SiO2 nanocomposite particles

Colloids and Surfaces A 530 (2017) 104–116

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Preparation of polymeric/inorganic nanocomposite particles in miniemulsions: II. Narrowly size-distributed polymer/SiO2 nanocomposite particles

MARK

⁎

Jia Yao, Zhihai Cao , Yi Shang, Qun Chen, Lin Yang, Yushan Zhang, Dongming Qi Key Laboratory of Advanced Textile Materials and Manufacturing Technology and Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China

G RA P H I C A L AB S T R A C T

A R T I C L E I N F O

A B S T R A C T

Keywords: Polymer/SiO2 nanocomposite particles Miniemulsion polymerization Narrow particle size distribution Surfactant

Narrowly size-distributed poly(styrene-co-methyl methacrylate) (poly(St-co-MMA))/SiO2 nanocomposite particles (NCPs) were efficiently prepared through miniemulsion polymerization by using a special use strategy of surfactants. A trace amount of sodium dodecyl sulfate (SDS) was post-added to monomer miniemulsions stabilized with polyoxyethylene (20) sorbitan monolaurate (Tween-20). The number fraction of the poly(St-coMMA)/SiO2 NCPs reached 97% in the as-synthesized emulsion stabilized with 0.15 g of Tween-20 and 2.5 mg of the post-added SDS. The highly efficient formation of poly(St-co-MMA)/SiO2 NCPs was realized through controlled mergence between the monomer droplets and latex particles during polymerization. The influence of the amount of post-added SDS, surface hydrophobic modification degree of SiO2 nanoparticles, and SiO2 content on the particle properties of latex particles was systematically investigated. The amount of post-added SDS and surface hydrophobic modification degree of SiO2 nanoparticles are two determining factors for the efficiency of polymer/SiO2 NCP formation.

⁎

Corresponding author. E-mail address: [email protected] (Z. Cao).

http://dx.doi.org/10.1016/j.colsurfa.2017.07.047 Received 11 April 2017; Received in revised form 13 July 2017; Accepted 13 July 2017 Available online 15 July 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.

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1. Introduction

2. Experimental section

Polymeric/inorganic nanocomposite particles (NCPs) may embody enhanced properties or novel functions through combining polymers with inorganic nanoparticles (NPs) in a nanosized scale [1,2]. By taking advantage of their excellent overall properties, the polymeric/inorganic NCPs may be widely used in the fields of catalysis, advanced coatings, colorants, sensors, solar cells, drug delivery, and disease diagnosis [3–6]. Heterophase polymerization techniques, such as emulsion polymerization, miniemulsion polymerization, microemulsion polymerization, and dispersion polymerization, are often used to prepare various polymeric/inorganic NCPs [7–13]. In recent years, miniemulsion polymerization has displayed a high flexibility to prepare versatile polymeric/inorganic NCPs by means of its predominant mechanism of droplet nucleation [4–9,14,15]. In a typical monomer miniemulsion, hydrophobic monomer droplets within the size range of 50–500 nm homogenously dispersed in an aqueous continuous phase [16–18]. Latex particles are mainly formed through the droplet nucleation [14–16]. Each nucleated latex particle can be regarded as a separated “nanoreactor” [19]. A typical preparation procedure for polymeric/inorganic NCPs can be as follows: I, surface hydrophobic modification of inorganic NPs; ii, dispersing of hydrophobically modified inorganic NPs in a monomer mixture; iii, preparation of monomer miniemulsion; iv, miniemulsion polymerization to form polymeric/inorganic NCPs [14,20]. Although the preparation of polymeric/inorganic NCPs through miniemulsion polymerization technique is convenient, versatile, and efficient, some plain polymer particles were also prepared in the polymerization process [21–23]. In some cases, the plain polymer particles even dominated in the final emulsions prepared from miniemulsions [23]. Bourgeat-Lami et al. reported that the plain polymer particles mainly form through droplet nucleation of plain monomer droplets that are produced in the sonication process [22]. Recently, we found that nearly all the latex particles are plain polymer particles in the emulsions prepared from miniemulsion polymerization systems stabilized with sodium dodecyl sulfate (SDS) when the monomer miniemulsions are prepared at low sonication power [23]. Although the number fraction of polymer/SiO2 NCPs (fNCP value) was improved to 40% with enhanced sonication power, the plain polymer particles still dominated in the final emulsion [23]. Very recently, we reported that polymer/SiO2 NCPs were efficiently prepared through miniemulsion polymerization by using polyoxyethylene (20) sorbitan monolaurate (Tween-20) as the sole surfactant [24]. The fNCP values in the as-synthesized emulsions could reach above 90%. The efficient formation of polymer/SiO2 NCPs is mainly contributed by the suppressed nucleation of plain monomer droplets through a controlled mergence of the monomer droplets with the latex particles [24]. However, the prepared polymer/SiO2 NCPs displayed a large particle size (> 300 nm) and a relative broad particle size distribution [24]. Although the particle size might be reduced by using a large amount of Tween-20, some plain polymer particles would be produced possibly through the droplet nucleation of the plain monomer droplets, leading to the decrease of the fNCP values [24]. Therefore, it is still highly desirable to improve the controllability over the particle size and particle size distribution of polymer/SiO2 NCPs under the circumstance that a high efficiency of polymer/SiO2 NCP formation is preserved. In this work, narrowly size-distributed poly(styrene-co-methyl methacrylate) (poly(St-co-MMA))/SiO2 NCPs were synthesized through miniemulsion polymerization by using a special use strategy of surfactants. A trace amount of SDS was post-added to the prepared monomer miniemulsions stabilized with Tween-20. The efficient formation of poly(St-co-MMA)/SiO2 NCPs was contributed by the controlled mergence between the monomer droplets and latex particles during polymerization. The efficiency of polymer/SiO2 NCP formation significantly depended on the amount of post-added SDS and the surface hydrophobic modification degree of SiO2 NPs.

2.1. Materials The monomers, styrene (St; AR grade; Tianjing Yongdao Chemical Co. Ltd.) and methyl methacrylate (MMA; AR grade; Tianjin Kermel Chemical Co., Ltd.), were distilled under reduced pressure and stored in a refrigerator before use. The silane coupling agent, 3-trimethoxysilyl propyl methacrylate (MPS; 97%; Acros Organics), was used as received. n-Hexadecane (HD; AR grade), potassium persulfate (KPS; AR grade), benzoyl peroxide (BPO; AR grade), SDS (AR grade), and Tween-20 (AR grade) were purchased from Aladdin Chemical Co. Ltd. Tetraethoxysilane (AR; Tianjin Kermel Chemical Co., Ltd.), ethanol (AR grade; Wuxi Zhanwang Chemical Co., Ltd.), and aqueous ammonia (25 wt%; Wuxi Zhanwang Chemical Co., Ltd.) were used as received. SiO2 NPs with a size of ∼27 nm were prepared through the Stöber method [25]. The dispersion of MPS-modified SiO2 NPs in MMA was made through a solvent replacement technique. The information on the preparation of SiO2 NPs, surface modification of SiO2 NPs, and preparation of SiO2 NP dispersion in MMA could refer to our previous study [23]. Demineralized water (resistivity: 18 MΩ cm) was used in all experiments. 2.2. Preparation of monomer miniemulsion and polymerization Tween-20 (0.15 g) was dissolved in 12.5 g water. St (1.5 g) and HD (0.09 g) were dissolved in 1.5 g of the SiO2 NP dispersion in MMA with various SiO2 contents (5–17 wt% relative to the mass of MMA). The surface of SiO2 NPs was modified with various amounts of MPS (5–20 wt% relative to the mass of SiO2 NPs). The two mixtures were combined and then pre-emulsified with magnetic agitation at 700 rpm for 15 min to form a crude emulsion. The crude emulsion was further sonicated at 95 W by applying a pulsed sequence (12 s work, 6 s break) for 7 min on a Scientz JY92-II DN sonifier to obtain a monomer miniemulsion. Subsequently, varied amounts of SDS (0–10 mg) were added to the prepared monomer miniemulsions and then the SDS-added monomer miniemulsions were magnetically stirred at an agitation rate of 400 rpm for 12 min under room temperature. KPS (0.075 g) was added to the prepared monomer miniemulsion. After purged with N2 for 5 min, the glass reactor was sealed and kept in a preheated oil bath at 70 °C. The polymerization was run for 6 h under magnetic agitation at 400 rpm. The recipes for the prepared poly(St-co-MMA)/SiO2 NCPs are listed in Table 1. For run 7, the oil-soluble initiator, BPO, was predissolved into the monomer mixture prior to preparation of the monomer miniemulsion. Table 1 Recipes for the prepared polymer/SiO2 NCPs.a runs

MPS modification degree of SiO2 NPs (wt%)

SiO2 content (wt%)c

Amount of post-added SDS (mg)

1 2 3 4 5 6 7b 8 9 10 11 12 13 14 15

20 20 20 20 20 20 20 5 7 10 15 20 20 20 20

8.5 8.5 8.5 8.5 8.5 8.5 8.5 3.5 3.5 3.5 3.5 3.5 2.5 5.0 6.5

2.5 0 5.0 7.5 8.8 10 10 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

a b c

105

The other polymerization parameters are provided in Section 2.2. The initiator used in this run was 0.03 g of BPO. Based on the overall mass of dispersed phase.

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sample with steel crucible, overall compounds in the emulsion, nonvolatile compounds in the emulsion, and monomer in the emulsion, respectively.

2.3. Characterization 2.3.1. Dynamic light scattering (DLS) The Z-average particle sizes and polydispersities (PDIs) of the monomer droplets and latex particles were measured by DLS (Zetasizer Nano series, Malvern Instruments) at 25 °C at a scattering angle of 90°. A drop of monomer miniemulsion or final emulsion was diluted with 2 mL distilled water. Particle sizes were reported as the average of three measurements. For evaluation of the colloidal stability of latex particles in the solutions with various NaCl concentrations, 0.015 mL of the assynthesized emulsion was added to 5 mL of the NaCl solutions with various concentrations (0–1.5 mol L−1). On one hand, the particle sizes of latex particles were measured by DLS immediately after the preparation of the dispersions. On the other hand, the prepared dispersions were stored for 24 h, and then the particle sizes of latex particles were measured.

2.3.6. Zeta potential measurement The zeta potential of the latex particles was measured on a Zetasizer (Nano series, Malvern Instruments) at 25 °C under the mode of zeta potential. Two microliter of the emulsion was diluted with 1 mL of aqueous solution of KCl (10−3 mol L−1). 3. Results and discussion 3.1. Preparation of narrowly size-distributed poly(St-co-MMA)/SiO2 NCPs In our previous paper, we found that polymer/SiO2 NCPs could be efficiently prepared in miniemulsion systems stabilized with a low amount of Tween-20 [24]. However, the particle size of the polymer/ SiO2 NCPs was larger than 300 nm, and moreover, the particle size distribution of the NCPs was relatively broad [24]. The large particle size and broad particle size distribution are highly related to the mergence extent of the monomer droplets and latex particles during polymerization. Therefore, it is critical to find an optimum mergence extent of the monomer droplets and latex particles to achieve a high efficiency of polymer/SiO2 NCP formation and to control the particle size and particle size distribution of the polymer/SiO2 NCPs. The colloidal stability of monomer droplets and latex particles in miniemulsion systems may be improved through adding a second dose of surfactants [28,29]. In order to adjust the mergence extent of the monomer droplets and latex particles, but not influence the initial state of monomer miniemulsion, a special use strategy of surfactants that a trace amount of SDS was post-added to monomer miniemulsions stabilized with Tween-20 was designed. In run 1, 2.5 mg of SDS was postadded to the monomer miniemulsion stabilized with 0.15 g of Tween20. In comparison with the polymer/SiO2 NCPs prepared in the system without post-added SDS (341 nm), the particle size of latex particles in this run was obviously decreased to 217 nm, indicative of the effectively suppressed mergence of the monomer droplets with the latex particles through the post-addition of SDS. Promisingly, the PDI of the latex particles was also markedly decreased from 0.308 to 0.050, indicative of significantly narrowing the particle size distribution of latex particles through the post-addition of SDS. The formation of narrowly size-distributed latex particles was also confirmed by the TEM and SEM images in Fig. 1A and B. More promisingly, nearly all the latex particles contained several SiO2 NPs, and the fNCP value of latex particles was ∼97%. The number distribution of SiO2 NPs among the latex particles is shown in Fig. 1C. About 43% of the latex particles contained 1–5 SiO2 NPs, ∼25% of the latex particles contained 6–10 SiO2 NPs, ∼14% of the latex particles contained 11–15 SiO2 NPs, ∼5% of latex particles contained 16–20 SiO2 NPs, and ∼10% of the latex particles contained more than 20 SiO2 NPs. In comparison, half of the latex particles prepared in the miniemulsion system solely stabilized with 0.15 g of Tween-20 contained more than 15 SiO2 NPs due to the higher mergence extent (Fig. S1). All these results indicated that the mergence extent in the system stabilized with Tween-20 and post-added SDS was much lower than that of the system solely stabilized with Tween-20. According to the SEM image (Fig. 1B), each polymer/SiO2 NCP had several protrusions with a brighter contrast on the surface. According to their size, we believe that these protrusions should be the SiO2 NPs. It meant that most of SiO2 NPs were attached to the surface of the particles. A similar particle morphology has also been observed in the particle samples from the emulsions solely stabilized with Tween-20 [24]. The reason for surface attachment of SiO2 NPs on the NCPs has not been fully understood yet, and is under investigation. The colloidal stability of polymer/SiO2 NCPs in the solutions with various NaCl concentrations was evaluated and the results are shown in

2.3.2. Conventional transmission electron microscopy (TEM) The particle morphology of particles was observed on a Hitachi HT7700 transmission electron microscope operated at 80 kV. The preparation of TEM samples was as follows: A drop of final emulsion was diluted with 2 mL of distilled water; a drop of the diluted sample was dropped on a 400-mesh carbon-coated copper grid and dried at room temperature. The polymer/SiO2 NCPs are particles containing at least one SiO2 NP. The fNCP value is defined as the percentage of polymer/ SiO2 NCPs relative to the overall number of particles. The number distribution of SiO2 NPs among the latex particles was also evaluated on the basis of the TEM images. All of the results were obtained by counting at least 200 particles. The number ratio between the SiO2 NPs and latex particles (Ns/Np) was roughly estimated by the formula

Ns / Np =

ms × ρp × dp3

mp × ρs × ds3

[26,27], in which Ns, Np, ms, mp, ρs, ρp, ds, and dp

represent the number of the SiO2 NPs and latex particles, mass of the SiO2 NPs and latex particles, densities of the SiO2 NPs and polymer, and number-average particle sizes of the SiO2 NPs and latex particles determined by TEM. 2.3.3. Cryo-TEM The morphology of monomer droplets in the miniemulsion of run 1 was observed on a Tecnai G2 F20 S-TWIN transmission electron microscope operated at 200 kV. The cryo-TEM sample was prepared using an FEI Vitrobot-type freezing granulator. One second adsorption time and single run were used; the humidity and temperature in the chamber were 100% and 22 °C, respectively. A copper grid was placed in the freezing granulator, and then 5 μL of the monomer miniemulsion that was diluted with the same amount of water was mounted onto the copper grid. The liquid film on the copper grid was vitrified by quenchfreezing in liquefied ethane after removal of excess liquid sample by blotting with filter paper. The frozen TEM sample was loaded onto a precooled sample holder, quickly transferred to the transmission electron microscope, and then observed at −180 °C in liquid nitrogen. 2.3.4. Field emission scanning electron microscopy (FESEM) The particle morphology of particles was also observed on a Zeiss Supra 55 field emission scanning electron microscope operated at an accelerating voltage of 5 kV. The preparation of FESEM samples was the same as that for the conventional TEM samples. 2.3.5. Measurement of monomer conversion The monomer conversion was measured by the gravimetric method. A certain amount of emulsion was taken at various time intervals in the polymerization process. The withdrawn emulsions were dried at 80 °C for 24 h. The monomer conversion was calculated through the equam3-m1 × mT-mS m -m

× 100%, in which m1, m2, m3, mT, mS, and mM tion, C = 2 1m M represent the mass of steel crucible, emulsions with steel crucible, dried 106

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Fig. 1. TEM (A) and FESEM (B) images of the latex particles prepared in the miniemulsion system stabilized with 0.15 g of Tween-20 and 2.5 mg of postadded SDS (see Table 1, run 1); (C) Number distribution of SiO2 NPs among the latex particles prepared in the miniemulsion system stabilized with 0.15 g of Tween-20 and 2.5 mg of post-added SDS (see Table 1, run 1); (D) Particle size of the latex particles as a function of NaCl concentration in the dispersions (see Table 1, run 1).

of latex particles did not change obviously. The slight decrement in particle size at the beginning of polymerization may be ascribed to the formation of latex particles through homogenous nucleation. We found that the surface tension of monomer miniemulsion with post-added SDS was lower than that of the initial monomer miniemulsion (Fig. S2). It means that in addition to being adsorbed onto the surface of monomer droplets, a part of SDS molecules dissolve in the aqueous phase. KPS, a water-soluble initiator, was used to initiate the copolymerization of St and MMA. Instead of being captured by the monomer droplets or latex particles, oligomeric radicals formed in the aqueous phase may aggregate to form primary particles through adsorbing the dissolved SDS molecules in the aqueous phase. Furthermore, the primary particles would continue to form latex particles during polymerization. It should be pointed out that the latex particles formed through homogenous nucleation should not contain any SiO2 NP. However, the absence of the plain polymer particles in the final product (Fig. 1A) indicates that these plain nucleated latex particles participate in the mergence with the monomer droplets to form SiO2-containing latex particles in the following polymerization process. As a result of the mergence, the particle size of latex particles gradually increased. As shown in Fig. 2A, the final conversion was only ∼83%. The appearance of limiting conversion may be ascribed to the vitrification effect [32], degenerative transfer of propagating radicals to hydroxyl groups on the surface of inorganic particles [33], and reduction of initiator efficiency through scavenging primary radicals by inorganic particles [34]. Further investigation on the polymerization kinetics will be carried out to clarify the exact reason for the limiting conversion and how the synthesis parameters influence the kinetic behavior of the miniemulsion polymerization systems containing SiO2 NPs. Only freely-distributed SiO2 NPs were observed in the conventional TEM images of the monomer miniemulsion due to the evaporation of monomers (Fig. S3). Therefore, the morphology of monomer droplets

Fig. 1D. The particle size of latex particles diluted in water was designated as P0, while the particle size of latex particles diluted in the solutions with various NaCl concentrations was designated as Pc. The particle sizes of latex particles in the freshly-prepared dispersions remained almost constant when the NaCl concentration of the dispersion was below 0.6 mol L−1. Further increase of the NaCl concentration led to the significant increase of particle size, indicative of a serious agglomeration of latex particles. In addition, the colloidal stability of latex particles also depends on the storage time. The onset of the NaCl concentration for agglomeration of latex particles obviously decreased to ∼0.25 mol L−1 in the dispersions after being stored for 24 h. In our case, both the anionic remaining groups of KPS and SDS can introduce negative charges to the latex particle, giving a zeta-potential of −52 mV. With the addition of NaCl, the electrostatic stabilization effect would be reduced because of the screening of the electrostatic interaction by counterions effect and compression of electrostatic double layer, and thus the latex particles underwent agglomeration [30]. Moreover, the presence of electrolytes may also compress the hydration layer of Tween-20 due to the electrostatic screening effect, leading to the decrease of steric stabilization and subsequent worsening the colloidal stability of latex particles [31]. 3.2. Particle size and morphology evolution during polymerization The particle size evolution of latex particles during polymerization were monitored for understanding the mergence process between the monomer droplets and latex particles. The ratio between the particle size of latex particles at time t and the droplet size of monomer droplets at time 0 was designated as Dt/Dm. As shown in Fig. 2A, the Dt/Dm ratio slightly decreased at the beginning of the polymerization, and then increased gradually along with the polymerization until 80% of conversion (Fig. 2A). After the conversion was above 80%, the particle size 107

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Fig. 2. (A) Curves of the particle size and conversion versus polymerization time (see Table 1, run 1); (B) CryoTEM image of the monomer droplets (see Table 1, run 1); (C–G) TEM images of the samples withdrawn at various conversions (C, 5%; D, 11%; E, 19%; F, 29%; G, 61%; see Table 1, run 1).

was further observed by cryo-TEM, and the result is shown in Fig. 2B. It should be pointed out that although this sample was observed by cryoTEM, the evaporation of monomer still took place during the sample

preparation. Therefore, we fail to observe the intact monomer droplets, including the monomer droplets with or without SiO2 NPs. Instead, we observed many SiO2 nanoaggregates with a relatively broad size 108

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although calculated through various statistic methods, the Z-average (Fig. 3A) and number-average (Fig. 3B) particle sizes of latex particles for the same sample were close, which could also be regarded as evidence for the narrow particle size distribution. The TEM and SEM images of latex particles in the emulsions with various amounts of post-added SDS are shown in Figs. 1 A, B, and 3 C–J. All the TEM and SEM images pointed to the narrow particle size distribution of latex particles regardless of the amount of post-added SDS. On the basis of the TEM images in Figs. 1 A, 3 D, F, G, and I, the fNCP values of latex particles in the emulsions with various amounts of postadded SDS were estimated, and the results are shown in Fig. 4A. With the increment in the amount of post-added SDS from 2.5 mg to 10 mg, the fNCP value of latex particles gradually decreased from 97% to 30%. As proposed in our previous paper [24], the plain monomer droplets could disappear through the mergence with monomer droplets or latex particles containing SiO2 NPs. Moreover, the mergence of several monomer droplets and latex particles containing SiO2 NPs would lead to the formation of latex particles containing many SiO2 NPs. Therefore, we used the number distribution of SiO2 NPs among the latex particles to evaluate the mergence extent of the monomer droplets with the latex particles. The corresponding results are shown in Figs. 1 C, 4 B–E. As shown in Fig. 1C, the percentage of latex particles containing more than 6 SiO2 NPs was 55% in the emulsion with 2.5 mg of post-added SDS, much higher than that of the other emulsions with more post-added SDS (Fig. 4B–E). These results support the suppressed mergence between the monomer droplets or latex particles during polymerization with the increase of the amount of post-added SDS. The Ns/Np ratios of the systems with various amounts of post-added SDS were calculated, and the results are shown in Fig. S4. Because of the increasingly suppressed mergence between the monomer droplets and latex particles, the particle size of the final particles decreased with the increase of the amount of post-added SDS (Fig. 3B). Therefore, the Ns/Np ratio obviously decreased with the increase of the amount of post-added SDS (Fig. S4). This result is consistent with the dependence of the number distribution of SiO2 NPs among the latex particles on the amount of post-added SDS. In the emulsions with 5–10 mg of the post-added SDS, the plain polymer particles accounted for 22–70%. We consider that the plain polymer particles in these systems were partially formed through the droplet nucleation of plain monomer droplets because of the suppressed mergence between the monomer droplets and latex particles. In addition, some of the plain polymer particles may form through homogenous nucleation. As discussed in Section 3.2, a part of post-added SDS molecules dissolved in the aqueous phase. Moreover, with the increase in the amount of the post-added SDS, more SDS molecules dissolved in the aqueous phase, because the surface tension of the monomer miniemulsion decreased with the increase of the amount of post-added SDS (Fig. S2). Therefore, more oligomeric radicals formed in the aqueous phase may aggregate to form primary particles through adsorbing the dissolved SDS molecules in the aqueous phase. Consequently, the number of the latex particles formed through homogenous nucleation may increase with the increase of the amount of post-added SDS. In order to confirm the occurrence of homogenous nucleation during polymerization, an oil-soluble initiator, BPO, was used to initiate the polymerization. The prepared latex particles were observed by TEM, and the result is shown in Fig. 5. As expected, due to the suppressed homogenous nucleation during the polymerization, the amount of the latex particles containing SiO2 NPs significantly increased. The fNCP value of this sample was ∼60%, much higher than that of the emulsion prepared by using KPS as the initiator (30%). It should be pointed out that the particle size distribution of the particles synthesized in the system initiated by BPO was much broader than that of the particles synthesized in the system initiated by KPS. In our previous work, we found that the initial monomer miniemulsion stabilized with Tween-20 was composed of SiO2-containing monomer droplets and plain monomer droplets [24]. Moreover, the droplet size of monomer

distribution in the cryo-TEM image (Fig. 2B). Moreover, the number of SiO2 NPs in each nanoaggregate also varied in a wide range. It is reasonable to assume that these SiO2 nanoaggregates were formed due to the monomer evaporation of the droplets containing various quantities of SiO2 NPs. Normally, one droplet of the as-synthesized emulsion is diluted by 2 mL of water for preparation of conventional TEM samples, otherwise, too many particles will deposit on the TEM grids. However, when we prepared the cryo-TEM sample, the monomer miniemulsion was only diluted with the same amount of water (the mass ratio between the monomer miniemulsion and water was 1:1.). It means that compared with the preparation of the conventional TEM samples of the polymer emulsions, we used a much higher concentration of monomer droplets to prepare the cryo-TEM sample. However, we did not observe many nano-objects in the cryo-TEM sample. Therefore, although we did not find many plain monomer droplets in the cryo-TEM image, we assume that the monomer miniemulsion still contains some plain monomer droplets, but they may disappear during the preparation of cryo-TEM sample. In addition, the morphological variation of latex particles during polymerization was also followed by conventional TEM. Many polymer/SiO2 NCPs could be observed in the TEM sample at the conversion of 5% (Fig. 2C). However, most of them displayed an irregular morphology, and some of them seemed to form a thin film on the copper grid. At this conversion, the amount of formed polymers was very low, and moreover, many remaining monomers promoted the diffusion of polymer chains to form a thin film in the drying process for preparation of the TEM sample. It should be pointed out that we might not observe the plain nucleated latex particles or latex particles containing a few SiO2 NPs in this sample due to the small amount of polymers and evaporation of monomers. When the conversion increased to 11%, the particle characteristics of latex particles became more distinguishable, and the contour of the particles could be well observed (Fig. 2D). Due to the low amount of polymers, many latex particles were mainly composed of SiO2 NPs. Some of the latex particles even displayed a capsule morphology due to the evaporation of the remaining monomers. Further increasing the conversion to 19% and 29%, well-defined polymer/SiO2 NCPs could be clearly observed (Fig. 2E and F). According to the particle morphology of the polymer/SiO2 NCPs, we can infer that it is possible to observe the plain polymer particles if they are present in the system. However, nearly all latex particles contained SiO2 NPs, indicating that a high fNCP value has already been achieved in this stage. It meant that most of the plain monomer droplets and plain nucleated latex particles formed through homogenous nucleation have already disappeared through the mergence with the SiO2-containing monomer droplets and latex particles in the initial stage of polymerization. Further increase of the conversion to 61%, the polymer/SiO2 NCPs displayed a clear spherical morphology (Fig. 2G). The particle morphology did not change obviously in the rest of polymerization time. 3.3. Variables influencing on particle properties of latex particles 3.3.1. Amount of the post-added SDS The mergence extent of the monomer droplets with the latex particles could be tuned through the amount of post-added SDS. With the increase in the amount of post-added SDS, the particle size of latex particles determined by both DLS and TEM gradually decreased (Fig. 3A and B). Considering the similar nature of the initial monomer miniemulsion, the decrement in the particle size could be regarded as evidence for the suppressed mergence between the monomer droplets and latex particles with post-addition of more SDS. Low PDI values of the latex particles determined by DLS (Fig. 3A) and small error bars in the number-average particle size of the latex particles determined by TEM (Fig. 3B) were observed in all the emulsions with various amounts of post-added SDS, indicating that the latex particles with a narrow particle size distribution have been synthesized in these runs. In addition, 109

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Fig. 3. (A) Z-average particle sizes and PDIs of latex particles determined by DLS in the emulsions with various amounts of the postadded SDS (see Table 1, runs 1–6); (B) Number-average particle sizes of the latex particles determined by TEM in the emulsions with various amounts of the post-added SDS (see Table 1, runs 1–6); TEM (C, E, G, I) and SEM (D, F, H, J) images of the latex particles stabilized with 0.15 g of Tween-20 and various amounts of the post-added SDS (C and D, 5 mg of SDS; E and F, 7.5 mg; G and H, 8.8 mg; I and J, 10 mg; see Table 1, runs 3–6).

droplets containing SiO2 NPs was much larger than that of plain monomer droplets, giving a relatively broad size distribution [24]. BPO, as an oil-soluble initiator, decomposes in the monomer droplets, and

then directly initiates the polymerization. It is reasonable to assume that more latex particles are expected to be formed through droplet nucleation in this case. It means that the broad size distribution of 110

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Fig. 4. (A) fNCP values of the latex particles in the emulsions with various amounts of the post-added SDS (see Table 1, runs 1, 3–6). (B–E) Number distribution of SiO2 NPs among the latex particles in the emulsions with various amounts of the post-added SDS (see Table 1, runs 3–6).

to increase the dispersibility of SiO2 NPs in hydrophobic monomer solutions [22]. The hydrophobic modification degree of SiO2 NPs could be conveniently tuned by the amount of MPS [35]. Accordingly, SiO2 NPs modified with 5–20 wt% of MPS were prepared. The SiO2 NPs were named as X%-MPS-SiO2, in which X% refers to the weight content of MPS relative to the amount of SiO2 NPs. The miniemulsion system stabilized with 0.15 g of Tween-20 and 2.5 mg of post-added SDS was used as the model polymerization system. The influence of the MPS modification degree of SiO2 NPs on the particle properties and the fNCP value was systematically investigated. The hydrophobicity of 5%-MPS-SiO2 and 7%-MPS-SiO2 was not high enough to prepare monomer dispersions with 8.5 wt% of SiO2 content. Therefore, the SiO2 content relative to the overall monomers

initial monomer droplets is well-preserved in this system, leading to a higher polydispersity of particles. It should be pointed out that the surface tensions of the monomer miniemulsions before and after the post-addition of SDS and the final emulsions were higher than that of the saturated aqueous solution of SDS (∼35 mN m−1 [23,28]; Fig. S2). It means that no micelles are present in the monomer miniemulsions with various amounts of postadded SDS, and therefore, the formation of plain polymer particles through micellar nucleation can be excluded. 3.3.2. MPS modification degree SiO2 NPs synthesized through the sol–gel process of TEOS are hydrophilic, and therefore, surface hydrophobic modification is necessary 111

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particle size of the latex particles. The number-average particle sizes of the latex particles with various SiO2 contents only varied in the small range of 188–214 nm (Fig. 7A). Both the low PDIs (Fig. 7A) and small error bars (Fig. 7B) pointed to the narrow particle size distribution of latex particles. According to the TEM images in Figs. 1 A, 6 G, and 7 C–E, most of latex particles contained SiO2 NPs in the samples with 2.5–8.5 wt% of 20%-MPS-SiO2. When 20%-MPS-SiO2 was 2.5 wt%, the fNCP value of latex particles was ∼77% (Fig. 7F). With the increase of the SiO2 content, the fNCP values slightly increased (Fig. 7F). In addition, according to the TEM results (Figs. 1 A, 6 G, and 7 C–E), we found that all the latex particles displayed a narrow particle size distribution regardless of the SiO2 content, consistent with the DLS results. 3.4. Discussion on the preparation of narrowly size-distributed polymer/ SiO2 NCPs A schematic representation of the preparation of polymer/SiO2 NCPs in the miniemulsion systems stabilized with Tween-20 and postadded SDS is shown in Fig. 8. In this work, monomer miniemulsions were firstly prepared by using Tween-20 as the sole surfactant. The monomer miniemulsions were composed of SiO2-containing monomer droplets and plain monomer droplets (Fig. 8A). In order to check the droplet stability of monomer droplets, the time-dependent size evolution of monomer droplets along with time in the system without adding any initiator was monitored by DLS. The size ratios between the monomer droplets at time t and 0 (Dt/D0) are shown in Fig. S5. All the Dt/D0 ratios were close to 1, and remained nearly constant. This behavior suggests that the monomer droplets have a good colloidal stability without polymerization. This result is consistent with the hypothesis that a miniemulsion system is a kinetically stable heterophase system, and that the monomer droplets are stably dispersed in the aqueous continuous phase for at least several hours [29]. At a low level of the post-added SDS, for example 2.5 mg, the mergence between the monomer droplets and latex particles takes place during polymerization. As a result, nearly all the plain monomer droplets disappear, giving a very high fNCP value (Fig. 8B). It should be pointed out that the particle size of latex particles prepared in the system with 2.5 mg of post-added SDS was obviously smaller than that of the latex particles prepared in the system without post-addition of SDS. It meant that the mergence extent in the system with 2.5 mg of the post-added SDS was obviously reduced in comparison with the system without post-added SDS. According to the dependence of the number distribution of SiO2 NPs among the latex particles on the amount of post-added SDS (Fig. 4), the mergence between the monomer droplets and latex particles would be suppressed with the increase of the amount of post-added SDS. Therefore, some plain polymer particles might be produced through the droplet nucleation of plain monomer droplets in the system with an intermediate amount of post-added SDS (Fig. 8C). In addition, we found that the surface tension of the monomer miniemulsions decreased with the increment in the amount of post-added SDS (Fig. S2). It meant that more SDS molecules dissolved in the aqueous phase with the increase of the amount of post-added SDS. In the systems with a high level of postadded SDS, oligomeric radicals produced in the aqueous continuous phase might more easily form primary particles through adsorbing SDS molecules. Thus, some plain polymer particles would be produced through homogenous nucleation. Therefore, the formation of plain polymer particles in the system with a high level of the post-added SDS might be contributed by the highly suppressed mergence between the monomer droplets and latex particles and the occurrence of homogenous nucleation simultaneously. As a result, the fNCP values were obviously decreased in the systems with a high amount of post-added SDS (Fig. 8D). It should be pointed out that using oil-soluble initiators, for example BPO, may promote the formation of polymer/SiO2 NCPs through suppression of the homogenous nucleation.

Fig. 5. TEM image of the latex particles prepared in the miniemulsion systems by using BPO as the initiator (see Table 1, run 7).

was kept at 3.5 wt% in this series of experiments. Both the DLS and TEM results indicated that the MPS modification degree did not have obvious influence on the particle size of the latex particles (Fig. 6A and B). The particle size of the latex particles only varied in a small range of 190–210 nm. The small PDIs of the latex particles indicated that latex particles with a relatively narrow particle size distribution have been prepared in a wide range of MPS modification degree. As shown in Fig. 6C, nearly all the latex particles were plain polymer particles when 5%-MPS-SiO2 were used, and the fNCP value of latex particles was only ∼2% (Fig. 6H). It should be pointed out that 5%-MPS-SiO2 was pre-dispersed in the monomer mixture prior to the preparation of monomer miniemulsion. It meant that 5%-MPS-SiO2 underwent re-distribution during polymerization. The surface modification of SiO2 NPs can influence the interfacial tension between the SiO2 NPs and polymers and that between the SiO2 NPs and water, leading to various thermodynamically stable particle morphologies [36]. Bourgeat-Lami et al. has reported that some of the SiO2 NPs distributed onto the surface of monomer droplets instead of staying inside the monomer droplets [22]. In our case, we inferred that 5%-MPS-SiO2 firstly transferred from the interior to the surface of monomer droplets due to the amphiphilic property of 5%-MPS-SiO2 during the sonication process under the drive of the thermodynamic force; subsequently, nearly all of them entered the aqueous continuous phase due to their relatively hydrophilic surface. When 7%-MPS-SiO2 was used, the fNCP value of latex particles increased to ∼30% (Fig. 6D, H). It meant that a part of SiO2 NPs could be incorporated in the latex particles with the increment in the hydrophobicity of SiO2 NPs. As expected, the fNCP values of latex particles increased to 71%, 80%, and 86% when 10%-MPS-SiO2, 15%-MPS-SiO2, and 20%-MPS-SiO2 were used, respectively (Fig. 6E–H). Consequently, the surface hydrophobic modification of SiO2 NPs is critical to the formation efficiency of the polymer/SiO2 NCPs. For achieving a high fNCP value, the SiO2 NPs should be modified at least with 10 wt% of MPS. 3.3.3. SiO2 content In this series of experiments, we still adopted the same surfactant addition strategy as Section 3.3.2, post addition of 2.5 mg of SDS to the monomer miniemulsion stabilized with 0.15 g of Tween-20. In addition, 20%-MPS-SiO2 was used as the model SiO2 NPs. As shown in Fig. 7A and B, the SiO2 content did not show obvious influence on the 112

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Fig. 6. (A) Z-average particle sizes and PDIs of latex particles determined by DLS in the emulsions with X %-MPS-SiO2 (see Table 1, runs 8–12); (B) Numberaverage particle sizes of the latex particles determined by TEM in the emulsions with X%-MPS-SiO2 (see Table 1, runs 8–12); TEM images (C–G) of the latex particles with X%-MPS-SiO2 (C, 5%-MPS-SiO2; D, 7%-MPS-SiO2; E, 10%-MPS-SiO2; F, 15%-MPS-SiO2; G, 20%-MPS-SiO2; see Table 1, runs 8–12); (H) fNCP values of the latex particles in the emulsions with X%-MPS-SiO2 (see Table 1, runs 8–12).

In addition to the amount of post-added SDS, the MPS modification degree of SiO2 NPs is also critical to achieving a high fNCP value of latex particles. On the basis of our results, we consider that the SiO2 NPs

should be modified at least with 10 wt% of MPS relative to the mass of SiO2 NPs. With a suitable amount of post-added SDS and a suitable surface MPS modification degree, a high fNCP value of latex particles 113

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Fig. 7. (A) Z-average particle sizes and PDIs of latex particles determined by DLS in the emulsions with various contents of 20%-MPS-SiO2 (see Table 1, runs 1, 12–15); (B) Number-average particle sizes of the latex particles determined by TEM in the emulsions with various contents of 20%-MPS-SiO2 (see Table 1, runs 1, 12–15); TEM images (C–E) of the latex particles with various contents of 20%-MPS-SiO2 (C, 2.5 wt%; D, 5.0 wt%; E, 6.5 wt%; see Table 1, runs 13–15); (F) fNCP values of the latex particles in the emulsions with various contents of 20%-MPS-SiO2 (see Table 1, runs 1, 12–15).

4. Conclusion

could be obtained in a wide range of the SiO2 content. According to the cryo-TEM image of the monomer miniemulsion [24], the droplet size distribution of monomer droplets was relatively broad. Moreover, on the basis of the TEM result of the particles prepared in the system using BPO as the initiator (Fig. 5), the particles formed mainly through droplet nucleation also displayed a relatively broad particle size distribution due to the inheritage of the characteristics of monomer droplets. In comparison, the particles synthesized in the systems using KPS as the initiator displayed a relatively narrow particle size distribution. As described in the previous sections, in these systems, the latex particle particles may be formed through droplet nucleation and (or) homogenous nucleation. In addition, the mergence between the monomer droplet and latex particles takes place during polymerization. Therefore, the formation mechanism of narrowly sizedistributed particles is complicated, and it has not been fully understood yet.

Narrowly size-distributed polymer/SiO2 NCPs were efficiently synthesized through miniemulsion polymerization technique by using a special use strategy of surfactants. Monomer miniemulsions were firstly prepared by using Tween-20 as the sole surfactant, and then a trace amount of SDS was added to the prepared monomer droplets. The fNCP value of latex particles reached 97% in the as-synthesized emulsion stabilized with 0.15 g of Tween-20 and 2.5 mg of the post-added SDS. The enhanced formation of polymer/SiO2 NCPs was realized through the controlled mergence between the monomer droplets and latex particles that could consume the plain monomer droplets and plain nucleated latex particles formed through homogenous nucleation. The amount of post-added SDS displayed a significant influence on the fNCP value of latex particles, which was decreased from 97% to 30% with the increase of the post-added SDS amount from 2.5 mg to 10 mg. The surface hydrophobicity of SiO2 NPs also had marked influence on the 114

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Fig. 8. Schematic representation of the preparation of polymer/SiO2 NCPs in miniemulsion systems stabilized with Tween-20 and post-added SDS.

fNCP value of latex particles. The SiO2 NPs should be modified at least with 10 wt% of MPS relative to the mass of SiO2 NPs for achieving a high fNCP value of latex particles. In comparison, the SiO2 content did not show an obvious influence on the fNCP value of latex particles, and high fNCP values of latex particles could be obtained in the range of 2.5–8.5 wt% of the SiO2 content. The reason for the narrow particle size distribution of particles has not been understood, and the related mechanistic investigation is ongoing.

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[7]

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Acknowledgements Financial supports from the Zhejiang Provincial Natural Science Foundation (LY16E030006), the National Natural Science Foundation of China (NNSFC) project (51573168), the Science Foundation of Zhejiang Sci-Tech University (14012208-Y), the Excellent Young Researchers Foundation (CETT2015001) of Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, the Young Researchers Foundation of Zhejiang Provincial Top Key Academic Discipline of Textile Science and Engineering (2015YXQN03), and Zhejiang Province’s Xinmiao Talent Plan (2016R406058 and 2017R406022) are gratefully acknowledged.

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Appendix A. Supplementary data

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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2017.07.047.

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