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Aqueous Polymeric Hollow Particles as Opacifier by Emulsion Polymerization using Macro-RAFT Amphiphiles Binh Pham, Duc Nguyen, Vien Huynh, Eh Hau Pan, Bhavna Shirodkar-Robinson, Michelle Carey, Algirdas Serelis, Gregory G. Warr, Tim Davey, Christopher H. Such, and Brian Stanley Hawkett Langmuir, Just Accepted Manuscript • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Aqueous Polymeric Hollow Particles as Opacifier by Emulsion Polymerization using Macro-RAFT Amphiphiles Binh T. T. Pham†; Duc Nguyen†; Vien T. Huynh†; Eh Hau Pan†a; Bhavna Shirodkar-Robinson†; Michelle Carey§b; Algirdas Serelis§; Gregory G. Warr†c; Tim Davey§; Christopher H. Such§ and Brian S. Hawkett†c* †

Key Centre for Polymers and Colloids, School of Chemistry and University of Sydney Nano Institute,

The University of Sydney, NSW 2006, Australia a

Current address: Centre for Advanced Macromolecular Design, The University of New South Wales,

NSW 2052 b

Current address: SDI Limited, Bayswater, VIC 3153, Australia

c

Australian Institute for Nanoscale Science and Technology, The University of Sydney, NSW 2006,

Australia §

DuluxGroup Australia, Clayton, VIC 3168, Australia

∗

[email protected]

KEYWORDS. Polymeric hollow particles, polymer capsules, amphiphiles, amphiphilic random copolymer, self-assembly, opacifier, vesicles, RAFT emulsion polymerization.

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ABSTRACT A robust polymerization technique that enables the surfactant-free aqueous synthesis of a high solid content latex containing polymeric hollow particles is presented. Uniquely designed amphiphilic macroRAFT copolymers were used as sole stabilizers for monomer emulsification as well as free radical emulsion polymerization. The polymerization was found to be under RAFT control, generating various morphologies from spherical particles, worm-like structures to polymer vesicles. The final particles were dominantly polymeric vesicles which had a substantially uniform and continuous polymer layer around a single aqueous filled void. They produced hollow particles once dried and were successfully used as opacifiers to impart opacity into polymer paint films. The method is simple, can be performed in a controllable and reproducible manner and may be performed using diverse procedures.


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INTRODUCTION Since the early 80s, especially over the last decade, natural polymer based and synthetic polymer based vesicles, polymer capsules and polymeric hollow particles have become a strong focus in both academic research and industry1-5. Such polymeric particles with an internal void can potentially be used in various applications such as drug delivery 2-4 and especially as opacifiers in paints1. For the later application, the opacity is generated by light scattering due to the difference between polymer and air void refractive indices1. Titanium dioxide pigment is traditionally used to impart the opacity to paint films. However, its mining and production processes come at a heavy cost to the environment1, 6. Therefore, hollow polymer particles can potentially reduce the amount of Titanium dioxide used in paint which helps to alleviate pigment environmental impact and reduce cost. Water based hollow particle synthetic methods all have similar precursors which are polymeric vesicles or latexes, consisting of polymer shells encapsulating aqueous cores1. Water removal during the drying process creates hollow structures of strong rigid polymer shells wrapping around air voids1. The most common method to synthesize single void polymeric hollow particles is by seeded emulsion polymerizations in which swell-able latex particles are coated with stretchable crosslinked polymer shells1,

7-11

. This approach was first pioneered by researchers at Rohm and Haas Co., whom have

successfully developed commercially available Ropaque polymeric hollow particles with monodisperse particles size distributions and well controlled shell thickness to replace titanium dioxide pigments in paints1, 7. However, this method consists of many complicated multiple steps and harsh swelling conditions of strong basic solution at elevated temperatures1. Furthermore, the final latexes also contained free surfactants, which might be undesirable for some applications. Another attractive route for hollow particle synthesis is via the synthesis of polymersomes in which amphiphilic polymers or macromolecules self-assemble to produce polymeric vesicles. The method was first studied by Eisenberg and co-workers12-18 and was further advanced by other research groups19-22. In this process, the polymeric vesicles could be formed by self-assembly of amphiphilic diblock copolymers through multiple solvent addition or exchange steps. Structures formed were dependent on ACS Paragon Plus Environment

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the packing parameter, which in turn were determined by ratios between the volume occupied by hydrophobic tails, the areas of hydrophilic heads and overall lengths of the diblocks23. Depending on the packing parameter, vesicle formation can be favored over spherical or rod-liked particles,23 as is also seen in the self-assembly of small-molecule surfactants24 and diblock copolymers25-26 into various micelle morphologies and lyotropic mesophases in water and other highly polar solvents27-28. However, the diblock copolymer concentrations used to create hollow particles were generally as low as 0.5 to 1 weight percent, resulting in products with very low solid contents. In many cases, the vesicles had very thin shells which were prone to collapse and often required a Cryo-Transmission Electron Microscope for imaging29-31. Recent developments of controlled living free radical polymerization in both organic solvent and water further advanced the self-assembly method in which various polymer nanostructures could be formed through chain extensions of amphiphilic living copolymers32-36. As in the previous case of diblock assembly in solvents, polymeric vesicles could be predominantly formed once critical packing parameters reached under suitable conditions33. However, to the best of our knowledge, no attempts have been made to use the present method to produce polymeric vesicles which can be used as hollow particles in paint applications. In our previous works, we have shown that Reversible Addition Fragmentation chain Transfer (RAFT) emulsion polymerization can be used to produce surfactant free latexes by ab-initio emulsion polymerization 37-38. In this method, chain extensions of amphiphilic macro-RAFT copolymers in water lead to their micellization and eventual formation of spherical nanoparticle latexes. The same principle has been expanded and applied to miniemulsion polymerization 39-40, encapsulation of solid particulates 41-46

and synthesis of anisotropic polystyrene particles

47-48

. Our miniemulsion work40 especially

demonstrated the potential for employing diblock macro-RAFT of poly(styrene)-block-poly(acrylic) to generate hollow particles. However, further investigations found that the conditions for such clean and straight forward diblocks to self-assemble and generate desired structures in the aqueous phase proved to be quite complex49. In this work, the ability to use amphiphilic diblock macro-RAFT copolymers in emulsion polymerization to spontaneously form polymeric vesicles at relatively high concentrations is ACS Paragon Plus Environment

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explored. The uniquely designed macro-RAFT copolymers of RAFT-(styrene)z-block-[(acrylic acid)aco-(butyl acrylate)b] (see Figure 1) would be first employed as dispersants to emulsify styrene monomer. The presence of butyl acrylate units in the stabilizing blocks reduces their overall hydrophilicity which allows the macro-RAFT copolymers to be easily blended with hydrophobic monomers prior to neutralization with base. For this reason, the molar ratio between butyl acrylate and acrylic acid units would be kept at 2 to 1. More importantly, the hydrophobic stabilizing groups also would make it easier to change the packing parameter even with small increases in polystyrene hydrophobic tail lengths which might favour vesicle formation. In this study, once emulsion polymerization was carried out, the amphiphilic macro-RAFT copolymers are chain-elongated by addition of styrene and self-assemble to produce vesiculated particles. Reinforcement by incorporation of divinyl benzene crosslinker improves hollow particle rigidity. The opacity of the resulting crosslinked polymeric vesicle latex is examined with an aim to supplement titanium dioxide in paint films.


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Figure

1.

Structures

of

RAFT

agents

dibenzyl

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trithiocarbonate

(DBTC)

(1),

2-

([(butylsulfanyl)carbonothioyl]sulfanyl) propanoic acid (PABTC) (2) and amphiphilic macro-RAFT copolymers Triblock A (3) and Diblock B (4) used in this work. Leaving groups of the RAFT agents are colored purple.

Table 1. Characterization of macro-RAFT copolymers Triblock A and Diblock B derived from RAFT DBTC and PABTC by NMR and GPC.

EXPERIMENTAL Materials. Milli Q water was used in the synthesis of all latexes. Acrylic acid (AA), butyl acrylate (BA), styrene (Sty), divinyl benzene (DVB), 1,4-dioxane were obtained from Aldrich. BA and Sty had the inhibitor removed by passing the monomer through an inhibitor removal column (Aldrich). 2,2’azobis-isobutyronitrile (AIBN, Fluka) was recrystallized from ethanol. The RAFT agents,

50

2-

([(butylsulfanyl)carbonothioyl]sulfanyl) propanoic acid (PABTC), dibenzyl trithiocarbonate (DBTC) (see Figure 1, Dulux Australia), sodium hydroxide (NaOH, Aldrich), deuterated dimethyl sulfoxide (DMSO, Sigma), 1,3,5-trioxane (Sigma), acetic acid (Sigma) and tetrahydrofuran (THF, Ajax, HPLC grade) were used as received. Dulux Aquanamel Gloss Extra Bright Base (39%) was purchased from Bunnings Warehouse and used as received. Leneta cards were obtained from Leneta Company. Synthesis of macro-RAFT Triblock A with [Sty]:[BA]:[AA]:[DBTC]:[AIBN]=83:122:61:1:0.5. Macro-RAFT Triblock A was synthesized as follows: RAFT DBTC (0.3g, 1.0 mmol), AIBN (0.04 g, 0.2 mmol), AA (4.5 g, 62.2 mmol), BA (15.9 g, 124.0 mmol), 1,3,5-trioxane (0.13 g, 1.4 mmol) in dioxane (31.0 g) was prepared in a 100 mL round bottom flask. This was stirred magnetically and ACS Paragon Plus Environment

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purged with nitrogen for 10 minutes. The flask was then heated at 70oC for 3 hours under constant stirring. At the end of the heating, BA and AA polymerization conversions were found to be 96.3 and 95.5% respectively by 1H-NMR (Figure S1). Sty (8.6 g, 82.9 mmol), AIBN (0.04 g, 0.2 mmol) and dioxane (12.0g) was added to the polymer solution. The flask was sealed, deoxygenated with nitrogen for 15 minutes and then heated at 70oC for another 12 hours under constant stirring. The final copolymer solution had 35.3 % solids while Sty polymerization conversion was found to be 58.8% by 1H-NMR (Figure S2). Synthesis of macro-RAFT Diblock B with [Sty]:[BA]:[AA]:[PABTIC]:[AIBN]=80:120:60:1:0.5. Macro-RAFT Diblock B was synthesized as follows: RAFT PABTC (0.3 g, 1.1 mmol), AIBN (0.04 g, 0.2 mmol), AA (4.6 g, 64.2 mmol), BA (16.4 g, 127.7 mmol), 1,3,5-trioxane (0.13 g, 1.4 mmol) in dioxane (45.2 g) was prepared in a 100 mL round bottom flask. This was stirred magnetically and purged with nitrogen for 10 minutes. The flask was then heated at 70oC for 3 hours under constant stirring. At the end of the heating, BA and AA polymerization conversions were found to be 90.0 and 91.0% respectively by 1H-NMR (Figure S3). Sty (8.9 g, 85.4 mmol), AIBN (0.04 g, 0.2 mmol) was added to the polymer solution. The flask was sealed, deoxygenated with nitrogen for 15 minutes and then heated at 70oC for another 12 hours under constant stirring. The final copolymer solution had 38.8 % solids while Sty polymerization conversion was found to be 54.5% by 1H-NMR (Figure S4).

Scheme 1. The procedure used to synthesize latexes containing polymeric hollow particles.


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Synthesis of surfactant free polymeric hollow particles using RAFT emulsion polymerizations. A typical procedure to make polymeric vesicles is described in Scheme 1 and carried out as follows: solutions of either of the macro-RAFT copolymers obtained from the typical syntheses given above (18.0 g, 0.2 mmol, containing approximately 15 mmol of AA units), Sty (45.0 g) and AIBN (0.4 g) was placed in a 400 mL beaker. To this macro-RAFT solution, 0.6 g (16 mmol) of NaOH dissolved in 18.0 g of water was added while the solution was stirred at 1000 rpm using an overhead stirrer (Labortechnik, IKA) to produce a thick yellowish white emulsion. After 30 minutes of stirring, 40 g of water was added using a pipette while stirring was maintained at 1000 rpm. After a further 5 minutes of stirring, 56 g of water was poured into the dispersion while stirring was maintained at 1000 rpm, to produce a viscous bright white emulsion. The emulsion was transferred to a 250 mL round bottom flask which was sealed and purged with nitrogen for 15 min. The whole flask was magnetically stirred for 3 hours while immersed in an oil bath with a temperature setting of 80oC. The final latex was found to be stable with a solid content of 29%, pH 9.5 and a Z-average particle size of 330 nm (PDI of 0.202; by DLS, Zetasizer, Malvern Instruments). Opacity measurements in latex films. For opacity measurements, hollow particles were synthesized in the same conditions as previously described to produce a 23.6% latex after filtering. The sample was further crosslinked with DVB by carrying out the emulsion polymerization of 100 g latex with 3.0 g of DVB in the presence of 0.05 g AIBN at 80oC over 3 hours to produce 25.8% latex. This latex was diluted with water to have the same solid content of 23.6% as the previous uncrosslinked latex. Both crosslinked and uncrosslinked latexes were then blended with Dulux Aquanamel Gloss Extra Bright Base latex at 1:1, 1:2 and 1:4 ratios using an overhead stirrer at 1000 rpm for 10 minutes. After blending, air bubbles were removed by placing the samples in vacuum for 5 minutes. Wet paint films were then applied on Leneta cards (Leneta Company) using a 50-micron drawdown bar coater (Sheen Instruments). The films were left to dry at the ambient condition (approximately 25oC) over 48 hours to produce polymer films. Opacity of the films on Leneta cards was then measured using Rhopoint NovoShade Duo Reflectometer (Rhopoint Instrumentation). ACS Paragon Plus Environment

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Sample characterization Gel Permeation Chromatography (GPC)was carried out using a Shimadzu system fitted with Waters HR4, HR3 and HR2 columns, 5% acetic acid in THF as eluent51-52, polystyrene standards, refractive index detection and Polymer Laboratories Cirrus™ software. Detailed procedures have been described previously51. Dry polymers were dissolved in the eluent in the sonic bath for 30 minutes to make approximately 20 mg/mL solutions. These were filtered prior to sample injection into the GPC. Polymers from the latex were not completely soluble in the eluent possibly due to salt formation of sodium with carboxylate groups on the polymers. Undissolved polymers were removed by filtration and therefore only parts of the samples could be characterized by GPC. For NMR characterization, macro-RAFT copolymers were dissolved in deuterated DMSO prior to 1H NMR measurements. Monomer emulsion droplets were studied by Light optical microscopy (LOM) in which the images were taken using Leica DM750 optical microscope. Particle sizes were measured by Dynamic Light Scattering (DLS). on a Zetasizer (Nano-ZS, Malvern Instruments). Samples were diluted with water to about 0.1% prior to measurements. Morphology of the particles formed was studied using Transmission Electron Microscopy (TEM, Philips CM120 Biofilter) and Scanning Electron Microscopy (SEM, Philips XL 30 CP). TEM and SEM samples were prepared by depositing a drop of the above DLS diluted samples on TEM grids or SEM stubs. The samples were allowed to dry at room temperature prior to examination. For SEM of the sectioned samples, the hollow particle latex was dried down on a glass slide to produce a white polymer film. A sharp razor was used to cut the film into very small chips which were then mounted on conductive carbon tape for further examination.

RESULTS AND DISCUSSION Macro-RAFT copolymers. As shown in Table 1, the macro-RAFT copolymer synthesis was under RAFT control. By NMR and GPC, Diblock B structures were estimated to be PABTC-Sty44-block(BA108-co-AA55) with an average molecular weight of 24,200 (g/mole) and a Ð of 1.5. Likewise, Triblock A structures were approximated to be DBTC-Sty49-block-(BA117-co-AA58) with an average ACS Paragon Plus Environment

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molecular weight of 22,900 (g/mole) and a Ð of 1.23. One key difference between the two macro-RAFT is the Triblock A contains two leaving groups while Diblock B has only one. If the two leaving groups of RAFT DBTC equally participated in the controlled living free radical polymerization, then the Triblock A structure is approximated by DBTC-[Sty25-block-(BA59-co-AA29)]2. True configurations of the two macro-RAFT copolymers would be more complicated if unreacted BA and AA during the hydrophilic block synthesis were taken into consideration. In our previous work, copolymerization of slow reacting sodium styrene sulfonate (StS) and fast reacting BA and AA in the presence of PABTC RAFT was found to produce statistical copolymers46. Such polymers had one ends containing primarily BA and AA which first polymerized and the other end was StS units which were the slowest to be added to the polymer chains. In this work, leftover BA and AA monomers were likely to first copolymerize with styrene during the initial stage of hydrophobic block synthesis. However, given their low concentrations compared to styrene in this step, the above simplified macro-RAFT structures are still our best estimate. Impacts on the activity of these two RAFT agents on vesicle formation will be assessed in this work. The macro-RAFT copolymer used in this work has unique architecture and is amphiphilic in nature. The hydrophilic block is a random copolymer of BA and AA containing both hydrophilic carboxylic (from AA) as well as hydrophobic butyl groups (from BA). After neutralization with base, anionic carboxylate groups will provide electrostatic stability to the dispersed oil droplets. However, due to the presence of more butyl groups, it is unlikely that these hydrophilic blocks will be fully extended into the water phase. Hydrophobic sections of the blocks are likely to reside at the oil water interface. By such design, the hydrophilic head groups occupy a large surface area on the monomer droplet or particle surfaces, efficiently stabilizing them. To these hydrophilic blocks, highly hydrophobic polystyrene blocks are attached and will function as the hydrophobic tails just as in the case of surfactants. However, our previous studies39-40 showed that polystyrene blocks with a similar chain length were sufficient to arrest Ostwald ripening and yield exceedingly stable miniemulsion droplets. Therefore, the macroRAFT copolymer used in this work is expected to be efficient in monomer emulsification. ACS Paragon Plus Environment

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As shown in Scheme 1, styrene was emulsified by simply adding a sodium hydroxide solution into the monomer solution containing the initiator, AIBN, and RAFT copolymers under mechanical stirring. Neutralization of macro-RAFT carboxylic groups with an approximately equivalent amount of NaOH provided charge to stabilize the monomer droplets. In our previous study37, macro-RAFT polymers were found to maintain their properties and functions as RAFT agents after neutralization with NaOH. Therefore, in this study, macro-RAFT copolymers were also expected to maintain RAFT functionality, which would be reflected by narrow polymer weight control. As shown in Figure 2, a relatively stable white emulsion (Figure 2A) with an approximated droplet size of around 2 microns (Figure 2B) was produced when Triblock A was used. A similar emulsion was also obtained in the case of Diblock B. This demonstrates the suitability of the macro-RAFT copolymers as stabilizers for emulsions.

Figure 2. Styrene emulsification using Triblock A as the emulsifier: A) emulsification using a mechanical stirrer; B) an optical image of the styrene emulsion. Free radical emulsion polymerization of styrene under RAFT control. The polymerization was carried out by heating the emulsion to 80oC under constant stirring. Decomposition of AIBN initiator ACS Paragon Plus Environment

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provided the free radical flux which initiated the polymerization. The kinetic study presented in Table 2 shows that the emulsion polymerization had a small inhibition period of 5-10 minutes and then proceeded rapidly. This is in contrast to the significant inhibition or retardation periods which were observed for RAFT mediated styrene polymerization by Charleux et al.47-48. Almost 100% conversion was reached after 3.5 hours which demonstrates the suitability of the process for industrial scale-up. The polymerization process is expected to involve the continual addition of styrene units to the hydrophobic ends of macro-RAFT Triblock A. This expectation was confirmed by GPC the curves in Figure 3 where narrow polymer molecular weight distributions are shown to increase in molecular weight with conversion. This shows the control and living effect of the amphiphilic macro-RAFT copolymers despite being anchored to the monomer/water interface of the emulsion droplets. This is in agreement with the findings in our previous works where the macro-RAFT amphiphilic property has never been an issue with polymer molecular weight control in free radical emulsion polymerization. Assuming total RAFT control, the average number of Sty units per RAFT molecule was calculated as a function of conversion and listed in Table 2.

Table 2. Conversion vs time data for the synthesis of hollow polystyrene particles using amphiphilic macro-RAFT Triblock A as sole stabilizer

Sample

Reaction Time (min)

Solids Content (%)

Conversion (%)

Sty Unit (Estimated)

1

0

3.8

0.0

49

2

8

4.3

1.8

79

3

15

7.1

12.6

294

4

22

13.0

35.4

683

5

30

16.2

48.3

902

6

40

18.2

56.1

1034

7

50

20.0

62.6

1145


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8

60

21.4

68.4

1244

9

210

28.9

97.3

1737

Emulsion polymerization of Sty in water at 80oC using Triblock A as the stabilizer and AIBN as the initiator.

Figure 3. Evolution of polymer molecular weight distributions versus time when macro-RAFT Triblock A was used as the stabilizer. In-situ formation of morphology and self-assembly mechanism during emulsion polymerization. Morphology evolution during the polymerization was studied by TEM and presented in Figure 4. At the low conversion of 12% (Sample 3), large aggregates as well as spherical structures were present. Such large aggregates and spherical particles had no clear morphologies due to the large amount of styrene monomer which plasticized such structures, making them unobservable when dry. When the conversion increased to 35% (Sample 4), polymer worm-like and vesicle nanostructures became clearly observable. This was because long polystyrene chain ends were now strong and rigid enough to retain their morphology. However, vesicles were mostly deformed, not spherical and had thin walls. This was probably because of the still abundant presence of unreacted Sty. The formation of worm-like nanostructures was found to coincide with an increase in the viscosity of the emulsion which was ACS Paragon Plus Environment

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indicated by a reduced vortex size. As conversions further increased from 48% to 63% (Samples 5-7), there was a clear decrease in the number of worm-like particles while presence of vesicles was dominant as shown in the TEM images. At a conversion of 97% (Sample 9), most of particles were spherical vesicles with diameters ranging from 100 nm to 1µm with an average particle size of 330 nm (by DLS). Shell thickness of approximately 60 nm was observed under TEM (see Figure 4). The particle wall maintained the consistent growth across the vesicles under the control of RAFT polymerization leading to the formation of uniform polymer shells. Polymer vesicles were also produced when emulsion polymerization of styrene was performed under the same conditions but using macro-RAFT Diblock B as the stabilizer. As shown Figure 5, the final latex contained polymer vesicles as well as nanoworms. This was despite the fact that Diblock B only had one living group as previously discussed. The result confirms the validity of this method in which the generation of polymer vesicles could be repeated when similar ratios between Sty/BA/AA are maintained in the macro-RAFT structures. However, the amount of nanoworms were found to be dominant and further optimizations are required to deliver mostly vesiculated particles for our particular purpose. Therefore, the macro-RAFT Triblock A copolymers were mainly used for the synthesis of hollow particles for our opacity assessment. The morphology evolution observed by TEM at various stages during the polymerization strongly suggests that hollow particles formed are the endpoint of changing self-assembly of growing amphiphilic macro-RAFT copolymers under the influence of solvents, temperature, stirring rates. Previous works

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by Eisenberg et al. have shown spherical, cylindrical, or bilayer morphologies

are governed by packing parameters of block copolymer amphiphiles. These depend on the ratio of the area covered by the hydrophilic heads to the length/volume occupied by the hydrophobic tails, which in turn depend on the degrees of polymerization of the two blocks and solvent quality, at least for the polar block. Micelle morphology can also change during polymerization of small-molecule surfactants into polysoaps54-55.


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In the present case, the packing parameter changes during emulsion polymerization as the styrene chain grows. At the beginning of the reaction the hydrophobic polymer chain consists of only ~49 Sty monomers compared to 117/58 BA/deprotonated AA in the hydrophilic block of the macro-RAFT copolymers. As a result, spherical micelles are formed, and mostly small globular particles are observed in the early stages (exemplified by 12% conversion). As Sty polymerization progresses, hydrophobic chains grow in length and volume at approximately constant occupied area, which monotonically increases the packing parameter. This leads first to the transformation into worm-like micelles (e.g., at 35% conversion) and then vesicles at even higher conversion, where each hydrophobic chain ultimately contains of order 1700 monomer units. As polymerization proceeds and Sty is transported from the emulsion droplets into the growing particles, increasing chain hydrophobicity renders the copolymers non-labile to exchange; We infer from Figure 4 that this occurs after the vesicles form, so that further conversion strengthens the particle shell by thickening the bilayer, greater cross-linking, and consuming plasticizing monomer. By changing the composition of the macro-RAFT agent, solvent, and/or polymerization temperature, it should be possible to arrest exchange at a smaller packing parameter in order to ‘lock-in’ cylindrical particles instead, as seen for the DBTC-based systems. This is outside the scope of the present work and would require systematic investigation. Similar results for the morphologies of amphiphilic copolymer self-assemblies have also been reported in recent literature23, 33 in which the effects of hydrophobic chains on the morphology were discussed extensively.


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Sample 3: 15min, 12% conv.




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Figure 4. TEM microscopy to study particle formation with reaction time/conversion. Sample numbers correspond to the numbers shown in Table 2. Macro-RAFT Triblock A was used as the stabilizer.


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Figure 5. Polymeric particles formed using macro- RAFT Diblock B as sole stabilizer.


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Figure 6. TEM and SEM images of hollow particles: A, B) with and C, D) without cross-linking to show the integrity of dried down hollow particles. Macro-RAFT Triblock A was used as the stabilizer. Opacity assessment of polymeric vesicles. As previously discussed, hollow particles can be used as opacifier due to their internal air voids which act as light scattering centers. In this work, hollow particles are formed when water in the internal void of vesicles is removed in the drying process. In order for the polymeric vesicles to impart opacity to a dry film, the air voids need to survive the drying process. This means the polymeric shell should remain intact without collapsing during the evaporation of the internal water. This is especially important in paint formulations which normally contain a certain amount of coalescing solvent. The solvent can soften the shells of the vesicles when they are blended during the paint manufacturing. After the initial synthesis, our polymeric shell was strengthened with 10 weight % of DVB as a cross-linker. In this case, DVB and AIBN initiator were mixed with the polymer vesicle latex for 10 minutes to allow for absorption into the polymer shells. Subsequent free radical emulsion polymerization at 80oC produced cross-linked polymer vesicle latex. TEM and SEM ACS Paragon Plus Environment

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characterization (see Figure 6) showed that both samples of hollow particles before and after the crosslink processes were mostly spherical in shape in the dry state. In Figure 7, sectioned, un-crosslinked hollow particles (SEM in Figure 6D) clearly showed the presence of internal air voids. They were therefore suitable to be tested for opacity. However, it was apparent that both samples contained cuplike structures. They could be originally formed as a by-product during the vesicle synthesis or just due to the collapse of some vesicle with thin shells. The presence of such structures was not desirable because their internal voids could be readily filled with polymer latex, preventing them from generating air voids.

Figure 7. SEM micrographs showing air voids of sectioned un-crosslinked polymer hollow particles: A) broad view; B) digitally magnification of the same image. Macro-RAFT Triblock A was used as the stabilizer. For opacity assessments, both latex samples of the un-crosslinked and crosslinked polymeric vesicles were blended with Dulux Aquanamel Gloss Extra Bright Base latex at different ratios under vigorous stirring. A wet paint film was applied on a Leneta card (Leneta Company) using a 50-micron drawdown bar coater (Sheen Instruments). The film was left to dry under ambient conditions (approximately 25oC) over 48 hours to produce polymer films (see Figure 8). The dry film (Figure 8B) was visually opaque in contrast to the translucence of the wet film (Figure 8A). The film opacity clearly demonstrates the ability of our polymeric vesicles to function as opacifier. In Figure 9, opacity measured at different hollow particle concentrations in paint films showed that the opacity increased with increasing particle ACS Paragon Plus Environment

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concentration. From the graph, it was observed that crosslinked particles consistently outperformed the un-crosslinked ones. It was probably due to the softening of the un-crosslinked polymer vesicles in the presence of coalescent in the commercial paint sample, which leads to the collapsing of the polymeric shell when dried. This reduced the number and size of the air voids, and therefore reduced the light scattering efficiency of the un-crosslinked hollow particles, leading to the lower opacity as measured.

Figure 8. Films formed on Leneta cards from sample shown in Figure 6A: A) Wet film; B) Dry film, after 48 hours drying time. Films were applied using a 50-micron wire wound drawdown bar coater.


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Figure 9. Measured film opacity versus hollow particle solids concentration in paint films. The ability of the polymer vesicles synthesized by our method to replace commercial hollow polymer particles was also investigated in the laboratory of Dulux Group Australia56. A paint sample containing commercial hollow particles, Ropaque UltraTM by Dow Chemical, was tested for comparison. As reported in this work, paint samples containing as low as 4 wt.% of hollow particles exhibited a 27% increase in opacity relative to paint samples without hollow particles56. Properties measured for films formed by paint samples containing hollow particles from this study were of the same order as those based on paint samples containing Ropaque UltraTM hollow particles. Work has been carried out to replace dioxane in the macro-RAFT synthesis with more environmentally friendly solvents which are commonly used in the paint industry.

CONCLUSIONS We have demonstrated that water based low VOC latexes containing hollow polymeric particles could be synthesized, using RAFT mediated emulsion polymerization. The latex particles are solely stabilized by amphiphilic macro-RAFT agents, without need of free surfactant. The procedures presented were ACS Paragon Plus Environment

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very simple, versatile, reproducible, and environmentally friendly with high solids content latexes at low cost. Through the control afforded by this method, the size, molecular weight, structure, thickness and polymer composition of the polymeric hollow particles can be tuned by using different monomers and amphiphilic macro-RAFT agents to obtain desired properties for the final product for a given application.

ASSOCIATED CONTENT SUPPORTING INFORMATION The supporting information is available free of charge on the ACS Publications website at DOI: … 1

H-NMR and monomer conversion calculation for the synthesis of triblock A’s hydrophilic block

(Figure S1), 1H-NMR and monomer conversion calculation for the synthesis of triblock A (Figure S2), 1

H-NMR and monomer conversion calculation for the synthesis of Diblock’s hydrophilic block (Figure

S3), 1H-NMR and monomer conversion calculation for the synthesis of Diblock (Figure S4) (PDF)

ACKNOWLEDGEMENT Financial supports of Dulux Australia and the ARC SPIRT Scheme are gratefully acknowledged. The authors also acknowledge the facilities and technical assistance of the Australian Centre for Microscopy and Microanalysis, the University of Sydney. The Key Centre for Polymers and Colloids was established and supported under the Australian Research Council’s Research Centers Program.


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