAFT copolymer concentration on z-averagediameter (A) and ζ potential (B) of the gibbsite platelets. UsedRAFT copolymers: 9, BA5-co-AA10; O, BA2.5-co-AA10; bluetriangle, BA7.5-co-AA10; red triangle, BA5-co-AA5.
Figure 5. (a) Encapsulated gibbsite platelets obtained by usingRAFT copolymer BA5-co-AA10 and a feed composition ratio ofMMA:BA= 10:1 for encapsulation. (b) A single polymer-encap-sulated gibbsite platelet obtained by using RAFT copolymer BA5-co-AA5 and a feed composition ratio of MMA:BA=10:1 forencapsulation at a higher magnification (black dots in the frameare 10 nm gold particles added for calibration).
(29) Bordi, F.; Cametti, C.; Diociaiuti, M.; Gaudino, D.; Gili, T.; Sennato, S.Complexation of anionic polyelectrolytes with cationic liposomes: evidence ofreentrant 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.
morphology in which the platelet is encapsulated inside a poly-meric shell (Figure 5a,b).
Effect of Hydrophilic-Lipophilic Balance of the RAFTCopolymer.The ratio of BA toAAunits in theRAFTcopolymerwas found to have a considerable influence on the final morphol-ogy, encapsulation efficiency, particle diameter, and colloidalstability of the composite latex. Figure 6 shows representativecryo-TEM images of the composite latex particles obtained byusing RAFT copolymers BA5-co-AA10, BA2.5-co-AA10, BA7.5-co-AA10, and BA5-co-AA5 for encapsulation with a 10:1 mixture ofthe monomers MMA and BA (entries 1 to 4, Table 2). Thesemicrographs reveal that platelet encapsulation is achieved in allcases and that the resultingmorphology, encapsulation efficiency,and latex stability depends on the structure of the RAFTcopolymer used. Figure 6a shows the composite latex particlesobtained by using RAFT copolymer BA5-co-AA10 for encapsula-tion (entry 1, Table 2). From the cryo-TEM images, a thickness ofaround 40-50 nm is estimated for the polymer shell formed onthe surface of the platelets. Particles with platelet basal planesoriented parallel and perpendicular to the electron beam can beseen, demonstrating that the composite latex particles are flatindeed. The near-hexagonal shape of the particles demonstratesthat 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 everyplatelet encapsulated and a negligible amount of gibbsite-freepolymer formed in the aqueous phase. From the DLS measure-ments, the final z-average particle diameter and the polydispersityindex were 209 nm and 0.03, respectively (entry 2, Table 4), ascompared to a z-average diameter of around 144 nm and thepolydispersity index of about 0.3 for the free gibbsite (entry 1,Table 4).
The encapsulation experiment with the more hydrophilicRAFT copolymer BA2.5-co-AA10 resulted in aggregation of thefinal encapsulated particles (Figure 6b and entry 2, Table 2),which suggests that there is a lack of surface charge for the
stabilization. This can be explained on the basis of the hydro-philicity of the RAFT copolymer. More hydrophilic copolymersare more likely to depart from the particle surface and migrateinto the aqueous phase. This would leave the particle surfacedeficient in the much needed stabilizing charge and hence theparticles aggregate. The aggregation of the encapsulated particlesis also evident from the dynamic light scattering measurementsgiving an average particle diameter of about 2 μm and a higherpolydispersity index of about 0.6 (entry 3, Table 4). Nevertheless,apart from the stability problems, the encapsulation of theplatelets is achieved even with this RAFT copolymer (Figure 6b).
As compared to BA5-co-AA10, increasing the hydrophobicityof the RAFT copolymer resulted in appreciable growth ofpolymer particles in the aqueous phase. Figure 6c shows thecomposite particles obtained by using RAFT copolymer BA7.5-co-AA10 (entry 3, Table 2) and it is clear that as a result of the freepolymer particles, the thickness of the polymer shell formed onthe surface of the platelets is relatively small compared to the oneobtained with the RAFT copolymer BA5-co-AA10 as is alsoevident from a relatively small particle diameter value of181 nm obtained by DLS (entry 4, Table 4). It is conceivablethat the RAFT copolymer with high BA content may containsome amphiphilic species which can self-assemble in the aqueousphase and cause secondary nucleation.
Figure 6d shows the composite latex particles obtainedbyusingtheRAFT copolymerBA5-co-AA5 (entry 4, Table 2). The absenceof any appreciable free polymer particles suggests thatmost of thepolymer is formed on the substrate surface. Substantial numbersof composite particles were also observed containing more thanone gibbsite platelet, which might result from the particle aggre-gation during encapsulation. This can be explained on the basis ofthe lower charge density of the RAFT copolymer BA5-co-AA5
because of which a larger amount would be needed to neutralizethe platelet surface charge and to get the charge inversion.26,31
This would leave the starting concentration of the RAFT copo-lymer in the water phase lower. As describe earlier, in order tomaintain the colloidal stability, sufficient RAFT copolymershould be present in the aqueous phase to adsorb onto thegrowing surface during encapsulation.19 Because of the insuffi-cient starting concentration of the RAFT copolymer it is likelythat during encapsulation its depletion causes the encapsulatedparticles to aggregate forming larger particles containing morethan one platelet.
Effect of Chain Length of RAFT Copolymer. The chainlength of the RAFT copolymers is expected to be of importancefor the success of the encapsulation by theRAFTapproach. It canbe expected that the longer RAFT copolymers having more
Figure 6. Cryo-TEM micrographs of the encapsulated gibbsiteparticles obtained by using different RAFT copolymers for theencapsulation: (a) BA5-co-AA10, (b) BA2.5-co-AA10, (c) BA7.5-co-AA10, and (d) BA5-co-AA5 (entries 1 to 4, Table 2). Monomer feedcomposition ratio in all the reactions was MMA:BA=10:1.
Table 4. Particle Diameters and ζ Potentials of the Composite
(31) Rojas, O. J.; Ernstsson, M.; Neuman, R. D.; Claesson, P. M. Effect ofpolyelectrolyte charge density on the adsorption and desorption behavior on mica.Langmuir 2002, 18, 1604-1612.
charged units can neutralize the platelet with a smaller number ofchains. This would lead to (a) a smaller number of RAFT groupson the surface and hence less control on polymer growth and (b)to more RAFT copolymers in the aqueous phase, which canpotentially give rise to secondary nucleation. Besides, watersolubility of the longer copolymers can also impart an adverseeffect on the encapsulation process.
To study the effect of the chain length, two more randomRAFT copolymers BA15-co-AA20 (with the same BA/AA ratio asBA7.5-co-AA10) andBA10-co-AA20 (with the sameBA/AA ratio asBA5-co-AA10) were synthesized and used for the encapsulationexperiments, using a 10:1 MMA/BA feed (entries 7 and 8,Table 2). Panels a and b of Figure 7 show the cryo-TEMmicrographs of the resulting latex particles. In both cases,appreciable growth of free polymer particles in the aqueous phaseis observed. This is also reflected in small particle diameters andhigh polydispersity index values obtained for these two latexes(entries 8 and 9, Table 4). Because of the formation of the freepolymer particles in the aqueous phase, with the same amount ofmonomer, the amount of polymer formed on the surface of theplatelets is reduced, resulting in a smaller particle diameter andhigher polydispersity index. Formation of free polymer particlesin the aqueous phase is an expected result for longer RAFTcopolymers. Owing to their limited solubility longer RAFTcopolymers can possibly collapse in the form of very smallparticles and induce secondary nucleation to give rise to thefree polymer particle formation. Longer copolymer chainsmay have self-assembling properties which can also give riseto secondary nucleation. Relatively higher viscosity of themedium in the presence of long-chain RAFT copolymers canalso give rise to the polymer particles in the aqueous phasebecause of a hindered monomer transport. It should be notedthat the extent of free polymer particle formation with thecopolymer BA15-co-AA20 is much higher than that with the morehydrophilic BA10-co-AA20, which is similar to what was observedfor 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 amaximumnumber of (RAFT group containing) chains on the substrate.This will facilitate the rapid transfer of the polymer growthbetween the polymer chains. However, RAFT copolymers thatare too small and possess a smaller number of anchoring chargeunits are more liable to migrate into the aqueous phase andmight cause problems of emulsion stability during the encapsula-tion reaction.
Effect of Monomer Feed Composition. To study the effectof monomer feed composition on the encapsulation, two morefeed compositions were explored (i.e., MMA:BA=7:3 and 100%
MMA)with use of RAFT copolymerBA5-co-AA10. A clear effecton the colloidal stability and themorphology of the latex particleswas observed as can be seen fromFigure 8, panels a and b (entries5 and 6, Table 2).
Control over the platelet orientation was completely lost andthemajority of the platelets were located outside the surface of thelatex particle in case of a higher butyl acrylate containing feed(MMA/BA= 7:3, Figure 8a). This effect can be attributed to thecomposition of the encapsulating polymer. The copolymer shellformedwith the feed composition containingmore BA is likely tobe more hydrophobic and with low glass transition temperature(Tg). High interfacial tension and the surface energy of thehydrophobic copolymer cause a minimization of the surface areaand drive the more hydrophilic gibbsite platelets toward thepolymer water interface. This process is aided by the low glasstransition temperature (Tg) of the copolymer, which providesenough mobility at the reaction temperature for the migration ofthe platelets toward the particle surface. Similar morphologytransformations were reported by Bon et al.32 and Armes et al.33
In these studies the migration of hydrophilic silica nanoparticlestoward the particle surface was observed for monomer composi-tions rich in more hydrophobic components such as BA32 andstyrene33 and the obtained morphologies were explained bysurface thermodynamics.
No segregation of the gibbsite platelets was observed for theparticles obtained with an all MMA monomer feed (entry 6,Table 2), as is clearly seen in Figure 8b. The surface of the polymershell on the platelet is uneven and the particles tend to aggregateas a result of insufficient colloidal stability. The aggregation is alsoevident from the larger particle diameters (870 nm) and higherpolydispersity index values (0.8) obtained with DLS (entry 7,Table 4). The uneven polymer growth is presumably caused byaggregation of secondary polymer particles onto the gibssitecontaining particles and because of the relatively high glasstransition temperature of the PMMA the polymer mobility isnot high enough for creating a smooth surface.Aggregation of theencapsulated gibssite particles is probably caused by the fact thatthe increased surface area of the uneven PMMA shell requiresmore RAFT copolymers for stabilization than are present in thesystem.
Figure 7. Cryo-TEMmicrographs of encapsulated gibbsite plate-lets obtainedbyusing theRAFTcopolymers (a)BA15-co-AA20and(b) BA10-co-AA20 for the encapsulation. Monomer feed composi-tion ratio was MMA:BA=10:1 for both reactions.
Figure 8. Cryo-TEMmicrographs of the composite latex samplesobtained by the encapsulation attempt of the gibbsite platelets byusing two different monomer compositions: (a) mixture ofMMA:BA=7:3 and (b)MMA only. RAFT copolymer BA5-co-AA10wasused for the encapsulation.
(32) Colver, P. J.; Colard, C. A. L.; Bon, S. A. F. Multilayered nanocompositepolymer 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 nano-composites using X-ray photoelectron spectroscopy. J. Mater. Chem. 2002, 12,697-702.
Particle Size Distributions.To probe the evolution of particlediameter and polydispersity index (PDI) as a function of theamount of monomer added, samples were taken at differentpercentages of monomer addition and were subjected to DLSmeasurements. In Figure 9a the obtained particle diameters andpolydispersity index (PDI) are shown as a function of the addedamount of monomer for the encapsulation reaction with use ofthe RAFT copolymer BA5-co-AA10 and a monomer feed com-position of MMA: BA=10:1 (entry 1, Table 2) . The z-averageparticle diameter increases during encapsulation indicating thegrowth of the polymer on the surface of the substrate; thepolydispersity index first increases during encapsulation and thendrops to a minimum at the final stages of the encapsulation. Apossible explanation for this trend is the aqueous phase growth ofthe RAFT copolymers. During encapsulation some of the excessRAFT copolymer chains present in the aqueous phase may alsogrow and at some stage become amphiphilic to form free polymerparticles causing the PDI to increase. These particles eventuallyadsorb onto the surface of the growing polymer shell on thesubstrate hence reducing the final PDI of the latex. The sametrend was also observed for the encapsulation reactions involvingRAFT copolymers BA7.5-co-AA10 and BA5-co-AA5 under thesame conditions. Figure 9b shows the cryo-TEMmicrographs at50%and 100%monomer addition. The free polymer particles areclearly seen at 50% monomer addition, but have virtuallydisappeared 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 thePDIof theparticles at the final stages
of the encapsulation reaction (around 75% of monomer added),
which as explained earlier might be due to the rearrangement
of the hydrophilic platelets in the more hydrophobic copolymer.Figure 10b shows the evolution of particle size and PDI for the
Figure 10. Evolution of particle size (9) and polydispersity index (PDI) (O) as a function of conversion for the encapsulation of gibbsiteplatelets by using (a) RAFT copolymer BA5-co-AA10 (monomer feed composition ratio MMA:BA = 7:3), (b) RAFT copolymer BA5-co-AA10 (monomer feed MMA only), and (c) RAFT copolymer BA15-co-AA20 (monomer feed composition ratio MMA:BA= 10:1).
Figure 9. (a) Evolution of particle diameter (9) and polydispersityindex (O) as a function of the amount of monomer added for theencapsulation of gibbsite platelets by usingRAFTcopolymerBA5-co-AA10 and monomer feed composition ratio of MMA:BA=10:1. (b) Cryo-TEM micrographs of the latex sample at 50% and100%monomer added.
encapsulation reaction with use of BA5-co-AA10 with a monomerfeed comprising MMA only (entry 6, Table 2).
A sharp increase in the particle diameter and PDI in the finalstage of the encapsulation might be attributed to the aggregationof the particles. Figure 10c shows the evolution of particle size andPDI as a function of the amount of monomer added for theencapsulation reaction with use of the long chain RAFT copoly-mer BA15-co-AA20 with a feed composition of MMA:BA=10:1(entry 7, Table 2). The increase in the particle diameter indicatesthe polymer growth, while the increase in polydispersity indexwascaused by the formation of a large number of free polymerparticles, as also shownby the cryo-TEMmicrograph (Figure 7a).
ζPotentials.Themeasured ζ potentials of the original gibbsiteplatelets and the composite latex particles are given in Table 4.The ζ potential of the gibbsite platelets was found to be around+50mV at the working pH of around 7 because of the ionizationof the surface AlOH groups to Al(OH)2
+. This value changes toaround -50 mV after the encapsulation, indicating that thecomposite latex particles are stabilized by a layer of negativelycharged carboxylic acid groups of the RAFT copolymer chains.
Molecular Weight Distributions. To probe whether polym-erization occurred under RAFT control, the molecular weightgrowth was followed during the polymerization by using GPC.Because the encapsulation reactions were conducted under
starved feed conditions, the amount of monomer added into thesystem can be approximated to be the monomer converted.Samples were taken at different amounts of monomer added intothe system and the GPC measurements were carried out on thetotal polymer present. Parts a and b of Figure 11 show themolecular weight distributions and the evolution of the molecularweight as a function of the added amount of monomer, respec-tively, for the encapsulation reaction, using RAFT copolymerBA5-co-AA10 and amonomer feed compositionMMA:BA=10:1(entry 1, Table 2). From the continuous increase of the numberaverage molecular weight with conversion, it is clear that thepolymerization happens under RAFT control up to high percen-tages ofmonomer conversion. Initially the obtained chain lengthsare larger than those predicted by eq 1, but at high conversionsthey become smaller. A reason for this behaviormay lie in the factthat only those RAFT copolymers that are adsorbed onto theGibbsite will participate in the polymerization process.19 Withincreasing conversion, more RAFT copolymers will adsorb ontothe growing particle from the aqueous phase, thus increasing thenumber of growing polymer chains and hence decreasing thevalue forMn. This effect and the relatively low [RAFT]0/[I]0 ratioalso explain the high PDI.Mechanism of Encapsulation. From the above observations,
the mechanism of the encapsulation with RAFT copolymer isproposed to be as follows. The amphipatic randomRAFT copoly-mers when dispersed in water can act as stabilizers and adsorb ontothe oppositely charged substrate by electrostatic interactions. Theresulting dispersion displays an overall charge of opposite sign totheone the substrate particle originally bears.The randomnature ofthe RAFT copolymer chains prevents them from self-assembling inthe aqueous phase, and thus reducing the formation of centers forsecondary particle nucleation. Once the polymerization is started,the adsorbed RAFT copolymer chains extend by undergoing rapidtransfer and incremental growth on the particle surface, resulting ina uniform coating over the entire surface. As the reaction proceeds,some of the RAFT copolymer present in the aqueous phasebecomes adsorbed onto the growing surface offered to them bythe growth of polymer on the surface of the substrate providing therequired stabilization. The RAFT copolymers present in the aqu-eous phase can also chain extend, become amphiphilic, and self-assemble to form centers for secondary particle nucleation. Thiswould generate a crop of new particles most of which may alsoeventually adsorb onto the surface of the substrate.
Conclusions
We presented a simple RAFT copolymer approach to synthe-size anisotropic polymer-inorganic nanocomposite latexparticles. In this approach the composite latex particles arestabilized by a layer of negatively charged carboxylic acid groupsof the RAFT copolymer chains thus avoiding the need for anyexternal surfactant. The approach was successful to encapsulatethe model clay gibbsite and as revealed by the cryo-TEMcharacterizations, the goal to synthesize anisotropic compositelatex particles by encapsulating single platelets inside a latexparticle was achieved with excellent control over the plateletorientation. The hydrophilic-lipophilic balance and chain lengthof the RAFT copolymers and monomer feed composition werefound to have a significant effect on the efficiency of theencapsulation reaction. More hydrophobic and long-chainRAFT copolymers give rise to more secondary particle forma-tion. A monomer feed comprising more hydrophobic monomerresults in the loss of morphology control during encapsulation,leading to an “armored” morphology. The best encapsulation
Figure 11. Molecular weight evolution during encapsulation byusing RAFT copolymer BA5-coAA10 and a feed composition ofMMA:BA=10:1 (entry 1, Table 2): (a) molecular weight distribu-tions and (b) Mn (3) and PDI (9). The straight line in panelb corresponds to the theoretical Mn vs conversion obtained byusing eq 1.
results were obtained by using RAFT copolymer BA5-co-AA10
with a monomer feed composition MMA:BA=10:1. The layerthickness can easily be controlled by the amount of monomerfed into the system. In the future the approach will be extendedto encapsulate the natural clays such as montmorilonite andlaponite to obtain composite latexes. Some preliminary resultsshow that the encapsulation of natural clays is also feasible throughthis route.
Acknowledgment. We gratefully acknowledge Dr. NicoSommerdijk and Paul Bomans of the Soft Matter cryo-TEMResearchUnit ofEindhovenUniversity ofTechnology for helpfuldiscussions about cryo-TEM and Dr. Laurence Couvreur ofArkema for a kind gift of a sample of the RAFT agent dibenzyltrithiocarbonate. S.I.A. is grateful for financial support by theHigher Education Commission, Government of Pakistan, underthe HEC-NUFFIC program.
