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Recent advances in the preparation of hybrid nanoparticles in miniemulsions
Recent Advances in the Preparation of Hybrid Nanoparticles in Miniemulsions Dongming Qi, Zhihai Cao, Ulrich Ziener PII: DOI: Reference:
S0001-8686(14)00195-X doi: 10.1016/j.cis.2014.06.001 CIS 1447
To appear in:
Advances in Colloid and Interface Science
Received date: Revised date: Accepted date:
13 March 2014 31 May 2014 1 June 2014
Please cite this article as: Qi Dongming, Cao Zhihai, Ziener Ulrich, Recent Advances in the Preparation of Hybrid Nanoparticles in Miniemulsions, Advances in Colloid and Interface Science (2014), doi: 10.1016/j.cis.2014.06.001
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ACCEPTED MANUSCRIPT Recent Advances in the Preparation of Hybrid Nanoparticles in Miniemulsions Dongming Qi1, Zhihai Cao1*, Ulrich Ziener2 1
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Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education,
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Zhejiang Sci-Tech University, Hangzhou, 310018, China 2
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Institute of Organic Chemistry III – Macromolecular Chemistry and Organic Materials, University of Ulm,
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Corresponding author: [email protected]
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Albert-Einstein-Allee 11, Ulm, 89081, Germany
Abstract. In this review, we summarize recent advances in the synthesis of hybrid in
miniemulsions
since
2009.
These
hybrid
nanoparticles
include
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nanoparticles
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organic–inorganic, polymeric, and natural macromolecule/synthetic polymer hybrid
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nanoparticles. They may be prepared through encapsulation of inorganic components or natural macromolecules by miniemulsion (co)polymerization, simultaneous polymerization of
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vinyl monomers and vinyl-containing inorganic precursors, precipitation of preformed polymers in the presence of inorganic constituents through solvent displacement techniques, and grafting polymerization onto, from or through natural macromolecules. Characterization, properties, and applications of hybrid nanoparticles are also discussed. Keywords: hybrid nanoparticles; miniemulsion; nanocomposites; polymerization.
1. Introduction Hybrid nanostructured materials that combine advantageous properties of at least two types of materials have attracted much attention for decades.[1, 1
2]
Polymer-based hybrid
ACCEPTED MANUSCRIPT nanoparticles may be the most frequently investigated hybrid nanostructured materials.[3, 4] For example, incorporation of inorganic nanomaterials into a nanoscale polymer matrix is a
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common method to prepare organic–inorganic hybrid nanoparticles. They may be used as
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building blocks to prepare hybrid films with enhanced mechanical, optical, and electric
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properties, thermal stability, UV-shielding properties, and with magnetism.[5, 6] Miniemulsion polymerization may be the most active branch of heterophase polymerization in recent years.
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One of the important factors that sustain activity of miniemulsion polymerization is its strong capability to prepare versatile hybrid nanoparticles. In a typical miniemulsion system, the
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monomer mixture is dispersed in a continuous phase through sonication or high-pressure
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homogenization.[7] The size range of monomer droplets is 50 to 500 nm. Droplet nucleation is
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the dominant pathway for forming latex particles in miniemulsions. Monomer droplets function as polymerization loci without requiring monomer diffusion. Miniemulsion reaction
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systems are highly suitable for the preparation of hybrid nanoparticles through encapsulation of a second moiety in the miniemulsion droplets, as they utilize the droplet nucleation
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mechanism.[8] In 2009, Landfester published a milestone review of the application of the miniemulsion technique in the preparation of structured hybrid nanoparticles.[8] In 2011, Wu et al. highlighted the development of the synthesis of organic–inorganic nanocomposites in miniemulsions.[9] In addition, the preparation of hybrid nanoparticles in miniemulsions is partially covered by other recent reviews.[4, 10-13] In the present review, recent advances in the preparation of hybrid nanoparticles in miniemulsions since 2009 are comprehensively summarized. The article is organized on the basis of the type of hybrid nanoparticles. Unlike previous reviews, this article also includes hybrid polymer nanoparticles and natural macromolecules/synthetic polymer hybrid 2
ACCEPTED MANUSCRIPT nanoparticles, as well as organic–inorganic hybrid nanoparticles and nanocapsules. Here, we consider nanoparticles that are composed of at least two types of polymers formed through
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2. Brief introduction to miniemulsion systems
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various polymerization mechanisms as hybrid polymer nanoparticles.
The basic concept, emulsification technique, stabilization mechanism, nucleation
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mechanism, reaction, and reaction kinetics of miniemulsions have been summarized in several classic reviews.[7, 14, 15] Therefore, we only give a brief introduction to miniemulsion in
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the present review. Normally, the preparation of nanoparticles from miniemulsion systems
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includes three steps: pre-emulsification of two heterogenous phases to prepare crude
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(macro)emulsions, homogenization of the crude emulsions to prepare miniemulsions, and reactions in miniemulsions to prepare nanoparticles (Figure 1). On the basis of polarity of the
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dispersed and continuous phase, miniemulsions can be divided into two types, namely inverse and direct miniemulsion systems. In direct miniemulsions, the polarity of the continuous
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phase is higher than that of the dispersed phase, while in inverse miniemulsions, the polarity of the continuous phase is lower than that of the dispersed phase. An aqueous solution of a surfactant is commonly used as continuous phase in direct miniemulsions. A hydrophobic solution of a surfactant is used as continuous phase in inverse miniemulsions. Commonly used hydrophobic solvents are cyclohexane, toluene, hexadecane, and Isopar M (a mixture of C12–C14 hydrocarbons). Direct miniemulsion systems can be used for the preparation of hydrophobic nanoparticles, while inverse miniemulsions can be applied for the preparation of hydrophilic ones.
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Figure 1. Schematic representation of preparation of and reaction (polymerization) in
D
miniemulsions.
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Surfactants are required for stabilizing miniemulsion systems preventing droplets from
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coalescence. In principle, surfactants with a high hydrophilic–lypophilic balance (HLB) value are suitable for direct systems, while surfactants with a low HLB value are used to stabilize
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inverse miniemulsions. Molecular diffusion between droplets driven by the Laplace pressure can destabilize the droplets. The diffusion can be suppressed by the addition of costabilizers to the dispersed phase, because costabilizers establish an osmotic pressure to counteract the Laplace pressure. Compounds with an extremely low solubility in water such as hexadecane are used as costabilizer in direct miniemulsions. On the contrary, compounds with an extremely low solubility in the hydrophobic continuous phase like hydrophilic salts were used as costabilizer in inverse miniemulsions. In principle, high-energy emulsification techniques are required for preparing droplets with a size of 50–500 nm. Sonication may be the most convenient technique to prepare 4
ACCEPTED MANUSCRIPT miniemulsions in a lab scale. For industrial applications, high-pressure homogenization may be more suitable than sonication.
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In the infancy of miniemulsions, free-radical polymerization of vinyl monomers were the
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most used reaction types in miniemulsions. In the meantime, various reaction types have been
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carried out in miniemulsions, for example controlled/living radical polymerization, polyaddition, anionic polymerization, polycondensation, sol–gel process, and precipitation
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reaction of inorganic precursors. Reactants can be introduced to the dispersed phase prior to preparation of the miniemulsion or through post-addition through the continuous phase.
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However, reactions only take place in the droplets or at the interface between the dispersed
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and continuous phases, but not in the continuous phase. For the preparation of hybrid
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nanoparticles, a second moiety can be pre-loaded in the droplets, and hybrid nanoparticles are
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formed through a subsequent reaction.
3. Inorganic nanoparticle–polymer nanoparticles prepared through nanoencapsulation
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in miniemulsions
3.1 Metal–polymer nanoparticles Metal nanoparticles may display various advantageous properties, including high catalytic activity, high optical absorption/emission, and (superpara)magnetism.[16, 17] They have wide applications in the field of semiconductors, catalysis, electronics, and so on. However, direct use of metal nanoparticles with a size of a few nanometers to several tens of nanometers is met with difficulties because metal nanoparticles with such a small size are subject to agglomeration. Incorporation of metal nanoparticles into a nanoscale polymeric matrix can prevent agglomeration of these small nanoparticles. Van Berkel and Hawker prepared noble 5
ACCEPTED MANUSCRIPT metal/poly(divinylbenzene) hybrid nanoparticles via the miniemulsion technique (Figure 2).[18] Surface modification of the metal nanoparticles with polymer chains is critical to the
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successful incorporation of metal nanoparticles in the polymeric matrix. The hybrid
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nanoparticles display good solvent resistance due to their highly cross-linked structure. They
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can be redispersed uniformly in polar solvents such as tetrahydrofuran after further
Figure
2.
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TE
D
MA
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modification with poly(ethylene glycol) through a thiol-ene reaction.[18]
Transmission
electron
microscopy
(TEM)
image
of
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gold–poly(divinylbenzene) hybrid nanoparticles. Inset at the top left shows a magnified image of a hybrid nanoparticle. Reprinted from ref.18 with permission.
Destabilization of Au nanoparticles may take place in some systems reacting at elevated temperatures because of desorption of alkanethiols from the gold surface.[19] To avoid such destabilization, Fuchs et al. prepared Au/poly(methyl methacrylate) (PMMA) hybrid nanoparticles through a photoinitiated miniemulsion polymerization process at low reaction temperature.[20] The thiol-capped Au nanoparticles were well dispersed in the PMMA matrix. The authors attributed the encapsulation and good dispersion of the Au nanoparticles in the 6
ACCEPTED MANUSCRIPT PMMA matrix to the low reaction temperature. Koh et al. prepared Ag-decorated polystyrene (PS) nanocapsules containing isopropyl myristate (IPM) in miniemulsions.[21] The
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dodecanethiol-capped Ag nanoparticles were predispersed in IPM prior to polymerization.
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Polyaniline has excellent electronic properties, but its insolubility and immiscibility with
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nonconducting polymers limits its application as electron-transport material. Modification of polyaniline with metal or metal oxides is an effective method of achieving improved
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performance of polyaniline as a sensor. Ag/polyaniline hybrid nanoparticles were prepared through miniemulsion polymerization.[22] The Ag/polyaniline nanocomposites were used to
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prepare ethanol-responsive films that showed higher stability and reproducibility in ethanol
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vapors compared with pure polyaniline.
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Normally, surface modification is required to improve the dispersion of metal nanoparticles in a monomer solution. Mamaghani et al. reported the encapsulation of unmodified
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hydrophilic silver nanoparticles through miniemulsion copolymerization of acrylic monomers.[23] However, morphological investigation by transmission electron microscopy
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(TEM) did not give strong evidence for the attachment of the silver nanoparticles to the polymer nanoparticles. Metal/hydrophilic polymer hybrid nanoparticles may be prepared through polymerization of hydrophilic monomers in inverse miniemulsions. For example, gold nanoparticles were incorporated in a hydrophilic polymer matrix through inverse miniemulsion atom transfer radical polymerization (ATRP) for biological applications.[24] The hybrid nanogels could be taken up by MC3T3-E4 mouse calvarial osteoprogenitor cells. Surface modification of the nanogels with glycine-arginine-glycine-aspartic acid-serine peptides improved the cell attachment and internalization. 7
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3.2 Nanoclay–polymer nanoparticles
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To improve the thermal stability and mechanical properties of polymers, inorganic
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nanofillers such as nanoclays are often combined with polymers to prepare organic–inorganic
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composites. Nanoscale blending of nanoclay and polymer may impart better thermal and mechanical properties to nanoclay/polymer composites than macroscale blending. The
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miniemulsion technique has proved to be an efficient method for preparing nanoclay/polymer nanocomposites. Large amounts of clay (30–50 wt%) have been loaded in the polymer matrix 26]
In this process, inorganic nanoparticles and
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by ad-miniemulsion polymerization.[25,
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monomers are emulsified separately to form two miniemulsions, which are then mixed and
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sonicated to form hybrid miniemulsions. The particle morphology is significantly influenced by the size of the clay particles and the reactivity of modifiers. Armored nanoparticles were
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formed in mixtures with large clay particles such as montmorillonite, irrespective of the type of modifier. In mixtures with small clay particles such as chemically modified laponite, the
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clay particles were encapsulated in the hybrid nanoparticles, while corresponding physically modified materials were mainly adsorbed to the surface of the hybrid nanoparticles. Cloisite 30B, an organically modified montmorillonite, was encapsulated with poly(styrene-co-butyl acrylate) through miniemulsion copolymerization.[27] Exfoliated Cloisite 30B was mainly located inside the nanocomposites at an encapsulation efficiency of about 73%. Encapsulation of organomodified montmorillonite by PMMA through ATRP in miniemulsions led to hybrid nanoparticles with a uniform size distribution.[28] Hatami et al. investigated the nucleation type, kinetics, and polymerization control of ATRP miniemulsion copolymerization of styrene (St) and methyl methacrylate (MMA) in the presence of 8
ACCEPTED MANUSCRIPT nanoclay.[29] They revealed that droplet nucleation was the dominant mechanism. The polymerization rate and molecular weight of the copolymer decreased with increased clay
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loading. Khezri et al. prepared Cloisite 30B/poly(styrene-co-methyl methacrylate)
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nanocomposites by using miniemulsion activators generated by electron transfer (AGET)
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ATRP.[30] Clay/poly(styrene-co-butyl acrylate) nanocomposites could also be prepared through ATRP in miniemulsions.[31] As expected, ATRP produced polymers with a molecular
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weight distribution narrower than that of polymers prepared through conventional free-radical polymerization. The resultant nanocomposites showed enhanced thermal stability compared
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with the pure polymers.
Capek investigated the kinetics of photo-initiated miniemulsion polymerization of butyl
TE
D
acrylate (BA) in the presence of clay.[32] He reported the polymerization in the presence of sodium dodecyl sulfate (SDS) with a final conversion of about 60%. However, the final
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conversion only reached about 20% when cetyltrimethylammonium bromide was used as surfactant.
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Interestingly, asymmetric saponite/PS nanoparticles (e. g. hemispheres or truncated spheres) have been prepared via miniemulsion polymerization using nanosaponites with a size larger than the equilibrated droplet size of the miniemulsion.[33] The authors suggest that the asymmetric nanoparticles are formed by asymmetric growth of polymer chains on one side of the nanosaponite platelets when monomer droplets only adsorbs on one side of the platelets. The asymmetric nanoparticles may also be propared through disintegration of intact nanoparticles symmetrically grown on both sides of the nanosaponite platelets into hemispheres.
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ACCEPTED MANUSCRIPT 3.3 SiO2–polymer nanoparticles Similarly to other inorganic nanoparticles, surface modification of SiO2 nanoparticles is
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important for the successful incorporation of SiO2 into a polymer matrix. The surface of silica
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nanoparticles may be modified chemically with various silanes or physically with ionic
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surfactants. Silane-modified SiO2 nanoparticles have been encapsulated with polyacrylate through miniemulsion polymerization.[34] With the increase in silica content in the
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nanocomposites, the tensile strength of hybrid films increased, while the elongation at break decreased. Silane-modified silica nanoparticles were also encapsulated through a reversible
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addition–fragmentation chain transfer (RAFT) polymerization process in miniemulsion.[35]
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Incorporation of SiO2 in the polymer matrix had positive influence on the thermal behavior of
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the polymer. The resultant polymers prepared by RAFT miniemulsion polymerization showed a narrow molecular weight distribution.
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SiO2 nanoparticles could also be modified with hydroxylated silicone oil. The modified particles were encapsulated with poly(dimethylsiloxane) (PDMS) through ring-opening of
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polymerization
octamethylcyclotetrasiloxane
in
miniemulsions.[36]
The
resultant
SiO2/PDMS nanocomposites displayed a well-defined core–shell morphology. The hybrid film prepared from the SiO2/PDMS nanocomposites showed improved mechanical and thermal properties compared with pure PDMS. The concentration of surfactant is a crucial parameter in miniemulsions, and may influence colloidal stability, droplet stability, nucleation mechanism, as well as droplet and particle size. Hecht et al. investigated the influence of surfactant concentration on miniemulsion polymerization for encapsulation of SiO2 nanoparticles.[37] The SiO2 nanoparticles were modified chemically with γ-trimethoxysilylpropylmethacrylate (MPS) or physically by 10
ACCEPTED MANUSCRIPT cetyltrimethylammonium chloride (CTMA-Cl) to achieve a lipophilic surface prior to encapsulation. They found that the presence of SiO2 nanoparticles lowered the required
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amount of surfactant. This decrease could be ascribed to the distribution of SiO2 nanoparticles
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at the interface, because it behaves as a colloidal stabilizer. In addition, organic modifiers that
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desorb from the SiO2 nanoparticles may act as additional surfactants.
In addition to sonication, high-pressure homogenization is sometimes used to prepare
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miniemulsions. In fact, high-pressure homogenization may be more suitable for industrial application than is sonication. Hecht et al. investigated the emulsification of droplets loaded
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with silica nanoparticles by high-pressure homogenization.[38] Compared with sonication,
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high-pressure homogenization was more efficient in emulsification of monomers with high
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silica content.
Recently, Bourgeat-Lami et al. investigated systematically the entire process of
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encapsulation of MPS-grafted silica nanoparticles by polymer matrixes in miniemulsions.[39] MPS-grafted silica nanoparticles may be well dispersed in MMA or an MMA/BA mixture, but
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a limited number of aggregates is formed when they are dispersed in BA. Such aggregation may be caused by the relatively large difference of hydrophobicity of MPS-grafted silica nanoparticles and BA. The influence of silica content on the size and morphology of the miniemulsion droplets was investigated by dynamic light scattering (DLS), small-angle X-ray scattering, and cryo-TEM. Results of these experiments indicate that the distribution of silica particles in the monomer droplets was not homogenous (Figure 3a and b). Some droplets were free of silica particles, whereas some contained one to several silica nanoparticles. It is interesting to find that the silica nanoparticles were mainly located at the water/oil interfaces of the miniemulsions. Similar to the miniemulsion droplets, hybrid nanoparticles formed 11
ACCEPTED MANUSCRIPT through subsequent polymerization also showed a heterogeneous distribution of the silica
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particles (Figure 3c).
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Figure 3. Cryo-TEM images of a 50/50 MMA/BA miniemulsion containing 10 wt % silica particles (a and b) and silica/poly(MMA-co-BA) nanoparticles with 3.5 wt% silica content (c). Reprinted from ref. 39 with permission.
3.4 Quantum dot (QD) –polymer nanoparticles QDs are promising materials for biological applications because of their luminescent properties. Among them, silicon QDs have high priority for use in biological applications due to their orange-red luminescence, which does not strongly interfere with cells. Harun et al. prepared silicon QD/polymer hybrid nanoparticles through miniemulsion polymerization.[40] 12
ACCEPTED MANUSCRIPT A
functional
comonomer,
4-vinylbenzaldehyde,
was
used
for
further
potential
functionalization of the nanocomposites. Incorporation of silicon QDs in the polymer matrix
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imparts high chemical stability and the potential for flexible functionalization but retains the
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fluorescent properties of silicon QDs.
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“Grafting from” is a commonly used method for preparing nanocomposites having an inorganic core decorated with polymer brushes. Esteves et al. prepared CdS QDs/PS
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nanocomposites through surface-initiated RAFT polymerization in miniemulsion.[41] QDs in the nanocomposites retained their quantum confinement effects. Polymer brushes on the
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surface of the QDs displayed uniform molecular weight. Living properties were confirmed by
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the formation of block polymer brushes through the addition of a second monomer.
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Polyfluorene nanoparticles were prepared through Suzuki–Miyaura polycondensation of 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-7-bromo-9,9-dioctylfluorene
in
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miniemulsion.[42] Polymerization of the monomer droplets containing CdSe/CdS core/shell QDs formed QDs/polyfluorene hybrid nanoparticles. The photostability of the QDs in the
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hybrid nanoparticles improved, and the o rster energy transfer from the polyfluorene shell to the QD core was efficient.
3.5 Inorganic nanoparticles–polymer nanoparticles with magnetic properties 3.5.1 Preparation of magnetic hybrid nanoparticles through miniemulsion polymerization Magnetic nanocomposites may be prepared in miniemulsion by incorporation of magnetic metal[43] or metal oxide nanoparticles into a nanoscale polymer matrix. Again, surface modification of the magnetic inorganic nanoparticles is crucial for their successful incorporation into polymer matrixes. Yan et al. investigated the influence of surface 13
ACCEPTED MANUSCRIPT modification of Fe3O4 nanoparticles on the preparation of Fe3O4/PS nanocomposites in miniemulsions.[44] They reported that the encapsulation efficiency of MPS-modified Fe3O4 was
higher
than
that
of
Fe3O4
nanoparticles
modified
with
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nanoparticles
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n-octadecyltrimethoxysilane. In addition, nanocomposites containing MPS-modified Fe3O4
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showed a regular morphology and narrow size distribution, probably because of the good compatibility between Fe3O4 and PS due to copolymerization between MPS and St. Fe3O4
polyvinylbenzylchloride,[46]
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nanoparticles modified with oleic acid can also be encapsulated with polyacrylonitrile,[45] PS,[47]
PMMA,[48]
poly(styrene-co-acrylamide),[49]
or
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poly(styrene-co-methacrylic acid)[50] in miniemulsions. Incorporation of chlorine as functionality provides the possibility to further functionalize the magnetic nanocomposites
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with various functional groups.[46] Addition of hydrophilic monomers such as acrylic acid may improve the colloidal stability of the system.[47] Magnetic poly(styrene-co-acrylamide)
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hybrid nanoparticles display a saturation magnetization of 27.1 emu·g-1.[49] Addition of acrylamide or methacrylic acid (MAA) as comonomer may lead to substitution of the hybrid
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nanoparticles with additional functional groups such as –CONH2 and –COOH.[ 49, 50] Fe3O4 nanoparticles modified with oleic acid were incorporated in a polyaniline matrix to prepare conductive and magnetic, dually functional nanocomposites.[51] The resultant nanocomposites showed a multicore/single-shell morphology. The increase in Fe3O4 content in the nanocomposites led to a reduction in conductivity and an increase in the saturation moment. Park
et
al.
nanocomposites
investigated prepared
the in
magnetorheological miniemulsions.[48]
characteristics They
found
of
Fe3O4/PMMA
that
Fe3O4/PMMA
nanocomposites in a nonmagnetic fluid displayed Bingham behavior under external magnetic fields. 14
ACCEPTED MANUSCRIPT Compared with thermal initiation, photoinitiated polymerization may be performed more rapidly at relatively low temperature. High reaction rate and low reaction temperature may be
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favorable for encapsulation of inorganic nanoparticles. Dou et al. loaded PMMA nanoparticles
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with about 85.1 wt% of magnetic nanoparticles via photoinitiated miniemulsion
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polymerization.[52] Recently, the same group prepared magnetic core–shell nanoparticles with a magnetic core of 250 nm ± 40 nm in diameter through photoinitiated polymerization in
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miniemulsions.[53] The size of magnetic particles used in this paper is relatively larger than that of magnetic nanoparticles prepared through coprecipitation (about 10 nm). Confinement
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of large magnetic nanoparticles in the dispersed phase was realized through a rapid increase in
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the viscosity of the dispersed phase. In addition, the high reaction rate of photoinitiated
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polymerization might minimize the influence of molecular diffusion between droplets on the droplet stability when hydrophilic monomers were used. Therefore, magnetic core–shell
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nanoparticles were functionalized with epoxy groups through photoinitiated miniemulsion polymerization using glycidyl methacrylate as the monomer.
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Ramos and Forcada encapsulated magnetic nanoparticles via surfactant-free miniemulsion polymerization.[54] Compared with hybrid nanoparticles obtained from SDS-stabilized miniemulsion, the magnetic nanoparticles distributed more homogenously in the polymeric matrix, and the resultant hybrid particles showed a narrower particle size distribution. The distribution of magnetic nanoparticles in the polymer matrix depended on the type of cross-linker. Use of a hydrophobic cross-linker led to core–shell hybrid particles with a core of aggregated magnetic nanoparticles. In contrast, use of a hydrophilic cross-linker resulted in a homogenous distribution of magnetic nanoparticles in the polymer matrix. Xu et al designed a modified miniemulsion system to prepare epoxy-functionalized 15
ACCEPTED MANUSCRIPT magnetic polymer latices with various morphologies.[55] No hydrophobe was added to the dispersed phase in the modified miniemulsion, resulting in micrometer-sized monomer
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droplets that formed via monomer diffusion. However, particles were formed through droplet
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nucleation by keeping the amount of surfactant below the critical micelle concentration. The
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particle morphology may be tuned by varying kinetic parameters such as cross-linker amount and reaction temperature. Epoxy groups on the surface of the hybrid particles enable further
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modifications.
Different from magnetic particles incorporated in the hybrid nanoparticles via
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miniemulsion polymerization, two polymerizable magnetic clusters, Mn12O12(4-vinylbenzoic acid (VBA))16 and Mn8Fe4O12(VBA)16, were copolymerized with St to form magnetic
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nanospheres in miniemulsion.[56] In contrast to the heterogenous distribution of magnetic particles in most of the magnetic hybrid nanoparticles, the magnetic clusters were
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homogenously distributed in the hybrid nanoparticles. The magnetic cores, Mn12 and Mn8Fe4, retained their identities after polymerization. The resulting magnetic nanoparticles have
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potential applications as contrast agents for magnetic resonance imaging.
3.5.2 Preparation of magnetic nanocomposites using preformed polymers in miniemulsions Polymer nanoparticles can be prepared by using preformed polymers through a combination of miniemulsification and solvent displacement techniques.[57] Urban et al. mixed superparamagnetic iron oxide nanoparticles and fluorescent molecules with a chloroform solution of polylactide (PLA).[58] The resulting hydrophobic dispersion was dispersed in an aqueous solution of SDS to form a miniemulsion. With subsequent evaporation of the solvent, superparamagnetic and fluorescent PLA-based hybrid 16
ACCEPTED MANUSCRIPT nanoparticles were formed. These hybrid nanoparticles displayed a regular shape and a narrow size distribution. The loading of magnetic nanoparticles in the nanocomposites was as
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high as 39 wt% relative to the weight of PLA.
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Janus magnetic nanoparticles have also been prepared through a combination of
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miniemulsification and solvent displacement techniques.[59] The Janus morphology was induced by incompatibility between oleic acid surrounding the magnetic nanoparticles and
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free PS chains, which caused phase separation between the magnetic nanoparticles and PS chains during evaporation of the St monomer. The heterogenous distribution of the surfactant
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on the droplet surface might be another significant reason for the formation of Janus
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nanoparticles.
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Dong et al. prepared thermosensitive cross-linked poly(di(ethylene glycol) methyl ether methacrylate) (PM(EO)2MA) nanogels in inverse miniemulsion.[60] To achieve a
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responsiveness to magnetic fields, hydrophobic magnetic nanoparticles stabilized with oleic acid were post-loaded into the nanogels. PM(EO)2MA nanogels with multiresponsive
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properties may find potential application in controlled drug release.
3.6 Carbon–polymer nanocomposites Pristine carbon black tends to aggregate because of its strong van der Waals forces. Preparation of carbon/polymer nanocomposites through incorporation of carbon black is an effective technique for alleviating the agglomeration of carbon black. Similar to other inorganic nanoparticles, carbon black requires modification prior to encapsulation. Han et al. reported a technique for preparing carbon black modified with oleic acid which was further encapsulated with PS through a miniemulsion polymerization process.[61] In their study, 17
ACCEPTED MANUSCRIPT carbon black was firstly hydroxylated by using KMnO4 in the presence of a phase-transfer catalyst, tetrabutyl ammonium bromide. Thereafter, the hydroxylated carbon black was
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reacted with oleic acid. Although pristine carbon black is a radical scavenger and thus may
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inhibit free-radical polymerization, free-radical polymerization could proceed in the presence
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of carbon black coated with oleic acid.
Graphene, a two-dimensional carbon nanomaterial, has become a popular material in recent
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years because of its remarkable electrical properties.[3, 62] It has been adopted extensively to prepare cost-effective conductive polymer composites. Direct incorporation of graphene oxide
MA
(GO) by utilizing hydrophobic polymers may be difficult due to the hydrophilic surface of
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GO sheets. To overcome this problem, Etmimi and Sanderson modified GO with a reactive
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surfactant, 2-acrylamido-2-methyl-1-propanesulfonic acid, to widen the interlayer distance and to promote the intercalation of monomers.[63] GO/poly(styrene-co-butyl acrylate)
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nanocomposites were prepared through a miniemulsion copolymerization technique in the presence of modified GO. Compared with the plain copolymer, nanocomposites containing
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GO showed improved mechanical properties and better thermal stability. In a succeeding study, Etmimi et al. modified GO with a RAFT agent, and then dispersed modified GO in a mixture of St and a hydrophobe.[64] The resultant GO dispersion was homogenized to form miniemulsions. GO/PS nanocomposites with a narrow particle size distribution were formed via RAFT polymerization in miniemulsion. Attachment of the RAFT agent to the surface of GO sheets resulted in covalent binding of polymer chains to the GO sheets. In addition, the molecular weight and polydispersity of the polymers could be controlled to some extent by taking advantage of the living characteristics of RAFT polymerization. The GO/PS nanocomposites showed improved thermal stability compared with the plain PS nanoparticles. 18
ACCEPTED MANUSCRIPT Tan et al. modified GO with poly(styrene-co-methyl methacrylate) by grafting copolymerization in miniemulsions.[65] The copolymer-grafted GOs were used as conducting
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fillers to impart conductive properties to the immiscible PS and PMMA blends. Furthermore,
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the compatibility between PS and PMMA was improved by the incorporation of the
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copolymer-grafted GO.
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3.7 Semiconductor nanoparticles–polymer hybrid nanoparticles ZnO nanoparticles have been widely used as photocatalysts, bactericides, UV-shielding
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materials, and photoluminescent materials.[66] Polypyrrole–zinc oxide (PPy/ZnO) hybrid
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nanoparticles were prepared by ultrasound-assisted miniemulsion polymerization.[67] The
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authors reported that sonication during polymerization reduced the size and narrowed the size distribution. The PPy/ZnO nanoparticles were used as a sensor for liquefied petroleum gas
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(LPG) with a more rapid response than the sensor using pure PPy. ZnO nanoparticles may also be encapsulated with common polymers such as PS in
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miniemulsions.[68] The encapsulation efficiency of ZnO nanoparticles can exceed 90% after modification with 3-aminopropyltriethoxysilane, which enhanced the hydrophobicity of the ZnO nanoparticles. The resulting ZnO/polymer nanocomposites could be used to prepare antibacterial hybrid films. Antimony-doped tin oxide (ATO) has numerous advantageous properties, including infrared (IR) light shielding, electrical conductivity, and high chemical and mechanical stability. Silane-modified ATO nanoparticles were encapsulated with poly(methyl methacrylate-co-n-butyl acrylate) in miniemulsion.[69] The encapsulation efficiency depended on synthesis parameters like comonomer ratio and amount of ATO nanoparticles, emulsifier, 19
ACCEPTED MANUSCRIPT and hydrophobe. An increase in the amount of ATO nanoparticles in the hybrid films led to a reduction of transmittance in the near-IR region but did not have an obvious influence in the
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UV–visible region. The resultant hybrid films have potential as transparent IR-light shielding
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films.
3.8 Pigment–polymer nanoparticles
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Mahdavian et al. prepared white nanocomposites by encapsulation of Al2O3 nanoparticles modified with oleic acid by polymerization in miniemulsions.[70] Yellow pigments were
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encapsulated by PS through miniemulsion polymerization to obtain hybrid nanoparticles with
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a pigment core and a polymer shell.[71] By post-polymerization with fluorescein
TE
isothiocyanate-modified (2-aminoethyl)methacrylate hydrochloride, the hybrid nanoparticles were encapsulated with a fluorescent shell to obtain luminescent electrophoretic particles
CE P
(EPs). The luminescent EPs could be used to prepare glowing electrophoretic displays
AC
applicable in dark conditions.
3.9 Inorganic nanoparticles–polymer nanocomposites prepared through in situ formation of inorganic nanoparticles in droplets Mineralization of inorganic precursors in a confined environment may be performed in the nanodroplets
of
miniemulsions.
Fukui
and
Fujimoto
prepared
nano-CaCO3
by
sonication-assisted mixing of two inverse miniemulsions with aqueous Ca(NO3)2 and Na2CO3 droplets, respectively.[72] Organic–inorganic hybrid nanoparticles were prepared through subsequent polymerization of the preloaded hydrophilic monomer, 2-hydroxyethyl methacrylate (HEMA), in the droplets of Ca(NO3)2. Interestingly, the morphology of 20
ACCEPTED MANUSCRIPT nano-CaCO3 could be tuned by modifying synthesis parameters such as the incubation time for the nucleation of CaCO3 and growth prior to polymerization and the polymerization rate
T
of HEMA. Such modification may inhibit the crystal growth and transformation of CaCO3.
SC R
CaCO3 core dictates the mechanical and optical properties.
IP
These hybrid nanoparticles were used as building blocks to prepare hybrid films. The inner
Systems that do not use organic solvents for the preparation of organic–inorganic hybrid
NU
materials are of particular interest because of their environmental benefits. Nabih et al. prepared inorganic nanoparticle–polymer hybrid particles in a system without any organic
MA
solvent.[73] They emulsified two aqueous solutions of inorganic precursors in a hydrophobic
D
monomer mixture to obtain two inverse miniemulsions. Combining and homogenizing both
TE
inverse miniemulsions led to a dispersion of inorganic nanoparticles (for example, zinc phosphate and calcium carbonate) in a polymerizable continuous phase. This inverse was
converted
CE P
miniemulsion
to
a
direct
miniemulsion.
Upon
polymerization,
inorganic–polymer hybrid nanoparticles were formed. Water-based emulsions of zinc
panels.[74]
AC
phosphate–polymer hybrid nanoparticles have been used as anticorrosive coatings for steel
Recently, Hamberger et al. designed a multistep miniemulsion process to prepare polyurethane (PU) nanocapsules containing inorganic nanoparticles.[75] First, they prepared two inverse miniemulsions with aqueous salt solution droplets. Second, water-insoluble inorganic nanoparticles were formed in the dispersed phase by co-sonication of the two inverse miniemulsions. Finally, the inorganic nanoparticles were encapsulated with PU shells through polyaddition of a polyol pre-dissolved in the dispersed phase and a diisocyanate added to the continuous phase. 21
ACCEPTED MANUSCRIPT
3.10 Section summary
T
The miniemulsion technique shows very high capability in the preparation of hybrid
IP
nanoparticles through polymer encapsulation of various inorganic nanoparticles. Generally, it
SC R
includes four steps to prepare inorganic nanoparticle–polymer nanoparticles through nanoencapsulation in miniemulsion: surface modification of inorganic nanoparticles to
NU
improve their dispersion ability in monomers, dispersion of inorganic nanoparticles in monomers, preparation of miniemulsions with droplets containing inorganic nanoparticles,
MA
and encapsulation of inorganic nanoparticles in a polymer matrix through miniemulsion
AC
CE P
TE
D
polymerization (Figure 4).
Figure 4. Schematic representation of surface modification of inorganic nanoparticles and their encapsulation in a polymer matrix.
The hybrid nanoparticles may show superior chemical or physical properties compared to the corresponding constituent inorganic or polymer nanoparticles. For example, encapsulation 22
ACCEPTED MANUSCRIPT of inorganic nanoparticles with polymers may improve their chemical stability and dispersibility, enhance the thermal stability and mechanical properties of the polymers,
T
facilitate modification of the inorganic nanoparticles by using comonomers with reactive
IP
groups, and impart additional functions such as fluorescence and magnetism to common
SC R
polymers. Functionalization of hybrid nanoparticles may be the most dominant trend in this sub-branch in recent years. For the syntheses using common materials such as silica
NU
nanoparticles and nanoclays, high pressure homogenization is recommended to prepare
MA
miniemulsions regarding the future large-scale industrial production.
D
4. Raspberry-like hybrid nanoparticles
TE
Pickering-type heterophase polymerization systems, such as Pickering (mini)emulsion, may be the most commonly used systems for preparing raspberry-like nanocomposites. In a typical
CE P
Pickering system, organic surfactants that are often used to avoid coalescence of particles or droplets are replaced with particulate stabilizers. Inorganic materials such as SiO2
AC
nanoparticles have been used as particulate stabilizers to prepare organic–inorganic nanocomposites. To improve the adsorption of particulate stabilizers on monomer droplets or polymer particles, additional interactions between stabilizers and droplets or particles are required. Electrostatic and acid–base interactions have often been adopted for this purpose. More information on the Pickering heterophase polymerization is provided in a recent review by Schrade et al.[76]
4.1 Preparation of raspberry-like nanocomposites in Pickering miniemulsion without organic surfactants 23
ACCEPTED MANUSCRIPT Inorganic nanoparticles or nanosheets may function as sole stabilizer for miniemulsions. Raspberry-like hybrid nanocapsules were synthesized through Pickering miniemulsion
T
copolymerization of St and 4-vinyl pyridine (4-VP) using silica sols as stabilizers and
IP
hexadecane as oil templates (see Figure 5).[77] The acid–base interaction between silica sol
SC R
and 4-VP contributes significantly to the formation of a raspberry-like morphology, which strongly depends on the suspension pH, size and amount of silica particles, and amount of
NU
4-VP. The formation of capsules was induced by internal phase separation of the polymers in
AC
CE P
TE
D
MA
the droplets.
Figure 5. TEM (a and b) and scanning electron microscopy (SEM) (c and d) images at various magnifications of hybrid capsules armored with silica particles synthesized in a Pickering miniemulsion. Reprinted from ref. 77 with permission.
Positively charged inorganic nanoparticles may also be used as a stabilizer to prepare raspberry-like
hybrid
nanoparticles
in
miniemulsions.
Schrade
et
al.
prepared
positively-charged, alumina-coated silica/PS hybrid nanoparticles via Pickering miniemulsion 24
ACCEPTED MANUSCRIPT polymerization.[78] Functional comonomers containing –COOH or –CONH2 groups were introduced to improve the adsorption of silica sols on the monomer droplets and, subsequently,
T
on polymer particles through hydrogen bonding between the comonomer and the silica
IP
surface. The particles’ properties and colloidal stability of the systems strongly depended on
SC R
the suspension pH, type of comonomers, and content and size of silica sols. Nanostructured cerium dioxide (CeO2) has attracted much attention because of its wide
NU
application in the fields of chemical catalysis, photocatalysis, sensors, oxygen permeation membranes, and biomedicine.[79] Similar to SiO2, CeO2 nanoparticles may be used as a
MA
stabilizer to prepare Pickering miniemulsions. Pickering miniemulsion and Pickering hybrid
D
nanoparticles stabilized by CeO2 nanoparticles were successfully prepared by Zgheib et al.[80]
TE
To obtain a stable CeO2-stabilized Pickering miniemulsion, MAA was added to the system. MAA establishes the interaction between monomer droplets and CeO2 nanoparticles. The
CE P
influence of the amount of MAA and CeO2 nanoparticles on the properties of droplets and nanoparticles, and on the colloidal stability of the system was systematically investigated. The
AC
CeO2 nanoparticles were only attached to the surface of the hybrid nanoparticles. Bonnefond et al. used sodium montmorillonite clay (NaMMT) as a stabilizer in the surfactant-free miniemulsion polymerization of n-BA/St aided by a functional macromonomer, poly(ethylene glycol) monomethacrylate (PEGMMA).[81] PEGMMA played a crucial role in the formation of NaMMT-armored hybrid nanoparticles after polymerization. PEGMMA may attach to the surface of NaMMT, improving the compatibility of NaMMT and polymers. After polymerization, NaMMT-armored hybrid nanoparticles were formed. These hybrid nanoparticles were used to prepare hybrid films with improved water resistance compared with that of hybrid films prepared from blends of polymer nanoparticles and NaMMT. 25
ACCEPTED MANUSCRIPT GO nanosheets have also been used as inorganic stabilizer for miniemulsions in the preparation of GO/polymer hybrid nanoparticles.[82] Although the size control was not as good
T
as in miniemulsions stabilized by nanoclays or silica particles, the colloidal stability of
IP
miniemulsion systems stabilized by GO nanosheets was still controllable. Similar to other
SC R
Pickering systems, GO sheets mainly distribute on the surface of the hybrid nanoparticles, creating a rough surface, as confirmed by SEM and elemental analysis by X-ray photoelectron
NU
spectroscopy.
For precious materials, a downscaled reaction system may be preferable. Kang et al.
MA
prepared dually labeled raspberry-like nanocomposites in a downscaled Pickering
D
miniemulsion system using silica nanoparticles labeled with a fluorescein dye and QDs.[83]
TE
Separate excitation of both fluorescence signals was confirmed by the emission spectra of the
CE P
dually labeled particles and by confocal laser scanning microscopy measurements.
4.2 Preparation of raspberry-like nanocomposites in Pickering miniemulsions in the
AC
presence of organic surfactants Ionic surfactants are often used in Pickering systems to establish electrostatic interactions between particulate stabilizers and monomer droplets or polymer particles. Normally, cationic surfactants are used for negatively-charged silica particles to promote adsorption of silica nanoparticles in the preparation of raspberry-like silica–polymer nanocomposites. For example, glycerol-functionalized silica sols were coated on the surface of PS nanoparticles in the presence of a small amount of cationic surfactant.[84] It should be pointed out that the use of cationic surfactants is not indispensable. For instance, Zhang et al. prepared raspberry-like silica/PS nanocomposites using an anionic surfactant and anionic silica particles.[85] For the 26
ACCEPTED MANUSCRIPT synthesis, a comonomer containing a basic amino group, 1-vinylimidazole (1-VID), was used. Adsorption of negatively charged silica particles on the surface of the nanocomposites was
T
driven by the strong acid–base interaction between the acidic hydroxyl groups of the silica
IP
particles and the basic, nitrogen-containing 1-VID. In addition, a cationic comonomer,
SC R
2-(methacryloyl) ethyltrimethylammonium chloride (MTC), was used to promote the adsorption of negatively charged SiO2 nanoparticles to the polymer nanoparticles through interaction
between
SiO2
nanoparticles
and
MTC.[86]
Raspberry-like
NU
electrostatic
SiO2/polymer hybrid nanoparticles were successfully prepared in a miniemulsion stabilized
MA
by anionic surfactants.
D
The use of naturally occurring materials to replace petroleum-derived ones is a common
TE
trend driven by environmental concerns. BelHaaj et al. used starch nanoplatelets (SNPs) as colloidal stabilizer to partially reduce the use of organic molecular surfactants to prepare
CE P
Pickering miniemulsions.[87] They found that the amount of cationic surfactant (dodecylpyridinium chloride) could be reduced from 2 wt% based on the monomer content
AC
for the system without SNPs to 0.5 wt% for the system with SNPs. This reduction is due to the synergistic stabilization effect with the cationic surfactant. It is worth mentioning that hybrid films prepared from Pickering nanoparticles stabilized with SNPs showed a markedly enhanced mechanical performance compared with hybrid films formed from simply mixed dispersions of SNPs and polymer particles.
4.3 Preparation of raspberry-like nanocomposites by in situ formation of inorganic nanoparticles Bowl-like or raspberry-like SiO2/PMMA hybrid hollow spheres have been synthesized by 27
ACCEPTED MANUSCRIPT simultaneous polymerization of MMA and hydrolysis–condensation of tetraethoxysilane (TEOS) in miniemulsion (Figure 6).[88] Three monomers, MMA, MPS, and TEOS, were
T
homogenized to form monomer miniemulsions (Figure 6a). Polymers formed through
IP
free-radical copolymerization underwent phase separation due to their low compatibility with
SC R
TEOS. Particles with various morphologies were produced by the use of different amounts of MPS, which could tune the compatibility between the polymer and TEOS. TEOS molecules
NU
can be expelled to the surface of the dispersed phase at a high MPS level or accumulated at the internal side of the dispersed phase at a low MPS level (Figure 6b). Subsequently, in the
MA
former case, SiO2 nanoparticles were formed through a sol–gel process of TEOS on the
AC
CE P
TE
D
surface to form raspberry-like nanoparticles (Figure 6c).
Figure 6. Schematic representation for the formation of silica/polymer nanoparticles via miniemulsion polymerization. Reprinted from ref. 88 with permission.
28
ACCEPTED MANUSCRIPT Similarly,
SiO2/poly(methyl
methacrylate-co-butyl
acrylate)[89]
SiO2/PS[90]
and
raspberry-like nanocomposites were also prepared through a similar synthetic process to that
T
outlined previously.[88] Furthermore, nanoparticles with more a complicated morphology, such
SC R
St using silica/PMMA nanocomposites as seeds.[91]
IP
as flower-like hybrid nanoparticles, were synthesized by seeded emulsion polymerization of
TiO2/PS raspberry-like hybrid nanoparticles could be prepared by simultaneous free-radical
NU
polymerization of St and hydrolysis–condensation of acetylacetone-chelated tetra-n-butyl titanate (TBT) in miniemulsions.[92] St and TBT were added to the dispersed phase of the
MA
miniemulsion. Free-radical polymerization of St mainly took place in the dispersed phase,
D
while TBT had to diffuse to the interface to undergo hydrolysis–condensation because of the
TE
absence of water in the monomer droplets. Consequently, most of the TiO2 nanoparticles distributed on the surface of the hybrid nanoparticles and formed a raspberry-like
CE P
morphology.
Raspberry-like nanocomposites may also be prepared by deposition of inorganic
AC
nanoparticles on the surface of preformed polymer nanoparticles. For example, poly(n-butyl methacrylate-co-N-vinyl-2-pyrrolidone) nanoparticles
were
prepared
(poly(BMA-co-NVP))
through
interfacially
amphiphilic
redox-initiated
core–shell
miniemulsion
polymerization.[93] Thereafter, poly(BMA-co-NVP) nanoparticles were decorated with NiS nanoparticles through the reaction of CH3CSNH2 with NiSO4 under γ-ray irradiation to form a raspberry-like morphology.
4.4 Section summary Raspberry-like
hybrid
nanoparticles
could 29
be
prepared
through
Pickering-type
ACCEPTED MANUSCRIPT miniemulsion polymerization, simultaneous reaction of vinyl monomers and inorganic precursors, or post-deposition of inorganic nanoparticles on the preformed polymeric
T
nanoparticles, respectively. The rough surface of raspberry-like hybrid nanoparticles may
IP
potentially be applied to prepare hybrid films with excellent superficial properties such as
SC R
superhydrophobicity or –hydrophilicity. Apart from common nonfunctional inorganic nanoparticles, functional inorganic nanoparticles, for example, fluorescent SiO2[83] or CeO2[80]
NU
nanoparticles, have been applied in recent years as stabilizers to prepare functional raspberry-like hybrid nanoparticles. These nanoparticles have great potential applications in
D
MA
catalysis, coatings, and biology.
TE
5. Hybrid nanoparticles consisting of polymer and metal salt or metal complex 5.1 Metal complex–polymer nanoparticles prepared in direct miniemulsions
CE P
For the preparation of metal complex–polymer nanoparticles, metal complexes are dissolved in monomers prior to the preparation of the miniemulsion, then the metal
AC
complex–polymer nanoparticles are prepared through polymerization of the metal complex containing monomer droplets in the miniemulsions. For example, Schreiber et al dissolved a series
of
hydrophobic
indium(III)acetylacetonate,
metal
complexes
such
as
zinc(II)tetramethylheptadionate,
platinum(II)acetylacetonate, zincphthalocyanine,
and
chromium(III)benzoylacetonate in monomers.[94] Narrowly size distributed metal-containing polymer nanoparticles were obtained through encapsulation of the metal complexes via miniemulsion polymerization.[94] These nanomaterials can form hexagonally well-ordered monolayers on various substrates because of their homogenous size distribution, leading to (potential) applications in nanolithography. Ordered arrays of metal or metal oxide 30
ACCEPTED MANUSCRIPT nanoparticles are formed through removing the organic components by plasma etching. The remaining metal (Pt) nanoparticles are employed as etching masks to prepare ordered arrays
T
of nanopillars by reactive ion etching.[95] Recently, Manzke et al. reported on nanoparticles
IP
containing two different metal complexes prepared through miniemulsion polymerization
AC
CE P
TE
D
MA
NU
SC R
finally resulting ordered arrays of Pt–Fe alloy nanoparticles (Figure 7). [96]
Figure 7. (A) Schematic representation of the miniemulsion process for the fabrication of metal complex containing PS nanoparticles. (B) Optical micrograph of a monolayer of Feand Pt-complex loaded PS nanoparticles coated by dip-coating onto a hydrophilic Si/SiO2 substrate. The inset of (B) presents an SEM image of PS nanoparticles at slightly reduced diameter by isotropic oxygen plasma treatment. (C) SEM image of hexagonally ordered oxidized FePt nanoparticles after further plasma and heat treatment. The inset of (C) shows that the FePt nanoparticles display a Gaussian size distribution with an average size of 6.8 ± 1.4 nm. Reprinted from ref. 96 with permission. 31
ACCEPTED MANUSCRIPT
Oil-soluble metal complexes have been incorporated in cross-linked polyacrylonitrile
T
nanoparticles through miniemulsion polymerization.[97] After thermal pyrolysis, inorganic
IP
nanoparticles were homogenously embedded in a porous, partially graphitic carbon matrix. A
SC R
wide variety of hybrid nanoparticles such as Fe3O4@C, Fe@C, and FeS2@C were synthesized through ex situ engineering of the embedded materials. Hybrid nanoparticles prepared
NU
through this technique showed enhanced lithium-storage performance compared with that of using bare materials.
MA
Hybrid nanoparticles for high-relaxivity magnetic resonance imaging-contrast agents have been prepared through miniemulsion polymerization with a gadolinium(III)-based
TE
D
metallosurfactant.[98] Magnetic resonance images produced by using the hybrid particles were much brighter than those obtained by using deionized water. Compared with that of the small
CE P
gadolinium-based molecule (Gd-DOTA), the relaxivity of the hybrid particles was markedly enhanced (2.4 times) probably because of the attachment of Gd(III) complexes to the slowly
AC
rotating colloids. In addition, double-modal imaging agents were synthesized by incorporating a fluorescent moiety with 5-acryloylaminofluorescein as the comonomer in the miniemulsions.
5.2 Metal salt–polymer hybrid nanoparticles prepared in inverse miniemulsions In inverse miniemulsions, the dispersed phase is composed of polar components. Hydrophilic metal salts used as lipophobes are required in these systems for achieving good droplet stability. Therefore, it is very convenient to incorporate hydrophilic salts in the polymeric nanogels through inverse miniemulsion polymerization. Cao et al. prepared a series 32
ACCEPTED MANUSCRIPT of transition-metal salt/poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hybrid nanogels in inverse miniemulsions (Figure 8).[99, 100] The size distribution of the hybrid nanogels was
T
narrow, and the loading of metal salts could be tuned within a wide range. Kobitskaya et al.
IP
prepared zinc-containing polyacrylamide hybrid nanogels in inverse miniemulsions.[101] The
SC R
hybrid nanogels, which showed a narrow size distribution, were used in nanolithography for a plasma-etching process by forming ordered monolayers of metal oxide nanoparticles. Silver
NU
salts were also incorporated in poly(HEMA) nanogels in inverse miniemulsions.[102] Silver/polymer hybrid nanogels were formed through in situ reduction of silver ions by
AC
CE P
TE
D
MA
gaseous diffusion of hydrazine to the dispersed phase.
33
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
Figure 8. Representative TEM images of narrowly size-distributed hybrid nanogels containing (A) 2.3 mol% Fe(BF4)2, (B) 5.4 mol% Co(NO3)2, (C) 6.5 mol% CoCl2, and (D) 4.3 mol% Fe(NO3)3. Nanogels were synthesized by inverse miniemulsion polymerization of HEMA. Reprinted from ref. 100 with permission.
Magnetic polyvinyl alcohol (PVA) nanogels were prepared in inverse miniemulsion through a multistep process.[103] First, an acidic aqueous solution of PVA and ferrous and ferric ions was dispersed in a hydrophobic continuous phase by sonication to form an inverse 34
ACCEPTED MANUSCRIPT miniemulsion. Second, the PVA molecules were cross-linked by addition of glutaraldehyde. Finally, Fe3O4 nanoparticles were formed on the surface through coprecipitation of Fe2+ and
IP
T
Fe3+ with triethylamine.
SC R
5.3 Section summary
Various metal salts or metal complexes may be conveniently incorporated into a polymer
NU
matrix in both direct and inverse miniemulsions. The metal salts or metal complexes must be dissolved in the monomer mixture prior to polymerization. Compared with the heterogeneous
MA
distribution of inorganic nanoparticles in miniemulsion droplets, dissolved metal salts or
D
metal complexes are homogenously distributed in miniemulsion droplets. It should be pointed
TE
out that the loading of hydrophobic metal complexes through direct miniemulsion polymerization is sometimes limited because of the low solubility of metal complexes in the
CE P
hydrophobic monomer mixture. In contrast, loading of hydrophilic metal salts in inverse miniemulsions is much easier because of the high solubility of metal salts in polar monomer
AC
mixtures. In addition, hydrophilic metal salts behave as lipophobes that improve droplet stability. Inorganic nanoparticle/polymer hybrid nanoparticles may be conveniently formed through reaction of the loaded metal salts. In contrast to direct encapsulation of inorganic nanoparticles by polymers, the indispensable surface modification of inorganic nanoparticles can be avoided.
6. Hybrid nanocapsules Miniemulsion polymerization has been proved to be an efficient method for the preparation of nanocapsules using liquid droplets as templates. Recent advances in this method have been 35
ACCEPTED MANUSCRIPT summarized in our review.[10] In the present contribution, we focus on the recent progress in research on hybrid nanocapsules, that is, nanocapsules with a hybrid shell or polymeric
6.1 Nanocapsules with a hybrid shell
SC R
IP
T
nanocapsules containing inorganic components or natural macromolecules.
The miniemulsion technique allows the preparation of nanoparticles of organometallic polymers.
A
miniemulsion-based
technique,
miniemulsion
periphery
NU
coordination
polymerization (MEPP), was designed by Wang’s group to prepare Prussian Blue (PB)
MA
nanoparticles with various particle morphologies (Figure 9).[104-106] In detail, an
D
organometallic surfactant, poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene
(EPE-Fe),
was
used
TE
glycol) (PEG-PPG-PEG) terminated with pentacyano(4-(dimethylamino)pyridine)ferrate to
stabilize
a
toluene
miniemulsion.
Polymerization
of
CE P
EPE-Fe-terminated PEG-PPG-PEG by addition of Fe3+ at the surface of the nanodroplets was carried out to form amorphous PB spherical nanoshells and crystallized cubic nanoshells by
AC
using EPE-Fe surfactant with 100% end functionality[104] and EPE-Fe surfactant with ≤60% end functionality,[105] respectively. Recently, PPy/PB core–shell nanoparticles were prepared in miniemulsions by simultaneous polymerization of PB on the surface of nanodroplets and of pyrrole in the nanodroplets.[106] Compared with that of PB nanoshells, the fluorescence of the PPy/PB core–shell particles was enhanced because of charge transfer from PPy to PB. Silyl-terminated surfactants could be also used as precursor to form organosilica nanocapsules through MEPP.[107] The shell of these nanocapsules was cross-linked, but the thickness of the shell was thin enough to allow ions to permeate. Organosilica nanocapsules may potentially be used as high-ion-conductivity nanoelectrolytes. 36
IP
T
ACCEPTED MANUSCRIPT
NU
MEPP. Reprinted from ref. 104 with permission.
SC R
Figure 9. Schematic representation of Prussian Blue (PB) nanoshells produced through
Organic–inorganic hybrid nanocapsules with an oily core were prepared through
MA
miniemulsion copolymerization of St, divinylbenzene (DVB), MPS, and N-isopropyl
D
acrylamide (NIPAM).[108] The inorganic moiety was formed through hydrolysis–condensation
TE
of MPS. Droplet formation and nucleation mechanisms of this process were investigated.
particles.
CE P
Homogenous nucleation was considered as the main reason for the formation of solid
Hollow spheres with polymer/silica double shells were prepared through polymerization of
AC
MMA and DVB and subsequent hydrolysis–condensation of TEOS in miniemulsions.[109] TEOS functioned as soft template for forming the voids in the polymerized MMA. Phase separation between the resulting polymers and TEOS played a critical role in the formation of the capsule morphology. Magnetic hollow spheres were synthesized by loading magnetic Fe3O4 nanoparticles in the droplets of the miniemulsions prior to the reaction. Through an analogous technique, Janus magnetic microcapsules were synthesized in a magnetic field. The magnetic field could drive the magnetic nanoparticles to move to one side of the nanodroplets of the miniemulsions.[110]
37
ACCEPTED MANUSCRIPT 6.2 Polymeric nanocapsules with inorganic or naturally occurring macromolecular containers
T
Analogously to MEPP, Utama et al. designed an inverse miniemulsion periphery RAFT
IP
polymerization technique to prepare hollow polymeric nanoparticles.[111] The key of this
SC R
technique is the use of an amphiphilic RAFT agent to confine the polymerization on the surface of droplets. A protein, bovine serum albumin (BSA), was encapsulated without
NU
denaturation through this technique. The permeability of the polymeric shell allows the release of BSA. Therefore, these polymeric nanocapsules are potential nanocarriers for
MA
protein delivery.
D
Starch dissolved in the aqueous droplets of inverse miniemulsions could be cross-linked
TE
with the hydrophobic reagent, 2,4-toluene diisocyanate, at the surface of droplets through polyaddition.[112] The resulting cross-linked starch nanoparticles displayed a capsule
CE P
morphology. Double-stranded DNA (dsDNA) could be encapsulated in the cross-linked starch nanocapsules through a similar process. Polymerase chain reaction (PCR) in the nanocapsules
AC
was carried out to amplify the incorporated dsDNA chains. Baier et al. prepared polymer nanocapsules containing dsDNA through a reverse method.[113] They first amplified a single-molecule dsDNA template by PCR in miniemulsion droplets. Subsequently, poly(n-butylcyanoacrylate) shells were formed on the miniemulsion droplets through interfacial anionic polymerization. Anionic (SDS), cationic (CTMA-Cl), and nonionic (Lutensol AT50) surfactants were used to prepare aqueous dispersions of the nanocapsules. HeLa cells showed higher uptake of nanocapsules stabilized by SDS compared with CTMA-Cl or Lutensol AT50. All the nanocapsules showed low cytotoxicity to HeLa cells. In addition, the incorporated DNA molecules could be released upon degradation of the 38
ACCEPTED MANUSCRIPT poly(n-butylcyanoacrylate) shells. Cao et al. prepared poly(N-isopropyl acrylamide) (PNIPAM) nanocapsules with narrow
T
size distribution in inverse miniemulsion by using aqueous metal-salt droplets as
IP
templates.[114] Formation of the capsule morphology was induced by internal phase separation
SC R
of the polymer in the droplets. The inorganic metal salts were encapsulated by a PNIPAM shell, which displayed reversible thermosensitivity. Similarly, pH- and thermo-responsive
NU
nanocapsules were prepared by inverse miniemulsion copolymerization of NIPAM and 4-VP
MA
using aqueous metal-salt droplets as templates.[115]
D
6.3 Section summary
TE
Because of its use of soft droplet templates, the miniemulsion technique shows high flexibility in the preparation of hybrid nanocapsules. A hybrid shell of nanocapsules could be
CE P
prepared by concurrent formation of polymeric and inorganic moieties by using monomers with different reactive groups, or by direct polymerization of hybrid organometallic molecules.
AC
Hybrid nanocapsules could be formed through encapsulation of preloaded inorganic salts or natural macromolecules in miniemulsion droplets. Hybrid capsules prepared in miniemulsions can be used in various fields, including biology, medicine, catalysis, coatings, and cosmetics.
7. Hybrid polymeric nanoparticles 7.1
Preparation
of
hybrid
polymeric
nanoparticles
through
miniemulsion
(co)polymerization in the presence of another type of polymer Nanoparticles composed of polymeric moieties prepared through various polymerization mechanisms may be regarded as another type of hybrid nanoparticles. PU–acrylic polymer 39
ACCEPTED MANUSCRIPT hybrid nanoparticles were prepared through simultaneous free-radical and addition polymerization in miniemulsion by using acrylic monomers, a hydroxyl-functionalized 117]
T
methacrylate, an isocyanate functional PU prepolymer, and a diol as reactants.[116,
IP
Covalent linkage of PU to the acrylic polymer could improve the compatibility between these
SC R
two types of polymers. Such hybrid polymer nanoparticles could be used as pressure-sensitive adhesives (PSAs). Introduction of PU to the hybrid copolymer could improve the cohesion of
NU
the copolymer through hydrogen bonds. The acrylic–PU PSAs showed enhanced shear resistance and reduced tackiness because of the high gel content and high elasticity of the
MA
hybrid network containing the PU segments.[116] Therefore, a balance between elastic and
D
viscous properties was required to produce PSAs with a better overall adhesive performance.
TE
Lopez et al. investigated the influence of the type of diol (chain extender) on the reaction kinetics, microstructure of the hybrid copolymer, and adhesive performance of acrylic–PU
CE P
PSAs.[117] They found that the PSAs using bisphenol A as chain extender showed better overall adhesive performance than those with other diols such as 1,6-hexanediol and
AC
1,4-cyclohexanediol. To achieve high tackiness, the presence of low-molecular-weight polymers is required because they can diffuse quickly to the interface. However, the polymer chains should be long enough to form entanglements, which can dissipate the energy during pulling of PSAs. The chain length of polymers could be conveniently tuned by the addition of a chain-transfer agent (CTA). Lopez et al. investigated the influence of the CTA, dodecylthiol, on the kinetics, polymer microstructure, particle morphology, and adhesive performance of acrylic–PU hybrid nanoparticles prepared through simultaneous free-radical and addition polymerization in miniemulsions.[118] They found that the optimum adhesive performance could be achieved by addition of a suitable amount of CTA. Promisingly, acrylic–PU hybrid 40
ACCEPTED MANUSCRIPT nanoparticles with a high solid content could be prepared in miniemulsions.[119] Photoinitiated polymerization may be carried out under mild conditions at a high rate. Daniloska et al.
T
prepared waterborne acrylic–PU hybrids through photoinitiated miniemulsion polymerization
IP
in a continuous tubular reactor.[120] Its high polymerization rate resulted in almost complete
SC R
conversion within a short residence time (< 5 min) at a low photoinitiator concentration (< 0.48 wt% to monomer) under modest irradiance (< 7 mW/cm2). The droplet nucleation
NU
mechanism dominated during the entire polymerization process.
Alkyds, a type of traditional binders for coatings, were added to the miniemulsion
MA
polymerization system of acrylic monomers to prepare alkyd/acrylic nanocomposites.[121]
D
Kinetic investigations showed that the polymerization reached a conversion limit. The limited
TE
conversion could be ascribed to chain transfer to alkyds forming unreactive radicals stabilized by conjugation of double bonds. The particle morphology strongly depended on the
CE P
hydrophobicity of the alkyds. Hydrophobic alkyds formed core–shell nanocomposites, whereas less hydrophobic alkyds formed hemispherical nanocomposites. Minari et al.
AC
established a method based on conventional size-exclusion chromatography to evaluate the degree of resin grafting, degree of acrylic grafting, number of reacted double bonds in the alkyd, gel content, and molecular-weight distribution of the sol part of the alkyd/acrylic hybrid nanoparticles prepared in miniemulsions.[122] Poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene],
a
fluorescent
conjugated
polymer with high quantum yield, was encapsulated with PS to prepare fluorescent hybrid nanoparticles through miniemulsion polymerization.[123] The emission wavelength of the fluorescent nanoparticles could be tuned by varying the synthesis parameters. The surface of the hybrid nanoparticles was modified with carboxyl groups using acrylic acid as a 41
ACCEPTED MANUSCRIPT comonomer.
Preparation
of
hybrid
polymeric
nanoparticles
through
miniemulsion
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7.2
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copolymerization
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Hybrid polymeric nanoparticles could be prepared via miniemulsion copolymerization of vinyl-containing silane and other vinyl monomers. MPS is often used as a vinyl-containing
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silane because of its good copolymerization capability with other vinyl monomers.[124, 125] MPS-containing hybrid particles may be used to prepare hybrid films. During film formation,
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the methoxy groups of MPS undergo hydrolysis and condensation to form cross-linked
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structures. Therefore, the number of MPS units in the copolymer has a significant influence
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on the mechanical properties of hybrid films. For example, Ramos-Fernández et al. found that
content.[126]
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the stiffness of hybrid films increased while the flexibility decreased with an increase in MPS
Preparation of biobased polymers is of high interest because fossil fuel resources dwindle.
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The journal Macromolecules had launched in 2009 a virtual issue “Polymers from Renewable Resources” to address the importance of design and synthesis of polymers from renewable resources.[127] Poly(n-butyl methacrylate)-g-polylactide nanoparticles were prepared via solvent-free miniemulsion copolymerization of methacryloyl-end-functionalized polylactide and n-butyl methacrylate.[128] The synthesis process excluded the use of organic solvents and the copolymer film showed elastic properties.
7.3 Section summary One method for preparing hybrid polymeric nanoparticles involves a process in which one 42
ACCEPTED MANUSCRIPT polymer is dissolved in a monomer mixture, which is then homogenized to form a miniemulsion. Thereafter, hybrid polymeric nanoparticles are prepared through miniemulsion Alternatively,
these
nanoparticles
may
be
prepared
through
T
(co)polymerization.
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copolymerization of two different types of monomer simultaneously or by grafting
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polymerization using a vinyl-containing natural polymer. Many properties of hybrid polymers may be improved through the synergistic contribution of the various polymeric components.
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For example, acrylic–PU hybrid nanoparticles may be used as high-performance PSAs. MPS-containing hybrid nanoparticles may be used as self-cross-linking coatings through
D
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hydrolysis–condensation of MPS in the film-formation processes.
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8. Natural macromolecule–synthetic polymer nanoparticles 8.1 Preparation of natural macromolecule–synthetic polymer nanoparticles
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Biomacromolecules like proteins or DNA are sensitive to external conditions such as solvent and temperature. Harsh environments may lead to the deactivation of enzymes and to
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denaturation of proteins. Incorporation of biomacromolecules into synthetic polymer matrixes may improve the chemical and biological stabilities of biomacromolecules in harsh external environments.[129-132] For example, Candida rugosa lipase behaves inactive in anhydrous dimethyl sulfoxide (DMSO), however, its original catalytic activity for transesterification can be retained after encapsulation in a polyacrylamide (PAAm) matrix.[129] The PAAm matrix helps to maintain the native configuration of incorporated lipase by shielding the extraction of essential water in anhydrous DMSO at elevated temperatures.[129] Protein–polymer hybrids have been intensively investigated for decades. To covalently link proteins with polymers, two steps are required in most cases, namely, attachment of vinyl groups to proteins and 43
ACCEPTED MANUSCRIPT copolymerization of modified proteins and common monomers. A commonly used modifier is N-hydroxysuccinimide (NHS) acrylate, because NHS can couple with amine groups to form
T
stable amide bonds.[133] Proteins usually contain more than one amine group which often
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originate from lysine residues, and NHS may react with amine groups randomly.[133]
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Therefore, binding of NHS to proteins is largely nonspecific, resulting in possible deactivation of the protein active sites. Recently, Averick et al. reported that the initiating
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amino acid could be expressed at a specific site of the green fluorescent protein (GFP) through genetic engineering; therefore, damage to the protein’s active sites and structure
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could be avoided during modification.[134] Subsequently, GFP-nanogels with a particle size of
D
240 nm were prepared through inverse miniemulsion AGET ATRP of hydrophilic monomers
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and modified GFP. According to the results of UV–vis and fluorescence spectroscopy, the activity of covalently attached GFP was well preserved. Poly(acrylic acid) (PAA)/gelatin
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interpenetrating polymer network (IPN) nanogels were prepared through inverse miniemulsion polymerization of acrylic acid in the presence of gelatin.[135] To prepare IPN
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nanogels, gelatin was cross-linked with glutaraldehyde, and PAA was cross-linked with N,N-methylene bisacrylamide. IPN nanogels that bear numerous functional groups such as –COOH and –NH2 have a high potential as vehicles for targeted drug delivery. Ethyl cellulose (EC)/acrylic hybrid polymeric nanoparticles were prepared through miniemulsion copolymerization of MMA and BA in the presence of EC.[136] The use of a water-soluble initiator did not yield hybrid nanoparticles because of incompatibility between EC and acrylic copolymers. The compatibility between EC and acrylic copolymers was improved by grafting of acrylic polymer radicals to EC using an oil-soluble initiator, dilauroyl peroxide. Furthermore, addition of a cross-linker can also promote the formation of 44
ACCEPTED MANUSCRIPT EC/acrylic hybrid polymer nanoparticles by entrapping EC physically in the acrylic polymer matrix. Compared with those of the plain acrylic copolymer and EC/acrylic copolymer blend,
improved. matrixes
may
be
reinforced
whiskers/poly(styrene-co-hexylacrylate)
using
cellulosic
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Polymer
IP
T
the thermal and mechanical properties of EC/acrylic hybrid polymer nanoparticles were
nanocomposites
were
substrates.
Cellulose
prepared
through
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miniemulsion copolymerization of St and hexylacrylate in the presence of the whiskers.[137] The loading of cellulose whiskers was varied from 1 to 5 wt% relative to the polymer weight.
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Addition of a vinyl-containing silane, MPS, could improve the colloidal stability of the
D
systems. The storage modulus of a hybrid film prepared from nanocomposites containing 5 wt%
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whiskers was significantly enhanced compared with that of the plain polymer film. In a subsequent report, the authors investigated the influence of silane content on the particle
films.[138]
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properties of the nanocomposites and on the thermal and mechanical properties of the hybrid
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Natural rubber latex (NRL) was modified through grafting polymerization of acrylic monomers in miniemulsion.[139] Thermal and morphological properties of the polymer-grafted NRL were investigated.
8.2 Section summary In the presence of natural macromolecules, natural macromolecule/synthetic polymer hybrid nanoparticles may be prepared through miniemulsion (co)polymerization. Such polymer hybrid nanoparticles may have improved chemical and biological stabilities compared with those of natural macromolecules such as proteins and DNA. They may also 45
ACCEPTED MANUSCRIPT have improved biocompatibility and thermal and mechanical properties compared with those
T
of hybrid nanoparticles.
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9. Concluding remarks and perspectives
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The miniemulsion technique shows high ability and flexibility in the preparation of versatile hybrid nanoparticles. It may be carried out through encapsulation of inorganic
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nanoparticles, inorganic metal salts, and natural macromolecules, or through adsorption of inorganic or natural-polymer nanoparticles. Various reactions such as free-radical
MA
polymerization, polyaddition, polycondensation, and sol–gel processes of inorganic
D
precursors may be performed in miniemulsion droplets. This allows the preparation of hybrid
TE
polymeric nanoparticles by simultaneous formation of at least two types of polymers through different reaction mechanisms. Compared with plain polymer nanoparticles, the hybrid
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nanoparticles may display better properties or additional functions. For example, they may show improved thermal stability and mechanical properties, optical, electrical, and magnetic
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properties, and environmental stimuli-responsiveness. This introduces possible applications of hybrid nanoparticles in various fields, such as biology, medicine, semiconductors, coatings, and cosmetics. Research has focused on the preparation of versatile hybrid nanoparticles in miniemulsions, however, the quest for applications has not been given as much effort. For example, application of magnetic or fluorescent hybrid nanoparticles as diagnostic tools is mainly restricted to in vitro cell experiments. For biological or medical applications, extensive biological research and in vivo experiments are necessary. More engineering research into aspects of the preparation of hybrid nanoparticles is required for industrial production. These 46
ACCEPTED MANUSCRIPT include reaction kinetics, kinetic modeling of reactions, and reactor type. Bourgeat-Lami et al. provided a good example for investigating the entire process of
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encapsulation of SiO2 nanoparticles through miniemulsion polymerization using cryo-TEM
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and other techniques.[39] However, a great number of mechanistic investigations on various
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reaction systems should be carried out to improve our understanding of these complicated systems. A deep understanding of the mechanisms would help improve control of the
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synthesis process and properties of the hybrid nanocomposites.
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Acknowledgement. Financial supports from National Natural Scientific Foundation of China
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(NNSFC) project (51003023) and open Foundation of Zhejiang Provincial Top Key Academic
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Graphical Abstract
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ACCEPTED MANUSCRIPT Highlights
were summarized. nanoparticles
include
organic-inorganic,
macromolecule/synthetic polymer nanoparticles.
polymeric,
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Hybrid
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The recent advances in the preparation of hybrid nanoparticles in miniemulsions since 2009
and
natural
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Hybrid nanoparticles can be prepared through encapsulation of a second moiety by polymers in miniemulsions.
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Hybrid nanoparticles can be prepared through in-situ formation of a second moiety in a
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nanoscale polymeric matrix in miniemulsion.
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Characterization, properties, and applications of hybrid nanoparticles are also discussed.
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S0001-8686(14)00195-X doi: 10.1016/j.cis.2014.06.001 CIS 1447
To appear in:
Advances in Colloid and Interface Science
Received date: Revised date: Accepted date:
13 March 2014 31 May 2014 1 June 2014
Please cite this article as: Qi Dongming, Cao Zhihai, Ziener Ulrich, Recent Advances in the Preparation of Hybrid Nanoparticles in Miniemulsions, Advances in Colloid and Interface Science (2014), doi: 10.1016/j.cis.2014.06.001
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ACCEPTED MANUSCRIPT Recent Advances in the Preparation of Hybrid Nanoparticles in Miniemulsions Dongming Qi1, Zhihai Cao1*, Ulrich Ziener2 1
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Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education,
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Zhejiang Sci-Tech University, Hangzhou, 310018, China 2
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Institute of Organic Chemistry III – Macromolecular Chemistry and Organic Materials, University of Ulm,
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Corresponding author: [email protected]
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Albert-Einstein-Allee 11, Ulm, 89081, Germany
Abstract. In this review, we summarize recent advances in the synthesis of hybrid in
miniemulsions
since
2009.
These
hybrid
nanoparticles
include
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nanoparticles
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organic–inorganic, polymeric, and natural macromolecule/synthetic polymer hybrid
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nanoparticles. They may be prepared through encapsulation of inorganic components or natural macromolecules by miniemulsion (co)polymerization, simultaneous polymerization of
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vinyl monomers and vinyl-containing inorganic precursors, precipitation of preformed polymers in the presence of inorganic constituents through solvent displacement techniques, and grafting polymerization onto, from or through natural macromolecules. Characterization, properties, and applications of hybrid nanoparticles are also discussed. Keywords: hybrid nanoparticles; miniemulsion; nanocomposites; polymerization.
1. Introduction Hybrid nanostructured materials that combine advantageous properties of at least two types of materials have attracted much attention for decades.[1, 1
2]
Polymer-based hybrid
ACCEPTED MANUSCRIPT nanoparticles may be the most frequently investigated hybrid nanostructured materials.[3, 4] For example, incorporation of inorganic nanomaterials into a nanoscale polymer matrix is a
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common method to prepare organic–inorganic hybrid nanoparticles. They may be used as
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building blocks to prepare hybrid films with enhanced mechanical, optical, and electric
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properties, thermal stability, UV-shielding properties, and with magnetism.[5, 6] Miniemulsion polymerization may be the most active branch of heterophase polymerization in recent years.
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One of the important factors that sustain activity of miniemulsion polymerization is its strong capability to prepare versatile hybrid nanoparticles. In a typical miniemulsion system, the
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monomer mixture is dispersed in a continuous phase through sonication or high-pressure
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homogenization.[7] The size range of monomer droplets is 50 to 500 nm. Droplet nucleation is
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the dominant pathway for forming latex particles in miniemulsions. Monomer droplets function as polymerization loci without requiring monomer diffusion. Miniemulsion reaction
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systems are highly suitable for the preparation of hybrid nanoparticles through encapsulation of a second moiety in the miniemulsion droplets, as they utilize the droplet nucleation
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mechanism.[8] In 2009, Landfester published a milestone review of the application of the miniemulsion technique in the preparation of structured hybrid nanoparticles.[8] In 2011, Wu et al. highlighted the development of the synthesis of organic–inorganic nanocomposites in miniemulsions.[9] In addition, the preparation of hybrid nanoparticles in miniemulsions is partially covered by other recent reviews.[4, 10-13] In the present review, recent advances in the preparation of hybrid nanoparticles in miniemulsions since 2009 are comprehensively summarized. The article is organized on the basis of the type of hybrid nanoparticles. Unlike previous reviews, this article also includes hybrid polymer nanoparticles and natural macromolecules/synthetic polymer hybrid 2
ACCEPTED MANUSCRIPT nanoparticles, as well as organic–inorganic hybrid nanoparticles and nanocapsules. Here, we consider nanoparticles that are composed of at least two types of polymers formed through
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2. Brief introduction to miniemulsion systems
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various polymerization mechanisms as hybrid polymer nanoparticles.
The basic concept, emulsification technique, stabilization mechanism, nucleation
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mechanism, reaction, and reaction kinetics of miniemulsions have been summarized in several classic reviews.[7, 14, 15] Therefore, we only give a brief introduction to miniemulsion in
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the present review. Normally, the preparation of nanoparticles from miniemulsion systems
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includes three steps: pre-emulsification of two heterogenous phases to prepare crude
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(macro)emulsions, homogenization of the crude emulsions to prepare miniemulsions, and reactions in miniemulsions to prepare nanoparticles (Figure 1). On the basis of polarity of the
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dispersed and continuous phase, miniemulsions can be divided into two types, namely inverse and direct miniemulsion systems. In direct miniemulsions, the polarity of the continuous
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phase is higher than that of the dispersed phase, while in inverse miniemulsions, the polarity of the continuous phase is lower than that of the dispersed phase. An aqueous solution of a surfactant is commonly used as continuous phase in direct miniemulsions. A hydrophobic solution of a surfactant is used as continuous phase in inverse miniemulsions. Commonly used hydrophobic solvents are cyclohexane, toluene, hexadecane, and Isopar M (a mixture of C12–C14 hydrocarbons). Direct miniemulsion systems can be used for the preparation of hydrophobic nanoparticles, while inverse miniemulsions can be applied for the preparation of hydrophilic ones.
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Figure 1. Schematic representation of preparation of and reaction (polymerization) in
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miniemulsions.
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Surfactants are required for stabilizing miniemulsion systems preventing droplets from
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coalescence. In principle, surfactants with a high hydrophilic–lypophilic balance (HLB) value are suitable for direct systems, while surfactants with a low HLB value are used to stabilize
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inverse miniemulsions. Molecular diffusion between droplets driven by the Laplace pressure can destabilize the droplets. The diffusion can be suppressed by the addition of costabilizers to the dispersed phase, because costabilizers establish an osmotic pressure to counteract the Laplace pressure. Compounds with an extremely low solubility in water such as hexadecane are used as costabilizer in direct miniemulsions. On the contrary, compounds with an extremely low solubility in the hydrophobic continuous phase like hydrophilic salts were used as costabilizer in inverse miniemulsions. In principle, high-energy emulsification techniques are required for preparing droplets with a size of 50–500 nm. Sonication may be the most convenient technique to prepare 4
ACCEPTED MANUSCRIPT miniemulsions in a lab scale. For industrial applications, high-pressure homogenization may be more suitable than sonication.
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In the infancy of miniemulsions, free-radical polymerization of vinyl monomers were the
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most used reaction types in miniemulsions. In the meantime, various reaction types have been
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carried out in miniemulsions, for example controlled/living radical polymerization, polyaddition, anionic polymerization, polycondensation, sol–gel process, and precipitation
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reaction of inorganic precursors. Reactants can be introduced to the dispersed phase prior to preparation of the miniemulsion or through post-addition through the continuous phase.
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However, reactions only take place in the droplets or at the interface between the dispersed
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and continuous phases, but not in the continuous phase. For the preparation of hybrid
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nanoparticles, a second moiety can be pre-loaded in the droplets, and hybrid nanoparticles are
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formed through a subsequent reaction.
3. Inorganic nanoparticle–polymer nanoparticles prepared through nanoencapsulation
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in miniemulsions
3.1 Metal–polymer nanoparticles Metal nanoparticles may display various advantageous properties, including high catalytic activity, high optical absorption/emission, and (superpara)magnetism.[16, 17] They have wide applications in the field of semiconductors, catalysis, electronics, and so on. However, direct use of metal nanoparticles with a size of a few nanometers to several tens of nanometers is met with difficulties because metal nanoparticles with such a small size are subject to agglomeration. Incorporation of metal nanoparticles into a nanoscale polymeric matrix can prevent agglomeration of these small nanoparticles. Van Berkel and Hawker prepared noble 5
ACCEPTED MANUSCRIPT metal/poly(divinylbenzene) hybrid nanoparticles via the miniemulsion technique (Figure 2).[18] Surface modification of the metal nanoparticles with polymer chains is critical to the
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successful incorporation of metal nanoparticles in the polymeric matrix. The hybrid
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nanoparticles display good solvent resistance due to their highly cross-linked structure. They
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can be redispersed uniformly in polar solvents such as tetrahydrofuran after further
Figure
2.
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modification with poly(ethylene glycol) through a thiol-ene reaction.[18]
Transmission
electron
microscopy
(TEM)
image
of
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gold–poly(divinylbenzene) hybrid nanoparticles. Inset at the top left shows a magnified image of a hybrid nanoparticle. Reprinted from ref.18 with permission.
Destabilization of Au nanoparticles may take place in some systems reacting at elevated temperatures because of desorption of alkanethiols from the gold surface.[19] To avoid such destabilization, Fuchs et al. prepared Au/poly(methyl methacrylate) (PMMA) hybrid nanoparticles through a photoinitiated miniemulsion polymerization process at low reaction temperature.[20] The thiol-capped Au nanoparticles were well dispersed in the PMMA matrix. The authors attributed the encapsulation and good dispersion of the Au nanoparticles in the 6
ACCEPTED MANUSCRIPT PMMA matrix to the low reaction temperature. Koh et al. prepared Ag-decorated polystyrene (PS) nanocapsules containing isopropyl myristate (IPM) in miniemulsions.[21] The
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dodecanethiol-capped Ag nanoparticles were predispersed in IPM prior to polymerization.
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Polyaniline has excellent electronic properties, but its insolubility and immiscibility with
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nonconducting polymers limits its application as electron-transport material. Modification of polyaniline with metal or metal oxides is an effective method of achieving improved
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performance of polyaniline as a sensor. Ag/polyaniline hybrid nanoparticles were prepared through miniemulsion polymerization.[22] The Ag/polyaniline nanocomposites were used to
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prepare ethanol-responsive films that showed higher stability and reproducibility in ethanol
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vapors compared with pure polyaniline.
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Normally, surface modification is required to improve the dispersion of metal nanoparticles in a monomer solution. Mamaghani et al. reported the encapsulation of unmodified
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hydrophilic silver nanoparticles through miniemulsion copolymerization of acrylic monomers.[23] However, morphological investigation by transmission electron microscopy
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(TEM) did not give strong evidence for the attachment of the silver nanoparticles to the polymer nanoparticles. Metal/hydrophilic polymer hybrid nanoparticles may be prepared through polymerization of hydrophilic monomers in inverse miniemulsions. For example, gold nanoparticles were incorporated in a hydrophilic polymer matrix through inverse miniemulsion atom transfer radical polymerization (ATRP) for biological applications.[24] The hybrid nanogels could be taken up by MC3T3-E4 mouse calvarial osteoprogenitor cells. Surface modification of the nanogels with glycine-arginine-glycine-aspartic acid-serine peptides improved the cell attachment and internalization. 7
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3.2 Nanoclay–polymer nanoparticles
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To improve the thermal stability and mechanical properties of polymers, inorganic
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nanofillers such as nanoclays are often combined with polymers to prepare organic–inorganic
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composites. Nanoscale blending of nanoclay and polymer may impart better thermal and mechanical properties to nanoclay/polymer composites than macroscale blending. The
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miniemulsion technique has proved to be an efficient method for preparing nanoclay/polymer nanocomposites. Large amounts of clay (30–50 wt%) have been loaded in the polymer matrix 26]
In this process, inorganic nanoparticles and
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by ad-miniemulsion polymerization.[25,
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monomers are emulsified separately to form two miniemulsions, which are then mixed and
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sonicated to form hybrid miniemulsions. The particle morphology is significantly influenced by the size of the clay particles and the reactivity of modifiers. Armored nanoparticles were
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formed in mixtures with large clay particles such as montmorillonite, irrespective of the type of modifier. In mixtures with small clay particles such as chemically modified laponite, the
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clay particles were encapsulated in the hybrid nanoparticles, while corresponding physically modified materials were mainly adsorbed to the surface of the hybrid nanoparticles. Cloisite 30B, an organically modified montmorillonite, was encapsulated with poly(styrene-co-butyl acrylate) through miniemulsion copolymerization.[27] Exfoliated Cloisite 30B was mainly located inside the nanocomposites at an encapsulation efficiency of about 73%. Encapsulation of organomodified montmorillonite by PMMA through ATRP in miniemulsions led to hybrid nanoparticles with a uniform size distribution.[28] Hatami et al. investigated the nucleation type, kinetics, and polymerization control of ATRP miniemulsion copolymerization of styrene (St) and methyl methacrylate (MMA) in the presence of 8
ACCEPTED MANUSCRIPT nanoclay.[29] They revealed that droplet nucleation was the dominant mechanism. The polymerization rate and molecular weight of the copolymer decreased with increased clay
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loading. Khezri et al. prepared Cloisite 30B/poly(styrene-co-methyl methacrylate)
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nanocomposites by using miniemulsion activators generated by electron transfer (AGET)
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ATRP.[30] Clay/poly(styrene-co-butyl acrylate) nanocomposites could also be prepared through ATRP in miniemulsions.[31] As expected, ATRP produced polymers with a molecular
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weight distribution narrower than that of polymers prepared through conventional free-radical polymerization. The resultant nanocomposites showed enhanced thermal stability compared
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with the pure polymers.
Capek investigated the kinetics of photo-initiated miniemulsion polymerization of butyl
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acrylate (BA) in the presence of clay.[32] He reported the polymerization in the presence of sodium dodecyl sulfate (SDS) with a final conversion of about 60%. However, the final
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conversion only reached about 20% when cetyltrimethylammonium bromide was used as surfactant.
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Interestingly, asymmetric saponite/PS nanoparticles (e. g. hemispheres or truncated spheres) have been prepared via miniemulsion polymerization using nanosaponites with a size larger than the equilibrated droplet size of the miniemulsion.[33] The authors suggest that the asymmetric nanoparticles are formed by asymmetric growth of polymer chains on one side of the nanosaponite platelets when monomer droplets only adsorbs on one side of the platelets. The asymmetric nanoparticles may also be propared through disintegration of intact nanoparticles symmetrically grown on both sides of the nanosaponite platelets into hemispheres.
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ACCEPTED MANUSCRIPT 3.3 SiO2–polymer nanoparticles Similarly to other inorganic nanoparticles, surface modification of SiO2 nanoparticles is
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important for the successful incorporation of SiO2 into a polymer matrix. The surface of silica
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nanoparticles may be modified chemically with various silanes or physically with ionic
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surfactants. Silane-modified SiO2 nanoparticles have been encapsulated with polyacrylate through miniemulsion polymerization.[34] With the increase in silica content in the
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nanocomposites, the tensile strength of hybrid films increased, while the elongation at break decreased. Silane-modified silica nanoparticles were also encapsulated through a reversible
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addition–fragmentation chain transfer (RAFT) polymerization process in miniemulsion.[35]
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Incorporation of SiO2 in the polymer matrix had positive influence on the thermal behavior of
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the polymer. The resultant polymers prepared by RAFT miniemulsion polymerization showed a narrow molecular weight distribution.
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SiO2 nanoparticles could also be modified with hydroxylated silicone oil. The modified particles were encapsulated with poly(dimethylsiloxane) (PDMS) through ring-opening of
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polymerization
octamethylcyclotetrasiloxane
in
miniemulsions.[36]
The
resultant
SiO2/PDMS nanocomposites displayed a well-defined core–shell morphology. The hybrid film prepared from the SiO2/PDMS nanocomposites showed improved mechanical and thermal properties compared with pure PDMS. The concentration of surfactant is a crucial parameter in miniemulsions, and may influence colloidal stability, droplet stability, nucleation mechanism, as well as droplet and particle size. Hecht et al. investigated the influence of surfactant concentration on miniemulsion polymerization for encapsulation of SiO2 nanoparticles.[37] The SiO2 nanoparticles were modified chemically with γ-trimethoxysilylpropylmethacrylate (MPS) or physically by 10
ACCEPTED MANUSCRIPT cetyltrimethylammonium chloride (CTMA-Cl) to achieve a lipophilic surface prior to encapsulation. They found that the presence of SiO2 nanoparticles lowered the required
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amount of surfactant. This decrease could be ascribed to the distribution of SiO2 nanoparticles
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at the interface, because it behaves as a colloidal stabilizer. In addition, organic modifiers that
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desorb from the SiO2 nanoparticles may act as additional surfactants.
In addition to sonication, high-pressure homogenization is sometimes used to prepare
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miniemulsions. In fact, high-pressure homogenization may be more suitable for industrial application than is sonication. Hecht et al. investigated the emulsification of droplets loaded
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with silica nanoparticles by high-pressure homogenization.[38] Compared with sonication,
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high-pressure homogenization was more efficient in emulsification of monomers with high
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silica content.
Recently, Bourgeat-Lami et al. investigated systematically the entire process of
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encapsulation of MPS-grafted silica nanoparticles by polymer matrixes in miniemulsions.[39] MPS-grafted silica nanoparticles may be well dispersed in MMA or an MMA/BA mixture, but
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a limited number of aggregates is formed when they are dispersed in BA. Such aggregation may be caused by the relatively large difference of hydrophobicity of MPS-grafted silica nanoparticles and BA. The influence of silica content on the size and morphology of the miniemulsion droplets was investigated by dynamic light scattering (DLS), small-angle X-ray scattering, and cryo-TEM. Results of these experiments indicate that the distribution of silica particles in the monomer droplets was not homogenous (Figure 3a and b). Some droplets were free of silica particles, whereas some contained one to several silica nanoparticles. It is interesting to find that the silica nanoparticles were mainly located at the water/oil interfaces of the miniemulsions. Similar to the miniemulsion droplets, hybrid nanoparticles formed 11
ACCEPTED MANUSCRIPT through subsequent polymerization also showed a heterogeneous distribution of the silica
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particles (Figure 3c).
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Figure 3. Cryo-TEM images of a 50/50 MMA/BA miniemulsion containing 10 wt % silica particles (a and b) and silica/poly(MMA-co-BA) nanoparticles with 3.5 wt% silica content (c). Reprinted from ref. 39 with permission.
3.4 Quantum dot (QD) –polymer nanoparticles QDs are promising materials for biological applications because of their luminescent properties. Among them, silicon QDs have high priority for use in biological applications due to their orange-red luminescence, which does not strongly interfere with cells. Harun et al. prepared silicon QD/polymer hybrid nanoparticles through miniemulsion polymerization.[40] 12
ACCEPTED MANUSCRIPT A
functional
comonomer,
4-vinylbenzaldehyde,
was
used
for
further
potential
functionalization of the nanocomposites. Incorporation of silicon QDs in the polymer matrix
T
imparts high chemical stability and the potential for flexible functionalization but retains the
IP
fluorescent properties of silicon QDs.
SC R
“Grafting from” is a commonly used method for preparing nanocomposites having an inorganic core decorated with polymer brushes. Esteves et al. prepared CdS QDs/PS
NU
nanocomposites through surface-initiated RAFT polymerization in miniemulsion.[41] QDs in the nanocomposites retained their quantum confinement effects. Polymer brushes on the
MA
surface of the QDs displayed uniform molecular weight. Living properties were confirmed by
D
the formation of block polymer brushes through the addition of a second monomer.
TE
Polyfluorene nanoparticles were prepared through Suzuki–Miyaura polycondensation of 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-7-bromo-9,9-dioctylfluorene
in
CE P
miniemulsion.[42] Polymerization of the monomer droplets containing CdSe/CdS core/shell QDs formed QDs/polyfluorene hybrid nanoparticles. The photostability of the QDs in the
AC
hybrid nanoparticles improved, and the o rster energy transfer from the polyfluorene shell to the QD core was efficient.
3.5 Inorganic nanoparticles–polymer nanoparticles with magnetic properties 3.5.1 Preparation of magnetic hybrid nanoparticles through miniemulsion polymerization Magnetic nanocomposites may be prepared in miniemulsion by incorporation of magnetic metal[43] or metal oxide nanoparticles into a nanoscale polymer matrix. Again, surface modification of the magnetic inorganic nanoparticles is crucial for their successful incorporation into polymer matrixes. Yan et al. investigated the influence of surface 13
ACCEPTED MANUSCRIPT modification of Fe3O4 nanoparticles on the preparation of Fe3O4/PS nanocomposites in miniemulsions.[44] They reported that the encapsulation efficiency of MPS-modified Fe3O4 was
higher
than
that
of
Fe3O4
nanoparticles
modified
with
T
nanoparticles
IP
n-octadecyltrimethoxysilane. In addition, nanocomposites containing MPS-modified Fe3O4
SC R
showed a regular morphology and narrow size distribution, probably because of the good compatibility between Fe3O4 and PS due to copolymerization between MPS and St. Fe3O4
polyvinylbenzylchloride,[46]
NU
nanoparticles modified with oleic acid can also be encapsulated with polyacrylonitrile,[45] PS,[47]
PMMA,[48]
poly(styrene-co-acrylamide),[49]
or
MA
poly(styrene-co-methacrylic acid)[50] in miniemulsions. Incorporation of chlorine as functionality provides the possibility to further functionalize the magnetic nanocomposites
TE
D
with various functional groups.[46] Addition of hydrophilic monomers such as acrylic acid may improve the colloidal stability of the system.[47] Magnetic poly(styrene-co-acrylamide)
CE P
hybrid nanoparticles display a saturation magnetization of 27.1 emu·g-1.[49] Addition of acrylamide or methacrylic acid (MAA) as comonomer may lead to substitution of the hybrid
AC
nanoparticles with additional functional groups such as –CONH2 and –COOH.[ 49, 50] Fe3O4 nanoparticles modified with oleic acid were incorporated in a polyaniline matrix to prepare conductive and magnetic, dually functional nanocomposites.[51] The resultant nanocomposites showed a multicore/single-shell morphology. The increase in Fe3O4 content in the nanocomposites led to a reduction in conductivity and an increase in the saturation moment. Park
et
al.
nanocomposites
investigated prepared
the in
magnetorheological miniemulsions.[48]
characteristics They
found
of
Fe3O4/PMMA
that
Fe3O4/PMMA
nanocomposites in a nonmagnetic fluid displayed Bingham behavior under external magnetic fields. 14
ACCEPTED MANUSCRIPT Compared with thermal initiation, photoinitiated polymerization may be performed more rapidly at relatively low temperature. High reaction rate and low reaction temperature may be
T
favorable for encapsulation of inorganic nanoparticles. Dou et al. loaded PMMA nanoparticles
IP
with about 85.1 wt% of magnetic nanoparticles via photoinitiated miniemulsion
SC R
polymerization.[52] Recently, the same group prepared magnetic core–shell nanoparticles with a magnetic core of 250 nm ± 40 nm in diameter through photoinitiated polymerization in
NU
miniemulsions.[53] The size of magnetic particles used in this paper is relatively larger than that of magnetic nanoparticles prepared through coprecipitation (about 10 nm). Confinement
MA
of large magnetic nanoparticles in the dispersed phase was realized through a rapid increase in
D
the viscosity of the dispersed phase. In addition, the high reaction rate of photoinitiated
TE
polymerization might minimize the influence of molecular diffusion between droplets on the droplet stability when hydrophilic monomers were used. Therefore, magnetic core–shell
CE P
nanoparticles were functionalized with epoxy groups through photoinitiated miniemulsion polymerization using glycidyl methacrylate as the monomer.
AC
Ramos and Forcada encapsulated magnetic nanoparticles via surfactant-free miniemulsion polymerization.[54] Compared with hybrid nanoparticles obtained from SDS-stabilized miniemulsion, the magnetic nanoparticles distributed more homogenously in the polymeric matrix, and the resultant hybrid particles showed a narrower particle size distribution. The distribution of magnetic nanoparticles in the polymer matrix depended on the type of cross-linker. Use of a hydrophobic cross-linker led to core–shell hybrid particles with a core of aggregated magnetic nanoparticles. In contrast, use of a hydrophilic cross-linker resulted in a homogenous distribution of magnetic nanoparticles in the polymer matrix. Xu et al designed a modified miniemulsion system to prepare epoxy-functionalized 15
ACCEPTED MANUSCRIPT magnetic polymer latices with various morphologies.[55] No hydrophobe was added to the dispersed phase in the modified miniemulsion, resulting in micrometer-sized monomer
T
droplets that formed via monomer diffusion. However, particles were formed through droplet
IP
nucleation by keeping the amount of surfactant below the critical micelle concentration. The
SC R
particle morphology may be tuned by varying kinetic parameters such as cross-linker amount and reaction temperature. Epoxy groups on the surface of the hybrid particles enable further
NU
modifications.
Different from magnetic particles incorporated in the hybrid nanoparticles via
MA
miniemulsion polymerization, two polymerizable magnetic clusters, Mn12O12(4-vinylbenzoic acid (VBA))16 and Mn8Fe4O12(VBA)16, were copolymerized with St to form magnetic
TE
D
nanospheres in miniemulsion.[56] In contrast to the heterogenous distribution of magnetic particles in most of the magnetic hybrid nanoparticles, the magnetic clusters were
CE P
homogenously distributed in the hybrid nanoparticles. The magnetic cores, Mn12 and Mn8Fe4, retained their identities after polymerization. The resulting magnetic nanoparticles have
AC
potential applications as contrast agents for magnetic resonance imaging.
3.5.2 Preparation of magnetic nanocomposites using preformed polymers in miniemulsions Polymer nanoparticles can be prepared by using preformed polymers through a combination of miniemulsification and solvent displacement techniques.[57] Urban et al. mixed superparamagnetic iron oxide nanoparticles and fluorescent molecules with a chloroform solution of polylactide (PLA).[58] The resulting hydrophobic dispersion was dispersed in an aqueous solution of SDS to form a miniemulsion. With subsequent evaporation of the solvent, superparamagnetic and fluorescent PLA-based hybrid 16
ACCEPTED MANUSCRIPT nanoparticles were formed. These hybrid nanoparticles displayed a regular shape and a narrow size distribution. The loading of magnetic nanoparticles in the nanocomposites was as
T
high as 39 wt% relative to the weight of PLA.
IP
Janus magnetic nanoparticles have also been prepared through a combination of
SC R
miniemulsification and solvent displacement techniques.[59] The Janus morphology was induced by incompatibility between oleic acid surrounding the magnetic nanoparticles and
NU
free PS chains, which caused phase separation between the magnetic nanoparticles and PS chains during evaporation of the St monomer. The heterogenous distribution of the surfactant
MA
on the droplet surface might be another significant reason for the formation of Janus
D
nanoparticles.
TE
Dong et al. prepared thermosensitive cross-linked poly(di(ethylene glycol) methyl ether methacrylate) (PM(EO)2MA) nanogels in inverse miniemulsion.[60] To achieve a
CE P
responsiveness to magnetic fields, hydrophobic magnetic nanoparticles stabilized with oleic acid were post-loaded into the nanogels. PM(EO)2MA nanogels with multiresponsive
AC
properties may find potential application in controlled drug release.
3.6 Carbon–polymer nanocomposites Pristine carbon black tends to aggregate because of its strong van der Waals forces. Preparation of carbon/polymer nanocomposites through incorporation of carbon black is an effective technique for alleviating the agglomeration of carbon black. Similar to other inorganic nanoparticles, carbon black requires modification prior to encapsulation. Han et al. reported a technique for preparing carbon black modified with oleic acid which was further encapsulated with PS through a miniemulsion polymerization process.[61] In their study, 17
ACCEPTED MANUSCRIPT carbon black was firstly hydroxylated by using KMnO4 in the presence of a phase-transfer catalyst, tetrabutyl ammonium bromide. Thereafter, the hydroxylated carbon black was
T
reacted with oleic acid. Although pristine carbon black is a radical scavenger and thus may
IP
inhibit free-radical polymerization, free-radical polymerization could proceed in the presence
SC R
of carbon black coated with oleic acid.
Graphene, a two-dimensional carbon nanomaterial, has become a popular material in recent
NU
years because of its remarkable electrical properties.[3, 62] It has been adopted extensively to prepare cost-effective conductive polymer composites. Direct incorporation of graphene oxide
MA
(GO) by utilizing hydrophobic polymers may be difficult due to the hydrophilic surface of
D
GO sheets. To overcome this problem, Etmimi and Sanderson modified GO with a reactive
TE
surfactant, 2-acrylamido-2-methyl-1-propanesulfonic acid, to widen the interlayer distance and to promote the intercalation of monomers.[63] GO/poly(styrene-co-butyl acrylate)
CE P
nanocomposites were prepared through a miniemulsion copolymerization technique in the presence of modified GO. Compared with the plain copolymer, nanocomposites containing
AC
GO showed improved mechanical properties and better thermal stability. In a succeeding study, Etmimi et al. modified GO with a RAFT agent, and then dispersed modified GO in a mixture of St and a hydrophobe.[64] The resultant GO dispersion was homogenized to form miniemulsions. GO/PS nanocomposites with a narrow particle size distribution were formed via RAFT polymerization in miniemulsion. Attachment of the RAFT agent to the surface of GO sheets resulted in covalent binding of polymer chains to the GO sheets. In addition, the molecular weight and polydispersity of the polymers could be controlled to some extent by taking advantage of the living characteristics of RAFT polymerization. The GO/PS nanocomposites showed improved thermal stability compared with the plain PS nanoparticles. 18
ACCEPTED MANUSCRIPT Tan et al. modified GO with poly(styrene-co-methyl methacrylate) by grafting copolymerization in miniemulsions.[65] The copolymer-grafted GOs were used as conducting
T
fillers to impart conductive properties to the immiscible PS and PMMA blends. Furthermore,
IP
the compatibility between PS and PMMA was improved by the incorporation of the
SC R
copolymer-grafted GO.
NU
3.7 Semiconductor nanoparticles–polymer hybrid nanoparticles ZnO nanoparticles have been widely used as photocatalysts, bactericides, UV-shielding
MA
materials, and photoluminescent materials.[66] Polypyrrole–zinc oxide (PPy/ZnO) hybrid
D
nanoparticles were prepared by ultrasound-assisted miniemulsion polymerization.[67] The
TE
authors reported that sonication during polymerization reduced the size and narrowed the size distribution. The PPy/ZnO nanoparticles were used as a sensor for liquefied petroleum gas
CE P
(LPG) with a more rapid response than the sensor using pure PPy. ZnO nanoparticles may also be encapsulated with common polymers such as PS in
AC
miniemulsions.[68] The encapsulation efficiency of ZnO nanoparticles can exceed 90% after modification with 3-aminopropyltriethoxysilane, which enhanced the hydrophobicity of the ZnO nanoparticles. The resulting ZnO/polymer nanocomposites could be used to prepare antibacterial hybrid films. Antimony-doped tin oxide (ATO) has numerous advantageous properties, including infrared (IR) light shielding, electrical conductivity, and high chemical and mechanical stability. Silane-modified ATO nanoparticles were encapsulated with poly(methyl methacrylate-co-n-butyl acrylate) in miniemulsion.[69] The encapsulation efficiency depended on synthesis parameters like comonomer ratio and amount of ATO nanoparticles, emulsifier, 19
ACCEPTED MANUSCRIPT and hydrophobe. An increase in the amount of ATO nanoparticles in the hybrid films led to a reduction of transmittance in the near-IR region but did not have an obvious influence in the
T
UV–visible region. The resultant hybrid films have potential as transparent IR-light shielding
SC R
IP
films.
3.8 Pigment–polymer nanoparticles
NU
Mahdavian et al. prepared white nanocomposites by encapsulation of Al2O3 nanoparticles modified with oleic acid by polymerization in miniemulsions.[70] Yellow pigments were
MA
encapsulated by PS through miniemulsion polymerization to obtain hybrid nanoparticles with
D
a pigment core and a polymer shell.[71] By post-polymerization with fluorescein
TE
isothiocyanate-modified (2-aminoethyl)methacrylate hydrochloride, the hybrid nanoparticles were encapsulated with a fluorescent shell to obtain luminescent electrophoretic particles
CE P
(EPs). The luminescent EPs could be used to prepare glowing electrophoretic displays
AC
applicable in dark conditions.
3.9 Inorganic nanoparticles–polymer nanocomposites prepared through in situ formation of inorganic nanoparticles in droplets Mineralization of inorganic precursors in a confined environment may be performed in the nanodroplets
of
miniemulsions.
Fukui
and
Fujimoto
prepared
nano-CaCO3
by
sonication-assisted mixing of two inverse miniemulsions with aqueous Ca(NO3)2 and Na2CO3 droplets, respectively.[72] Organic–inorganic hybrid nanoparticles were prepared through subsequent polymerization of the preloaded hydrophilic monomer, 2-hydroxyethyl methacrylate (HEMA), in the droplets of Ca(NO3)2. Interestingly, the morphology of 20
ACCEPTED MANUSCRIPT nano-CaCO3 could be tuned by modifying synthesis parameters such as the incubation time for the nucleation of CaCO3 and growth prior to polymerization and the polymerization rate
T
of HEMA. Such modification may inhibit the crystal growth and transformation of CaCO3.
SC R
CaCO3 core dictates the mechanical and optical properties.
IP
These hybrid nanoparticles were used as building blocks to prepare hybrid films. The inner
Systems that do not use organic solvents for the preparation of organic–inorganic hybrid
NU
materials are of particular interest because of their environmental benefits. Nabih et al. prepared inorganic nanoparticle–polymer hybrid particles in a system without any organic
MA
solvent.[73] They emulsified two aqueous solutions of inorganic precursors in a hydrophobic
D
monomer mixture to obtain two inverse miniemulsions. Combining and homogenizing both
TE
inverse miniemulsions led to a dispersion of inorganic nanoparticles (for example, zinc phosphate and calcium carbonate) in a polymerizable continuous phase. This inverse was
converted
CE P
miniemulsion
to
a
direct
miniemulsion.
Upon
polymerization,
inorganic–polymer hybrid nanoparticles were formed. Water-based emulsions of zinc
panels.[74]
AC
phosphate–polymer hybrid nanoparticles have been used as anticorrosive coatings for steel
Recently, Hamberger et al. designed a multistep miniemulsion process to prepare polyurethane (PU) nanocapsules containing inorganic nanoparticles.[75] First, they prepared two inverse miniemulsions with aqueous salt solution droplets. Second, water-insoluble inorganic nanoparticles were formed in the dispersed phase by co-sonication of the two inverse miniemulsions. Finally, the inorganic nanoparticles were encapsulated with PU shells through polyaddition of a polyol pre-dissolved in the dispersed phase and a diisocyanate added to the continuous phase. 21
ACCEPTED MANUSCRIPT
3.10 Section summary
T
The miniemulsion technique shows very high capability in the preparation of hybrid
IP
nanoparticles through polymer encapsulation of various inorganic nanoparticles. Generally, it
SC R
includes four steps to prepare inorganic nanoparticle–polymer nanoparticles through nanoencapsulation in miniemulsion: surface modification of inorganic nanoparticles to
NU
improve their dispersion ability in monomers, dispersion of inorganic nanoparticles in monomers, preparation of miniemulsions with droplets containing inorganic nanoparticles,
MA
and encapsulation of inorganic nanoparticles in a polymer matrix through miniemulsion
AC
CE P
TE
D
polymerization (Figure 4).
Figure 4. Schematic representation of surface modification of inorganic nanoparticles and their encapsulation in a polymer matrix.
The hybrid nanoparticles may show superior chemical or physical properties compared to the corresponding constituent inorganic or polymer nanoparticles. For example, encapsulation 22
ACCEPTED MANUSCRIPT of inorganic nanoparticles with polymers may improve their chemical stability and dispersibility, enhance the thermal stability and mechanical properties of the polymers,
T
facilitate modification of the inorganic nanoparticles by using comonomers with reactive
IP
groups, and impart additional functions such as fluorescence and magnetism to common
SC R
polymers. Functionalization of hybrid nanoparticles may be the most dominant trend in this sub-branch in recent years. For the syntheses using common materials such as silica
NU
nanoparticles and nanoclays, high pressure homogenization is recommended to prepare
MA
miniemulsions regarding the future large-scale industrial production.
D
4. Raspberry-like hybrid nanoparticles
TE
Pickering-type heterophase polymerization systems, such as Pickering (mini)emulsion, may be the most commonly used systems for preparing raspberry-like nanocomposites. In a typical
CE P
Pickering system, organic surfactants that are often used to avoid coalescence of particles or droplets are replaced with particulate stabilizers. Inorganic materials such as SiO2
AC
nanoparticles have been used as particulate stabilizers to prepare organic–inorganic nanocomposites. To improve the adsorption of particulate stabilizers on monomer droplets or polymer particles, additional interactions between stabilizers and droplets or particles are required. Electrostatic and acid–base interactions have often been adopted for this purpose. More information on the Pickering heterophase polymerization is provided in a recent review by Schrade et al.[76]
4.1 Preparation of raspberry-like nanocomposites in Pickering miniemulsion without organic surfactants 23
ACCEPTED MANUSCRIPT Inorganic nanoparticles or nanosheets may function as sole stabilizer for miniemulsions. Raspberry-like hybrid nanocapsules were synthesized through Pickering miniemulsion
T
copolymerization of St and 4-vinyl pyridine (4-VP) using silica sols as stabilizers and
IP
hexadecane as oil templates (see Figure 5).[77] The acid–base interaction between silica sol
SC R
and 4-VP contributes significantly to the formation of a raspberry-like morphology, which strongly depends on the suspension pH, size and amount of silica particles, and amount of
NU
4-VP. The formation of capsules was induced by internal phase separation of the polymers in
AC
CE P
TE
D
MA
the droplets.
Figure 5. TEM (a and b) and scanning electron microscopy (SEM) (c and d) images at various magnifications of hybrid capsules armored with silica particles synthesized in a Pickering miniemulsion. Reprinted from ref. 77 with permission.
Positively charged inorganic nanoparticles may also be used as a stabilizer to prepare raspberry-like
hybrid
nanoparticles
in
miniemulsions.
Schrade
et
al.
prepared
positively-charged, alumina-coated silica/PS hybrid nanoparticles via Pickering miniemulsion 24
ACCEPTED MANUSCRIPT polymerization.[78] Functional comonomers containing –COOH or –CONH2 groups were introduced to improve the adsorption of silica sols on the monomer droplets and, subsequently,
T
on polymer particles through hydrogen bonding between the comonomer and the silica
IP
surface. The particles’ properties and colloidal stability of the systems strongly depended on
SC R
the suspension pH, type of comonomers, and content and size of silica sols. Nanostructured cerium dioxide (CeO2) has attracted much attention because of its wide
NU
application in the fields of chemical catalysis, photocatalysis, sensors, oxygen permeation membranes, and biomedicine.[79] Similar to SiO2, CeO2 nanoparticles may be used as a
MA
stabilizer to prepare Pickering miniemulsions. Pickering miniemulsion and Pickering hybrid
D
nanoparticles stabilized by CeO2 nanoparticles were successfully prepared by Zgheib et al.[80]
TE
To obtain a stable CeO2-stabilized Pickering miniemulsion, MAA was added to the system. MAA establishes the interaction between monomer droplets and CeO2 nanoparticles. The
CE P
influence of the amount of MAA and CeO2 nanoparticles on the properties of droplets and nanoparticles, and on the colloidal stability of the system was systematically investigated. The
AC
CeO2 nanoparticles were only attached to the surface of the hybrid nanoparticles. Bonnefond et al. used sodium montmorillonite clay (NaMMT) as a stabilizer in the surfactant-free miniemulsion polymerization of n-BA/St aided by a functional macromonomer, poly(ethylene glycol) monomethacrylate (PEGMMA).[81] PEGMMA played a crucial role in the formation of NaMMT-armored hybrid nanoparticles after polymerization. PEGMMA may attach to the surface of NaMMT, improving the compatibility of NaMMT and polymers. After polymerization, NaMMT-armored hybrid nanoparticles were formed. These hybrid nanoparticles were used to prepare hybrid films with improved water resistance compared with that of hybrid films prepared from blends of polymer nanoparticles and NaMMT. 25
ACCEPTED MANUSCRIPT GO nanosheets have also been used as inorganic stabilizer for miniemulsions in the preparation of GO/polymer hybrid nanoparticles.[82] Although the size control was not as good
T
as in miniemulsions stabilized by nanoclays or silica particles, the colloidal stability of
IP
miniemulsion systems stabilized by GO nanosheets was still controllable. Similar to other
SC R
Pickering systems, GO sheets mainly distribute on the surface of the hybrid nanoparticles, creating a rough surface, as confirmed by SEM and elemental analysis by X-ray photoelectron
NU
spectroscopy.
For precious materials, a downscaled reaction system may be preferable. Kang et al.
MA
prepared dually labeled raspberry-like nanocomposites in a downscaled Pickering
D
miniemulsion system using silica nanoparticles labeled with a fluorescein dye and QDs.[83]
TE
Separate excitation of both fluorescence signals was confirmed by the emission spectra of the
CE P
dually labeled particles and by confocal laser scanning microscopy measurements.
4.2 Preparation of raspberry-like nanocomposites in Pickering miniemulsions in the
AC
presence of organic surfactants Ionic surfactants are often used in Pickering systems to establish electrostatic interactions between particulate stabilizers and monomer droplets or polymer particles. Normally, cationic surfactants are used for negatively-charged silica particles to promote adsorption of silica nanoparticles in the preparation of raspberry-like silica–polymer nanocomposites. For example, glycerol-functionalized silica sols were coated on the surface of PS nanoparticles in the presence of a small amount of cationic surfactant.[84] It should be pointed out that the use of cationic surfactants is not indispensable. For instance, Zhang et al. prepared raspberry-like silica/PS nanocomposites using an anionic surfactant and anionic silica particles.[85] For the 26
ACCEPTED MANUSCRIPT synthesis, a comonomer containing a basic amino group, 1-vinylimidazole (1-VID), was used. Adsorption of negatively charged silica particles on the surface of the nanocomposites was
T
driven by the strong acid–base interaction between the acidic hydroxyl groups of the silica
IP
particles and the basic, nitrogen-containing 1-VID. In addition, a cationic comonomer,
SC R
2-(methacryloyl) ethyltrimethylammonium chloride (MTC), was used to promote the adsorption of negatively charged SiO2 nanoparticles to the polymer nanoparticles through interaction
between
SiO2
nanoparticles
and
MTC.[86]
Raspberry-like
NU
electrostatic
SiO2/polymer hybrid nanoparticles were successfully prepared in a miniemulsion stabilized
MA
by anionic surfactants.
D
The use of naturally occurring materials to replace petroleum-derived ones is a common
TE
trend driven by environmental concerns. BelHaaj et al. used starch nanoplatelets (SNPs) as colloidal stabilizer to partially reduce the use of organic molecular surfactants to prepare
CE P
Pickering miniemulsions.[87] They found that the amount of cationic surfactant (dodecylpyridinium chloride) could be reduced from 2 wt% based on the monomer content
AC
for the system without SNPs to 0.5 wt% for the system with SNPs. This reduction is due to the synergistic stabilization effect with the cationic surfactant. It is worth mentioning that hybrid films prepared from Pickering nanoparticles stabilized with SNPs showed a markedly enhanced mechanical performance compared with hybrid films formed from simply mixed dispersions of SNPs and polymer particles.
4.3 Preparation of raspberry-like nanocomposites by in situ formation of inorganic nanoparticles Bowl-like or raspberry-like SiO2/PMMA hybrid hollow spheres have been synthesized by 27
ACCEPTED MANUSCRIPT simultaneous polymerization of MMA and hydrolysis–condensation of tetraethoxysilane (TEOS) in miniemulsion (Figure 6).[88] Three monomers, MMA, MPS, and TEOS, were
T
homogenized to form monomer miniemulsions (Figure 6a). Polymers formed through
IP
free-radical copolymerization underwent phase separation due to their low compatibility with
SC R
TEOS. Particles with various morphologies were produced by the use of different amounts of MPS, which could tune the compatibility between the polymer and TEOS. TEOS molecules
NU
can be expelled to the surface of the dispersed phase at a high MPS level or accumulated at the internal side of the dispersed phase at a low MPS level (Figure 6b). Subsequently, in the
MA
former case, SiO2 nanoparticles were formed through a sol–gel process of TEOS on the
AC
CE P
TE
D
surface to form raspberry-like nanoparticles (Figure 6c).
Figure 6. Schematic representation for the formation of silica/polymer nanoparticles via miniemulsion polymerization. Reprinted from ref. 88 with permission.
28
ACCEPTED MANUSCRIPT Similarly,
SiO2/poly(methyl
methacrylate-co-butyl
acrylate)[89]
SiO2/PS[90]
and
raspberry-like nanocomposites were also prepared through a similar synthetic process to that
T
outlined previously.[88] Furthermore, nanoparticles with more a complicated morphology, such
SC R
St using silica/PMMA nanocomposites as seeds.[91]
IP
as flower-like hybrid nanoparticles, were synthesized by seeded emulsion polymerization of
TiO2/PS raspberry-like hybrid nanoparticles could be prepared by simultaneous free-radical
NU
polymerization of St and hydrolysis–condensation of acetylacetone-chelated tetra-n-butyl titanate (TBT) in miniemulsions.[92] St and TBT were added to the dispersed phase of the
MA
miniemulsion. Free-radical polymerization of St mainly took place in the dispersed phase,
D
while TBT had to diffuse to the interface to undergo hydrolysis–condensation because of the
TE
absence of water in the monomer droplets. Consequently, most of the TiO2 nanoparticles distributed on the surface of the hybrid nanoparticles and formed a raspberry-like
CE P
morphology.
Raspberry-like nanocomposites may also be prepared by deposition of inorganic
AC
nanoparticles on the surface of preformed polymer nanoparticles. For example, poly(n-butyl methacrylate-co-N-vinyl-2-pyrrolidone) nanoparticles
were
prepared
(poly(BMA-co-NVP))
through
interfacially
amphiphilic
redox-initiated
core–shell
miniemulsion
polymerization.[93] Thereafter, poly(BMA-co-NVP) nanoparticles were decorated with NiS nanoparticles through the reaction of CH3CSNH2 with NiSO4 under γ-ray irradiation to form a raspberry-like morphology.
4.4 Section summary Raspberry-like
hybrid
nanoparticles
could 29
be
prepared
through
Pickering-type
ACCEPTED MANUSCRIPT miniemulsion polymerization, simultaneous reaction of vinyl monomers and inorganic precursors, or post-deposition of inorganic nanoparticles on the preformed polymeric
T
nanoparticles, respectively. The rough surface of raspberry-like hybrid nanoparticles may
IP
potentially be applied to prepare hybrid films with excellent superficial properties such as
SC R
superhydrophobicity or –hydrophilicity. Apart from common nonfunctional inorganic nanoparticles, functional inorganic nanoparticles, for example, fluorescent SiO2[83] or CeO2[80]
NU
nanoparticles, have been applied in recent years as stabilizers to prepare functional raspberry-like hybrid nanoparticles. These nanoparticles have great potential applications in
D
MA
catalysis, coatings, and biology.
TE
5. Hybrid nanoparticles consisting of polymer and metal salt or metal complex 5.1 Metal complex–polymer nanoparticles prepared in direct miniemulsions
CE P
For the preparation of metal complex–polymer nanoparticles, metal complexes are dissolved in monomers prior to the preparation of the miniemulsion, then the metal
AC
complex–polymer nanoparticles are prepared through polymerization of the metal complex containing monomer droplets in the miniemulsions. For example, Schreiber et al dissolved a series
of
hydrophobic
indium(III)acetylacetonate,
metal
complexes
such
as
zinc(II)tetramethylheptadionate,
platinum(II)acetylacetonate, zincphthalocyanine,
and
chromium(III)benzoylacetonate in monomers.[94] Narrowly size distributed metal-containing polymer nanoparticles were obtained through encapsulation of the metal complexes via miniemulsion polymerization.[94] These nanomaterials can form hexagonally well-ordered monolayers on various substrates because of their homogenous size distribution, leading to (potential) applications in nanolithography. Ordered arrays of metal or metal oxide 30
ACCEPTED MANUSCRIPT nanoparticles are formed through removing the organic components by plasma etching. The remaining metal (Pt) nanoparticles are employed as etching masks to prepare ordered arrays
T
of nanopillars by reactive ion etching.[95] Recently, Manzke et al. reported on nanoparticles
IP
containing two different metal complexes prepared through miniemulsion polymerization
AC
CE P
TE
D
MA
NU
SC R
finally resulting ordered arrays of Pt–Fe alloy nanoparticles (Figure 7). [96]
Figure 7. (A) Schematic representation of the miniemulsion process for the fabrication of metal complex containing PS nanoparticles. (B) Optical micrograph of a monolayer of Feand Pt-complex loaded PS nanoparticles coated by dip-coating onto a hydrophilic Si/SiO2 substrate. The inset of (B) presents an SEM image of PS nanoparticles at slightly reduced diameter by isotropic oxygen plasma treatment. (C) SEM image of hexagonally ordered oxidized FePt nanoparticles after further plasma and heat treatment. The inset of (C) shows that the FePt nanoparticles display a Gaussian size distribution with an average size of 6.8 ± 1.4 nm. Reprinted from ref. 96 with permission. 31
ACCEPTED MANUSCRIPT
Oil-soluble metal complexes have been incorporated in cross-linked polyacrylonitrile
T
nanoparticles through miniemulsion polymerization.[97] After thermal pyrolysis, inorganic
IP
nanoparticles were homogenously embedded in a porous, partially graphitic carbon matrix. A
SC R
wide variety of hybrid nanoparticles such as Fe3O4@C, Fe@C, and FeS2@C were synthesized through ex situ engineering of the embedded materials. Hybrid nanoparticles prepared
NU
through this technique showed enhanced lithium-storage performance compared with that of using bare materials.
MA
Hybrid nanoparticles for high-relaxivity magnetic resonance imaging-contrast agents have been prepared through miniemulsion polymerization with a gadolinium(III)-based
TE
D
metallosurfactant.[98] Magnetic resonance images produced by using the hybrid particles were much brighter than those obtained by using deionized water. Compared with that of the small
CE P
gadolinium-based molecule (Gd-DOTA), the relaxivity of the hybrid particles was markedly enhanced (2.4 times) probably because of the attachment of Gd(III) complexes to the slowly
AC
rotating colloids. In addition, double-modal imaging agents were synthesized by incorporating a fluorescent moiety with 5-acryloylaminofluorescein as the comonomer in the miniemulsions.
5.2 Metal salt–polymer hybrid nanoparticles prepared in inverse miniemulsions In inverse miniemulsions, the dispersed phase is composed of polar components. Hydrophilic metal salts used as lipophobes are required in these systems for achieving good droplet stability. Therefore, it is very convenient to incorporate hydrophilic salts in the polymeric nanogels through inverse miniemulsion polymerization. Cao et al. prepared a series 32
ACCEPTED MANUSCRIPT of transition-metal salt/poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hybrid nanogels in inverse miniemulsions (Figure 8).[99, 100] The size distribution of the hybrid nanogels was
T
narrow, and the loading of metal salts could be tuned within a wide range. Kobitskaya et al.
IP
prepared zinc-containing polyacrylamide hybrid nanogels in inverse miniemulsions.[101] The
SC R
hybrid nanogels, which showed a narrow size distribution, were used in nanolithography for a plasma-etching process by forming ordered monolayers of metal oxide nanoparticles. Silver
NU
salts were also incorporated in poly(HEMA) nanogels in inverse miniemulsions.[102] Silver/polymer hybrid nanogels were formed through in situ reduction of silver ions by
AC
CE P
TE
D
MA
gaseous diffusion of hydrazine to the dispersed phase.
33
AC
CE P
TE
D
MA
NU
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IP
T
ACCEPTED MANUSCRIPT
Figure 8. Representative TEM images of narrowly size-distributed hybrid nanogels containing (A) 2.3 mol% Fe(BF4)2, (B) 5.4 mol% Co(NO3)2, (C) 6.5 mol% CoCl2, and (D) 4.3 mol% Fe(NO3)3. Nanogels were synthesized by inverse miniemulsion polymerization of HEMA. Reprinted from ref. 100 with permission.
Magnetic polyvinyl alcohol (PVA) nanogels were prepared in inverse miniemulsion through a multistep process.[103] First, an acidic aqueous solution of PVA and ferrous and ferric ions was dispersed in a hydrophobic continuous phase by sonication to form an inverse 34
ACCEPTED MANUSCRIPT miniemulsion. Second, the PVA molecules were cross-linked by addition of glutaraldehyde. Finally, Fe3O4 nanoparticles were formed on the surface through coprecipitation of Fe2+ and
IP
T
Fe3+ with triethylamine.
SC R
5.3 Section summary
Various metal salts or metal complexes may be conveniently incorporated into a polymer
NU
matrix in both direct and inverse miniemulsions. The metal salts or metal complexes must be dissolved in the monomer mixture prior to polymerization. Compared with the heterogeneous
MA
distribution of inorganic nanoparticles in miniemulsion droplets, dissolved metal salts or
D
metal complexes are homogenously distributed in miniemulsion droplets. It should be pointed
TE
out that the loading of hydrophobic metal complexes through direct miniemulsion polymerization is sometimes limited because of the low solubility of metal complexes in the
CE P
hydrophobic monomer mixture. In contrast, loading of hydrophilic metal salts in inverse miniemulsions is much easier because of the high solubility of metal salts in polar monomer
AC
mixtures. In addition, hydrophilic metal salts behave as lipophobes that improve droplet stability. Inorganic nanoparticle/polymer hybrid nanoparticles may be conveniently formed through reaction of the loaded metal salts. In contrast to direct encapsulation of inorganic nanoparticles by polymers, the indispensable surface modification of inorganic nanoparticles can be avoided.
6. Hybrid nanocapsules Miniemulsion polymerization has been proved to be an efficient method for the preparation of nanocapsules using liquid droplets as templates. Recent advances in this method have been 35
ACCEPTED MANUSCRIPT summarized in our review.[10] In the present contribution, we focus on the recent progress in research on hybrid nanocapsules, that is, nanocapsules with a hybrid shell or polymeric
6.1 Nanocapsules with a hybrid shell
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IP
T
nanocapsules containing inorganic components or natural macromolecules.
The miniemulsion technique allows the preparation of nanoparticles of organometallic polymers.
A
miniemulsion-based
technique,
miniemulsion
periphery
NU
coordination
polymerization (MEPP), was designed by Wang’s group to prepare Prussian Blue (PB)
MA
nanoparticles with various particle morphologies (Figure 9).[104-106] In detail, an
D
organometallic surfactant, poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene
(EPE-Fe),
was
used
TE
glycol) (PEG-PPG-PEG) terminated with pentacyano(4-(dimethylamino)pyridine)ferrate to
stabilize
a
toluene
miniemulsion.
Polymerization
of
CE P
EPE-Fe-terminated PEG-PPG-PEG by addition of Fe3+ at the surface of the nanodroplets was carried out to form amorphous PB spherical nanoshells and crystallized cubic nanoshells by
AC
using EPE-Fe surfactant with 100% end functionality[104] and EPE-Fe surfactant with ≤60% end functionality,[105] respectively. Recently, PPy/PB core–shell nanoparticles were prepared in miniemulsions by simultaneous polymerization of PB on the surface of nanodroplets and of pyrrole in the nanodroplets.[106] Compared with that of PB nanoshells, the fluorescence of the PPy/PB core–shell particles was enhanced because of charge transfer from PPy to PB. Silyl-terminated surfactants could be also used as precursor to form organosilica nanocapsules through MEPP.[107] The shell of these nanocapsules was cross-linked, but the thickness of the shell was thin enough to allow ions to permeate. Organosilica nanocapsules may potentially be used as high-ion-conductivity nanoelectrolytes. 36
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ACCEPTED MANUSCRIPT
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MEPP. Reprinted from ref. 104 with permission.
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Figure 9. Schematic representation of Prussian Blue (PB) nanoshells produced through
Organic–inorganic hybrid nanocapsules with an oily core were prepared through
MA
miniemulsion copolymerization of St, divinylbenzene (DVB), MPS, and N-isopropyl
D
acrylamide (NIPAM).[108] The inorganic moiety was formed through hydrolysis–condensation
TE
of MPS. Droplet formation and nucleation mechanisms of this process were investigated.
particles.
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Homogenous nucleation was considered as the main reason for the formation of solid
Hollow spheres with polymer/silica double shells were prepared through polymerization of
AC
MMA and DVB and subsequent hydrolysis–condensation of TEOS in miniemulsions.[109] TEOS functioned as soft template for forming the voids in the polymerized MMA. Phase separation between the resulting polymers and TEOS played a critical role in the formation of the capsule morphology. Magnetic hollow spheres were synthesized by loading magnetic Fe3O4 nanoparticles in the droplets of the miniemulsions prior to the reaction. Through an analogous technique, Janus magnetic microcapsules were synthesized in a magnetic field. The magnetic field could drive the magnetic nanoparticles to move to one side of the nanodroplets of the miniemulsions.[110]
37
ACCEPTED MANUSCRIPT 6.2 Polymeric nanocapsules with inorganic or naturally occurring macromolecular containers
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Analogously to MEPP, Utama et al. designed an inverse miniemulsion periphery RAFT
IP
polymerization technique to prepare hollow polymeric nanoparticles.[111] The key of this
SC R
technique is the use of an amphiphilic RAFT agent to confine the polymerization on the surface of droplets. A protein, bovine serum albumin (BSA), was encapsulated without
NU
denaturation through this technique. The permeability of the polymeric shell allows the release of BSA. Therefore, these polymeric nanocapsules are potential nanocarriers for
MA
protein delivery.
D
Starch dissolved in the aqueous droplets of inverse miniemulsions could be cross-linked
TE
with the hydrophobic reagent, 2,4-toluene diisocyanate, at the surface of droplets through polyaddition.[112] The resulting cross-linked starch nanoparticles displayed a capsule
CE P
morphology. Double-stranded DNA (dsDNA) could be encapsulated in the cross-linked starch nanocapsules through a similar process. Polymerase chain reaction (PCR) in the nanocapsules
AC
was carried out to amplify the incorporated dsDNA chains. Baier et al. prepared polymer nanocapsules containing dsDNA through a reverse method.[113] They first amplified a single-molecule dsDNA template by PCR in miniemulsion droplets. Subsequently, poly(n-butylcyanoacrylate) shells were formed on the miniemulsion droplets through interfacial anionic polymerization. Anionic (SDS), cationic (CTMA-Cl), and nonionic (Lutensol AT50) surfactants were used to prepare aqueous dispersions of the nanocapsules. HeLa cells showed higher uptake of nanocapsules stabilized by SDS compared with CTMA-Cl or Lutensol AT50. All the nanocapsules showed low cytotoxicity to HeLa cells. In addition, the incorporated DNA molecules could be released upon degradation of the 38
ACCEPTED MANUSCRIPT poly(n-butylcyanoacrylate) shells. Cao et al. prepared poly(N-isopropyl acrylamide) (PNIPAM) nanocapsules with narrow
T
size distribution in inverse miniemulsion by using aqueous metal-salt droplets as
IP
templates.[114] Formation of the capsule morphology was induced by internal phase separation
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of the polymer in the droplets. The inorganic metal salts were encapsulated by a PNIPAM shell, which displayed reversible thermosensitivity. Similarly, pH- and thermo-responsive
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nanocapsules were prepared by inverse miniemulsion copolymerization of NIPAM and 4-VP
MA
using aqueous metal-salt droplets as templates.[115]
D
6.3 Section summary
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Because of its use of soft droplet templates, the miniemulsion technique shows high flexibility in the preparation of hybrid nanocapsules. A hybrid shell of nanocapsules could be
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prepared by concurrent formation of polymeric and inorganic moieties by using monomers with different reactive groups, or by direct polymerization of hybrid organometallic molecules.
AC
Hybrid nanocapsules could be formed through encapsulation of preloaded inorganic salts or natural macromolecules in miniemulsion droplets. Hybrid capsules prepared in miniemulsions can be used in various fields, including biology, medicine, catalysis, coatings, and cosmetics.
7. Hybrid polymeric nanoparticles 7.1
Preparation
of
hybrid
polymeric
nanoparticles
through
miniemulsion
(co)polymerization in the presence of another type of polymer Nanoparticles composed of polymeric moieties prepared through various polymerization mechanisms may be regarded as another type of hybrid nanoparticles. PU–acrylic polymer 39
ACCEPTED MANUSCRIPT hybrid nanoparticles were prepared through simultaneous free-radical and addition polymerization in miniemulsion by using acrylic monomers, a hydroxyl-functionalized 117]
T
methacrylate, an isocyanate functional PU prepolymer, and a diol as reactants.[116,
IP
Covalent linkage of PU to the acrylic polymer could improve the compatibility between these
SC R
two types of polymers. Such hybrid polymer nanoparticles could be used as pressure-sensitive adhesives (PSAs). Introduction of PU to the hybrid copolymer could improve the cohesion of
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the copolymer through hydrogen bonds. The acrylic–PU PSAs showed enhanced shear resistance and reduced tackiness because of the high gel content and high elasticity of the
MA
hybrid network containing the PU segments.[116] Therefore, a balance between elastic and
D
viscous properties was required to produce PSAs with a better overall adhesive performance.
TE
Lopez et al. investigated the influence of the type of diol (chain extender) on the reaction kinetics, microstructure of the hybrid copolymer, and adhesive performance of acrylic–PU
CE P
PSAs.[117] They found that the PSAs using bisphenol A as chain extender showed better overall adhesive performance than those with other diols such as 1,6-hexanediol and
AC
1,4-cyclohexanediol. To achieve high tackiness, the presence of low-molecular-weight polymers is required because they can diffuse quickly to the interface. However, the polymer chains should be long enough to form entanglements, which can dissipate the energy during pulling of PSAs. The chain length of polymers could be conveniently tuned by the addition of a chain-transfer agent (CTA). Lopez et al. investigated the influence of the CTA, dodecylthiol, on the kinetics, polymer microstructure, particle morphology, and adhesive performance of acrylic–PU hybrid nanoparticles prepared through simultaneous free-radical and addition polymerization in miniemulsions.[118] They found that the optimum adhesive performance could be achieved by addition of a suitable amount of CTA. Promisingly, acrylic–PU hybrid 40
ACCEPTED MANUSCRIPT nanoparticles with a high solid content could be prepared in miniemulsions.[119] Photoinitiated polymerization may be carried out under mild conditions at a high rate. Daniloska et al.
T
prepared waterborne acrylic–PU hybrids through photoinitiated miniemulsion polymerization
IP
in a continuous tubular reactor.[120] Its high polymerization rate resulted in almost complete
SC R
conversion within a short residence time (< 5 min) at a low photoinitiator concentration (< 0.48 wt% to monomer) under modest irradiance (< 7 mW/cm2). The droplet nucleation
NU
mechanism dominated during the entire polymerization process.
Alkyds, a type of traditional binders for coatings, were added to the miniemulsion
MA
polymerization system of acrylic monomers to prepare alkyd/acrylic nanocomposites.[121]
D
Kinetic investigations showed that the polymerization reached a conversion limit. The limited
TE
conversion could be ascribed to chain transfer to alkyds forming unreactive radicals stabilized by conjugation of double bonds. The particle morphology strongly depended on the
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hydrophobicity of the alkyds. Hydrophobic alkyds formed core–shell nanocomposites, whereas less hydrophobic alkyds formed hemispherical nanocomposites. Minari et al.
AC
established a method based on conventional size-exclusion chromatography to evaluate the degree of resin grafting, degree of acrylic grafting, number of reacted double bonds in the alkyd, gel content, and molecular-weight distribution of the sol part of the alkyd/acrylic hybrid nanoparticles prepared in miniemulsions.[122] Poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene],
a
fluorescent
conjugated
polymer with high quantum yield, was encapsulated with PS to prepare fluorescent hybrid nanoparticles through miniemulsion polymerization.[123] The emission wavelength of the fluorescent nanoparticles could be tuned by varying the synthesis parameters. The surface of the hybrid nanoparticles was modified with carboxyl groups using acrylic acid as a 41
ACCEPTED MANUSCRIPT comonomer.
Preparation
of
hybrid
polymeric
nanoparticles
through
miniemulsion
T
7.2
IP
copolymerization
SC R
Hybrid polymeric nanoparticles could be prepared via miniemulsion copolymerization of vinyl-containing silane and other vinyl monomers. MPS is often used as a vinyl-containing
NU
silane because of its good copolymerization capability with other vinyl monomers.[124, 125] MPS-containing hybrid particles may be used to prepare hybrid films. During film formation,
MA
the methoxy groups of MPS undergo hydrolysis and condensation to form cross-linked
D
structures. Therefore, the number of MPS units in the copolymer has a significant influence
TE
on the mechanical properties of hybrid films. For example, Ramos-Fernández et al. found that
content.[126]
CE P
the stiffness of hybrid films increased while the flexibility decreased with an increase in MPS
Preparation of biobased polymers is of high interest because fossil fuel resources dwindle.
AC
The journal Macromolecules had launched in 2009 a virtual issue “Polymers from Renewable Resources” to address the importance of design and synthesis of polymers from renewable resources.[127] Poly(n-butyl methacrylate)-g-polylactide nanoparticles were prepared via solvent-free miniemulsion copolymerization of methacryloyl-end-functionalized polylactide and n-butyl methacrylate.[128] The synthesis process excluded the use of organic solvents and the copolymer film showed elastic properties.
7.3 Section summary One method for preparing hybrid polymeric nanoparticles involves a process in which one 42
ACCEPTED MANUSCRIPT polymer is dissolved in a monomer mixture, which is then homogenized to form a miniemulsion. Thereafter, hybrid polymeric nanoparticles are prepared through miniemulsion Alternatively,
these
nanoparticles
may
be
prepared
through
T
(co)polymerization.
IP
copolymerization of two different types of monomer simultaneously or by grafting
SC R
polymerization using a vinyl-containing natural polymer. Many properties of hybrid polymers may be improved through the synergistic contribution of the various polymeric components.
NU
For example, acrylic–PU hybrid nanoparticles may be used as high-performance PSAs. MPS-containing hybrid nanoparticles may be used as self-cross-linking coatings through
D
MA
hydrolysis–condensation of MPS in the film-formation processes.
TE
8. Natural macromolecule–synthetic polymer nanoparticles 8.1 Preparation of natural macromolecule–synthetic polymer nanoparticles
CE P
Biomacromolecules like proteins or DNA are sensitive to external conditions such as solvent and temperature. Harsh environments may lead to the deactivation of enzymes and to
AC
denaturation of proteins. Incorporation of biomacromolecules into synthetic polymer matrixes may improve the chemical and biological stabilities of biomacromolecules in harsh external environments.[129-132] For example, Candida rugosa lipase behaves inactive in anhydrous dimethyl sulfoxide (DMSO), however, its original catalytic activity for transesterification can be retained after encapsulation in a polyacrylamide (PAAm) matrix.[129] The PAAm matrix helps to maintain the native configuration of incorporated lipase by shielding the extraction of essential water in anhydrous DMSO at elevated temperatures.[129] Protein–polymer hybrids have been intensively investigated for decades. To covalently link proteins with polymers, two steps are required in most cases, namely, attachment of vinyl groups to proteins and 43
ACCEPTED MANUSCRIPT copolymerization of modified proteins and common monomers. A commonly used modifier is N-hydroxysuccinimide (NHS) acrylate, because NHS can couple with amine groups to form
T
stable amide bonds.[133] Proteins usually contain more than one amine group which often
IP
originate from lysine residues, and NHS may react with amine groups randomly.[133]
SC R
Therefore, binding of NHS to proteins is largely nonspecific, resulting in possible deactivation of the protein active sites. Recently, Averick et al. reported that the initiating
NU
amino acid could be expressed at a specific site of the green fluorescent protein (GFP) through genetic engineering; therefore, damage to the protein’s active sites and structure
MA
could be avoided during modification.[134] Subsequently, GFP-nanogels with a particle size of
D
240 nm were prepared through inverse miniemulsion AGET ATRP of hydrophilic monomers
TE
and modified GFP. According to the results of UV–vis and fluorescence spectroscopy, the activity of covalently attached GFP was well preserved. Poly(acrylic acid) (PAA)/gelatin
CE P
interpenetrating polymer network (IPN) nanogels were prepared through inverse miniemulsion polymerization of acrylic acid in the presence of gelatin.[135] To prepare IPN
AC
nanogels, gelatin was cross-linked with glutaraldehyde, and PAA was cross-linked with N,N-methylene bisacrylamide. IPN nanogels that bear numerous functional groups such as –COOH and –NH2 have a high potential as vehicles for targeted drug delivery. Ethyl cellulose (EC)/acrylic hybrid polymeric nanoparticles were prepared through miniemulsion copolymerization of MMA and BA in the presence of EC.[136] The use of a water-soluble initiator did not yield hybrid nanoparticles because of incompatibility between EC and acrylic copolymers. The compatibility between EC and acrylic copolymers was improved by grafting of acrylic polymer radicals to EC using an oil-soluble initiator, dilauroyl peroxide. Furthermore, addition of a cross-linker can also promote the formation of 44
ACCEPTED MANUSCRIPT EC/acrylic hybrid polymer nanoparticles by entrapping EC physically in the acrylic polymer matrix. Compared with those of the plain acrylic copolymer and EC/acrylic copolymer blend,
improved. matrixes
may
be
reinforced
whiskers/poly(styrene-co-hexylacrylate)
using
cellulosic
SC R
Polymer
IP
T
the thermal and mechanical properties of EC/acrylic hybrid polymer nanoparticles were
nanocomposites
were
substrates.
Cellulose
prepared
through
NU
miniemulsion copolymerization of St and hexylacrylate in the presence of the whiskers.[137] The loading of cellulose whiskers was varied from 1 to 5 wt% relative to the polymer weight.
MA
Addition of a vinyl-containing silane, MPS, could improve the colloidal stability of the
D
systems. The storage modulus of a hybrid film prepared from nanocomposites containing 5 wt%
TE
whiskers was significantly enhanced compared with that of the plain polymer film. In a subsequent report, the authors investigated the influence of silane content on the particle
films.[138]
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properties of the nanocomposites and on the thermal and mechanical properties of the hybrid
AC
Natural rubber latex (NRL) was modified through grafting polymerization of acrylic monomers in miniemulsion.[139] Thermal and morphological properties of the polymer-grafted NRL were investigated.
8.2 Section summary In the presence of natural macromolecules, natural macromolecule/synthetic polymer hybrid nanoparticles may be prepared through miniemulsion (co)polymerization. Such polymer hybrid nanoparticles may have improved chemical and biological stabilities compared with those of natural macromolecules such as proteins and DNA. They may also 45
ACCEPTED MANUSCRIPT have improved biocompatibility and thermal and mechanical properties compared with those
T
of hybrid nanoparticles.
IP
9. Concluding remarks and perspectives
SC R
The miniemulsion technique shows high ability and flexibility in the preparation of versatile hybrid nanoparticles. It may be carried out through encapsulation of inorganic
NU
nanoparticles, inorganic metal salts, and natural macromolecules, or through adsorption of inorganic or natural-polymer nanoparticles. Various reactions such as free-radical
MA
polymerization, polyaddition, polycondensation, and sol–gel processes of inorganic
D
precursors may be performed in miniemulsion droplets. This allows the preparation of hybrid
TE
polymeric nanoparticles by simultaneous formation of at least two types of polymers through different reaction mechanisms. Compared with plain polymer nanoparticles, the hybrid
CE P
nanoparticles may display better properties or additional functions. For example, they may show improved thermal stability and mechanical properties, optical, electrical, and magnetic
AC
properties, and environmental stimuli-responsiveness. This introduces possible applications of hybrid nanoparticles in various fields, such as biology, medicine, semiconductors, coatings, and cosmetics. Research has focused on the preparation of versatile hybrid nanoparticles in miniemulsions, however, the quest for applications has not been given as much effort. For example, application of magnetic or fluorescent hybrid nanoparticles as diagnostic tools is mainly restricted to in vitro cell experiments. For biological or medical applications, extensive biological research and in vivo experiments are necessary. More engineering research into aspects of the preparation of hybrid nanoparticles is required for industrial production. These 46
ACCEPTED MANUSCRIPT include reaction kinetics, kinetic modeling of reactions, and reactor type. Bourgeat-Lami et al. provided a good example for investigating the entire process of
T
encapsulation of SiO2 nanoparticles through miniemulsion polymerization using cryo-TEM
IP
and other techniques.[39] However, a great number of mechanistic investigations on various
SC R
reaction systems should be carried out to improve our understanding of these complicated systems. A deep understanding of the mechanisms would help improve control of the
NU
synthesis process and properties of the hybrid nanocomposites.
MA
Acknowledgement. Financial supports from National Natural Scientific Foundation of China
D
(NNSFC) project (51003023) and open Foundation of Zhejiang Provincial Top Key Academic
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miniemulsion polymerization. Colloid Polym Sci 2011; 289:229–35.
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Graphical Abstract
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ACCEPTED MANUSCRIPT Highlights
were summarized. nanoparticles
include
organic-inorganic,
macromolecule/synthetic polymer nanoparticles.
polymeric,
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Hybrid
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The recent advances in the preparation of hybrid nanoparticles in miniemulsions since 2009
and
natural
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Hybrid nanoparticles can be prepared through encapsulation of a second moiety by polymers in miniemulsions.
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Hybrid nanoparticles can be prepared through in-situ formation of a second moiety in a
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nanoscale polymeric matrix in miniemulsion.
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Characterization, properties, and applications of hybrid nanoparticles are also discussed.
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