pigment encapsulation by emulsion polymerization using macro-raft copolymers

Pigment Encapsulation by Emulsion Polymerization UsingMacro-RAFT Copolymers

Duc Nguyen,† Hollie S. Zondanos,† Jason M. Farrugia,‡ Algirdas K. Serelis,‡Chris H. Such,‡ and Brian S. Hawkett*,†

Key Centre for Polymer Colloids, Chemistry F11, UniVersity of Sydney, NSW 2006, Australia, and DuluxAustralia, 1970 Princes Highway, Clayton, VIC 3168, Australia

ReceiVed September 5, 2007. In Final Form: NoVember 2, 2007

A new method is described, based on living amphipathic random macro-RAFT copolymers, which enables theefficient polymeric encapsulation of both inorganic and organic particulate materials via free-radical polymerization.The mechanism for this new approach is examined in the context of the polymer coating of zirconia- and alumina-coated titanium dioxide particles and its breadth of application demonstrated by the coating of organic phthalocyanineblue pigment particles. The particulate materials were first dispersed in water using a macro-RAFT copolymer as astabilizer. Monomer and water-soluble initiator were then added to the system, and the monomer polymerized to formthe coating. If nucleation of new polymer particles in the aqueous phase was to be avoided, it was found necessaryto use a macro-RAFT copolymer that did not form micelles; within this constraint, a broad range of RAFT agentscould be used. The macro-RAFT agents used in this work were found not to transfer competitively in the aqueousphase and therefore did not support growth of aqueous-phase polymer. Successful encapsulation of particles wasdemonstrated by TEM. The process described enables 100% of the particles to be encapsulated with greater than 95%of the polymer finishing up in the polymeric shells around the particles. Moreover, the coating reaction can be carriedout at greater than 50% solids in many cases and avoids the agglomeration of particles during the coating step.

Introduction

Pigment and polymer latex are the most important ingredientsin water-based paint and ink formulations.1During film formation,latex particles coalesce to form a polymer film covering thesubstrate surface while the presence of pigment particles in thefilm provides the final coating with color and influences otherappearance properties such as opacity and gloss.1-14 With suchan important role, pigments are typically manufactured with aprimary particle size that is designed to deliver optimum effectsin the paint film.1 However, due to their surface properties andsmall size, usually sub-micrometer, one of the most challengingtasks for the paint technologist is to disperse pigments to their

primary particle size and to maintain the quality of that dispersionthroughout the manufacturing, storage, and most importantly,through the film formation process to the final coating.2,3,10-12,15

A general problem with latex paints is that pigment agglomerationoccurs during the film formation process, forming pigmentaggregates; such aggregates have similar effectiveness to singlepigment particles.10,11Pigment agglomeration thus significantlyreduces the pigment efficiency, resulting in a lower quality productat higher cost.7 Moreover, the appearance of aggregates on thefilm surface reduces surface smoothness and leads to lowergloss.2,10,11

An ultimate solution that avoids pigment agglomeration is toencapsulate the primary pigment particles with a layer of binderpolymer, creating polymer shells that ensure that the pigmentparticles remain separated during film formation. For thisapproach to be effective, the process needs to be very efficient,ensuring that all pigment particles are coated and avoiding suchproblems as pigment agglomeration during the encapsulationprocess.

With so much potential to offer, considerable researcheffort16-49 has been applied to the problem of encapsulatingindividual pigment particles in a cost-effective and efficient

* To whom correspondence should be addressed. E-mail: [email protected].

† Key Centre for Polymer Colloids.‡ Dulux Australia.(1) Dieter Urban; Takamura, K.Polymer Dispersions and Their Industrial

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10.1021/la7027466 CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 01/16/2008

manner. Most past attempts have involved the use of aqueousfree-radical polymerization methods such as emulsion16-18,21,34,35,46

or miniemulsion30-33,38,39,48polymerization. Haga et al.16 at-tempted to encapsulate titanium dioxide with poly(methylmethacrylate) and polystyrene by carrying out emulsion po-lymerization of monomers in the presence of pigment dispersions.They found that the pH of polymerization, initiator types, andcharges on polymer chains and pigment surfaces greatlyinfluenced encapsulation outcomes. Employing a monomer-starvefeed process, Viala et al.35 encapsulated a number of inorganicpigments by slowly adding a mixture of hydrophilic andhydrophobic monomers, such as methacrylic acid, methacrylateesters, and vinyl acetate, into pigment dispersions. Carboxylicacid groups on the surface of encapsulated pigment particleswere subsequently partially neutralized with base to make thefinal products dispersible in water. Pursuing a different approach,Janssen et al.19,21and Oliveira et al.34,46modified the surface oftitanium dioxide to make it hydrophobic. Encapsulation of theresulting pigment was carried out by emulsion polymerizationof methyl methacrylate or styrene. However, it was reported thatthe encapsulated pigment had incomplete polymer coverage,displaying a raspberrylike core-shell morphology.46 The use ofminiemulsion polymerization for pigment encapsulation has alsobeen extensive. Erdem et al.30-32 dispersed titanium dioxidepigment in styrene using a steric stabilizer. To this dispersionhydrophobic species such as hexadecane and polystyrene wereadded, and the whole mixture was in turn sonically dispersed ina surfactant solution to form pigment-containing miniemulsiondroplets. Subsequent polymerization of the miniemulsion dropletsproduced encapsulated pigment. Employing a similar process,

successful polymer encapsulation of carbon black,33 phthalo-cyanine blue,39 calcium carbonate,49 and other pigments48 hasbeen reported. However, pigment encapsulation methods usingfree-radical emulsion or miniemulsion polymerization suffer froma number of problems such as low efficiency16,21,30-32,39,46andmay require complex treatment of pigment surface19,21,34,46,49

prior to encapsulation. In the case of miniemulsion poly-merization, the presence of hydrophobes such as hexadecaneand polystyrene is essential and may alter properties of finalproducts.30-33,38,39,48,49Most methods utilized low concentrationsof pigments,16,30-32,34,39,46rendering the processes unsuitable forindustrial production. Furthermore, pigment particles often losestability and form aggregates.16,35In most cases, due to extensiveuse of surfactants, nucleation of new particles is difficult to avoid,producing polymer particles as byproducts which reduceencapsulation efficiency.30-32,39,46

Recent developments50-55 in the use of Reversible AdditionFragmentation chain Transfer (RAFT) controlled radical po-lymerization in disperse phase media have paved the way for anew approach to the encapsulation of particles. For an example,Ferguson et al.50,52first described the use of amphipathic macro-RAFT agents to synthesize self-assembling oligomeric diblockscopolymers in water which can be further grown to formmonodisperse polymer latex particles. The latexes produced bythis method are stabilized by anchored units of hydrophilicmonomer, e.g., acrylic acid, eliminating the need for addedsurfactant. Pham et al.51 further developed use of amphipathicmacro-RAFT agents for miniemulsion polymerization. In theirwork, diblock macro-RAFT agents were synthesized andemployed to emulsify hydrophobic monomers by sonication.The resulting miniemulsion was found to be stable against Ostwaldripening despite the absence of a hydrophobic stabilizer.Polymerization of the miniemulsion produced latex with a one-to-one correspondence between droplets and particles and goodmolecular weight control. In this system the final latex particlesare stabilized by the hydrophilic blocks originally present in theamphipathic macro-RAFT agents used to stabilize the originalminiemulsion. This work suggested that living amphipathicmacro-RAFT agents could potentially be used as dispersants forpigments. Moreover, since RAFT controlled polymerizationallows rapid transfer of growth from polymer chain to polymerchain with the addition of few monomer units per growthsequence, it is feasible to encapsulate dispersed pigment particlesby further growth of the amphipathic macro-RAFT agents usedto stabilize the original pigment dispersion.

The use of amphipathic random copolymers to disperse pigmentparticles has been well documented.1 One typical example isacrylic copolymers which contain acrylic acid along withhydrophobic acrylate monomer units randomly distributed alongpolymer chains.1,10,11 Neutralized solutions of such polymershave been found to be effective dispersants for both organic andinorganic pigments.1,10,11 By employing RAFT-controlled po-lymerization, random copolymers with similar molecular weightand properties can be readily prepared. Moreover, the macro-

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Pigment Encapsulation by Emulsion Polymerization Langmuir, Vol. 24, No. 5, 20082141

RAFT copolymers thus synthesized have the clear advantage ofcarrying living ends, which allow further chain extension via theaddition of desired monomer units at controlled rates. Unlikewater-soluble amphipathic block copolymers, water-solublerandom macro-RAFT agents cannot form micelles. The absenceof micelles potentially minimizes new particle formation, thusensuring that more of the polymer formed goes onto the pigmentparticle surface, leading to more efficient pigment encapsulation.Therefore, in this paper, we describe a novel method for theencapsulation of solid particulate material wherein livingamphipathic random macro-RAFT copolymers are used tostabilize dispersions of inorganic and organic pigments and grownon to form an encapsulating polymer shell. The method involvesdispersion of pigments in a neutralized macro-RAFT solution,which is followed by an encapsulation process. During encap-sulation, hydrophobic monomers, such as methyl methacrylateand butyl acrylate, are slowly added into the dispersion underpolymerization conditions, forming encapsulating polymer shellsaround the pigment particles.

Experimental

Reagents.Milli RO water was used in the synthesis of latexesand acrylic acid-containing RAFT agents. Acrylic acid (AA) (Aldrich)was purified by distillation under reduced pressure. Butyl acrylate(BA) (Aldrich) and methyl methacrylate (MMA) (Aldrich) hadinhibitor removed by passing them through an inhibitor-removalcolumn (Aldrich). Alumina and zirconia-coated titanium dioxide(Tioxide TR-92) (Huntsman Corporation), phthalocyanine bluepigment (Heliogen Blue L6900) (BASF), sodium hydroxide (NaOH),ammonium hydroxide (NH4OH) (Aldrich), carbon disulfide(Aldrich), acetone (Aldrich), tetrabutylammonium bromide(Aldrich), 2-bromopropanoic acid (Aldrich), diethyl ether (Aldrich,Univar), hydrochloric acid (Aldrich, Univar), sodium sulfate(Aldrich), tetrahydrofuran (THF) (Aldrich), trifluoroacetic acid (TFA)(Aldrich), 2,2′-azobisisobutyronitrile (AIBN) (Wako), and 4,4′-azobis(4-cyanopentanoic acid) (V-501) (Wako) were used asreceived. Dioxane (Aldrich) was distilled under reduced pressurebefore use.

The RAFT agents, 2-{[(butylsulfanyl)carbonothioyl]sulfanyl}-propanoic acid52(RAFT-V, Figure 1) and 2-[(ethoxycarbonothioyl)-sulfanyl]succinic acid56 (RAFT-XIII, Figure 1) were synthesized aspreviously described. The RAFT agent 2,2′-(carbonothioyldisul-fanediyl)dipropanoic acid (RAFT-XIV, Figure 1) was synthesizedas described below.

Synthesis of 2,2′-(Carbonothioyldisulfanediyl)dipropanoicAcid (RAFT-XIV). Sodium sulfide nonahydrate (24.0 g, 100 mmol)was stirred with water (40 mL) until it had all dissolved.Tetrabutylammonium bromide (3.22 g, 10.0 mmol) was added,followed by acetone (200 mL), and then carbon disulfide (20 mL,

25.3 g, 333 mmol). The resulting red two-phase mixture was stirredvigorously at room temperature for 19 h, then treated with the additionof a freshly prepared solution of sodium 2-bromopropanoate madeby slowly adding 2-bromopropanoic acid (30.6 g, 200 mmol) toice-cooled 25% sodium hydroxide solution (32.0 g, containing 8.00g, 200 mmol NaOH) at such a rate that the temperature did notexceed ca. 32°C. The resulting orange-yellow reaction was stirredat ambient ca. temperature for 3 h, then acidified with 3 M HCl (80mL, 240 mmol) and evaporated to dryness by rotary evaporator. Theorange and white residue was partitioned between water (100 mL)and 3:1 ether/dichloromethane (250 mL). The organic layer wasdried (sodium sulfate) and evaporated to give an orange solid, 27.12g. The crude product was recrystallized from ethyl acetate/toluenein an open flask over 4 days to give a hard orange cake which wasbroken up, filtered, ground, and air-dried to give a yellow powder,comprising a ca. 82:18 mixture (from13C intensities) of therac- andmeso-diastereoisomers of RAFT-XIV, 11.39 g, 42%, mp 143-8°C.The mother liquors were evaporated, affording an orange oil whichwas crystallized from toluene to give a hard orange cake which wasbroken up and filtered to give RAFT-XIV, now as a ca. 38:62 mixtureof rac- and meso-diastereoisomers, as a yellow powder, 6.51 g,24%, mp 91-8 °C. Concentrating the mother liquors gave a secondcrop of RAFT-XIV (ca. 29:71rac- andmeso-), 4.61 g, 17%, mp93-8 °C. δH(chloroform-d) 4.72 (1H, q, J 7.5, rac-CH), 4.65 (1H,q, J 7.5,meso-CH), 1.66 (3H, d, J 7.6,rac-CH3), 1.64 (1H, d, J 7.6,meso-CH3). δC(chloroform-d + DMSO-d6) 220.2 (rac-, meso-CdS), 172.74 (rac-CdO), 172.67 (meso-CdO), 48.61 (rac-CH), 48.56(meso-CH), 17.14 (meso-CH3), 17.10 (rac-CH3).

Electrospay Mass Spectrometer Analysis.Analysis was carriedout using a Finnigan Mat LCQ MS detector with Finnigan LCQData Processing and Instrument Control Software. Ten microgramsamples were dissolved in 10 mL of 50:50 methanol/H2O and fedinto the electrospray ionization unit at 0.2 mL min-1. The electrosprayvoltage was 5 kV, the sheathing gas was nitrogen at 415 kPa, andthe heated capillary was at 200°C.

Preparation of Macro-RAFT Agents. Random macro-RAFTagents were synthesized by reacting RAFT agents with a monomermixture of butyl acrylate and acrylic acid in the presence of initiator.A macro-RAFT agent containing, on average, 5 butyl acrylate and10 acrylic acid units using the RAFT agent having a butanethiol Zgroup, hence referred to as macro-RAFT-V(5BA-co-10AA) wassynthesized as follows. A solution of RAFT-V (1.11 g, 4.6 mmol)and 2,2′-azobisisobutyronitrile (0.07 g, 0.4 mmol), acrylic acid (3.39g, 47.0 mmol), and butyl acrylate (3.16 g, 24.6 mmol) was preparedin dioxane (7.58 g) in a 50 mL round-bottomed flask. This wasstirred magnetically and sparged with nitrogen for 10 min. The flaskwas then heated to and maintained at 70°C for 3 h underconstant stirring. The resulting product was characterized byelectrospray mass spectrometry. Electrospray mass spectroscopyshowed a distribution of copolymer species of various masses andcompositions, giving a semiquantitative indication of their relativeabundance.

Macro-RAFT agents with different BA/AA ratios were synthesizedin a similar manner. Macro-RAFT agents containing, on average,10 acrylic acid units and 2.5 BA units (RAFT-V(2.5BA-co-10AA));7.5 BA units and 10 AA units (RAFT-V(7.5BA-co-10AA)) weresynthesized for adsorption studies on the titanium dioxide pigment.Macro-RAFT agent with five acrylic acid and five butyl acrylateunits (RAFT-V(5BA-co-5AA)) was prepared and used for dispersionand encapsulation of phthalocyanine blue pigment.

To demonstrate the versatility of the encapsulation method, otherRAFT agents, RAFT-XIII and RAFT-XIV, were used to synthesizerandom macro-RAFT copolymers. The synthesis was similar to thatusing RAFT-V. On average, the macro-RAFT synthesized fromRAFT agent RAFT-XIII contained 10 units of acrylic acid and 5units of butyl acrylate (RAFT-XIII(5BA-co-10AA)). The macro-RAFT synthesized from RAFT agent RAFT-XIV contained 10 unitsof acrylic acid and 10 units of butyl acrylate (RAFT-XIV(10BA-co-10AA)). These macro-RAFT agents were both used for thedispersion and encapsulation of titanium dioxide.

(56) Hawkett, B. S.; Such, C. H.; Nguyen, D. N.; Farrugia, J. M.; MacKinnon,O. M. WO 2005-AU1512, 2006037161, 2006; p 145.

Figure 1. RAFT agents used in the synthesis of random macro-RAFT copolymers.

2142 Langmuir, Vol. 24, No. 5, 2008 Nguyen et al.

Pigment Dispersion.Pigment dispersion was normally carriedout by mechanical milling in a bead mill57 or by ultrasonication.57,58

The main aim of this process was to apply external force to breakup pigment aggregates, reducing them to primary particle size. Duringthe dispersion process pigment particles interact with stabilizermolecules in the dispersion medium, so that by the end of the processthey acquired a sufficiently thick steric layer or enough surfacecharge to remain in stable dispersion. Very often, considerable energyis required to disperse pigments to their primary particle size,depending on the primary particle size and the nature of the pigmentsurface. The hydrophilic titanium dioxide pigment could be readilydispersed using ultrasonication, but the phthalocyanine blue pigment,with a much smaller primary particle size and hydrophobic surface,required a bead mill run at high milling speed.

To obtain the necessary dispersion stability, it is of utmostimportance that there are interactions between stabilizers and pigmentsurfaces. By adsorbing onto the pigment surface, layers of chargesor hydrophilic hairs are generated on the pigment particle surface,which help pigment particles repel each other and diminish the chanceof them reaggregating. The ability of stabilizers to adsorb onto thepigment particle surface is greatly influenced by the nature of thatsurface, such as its hydrophobicity and the presence of functionalgroups and charges. In order to achieve a stable dispersion the choiceof compatible stabilizers must be made accordingly. We have foundthat more hydrophobic surfaces generally require a more hydrophobicstabilizer than hydrophilic surfaces while a charged surface generallyinteracts well with stabilizers having opposite charge.

Pigment Encapsulation. The encapsulation of the dispersedpigment particles was readily achieved by feeding monomer into thepigment dispersion in the presence of a water-soluble initiator at areaction temperature of 70°C, as depicted schematically in Figure2. The macro-RAFT copolymer molecules adsorbed onto andstabilized the pigment particle surface, where they facilitate therapid transfer of hydrophobic polymer growth from molecule tomolecule over the entire pigment surface. This orderly extension ofeach polymer chain resulted in an even build-up of polymer in thelayer surrounding the pigment particles, leading to their encapsulation.Throughout the encapsulation process, the ionic outer layer derivedfrom deprotonated acrylic acid groups maintained charge stabilizationof the encapsulated particles.

Dispersion and Encapsulation of Titanium Dioxide. Thedispersion and encapsulation process is depicted schematically inFigure 2. A detailed description of the dispersion and encapsulationof titanium dioxide using RAFT-V(5BA-co-10AA) is as follows. Asolution containing RAFT-V(5BA-co-10AA) (0.93 g, 0.3 mmol),water (51.81 g), and sodium hydroxide (0.11 g, 2.6 mmol) wasprepared in a 100 mL beaker. To this solution, TiO2 pigment (10.69g) was added, mixed, and then further thoroughly dispersed usinga Vibra-Cell Ultrasonic Processor (Sonics and Materials, Inc.)standard probe at 30% amplitude for 10 min. During the sonicationprocess, the dispersion was stirred magnetically and cooled in awater bath. The white dispersion (58.34 g) was then transferred toa 100 mL round-bottomed flask containing 4,4′-azobis(4-cyanovaleric

acid) (0.029 g, 0.1 mmol) and was deoxygenated by nitrogen sparging.The whole flask was immersed in an oil bath with a temperaturesetting of 70°C and stirred magnetically while a deoxygenatedmixture of butyl acrylate (1.39 g, 10.9 mmol) and methyl methacrylate(3.24 g, 32.4 mmol) was fed in at 0.92 g h-1 over 5 h. After monomeraddition was complete, the heating was continued overnight, afterwhich time polymerization was found to be complete. Theencapsulating polymer layer around the TiO2 particles was furtherincreased by adding another deoxygenated mixture of butyl acrylate(1.39 g, 10.9 mmol) and methyl methacrylate (3.24 g, 32.4 mmol)at 2.3 g h-1 into the above latex (55.95 g) in the presence of 4,4′-azobis(4-cyanovaleric acid) (0.025 g, 0.1 mmol) at 70°C over 2 h.After monomer addition was complete, the temperature wasmaintained for a further 1 h to effect full polymerization. Afterfiltering, the resulting latex was characterized by light scattering(for particle sizes and particle size distribution, HPPS, MalvernInstruments, Ltd.) and transmission electron microscopy (TEM,Biofilter, Philips).

Titanium dioxide encapsulation using RAFT-XIII(5BA-co-10AA)and RAFTXIV(10BA-co-10AA) was carried out in the same manner.The resulting latexes were also characterized by light scattering andTEM.

DispersionandEncapsulationofPhthalocyanineBluePigment.The dispersion of the phthalocyanine blue pigment was much moredifficult than the dispersion of the titanium dioxide pigment andcould not be accomplished using a sonic probe. The dispersion andencapsulation of phthalocyanine was carried out as follows. A solutioncontaining RAFT-V(5BA-co-5AA) (0.73 g, 0.3 mmol), ethyleneglycol (19.08 g), and methanol (3.10 g) was prepared in a 50 mLbeaker. To this solution, water (10.49 g) and then sodium hydroxide(0.06 g, 1.48 mmol) was added, mixed, and sonicated in a sonic bathfor 2 min. The solution was transferred to a water-jacketed millingvessel (Dispermat AE 3C laboratory dissolver fitted with an APS250 milling system, VMA-Getzmann) containing phthalocyanineblue pigment (5.01 g) and 1 mm diameter glass beads (101 g). Thebath jacket temperature was maintained at 20°C. The milling wasinitially at 1000 rpm for 60 min to produce a viscous blue dispersion.Then, more water (20.00 g) and glass beads (50 g) were added intothe milling vessel and the milling speed was raised to 6000 rpm for60 min. At the end of the milling period, another portion of water(70.17 g) was mixed into the pigment dispersion. Foam and glassbeads were then separated from the dispersion using a plastic meshwhile any remaining pigment aggregates were removed by cen-trifugation at 2000 rpm for 5 min (MSE MK2 centrifuge, ThomasOptical and Scientific Co. Pty Ltd). The pigment dispersion (50.64g) was transferred into a 100 mL round-bottomed flask containing4,4′-azobis(4-cyanovaleric acid) (0.025 g, 0.1 mmol). The flask wassealed, sparged with nitrogen for 15 min, placed in an oil bathmaintained at 70°C, and stirred magnetically. A deoxygenatedsolution of butyl acrylate (1.39 g, 10.9 mmol) and methyl methacrylate(3.24 g, 32.4 mmol) was injected into the flask at a rate of 1 mLh-1 over 5 h. Addition commenced 10 min after completion of theinitial sparge. After monomer addition was complete bath temperaturewas maintained at 70°C overnight, after which polymerization wasfound to be complete. After filtering, a stable blue latex wascharacterized using dynamic laser light scattering (for particle sizes

(57) Merrington, J.; Hodge, P.; Yeates, S.Macromol. Rapid Commun.2006,27, 835-840.

(58) Schutyser, P.; Van Eecke, M. C.; Piens, M.FATIPEC Congress2000,25, 197-214.

Figure 2. Schematic representation of the dispersion and encapsulation of pigment particles using macro-RAFT random copolymers.


and particle size distribution, HPPS, Malvern Instruments, Ltd.) andTEM (Biofilter, Philips).

Size-Exclusion Chromatography (SEC).Measurements wereperformed on a Shimadzu system fitted with a series of Waterscolumns (HR4, HR3, HR2, and HR1) and a DRI detector. PolymerLaboratories Cirrus Software was used with all molecular weightsbeing relative to polystyrene standards. THF mixed with 0.1 wt %TFA was used as eluent to block carboxylic acid interactions dueto the presence of acrylic acid groups in the random macro-RAFTcopolymer.

Samples for SEC measurements were taken from the titaniumdioxide encapsulation reaction after 0%, 23%, 41%, 74%, and 100%of the monomer had been added. The polymer was extracted fromthe samples using the THF/TFA eluent mixture over a 24 h period.The samples were then centrifuged and filtered to remove titaniumdioxide.

Results and Discussion

Dispersion and Encapsulation of Titanium Dioxide.Macro-RAFT Random Copolymers.The living random macro-RAFT copolymer RAFT-V(5BA-co-10AA) was prepared asdescribed above. The reaction was very straight forward involvingthe heating of the monomer/RAFT/dioxane mixture in thepresence of initiator at 70°C under inert gas. Electrospray massspectroscopy showed that polymerization was under RAFTcontrol with a narrow molecular weight distribution (Figure 3).However, as demonstrated elsewhere,52 the overall molecularweight distribution for a copolymer is comprised of several sub-distributions and is generally broader than when a single monomersuch as AA or BA is used.

One of the key requirements for the macro-RAFT agents usedfor encapsulation is that they do not form micelles when dispersedin the reaction medium. To satisfy this, the synthesized macro-RAFT copolymers should ideally have a random distribution ofhydrophobic and hydrophilic units. In order to verify therandomness of these copolymers, a polymerization experimentwas carried out in which the composition of the residual monomermix in the reaction was monitored by proton NMR as a functionof conversion. 1,3,5-Trioxane was used as an internal standard;samples were taken at 30 min intervals and immediately quenchedin iced water. Residual acrylic acid and butyl acrylate monomerlevels in each sample were calculated from the signal ratiosbetween monomer and trioxane and taken to represent monomerlevels in the reactor at the time of sampling. As shown in Figure4, the extent of conversion of butyl acrylate and acrylic acidwere almost identical, confirming the random nature of thecopolymer. Thus, a living random copolymer of reasonablynarrow molecular weight distribution was obtained. These resultsare consistent with the very similar reactivity ratios for these twomonomers.59

Two further macro-RAFT agents with different molar ratiosof butyl acrylate and acrylic acid were synthesized for adsorption

studies on the titanium dioxide pigment surface. They containedon average 10 acrylic acid units and 7.5 (RAFT-V(7.5BA-co-10AA)) and 2.5 (RAFT-V(2.5BA-co-10AA)) butyl acrylate units,respectively. In order to illustrate the versatility of the approachother macro-RAFT agents, RAFT-XIII(5BA-co-10AA) andRAFT-XIV(10BA-co-10AA), were also synthesized for theencapsulation of titanium dioxide. All macro-RAFT copolymerswere prepared in the same manner as RAFT-V(5BA-co-10AA)and could therefore be reasonably expected to also have randommonomer distributions. In all experiments, mass spectroscopyof the macro-RAFT copolymers formed showed narrow molecularweight distributions, indicating good control by the RAFT agents.

Dispersion of Titanium Dioxide.The titanium dioxide thatwas used in these experiments is representative of pigments withhydrophilic surfaces and which generally carry surface charge.In this study, a general purpose zirconia and alumina-coatedtitanium dioxide pigment was used. To disperse this pigment,attention was paid to the charge on the surface of the pigmentparticles at various pH’s. Liang et al.60studied the surface chargeof this pigment and found that it has an isoelectric point at pH7.3 with a net positive charge on the particle surface at lowerpH’s and a net negative charge above that point. Hence, RAFT-V(5BA-co-10AA) was used as a stabilizer to disperse the titaniumdioxide pigment in water in the pH range from 5.5 to 8. In thispH range, the net pigment surface charge was positive or closeto zero and was found to facilitate the adsorption of the negativecharge carrying macro-RAFT copolymer. It was found that, afterultrasonication, pigment particles were well dispersed at pH 5.5with aZ-averaged size of 278 nm (Table 1). The dispersion wasfound not to sediment for over 24 h, thus providing sufficienttime to carry out the encapsulation step. Even toward the endof the working pH range, e.g., pH 8 (just above the point of zerocharge), a good dispersion of titanium dioxide pigment was stillachieved, indicating an adequate level of adsorption of the macro-RAFT onto pigment particle surface at that pH.

In order to better understand the ability of different macro-RAFT copolymers to adsorb onto the zirconia and alumina-coated titanium dioxide pigment surface at different pH’s, anadsorption study of three macro-RAFT copolymers, RAFT-V(2.5BA-co-10AA), RAFT-V(5BA-co-10AA), and RAFT-V(7.5BA-co-10AA), was carried out at pH 6, 7, 8, 9, and 10. In

(59) Brandrup, J. I., Edmund H.; Grulke, Eric A.; Abe, Akihiro; Bloch, DanielR. Polymer Handbook, 4th ed.; 2005.

(60) Liang, G. G.; Hawkett, B. S.; Tanner, R. I.J. Dispersion Sci. Technol.2005, 26, 469-472.

Figure 3. Mass electrospray of RAFT-V(5BA-co-10AA).

Figure 4. Conversion of acrylic acid and butyl acrylate duringRAFT-V(5BA-co-10AA) copolymer synthesis. AIBN was used asan initiator. The polymerization was carried out at 60°C in dioxane(49 wt %) with the presence of 1,3,5-trioxane (1.5 wt %) as aninternal reference for NMR measurement.


each of these experiments. the concentration of macro-RAFTwas maintained at the same level as in the encapsulationexperiment and dispersion was carried out in the same manner.Figure 5 clearly shows that the amount of macro-RAFT copolymeradsorbed onto the pigment surface decrease with increasing pH,especially at a pH of 9 and 10. This trend is consistent with asignificant build up of a net negative charge (and possibly moreimportantly, significant reduction in positive surface charge) onthe pigment surface with increasing pH, which significantlyreduced the adsorption of the negative charge carrying macro-RAFT copolymers. Figure 5 also shows that adsorption of macro-RAFT copolymers was very dependent on their overall hydro-phobicity. Over the studied pH range, the macro-RAFT RAFT-V(7.5BA-co-10AA) is best adsorbed onto pigment surfacefollowed by RAFT-V(5BA-co-10AA). Macro-RAFT RAFT-V(2.5BA-co-10AA) displays poor adsorption which declinesrapidly to zero at pH 10. This indicates copolymers with a largenumber of hydrophobic units are more surface active and preferto interact with the pigment surface, resulting in good adsorptioncharacteristics. The configuration that the adsorbed macro-RAFTagents on the surface is not yet understood and warrants furtherinvestigation. However, we believe that they adsorb as a bilayer,with the inner layer anchoring the coating to the surface and theouter layer providing stabilization to the particles as the particlegrows. As the surface area of the coated particle increases furthermacro-RAFT is adsorbed from the aqueous phase that helpsmaintain stability. On the other hand, as the numbers of BA unitsreduce, the copolymers become very labile and are more likelyto remain in the water phase, especially at high pH. However,macro-RAFT copolymers containing high BA content are notalways desirable for encapsulation purpose. Such copolymersmay contain species that can self-assemble in the aqueous phase,leading to the formation of polymer particles which directlycompete with the encapsulation process.

The importance of macro-RAFT copolymer composition ontheir tendency to adsorb onto the titanium dioxide pigment surfacewas also reflected in Figure 6, which contains electrospray spectraof (a) a solution of RAFT-V(5BA-co-10AA) copolymer at pH7 before the addition of the titanium dioxide and (b) the macro-RAFT copolymer in the supernatant after the titanium dioxidehad been dispersed in the macro-RAFT solution and subsequently

removed by centrifugation. Comparison between spectra (a) and(b) clearly shows that the molecular weight distribution in (b)is slightly shifted to lower molecular weight relative to thedistribution in (a). A detailed analysis (included in the SupportingInformation) of the two spectra indicates that there is preferentialadsorption of macro-RAFT copolymers which are rich in BAcontent onto the titanium dioxide surface. This preferentialadsorption is in good agreement with the previous result in Figure5 where the more hydrophobic macro-RAFT was found to bebest adsorbed.

The effect of molecular weight of copolymers on their abilityto adsorb onto the titanium dioxide surface was not clear fromthis work. However, there are two reasons to maintain themolecular weight of the macro-RAFT as low as possible: (i) itis desirable to maximize the number of macro-RAFT agents onthe surface of the particles in order to facilitate rapid transfer ofpolymer growth between polymer chains and (ii) since thecopolymers are necessarily water soluble, minimizing themolecular weight also minimizes the hydrophilic polymer blocklength and with it the water sensitivity of the final particle coating.To avoid the use of solvents in the encapsulation process, it isdesirable to ensure the water solubility of the macro-RAFT agent.However, for theencapsulationof thehydrophobicphthalocyanineblue pigment (see below), best results were obtained with a macro-RAFT agent of sufficient hydrophobicity to require the use ofsolvent.

Table 1. Particle Size and Size Distributions of Encapsulated TiO2 Pigment Particles

RAFT-V(5BA-co-10AA) RAFT-XIII(5BA- co-10AA) RAFT-XIV(10BA-co-10AA)

size(Z-averaged, nm) PDI



before encapsulation 278 0.154 270 0.179 297 0.146after initial step 365 0.173 430 0.159 372 0.117after growth step 493 0.096 - - 464 0.142

Figure 5. Adsorption of macro-RAFT copolymers on zirconia andalumina-coated titanium dioxide as a function of pH.

Figure 6. Mass spectroscopy of RAFT-V(5BA-co-10AA) copoly-mer in (a) a solution before addition of titanium dioxide and (b)supernatant solution after all dispersed titanium dioxide particleswere removed by ultracentrifugation.


Titanium Dioxide Encapsulation.Zirconia and alumina-coatedtitanium dioxide pigment particles were encapsulated as describedabove. TEM images in Figure 7 of latex samples taken afterdifferent amounts of monomer were added to the reaction clearlyshow pigment particles of various shapes and sizes being centrallyencased in polymer shells. When the amount of monomer addedwas small (Figure 7A), the coating was thin and uniform,following the contours of the particle shape, indicating an evengrowth of each macro-RAFT molecule around the particle surface.However, with further addition of monomer, the coatings becamethicker (Figure 7B) and the pigment-containing latex particlesbecame more spherical in their drive to minimize their interfacialenergy. It is also very important to note the absence of un-encapsulated titanium dioxide particles, as well as the presenceof an extremely low number of new polymer particles. Thus, avery efficient encapsulation process has been demonstrated,with 100% of pigment particles encapsulated while almost allthe polymer formed was in the polymer coatings around theparticles.

The size measurements in Table 1 provide a further demon-stration of uniform encapsulation. The average particle sizeincreased from 278 nm for the uncoated pigment to 365 nm forthe thin coated sample while further growth of the polymer shellincreased the average particle size to 493 nm. Indeed, the pigmentcoating can be increased by further polymerization as long asthere is sufficient charge or steric stabilizer on the particle surfaceto maintain their stability. This was demonstrated by TEM imagesin Figure 8 where polymer shells became so thick that allencapsulated pigment particles became spherical.

The data in Table 1 show a significant decrease in thepolydispersity of the coated pigment particles (as measured byHPPS) as the coating thickness is increased. The original titaniumdioxide dispersion had quite a broad particle size distribution.However, as coating thickness increased, the distribution becamemuch narrower. This narrowing of the particle size distributionas the particles grow is consistent with the titanium dioxideparticles behaving as a seed and growing without new nucleationof pure polymer particles. Indeed, very few new polymer particlescould be found when samples were observed by TEM. As withnormal seed latex growth, significant new particles would beexpected to broaden the particle size distribution.

Effect of pH on Encapsulation.As previously discussed, thepH of the dispersion was found to influence adsorption of macro-RAFT copolymer onto the titanium dioxide surface, which inturn affected the stability of the final encapsulated pigment latex.To study this effect, a series of encapsulating experiments wascarried out with the pH of the pigment dispersions ranging from5.5 to 9.1 (Table 2). It was found that good encapsulation wasobserved from pH 5.5 to 8.1 with very high encapsulationefficiencies, with between 90% and 94% of the monomer addedfinishing up as polymer in the encapsulating shells around theparticles. At pH 9.1, the reacting dispersion became heavilycoagulated in the early stage of encapsulation. This result isconsistent with the adsorption study in Figure 5 where it wasreported that only a small amount of macro-RAFT copolymeradsorbed onto the pigment surface at this pH. At pH 5.5, thepresence of pigment aggregates in the final encapsulated latexindicates the instability of the pigment particle dispersion during

Figure 7. Poly(methyl methacrylate-co-butyl acrylate) encapsulatedtitanium dioxide latex particles with (A) 26% monomer by weightand (B) 42% monomer by weight. Macro-RAFT RAFT-V(5BA-co-10AA) was used as a dispersant. Emulsion polymerization wascarried out at 70°C using MMA/BA (7:3 by weight) monomermixture and V-501 initiator.

Figure 8. Encapsulated titanium dioxide pigment particles becomespherical after the polymerization of 68% monomer by weight.Macro-RAFT RAFT-V(5BA-co-10AA) was used as a dispersant.Emulsion polymerization was carried out at 70°C using MMA/BA(7:3 by weight) monomer mixture and V-501 initiator.


the encapsulation process. This was most likely due to a lowlevel of ionization of the carboxylic groups on the surface, leadingto insufficient surface charge to provide a stable product.

Effect of RAFT Agents on Encapsulation.RAFT agents canpotentially have a large effect on the encapsulation process ifthey influence monomer addition rates to the living RAFT endsof the macro-RAFT copolymers. Ideally, monomers shouldimmediately be consumed in the RAFT-mediated polymerizationprocess as they are injected into the pigment dispersion. Failureto do so will lead to a build-up of monomer in the aqueous phaseand the eventual formation of monomer droplets. Dropletformation may strip macro-RAFT copolymer stabilizers fromthe pigment surface or cause pigment particles to adsorb ontothe surface of droplets, resulting in pigment agglomeration. Aspreviously mentioned, macro-RAFT copolymers, RAFT-XIII-(5BA-co-10AA) and RAFT-XIV(10BA-co-10AA), were pre-pared and used for the dispersion and encapsulation of titaniumdioxide pigment in the same manner as using RAFT-V(5BA-co-10AA). The encapsulation was found to be successful,producing stable latexes which contain pigment particles coatedin thick layers of poly(MMA-co-BA) copolymer (Figure 9 A-B).Size measurements in Table 1 further demonstrate uniform coatingin both cases, with the size of encapsulated particles allsignificantly bigger than the initial pigment particles. These resultsshow the versatility of the encapsulation method which can beapplied with a broad range of RAFT agents, provided that theydo not have sufficient surfactant-like properties to support thenucleation of new polymer particles.

Macro-RAFT Copolymers in the Supernatant during Encap-sulation.In previous publications it has been taken for grantedthat macro-RAFT copolymers dissolved in the aqueous phasewould be active and able to add on monomer units as long asthere is a supply of monomer and free radicals.52 The additionof hydrophobic monomer to one end of the water-soluble randommacro-RAFT copolymer would be expected to generate asurfactant species capable of supporting nucleation of newpolymer particles. That no significant amounts of new polymerparticles are formed in these systems suggests that insignificantaqueous phase chain extension of the macro-RAFT agents occurs.It is, therefore, a matter of considerable interest to understandevents involving the macro-RAFT copolymers in the aqueousphase during encapsulation.

In order to investigate aqueous-phase events during theencapsulation process, pigment particles were removed bymoderate centrifugation and the resultant supernatant examined.UV spectroscopy and gravimetry were employed to monitorchanges in macro-RAFT copolymer concentrations and freepolymer solids content in the supernatant during the encapsulationprocess. The CdS double bond in the growing end of the macro-RAFT copolymer absorbs in the area from 265 to 365 nm andtherefore can be calibrated to relate back to concentrations ofRAFT ends in the supernatant. As presented in Figure 10, it wasfound that∼75% of the macro-RAFT copolymer remained free

in the aqueous phase at the commencement of the encapsulationprocess and∼19% of this free macro-RAFT copolymer becameadsorbed onto the particle surface during the encapsulationprocess. The balance of the macro-RAFT remained in the aqueousphase at the end of the reaction. By gravimetry, the free polymersolids content of the supernatant (determined after removing thedense titanium dioxide particles by moderate centrifugation) wasfound to remain relatively unchanged during the encapsulationprocess, indicating an insignificant amount of aqueous-phasegrowth. Furthermore, mass spectra of the macro-RAFT copolymerin the supernatant before and after encapsulation showed that itsmolecular weight and molecular weight distribution remainedessentially unchanged during the course of the reaction (Figure11), indicating an insignificant amount of aqueous phase chainextension of macro-RAFT copolymer during the reaction, despitea constant supply of monomer and free radicals. It was alsofound that the macro-RAFT copolymer remaining in thesupernatant could be recovered and used to disperse andencapsulate a further amount of titanium dioxide pigment, thusdemonstrating that the labile macro-RAFT copolymer remaining

Table 2. Effect of pH on Encapsulation of Titanium DioxidePigment Using RAFT-V(5BA-co-10AA)

pH encapsulation aggregate formationpolymer percentage

in shells (%)

5.5 yes pigment aggregatesin the final latex

94

6.5 yes no 907.6 yes no 918.1 yes no 919.1 no heavy coagulation in

the very early stage ofencapsulation

Figure 9. TEM images of poly(MMA-co-BA)-encapsulated TiO2pigment particles using macro-RAFT agents synthesized from (A)RAFT-XIII and (B) RAFT-XIV as stabilizers. Emulsion polymer-ization was carried out at 70°C using MMA/BA (7:3 by weight)monomer mixture and V-501 initiator.


in the aqueous phase during the encapsulation process maintainedits livingness and ability to adsorb onto the pigment surface andmediate the growth of polymeric coatings. This evidence stronglysupports the notion that chain extension of labile macro-RAFTcopolymers in water is not a favored event.

Molecular Weight Distribution.GPC measurements of mo-lecular weight were carried out on the total polymer present inthe reacting system as a function of the amount of monomeradded to the system. As the reaction was conducted under starve-feed conditions, the amount of monomer added corresponds tothe conversion of monomer to polymer to a reasonable ap-proximation. These chromatograms are plotted together in Figure12. The series of peaks at low retention time represents a measureof the molecular weight of the polymer in the encapsulatingshells of the particles. Note that no corresponding peak existsin this position at zero monomer added. The retention time forthe polymer component represented by these peaks increases asthe reaction progresses and demonstrates that the polymer grownon the particle surface is under RAFT control. This conclusionis supported by the plots of theoretical and experimental numberaverage molecular weight depicted in Figure 13. The agreementis quite surprising, and possibly fortuitous, as polystyrenestandards were used. However, the Mark-Houwink parametersfor poly(methyl methacrylate) (the main component of thecopolymer) and polystyrene are similar in THF (the maincomponent of the eluent solvent).59 To generate the theoreticalplot of number average molecular weight, account was taken of

the number of macro-RAFT agents initially adsorbed onto thesurface of the pigment particles and also that which adsorbedduring the progress of the reaction. The second peak in Figure12, at somewhat higher retention times, represents the originalmacro-RAFT agents that have not undergone chain extension.This is the only significant peak at zero monomer added, and thesignal due to this peak diminishes during the course of the reaction.The very small peak in Figure 12, close to the solvent peak, isof unknown origin but may represent initiator derived terminationproducts. These results provide further evidence that the macro-RAFT agents do not undergo significant chain extension in theaqueous phase despite a supply of monomer and initiator radicalsand the abundance of unadsorbed macro-RAFT copolymer inthe water. We note that the molecular weight distribution ofpolymer in the growing encapsulating shells becomes broaderas the reaction proceeds, reflected in a PDI (Figure 13) of 1.7at the end of the reaction. This increase in PDI is expected andis most probably due to some chain branching due to butyl acrylatein the growing chains and adsorption of low-molecular-weightmacro-RAFT agents during the course of the reaction, alsoresulting in the tailing of distributions as displayed in GPCchromatograms in Figure 12.

Model forEncapsulation.Onthebasisofexperimental evidencepresented above, a model for the encapsulation of titanium dioxideusing amphipathic random macro-RAFT copolymers can beproposed as follows. During encapsulation, as monomer was fedinto the reacting dispersion of titanium dioxide, only adsorbedmacro-RAFT agents chain extend to form polymer shellsencapsulating pigment particles. The macro-RAFT copolymers

Figure 10. Changes in macro-RAFT and polymer solids concentra-tion in the supernatant during titanium dioxide encapsulation.

Figure 11. Mass spectra of RAFT-V(5BA-co-10AA) copolymerin supernatant before and after encapsulation of titaniumdioxide.

Figure 12. GPC chromatograms of all polymer present in the reactingsystem after various amounts of monomer had been added.

Figure 13. Growth of polymer in the encapsulating shells at variouspoints of monomer addition.


present in the aqueous phase do not chain-extend to becomesurfactant-like and therefore do not support the nucleation ofnew particles. As the reaction proceeds, some of the aqueous-phase macro-RAFT copolymer becomes adsorbed onto thegrowing polymer shells on the surface of the titanium dioxideparticles. However, the labile macro-RAFT copolymer thatremains in the aqueous phase remains unchanged during thecourse of the encapsulation process. Uncompetitive growth oflabile macro-RAFT copolymer in the aqueous phase was believedto be a very significant factor in the high encapsulation efficienciesachieved in this work. The use of water-soluble randomcopolymers avoided aqueous-phase self-assembly and therebyavoided surfactant species that would support new particlenucleation. The rapid transfer of chain growth among polymerchains on the particle surface ensures that growth of polymeroccurs over the entire surface of the particle and ensures an evenpolymer coating on the pigment particle surface.

Dispersion and Encapsulation of Phthalocyanine BluePigment.Macro-RAFT Copolymer.The macro-RAFT copolymer used forthe dispersion and encapsulation of phthalocyanine blue pigmentwas prepared in a similar manner as above, using the same RAFTagent but with the final product containing, on average, fivebutyl acrylate and five acrylic acid units (RAFT-V(5BA-co-5AA)). This copolymer was also expected to contain a randomdistribution of butyl acrylate and acrylic acid. This macro-RAFTcopolymer was more hydrophobic than that used for theencapsulation of titanium dioxide due to the more hydrophobicnature of the blue pigment surface.

Dispersion of Phthalocyanine Blue Pigment.While butylacrylate units help the macro-RAFT copolymers to adsorb ontothe pigment surface, neutralized acrylic acid units providestabilizing charge. However, a consequence of higher hydro-phobicity is that the copolymer by itself was not soluble in watereven when fully neutralized. To overcome this problem, thedispersion of phthalocyanine blue pigment was carried out intwo steps. In the first step, the pigment was dispersed in aneutralized macro-RAFT solution containing ethylene glycol andmethanol to solubilize the macro-RAFT in the water. In thesecond step, extra amounts of water were added during thedispersion to reduce the organic solvent concentration, makingthe copolymer insoluble and locking it onto pigment surfaces.

Phthalocyanine Encapsulation.Once the pigment has beendispersed, the encapsulation of phthalocyanine blue pigment wascarried out in a similar manner as the encapsulation of titaniumdioxide. TEM images in Figure 14A show phthalocyanine bluepigment of different shapes and sizes centrally encapsulated bypolymer. The polymer shells are sufficiently thick that encap-sulated particles have become spherical. As in the case of titaniumdioxide, there is an absence of pigment particles without anyencapsulating polymer, indicating the process is efficient inencapsulating 100% of pigment particles. As for titanium dioxide,there is a very small population of polymer particles that do notcontain pigment particles. Figure 14B shows the versatility andeffectiveness of the encapsulating process, as even particles withlarge aspect ratios can be uniformly coated with polymer.

Conclusion

We have presented a new method for the encapsulation ofboth hydrophilic inorganic (represented by zirconia and alumina-coated titanium dioxide) and hydrophobic organic (representedby phthalocyanine blue) pigments with poly(methyl methacrylate-co-butyl acrylate) using living amphipathic random macro-RAFTcopolymers to both stabilize the initial pigment dispersion andto facilitate the uniform growth of polymer on the pigment particlesurface. Encapsulated composite particles were formed that hada core-shell morphology with pigment particles at the centersurrounded by thick polymer shells. The encapsulated particleswere stabilized in the aqueous phase by an anchored hydrophiliclayer of negatively charged carboxyl groups on the surface. Themethod was found to be incredibly efficient in that 100% of thepigment particles were encapsulated and almost all of the polymergrowth was within the encapsulating polymer shells. The highencapsulation efficiency is first attributed to the use of livingamphipathic random macro-RAFT copolymers as stabilizers thatdo not self-assemble in the aqueous phase and, therefore, do notform centers for secondary particle nucleation. Second, the growthof the macro-RAFT agents in the aqueous phase was notcompetitive and, hence, macro-RAFT stabilizers did not growto become micelle forming and thus did not facilitate new particlenucleation. Only adsorbed macro-RAFT copolymers were found

Figure 14. TEM images of poly(methyl methacrylate-co-butylacrylate) encapsulated phthalocyanine particles: (A) poly(methylmethacrylate-co-butyl acrylate) encapsulated phthalocyanineblue latex particles and (B) an even coating over a particle withlarge aspect ratio. Macro-RAFT RAFT-V(5BA-co-5AA) was usedas a dispersant. Emulsion polymerization was carried out at 70°Cusing MMA/BA (7:3 by weight) monomer mixture and V-501initiator.


to undergo rapid transfer and incremental growth, leading to theformation of a uniform coating over the entire particle surface.This encapsulation approach is simple and can be readily scaledup for industrial production.

Acknowledgment. We thank Australian Research Counciland Dulux Australia for financial support of this study.

Supporting Information Available: Detailed analysis of massspectra of macro-RAFT-V(5BA-co-10AA) copolymers in solutionsbefore the addition of titanium dioxide in supernatants after the dispersionand after the encapsulation of the aforementioned pigment. This materialis available free of charge via the Internet at http://pubs.acs.org.

LA7027466


pigment encapsulation by emulsion polymerization using macro-raft copolymers

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