Morphological Control of Multihollow Polymer Latex Particles through

Nov 11, 2013 - during the radiation-induced seeded emulsion polymerization. The ... cross-linked polystyrene (SCPS) seed microspheres were swollen by...
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Morphological Control of Multihollow Polymer Latex Particles through a Controlled Phase Separation in the Seeded Emulsion Polymerization Bingxin Li, Yongfei Xu, Mozhen Wang,* and Xuewu Ge* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: In this work, we first reported that the phase separation can take place both inside and outside of a multihollow-structured cross-linked seed microspheres swollen by styrene monomers in water during the radiation-induced seeded emulsion polymerization. The phase separation process in these two opposite directions will determine the morphology of final latex particles. First, sulfonated cross-linked polystyrene (SCPS) seed microspheres were swollen by styrene in water. Water will permeate into the SCPS seed microspheres during the swelling process, forced by the osmotic pressure produced by the strong hydrophilicity of the sulfonic acid groups. New aqueous phases are created and stabilized by the hydrophilic −SO3H groups, resulting in a multihollow structure of swollen SCPS seed microspheres. When the polymerization of styrene is induced by 60Co γ-ray radiation, the phase separation of newly formed polystyrene phase will occur at the seed microsphere-water interface inside and/or outside of the SCPS seed microspheres through adjusting the diameter of seed microsphere, the content of cross-link agent, and the sulfonation degree of SCPS seed microspheres. As a result, SCPS latex particles with a variety of special morphologies, such as spherical multihollow, plum-like, and walnut-like latex particles were obtained. The results of this study provide not only a simple and interesting way to design and synthesize multihollow polymer latex particles with controllable surface morphologies but also a better understanding on phase separation mechanism during the swelling and polymerization of monomers in cross-linked amphiphilic polymer networks.

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INTRODUCTION In recent decades, polymer microspheres with various of morphologies, from the classic core−shell structure to hollow and porous structures, as well as a variety of anisotropic microspheres, have been created by means of the phase separation in the seeded emulsion polymerization process forced by the thermodynamic and kinetic factors.1−8 The thermodynamic factors determine the equilibrium morphology of the final microspheres because they will drive the system toward a minimum in free energy. The kinetic factors, e.g., the viscosity in the seed microspheres, determine the ease with which the thermodynamically favored morphology can be achieved. The actual morphology of the final microspheres should be the result from the competition between these two factors. Therefore, extensive investigations on the effects of the polarity of monomers in the second-stage polymerization, the types of dispersion phase, and the nature of seed particles in themselves, such as the composition, the size, and the crosslinking density, have been reported in the literatures.9−12 The emulsion polymerization of styrene (St) in the presence of noncross-linked polymer seeds particles has been studied early and extensively as one of classic emulsion polymerization systems.13−17 The kinetic evidence supported a heterogeneous © 2013 American Chemical Society

core−shell model for the growing monomer-swollen latex particle; i.e., the particle consists of an expanding polymer-rich core surrounded by a monomer-rich shell, and the outer shell serves as the major locus of polymerization. As a consequence, a variety of core−shell structured polystyrene (PS)-based composited latexes have been designed and fabricated.18 When the seed particle is cross-linked, the kinetic of the polymerization of the second-stage monomer will be more sophisticated. Sheu et al. reported some uniform nonspherical particles created by seeded emulsion polymerization of styrenedivinylbenzene mixtures in cross-linked monodisperse PS seed particles as early as 1990.19 These nonspherical shapes include ellipsoidal and egg-like singlets, symmetry and asymmetric doublets, and ice cream cone-like and popcorn-like multiplets, which were formed by separation of the second-stage monomer from the cross-linked seed particles during swelling and polymerization. The degree of phase separation increased with increasing monomer/polymer swelling ratio, the size and degree of cross-linking of the seed particles, and the Received: August 16, 2013 Revised: October 15, 2013 Published: November 11, 2013 14787

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and reagent grade concentrated sulfuric acid (>98%) were all purchased from Shanghai Chemical Reagents Corporation of China, and used without further purification. Distilled water was used in all experiments. Synthesis of Cross-Linked Polystyrene (CPS) Microspheres. A. Synthesis of Submicrosized CPS Microspheres. Monodisperse cross-linked PS microspheres with an average diameter of 172 nm were synthesized by a soap-free emulsion polymerization at boiling temperature in a three-necked flask equipped with a reflux condenser and a mechanical stirrer according to the literature.34 In this work, a mixed monomer containing St (6.0 g), DVB (0.03 g) and 100 mL of aqueous solution of AA (0.9 wt %) was added into the three-necked flask under mechanical stirring (300 rpm). The mixture was heated to boiling and kept for 10 min before the initiator KPS was added. The reaction stopped after 4 h. The obtained microspheres were collected by centrifugation and washed with ethanol for three times, then dried in a vacuum oven at 40 °C. B. Synthesis of Microsized CPS Microspheres. Monodisperse CPS microspheres with a size of 3.1 μm were synthesized through a classic seeded polymerization.35,36 A monomer solution was first prepared by mixing St, DVB, and AIBN. The detailed recipes of the mixed monomer solution is listed in Table 1. Then the monomer solution

polymerization temperature. The kinetics of phase separation investigated using optical microscopy revealed that the phase separation was induced by the relaxation of the polymer chains before the polymerization and was enhanced by increased conversion.11 A thermodynamic analysis of the swelling of cross-linked latex particles describes the mechanism of phase domain formation as a consequence of the equilibrium of three free energy change during swelling and polymerization process, i.e., the monomer-polymer mixing force, the polymer elasticretractive force, and the interfacial pressure across the particle− water interface.11 The above studies inspired later works on the design and synthesis of PS or polyacrylate-based anisotropic particles of different morphologies.20−24 On the other hand, polymer microspheres having hollow(s) inside have also been found to be fabricated through seeded emulsion polymerization of styrene or other acrylic monomers in the presence of noncross-linked PS seed particles.25−28 The hollow(s) originated from the expansion of the seed particles by osmotic swelling of water driven by the dissolution of the core part of seed particles in alkali solutions25−27 or the strong hydrophilic groups located at the surface of the seed particles.28 In the previous work of our group, cage-like hollow microspheres have been created by the seeded emulsion polymerization of styrene or acrylic monomers in the presence of sulfonated polystyrene (SPS) seed particles.29−31 Because of the strong hydrophilicity of −SO3H groups on seed particles, water molecules will diffuse from the outer aqueous phase into the seed particles under the osmotic pressure, and new individual water phases form in the inner part of SPS seed particles, which could be stabilized by the amphiphilic SPS molecular chains. Further studies on the effect of the crosslinking of SPS seed particles on the final latex particles have also been carried out in our group.32,33 The results show that the morphology of final latex particles greatly depends on the size, the degree of sulfonation, and cross-linking of the seed particles. Under the same sulfonation time in concentrated sulfuric acid and cross-linking agent (divinylbenzene) content of PS seed particles, walnut-like multihollow polymer particles can be obtained when the diameter of SCPS seed particles is 4.2 μm.32 The kinetics of the polymerization of the secondstage monomer in the presence of SCPS seed particles should be more complicated since the diffusion of water into the seed particles should be considered. Evidently, more work is needed for a further understanding of the kinetics of phase separation during the swelling and polymerization of monomer in the presence of amphiphilic cross-linked seed particles. Therefore, in this work, we prepared two kinds of SCPS seed microspheres with an average diameter of 170 nm and 3 μm, respectively. After the SCPS seed microspheres are swelled in a mixture of water and styrene for a certain time at room temperature, the polymerization of St is initiated by γ-ray radiation. The morphological formation mechanism of final latex particles is discussed based on a series of investigations on the effect of the size, the degree of sulfonation and cross-linking of the seed particles on the morphological change.

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Table 1. Detailed Recipes for the Preparation of the Microsized CPS Seed Microspheres composition of swelling monomer solution sample ID

styrene (g)

DVB (g)

AIBN (g)

monodisperse PS microspheres (g)

S1 S2

0.99 0.97

0.01 0.03

0.02 0.02

0.5 0.5

theoretical DVB weight content in the obtained CPS microspheres 0.7% 2.0%

was mixed with SDS solution (60 mL, 0.25 wt %) ultrasonically. 0.5 g of PS microspheres preprepared through dispersion polymerization37 (see Supporting Information) were dispersed into the above emulsion ultrasonically. Then the mixture was stirred mechanically for 15 h in order to let the PS microspheres fully swollen by monomer. An aqueous solution of PVA (50 mL, 1 wt %) was added at the end of swelling process. Finally, the polymerization was performed for 10 h at 70 °C with a mechanical stirring (300 rpm) and bubbling N2. The obtained microsized CPS microspheres were collected by centrifugation, and washed with ethanol for three times, then dried in a vacuum oven at 40 °C. C. Preparation of Sulfonated Cross-linked Polystyrene (SCPS) Seed Microspheres. For the preparation of SCPS microspheres, the above as-prepared CPS microspheres (0.5 g) were sulfonated in concentrated sulfuric acid (20 mL) at 40 °C under magnetic stirring. The product were collected by centrifugation after the reaction system was diluted by distilled water. After being washed with ethanol for three times, and then dried in vacuum oven for 48 h at 40 °C, the final SCPS seed microspheres were obtained. The Emulsion Polymerization of St in the presence of SCPS Seed Microspheres. A 0.05 g sample of as-prepared SCPS seed particles was added into a mixture of St (0.05 g) and water (20 mL) under mechanical stirring. Then, the emulsion was stirred magnetically for another 24 h to let the SCPS seed microspheres swollen by St. The resultant mixture was irradiated by 60Co γ-ray (located in USTC, China) at a dose rate of 35 Gy/min and a total absorbed dose of 45 kGy after bubbling N2 to remove the oxygen for 10 min. The products were collected by centrifugation, and washed with ethanol for three times, then dried in a vacuum oven at 40 °C to a constant weight. Characterizations. The morphology of the prepared latex particles was studied by SEM (JEOL JSM6700F, 5.0 kV) and TEM (H-7650, JEOL2011, 100 kV). Samples for SEM and TEM analysis were prepared by dispersing one drop of the ethanol solution of the sample on copper grids and then drying in air. The number-average diameter (Dn), weight-average diameter (Dw), and polydispersibility index (PDI) of the microspheres were calculated by the following equations

EXPERIMENTAL SECTION

Materials. Styrene (St) and acrylic acid (AA) were distilled under reduced pressure before use. Divinylbenzene (DVB) was purified by passing through an alumina column to remove the inhibitor before use. Reagent-grade 2,2-azobis(isobutyronitrile) (AIBN) was purified by recrystallization in ethanol. Ethanol (99.5%), polyvinylpyrrolidone (PVP, K30), sodium dodecyl sulfate (SDS), polyvinyl alcohol (PVA), 14788

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Figure 1. TEM images of (A) CPS microspheres containing 0.5% of DVB prepared by a boiling temperature soap-free emulsion polymerization, (B) CPS microspheres swollen by St for 15 h, and (C) snowman-like latex particles observed after the polymerization of St in CPS microspheres in part B initiated by 60Co γ-ray.

Figure 2. TEM images of (A) SCPS seed microspheres (the DVB content: 0.5%; the sulfonation time: 5 h), SCPS microspheres swollen by St for 4 h (B), 8 h (C), and 15 h (D), as well as (E) multihollow latex particles after the polymerization of St in SCPS microspheres of part D initiated by 60 Co γ-ray (The white arrows indicate the solid microspheres), and (F) the SEM images of multihollow latex particles in part E. with the diameters of at least 100 particles measured in the SEM and TEM images: Dn =

∑ niDi ; ∑ ni

Dw =

∑ niDi4 ∑ niDi 3

;

PDI =

Dw Dn

However, it is very interesting that the morphology of the final latex particles totally changed if the CPS microspheres were sulfonated in concentric sulfuric acid and then used as the seed microspheres in the same seeded emulsion polymerization process, as clearly displayed in Figure 2. EA analysis results show that SCPS microsphere contains 7.428 wt % of sulfur and 63.17 wt % of carbon. However, the morphology of SCPS microspheres has little change before and after the sulfonation, as exhibited in Figure 2A. The BET surface area and pore volume of the SCPS microspheres are 1.67 m2/g and 0.0014 cm3/g, respectively. The bulk and tapped density of SCPS seed microspheres are 0.662 g/cm3 and 0.751 g/cm3, respectively (see Table 2). After being swelled by St monomer for different time, the SCPS seed microspheres also have a volume expansion. The diameter of microspheres increases to 180

(1)

Here ni is the number of particles with a diameter of Di. The elemental analysis (EA) of latex particles was carried out by a VARIO ELIII analyzer (Elemental analysis system Co. Ltd., Germany) to get the composition of S, C, H, and O in microspheres. The specific surface area was analyzed by the nitrogen adsorption at 77.3 K on a Micromeritics Tristar II 3020 M V1.03 analyzer. Samples were degassed at 60 °C for 300 min under vacuum prior to data collection. Surface area data were collected over 0.005−0.994 p/p0 via the BET (Brunauer−Emmett−Teller) method.

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RESULTS AND DISCUSSION The Emulsion Polymerization of St in the Presence of Submicrometer-Sized SCPS Seed Microspheres. To obtain monodisperse submicrometer-sized SCPS seed microspheres, a soap-free emulsion polymerization in a boiling liquid medium had been adopted to prepare uniform CPS microspheres first, as shown in Figure 1A. The prepared CPS microspheres have a diameter of 172 nm with a narrow size distribution (PDI = 1.00). These CPS microspheres could be swollen by St monomer, and have a volume expansion (Dn = 196 nm), indicated in Figure 1B. After the polymerization of St monomer in the swollen CPS microspheres was initiated by the radiation of 60Co γ-ray, snowman-like latex particles have been fabricated (see Figure 1C), as expected according to the previous research work.11,19−21

Table 2. Bulk and Tapped Density of SCPS Seed Microspheres and the Prepared Latex Particles sample SCPS (172 nm) spherical multihollow latex particles SCPS (3 μm) walnut-like latex particles plum-like latex particles

bulk density (g/cm3)a

tapped density (g/cm3)a

0.662 0.562

0.751 0.658

0.589 0.443 0.402

0.707 0.532 0.530

a

The measurement of the density is described in Supporting Information. 14789

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elastic-retractile force associated with the change in configuration of the polymer network upon swelling, which can be described by the Flory−Rehner equation.39 ΔG̅ t is the microsphere-water interfacial tension force caused by the increase of the surface area on swelling, which can be calculated by the Morton equation.40 Thus, at saturation swelling equilibrium, ΔG̅ m,p should be given as follows:

nm when the swelling time is 15 h. In addition, a lot of voids can be seen clearly inside the expanded SCPS microspheres (Figure 2B−D). These voids could be remained after the polymerization of St initiated by 60Co γ-ray, as shown in Figure 2E. As a consequence, spherical multihollow latex particles, rather than nonspherical snowman-like latex particles, are created. The BET surface area and pore volume of the final latex particles are 7.46 m2/g and 0.0064 cm3/g, respectively, and more than four times of the SCPS seed microspheres. At the same time, the bulk and tapped density of the final latex particles decrease to 0.562 g/cm3 and 0.658 g/cm3 respectively (see Supporting Information). All these results indicate the formation of pores in the seed microspheres. Furthermore, the SEM images in Figure 2 (F) reveal that the final latex particles have a rough surface morphology. The Formation Mechanism of the Spherical Multihollow Latex Particles in the Presence of Submicrometer-Sized SCPS Seed Microspheres. 1. Phase Separation in Solid Cross-Linked Polymer Seed Microspheres Swollen by Monomers. The thermodynamic analysis on the phase separation for the system comprising CPS seed microspheres/St/aqueous phase at swelling equilibrium can be established under a condition that the chemical potential, ΔG̅ m,p, of a monomer in the cross-linked polymer seed microspheres should be equal to 0 according to the literatures.11 ΔG̅ m,p is the sum of following three contributions, as illustrated in Scheme 1: ΔGm̅ , p = ΔGm̅ + ΔGel̅ + ΔGt̅

ΔGm̅ , p = RT[ln(1‐νp) + νp + χmp νp 2] + RTN Vm(νp1/3 − νp/2) + 2Vmγ /r = 0

(3)

Here R is the gas constant, T is the absolute temperature, νp is the volume fraction of polymer in the swollen seed microsphere, χmp is the monomer-polymer interaction parameter, N is the effective number per unit volume of chains in the network, Vm is the molar volume of the monomer, γ is the microspherewater interfacial tension, and r is the radius of the swollen seed microsphere. It is seen that when ΔG̅ m,p < 0, the swelling of polymer microspheres would take place, while expulsion of monomer (to give phase separation) takes place when ΔG̅ m,p > 0. Therefore, supposed the swelling monomer is totally absorbed into all the seed microspheres uniformly, i.e., νp, χmp, and Vm are fixed at a constant feed ratio of St monomer to SPS seed microsphere (here is 1:1), the smaller radius of the swollen seed microsphere (r) will lead to the larger microsphere-water interfacial tension force (ΔG̅ t) according to the Morton equation ΔG̅ t = 2Vmγ/r.40 As a result, ΔG̅ m,p will be readily exceed zero according to eq 3, resulting in the occurrence of the phase separation, which is in accord with the formation of the snowman-like latex particles in the presence of CPS seed microspheres. 2. Phase Separation in Multihollow Sulfonated CrossLinked Polymer Seed Microspheres Swollen by Monomers. For the SCPS seed microspheres, the introduction of −SO3H groups on the PS chains via the sulfonation reaction makes the hydrophobic seed have a certain hydrophilicity. It is well-known that sulfonation of polymer can strongly increase the hydraulic water permeability and water vapor transport rate due to the increased hydrophilicity of the polymer membrane.41 Similarly, water will permeate into the SCPS seed microspheres while they are being swelled by St monomer. In the St-swollen seed microspheres, the polymer chains have good mobility so that

(2)

Scheme 1. Schematic Illustration of the Forces Acting to Expand or Shrink the Microspheres19

Here ΔG̅ m is the monomer-polymer mixing force arising from the absorption of monomer, which is given by the familiar Flory−Huggins expression.38 ΔG̅ el is the cross-linking network

Scheme 2. Schematic Illustration of the Formation of Spherical Multihollow Latex Particles via a Radiation-Induced Seeded Emulsion Polymerization of St in the Presence of Submicrometer-Sized SCPS Seed Microspheres

14790

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more and more water diffuse into the seed microsphere forced by the osmotic pressure, and form new aqueous phase in the inner part of the seed microspheres stabilized by the strong hydrophilic−SO3H groups, as the case shown in Figure 2B−D and illustrated in Scheme 2. Obviously, the seed microsphere− water interface exists not only at the outside surface of the seed microspheres, also inside the seed. Furthermore, the diameter of the newly formed inner water phase will be enlarged with the increase of the swelling degree of seed microspheres (see Figure 2B−D), which means the seed microsphere−water interface inside the seed will also be enlarged during the swelling process. Therefore, it can be inferred that the separation of second-stage monomer or newly formed polymer will also appear at the interface in the inner water phase, as illustrated in Scheme 2. Evidently, the radius of the inner aqueous phases are smaller than that of SCPS seed microspheres. According to the Morton equation,40 the trend of the phase separation of second-stage monomer or newly formed polymer at the interface in the inner water phase during the swelling process may be more obvious. Under the comprehensive effect of phase separation both inward and outward the seed microspheres, spherical multihollow latex particles with a similar size of the St-swollen seed microspheres before polymerization, rather than snowman-like particles, are the main product of the seeded emulsion polymerization, as shown in Figure 2, parts E and F. We can see in Figure 2E that the voids in some latex particles are occupied by the newly formed polymer so that solid latex particles could also be produced. These solid latex particles have the similar size to the swollen SCPS seed microspheres and the final spherical multihollow latex particles. It means they could not be the polystyrene latex particles from the secondary nucleation in the seed emulsion, whose size is generally much smaller than that of the swollen seeds. Effect of the Sulfonation Time on the Morphology of Latex Particles in the presence of Submicrometer-Sized SCPS Seed Microspheres. As discussed above, the introduction of −SO3H groups is crucial to the formation of the multihollow structure. So the sulfonation degree of SCPS seed microspheres should influence the morphology of the final latex particles. The SEM and TEM images of St-swollen SCPS seed microspheres are shown in Figure 3, parts A1 and B1. The sulfonation time is 5 h (part A1) and 8 h (part B1), respectively. The corresponding final spherical multihollow latex particles via a radiation-induced seeded emulsion polymerization are displayed in Figure 3, parts A2 and B2. The DVB content in SCPS seed microspheres is 0.5% for both samples. Generally, the contents of −SO3H groups will increase with the sulfonation time. Therefore, more voids appear in the St-swollen SCPS seed microspheres with a sulfonation time of 8 h. On the other hand, the solubility of seed microsphere in water seems to be improved with the increase of the sulfonation degree. More sulfonated PS chains in the noncross-linked area would separate from the microspheres during the swelling process so that a severe collapse can be observed for St-swollen SCPS seed microspheres with a sulfonation time of 8 h, leading to the final solid latex particles with a smaller size. Effect of the Size of SCPS Seed Microspheres on the Morphology of Final Latex Particles. The size of the seed particle is another essential factor influencing the morphology of final latex particles. According to the thermodynamic analysis on the phase separation for the system comprising polymer seed microspheres/monomer/aqueous phase at swelling

Figure 3. SEM and TEM images of St-swollen SCPS seed microspheres (A1, B1) and the corresponding final spherical multihollow latex particles via a radiation-induced seeded emulsion polymerization of St (A2, B2). The sulfonation time of SCPS seed microspheres is 5 h for sample A, and 8 h for sample B. The DVB content in SCPS seed microspheres is 0.5% for both samples.

equilibrium in the literature,11 the contributions of ΔG̅ t is near-negligible when the seed particles have a large size of above 2 μm. When a particle size is smaller than 2 μm, ΔG̅ t becomes more significant. Thus, microsized CPS microspheres were prepared by conventional seeded polymerization, shown in Figure 4B. The diameter of the CPS microspheres is about 3.08 μm (PDI = 1.00). After this microsized CPS microspheres was swollen by St, and then irradiated by 60Co γ-ray, no phase separation could be observed in the system, and only polydispersed spherical solid latex particles have been prepared (See Figure 4C). The microsized CPS microspheres were also sulfonated in concentrated H2SO4 for 5 h (see Figure 5A1. The bulk and tapped density of the SCPS seed microspheres are 0.589 g/cm3 and 0.707 g/cm3 respectively (see Table 2). The morphology of microsized SCPS swollen by St in aqueous solution is displayed in Figure 5A2. It looks similar to the case of submicrometersized SCPS seed microspheres (See Figure 2D) with nearly the same DVB content (∼0.5 wt %), i.e., multihollow SCPS microspheres are obtained. However, after the polymerization of St monomers swollen in the microsized SCPS is initiated by γ-ray radiation, the obtained final latex particles look like walnut, having grooved surface structure (Figure 5A3), which is very different from spherical multihollow latex particles prepared from submicrometer-sized SCPS microspheres. The above results verified that the size of the cross-linked seed particle plays an important role in the phase separation of the second-stage monomer and newly formed polymer during the seeded emulsion polymerization. The grooved surface structure should be the result of the shrink of St-swollen SCPS seed microspheres during the polymerization process since the separation of the newly formed PS only occurs in the small inner aqueous phase of the large seed microspheres. The polymerization and phase separation induce the volume shrink in the inner part of seed microspheres, resulting in the formation of the wrinkles on the surface, as illustrated in Scheme 3. 14791

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Figure 4. SEM images of (A) microsized PS microspheres prepared by dispersion polymerization, (B) CPS microspheres prepared by seeded polymerization (the DVB content is 2%), and (C) latex particles obtained from the seeded emulsion polymerization of St in the presence of CPS microspheres (part B) as the seed.

Figure 5. Effect of the DVB content in microsized SCPS seed microspheres on the morphology of the latex particles. SEM images of A1, A2, A3 show the morphology of SCPS seed microspheres containing 0.7 wt % of DVB (sample S1 in Table 1), after being swollen in St for 15 h and then after polymerization is induced by γ-ray radiation, respectively. SEM images of B1, B2, B3 show the morphology of SCPS seed microspheres containing 2 wt % of DVB (sample S2 in Table 1), after being swollen in St for 15 h and then after polymerization is induced by γ-ray radiation, respectively. The sulfonation times for both SCPS seed microspheres are 5 h.

Scheme 3. Schematic Illustration of the Formation of Latex Particles with Grooved Surface Structure via a Radiation-Induced Seeded Emulsion Polymerization of St in the Presence of Microsized SCPS Seed Microspheres

in SCPS seed microspheres rises to 2 wt % (see Figure 5B1), the collapse of the microspheres occurs while the multihollow structure forms, as shown in Figure 5B2. It can be explained

According to the analysis, we can expect to tune the surface structure by adjusting the cross-linking density of the microsized SCPS seed microspheres. When the DVB content 14792

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Notes

that the separation of monomer from the swollen microspheres to the inner water phase is enhanced with the increase of DVB content because ΔG̅ el increases as the cross-linking density of seed microspheres rises. As a consequence, the microspheres will collapse inward, leading to the appearance of the collapse on the surface (as illustrated in Scheme 3). When St monomers polymerize induced by γ-ray radiation, plum-like multihollow microspheres are formed under the enhanced phase separation process during the polymerization,19,20 as verified in Figure 5B3. The bulk and tapped density of walnut-like and plum-like latex particles (see Table 2) are almost the same, but both of them are much smaller than the SCPS seed microspheres. It should be attributed to the formation of grooved surface and pores inside the seed microspheres.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 51073146, 51103143, 51173175), and Fundamental Research Funds for the Central Universities (No. WK2060200005, 2010).

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CONCLUSIONS In this work, we found that the phase separation could occur both inside and outside of the styrene-swollen sulfonated crosslinked polystyrene (SCPS) seed microspheres during the radiation-induced seeded emulsion polymerization of styrene monomers. The phase separation process in these two opposite directions will determine the morphology of final latex particles. First, when SCPS seed microspheres are swollen by St monomers in water, new aqueous phase could be created and stabilized in the inner part of the SCPS seed microspheres due to the hydrophobic PS network and the strong hydrophilic sulfonic acid groups. During the polymerization process of St monomers induced by γ-ray, the separation of second-stage monomer or newly formed polymer in submicrometer-sized SCPS seed microspheres appears at the seed microsphere-water interface both inside and outside of the SCPS seed microspheres. As a result, submicrometer-sized spherical multihollow latex particle can be created. However, in the St-swollen microsized (above 3 μm) SCPS seed microspheres, phase separation only occurs at the seed microsphere-water interface inside the seed microspheres due to the thermodynamic reason, which induces the volume shrink in the inner part of seed microspheres, resulting in the formation of grooved surface structure. Through adjusting the content of cross-link agent DVB in SCPS seed microspheres, fine surface structure, such as plum-like and walnut-like latex particles are obtained. The results of this study provide a better understanding on phase separation mechanism during the swelling and polymerization of polymerizable monomers in cross-linked amphiphilic polymer networks. It also gives a practical guide to design and synthesis of multihollow polymer latex particles with tunable surface morphologies.

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ASSOCIATED CONTENT

S Supporting Information *

The preparation of amorphous polystyrene seed microspheres through dispersion polymerization, and the measurement of bulk and tapped density of the SCPS seed microspheres and the final latex particles. This material is available free of charge via the Internet at http://pubs.acs.org/.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Telephone: +86-551-63600843. Fax: +86-551-63601592. 14793

dx.doi.org/10.1021/la403166q | Langmuir 2013, 29, 14787−14794

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