Formation of Monodisperse Polymer@SiO2 Core–Shell Nanoparticles

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Formation of Monodisperse Polymer@SiO2 Core−Shell Nanoparticles via Polymerization in Emulsions Stabilized by Amphiphilic Silica Precursor Polymers: HLB Dictates the Reaction Mechanism and Particle Size Zhi Chen,† Yongliang Zhao,‡ Xiaomin Zhu,*,† and Martin Möller†

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†

DWILeibniz-Institute for Interactive Materials e.V. and Institute for Technical and Macromolecular Chemistry of RWTH Aachen University, Forckenbeckstraße 50, Aachen 52056, Germany ‡ Shanghai Dilato Materials Co., Ltd, Guohe Road 60, Shanghai 200433, P. R. China S Supporting Information *

ABSTRACT: Heterophase polymerization is the main tool to prepare polymer dispersions. In this work, we report on a new type of heterophase polymerization that allows synthesizing in one step monodisperse polymer@SiO2 core−shell particles in a wide size range from tens to hundreds of nanometers. The strategy utilizes amphiphilic silica precursor polymers, namely, PEGylated polyethoxysiloxanes, which can reduce the interfacial tension between oil and water close to zero. An oil phase containing such a kind of surfactants can be emulsified in water spontaneously or just under low-energy stirring. Polymerization of styrene in the resulting emulsions using an oil-soluble initiator leads to the formation of monodisperse polystyrene@SiO2 particles, whose size can be precisely adjusted by the PEGylation degree of precursor molecules and can reach as small as 30 nm. It is demonstrated that the PEGylation degree, that is, the HLB, also dictates the reaction mechanism that varies from suspension polymerization with breakup of monomer droplets and miniemulsion polymerization to microemulsion polymerization leading to exact “copying” of initial emulsion droplets. This technique paves a new avenue for simple, controllable, and environmentally friendly production of dispersions of composite polymer particles.

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INTRODUCTION Heterophase polymerization is a generic term describing polymerization reactions taking place in heterogeneous media.1 Various kinds of heterophase polymerization techniques are generally used for manufacturing polymer dispersions where polymer particles are dispersed in a liquid continuous phase.2,3 Aqueous emulsion polymerization is by far the most relevant, but also a special case, as the polymer particles are formed via nucleation in the continuous aqueous phase.4,5 In contrast, polymerization occurs in monomer droplets during suspension and miniemulsion polymerization. These two methods differ mainly in the droplet size and subsequently the resulting polymer particle size. In suspension polymerization, the monomer is dispersed as liquid droplets, typically with diameters in the range of 10 μm to 5 mm, with the steric stabilizer and vigorous stirring, which is maintained during polymerization, to produce polymer particles as a dispersed solid phase.6,7 Monomer-in-water miniemulsions suitable for miniemulsion polymerization contain submicron monomer droplets stabilized against both diffusional degradation and droplet coagulation by using a water-insoluble compound (hydrophobe) and an efficient surfactant.8,9 This method allows obtaining complex polymer nanoparticles and encapsulating different materials into a polymer shell.10,11 The main © XXXX American Chemical Society

impediment of the miniemulsion polymerization technique to industrial exploitation is that high-energy emulsification is generally required for the formation of initial thermodynamically unstable emulsions. In contrast to conventional emulsions as well as miniemulsions, microemulsions are thermodynamically stable systems and often form spontaneously,12−14 however, a big amount of the surfactant frequently in combination with a cosurfactant (usually an alcohol) is needed for their stabilization. Polymerization was carried out in microemulsions aiming to obtain thermodynamically stable dispersions of ultrasmall polymer particles.13,15,16 It is, however, complicated by the fact that the initial microemulsion may not stay in the thermodynamic equilibrium through the whole polymerization reaction. In addition to the entropic factors contributing to the destabilization of microemulsions during polymerization, the compatibility between polymer and cosurfactant can also affect the system.17 It was demonstrated that particle nucleation was continuous throughout the whole process in contrast to emulsion polymerization.18−20 For the case of oil-in-water (O/ Received: April 24, 2019 Revised: July 3, 2019

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DOI: 10.1021/acs.macromol.9b00841 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Recipes for Polymerization in 100 g of Water. The Amount of Initiator AIBN is 0.5 wt % of the Oil Phase run

styrene, (g)

PEGx-PEOS, (g)

1 2 3 4 5c 6 7 8 9 10c 11 12

5.0 10.0 10.0 5.0 5.0 5.0 5.0 10.0 5.0 5.0 5.0 5.0

PEG3-PEOS, 4.0 PEG5-PEOS, 2.15 PEG5-PEOS, 4.3 PEG5-PEOS, 4.3 PEG5-PEOS, 4.3 PEG5-PEOS, 8.6 PEG7-PEOS, 4.7 PEG10-PEOS, 5.5 PEG10-PEOS, 5.5 PEG10-PEOS, 5.5 PEG10-PEOS, 11.0 PEG15-PEOS, 7.3

Dhb of emulsion droplet, nm (PDIb) particle sizea, (nm) Dhb of PS particles, nm (PDIb) Mn of PS (Mw/Mn) 4409 1128 (0.294) 1077 (0.288) 849 (0.299) 849 (0.299) 723 (0.360) 303 (0.210) 515 (0.172) 110 (0.18) 110 (0.18) 122 (0.274) 34 (0.150)

494 ± 6

529 (0.021)

39 000 (2.00)

502 ± 53 347 ± 7 1065 ± 387 336 ± 18 203 ± 6 355 ± 6 97 ± 4 99 ± 7 153 ± 21 23 ± 4

572 (0.195) 367 (0.037) 1163 (0.258)

32 900 (3.21) 44 400 (2.64) 26 100 (2.98) 44 100 (2.79) 54 900 (3.68) 46 300 (3.23) 92 400 (2.69) 89 200 (2.40) 81 400 (2.10) 157 000 (1.86)

215 375 117 140

(0.025) (0.068) (0.097) (0.106)

34 (0.138)

a

Particle size was estimated by averaging diameter of 1000 particles in electron micrographs. bDh (hydrodynamic diameter) and PDI (polydispersity index) were obtained via dynamic light scattering (DLS) measurements. cEmulsion was stirred at room temperature for 24 h before heating.

investigated and the influence of the degree of PEG substitution, that is the hydrophilic−lipophilic balance (HLB) value of these materials, as well as the precursor-tomonomer ratio on the size and morphology of the resulting latex is the main focus of this study.

W) microemulsions, the initiation can take place in both the aqueous phase and monomer reservoir, depending mainly on the solubility of the monomer in water. As a general rule, the particles prepared in microemulsions are much smaller than those obtained by emulsion as well as miniemulsion polymerization; however, their size still significantly exceeds that of the parent microemulsion droplets. Recently, we developed new one-pot heterophase polymerization techniques to synthesize well-defined nanostructured hybrid polymer particles.21−23 Instead of classical organic surfactants, hyperbranched polyethoxysiloxane (PEOS) was used as both the silica precursor and emulsion stabilizer. When styrene was employed as the monomer, monodisperse core− shell polystyrene@SiO2 nanoparticles were obtained via a miniemulsion polymerization mechanism.21 In the case of the methyl methacrylate (MMA)−PEOS system, the formation of monodisperse core−shell polymer@SiO2 nanoparticles was the result of emulsion polymerization.23 The difference in the reaction mechanism is associated with the different polarity and water solubility of these two monomers. In both cases, PEOS, which was per se a water-insoluble liquid, became amphiphilic upon hydrolysis at the oil/water interface.24 The partially hydrolyzed PEOS molecules, however, could not stabilize MMA droplets in water because of the low stimulus for their adsorption on the MMA/water interface. For more hydrophobic styrene, a high-energy homogenizer was required to create submicron oil droplets in water, so that the hydrolysis of PEOS could be accelerated. It can be expected that intrinsically amphiphilic silica precursor polymers should significantly facilitate the emulsification process by acting as efficient surfactants.25 The interfacial dilational modulus, which is of great importance during emulsification, can also be reduced, because time-consuming chemical reactions are no more needed to form surfactant molecules. We have synthesized amphiphilic PEOS derivatives by substituting PEOS with poly(ethylene glycol) monomethyl ether (PEG).26 It has been shown that depending on the degree of modification, these materials self-assembled in water to form crew-cut micelles, monolamellar vesicles or micelles, which were converted under basic conditions to mesoporous silica microparticles, nanocapsules, or ultrasmall particles. In this work, PEOS derivatives substituted with PEG are employed as both the silica precursor and surfactant for heterophase polymerization of styrene. The polymerization mechanism is

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EXPERIMENTAL SECTION

Materials. Tetraethoxysilane (TEOS, GPR RECTAPUR, VWR), acetic anhydride (ACS reagent, ≥98.0%, Sigma-Aldrich), PEG monomethyl ether (average molecular weight 350, Sigma-Aldrich), tetraethyl orthotitanate (Sigma-Aldrich), sodium hydroxide (EMSURE zur Analyse, VWR), tetrahydrofuran (THF, EMPARTA ACS zur Analyse, VWR) were used as received. Styrene (ReagentPlus, ≥99.0%, Sigma-Aldrich) was purified by passing through a column filled with aluminum oxide (activated, neutral, Sigma-Aldrich) and 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%, Sigma-Aldrich) was recrystallized from ethanol. Deionized water was used for all the experiments. Synthesis of PEGylated PEOS (PEGx-PEOS). PEGx-PEOS were synthesized according to the method published elsewhere.26 The recipes used to synthesize different PEGx-PEOS are shown in Table S1. TEOS, acetic anhydride and tetraethyl orthotitanate were added under a nitrogen atmosphere into a 1 L three-neck round-bottom flask equipped with a mechanical stirrer and a 30 cm long refluxing column connected with a distillation bridge. The mixture was heated to 135 °C in an oil bath. The resulting ethyl acetate was continuously distilled off. When the distillation of ethyl acetate stopped, the system was connected to vacuum. The product was dried in vacuum at 135 °C for 2 h. After cooling to room temperature, PEG monomethyl ether was added. The resulting mixture was heated to 135 °C under intensive stirring until the termination of ethanol distillation. Volatile fractions were eventually removed in vacuum at this temperature. A transparent, yellowish oily liquid was obtained in all cases. Polymerization Procedure. The recipes for polymerization of styrene/PEGx-PEOS systems in water are summarized in Table 1. The reaction was typically carried out as follows. Water was added to a 250 mL three-necked round-bottom flask equipped with a reflux condenser and a nitrogen inlet. Under a nitrogen atmosphere and magnetic stirring (800 rpm), an oil phase consisting of styrene, PEGPEOS, and AIBN was added to water. Afterward, the resulting emulsion was heated to 70 °C, and the reaction was allowed to proceed for another 24 h under stirring at 500 rpm. In order to remove the silica coating, the core−shell particles were first isolated by centrifugation at a speed of 11 000 rpm for 60 min, and then dispersed in an aqueous solution of NaOH (3 M). The resulting polystyrene (PS) particles were separated from the aqueous phase, washed three times with deionized water, and dried. To remove the PS core, the core−shell particles were dispersed in THF and stirred at B

DOI: 10.1021/acs.macromol.9b00841 Macromolecules XXXX, XXX, XXX−XXX

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Table 2. Chemical Structure and HLB of PEGx-PEOS According to 1H NMR Spectroscopy and Elemental Analysis Data sample

molar fraction of ethoxy-groups replaced by PEG, %

SiO2 content, %

Brutto formula

HLB

PEG3-PEOS PEG5-PEOS PEG7-PEOS PEG10-PEOS PEG15-PEOS

2.95 5.04 7.08 10.04 14.91

45.0 41.6 38.5 33.0 24.9

SiO1.203(OEt)1.547(PEG)0.047 SiO1.196(OEt)1.527(PEG)0.081 SiO1.182(OEt)1.520(PEG)0.116 SiO1.099(OEt)1.621(PEG)0.181 SiO0.903(OEt)1.867(PEG)0.327

2.5 3.9 5.2 7.0 9.5

room temperature overnight. The resulting hollow silica particles were isolated by centrifugation, washed three times with water, and then redispersed in water for further investigation. Interfacial Tension Measurements. Interfacial tension (IFT) between the water and oil phase was measured with a Krüss spinning drop tensiometer at 25 °C. During the measurements a droplet of an oil phase consisting of styrene and PEOS derivatives of 4 μL was positioned in the water phase placed in a glass capillary with a diameter of 4.6 mm. IFT was calculated using the Vonnegut method. Field-Emission Scanning Electron Microscopy. Field-emission scanning electron microscopy (FE-SEM) measurements were performed on a Hitachi S4800 high-resolution field emission scanning electron microscope with an accelerating voltage of 1.5 kV for samples prepared with PEG3-PEOS to PEG10-PEOS and 5.0 kV for samples prepared with PEG15-PEOS. A drop of diluted sample dispersion was placed on a silicon wafer substrate and air-dried under ambient conditions. Transmission Electron Microscopy. Transmission electron microscopy (TEM) measurements were carried out on a Zeiss Libra 120 transmission electron microscope. The accelerating voltage was set at 120 kV. The samples were prepared by placing a drop of diluted sample dispersion on a Formvar−carbon-coated copper grid of 200 meshes. Energy-Dispersive X-ray Spectroscopy−Scanning TEM. Energy-dispersive X-ray spectroscopy (EDX)−scanning TEM (STEM) measurements were performed for the core−shell particles using a Hitachi SU9000 field emission scanning electron microscope, equipped with Oxford Xmax 80 EDX-detector and operated at 30 kV. The Si−K and C−K edges were used to collect chemical information of individual elements. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) measurements were conducted on a PerkinElmer STA 6000 unit operating under a nitrogen atmosphere with a flow rate of 20 mL/min. Dried sample (5−10 mg) was placed in a standard PerkinElmer alumina 85 μL crucible for the measurements. The heating rate was 10 K/min. DLS Measurements. Hydrodynamic diameter of particles in water was measured with a Malvern Zetasizer Nano Series at a scattering angle of 173° at 25 °C. Before measurements, the dispersions were diluted to a particle concentration of 1.5 wt ‰. Gel Permeation Chromatography. Gel permeation chromatography (GPC) was performed on a GPC system consisting of a highperformance liquid chromatography pump (Bischoff HPLC), a Jasco 2035-plus RI detector, and four MZ-DVB gel columns (30, 100, and 2 × 3000 Å) in series. THF containing 1.0 g·L−1 LiBr was used as the eluent at a flow rate of 1.0 mL min−1. A calibration plot, constructed with PS standards, was used to determine the molecular weight.

where MPEG is the molecular weight of PEG and M is the molecular weight of the brutto formula. As described in our previous work, depending on the degree of PEG substitution, that is HLB value, PEGx-PEOS derivatives self-assemble into different supramolecular structures in water.26 The HLB is a parameter that describes the emulsification behavior of surfactants. It is known that O/W emulsions are stable when they are stabilized by surfactants with HLB in the range of 8− 18.27 The HLB values of PEGx-PEOS synthesized in this work lie mostly below this range (cf. Table 2). Nevertheless, the solution of PEGx-PEOS in styrene can be easily emulsified in water. The deviation between the HLB values and the observations can be explained as follows. Eq 1 is well suitable for homologues series of aliphatic esters and ethers of polyhydric alcohols and ethoxylated derivatives, where the hydrophobic and hydrophilic fragments have similar density.28 In our case, the HLB values are underestimated because oligoethoxysiloxane is less lipophilic than the aliphatic chains and has a higher density. Furthermore, ethoxy groups hydrolyze upon contact with water, leading to the formation of ionic silanolate groups. In contrast to the PEOS/styrene system, where high-energy homogenization is needed for emulsification, the styrene/ PEGx-PEOS mixtures form stable O/W emulsions under magnetic stirring. At high degrees of PEGylation, the emulsification process requires almost no energy input and O/W emulsions form spontaneously. This indicates that the emulsifying ability of the homologues series of amphiphiles PEGx-PEOS increases with increasing the PEGylation degree (i.e., increasing HLB) because of the enhanced ability to reduce IFT (Figure 1). According to the dynamic light scattering (DLS) data shown in Table 1, the size and size distribution of the emulsion droplets prepared under the same conditions decrease with the increase of the HLB value. With PEG15-PEOS, the droplet size can reach the value as small as 34 nm. This phenomenon is related to the increase of the limited area per molecule A, which scales as N1/2, where N is the number of ethylene oxide fragments.29,30 The interfacial activity of PEGx-PEOS in the styrene/water system was studied at room temperature using a spinning drop tensiometer, which is able to measure very low IFT. As shown in Figure 1a, the IFT arising from the styrene−water interface is measured to be 26 mN/m. After adding nonmodified PEOS, the IFT is reduced to 13 mN/m. The solutions containing PEGx-PEOS exhibit significantly reduced IFT in water. By dissolving less than 10 wt % PEG3-PEOS in styrene, the IFT becomes 4.6 mN/m. The IFT decreases with the increase of PEG3-PEOS concentration and reaches the plateau (∼0.7 mN/m) at the concentration of around 17 wt %. PEGx-PEOS with higher PEGylation degrees show much higher interfacial activity (Figure 1b). The styrene solution containing less than 5 wt % PEG5-PEOS has an IFT of 4.2 mN/m, and it decreases to a value as low as 0.05 mN/m when its concentration exceeds above 17 wt %. The measurements for PEGx-PEOS

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RESULTS AND DISCUSSIONS Table 2 summarizes the amphiphilic silica precursor polymers synthesized in this work. Their chemical structure was determined by 1H NMR spectroscopy and elemental analysis. The products are designated as PEGx-PEOS, where x% is the molar fraction of ethoxy groups, which have been replaced by PEG groups. HLB of PEGx-PEOS with a brutto formula SiOm(OC2H5)n(PEG)p was calculated according to the following equation HLB = 20 × MPEG /M

(1) C

DOI: 10.1021/acs.macromol.9b00841 Macromolecules XXXX, XXX, XXX−XXX

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The styrene-in-water emulsions stabilized by PEGx-PEOS with different PEGylation degrees were subjected to radical polymerization using AIBN as the initiator. The choice of reaction compositions (i.e., the mass ratio of styrene to silica precursor) is based on the result of our previous publication.21 In that work, PS@SiO2 core−shell particles with the narrowest size distribution formed when the mass ratio of styrene to PEOS was 1:1, and the reaction composition was varied around this ratio. In order to compare the behavior of PEOS and PEGx-PEOS, similar reaction compositions have been chosen in this study. We compared the reaction systems with the same content of styrene and silica converted from PEGxPEOS (runs 1, 4, 7, 9, and 12 in Table 1). The emulsions were heated to 70 °C as soon as they formed. In all cases, polymer particles of very narrow size distribution were obtained. As shown in Figure 2a, the hydrodynamic diameter (Dh) of the resulting particles measured by DLS is smaller than that of the initial emulsion droplets for PEGx-PEOS with lower modification degrees (x = 3, 5, and 7), and the shrinkage becomes much less significant with the increase of the PEGylation degree. For PEGx-PEOS with higher degrees of PEG modification (x = 10 and 15), polymerization only leads to slight narrowing of the size distribution, and the mean particle size remains almost unchanged. The change of size distribution in the styrene/PEG5-PEOS-in-water and styrene/ PEG10-PEOS-in-water emulsions during the polymerization was followed by DLS. For the system containing PEG5-PEOS, the particle size decreases significantly within the first 30 min, while the size distribution remains almost constant (cf. Figure 2b). Afterward, both size and size distribution decrease and finally almost monodisperse polymer particles are formed. The change of the particle size distribution during polymerization in the styrene/PEG10-PEOS-in-water emulsion is insignificant (cf. Figure 2c). Interestingly, the molecular weight of the obtained PS increases with the increase of hydrophilicity of the silica precursor polymer, and the maximal polydispersity is observed with PEG7-PEOS (Figure 3). It seems that polymerization in different styrene/PEGx-PEOS-in-water emulsions proceeds according to two different mechanisms, and PEG7-PEOS probably marks the border between them. The resulting polymer particles were analyzed by means of FE-SEM, TEM, EDX−STEM, and TGA. Similar to the DLS data, electron microscopy displays the formation of monodisperse polymer particles in all cases (Figure 4). Furthermore, the particle sizes measured by both DLS and electron microscopy are similar (Table 1). After washing away PS by THF, hollow particles with a broken shell that is caused most probably by osmotic pressure are observed. EDX−STEM elemental mapping (Figure 5) shows that the silicon atoms are located only at the periphery of the particles, indicating clearly a PS@SiO2 core−shell structure. For the system with PEG15PEOS, the core−shell structure can hardly be directly visualized because of the small particle size and limited resolution of the EDX−STEM technique; nevertheless, the overlapping of the TEM micrograph, C-mapping and Simapping of the polymer particles (Figure S1) implies the formation of such a kind of the structure. As can be seen from the EDX−STEM micrographs (Figure 5), the silica layer that covers the PS core is very thin, and the dependence of its thickness (d) on the diameter of the core−shell particle (Ø) can be well described by the following equation: d = 0.033 Ø, in good agreement with the theoretically calculated values by assuming that PEGx-PEOS is fully converted to a layer of

Figure 1. IFT of the oil phase containing styrene and different amounts of PEOS and PEGx-PEOS with water of pH 7 vs time: (a) pure styrene and styrene with PEOS and PEG3-PEOS; (b) styrene with PEG5-PEOS.

derivatives with the PEG substitution degree above 5 mol % were not successful even at very low concentration. The oil drop consisting of styrene and PEGx-PEOS (x > 5) broke up to very small droplets under very gentle spinning, indicating extremely low IFT in these systems. In contrast to the styrene/PEOS system, where amphiphilic molecules form at the oil/water interface via hydrolysis of the ethoxysilane groups,21 PEGx-PEOS molecules are intrinsically amphiphilic and exhibit extremely high interfacial activity with IFT close to zero. From the thermodynamic point of view, the amount of work needed to form emulsion droplets is small, when IFT is small. In this case, the stability of the emulsion should increase with the decrease of the droplet size because of the increased entropy of mixing.31 When a styrene/PEGxPEOS (x ≤ 5) mixture is added to water under stirring, stable emulsion is formed. For PEGx-PEOS with higher degree of PEG substitution (x ≥ 7), spontaneous emulsification31−33 takes place, which is responsible for the fact that the IFT could not be measured with the spinning drop tensiometer. The spontaneous emulsification in this case can be explained by the combination of following mechanisms. First, in the presence of surfactant molecules Marangoni flow leading to breakoff of droplets is produced by IFT gradients arising from an instability that often develops when diffusion occurs near an interface.34 Second, diffusion of PEGx-PEOS molecules from the oil phase to water can cause local supersaturation and consequently nucleation of emulsion droplets.35 Because the equilibrium IFT in these systems is very close to zero, a transient negative IFT induced by fluctuation may also result in spontaneous interfacial expansion.32 D

DOI: 10.1021/acs.macromol.9b00841 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. Number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of PS obtained via polymerization of styrene/PEGx-PEOS-in-water emulsions (runs 1, 4, 7, 9, and 12 in Table 1).

Figure 2. (a) Hydrodynamic diameter (Dh) of emulsion droplets and resulting polymer particles prepared under the same conditions from reaction systems with the same content of styrene and silica converted from PEGx-PEOS (runs 1, 4, 7, 9, and 12 in Table 1). Evolution of distribution of Dh during polymerization in styrene/PEG5-PEOS-inwater (b, run 4 in Table 1) and styrene/PEG10-PEOS-in-water (c, run 9 in Table 1) emulsions.

Figure 4. FE-SEM and TEM micrographs of polymer particles synthesized by polymerization of styrene/PEGx-PEOS systems in water and particles obtained after the treatment with THF. (a) PEG3PEOS (run 1 in Table 1); (b) PEG5-PEOS (run 4 in Table 1); (c) PEG7-PEOS (run 7 in Table 1); (d) PEG10-PEOS (run 9 in Table 1), and (e) PEG15-PEOS (run 12 in Table 1). The scale bars represent 200 nm.

hydrated silica with a density of 1.65 g/cm3. Meanwhile, TGA data demonstrate that all the PS@SiO2 particles considered here contain around 26 wt % silica (Figure S3) that well coincides with the value calculated from the recipe, confirming the full incorporation of silica converted from PEGx-PEOS into the PS@SiO2 core−shell particles. For PEG5-PEOS, its ratio to styrene was systematically varied (run 2−6 in Table 1). The size of emulsion droplets decreases with the increase of the PEG5-PEOS concentration in the oil phase. Although stable emulsions can form at a pretty low PEG5-PEOS concentration (e.g., run 2 in Table 1), they break down during the polymerization process by heating to 70 °C and PS precipitates from the reaction medium in the form of big blocks (Figure S2). When the concentration of PEG5PEOS in the oil phase increases to 30 wt % (run 3 in Table 1),

a stable polymer dispersion is obtained after polymerization. The mean particle size is ca. 500 nm according to the electron microscopy, and the particle size distribution is quite broad. By raising the PEG5-PEOS concentration to 46.2 wt %, both the size and size distribution decrease (Figure 6a). However, the particle size cannot be further reduced by further increase of the precursor concentration. Furthermore, the particles prepared with the PEG5-PEOS concentration of 63.2 wt % sediment (Figure S2), nevertheless, they can be easily redispersed in water by shaking. According to the FE-SEM data (Figure 7a), the particles tend to form aggregates. Most E

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Figure 5. EDX−STEM micrographs of polymer particles synthesized by polymerization of styrene/PEGx-PEOS systems in water. (a) PEG3-PEOS (run 1 in Table 1); (b) PEG5-PEOS (run 4 in Table 1); (c) PEG7-PEOS (run 7 in Table 1); and (d) PEG10-PEOS (run 9 in Table 1). Red color indicates the presence of carbon atoms and blue color corresponds to silicon atoms. The scale bars represent 250 nm.

Figure 7. FE-SEM micrographs of polymer particles obtained via polymerization in styrene/PEG5-PEOS-in-water (a) and styrene/ PEG10-PEOS-in-water (b) emulsions. Weight fraction of PEG5PEOS and PEG10-PEOS in the oil phase is 63.2 wt % (run 6 in Table 1) and 68.8 wt % (run 11 in Table 1), respectively. The scale bars represent 500 nm. Figure 6. Influence of weight fraction of PEG5-PEOS (a) and PEG10-PEOS (b) in the oil phase with styrene on the hydrodynamic diameter (Dh) of emulsion droplets and size of resulting polymer particle (Ø) measured by electron microscopy.

the oil phase, the size of the formed emulsion droplets and the resulting polymer particles after polymerization first decreases and then slightly increases. At the PEG10-PEOS concentration of 68.8 wt %, the agglomeration of the resulting polymer particles is again observed because of the formation of ultrasmall particles from excess silica (Figure 5b). It seems that the silica content in the core−shell particles can be adjusted only in a relatively small range in the case of PEGxPEOS/styrene-in-water emulsion systems. At high PEGxPEOS fractions in the oil phase, a certain amount of PEGxPEOS does not contribute to the coating of the PS particles,

probably, excess silica, which is converted from PEG5-PEOS but is not integrated into the core−shell particles, forms ultrasmall silica particles, hence phase separation occurs because of the depletion interaction in the bimodal colloidal suspension with a large size difference.36−38 In the case of PEG10-PEOS, with the increase of the precursor fraction in F

DOI: 10.1021/acs.macromol.9b00841 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules instead they are converted to small silica particles that cause flocculation of the PS@SiO2 core−shell particles. It seems that there exists a concentration window for each PEGx-PEOS, in which well-defined core−shell particles are formed with all silica integrated. In order to study the stability of styrene/PEGx-PEOS-inwater emulsions, they were stirred at room temperature for 24 h, and afterward, they were subjected to radical polymerization by increasing the temperature to 70 °C. The change of size distribution with time was then monitored by DLS (Figure 8).

Figure 9. FE-SEM and TEM images of PS@SiO2 core−shell particles obtained via stirring styrene/PEG5-PEOS-in-water (a, run 5 in Table 1) and styrene/PEG10-PEOS-in-water (b, run 10 in Table 1) at room temperature for 24 h and subsequent polymerization at 70 °C for 24 h. The scale bars represent 500 nm.

called “nanorattle” or “yolk−shell” particles39 are formed (Figure 9b). Such a structure appears because of the formation of a solid silica shell surrounding the monomer droplets before the start of the polymerization. The experimental data indicate that there must be a synergistic effect between the polymerization of styrene and PEGx-PEOS conversion. PEGx-PEOS derivatives are miscible with styrene, however, they have an increasing tendency to migrate to the oil/water interface with the increase of the PEGylation degree as indicated by low and decreasing IFT values, and their compatibility with PS is even more significantly reduced. The proposed mechanism of polymerization in styrene/PEGx-PEOS-in-water emulsions is depicted in Scheme 1. The emulsion droplets resemble the self-

Figure 8. Change of hydrodynamic diameter (Dh) of emulsion droplets of styrene/PEG5-PEOS-in-water (a, run 5 in Table 1) and styrene/PEG10-PEOS-in-water (b, run 10 in Table 1) emulsions during stirring at room temperature for 24 h and subsequent polymerization at 70 °C for 24 h.

Scheme 1. Schematic Illustration of Mechanisms of Polymerization in Styrene/PEGx-PEOS-In-Water Emulsions. (a) PEG3-PEOS and PEG5-PEOS, (b) PEG7PEOS, (c) PEG10-PEOS and PEG15-PEOS

For the styrene/PEG5-PEOS-in-water emulsion, the droplets grow with time; however, the size distribution slightly decreases. After 24 h, the droplet size increases for about 35%. Subsequent polymerization of the styrene monomer leads to broadening of the particle size distribution and the mean size remains almost unchanged. The increase of the size of the emulsion droplets should be the result of the interplay between Ostwald ripening and formation of a silica shell around the liquid droplets preventing particle coalescence. The wrinkled particle surface can be related to the shrinkage of the droplets upon polymerization (Figure 9a). In the case of the styrene/ PEG10-PEOS-in-water emulsion, the growth of the particle size during stirring at room temperature for 24 h is 27%, less significant than that with PEG5-PEOS, and narrowing of the size distribution is also observed. Subsequent polymerization of styrene almost does not cause any change in the size distribution. The TEM data show that there is a void between the polymer core and the surrounding shell, that is, the soG

DOI: 10.1021/acs.macromol.9b00841 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules assembled structures of PEGx-PEOS in water26 that are swollen by the hydrophobic liquid. PEGx-PEOS with a lower degree of PEG modification (x ≤ 5) are quite hydrophobic, so in the emulsion, the precursor molecules are located not only at the oil/water interface, but also inside the droplets, forming styrene-swollen crew-cut micelles. As soon as the polymerization is initiated, phase separation occurs in the emulsion droplets and the PEGx-PEOS molecules continuously diffuse to the interface leading to local supersaturation. Meanwhile, the precursor molecules become more hydrophilic with time because of further hydrolysis at the interface. The breakup of the big droplets to small ones at the beginning of polymerization in the emulsions stabilized by PEG3-PEOS and PEG5PEOS should result from the synergy of these two effects. Further particle shrinkage can be attributed for the most part to the conversion of PEGx-PEOS dissolved in styrene to silica, which is accompanied by more than 50% weight loss.21 The PEG substitution certainly decreases the solubility of precursor molecules in a styrene/PS mixture, therefore, excess PEGxPEOS can neither be dissolved in droplet bulk nor be at the oil/water interface because of the limited interfacial area, and they are expelled to the water phase and form free small silica particles. Emulsions containing PEG10-PEOS or PEG15PEOS form spontaneously. They are stable with time and contain very small droplets (≤100 nm), therefore, can be considered as microemulsions.40 In these emulsions, the precursor molecules are located exclusively at the oil/water interface forming a monolayer because of their extremely high interfacial activity, resembling styrene-swollen micelles. Thus, polymerization does not give rise to any significant size change, and a direct translation of monomer droplets into core−shell particles, that is one-to-one copy, is observed. For the intermediate precursor PEG7-PEOS, from the size change during polymerization, it can be concluded that the precursor molecules are distributed between the interface and droplet bulk, however, the migration of these molecules to the interface and their further hydrolysis do not break up the droplets because of their high stability. The dependence of the molecular weight of PS on the chemical structure of PEGxPEOS can be well explained by the above mechanism. The kinetic chain length of radical polymerization (Xn) can be described using the following equation: X n = K [M]/[I]1/2

SiO2 core−shell nanoparticles, whose size can be precisely adjusted by the PEGylation degree of the precursor molecules in a wide range from tens to hundreds of nanometers and can reach as small as 30 nm. It has been shown that the HLB of PEGx-PEOS dictates the polymerization mechanism. For PEGx-PEOS with lower HLB, the initial microsized emulsion droplets break up during polymerization and PS@SiO2 core− shell particles of hundreds of nanometers are eventually formed. In the case of PEGx-PEOS, with higher HLB, microemulsions are formed where precursor molecules are located exclusively at the oil/water interface. After polymerization, a direct translation of monomer droplets into core− shell particles, that is one-to-one copy, is observed. This technique paves a new avenue for simple, controllable, and environmentally friendly production of dispersions of composite polymer particles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00841. Additional experimental details, EDX−STEM micrographs, optical photos, and TGA data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49-241-8023341. Fax: +49-241-8023301. ORCID

Xiaomin Zhu: 0000-0002-3887-6791 Martin Möller: 0000-0002-5955-4185 Author Contributions

The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. Notes

The authors declare the following competing financial interest(s): Some portion of the results presented in this paper form a part of a patent submitted by Y.Z., X.Z., and M.M. The remaining authors declare no competing interests.

(2)

where [M] and [I] are concentration of monomer and initiator, respectively. Dilution of the reaction medium by PEGx-PEOS in the case of low degrees of PEG substitution certainly leads to the decrease of the molecular weight of the final product.

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ACKNOWLEDGMENTS Y.Z. thanks the financial support of Shanghai Pujiang Talent Program (no. 18PJ1432900).

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CONCLUSIONS In summary, a series of amphiphilic silica precursor polymers, namely, PEGylated PEOSs (PEGx-PEOS) with different modification degrees, which can reduce the IFT between oil and water close to zero, have been synthesized. Their interfacial activity increases with the increase of the PEGylation degree, that is, HLB value. Styrene containing such kind of material can form spontaneously microemulsions in water for PEGx-PEOS with higher HLB or emulsions just under magnetic stirring in the case of lower HLB. Polymerization of styrene in the resulting emulsions using an oilsoluble initiator leads to the formation of monodisperse PS@

ABBREVIATIONS HLB, hydrophilic-lipophilic balance; PEG, poly(ethylene glycol); PEOS, polyethoxysiloxane; MMA, methyl methacrylate; TEOS, tetraethoxysilane; AIBN, 2,2′-azobis(2-methylpropionitrile); THF, tetrahydrofuran; PDI, polydispersity index; IFT, interfacial tension; DLS, dynamic light scattering; FESEM, field emission-scanning electron microscopy; TEM, transmission electron microscopy; EDX−STEM, energydispersive X-ray spectroscopy−scanning transmission electron microscopy; TGA, thermogravimetric analysis; GPC, gel permeation chromatography H

DOI: 10.1021/acs.macromol.9b00841 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

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DOI: 10.1021/acs.macromol.9b00841 Macromolecules XXXX, XXX, XXX−XXX