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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Double in situ Preparation of Raspberry-like Polymer Particles Jiawei Li, Susanne Sihler, and Ulrich Ziener Langmuir, Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019
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Double in situ Preparation of Raspberry-like Polymer Particles Jiawei Li,a,b Susanne Sihler,b Ulrich Zienerb*
aEngineering
Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China
bInstitute
of Organic Chemistry III – Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany
*To whom correspondence should be addressed. Ulrich Ziener: E-mail:
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ABSTRACT
Well-defined raspberry-like poly(styrene-co-4-vinylpyridine)-SiO2 nanocomposite particles with a diameter of around 200 nm were easily prepared by a double in situ process in nanoemulsion with the water-soluble dye Eosin Y as the stabilizer. During radical polymerization of the nanodroplets comprising styrene (St), 4-vinylpyridine (4-VP) and tetraethoxysilane (TEOS), the silane phase is expelled from the polymer phase to the oil/water (o/w) interface. In the later polymerization stage, SiO2 nanoparticles with a size of around 25 nm were produced via the in situ sol-gel reaction of TEOS at the o/w interface promoted by the negatively charged dye. The pyridine moieties in the copolymer fix the SiO2 nanoparticles on the surface of the polymer particles by electrostatic interactions without any sign of free unbound silica particles as proven by TEM.
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INTRODUCTION Research on colloidal polymer/silica nanocomposite materials with defined morphologies and properties has progressed rapidly since the past decades. The morphologies range from raspberry-like over core-shell, currant-bun-like, hedgehog, dumbbell-like, snowman-like, daisy-shaped, to multipod-like structures, exhibiting remarkable mechanical, electrical, optical, chemical, rheological etc. properties.1-8 In this fast-growing field, considerable effort is devoted to the development of raspberry-like colloidal particles with hierarchical structures, which consist of smaller corona particles attached to the surface of larger core particles.9-12 They are intriguing due to their potential application in catalysis,13-17 selfassembly,18-20 and functional coatings.21-26 In general, there are two different strategies for the fabrication of the raspberry-like organic-inorganic nanocomposite particles (NCPs). Either they are formed from small corona particles which become anchored to larger pre-synthesized core particles in situ,27-29 or from polymerization in a Pickering emulsion.11, 30-42 However, these are multi-step processes or require presynthesized (inorganic) particles.10,41,45 To the best of our knowledge research is still in search of a onestep “all in situ” method to prepare raspberry-like NCPs. A one-step soap-free synthesis of colloidal NCPs can be controlled by the phase separation of inorganic and organic components.9-11, 13, 41-48 Sun et al.9 have reported a facile method to prepare raspberry-like NCPs by copolymerization of styrene (St), acrylic acid (AA) and 3-(trimethoxysilyl) propyl methacrylate (MPS). The hydrophilicity of hydrolyzed MPS causes a separation from the hydrophobic polystyrene (PSt) segments accompanied by a movement to the surface of the particles, and results in the formation of corona silica particles. Zhao et al.47 also reported the one-step preparation of monodisperse PSt@SiO2 NCPs with a well-defined core-shell structure by the polymerization of an emulsion of St stabilized by hyperbranched polyethoxysiloxane (PEOS). Nevertheless, the simultaneous formation of both inorganic and organic components with controllable structures and morphologies using a “one-pot” preparation is complex, because the conditions for polymerization and precipitation of inorganic materials are mostly very different.10, 48
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Recently, we have reported on aggregates of very common dyes as molecular scale Pickering stabilizers for nano- or miniemulsions which do not only allow miniemulsion polymerization but also constitute a platform to generate well controlled sub-20 nm silica particles by an interfacial sol-gel process.49-51 In the present work we combine the formation of small silica nanoparticles and St polymerization in dyestabilized emulsions to end up with an “all in situ” strategy for the facile one-step preparation of organicinorganic hybrid nanoparticles. A miniemulsion of St, 4-vinylpyridine (4-VP) and TEOS in water was stabilized by the water soluble dye Eosin Y disodium salt (E) as outlined in Scheme 1. Both the polymerization and the sol-gel process will take place in close vicinity, i.e. within the monomer/inorganic precursor droplets or at the o/w interface, respectively. Here, the question arose whether by a subtle control of the reaction conditions a raspberry-like morphology can be obtained. The effects of the initial amounts of 4-VP, the ratio of St/TEOS, temperature of the reaction, pH of the aqueous medium, etc. on the size and morphology of the colloidal particles were investigated, and a mechanism of formation is proposed.
EXPERIMENTAL Materials Styrene (St, Merck, 99 %) was purified by passing through a column filled with alumina and stored in the refrigerator before use. 4-Vinylpyridine (4-VP, Aldrich, 95 %) was purified by vacuum distillation and stored at -30 oC. The initiator, α,α'-azoisobutyronitrile (AIBN, Merck, 98 %), was purified by recrystallization from ethanol. The dye Eosin Y disodium salt (E, Sigma-Aldrich), the inorganic precursor tetraethoxysilane (TEOS, 99.93 %, VWR Prolabo), the surfactant sodium dodecylsulphate (SDS, AppliChem), the initiators 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70, 98 %, Wako pure chemical industries Ltd.) and 2,2'-azobis[n-(2-carboxyethyl)-2-methylpropionamidine] n-hydrate (VA057, 98 % Wako pure chemical industries Ltd.) and n-hexadecane (HD, >98 %, TCI) were used without further purification. Milli-Q grade water (resistivity: 18 MΩ) was used in all processes. Preparation of raspberry-like PSt-SiO2 NCPs ACS Paragon Plus Environment
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The raspberry-like PSt-SiO2 NCPs were prepared in one step from a dye stabilized miniemulsion. For a typical experiment, the aqueous phase (continuous phase, CP) was prepared by dissolving 200 mg of Eosin Y disodium salt in 100 g water at pH 6 to reach a concentration of 2.0 mg/mL (dye/water). The oil phase consisting of 0.35 g of St, 0.15 g of TEOS, 20 mg of HD, 20 mg of AIBN and various amounts of 4-VP (given in percentage as content of the copolymer PS-co-P4VP) was prepared by dissolution with the help of ultrasound and subsequently mixed with the water phase (9.5 g of aqueous dye solution) in a 20 mL screw cap jar. The mixture was homogenized by direct ultrasonication using a Branson W450 digital sonifier for 8 min under ice cooling (70 % amplitude, 1/4″tip). After that, the system was degassed by bubbling argon for 15 min. Finally, the emulsion was polymerized at 60 °C in an oil bath for 1 d under magnetic stirring to produce nanocomposite particles. The detailed recipes and experimental conditions are described in Table 1.
Scheme 1. Schematic illustration of the one-step preparation process of raspberry-like PSt-SiO2 NCPs via a dye stabilized miniemulsion with subsequent polymerization and sol-gel process.
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Table 1. Summary of formulations, particle sizes and PDIs for the PSt-SiO2 NCPs. Runs
St/g
TEOS/g
4-VP/mg
HD/mg
Initiator/mg
c(stabilizer)/mg/mL
Reaction conditions
z-Diameter/nm (Dz)a
Polydispersity (PDI)a
1
0.35
0.15
0
20
AIBN/20
2 (E)
60 oC, 1 d
155
0.125
2 (E)
60
oC,
1d
161
0.119
oC,
1d
165.5
0.100
2
0.35
0.15
25
20
AIBN/20
3
0.35
0.15
50
20
AIBN/20
2 (E)
60
4
0.35
0.15
75
20
AIBN/20
2 (E)
60 oC, 1 d
238
0.122
2 (E)
60
oC,
5h
255
0.143
60
oC,
1d
163.5
0.141
60
oC,
1d
139
0.120
60
oC,
1d
187
0.174
5b 6 7
2 0.45 0.4
0 0.05 0.1
0 25 25
80 20 20
AIBN/80 AIBN/20 AIBN/20
2 (E) 2 (E)
8
0.3
0.2
25
20
AIBN/20
2 (E)
9
0.35
0.15
25
20
AIBN/20
1 (SDS)
60 oC, 1 d
80
0.062
1 (SDS)
60
oC,
1d
88
0.127
40
oC,
3d
-
-
40
oC,
3d
10 11c 12 13
0.35 0.35 0.35 0.35
0.15 0.15 0.15 0.15
50 25 50 25
20 20 20
AIBN/20 V-70/20 V-70/20
2 (E) 2 (E)
oC,
pH = 9, 1
204
0.283
dd
142
0.192
225
0.142
20
V-70/20
2 (E)
40
14 0.35 0.15 50 20 Determined by DLS. b The monomer concentration is fourfold (20 wt%).
V-70/20
2 (E)
40 oC, pH = 9, 1 dd
a
The emulsion displayed phase separation. d The continuous phase was buffered with 0.01 M NH *NH Cl at pH of 9. 3 4 c
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Characterization Methods. Dynamic Light Scattering (DLS). The particle size and distribution (polydispersity index (PDI)) of PSt-SiO2 NCPs was analyzed by dynamic light scattering (Malvern Instruments Zetasizer Nanos-6, UK) at 25 °C with a scattering angle of 173° and a wavelength of λ = 633 nm. A volume of 10 μL of the emulsion was diluted with 1.5 mL of water and put into a polystyrene cuvette before measuring. Particle size and PDIs are given as the average of ten measurements. Transmission Electron Microscopy (TEM). To visually observe the morphology of PSt-SiO2 NCPs, a small drop of emulsion was deposited onto a carbon-coated copper grid, dried overnight under ambient conditions and analyzed by transmission electron microscopy (Zeiss EM10, Germany) in the conventional transmission mode using 120 kV acceleration voltage. Energy-Dispersive X-ray Spectroscopy-Scanning Transmission Electron Microscopy (EDXSTEM). EDX-STEM mapping was performed for PSt-SiO2 NCPs on a FEI G2 F30 transmission electron microscope operating at 300 kV and equipped with a detector for EDX mapping. Scanning Electron Microscopy (SEM). The morphology of PSt-SiO2 NCPs was also investigated by scanning electron microscopy (SEM, FEI Quanta 400 FEG); all samples were deposited onto a carboncoated copper grid before examination.
RESULTS AND DISCUSSION Preparation and Structure of Raspberry-Like PSt-SiO2 NCPs The process for the preparation of PSt-SiO2 NCPs with a raspberry-like morphology in one step via dye stabilized mini- or nanoemulsion with subsequent polymerization and sol-gel process is depicted in Scheme 1. We assume a significant deviation from the mechanism of pure miniemulsion polymerization although the starting point is a miniemulsion with all the typical characteristics of such emulsion (see below). Therefore, it is called a one-step miniemulsion based heterophase polymerization. At first, the o/w miniemulsion was prepared by ultrasound with the monomers St (0.35 g), 4-VP (25 mg) and TEOS ACS Paragon Plus Environment
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(0.15 g), the osmotic pressure agent HD (20 mg) and the initiator AIBN (20 mg) as dispersed phase. The continuous phase consisted of an aqueous solution (9.5 g) of Eosin Y (2 mg mL-1). After stirring for 1 d at 60 oC under argon (Table 1, run 2), the as-obtained PSt-SiO2 NCPs were analyzed via SEM and TEM (Figure 1).
Figure 1. Typical (a) SEM and (b) TEM images of the raspberry-like PSt-SiO2 NCPs; inset magnified detail of (b) (Table 1, run 2). Figure 1 displays more or less spherical objects decorated with irregular spots indicative of PSt-SiO2 NCPs with a typical raspberry-like morphology. The high resolution SEM (Figure 1(a)) and TEM (Figure 1 (b)) images show that corona particles with a size of around 25 nm are randomly distributed on the surface of core particles reasonably attributed to silica (corona) and copolymer particles (core), respectively. Remarkably, no free silica particles are detectable neither by TEM nor SEM in contrast to our previous findings.19 There the raspberry-like morphology was generated from preformed silica particles leading unavoidably to free particles which had to be removed by centrifugation. The statistical evaluation of the TEM images delivers a mean diameter of 135 nm of the dried nanocomposite particles (Figure 2). The results from DLS of the sample in dispersion reveal a z-average size of 161 nm with a distribution (PDI 0.12, see Figure S1) in good agreement with the results from TEM. The somewhat increased diameter of the nanocomposite particles measured by DLS is attributed to the presence of a ACS Paragon Plus Environment
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hydration layer and a certain swelling because of the hydrophilic comonomer in contrast to the dried particles in TEM.9, 10
Figure 2. Histogram of raspberry-like PSt-SiO2 nanocomposite particles determined by TEM (see Table 1, run 2).
Effects of the mass fraction of 4-VP To reveal the necessity of the co-monomer the mass fraction of 4-VP was systematically varied between 0 and 75 mg corresponding to 0 to 18 wt% as content of the copolymer PS-co-P4VP (see Table 1, runs 1 – 4). Figure 3 shows the corresponding TEM images of the PSt-SiO2 NCPs. If 4-VP is omitted (Table 1, run 1), plenty of free SiO2 nanoparticles are found and only few inorganic nanoparticles can be observed on the surface of the polymer particles (Figure 3(a)). As mentioned above with 25 mg of 4-VP (7 wt%), raspberry-like NCPs are produced easily and no free SiO2 nanoparticles are found in the TEM picture (Figure 3(b)). Thus, for a successful attachment of the acidic silica particles a minimum amount of the basic comonomer is required in accordance with the literature.31, 33 Interestingly, Wu33 has reported that at least 10 wt% of 4-VP is necessary for a successful formation of raspberry-like polymer/SiO2 ACS Paragon Plus Environment
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organic-inorganic NCPs. Remarkably, in the present system with Eosin Y as stabilizer, 7 wt% of 4-VP is enough to anchor the silica nanoparticles. This finding demonstrates the high efficiency of the in-situ process for the preparation of the hybrid particles. With a further increase of the feeding amount of 4-VP to 13 wt% (50 mg, run 3, Figure 3 (c)) and 18 wt% (75 mg, run 4, Figure 3 (d)), the raspberry-like NCPs become less spherical and their sizes increase from 161 (run 2) to 238 nm (run 4) determined by DLS (see Figure S1). Apparently, the nanoreactor concept of the miniemulsions is partially corrupted at the higher contents of 4-VP which we ascribe to the hydrophilicity of 4-VP. Substantial amounts thereof will move to the aqueous phase and might cause a net mass exchange between the droplets. This might be even enhanced by the release of ethanol from hydrolyzed TEOS (see below).
Figure 3. TEM images of PSt-SiO2 NCPs with different amounts of 4-VP: (a) 0, (b) 25, (c) 50 and (d) 75 mg (see Table 1, runs 1-4).
Effects of the Ratio of St and TEOS
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To elucidate the role of the silane precursor on the amount and morphology of silica particles on the surface of the NCPs the ratio of St and TEOS was varied. Figure 4 shows TEM images and DLS size distributions of the as-obtained PSt-SiO2 NCPs as a function of the initial mass ratio St/TEOS. Without TEOS, round and uniform particles of neat PSt (run 5) are formed with a z-average size of 255 nm and a relatively low PDI value of 0.14 (see Table 1 and Figure 4 (a)). It shall be mentioned that the monomer concentration of St was 20 wt% of the total emulsion and the conversion 93 % after 5 h reaction time determined by gravimetric measurement. The numbers are in the typical range of miniemulsion polymerizations indicating an efficient stabilization of the emulsion by Eosin Y with a concentration of 2 mg/mL similar to our previous findings with Alizarin yellow R as stabilizer. Still, a significant deviation from the mechanism of pure miniemulsion polymerization is assumed (see below).50, 52 Introducing TEOS with a mass ratio of St/TEOS 90/10 (Table 1, run 6) and 4-VP (25 mg, also for the following runs) generates some SiO2 nanoparticles with a size of around 25 nm anchored on the surfaces of the core particles with a raspberry-like structure as shown in Figure 4 (b). An increase of the mass fraction of TEOS to 20 % leads to more SiO2 nanoparticles on the surface of the copolymer particles. The bestdefined raspberry-like PSt-SiO2 NCPs (Figure 4 (d)), were produced with 30 wt% of TEOS (Table 1, run 2). However, both the raspberry-like morphology and the size of the SiO2 nanoparticles became less uniform by further increasing the mass fraction of TEOS to 40 wt% (Table 1, run 8 and Figure 4 (e)). This can be also observed in the DLS size distribution of PSt-SiO2 NCPs in Figure 4 (f). For the mass fractions of TEOS from 10 wt % to 30 wt % a monomodal size distribution is obtained while at 40 wt% the distribution appeared bimodal exhibiting two peaks centered at 203 nm (98 %) and 4800 nm (2 %), respectively. Apparently, the initial St and TEOS composition plays a vital role in the formation of the raspberry-like NCPs. This could be attributed to a competition between diffusion and hydrolysis processes of the styrene and TEOS molecules, respectively. At the interface the TEOS releases ethanol molecules by hydrolysis which enhance the solubility of the styrene molecules in the continuous phase and, thus, promote net diffusion and initiate secondary nucleation. Therefore, the size distribution of the NCPs
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broadens with increasing content of TEOS (Figure 4 (e)) and a few (smaller) polymer particles do not seem to bear silica particles on their surface (e. g. Figure 4 (c)).
Figure 4. TEM images of the as-obtained PSt-SiO2 NCPs with different mass ratios St/TEOS: (a) 100/0, (b) 90/10, (c) 80/20, (d) 70/30, (e) 60/40 and (f) corresponding size distributions of the NCPs (see Table 1, runs 2 and 5-8).
Effects of the Stabilizer and Preparation Conditions ACS Paragon Plus Environment
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Effects of the Stabilizer: In our previous study, cationic water-soluble dyes like crystal violet and malachite green-oxalate promoted the formation of silica capsules while anionic dyes delivered sub-20nm SiO2 nanoparticles. However, a three-dimensional gel was formed by employing the classical surfactant SDS as the stabilizer for TEOS-in-water miniemulsions.53 In order to shine light on the role of the dye in the present system a further set of experiments was conducted with SDS (1 mg/ mL) as stabilizer. If a small amount of 4-VP was added to the reaction system (25 mg), surprisingly silica nanoparticles were found on the surface of the polymer particles (see Figure S2 a) but much less than in the comparable run 2 with Eosin Y as stabilizer (Figure 1). Larger amounts of 4-VP (50 mg) were associated with more and randomly distributed SiO2 nanoparticles on the surface of the copolymer particles (see Figure S2 b). Apparently, the presence of 4-VP promotes the formation of the silica particles. However, the resulting NCPs do not exhibit a typical raspberry-like morphology, because the silica nanoparticles are not uniformly distributed. These findings are in accordance with the suggested mechanism of formation of the silica particles. 51 While in the case of Eosin Y the locally enhanced pH at the o/w interface is sufficient to accelerate the sol-gel process, SDS alone does not lead to silica particles and only with the addition of 4-VP acting as basic catalyst silica particles are obtained. Thus, the dye stabilized emulsions comprise a further degree of freedom as the amount of 4-VP can be varied independently and already low amounts are sufficient to generate the desired morphology while for SDS much larger amounts are required. Effect of Temperature: Temperature has a strong influence on reaction kinetics and presents a powerful tool to fairly decouple the rate of the sol-gel process and the radical polymerization. While a lowering of the temperature strongly slows down the hydrolysis and condensation rates of the sol-gel process the rate determining decomposition of the radical initiator can be roughly maintained by choosing an initiator with a lower decomposition temperature. Instead of AIBN, the oil soluble initiator V-70 was used with a halflife time of 5 h at 40 °C and the polymerization temperature was reduced from 60 to 40 °C. It appeared that the emulsion with a content of 7 wt% of 4-VP at 40 °C is not stable and precipitation occurs after 3 d (Table 1, run 11). If the amount of 4-VP is doubled to 13 wt% (Table 1, run 12), the visual stability of the emulsion is maintained after 3 d reaction at 40 °C but relatively broad distributions of the NCPs with ACS Paragon Plus Environment
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a z-diameter of 204 nm and a PDI value of 0.28 are obtained (see Figure S3). Presumably, the formation rate of the SiO2 nanoparticles is slowed down to such an extent that the critically stabilized miniemulsion destabilizes before the sol-gel process is finished. The same trend had already been observed for the formation of SiO2 nanoparticles at room temperature with congo red as stabilizer at a concentration of 0.1 mg/mL.51 There, we assumed destabilization of the emulsion caused by protonation of the dye, resulting from the lowered pH because of the formation of silicic acid. The latter might have been enriched because of a reduced condensation rate. Hence, the hydrolysis and condensation rate of TEOS displays a strong effect on the emulsion stability in this system. Effect of the pH value: In our previous study the pH value played a pivotal role in the sol-gel process and an increase of the pH value to 9 led to an increase of the hydrolysis rate and subsequently the formation of smaller particles.51 Therefore, in the present system the plain water of the continuous phase was replaced by ammonia buffer with a pH of 9 (Table 1, runs 13 and 14). As shown in Figure 5 (a) and (a’), the raspberry-like NCPs grow less spherical and more anomalous with a z-diameter of 142 nm and a PDI value of 0.19 (see Table 1) with a mass fraction of 4-VP of 7 wt% at 40 °C. With increasing the mass fraction of 4-VP to 13 wt%, the raspberry-like NCPs display a more spherical and uniform appearance (see Figure 5 (b) and (b’)), and the z-average increases to 225 nm while the PDI value decreases to 0.14 (see Table 1). Those results are consistent with the corresponding reactions in plain water at 60 °C with 13 wt% and 18 wt% of 4-VP, respectively (Table 1, runs 3 and 4). This behavior is ascribed to an increase of the mass fraction of 4-VP molecules staying in the droplets due to the lower solubility of 4-VP in water at elevated pH values.37 On the other hand, in contrast to run 11 and 12, an increase in the hydrolysis rate of TEOS at higher pH values prevailed over the destabilization of the emulsion droplets. Consequently, many hydrophilic SiO2 nanoparticles were formed early followed by anchoring of basic nitrogen atoms from the copolymer on the surface of the core particles, enhancing the colloidal stability.37
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Figure 5. TEM images of the PSt-SiO2 NCPs at 40 °C and pH = 9 with different magnifications from (a), (a’) run 13 and (b), (b’) run 15 (Table 1).
NCPs by Postpolymerization: A general approach to prepare polymer-silica NCPs starts from the preparation of o/w Pickering emulsions of monomers with silica nanoparticles as stabilizer which is subsequently polymerized.37,
38
To investigate whether this principle can be transferred to the dye
stabilized miniemulsions, both processes (silica formation and polymerization) were performed consecutively but in one pot. After the formation of the SiO2 nanoparticles in the Eosin Y stabilized emulsion with ammonia buffer at pH 9 for 3 days at room temperature (RT), a certain mass fraction of 4VP was added to the emulsion. Then the polymerization was started by adding the water soluble initiator V-057 and heating to 60 °C for 5 h (see Table S1). As shown in Figure S4, the formation of many highly uniform SiO2 nanoparticles with a diameter below 20 nm is observed before the radical polymerization was started. Figure S5 presents the morphology of the PSt-SiO2 NCPs after completion of the free radical polymerization. When 50 mg of 4-VP (13 wt%) was employed, a small number of free silica nanoparticles ACS Paragon Plus Environment
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can be found besides the desired PSt-SiO2 NCPs (see Figure S5 (a)), which suggests a poor adhesion of the silica particles to the copolymer particles. After increasing the amount of 4-VP to 75 mg (18 wt%), no more free silica nanoparticles are observed but raspberry-like NCPs are obtained (see Figure S5 (bd)). Simultaneously, the number of SiO2 nanoparticles anchored to the surface of the copolymer core particles increased upon increasing the mass fraction of TEOS. In addition, it is noteworthy that the NCPs exhibit a quite uniform appearance with a narrow size distribution. For example, run S5 delivers particles with a z-diameter of 132 nm and a PDI of 0.098 (see Figure S6). This is attributed to the fact that in the postpolymerization method, both dye and hydrophilic SiO2 nanoparticles can act as stabilizer for the Pickering-type emulsion polymerization, which enhances the stability and uniformity of the colloidal system.
Formation Mechanism of the Raspberry-Like NCPs On the basis of the discussions above, the resulting morphologies of the PSt-SiO2 NCPs highly depend on the mass fraction of 4-VP, the weight ratio of St/TEOS, stabilizer, temperature, pH value and the order of reaction (polymerization method). To better understand the mechanism of formation of these raspberrylike NCPs, the system with a weight ratio of St/TEOS of 70/30 and 7 wt% of 4-VP was investigated timedependent. As shown in Figure 6, after ultrasonication for 8 min (0 min of polymerization), droplets up to the micrometer range with a z-diameter of 331 nm and a relatively high polydispersity (PDI 0.26) were formed. During the next 5 h, the droplet or particle size, respectively, decreased to 147 nm together with the size distribution (PDI 0.173). In the following period from 5 to 24 h, the particle size and size distribution reach a plateau and remain almost unchanged during further polymerization. This reaction can be considered as a one-step process of dye stabilized Pickering miniemulsions49 with simultaneous polymerization of the organic monomers and the sol-gel process of the inorganic precursor. Presumably, already in the early phase of the polymerization the TEOS molecules are hydrolyzed and ethanol is released. The alcohol enhances the solubility of the styrene in the continuous phase and, thus, diffusion will take place with partial homogenous nucleation of polymer particles, leading to a significant reduction ACS Paragon Plus Environment
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of the average droplet size.54 Silica free polymer particles are still well dispersed and stabilized by the remaining dye molecules. Figure 7 shows the evolutional morphologies with polymerization time. As displayed in Figure 7 (a), some soft polymer capsules are formed already after 1 h probably because of incomplete polymerization and evaporation of the monomer in the TEM. After 5 h polymerization time, monodisperse homogeneous spherical particles are observed with a rather smooth surface and seemingly no sign of silica particle generation (Figure 7 (b)).9, 10, 13, 45, 47 However, a closer look by EDX-STEM mapping of PSt-SiO2 NCPs (see Figure S7) reveals that the silicon atoms are concentrated at the periphery of the particles, and the carbon atoms are mostly observed in the particle core. Thus, a very thin silicalike layer on the particle surface after polymerization times of 5 h is formed which cannot be observed by TEM with normal resolution.47 When the reaction lasted 12 h, some small bulges emerge on the particle surface (Figure 7 (c)). As the polymerization reaction continued, more silica nanoparticles are observed on the latex surface and become clearly corona particles (Figure 7 (e-f)). This result is consistent with a reference sample where St and 4-VP were replaced by toluene (Figure S8). After the sol-gel reaction for 24 h a large number of silica nanoparticles can be clearly seen in Figure S8 (d). The corona particles mentioned above are only present on the surface of colloidal particles, and no free silica nanoparticles are found in the aqueous phase. This result indicates that the raspberry-like NCPs are formed by double in situ polymerization rather than by coagulation of the different types of particles.
Figure 6. Size distributions and z-diameters of the PSt-SiO2 NCPs with an initial ratio of St/TEOS of 70/30 and 25 mg of 4-VP (Table 1, run 2) at different polymerization times determined by DLS. ACS Paragon Plus Environment
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Figure 7. TEM images of the PSt-SiO2 NCPs at different polymerization times determined with an initial ratio of St/TEOS of 70/30 and 25 mg of 4-VP (Table 1, run 2): (a) 1 h, (b) 5 h, (c) 12 h, (d) 15h, (e) 18 h and (f) 21 h. Based on the experimental data presented above we suggest a mechanism of formation of the raspberrylike PSt-SiO2 NCPs shown in Scheme 2. After ultrasonication, the inorganic precursor TEOS and the monomers St and 4-VP are homogenously mixed within the dye stabilized droplets. Meanwhile, water molecules are incorporated within the self-assembled water soluble dye layer at the o/w interface. The latter serves as the zone into which TEOS molecules diffuse and start to form the silica phase. The partly hydrolyzed TEOS can no longer be dissolved in the emulsion droplets, as it is too hydrophilic and its compatibility with both St and PSt is reduced. The solubility parameters of PSt and TEOS are ca. 17-2155 and 14.6 MPa1/2, 56 respectively. As the polymerization proceeds, driven by the osmotic pressure and incompatibility with PSt, TEOS molecules continuously migrate towards the dye stabilized o/w interface, where they are consumed.39, 40, 42On the other hand, the local pH at the o/w interface is significantly higher than in the bulk aqueous phase owing to the negatively charged dyes. This promotes the controlled solACS Paragon Plus Environment
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gel process and growth of the silica particles continues.51 Therefore, the as-fabricated hierarchically structured raspberry-like PSt-SiO2 NCPs can be successfully prepared via a one-step heterophase polymerization process accompanied by an interfacial sol-gel reaction.
Scheme 2. Schematic illustration of the formation of raspberry-like PSt-SiO2 NCPs via a one-step heterophase polymerization process accompanied by an interfacial sol-gel reaction.
CONCLUSIONS In summary, a one-step miniemulsion based heterophase polymerization technique has been developed for the preparation of PSt-SiO2 raspberry-like NCPs using the dye Eosin Y not only as a miniemulsion stabilizer, but also as a great platform for the controllable synthesis of silica nanoparticles. The influence of the reaction conditions such as content of the comonomer 4-VP, the ratio of St and TEOS, reaction temperature, pH of the aqueous medium, and the size and morphology of the resulting particles has been systematically investigated. The mechanism by which the raspberry-like structures form could be elucidated by the result of a delicate interplay between the TEOS hydrolysis and condensation rate, emulsion stability, and phase separation in the emulsion droplets. This simple and environmentallyfriendly “all in situ” strategy could be useful in synthesizing other functional nanocomposite particles in large scale.
ASSOCIATED CONTENT Supporting Information. ACS Paragon Plus Environment
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs. Summary of formulations, particle sizes and PDIs for the PSt-SiO2 NCPs prepared in the postpolymerization method using the waterborne initiator V-057; DLS size distribution of raspberry-like PSt-SiO2 NCPs (run 2, 4 and 12 in Table 1, run S5 in Table S1); TEM images of the as-obtained PSt-SiO2 NCPs (runs 9, 10 and 12 in Table 1), before radical polymerization (run S2 in Table S1) and after radical polymerization (runs S2-S5 in Table S1); TEM images of SiO2 particles after different times of the solgel process from emulsions with TEOS and toluene as dispersed phase; EDX element mappings (C, Si) of PSt-SiO2 NCPs (run 2 in Table 1) at a polymerization time of 5 h.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. ORCID Ulrich Ziener: 0000-0001-7582-1143 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from National Natural Science Foundation of China (NSFC, grant No.51703200) and Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology for J. L. is gratefully acknowledged.
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For Table of Contents Only
“All in Situ” Strategies
St TEOS 4-VP
H2O
O
60 C
raspberry-like PSt@SiO2 Sol-gel reaction + radical polymerization
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