Synthetic strategies for raspberry-like polymer composite particles

Hua Zou * and Shuxia Zhai
School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China. E-mail: hua.zou@usst.edu.cn

Received 17th March 2020 , Accepted 1st May 2020

First published on 1st May 2020


Raspberry-like polymer composite particles, which refer to the particles consisting of smaller corona particles attached to the surface of larger core particles with the core and/or the corona containing the polymer component, have attracted much interest over the past two decades. Syntheses of these particles typically involve seeded polymerization, Pickering polymerization, formation of inorganic coronas on the polymer core particles, and heterocoagulation, although some other techniques have also been developed. This review summarizes the strategies used for the preparation of raspberry-like polymer composite particles with a focus on the representative and latest developments, followed by a brief discussion of their applications and an outlook on future developments.


image file: d0py00394h-p1.tif

Hua Zou

Hua Zou received his Ph.D. in polymer chemistry and physics from Nanjing University in 2008. After that, he had been a postdoctoral researcher in the University of Sheffield, Max Planck Institute of Colloids and Interfaces, and Adophe Merkle Institute subsequently. He joined the University of Shanghai for Science and Technology in 2014 and currently is an associate professor. His research interests focus on polymer colloids and polymer composites. His recent project includes the development of nonspherical polymer composite particles.

1. Introduction

Controlling the morphology of colloidal particles at the nano/microscale is very important to master their physicochemical properties. Undoubtedly, spherical is the most common morphology, while raspberry-like particles can be regarded as one of the most common non-spherical particles.1,2 As the name suggests, raspberry-like polymer particles (usually in the form of composite particles) refer to the particles consisting of smaller corona particles attached to the surface of larger core particles, with the core and/or the corona containing the polymer component. Many benefits have been claimed for this unique morphology, including hierarchical surface roughness, high specific surface areas and large amounts of scattering.3 Based on their chemical composition, there are three possible particle structures, namely, inorganic–polymer, polymer–inorganic, and polymer–polymer core–corona structures.

On the other hand, according to the formation sequence of the core and corona particles, the strategies used to prepare raspberry-like particles can be roughly classified into four categories: (i) seeded polymerization in the presence of large core particles, in which small polymer corona particles are formed in situ on the core particles; or small inorganic coronas are formed in situ on the core particles; (ii) Pickering polymerization in the presence of small corona particles; (iii) formation of the core and corona particles simultaneously in one-step polymerization; (iv) heterocoagulation of large core particles and small corona particles by noncovalent or covalent interactions.

Although scattered reports on raspberry-like polymer composite particles appeared in the literature as early as in 1970s, significant advances in this field have been made until the past two decades. Several reviews on colloidal composite particles have partially devoted to raspberry-like polymer composite particles.4–7 However, the topic of raspberry-like polymer composite particles has never been summarized. This article aims to give an up-to-date overview of the synthetic strategies of raspberry-like polymer composite particles with a particular focus on the formation mechanisms of raspberry-like morphology, followed by a brief discussion of their applications. Finally, an outlook on future developments is given. Some selected examples, especially the latest developments in this field are highlighted. This review concentrates on polymer composite particles, while some examples of one-component polymer particles are also referred to. It is perhaps noteworthy that the expression of “raspberry-like” particles seems to be ambiguous in some literatures: they are often merely called “core–shell” particles. These works are generally not included in this review. In addition, raspberry-like inorganic–inorganic composite particles are also excluded from this review.

2. Polymerization methods

2.1. Seeded polymerization in the presence of larger core particles

In this strategy, raspberry-like particles are prepared via heterophase polymerization of monomer(s) in the presence of preformed large polymer or inorganic particles as seeds, which in turn become the cores of the raspberry particle. Various heterogeneous polymerization techniques including emulsion polymerization,8–32 surfactant-free emulsion polymerization,33–41 miniemulsion polymerization,42–50 dispersion polymerization,51–67 and precipitation polymerization68 have been employed.

In the context of heterogeneous polymerization, it is worth noting that different polymerization techniques can lead to polymer (composite) particles with different size ranges. There are three basic types of heterogeneous polymerization: emulsion (also including miniemulsion and surfactant-free emulsion), precipitation (also including dispersion) and suspension polymerization. The polymer particles prepared by emulsion polymerization generally have a diameter in the range of 0.05 to 1 μm; while the polymer particles prepared by precipitation polymerization generally have a much larger diameter (in the range 0.1 to 10 μm). The suspension polymerization can produce polymer beads with an average diameter close to that of the initial monomer droplets (0.01 to 2 mm), and there might be a wide bead size distribution.69,70

Based on the nature of the seeds, seeded polymerization can be divided into polymer seeds-based and inorganic seeds-based polymerizations. When polymer particles are used as seeds, cross-linking of the seed particles is usually a prerequisite. Typically, cross-linked polymer spheres are first swollen with hydrophobic monomers in an aqueous environment. Upon relaxation of the stretched cross-linked polymer network by heating, phase separation occurs in the swollen polymer spheres, yielding liquid protrusions consisting of monomers on the seeds. After the protrusions are polymerized, nonspherical (such as raspberry-like) composite particles are formed. However, there are also a few literature examples in which non-cross-linked polymer particles act as seeds. A wide range of polymer particles have been used as seeds, such as poly(butyl acrylate) (PBA),8–10,13,14 poly(methyl methacrylate) (PMMA) and PMMA-based copolymers,11,12,19,36,38,51,60 poly(N-isopropylacrylamide) (PNIPAm) and PNIPAm-based copolymer,15–17,21–23,54,61 polystyrene (PS) and PS-based copolymers,17,18,34,50,52,53,55,63–65,67,68 poly(glycidyl methacrylate) (PGMA),20 poly(2-acetoxyethyl methacrylate) (PAEMA),33 poly(vinyl chloride-co-acetoacetoxyethyl methacrylate) (P(VC-co-AAEM),35 poly(vinylidene fluoride) (PVDF),37 starch,39 and poly(n-butyl methacrylate) (PBMA).53 When inorganic particles are used as seeds, functionalization of the seeds either by surface modification or adsorption of the polymerization composition (e.g. comonomer or initiator) is usually necessary to increase their chemical affinity with the polymer coronas.1 Most of literature examples with inorganic seeds have focused on silica particles.

2.1.1. Seeded emulsion polymerization.
(a) Based on polymer seeds8–23. As far as we are aware, the first report using the term “raspberry-like” to describe polymer particles appeared in 1976,8–11 when Okubo et al. found that the particles having an uneven raspberry-like surface were formed by the seeded emulsion polymerization of styrene (St) in the presence of non-cross-linked poly(alkyl acrylate) such as PBA emulsion as the seed. This phenomenon was explained by the assumption that such particles consisting of PBA and PS were ruptured at the end period of polymerization.

Kraft et al. presented a method to make anisotropic polymer particles based on the formation of multiple protrusions by seeded emulsion polymerization.17 As shown in Fig. 1, polymer particles with multiple protrusions on the surface were first prepared based on cross-linked polymer seed particles. After the formation of the multiple protrusions, the interfacial tension between the droplets and the aqueous phase determined the driving force for merging. The larger the interfacial tension, the larger the surface free energy and the stronger the driving force to minimize the total interfacial area. Changing the stabilization of the protrusions by the amount of a surfactant allowed for control over the final number of protrusions, or branching anisotropy of the particles. High surfactant concentration could prevent the droplets from coalescing, thus yielding raspberry- or popcorn-like particles. The generality of the approach was demonstrated by using different sizes of cross-linked PNIPAm, PS, and magnetite filled PNIPAm as seed particles.


image file: d0py00394h-f1.tif
Fig. 1 Schematic of the synthesis technique and systematic influence of several factors on the resulting patchy particles. Reproduced with permission from ref. 17. Copyright 2011, American Chemical Society.

PGMA is a versatile polymer with pendant epoxide groups, which have the potential to offer interesting properties for biological applications. Zhang et al. prepared monodispersed PGMA-PS anisotropic microparticles via modified seeded emulsion polymerization of St using swelled PGMA particles as seeds, which had a unique phenomenon—a single hole appeared gradually on the surface of PGMA seed particles during swelling.20 Unlike traditional seeded polymerization, the PGMA seeds were non-cross-linked. Therefore, the swelling agent dibutyl phthalate and the monomer styrene can be regarded as plasticization solvents to liquefy the PGMA particles. The formation of the single hole was attributed to phase separation of the swelling agent with the seed polymer. By regulating the amount of monomer and monomer/seed weight ratio, anisotropic PGMA-PS microparticles including raspberry-shaped with various sizes of protrusions were fabricated (see Fig. 2).


image file: d0py00394h-f2.tif
Fig. 2 Schematic synthesis of the morphology transformation process. (i) The swelling PGMA seed particles with a single hole (ii) was formed by the action of swelling agent dibutyl phthalate and styrene monomer. Then, the Janus (iii) and raspberry-shaped PGMA/PS microparticles (v) were obtained with different ratios of monomer/seeds; after washing with ethanol, the one face of Janus particles was removed and transformed into large-hole microparticles (iv), and the size of protrusions of raspberry-shaped microparticles can be regulated (vi). Reproduced with permission from ref. 20. Copyright 2015, American Chemical Society.

Besides “solid” seeds, seeded emulsion polymerization can also be conducted with soft microgels particles, which are water-swellable colloidal particles with cross-linked hydrophilic or amphiphilic polymer chains. Suzuki et al. reported the synthesis of raspberry-shaped composite microgels by seeded emulsion polymerization of St in the presence of PNIPAm microgels as seeds. In the absence of surfactant, the PS layers did not completely cover the PNIPAm core microgels, resulting in temperature-induced deswelling of the composite microgels. Conversely, composite microgels synthesized with sodium dodecyl sulfate (SDS) ([SDS] > 6.5 mM) did not exhibit thermoresponsive deswelling behavior because PS particles covered the core microgels. The diameter of the PS particles decreased with increasing SDS concentration during the seeded emulsion polymerization. In particular, PS particles formed composites on the microgel surface as well as inside the microgels when the SDS concentration exceeded a critical value for core microgel swelling at 70 °C.21 The same group also prepared raspberry-like composite microgels via the seeded emulsion polymerization of St using poly(NIPAm-co-methacrylic acid) (P(PNIPAm-co-MAA)) microgels with carboxyl groups located in the center of the microgels,22 and poly(NIPAm-co-fumaric acid) (P(NIPAm-co-FAc)) microgels with carboxyl groups located on their surfaces as seeds,23 respectively.


(b) Based on inorganic seeds24–32. An early example which appeared in 1986 described the synthesis of composite PS latices with silica particles in the core under various conditions.24 When hydroxyl propyl cellulose-coated silica particles were used as the seeds, fairly monodispersed composite latices including silica particles were prepared. Furthermore, in the high concentration runs of the SDS, the composite with a raspberry shape was generated with a high product yield. It is assumed that nucleation of new polymer particles occurs above the critical micelle concentration of the surfactant and that one portion of the new polymer particles coagulate heterogeneously with the hydroxyl propylcellulose-coated silica particles after some period of the seeded polymerization.

In 2002, Ravaine et al. reported the synthesis of raspberry-like silica-PS particles consisting of spherical silica beads supporting smaller PS particles through seeded emulsion polymerization of St.25 First, micrometer-sized silica particles were synthesized via a base-catalyzed sol–gel process. Second, a hydrophilic poly(ethylene glycol) macromonomer, i.e., a macromolecule with a polymerizable group, was adsorbed on the surface of the silica beads. Finally, emulsion polymerization of St was performed with a nonionic surfactant and sodium persulfate initiator (see Fig. 3). The adsorption of the macromonomer on the surface of the silica beads is essential to control the morphology of the particles: the macromonomer is expected to react with the growing polystyryl radicals and thus promotes anchoring of the polymer chains on the silica surface. Further work indicated the ratio between the number of silica seeds and the number of growing nodules was another key parameter to control the morphology of the particles.26 When the number of silica seeds was much smaller than the number of polymer nodules, raspberry-like particles were obtained.


image file: d0py00394h-f3.tif
Fig. 3 Schematic representation of the process involved in the synthesis of the raspberry-like silica/polystyrene nanohybrids. Reproduced with permission from ref. 25. Copyright 2002, American Chemical Society.

The strategy of absorption of a comonomer to the silica surface was also adopted by Wu et al.29 The use of 4-vinylpyridine (4VP) as an auxiliary monomer allowed the preparation of SiO2-PMMA composite particles with various (including raspberry-like) morphologies by emulsion polymerization, since there is a acid–base interaction between the silanol groups of unmodified silica particles and the amino groups of 4VP.

The strategy of absorption of the initiator instead of the comonomer was used by Bourgeat-Lami and coworkers.27 They synthesized silica–PMMA nanocomposite particles by emulsion polymerization in the presence of silica beads with different sizes using a cationic initiator, 2,2′-azobis(isobutyramidine)dihydrochloride (AIBA), and a nonionic polyoxyethylenic surfactant (NP30) (see Fig. 4). Coating of the silica particles with PMMA took place in situ during polymerization, resulting in the formation of colloidal nanocomposites with a raspberry-like or a core–shell morphology, depending on the size and nature of the silica beads. A raspberry-like morphology was formed when using smaller silica beads with a diameter of 68 nm. This suggested that the PMMA particles were first nucleated in the continuous phase and soon after heterocoagulated on the mineral surface. Both the cationic initiator and nonionic surfactant were proved to be helpful in conducting the polymerization at the inorganic surface through either electrostatic or hydrophobic interactions. Continuing this work, the preparation of nano-sized silica-PMMA composite latexes either in situ by using a cationic AIBA initiator or ex situ by mixing together anionic silica particles and preformed cationic PMMA latexes were demonstrated.28 In the in situ method, two different procedures were investigated. One was performed directly in an aqueous suspension of the silica beads using a NP30 surfactant initiated by AIBA. In another route, AIBA was first adsorbed on the silica surface, and the free amount of initiator was discarded from the suspension. The silica-adsorbed AIBA adduct was suspended in water with the help of surfactant, and used to initiate the emulsion polymerization of MMA. In each case, the nanocomposites exhibited a raspberry-like morphology.


image file: d0py00394h-f4.tif
Fig. 4 Schematic representation of the polymerization reaction initiated with AIBA at the surface of the silica beads. Reproduced with permission from ref. 27. Copyright 2002, Elsevier.
2.1.2 Seeded surfactant-free emulsion polymerization. Surfactant-free emulsion polymerization refers to the emulsion polymerization process without adding or adding only a small amount of emulsifier (below the critical micelle concentration). Seeded surfactant-free emulsion polymerization can also be divided into polymer seeds-based and inorganic seeds-based polymerizations, as discussed below.
(a) Based on polymer seeds33–39. In 2003, Kuroda et al. found that the PAEMA-PS latex prepared by soap-free emulsion polymerization of St on cross-linked PAEMA were all non-spherical, assuming a wide variety of morphologies including raspberry-like morphology.33 It was inferred that the network's uniformity was responsible for the formation of such anomalous particles. Also based on cross-linked polymer seeds, Liu et al. prepared submicron-sized raspberry-like PS-polyacrylonitrile particles via γ-radiation-induced seeded emulsion polymerization at room temperature, in which the monodispersed cross-linked P(St-DVB-AA) particles were used as seed particles and acrylonitrile as the second monomer.34 The phase separation and the capability of the cross-linked network structure to accommodate the swelling monomer are crucial for the formation of raspberry-like particles.

Pan et al. reported a method to fabricate anisotropic P(VC-co-AAEM)-PS nanoparticles via an emulsifier-free seeded emulsion polymerization of St with non-cross-linked P(VC-co-AAEM) nanoparticles as seeds.35 As shown in Fig. 5, the P(VC-co-AAEM) seed was first swollen by St at room temperature for a preset time. Second, the mixture was heated to 70 °C, and the water-soluble potassium persulfate was introduced to initiate the radical polymerization of St. Then, the phase separation in composite particles happened through the polymerization process. The AAEM content in the seeds is crucial to control the phase separation and the number of bulges of the obtained nanoparticles. By adjusting the AAEM content and the reaction time of seeded emulsion polymerization, nanoparticles with diverse morphologies including raspberry-like were achieved. Moreover, the thermodynamic immiscibility between PVC and PS was the driving force for the formation of PS bulges onto the P(VC-co-AAEM) seeds.


image file: d0py00394h-f5.tif
Fig. 5 Schematic illustration of synthesis of anisotropic nanoparticles via emulsifier-free seeded emulsion polymerization. Reproduced with permission from ref. 35. Copyright 2013, Wiley-VCH.

Single electron transfer radical polymerization (SET-RP) is a kind of living radical polymerization method, which offers some advantages in terms of mild reaction conditions, easy removal of catalyst and colorless polymer product. A later report from the same group reported an approach to prepare asymmetric PVDF-PS composite latex particles with controllable morphologies via seeded SET-RP in surfactant-free emulsion of St at the surface of PVDF seed particles.37 It was observed that the morphology of the composite particles was influenced mainly by the St/PVDF feed ratio, the polymerization temperature, and the length of the catalyst Cu(0) wire (Φ 1.00 mm). The raspberry-like composite particles could be prepared at a higher polymerization temperature or a smaller length of the Cu(0) wire. The formation of nonspherical composite nanoparticles could be ascribed to the surface nucleation of PS bulges following the SET-RP.

Lu and Urban synthesized “gibbous” copolymer nanoparticles with raspberry-like morphologies by surfactant-free seeded emulsion polymerization of pentafluorostyrene/n-butyl acrylate in the presence of PMMA, P(MMA/nBA) or SiO2-PMMA seed nanoparticles.38 The synthesis consisted of swelling the seed with monomers followed by polymerization of the monomers swollen in the seed. The topography of the gibbous phase can be controlled by the copolymer composition and polymerization conditions. Furthermore, when pH-sensitive monomers such as MAA were copolymerized onto surface bulges, pH changes resulted in localized gibbous phase dimensional changes. Facilitated by monomer diffusion into interfacial particle seed solution regions, localized polymerization near the surface is responsible for the formation of phase-separated gibbous topographies.

In a recent study,39 particles derived from natural biopolymers have also been employed as seeds. Shape-tunable starch-PS composite particles were fabricated by seeded polymerization of St using a non-cross-linked starch-based seed. At a low monomer feed ratio, the polymerization process was surface-initiated and the degree of surface nucleation increased, leading to raspberry-like particles.


(b) Based on inorganic seeds40,41. The polymerization-induced self-assembly (PISA) process involves chain extending a hydrophilic polymer precursor prepared via controlled radical polymerization with hydrophobic monomer(s), which leads to amphiphilic block copolymers that self-assembly in situ into nano-objects. Given the recent advances in PISA, a new technique for the synthesis of polymer-encapsulated inorganic particles has been developed. For example, Bourgeat-Lami et al. reported nitroxide-mediated synthesis of silica–polymer composite latexes by PISA of amphiphilic block copolymers in aqueous media.41 The strategy involved the use of a water-soluble brush-type PEO-based macroalkoxyamine initiator, which was synthesized and physically adsorbed on the surface of silica particles through hydrogen-bonding interactions. The adsorbed macroalkoxyamine initiator was subsequently employed to initiate the emulsion polymerization of BMA with a small amount of St under mild conditions (85 °C). Neither chemical modification nor a surfactant was needed for the polymerization. The resulting self-assembled block copolymers formed polymer nodules randomly distributed around the central silica spheres. Varying the macroinitiator concentration or the silica particle size enabled the formation of hybrid particles with dumbbell-, daisy-, or raspberry-like morphologies.
2.1.3 Seeded miniemulsion polymerization. The feature of miniemulsion polymerization is that the particle nucleation occurred primarily within the submicrometer monomer droplets. The seeded miniemulsion polymerizations are typically conducted with silica particles as seeds. For example, Wu et al. presented a controlled synthesis of SiO2-PS nanocomposite particles through miniemulsion polymerization by using sodium lauryl sulfate surfactant, hexadecane costabilizer in the presence of silica particles coated with (methacryloxy)propyltrimethoxysilane (MPS).42 The size and morphology of nanocomposite particles could be tuned by adjusting the silica particle size and surfactant concentration. For 200 nm silica particles, some raspberry-like morphology was observed. This work was subsequently extended to include P(St-co-BA) coronas43 or PS-SiO2 cores.44

Ge et al. reported a strategy on the fabrication of raspberry SiO2-PS particles via radiation miniemulsion polymerization. Starting from MPS-functionalized silica particles (176 nm), raspberry SiO2-PS particles (257 nm) with a submicron SiO2 core decorated by nano-sized PS latex particles (58 nm) were obtained after γ-ray induced miniemulsion polymerization of St.48

2.1.4 Seeded dispersion polymerization. In a dispersion polymerization process, the monomer that dissolves in an organic solvent (or water) is polymerized to form a polymer insoluble in the solvent, and a colloidally stable dispersion is formed with the aid of a dispersant. Similar to the cases of seeded emulsion and surfactant-free emulsion polymerizations, both polymers and inorganics can be used as seeds in seeded dispersion polymerization.
(a) Based on polymer seeds. There are already several examples of seeded dispersion polymerization formulations with polymer seeds, including PMMA–PMMA,51 P(St-co-MPS)-PS,52 PBMA-PS,53 poly(lauryl methacrylate)-PS,53 PNIPAm-PS,54 and P(St-DVB-trifluoroethyl methacrylate)-P(St-ethylene glycol dimethacrylate).55

Shi et al. prepared raspberry-like PMMA particles (strictly speaking this is not an example of polymer composite particles) by seeded dispersion polymerization of MMA in the presence of PMMA seed particles.51 It was found that the following polymerization conditions were necessary to prepare this kind of nonspherical particles: a relatively low temperature, an appropriate ratio of seed/MMA, an initiator with a relatively low decomposition rate, and a relatively low initiator concentration. These particles showed a very good morphological stability at room temperature, but they changed to the spherical ones at 60 °C in a methanol solution of MMA. The prepared PMMA particles were kinetically favored and the localized polymerization of the MMA monomer on PMMA seed particle surface was responsible for the formation of the raspberry-like particles.

Liu et al. presented an approach to fabricate raspberry-like particles via seeded dispersion polymerization with hydrolyzed-MPS as the cross-linking agent, P(St-co-MPS) latex as the seed and only St as a monomer by just changing the ratio of monomer St to seed (see Fig. 6).52 A possible formation mechanism of raspberry-like particles is as follows. During ultrasonic dispersion, a part of monomers is still dispersed in water, while the rest will swell the seed particles. Therefore, the elastic-retractile force in the limited swollen seed particles is not sufficient to extrude monomer. The St-swollen seed particles become deflated due to the incomplete polymerization of St swollen in seed particles. At the same time, they also adsorb PS particles self-nucleated in the dispersion medium during seeded polymerization. The appearance of pits in the surface of individual particles may be attributed to the shedding of PS particles during post treatment of sonication.


image file: d0py00394h-f6.tif
Fig. 6 The procedure for preparation of anisotropic particles. Reproduced with permission from ref. 52. Copyright 2013, American Chemical Society.

There are also several literature examples of seeded dispersion polymerization in combination with chemical oxidative polymerization. The reported core–corona systems with polymer core include PNIPAm-poly(N-methylpyrrole),56 P(St-co-MAA)-Ag@polypyrrole,57 PS-polypyrrole,58 PS-Au@polyaniline,59 sulfonated PS-polyaniline,60 and PS–polydopamine (PDA).61 For example, raspberry-like particles composed of a PS core and PDA nodules were prepared through the self-oxidative polymerization of DA on the PS core particles in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 water/methanol solution in the presence of compounds with carboxylic acid groups, e.g., caffeic acid (CF), as shown in Fig. 7.61 The size of the PDA nodules formed on the PS particles could be controlled from 87 to 140 nm by varying the amount of CF and the reaction time. The results showed that the carboxylic acid group of CF was a key moiety to form raspberry-like particles and that the catechol moiety did not affect the formation of raspberry-like particles. Since DA monomer complexes as cross-linking points in PDA become larger through hydrogen bonding between CF and DA, the PDA nodules formed on the PS core particles were mainly grown in the vertical direction, leading to raspberry-like particles.


image file: d0py00394h-f7.tif
Fig. 7 Schematic illustration of the procedure used for the fabrication of raspberry-like particles. Reproduced with permission from ref. 61. Copyright 2014, Elsevier.

(b) Based on inorganic seeds62–67. Ravaine et al. synthesized SiO2-PS colloidal supraparticles through seeded dispersion polymerization by using polyvinylpyrrolidone (PVP) as a stabilizer in the presence of silica particles modified with methacryloxymethyltriethoxysilane.62 SEM and TEM studies indicated that dumbbell-like or snowman-like supraparticles could be prepared by taking advantage of the coalescence phenomenon which occurred between growing polymer nodules in pure alcoholic medium, at a high monomer concentration and/or at a high temperature. When both St concentration and temperature were decreased, the morphology of the supraparticles could be varied from core–shell to raspberry-like morphologies by increasing the water content in the alcoholic reaction medium (see Fig. 8).
image file: d0py00394h-f8.tif
Fig. 8 Schematic representation of the synthesis of PS/silica supraparticles of various morphologies by dispersion polymerization of St in an ethanol (100 − x)/water (x) % medium. Reproduced with permission from ref. 62. Copyright 2010, American Chemical Society.

ZIF-8 is a prototype zeolitic imidazolate frameworks (ZIFs), which are a family of metal–organic frameworks with excellent thermal and chemical stabilities. Zhu et al. reported a method for the preparation of raspberry-like ZIF-8-PS composite spheres through dispersion polymerization of St in methanol, using ZIF-8 and PVP as co-stabilizers.65 ZIF-8 nanoparticles stabilize PS particles when PS chains precipitate out from the dispersion polymerization, resulting in the final products having raspberry-like structure.

Seeded dispersion polymerizations based on inorganic core in combination with chemical oxidative polymerization were also reported. For example, Yang et al. reported a way to prepare raspberry-like polypyrrole composites by coating the silver/polypyrrole core/shell composites onto the surface of silica spheres via oxidation polymerization of pyrrole monomer.66 The oxidation polymerization of pyrrole and reduction of [Ag(NH3)2]+ ions took place on the surface of silica spheres simultaneously, and no stabilizer was used in the whole process. The silica spheres loaded with [Ag(NH3)2]+ ions played a role as the source of oxidant, and [Ag(NH3)2]+ ion played a key role as a weak oxidant in the fabrication of raspberry-like composites.

2.2 Pickering polymerization in the presence of small corona particles

In this strategy, raspberry-like particles are prepared via heterophase polymerization of monomer(s) in the presence of preformed small polymer or inorganic particles as stabilizers, which in turn become the coronas of the raspberry particles. According to the type of the stabilizer, Pickering polymerization might also be categorized into inorganic particles-based and polymer particles-based polymerizations. Undoubtedly, silica nanoparticle is the most common, if not the only, inorganic particles used to generate raspberry-like particles via Pickering polymerization. When silica nanoparticles are used as Pickering emulsifiers, it is usually necessary to use an auxiliary comonomer or costabilizer to promote a strong interaction between the silica sols and the generated polymer phase.

Various Pickering polymerization techniques including emulsion polymerization,71–93 miniemulsion polymerization,94–99 dispersion polymerization100–102 and suspension polymerization103,104 have been developed to produce raspberry-like polymer composite particles. However, most examples of the particles syntheses are based on emulsion polymerization and miniemulsion polymerization, and there has been relatively little work on dispersion polymerization and suspension polymerization.

2.2.1 Pickering emulsion polymerization.
(a) Based on inorganic particle stabilizers71–86. The Armes group pioneered the polymerization of monomers in the presence of ultrafine silica sols as a facile synthetic route to colloidal nanocomposite particles. Since the early 1990s, they reported the preparation of a series of conducting polymer–silica nanocomposite particles based on either polyaniline,105–107 polypyrrole108,109 or poly(3,4-ethylenedioxythiophene)110 by chemical oxidative polymerization of the corresponding monomer in the presence of ultrafine silica sols. Recently, our own group also prepared poly(2-aminothiazole)-silica nanocomposite particles based on a relatively new heterocyclic monomer 2-aminothiazole.111 In this approach, the silica particles act as a high-surface-area colloidal substrate for the precipitating polymer and “glued” them together, and the silica nanoparticles on the surface of the nanocomposite particles are responsible for their colloidal stability by charge stabilization mechanism. Such nanocomposite particles typically have a raspberry morphology. It is important to note that, the “raspberry” morphology herein has a stricter definition, which means the ultrafine silica particles are present not only on the particle surface but also distributed throughout the interior of the particles.2

Later, Armes et al. extended this colloidal nanocomposite route to vinyl monomers via the free radical (co)polymerization of 4-vinylpyridine and vinyl monomers in the presence of an ultrafine silica sol as a Pickering emulsifier.71,72 A strong acid–base interaction between the (co)polymer and the silica sol appears to be a prerequisite for nanocomposite formation. TEM studies indicated a “currant-bun” morphology for the P(4VP)-SiO2 nanocomposites, while a raspberry morphology for the P(St-4VP)-SiO2 nanocomposites with 10 mol% 4VP in comonomer feed.

Following a similar strategy, Wu et al. reported the synthesis of raspberry-like PMMA-SiO2 nanocomposite particles in the presence of ultrafine silica aqueous sols as a Pickering emulsifier with 1-vinylimidazole (1-VID) as an auxiliary monomer. These particles had a reasonably narrow size distribution, and the average particle sizes were in the range of 120–350 nm and silica contents were up to 47%. The strong acid–base interaction between hydroxyl groups (acidic) of silica surfaces and amino groups (basic) of 1-VID was responsible for promoting the formation of PMMA-SiO2 nanocomposite particles.73 In a follow-up work, raspberry-like PMMA-SiO2 hybrid microspheres were prepared using cationic 2-(methacryloyl)ethyltrimethylammonium chloride (MTC) as an auxiliary monomer in the presence of ultrafine silica aqueous sols. The silica nanoparticles were adsorbed onto the surfaces of organic particles via electrostatic interaction to form nanocomposite particles with a raspberry-like morphology.74


(b) Based on polymer particle stabilizers87–93. There are relatively rare examples of Pickering polymerization being conducted in the presence of a polymeric stabilizer. For example, Yang et al. applied cross-linked poly(divinylbenzene-alt-maleic anhydride) nanoparticles (PDMNPs) as a very effective oil-in-water-type stabilizer for Pickering polymerization of St.87 The PDMNPs have hydrophilic anhydride groups (easily to be hydrolyzed to improve the water affinity) and pendant reactive vinyl groups (potentially to copolymerize with other vinyl groups) on the particle surface. It was found that the PS particles presented different morphologies with increasing amounts of PDMNPs. Notably, the PS particles exhibited a typical raspberry-like morphology with granules located evenly on the particle surface when the PDMNPs content was only 0.5 wt% relative to St, which was much lower than that of inorganic nanoparticle stabilizers (10–30 wt%) in a conventional Pickering emulsion polymerization.

Nanoparticles derivated from natural polymer had also be used as emulsifiers for stabilization of Pickering polymerization. Tan et al. reported the Pickering polymerization of St with starch-based nanospheres derivated from acetylated starch octenyl succinicester as the sole stabilizer. It was shown that the nanospheres concentration, size and pH played important roles in the stabilization process and determined the polymer particle morphology. Raspberry-like PS particles were obtained when the starch-based nanospheres concentration was above 0.7% with a pH value of 8.88

Pan et al. showed that Janus polymer composite particles could be used as surfactants for in situ Pickering-like emulsion polymerization to prepare raspberry-like colloidal particle clusters through a one-pot approach.90 As shown in Fig. 9, under appropriate conditions, PVDF-P(St-co-tBA) Janus particles, which were formed in situ in the early stage polymerization, could self-assemble into the Pickering-like emulsion. The hydrophobicity of the P(St-co-tBA) domains and the affinity of PVDF to the aqueous environment were considered to be the driving force for the self-assembly of the in situ formed PVDF-P(St-co-tBA) Janus particles. Further polymerization eventually led to raspberry-like colloidal particle clusters (≈600 nm) with P(St-co-tBA) as the core and PVDF particles protruding outward. The tBA/St/PVDF feed ratios and polymerization temperature played an important role in achieving relatively well-defined colloidal particle clusters. Subsequently, the same team fabricated raspberry-like PVDF-PS colloidal particle clusters with PVDF seed particles protruding outward via self-assembly of in situ generated PVDF-PS Janus particles.91,92


image file: d0py00394h-f9.tif
Fig. 9 Schematic morphology evolution of PVDF/P(St-co-tBA) complex colloidal particles via surfactant-free seeded emulsion copolymerization. Reproduced with permission from ref. 90. Copyright 2016, Wiley-VCH.

It is perhaps worth emphasizing that Yang et al. demonstrated the preparation of anisotropic microparticles using PS latex particle-stabilized oil-in-water emulsions with UV-curable ethoxylated trimethylolpropane triacrylate (ETPTA) as the oil phase.93 The particle-armored oil droplets could be solidified by UV irradiation within a few seconds to produce ETPTA-PS composite microparticles. Large armored emulsion drops became raspberry-like particles, while small emulsion drops were transformed into colloidal clusters.

2.2.2 Pickering miniemulsion polymerization. In 2001, soon after the emergence of miniemulsion polymerization, this technique was used to synthesize polymer/silica nanocomposite particles.94 The polymerization was conducted with a variety of monomers (including St, BA, and MMA) in the presence of a basic comonomer 4VP, a hydrophobe, and silica nanoparticles as Pickering stabilizers. Depending on the reaction conditions and the surfactants employed, different morphologies were obtained. When a very large amount of cationic surfactant was used to hydrophobize the complete silica surface, a raspberry morphology was obtained.

Ziener et al. prepared raspberry-like hybrid nanocapsules by the copolymerization of St and 4VP in Pickering miniemulsion by using negatively charged silica particles as the sole emulsifier and hexadecane as a liquid template.95 The formation of the capsules was caused by phase separation of the polymers in the droplets. The adsorption of silica particles by the droplets and polymeric particles relied on the interaction between 4VP and silica particles. When compared with conventional Pickering miniemulsions and Pickering suspensions, the colloidal stability of the systems was much more sensitive to the variation of reaction parameters such as pH, size, amount of silica particles, and content of 4VP. The systems without coagulum were only achieved in a narrow pH range at around 9.5 and by using 12 nm silica particles as the emulsifiers.

2.2.3 Pickering dispersion polymerization. 2-Hydroxypropyl methacrylate (HPMA) is one of the few water-miscible vinyl monomers that are suitable for aqueous dispersion polymerization. In 2012, we reported the preparation of poly(2-hydroxypropyl methacrylate)-silica nanocomposite particles by aqueous dispersion polymerization at 60 °C using a binary mixture of an ultrafine aqueous silica sol and PVP as the stabilizer system.101 Optimization of the initial silica sol concentration allowed relatively high silica incorporation efficiencies (close to 100%) to be achieved. The presence of the ultrafine silica at the surface of the nanocomposite particles was confirmed by SEM studies, which revealed a distinctive raspberry morphology compared to the relatively smooth latex particles prepared in the absence of any silica.
2.2.4 Pickering suspension polymerization. Bon et al. demonstrated that colloidosomes made from PMMA microgels could be used as reaction vessels to prepare raspberry microcapsules.103 The PMMA microgels were first used to generate micron-sized colloidosomes dispersed in water which had an interior phase composed of monomers and porogens. Subsequently a Pickering suspension polymerization process was carried out inside the colloidosomes.

Very recently, Shipp et al. demonstrated the thiol–ene suspension polymerization of monomers 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione and pentaerythritol tetrakis(3-mercaptopropionate) stabilized with silica nanoparticles (≈80 nm) to create raspberry-like colloidal composite particles that have a rough silica shell and a cross-linked poly(thioether) core.104 The use of a costabilizer was also found to be beneficial, and cetyl alcohol was most effective.

2.3 Formation of core and corona particles simultaneously by one-step polymerization

In this strategy, raspberry-like particles are typically prepared by heterophase copolymerization of the monomers, in which the core and corona particles formed simultaneously in one-step. This is different to the seeded polymerization with preformed seeds as cores and the Pickering polymerization with preformed stabilizer particles as coronas, as discussed above. The phase separation between different components within the copolymer is normally responsible for the formation of the raspberry-like morphology. Such method usually requires special formulations, and thus is generally not widely used. Until now, surfactant-free emulsion polymerization,112–116 emulsion polymerization,117 miniemulsion polymerization,118 and dispersion polymerization119–128 have been used to prepare polymer–polymer raspberry-like particles, while double in situ miniemulsion polymerization129–131 has been developed to prepare polymer–silica raspberry-like particles.
2.3.1 Surfactant-free emulsion polymerization. The formation of polymer particles with a raspberry-like morphology by a one-step polymerization is not in itself novel. As early as in 1986, it was found that copolymer particles produced by emulsifier-free emulsion copolymerization of St and HEMA had an anomalous shape with an uneven surface as phase separation occurred in the particles, resulting in the heterogeneous structure.112 More specially, in the cases of 5 and 10 mol% HEMA contents, raspberry-like particles were observed.

Wu et al. reported a one-step synthesis of monodispersed raspberry-like colloidal polymer particles using surfactant-free emulsion polymerization of St, MPS and acrylic acid (AA).113 In this method, when St, MPS and AA were mixed with water and stirred at a certain temperature for a period of time, large amounts of raspberry-like P(St–MPS–AA) particles with uniform shapes and sizes were produced (see Fig. 10). Both the larger core particles and the smaller corona particles of the core surfaces were formed in situ. The formation mechanism of the raspberry-like P(S–MPS–AA) particles is believed to involve the second nucleation and phase separation induced by the MPS-enriched segments on the surfaces.


image file: d0py00394h-f10.tif
Fig. 10 Schematic illustration of raspberry-like P(S-MPS-AA) colloidal particles. Reproduced with permission from ref. 113. Copyright 2013, Royal Society of Chemistry.

Similarly, Zhang et al. reported the synthesis of raspberry-like polymer particles via a one-step surfactant-free emulsion polymerization process of St and GMA.114 In this method, when St, GMA, and potassium peroxodisulfate were mixed with water and stirred at a certain temperature for a period of time, raspberry-like P(St–GMA) particles with uniform shapes and sizes were produced. The formation of raspberry-like particles is caused by phase separation in the later stage of the polymerization. At the beginning of the polymerization, GMA-enriched copolymers are produced to form GMA-enriched core particles. As the polymerization continues, the copolymers gradually turn to St-enriched in the later polymerization stage. The St-enriched copolymers are incompatible with GMA-enriched core particles and migrate from the core particles to form corona particles on the surface of core particles. The size of the corona particles and the roughness of the raspberry-like particles could be easily tailored by adjusting the amount of St, GMA, and divinylbenzene (DVB).

2.3.2 Emulsion polymerization. Different to copolymer particles, homopolymer particles with a raspberry-like morphology (strictly speaking, they are not polymer composite particles) are not commonly found in the literature as no phase separation occurs. However, Okubo et al. proposed that submicrometer-sized raspberry-like PS particles could be prepared by emulsion polymerization with polyoxyethylene nonylphenyl ether nonionic emulsifier (Emulgen 910) and potassium persulfate initiator, contained 8.5 vol% (relative to the particle) of water and 5.5 wt% (relative to PS) of Emulgen 910 in the inside obtained.117 The nonspherical shape is ascribed to a heterocoagulation between large preexisting particles and new small particles caused by an unincorporated portion of the emulsifiers released from the monomer droplets (i.e., emulsifier in the aqueous phase).
2.3.3 Miniemulsion polymerization. We are aware of only one literature example in which raspberry-like polymer particles were prepared by a one-step miniemulsion polymerization. In a hybrid miniemulsion polymerization of acrylic monomers in the presence of alkyd resin, a raspberry-like morphology consisting of a full coverage of small (∼25 nm) PMMA spheres anchored to alkyd core particle surface was found to form through grafting.118 Migration of the PMMA spheres to composite particle surface is thought to be induced by phase separation.
2.3.4 Dispersion polymerization. There are several examples of raspberry-like particles prepared by dispersion polymerization in one step. For example, Hu et al. reported the preparation of poly((glycidyl methacrylate)-co-(ethylene glycol dimethacrylate)) P(GMA-co-EGDMA) raspberry-like colloidal particles bearing micro-/nano-scale surface roughness via the one-pot dispersion polymerization of GMA and a cross-linkable monomer EGDMA using PVP as a stabilizer and azobisisobutyronitrile (AIBN) as an initiator in EtOH.119 The raspberry-like structure is believed to arise from the introduction of a high content of EGDMA cross-linker, which copolymerizes with the GMA in a heterogeneous fashion. Du et al. reported a one-step synthesis of highly cross-linked raspberry-like nano/microspheres using the dispersion polymerization of N-vinylimidazole(VI) in the presence of two cross-linking agents (PEGDMA and EGDMA).120–122

Recently, Lan et al. synthesized raspberry colloids by injecting a ternary mixture of acrylate monomers (AMs), St and the cross-linker DVB into a water–ethanol solution (80/20 v/v) containing the initiator AIBA at 70 °C.123 Due to the different solubilities of AM and St monomers in water–ethanol mixtures, dispersion polymerization of the AMs and surfactant-free emulsion polymerization of St-rich emulsion droplets occurred simultaneously, resulting in ∼45 nm-sized polyacrylate (PA) particles and ∼200 nm-sized particles containing mainly PS, respectively. The 45 nm PA and 200 nm PS-rich particles then fused to the final shape of raspberry colloids in the presence of cross-linker DVB. Mechanism of the one-step polymerization approach for raspberry particles is shown in Fig. 11. At the beginning, AM-rich solution and St-rich heterogeneous monomer droplets coexist in the continuous phase (c). Upon initiation, nanoscale PA-rich particles and microscale PS-rich particles form in the same system (d), which are brought together in the presence of cross-linker DVB, forming raspberry colloids (e). The solvent conditions and the cross-linker were crucial in the formation of the raspberry shape.


image file: d0py00394h-f11.tif
Fig. 11 One-step synthesis of raspberry colloids. (a) 2.0, 20.0 and 1.0 mmol of acrylate monomer, St and DVB are polymerised in 50 mL of a water–ethanol mixture (80/20 v/v), containing 0.2 mmol AIBA as initiator at 70 °C. Raspberry particles are formed as cross-linked hybrid polymers with polyacrylate (PA) and PS sections. (b) TEM image of the raspberry particle. (c–e) Mechanism of the one-step polymerization approach for raspberry particles. Reproduced with permission from ref. 123. Copyright 2018, Springer Nature Limited.

It is also noteworthy that, aqueous dispersion polymerization, either by conventional free radical polymerization or living radical polymerization, is an attractive technique to prepare nanostructures with different morphologies including framboidal (i.e. raspberry-like) morphology in a convenient one-shot batch process, although the polymer particles do not belong to composite particles in the strict sense.

For example, Hasegawa et al. reported the syntheses of phenylboronic acids-containing framboidal nanoparticles via one-step aqueous dispersion polymerization of N-acryloyl-3-aminophenylboronic acid in the presence of a polymerizable poly(ethylene glycol)acrylamide dispersant and a N,N′-methylenebis(acrylamide) cross-linker.124 Recent advances in PISA, as mentioned in the section 2.1.2, have enabled a wide range of block copolymer nanoparticles to be prepared directly in aqueous solution at relatively high solids via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerisation. For example, poly(glycerol monomethacrylate-block-2-hydroxypropyl methacrylate) [PGMA–PHPMA] diblock copolymer vesicles can be prepared by RAFT aqueous dispersion polymerization of HPMA using a PGMA macromolecular chain transfer agent. PISA occurs to form nanoparticles with PHPMA cores that are stabilised by the water-soluble PGMA chains. Armes et al. reported that a further chain extension of such PGMA–PHPMA precursor vesicles using a water insoluble monomer (benzyl methacrylate)(PBzMA) via seeded RAFT emulsion polymerization resulted in the formation of ABC triblock copolymer vesicles with a distinctive framboidal morphology, which is attributed to microphase separation between the PHPMA and PBzMA hydrophobic blocks.125,126 Similarly, pH-responsive framboidal ABC triblock copolymer vesicles were prepared simply by using 2-(diisopropylamino)ethyl methacrylate instead of benzyl methacrylate,127 which have potential applications such as targeting triple-negative breast cancer cells.128

2.3.5 Double in situ miniemulsion polymerization. Double in situ miniemulsion polymerization refers to the in situ miniemulsion polymerization of monomers and the in situ hydrolysis-condensation of tetraethyl orthosilicate (TEOS) under basic conditions. The simultaneous formation of both polymer and inorgainc components with controllable morphologies (e.g. raspberry-like) using this one-pot method is somewhat challenging because there is a significant difference between the conditions for polymerization and formation of inorganic materials.

Ge et al. fabricated P(MMA-b-BA)-SiO2 microspheres in a one-step process. The method involved in situ miniemulsion polymerization of MMA/BA using MPS as functional monomer in the presence of TEOS.129 The resulting 200–400 nm nanocomposite microspheres had a raspberry-like morphology with silica nanoparticles of ∼20 nm on the shells. The formation mechanism can be attributed to the phase separation between the growing polymer particles and TEOS. During the polymerization of organic monomers, the TEOS phase is extruded out from the miniemulsion droplets as nodules by phase separation. Due to the low miscibility between TEOS and polymer, the nodules will not come together to form a continuous shell of the polymer particles. Therefore, TEOS nodules nucleate and then grow into silica nanoparticles under basic conditions.

Very recently, Ziener et al. developed a similar method to prepare raspberry-like poly(St-co-4VP)-SiO2 nanocomposite particles with a diameter of around 200 nm.131 As shown in Fig. 12, this was done easily in a miniemulsion of St, 4VP and TEOS in water stabilized by the water-soluble dye Eosin Y disodium salt. During radical polymerization of the nanodroplets, the silane phase was expelled from the polymer phase to the oil/water interface. In the later polymerization stage, silica nanoparticles with a size of around 25 nm were produced via the in situ sol–gel reaction of TEOS at the o/w interface promoted by the negatively charged dye. The pyridine moieties in the copolymer could fix the silica nanoparticles on the surface of the polymer particles by electrostatic interaction, which allowed no free-silica nanoparticles to be produced.


image file: d0py00394h-f12.tif
Fig. 12 Schematic illustration of the formation of raspberry-like poly(St-co-4VP)-SiO2 nanocomposite particles via a one-step heterophase polymerization process accompanied by an interfacial sol–gel reaction. Reproduced with permission from ref. 131. Copyright 2019, American Chemical Society.

3. Formation of inorganic coronas on the polymer core particles

In this method, inorganic precursors are controllably precipitated onto the polymer core particle to form inorganic-armored composite particle. Typically, silica coronas are formed by a sol–gel process using the well-known precursor TEOS. In order to circumvent the inherent incompatibility between the polymers and the inorganics, the formation of raspberry-like microspheres usually requires functionalization or modification of the polymer particles. This can be realized by functionalization or modification of the preformed polymer particles, or functionalization of the polymer by the use of functional ingredients (i.e., the surfactant, monomer, or initiator) in polymerization recipes.132

So far, the reported polymer–inorganic core–corona systems include poly((2-(methacryloyloxy)ethyl)trimethylammonium iodide)-SiO2,133 PS-SiO2,134–145 PNIPAm-SiO2,146–148 PMMA-SiO2,149 PS@poly(2-(diethylamino)ethyl methacrylate)-SiO2,150 P(St-co-GMA)@Fe3O4-SiO2,151 poly(N-benzophenoyl methacrylamide-co-N-hydroxyethyl acrylamide)-SiO2,152 poly(ethylene oxide)-functionalized latexes-SiO2,153 PS-ZnO,154–156 PS-In(OH)3,157 poly(vinyl chloride)-ZnO,158 P(MMA-AA-DVB)-Fe3O4,159 PS-CeO2,160 PS-Au,161 dopamine acrylamide copolymer-coated SiO2-Ag,162 poly(St-co-sodium 4-vinylbenzenesulfonate)-Ag,163 poly(2-hydroxyethyl methacrylate)-Ag,164etc. As it can be seen, besides SiO2, a variety of other inorganics including ZnO, In(OH)3, Fe3O4, CeO2, Au and Ag have been used as the corona materials.

He et al. fabricated monodispersed raspberry-like PS@SiO2 composite nanoparticles by a single-step sol–gel process using oxygen plasma-treated PS spheres (∼615 or 270 nm) as cores (see Fig. 13).134 However, control experiments conducted using other PS spheres did not result in raspberry-like composite particles. When PVP-modified PS spheres were used as core, particles with an inhomogeneous coverage of silica particles were obtained; when the surface of PS spheres was positively charged or functionalized with silanol groups, silica sols could easily nucleate on the surface of PS spheres and eventually merge and grow into a thin shell of uniform thickness. The presence of surface hydroxyl groups as produced by oxygen plasma treatment was responsible for the formation of raspberry-like nanoparticles with a protruding surface morphology. This can be explained as follows: when oxygen plasma-treated PS spheres are used as core, the co-condensation reaction between C-OH groups attached to polymer chains and TEOS is slower than that between Si-OH groups and TEOS, since the hydroxyl groups (C-OH) attached to polymer chains are less reactive than silanol groups. Thus, raspberry-like composite particles of a protruding surface are obtained. The diameter of silica nanoparticles and their coverage degree on the surface of PS spheres could be easily tailored by adjusting the TEOS concentration.


image file: d0py00394h-f13.tif
Fig. 13 Schematic illustration of the fabrication of PS@SiO2 composite particles with different kinds of morphology with different PS spheres as core. Reproduced with permission from ref. 134. Copyright 2009, American Chemical Society.

Zhang et al. prepared raspberry-like particles by introducing poly(acrylic acid) (PAA)-functionalized PS particles into hydrolysis reaction of TEOS.135 The PAA-functionalized PS particles with ∼300 nm in diameter were used as cores and nanosized silica particles were then assembled at the surface of cores in the hydrolysis reaction to construct the raspberry-like particles. With the increase of PAA content from 11 wt% to 20 wt% at the surface of latexes, the diameter of the silica particles assembled at the surface of cores could be adjusted from 124 nm to 36 nm.

Schurtenberger et al. reported the synthesis of hybrid raspberry microgels with a temperature-responsive PNIPAm core and a shell made of in situ formed silica nanoparticles.146 This was achieved by copolymerizing MPTMS with the NIPAM in the outer shell of the polymer particles to promote the silica anchoring on the PNIPAm microgel.

Both ZnO and In(OH)3 are important semiconductor materials that can offer interesting properties for various applications. Agrawal et al. reported the preparation of PS-ZnO composite particles with core–shell or raspberry-like morphology by controlling critical reaction parameters.154 The interaction between ZnO nanoparticles and β-diketone groups, present on the surface of PS beads, was the driving force for the preparation of these composite particles. As shown in Fig. 14, first, the ZnO precursor (Zn(Ac)2·2H2O) interacts with the β-diketone groups of PS beads and is adsorbed on their surface; second, these precursors are converted into ZnO nanoparticles after reacting with NaOH. It was demonstrated that higher NaOH concentration (1 M) and higher reaction temperatures (55 and 70 °C) favored the raspberry-like morphology because of the accelerated nucleation and growth processes as well as an induced coarsening effect of ZnO nanoparticles. In a follow-up work, PS-In(OH)3 composite particles were fabricated by hydrolyzing the In(OC3H7)3 salt in the presence of β-diketone functionalized PS colloidal particles.157 SEM and TEM results illustrated that variation the In(OC3H7)3 concentration could effectively tune the morphology of resulting composite particles between core–shell and raspberry-like.


image file: d0py00394h-f14.tif
Fig. 14 Schematic presentation of the synthesis of PS-ZnO composite particles with raspberry and core–shell morphology. Reproduced with permission from ref. 154. Copyright 2007, American Chemical Society.

Raspberry-like composite particles with nobel metal (e.g. gold, silver) nanoparticle coronas on polymer substrates have also received widespread attention.161–164 For example, Kang et al. described the synthesis of raspberry-like SiO2@poly(dopamine acrylamide-co-methacrylic acid-co-ethylene glycol dimethacrylate)-Ag (or SiO2@PDA-Ag) composite nanospheres with a SiO2 nanocore, PDA shell, and Ag satellite nanoparticles.162 In this approach, SiO2@poly(pentafluorophenyl acrylate-co-methacrylic acid-co-ethylene glycol dimethacrylate) core–shell nanospheres with reactive pentafluorophenyl esters were first prepared by distillation–precipitation polymerization, using the MPS modified-SiO2 nanospheres as seeds. Postfunctionalization of reactive pentafluorophenyl ester groups with dopamine produced the catecholamine-containing SiO2@PDA nanospheres. The catecholamine moieties in the SiO2@PDA nanospheres were utilized for simultaneous reduction of Ag+ ions and coordinative binding of the metal nanoparticles. Surface coverage of the Ag satellite nanoparticles on the SiO2@PDA nanospheres could be easily tuned by varying the AgNO3 salt concentration for reduction.

Compared to metal and metal oxide nanoparticles, the loading of hydrophilic metal salts into the polymer particles is much more convenient. Especially in inverse systems, metal salts can improve the droplet stability and narrow the particle size distribution. In addition, the maximum loading amount of metal salts in inverse miniemulsion is much higher due to their high solubility in the polar dispersed phase. Poly(2-hydroxyethyl methacrylate) (PHEMA)-Ag hybrid particles were synthesized via the combination of inverse miniemulsion and silver ion reduction.164 A large amount of silver salt (3.2 to 12.9 mol% to monomer) was first loaded into the PHEMA particles by inverse miniemulsion polymerization of HEMA with silver tetrafluoroborate (AgBF4) as a lipophobe. The PHEMA-Ag hybrid particles with a raspberry-like morphology were subsequently formed via a gas-phase in situ reduction of AgBF4 by hydrazine on the surfaces of the PHEMA/silver salt particles (see Fig. 15). It was assumed that the formation of raspberry-like hybrid particles was caused by the slow diffusion of hydrazine to the dispersed phase and the fast reduction of silver particles by hydrazine.


image file: d0py00394h-f15.tif
Fig. 15 Synthesis of poly(2-hydroxyethyl methacrylate)-Ag particles via a combination of inverse miniemulsion and silver ion reduction. Reproduced with permission from ref. 164. Copyright 2011, American Chemical Society.

4. Heterocoagulation

In a heterocoagulation process, the raspberry-like particles are prepared by interaction of preformed core and corona particles. In view of the nature of the interaction between the core particles and corona particles, heterocoagulation can be divided into noncovalent and covalent attachments.

4.1. Noncovalent attachment165–192

Typically, electrostatic interaction165–177 or hydrogen bonding178–185 is the driving force for noncovalent attachment between particles. Other interactions such as charge compensation interactions,186,187 steric stabilization,188 π–π interactions,189–191 and supramolecular interactions192 have also been utilized for the preparation of raspberry-like polymer composite particles. Examples of raspberry-like particles prepared by noncovalent attachments are listed in Table 1.
Table 1 Summary of raspberry-like polymer composite particles prepared by noncovalent attachments
Type of interaction Core particle Corona particles Ref.
electrostatic interaction Cationic P(DVB-co-StMTMACl) Anionic P(EGDMA-co-AcNa) 167
Positively charged PS Negatively charged Fe3O4@SiO2 168
Amine-modified PS Negatively charged silica, gold, maghemite 169
Amine-silanized SiO2 Negatively charged P(MAA-co-MMA) 170
Amino-terminated polymer Negatively charged gold 171
Positively charged PGMA Negatively charged gold 172
Anionic PMMA Cationic PS 175
Positively charged PS Negatively charged PS 176
Positively charged PS Negatively charged SiO2 177
Hydrogen bonding P(EGDMA-co-VPy) P(EGDMA-co-AA) 178
PAA-stabilized PS PVP-stabilized PS 184
PEO-b-PBA PAA-b-PBA 185
Charge compensation P(DVB-co-StMPyCl) P(EGDMA-co-AA) 186
PVDF/P4VP P(St-co-AA) 187
Steric stabilization PS SiO2 188
π–π interaction PS Carbon black 189
Supramolecular interaction Azobenzene-functionalized SiO2 Viologen-functionalized polymer 192


4.1.1 By electrostatic interactions. Both of raspberry-like polymer–polymer particles and polymer–inorganic particles can be fabricated through electrostatic interactions. It is perhaps noteworthy that the term of “heterocoagulation” originally referred to irreversible contact between particles typically with opposite charges through electrostatic interactions, while this definition is not strictly followed later.201,202 The use of electrostatic interactions for the preparation of raspberry-like polymer particles was first reported by Furusawa and Anzai in 1992.165 It was found that uniform composite particles composed of amphoteric latex particles adhering to large core silica particle were obtained when the particle size ratio between the large and small particles was higher than 3. Later, Ottewill et al. investigated the heterocoagulation of anionic PS particles with smaller electrosterically-stabilized cationic poly(butyl methacrylate) particles.166 Yang et al. described a route to raspberry-like polymer composite particles via the heterogeneous self-assembly of poly(ethyleneglycol dimethacrylate-co-sodium acrylate) P(EGDMA-co-AcNa) corona nanospheres and poly(divinylbenzene-co-styrylmethyl trimethyl ammonium chloride) (P(DVB-co-StMTMACl)) core microspheres by an electrostatic interaction between the sodium acrylate and the quaternary ammonium chloride groups.167

Raspberry-like composite particles with metal (e.g. gold) nanoparticles on polymer substrates can also be conveniently fabricated by heterocoagulation, which has been suggested to overcome the problem of formation of irregular metal nanoparticles in in situ metal reduction method.169,171–174 For example, Yabu et al. fabricated gold nanoparticle–polymer composite particles with raspberry morphology via electrostatic interactions between the positively charged amino-terminated PS or amino-terminated 1,2-polybutadiene particles and negatively charged Au nanoparticles.171 Chen et al. synthesized raspberry-like gold nanoparticles-coated composite spheres based on chemically reactive PGMA cores.172 Specifically, negatively charged PGMA spheres were modified with a layer of weakly hydrophobic cationic polyelectrolyte film. Then negatively charged gold nanoparticles were coated onto the modified PGMA colloids in raspberry-like fashion.

Different to common raspberry-like particles based on core of spherical particle, Wagner et al. presented a type of raspberry-like particles with dual surface roughness based on core of PS particle cluster by two steps. First, amine-modified PS particles with 154 nm diameter were assembled into clusters of well-defined configurations. Second, oppositely charged inorganic particles with diameters of only a few nanometres were deposited via electrostatic attraction.169

4.1.2 By hydrogen bonding interactions. The hydrogen bonding interaction is widely utilized to prepare raspberry-like polymer–polymer composite particles. In 2006, Yang et al. described the heterocoagulation based on a hydrogen interaction process to prepare anomalous core–corona polymer composites with a raspberry-like morphology having uneven surfaces through the self-assembly of poly(ethylene glycol dimethacrylate-co-acrylic acid) (P(EGDMA-co-AA)) small microspheres on poly(ethylene glycol dimethacrylate-co-4-vinylpyridine) (P(EGDMA-co-VPy)) surfaces.178 The hydrogen-bonding interaction of pyridine and carboxylic acid between the core and corona particles played a key role as the driving force during the heterocoagulation. However, the hydrogen bonding interaction between the two polymers often induced the formation of precipitated interpolymer complexes in aqueous solution.

Bouteiller et al. described a strategy towards the high yield preparation of raspberry-like polymer particles by simple mixing PAA-b-PBA core–shell nanoparticles (∼80 nm) and larger PEO-b-PBA core–shell (∼230 nm) at high solids content (23 wt%) at room temperature without any particular precaution (no dropwise addition, no pH adjustment).185 The heteroaggregation process is probably driven by the hydrogen-bond interactions between the PAA and the PEO shells of the particles.

It is also worth mentioning that Minami et al. demonstrated the preparation of raspberry-like particles by simply mixing large PS particles stabilized by PAA (as the core) and small PS particles stabilized by PVP (as the corona) utilizing the hydrogen bonding interaction between PAA and PVP (see Fig. 16),184 although strictly speaking this is not an example of polymer composite particles as both the core and corona particles are PS. The morphology of the raspberry-like particles was controlled by the addition of the free stabilizer and by tuning the molecular weight of the stabilizers. Moreover, various composite raspberry-like particles of various coverages could be formed easily by mixing various polymer particles stabilized by PVP and PAA.


image file: d0py00394h-f16.tif
Fig. 16 Preparation of raspberry-like polymer particles by a heterocoagulation technique utilizing hydrogen bonding interactions between steric stabilizers. Reproduced with permission from ref. 184. Copyright 2013, American Chemical Society.
4.1.3 By charge compensation interactions. Raspberry-like polymer–polymer composite particles could also be formed based on the charge compensation interactions (i.e. acid–base interactions). Yang et al. prepared raspberry-like polymer composite particles by a self-assembled heterocoagulation of nano-sized poly(ethyleneglycol dimethacrylate-co-acrylic acid) (P(EGDMA-co-AA)) particles on poly(divinylbenzene-co-styryl methyl pyridinium chloride) (P(DVB-co-StMPyCl)) surfaces based on a charge compensation mechanism.186 To be more specific, the interparticle-complexation of polymer particles were achieved through the affinity complex between the lone electron pair of carboxylic acid (as electron-donating group) and pyridinium group with strong electron-accepting property. It was also verified that the driving force for the formation of the raspberry-like polymer composite was not the electrostatic interaction between the carboxylic acid and the pyridinium groups. A similar example was reported by Pan et al., who performed the self-assembly of snowman-like PVDF/P4VP particles and P(St-co-AA) particles to obtain raspberry-like colloidal particle clusters with PVDF bulges protruding outward.187 This is driven by the strong interactions between the carboxylic acid groups in PAA and the pyridine groups in P4VP.
4.1.4 By steric stabilization. A heterocoagulation strategy based on colloidal steric stabilization theory has been developed by Wang et al., through which PS and silica particles without any surface modification or functionalization self-assembled rapidly via solution to afford nanocomposite particles with raspberry-like morphology.188 An overview of the proposed heterocoagulation protocol is shown in Fig. 17. The first procedure (a) was purification of the PS host particles by washing off the PVP left on the particle surface, followed by transfer into pure water from 2-propanol. The PS particles were thus in a metastable state due to the lack of steric stabilizer and enhanced interfacial tension. After blending with the guest SiO2 (b) at ambient temperature, colloidal self-assembly occurred instantaneously (c). It seemed that small, negative-charged silica particles readily took the place of soluble PVP as the steric stabilizer to protect the “naked” PS particles suspended in water; i.e., the heterocoagulation was probably driven by the requirement of colloidal steric stabilization.
image file: d0py00394h-f17.tif
Fig. 17 Schematic representation of the proposed heterocoagulation strategy. Reproduced with permission from ref. 188. Copyright 2008, American Chemical Society.
4.1.5 By π–π interactions. Carbon black is a low-cost inorganic filler that widely used in the preparation of polymer composites. Raspberry-like PS-carbon black composite microspheres have been prepared via π–π interactions between the grafting hindered phenol antioxidant Irganox 1330 on carbon black surface and the benzene ring of PS molecules, as reported by Huang et al.189–191 Scanning electron microscopy demonstrated that modified carbon black could be homogenously adsorbed on the surface of PS microspheres.
4.1.6 By supramolecular interactions. Based on the unique ternary complexation ability of cucurbit[8]uril (CB[8]), which is a well-known macrocyclic host molecule that can simultaneously accommodate two guest molecules,193 raspberry-like polymer composite particles with an inorganic core have been prepared. Scherman et al. reported the preparation of hybrid raspberry-like colloids bearing silica microspheres as the core and polymeric nanoparticles as the corona by CB[8] host–guest complexation in water.192 CB[8] was employed as the supramolecular linking agent to lock methyl viologen functionalized polymeric nanoparticles (MV-NP corona) onto 4-hydroxyazobenzene-(Azo-) functionalized silica microspheres (Azo-silica core) by forming the (MV/trans-azo)@CB[8] ternary complexes (see Fig. 18). Significantly, the formed raspberry-like colloids were photoresponsive and could be reversibly disassembled upon light irradiation.
image file: d0py00394h-f18.tif
Fig. 18 (a) Stepwise formation of (MV/trans-azo)@CB[8] ternary complex and light driven reversible disassembly of the ternary complex. (b) HRCs obtained by the formation of (MV/trans-azo)@CB[8] ternary complexes and light-driven reversible disassembly of the HRCs. Reproduced with permission from ref. 192. Copyright 2014, Wiley-VCH.

4.2. Covalent attachment194–200

As is well known, covalent interaction is much stronger than noncovalent interaction, which provides strong stability to the obtained raspberry-like particles. A few examples of raspberry-like particles prepared by covalent interactions between particles with different functional groups have been reported. The used strategies include pyridinium reaction,194 azide–alkyne click reactions,195 epoxy-amine reactions,196–198 silanol-alkoxysilane reaction,199 and UV-curing,200 as summarized in Table 2.
Table 2 Summary of raspberry-like polymer composite particles prepared by covalent attachments
Type of reaction Core particle Corona particles Ref.
Reaction between chloromethyl and pyridyl groups Poly(DVB-co-CMSt) Poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy) 194
Azide–alkyne click reaction Azide-functionalized P(EGDMA-co-HEMA) Alkynyl-modified P(MBA-co-MAA) 195
Reaction between amino and epoxy groups Amino-functionalized PS particles Epoxy-functionalized PS 196
Epoxy-functionalized poly(vinylphenylglycidyl ether) Amino-functionalized SiO2 197
Amino-functionalized P(DVB-co-GMA)-NH2 Epoxy-functionalized SiO2/P(DVB-co-GMA) 198
Reaction between silanol group and alkoxysilane SiO2 Trialkoxysilanes-functionalized polymer 199
UV coupling Aminobenzophenone-functionalized PS PNIPAm-based microgels 200


In 2008, Yang et al. synthesized core–corona polymer hybrids with a raspberry-like structure by a self-assembly process between the chloromethyl groups of poly(divinylbenzene-co-chloromethyl styrene) (P(DVB-co-CMSt)) core microspheres and the pyridyl groups of poly(ethylene glycol dimethacrylate-co-methacrylic acid)@poly(ethylene glycol dimethacrylate-co-4-vinylpyridine) (P(EGDMA-co-MAA)@P(EGDMA-co-VPy)) small corona particles.194 The resultant core–corona polymer hybrids had a high coverage when the mass ratio of corona particles to core microspheres was higher than 1/10. FT-IR spectra demonstrated that the nature of the construction for raspberry-like polymer hybrids was the heterocoagulated pyridinium reaction between the chloromethyl group of the core microsphere and the pyridyl group of the corona particle. In a subsequent study, raspberry-like microspheres were prepared through a click reaction between azide group and alkynyl group.195 The azide-functionalized poly(ethylene glycol dimethacrylate-co-2-hydroxyethyl methacrylate) (P(EGDMA-co-HEMA)) microspheres behaved as the core and the alkynyl-modified poly(N,N-methylene diacrylamide-co-methacrylic acid) (P(MBA-co-MAA)) microspheres acted as the corona.

Liu et al. presented the preparation of a type of raspberry-like particles using two glycidyl-bearing particles with different sizes (212 and 332 nm, respectively).196 The smaller particles (s-GMA) were prepared from the surfactant-free emulsion copolymerization of St, GMA and DVB. Meanwhile, the larger l-GMA particles were prepared via surfactant-free seeded emulsion polymerization. The cores of the l-GMA particles were prepared through the copolymerization of St and DVB (A → B, Fig. 19). Subsequently, polymerization of GMA, St, and DVB onto these core particles yielded a GMA-based shell (B → C). Reaction between the glycidyl groups of the l-GMA particles and 2,2′-(ethylenedioxy)bis-(ethylamine) produced large amine-functionalized particles (l-NH2)(C → D). Finally, the raspberry-like particles were prepared by reacting an excess of the s-GMA particles with the l-NH2 particles (D → E).


image file: d0py00394h-f19.tif
Fig. 19 Schematic illustration of the procedure used to prepare the raspberry-like particles. Reproduced with permission from ref. 196. Copyright 2014, American Chemical Society.

Similarly, raspberry-like hybrids with a polymer core (348 nm) and smaller silica shell were prepared by utilization of the reaction of poly(vinylphenylglycidyl ether)-containing core/shell particles bearing epoxy groups and amino-functionalized silica particles.197 Raspberry-like particles were also prepared through the reaction between the epoxy groups of silica/poly(divinylbenzene-co-glycidylmethacrylate) (SiO2/P(DVB-co-GMA)) core–shell microspheres and the amino groups of amino-modified poly(divinylbenzene-co-glycidyl methacrylate)(P(DVB-co-GMA)-NH2) microspheres, in which P(DVB-co-GMA)-NH2 acted as the core and SiO2/P(DVB-co-GMA) acted as the corona.198

Zhang et al. developed a strategy to covalently anchor polymer nanoparticles onto the surface of silica for the fabrication of raspberry-like composite particles based on silanol-alkoxysilane reaction.199 First, the trimethoxysilane-end-capped poly(dimethylaminoethyl methacrylate)-block-PS (Tsi-PDMAEMA-PS) was readily prepared via a RAFT polymerization. The amphiphilic Tsi-PDMAEMA-PS could self-assemble in water to obtain the positively charged trialkoxysilanes-functionalized polymer nanoparticles, which were able to effectively deposit on the surface of negatively charged silica microspheres due to electrostatic heterocoagulation and then form the covalent bonds between alkoxysilanes and surface silanols.

5. Miscellaneous methods

Apart from the above-mentioned methods, various alternative techniques have been developed for raspberry-like polymer particles syntheses.203–208 Most of the methods involve post-treatment of the preformed polymer (composite) particles. Several examples are discussed in the following section.

Polyelectrolyte multilayers (PEMs), prepared via the layer-by-layer (LbL) technique based on sequential deposition of oppositely charged polyelectrolytes on a surface, could be used as platforms for producing raspberry-like particles.203 As shown in Fig. 20, PEM-coated melamine–formaldehyde (MF) particles were first prepared via the LbL self-assembly technique, and then used as a template to synthesize Au nanoparticles by absorption of HAuCl4 and reduction with NaBH4. Subsequently, a silica shell was deposited. Finally, the silica-shell particles were reacted with KCN, which dissolved the Au nanoparticles and also changed the silica shell significantly, leading to raspberry-like particles.


image file: d0py00394h-f20.tif
Fig. 20 (a) Schematic illustration for the synthesis of the raspberry-like particles. SEM images of each step for the formation of raspberry-like particles: (b) PEM-coated MF particles (PEMPs), (c) Au nanoparticles-coated PEMPs (particle diameter 1.73 μm), (d) Au/SiO2-coated PEMPs (particle diameter 1.91 μm), (e) raspberry-like particles formed by the KCN treatment of particles in (d). Reproduced with permission from ref. 203. Copyright 2007, Royal Society of Chemistry.

Ziener et al. reported the preparation of raspberry-like P(St-co-4VP) nanocapsules via the combination of Pickering emulsification and solvent displacement technique with P(St-co-4VP) as shell polymer, silica particles as stabilizer and hexadecane (HD) as soft template.205 The formation of the capsule morphology was caused by the phase separation of P(St-co-4VP) in the droplets. In detail, the P(St-co-4VP) underwent phase separation at the interface to form a shell on the soft template (HD) due to the diffusion of the THF, a good solvent of copolymer, into the aqueous phase. The decrease of the capsules’ size by stirring was caused by the diffusion of HD between the droplets driven by the Ostwald ripening and the coagulation of large droplets to form a macroscopic oil layer on the surface of the dispersion. The electrostatic and acid–base interactions between the nitrogen atom of 4VP units in the copolymer and the hydroxyl groups on the surface of silica particles were crucial for the adsorption of silica particles on the surface of nanocapsules to provide colloidal stability to and control the size of the nanocapsules.

Template-assisted polymerization, which means polymer particles are employed as templates and reaction vessels inside which polymers are formed, is a powerful and scalable technique to prepare polymer particles with advanced structures. Very recently, Scherman et al. presented the synthesis of raspberry-like PMMA particles via template-assisted polymerization.207 As shown in Fig. 21, non-cross-linked PS particles were employed as templates and reaction vessels for the polymerization of MMA and the cross-linking agent DVB. Cross-linked PMMA with internally-connected nano domains were then formed inside the reaction vessels. Due to phase separation and proper elastic restriction from the PS template, monodispersed PMMA raspberry-like particles were obtained after removal of the PS template with DMF, which is a good solvent for non-cross-linked PS at room temperature. Functional PMMA raspberry-like particles bearing methyl viologen groups could also be prepared using this method.


image file: d0py00394h-f21.tif
Fig. 21 Template-assisted polymerization method for the preparation of PMMA-based raspberry-like particles. Reproduced with permission from ref. 207. Copyright 2019, Royal Society of Chemistry.

More recently, Kanai et al. reported a facile method for preparing monodispersed hybrid smart gel particles with various morphologies via the combination of microfluidic technique and swelling–shrinking phenomenon of PNIPAm gel particles by varying only the flow rate and temperature.208 Monodispersed droplets of acrylamide aqueous solution containing one or two PNIPAm gel particles could be generated through the microfluidic device. The core–shell structures could be transformed into snowman-like, raspberry-like, and dumbbell-like shapes thorough the swelling-shrinking phenomenon of PNIPAm gel particles by altering the temperature.

6. Summary and outlook

During the past two decades, there has been increasing interest in the synthesis of raspberry-like polymer composite particles, which have inorganic–polymer, polymer–inorganic or polymer–polymer core–corona structures. As summarized in this review, such syntheses typically involve seeded polymerization, Pickering polymerization, formation of inorganic coronas on the polymer core particles, and heterocoagulation, although other techniques such as one-step polymerization and template-assisted polymerization have also been described. More specifically, the seeded polymerization can lead to inorganic–polymer or polymer–polymer composite particles, depending on the nature of the seed particles; the Pickering polymerization method can lead to polymer–inorganic or polymer–polymer composite particles, depending on the nature of the stabilizer particles; the method of formation of inorganic coronas on the polymer core particles exclusively results in polymer–inorganic composite particles; while raspberry-like polymer composite particles with all the three structures can be produced by the heterocoagulation method.

To date, the more popular and versatile approach appears to be seeded polymerization, and most of the raspberry-like polymer particles reported are polymer–inorganic or inorganic–polymer composite particles. In this context, it is important to note that there are subtle differences between the “raspberry-like” particles prepared by different methods. The raspberry-like polymer composite particles prepared by seeded polymerization from monomer-swollen seeds might have a surface completely covered by the coronas,20 while for the raspberry-like polymer composite particles prepared by Pickering polymerization with ultrafine silica sols as stabilizer, a fraction of silica nanoparticles might locate within the interior of the nanocomposite particles.101 It is perhaps also worth emphasizing that there are significant differences between inorganic–polymer and polymer–inorganic composite particles in terms of properties. Generally speaking, for inorganic–polymer composite particle, the polymer coronas can protect the inorganic core and/or improve its processability. While for polymer–inorganic composite particle, the inorganic coronas can impart hydrophilicity, biocompatibility and modifiability to the polymer core; moreover, the chemical and thermal stability of the polymer core can be significantly improved.209

It is well-known that the raspberry-like particles have some attractive characteristics such as high surface roughness, large specific surface area and high degree of scattering when compared to the spherical ones, and they often exhibit improved physical and/or chemical properties over their single-component counterparts.161 Whatever the synthetic method, controlling the surface roughness of the core–corona particles is usually the major concern. Current studies have tended to focus on tuning the surface roughness in situ by changing the synthetic parameters. Alternatively, this can be done ex situ, such as surface smoothing via fusion of preformed composite particles with polymer coronas.170,210

Based on the precise nature of the polymer component and/or the inorganic component, various potential applications have been suggested for raspberry-like polymer composite particles, such as immunodiagnostic assays,211 synthetic mimics for silicate-based micrometeorites,212 superhydrophobic coatings,48,55,119,135,137,196 superhydrophilic coatings,134 oil–water separation,145 catalysis,66,161–163,172 sensors,154,167 rubber-reinforcing agents,81 peptide adsorption,120,121 surface enhanced Raman scattering,161 protein delivery,204 logic gate,206 just to name a few. Some selected examples are presented here.

In an early example, conducting polypyrrole-silica nanocomposite with raspberry morphology had been evaluated as highly coloured marker particles for immunodiagnostic assays, which was mainly based on the intense optical absorbance property of polypyrrole.211 Raspberry-like polypyrrole-silica and polyaniline-silica nanocomposite particles were also used as synthetic mimics for understanding the behavior of silicate-based micro-meteorites since the conducting polymers enable such particles to acquire sufficient surface charge to enable electrostatic acceleration.212

Given their intrinsic dual-size rough surfaces, the most important application of raspberry-like composite nanoparticles with SiO2 coronas is as building blocks for the fabrication of superhydrophilic/superhydrophobic coatings. For example, Zhang et al. prepared raspberry-like PS-SiO2 composite particles by a sol–gel process using PAA-functionalized PS spheres as cores.135 Subsequent deposition of the raspberry-like particles on a glass substrate, a hierarchically structured surface was fabricated. After surface modification with dodecyltrichlorosilane, superhydrophobic surface was obtained and the contact angle of water on the surface could reach 162.1°. Moreover, the superhydrophobic raspberry-like polymer composite particles can be utilized for oil–water separation. For example, a recent report described the preparation of superhydrophobic surfaces via hydrophobization modification of dual-scaled raspberry-like PS-SiO2 microspheres, exhibiting a water contact angle of 163.3° and a water contact angle hysteresis of 4°.145 The hydrophobic raspberry microsphere treated steel mesh demonstrated excellent oil–water separation efficiency and reusability. Catalysis has turned to be another important application field of the raspberry-like composite particles. For example, raspberry-like composites with silver-polypyrrole core–shell composites on the surface of silica spheres exhibited very good catalytic performance based on the reduction results of methylene blue dye with NaBH4 as the reducing agent.66 Raspberry-like PGMA-gold composite spheres exhibited an excellent catalytic activity with an average turnover frequency of 250.5 mol mol−1 s−1 and the fastest reported reduction of 4-nitrophenol with NaBH4 was achieved within 76 s.172 In addition, raspberry-like SiO2@polymer composite particles were reported to reinforce silicone rubber.81 Bearing both polymer moieties and Si-OH groups on the surface, these raspberry-like particles were better dispersed in the silicone rubber matrix, resulting in significantly enhanced mechanical performance.

While a lot has already been done, there is still much to do on raspberry-like polymer composite particles, especially when compared to the most common and widely studied spherical core–shell composite particles. Currently, only a limited types of polymers and inorganics have been utilized to synthesize raspberry-like polymer composite particles. Especially, there are relatively few reports on polymer–polymer composite particles. Thus, future work is likely to focus on developing composite particles based on new polymers and inorganics, as well as novel formation mechanisms. On the other hand, there are still very few reports on raspberry-like polymer composite particles in unusual forms (such as capsules and vesicles), and there are many potential systems to be explored. Finally, it is worth bearing in mind that such raspberry-like polymer composite particles should be technologically useful materials when designing the formulations.

Abbreviations

4VP4-Vinylpyridine
AAAcrylic acid
AAEMAcetoacetoxyethyl methacrylate
AIBA2,2′-Azobis(isobutyramidine)dihydrochloride
AIBNAzobisisobutyronitrile
BA n-Butyl acrylate
BMA n-Butyl methacrylate
DVBDivinylbenzene
EGDMAethylene glycol dimethacrylate
GMAGlycerol monomethacrylate
HEMA2-Hydroxyethyl methacrylate
HPMA2-Hydroxypropyl methacrylate
MBA N,N-Methylene diacrylamide
MMAMethyl methacrylate
MPS(Methacryloxy)propyltrimethoxysilane
P∼Poly∼
PAEMAPoly(2-acetoxyethyl methacrylate)
PDAPolydopamine
PDMNPsPoly(divinylbenzene-alt-maleic anhydride) nanoparticles
PISAPolymerization-induced self-assembly
PNIPAmPoly(N-isopropylacrylamide)
PSPolystyrene
PVDFPoly(vinylidene fluoride)
PVPPolyvinylpyrrolidone
RAFTReversible addition–fragmentation chain transfer
SDSSodium dodecyl sulphate
SEMScanning electron microscopy
StStyrene
TEMTransmission electron microscopy
TEOSTetraethyl orthosilicate
VPy4-Vinylpyridine

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51973117, 51503123), Program of Young Eastern Scholar from Shanghai Institutions of Higher Learning (QD2015014) and University of Shanghai for Science and Technology.

Notes and references

  1. H. Zou, S. Wu and J. Shen, Chem. Rev., 2008, 108, 3893–3957 CrossRef CAS PubMed .
  2. J. A. Balmer, A. Schmid and S. P. Armes, J. Mater. Chem., 2008, 18, 5722–5730 RSC .
  3. W. Ming, D. Wu, R. van Benthem and G. de With, Nano Lett., 2005, 5, 2298–2301 CrossRef CAS PubMed .
  4. E. Bourgeat-Lami and M. Lansalot, Adv. Polym. Sci., 2010, 233, 53–123 CrossRef CAS .
  5. A. Schrade, K. Landfester and U. Ziener, Chem. Soc. Rev., 2013, 42, 6823–6839 RSC .
  6. S. C. Thickett and G. H. Teo, Polym. Chem., 2019, 10, 2906–2924 RSC .
  7. L. C. Hsiao and S. Pradeep, Curr. Opin. Colloid Interface Sci., 2019, 43, 94–112 CrossRef CAS .
  8. T. Matsumoto, M. Okubo and S. Shibao, Kobunshi Ronbunshu, 1976, 33, 575–583 CrossRef CAS .
  9. M. Okubo, Y. Katsuta, A. Yamada and T. Matsumoto, Kobunshi Ronbunshu, 1979, 36, 459–464 CrossRef CAS .
  10. M. Okubo, Y. Katsuta and T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 1980, 18, 481–486 CrossRef CAS .
  11. M. Okubo, A. Yamada and T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 1980, 16, 3219–3228 CrossRef .
  12. I. Cho and K. W. Lee, J. Appl. Polym. Sci., 1985, 30, 1903–1926 CrossRef CAS .
  13. F. Sommer, T. M. Duc, R. Pirri, G. Meunier and C. Quet, Langmuir, 1995, 11, 440–448 CrossRef CAS .
  14. S. Tolue, M. R. Moghbeli and S. M. Ghafelebashi, Eur. Polym. J., 2009, 45, 714–720 CrossRef CAS .
  15. L. Zhang, E. S. Daniels, V. L. Dimonie and A. Klein, J. Appl. Polym. Sci., 2010, 118, 2502–2511 CrossRef CAS .
  16. L. Zhang, E. S. Daniels, V. L. Dimonie and A. Klein, J. Appl. Polym. Sci., 2014, 131, 40124 Search PubMed .
  17. D. J. Kraft, J. Hilhorst, M. A. P. Heinen, M. J. Hoogenraad, B. Luigjes and W. K. Kegel, J. Phys. Chem. B, 2011, 115, 7175–7181 CrossRef CAS PubMed .
  18. X. Zhang, X. H. Yao, X. M. Wang, L. Feng, J. Y. Qu and P. G. Liu, Soft Matter, 2014, 10, 873–881 RSC .
  19. D. Zhao, M. Z. Wang, Y. F. Xu, Z. C. Zhang and X. W. Ge, Surf. Coat. Technol., 2014, 238, 15–26 CrossRef CAS .
  20. L. Tian, X. Li, P. Zhao, X. Chen, Z. Ali, N. Ali, B. Zhang, H. Zhang and Q. Zhang, Macromolecules, 2015, 48, 7592–7603 CrossRef CAS .
  21. D. Suzuki and C. Kobayashi, Langmuir, 2014, 30, 7085–7092 CrossRef CAS PubMed .
  22. C. Kobayashi, T. Watanabe, K. Murata, T. Kureha and D. Suzuki, Langmuir, 2016, 32, 1429–1439 CrossRef CAS PubMed .
  23. T. Watanabe, C. Kobayashi, C. Song, K. Murata, T. Kureha and D. Suzuki, Langmuir, 2016, 32, 12760–12773 CrossRef CAS PubMed .
  24. K. Furusawa, Y. Kimura and T. Tagawa, J. Colloid Interface Sci., 1986, 109, 69–76 CrossRef CAS .
  25. S. Reculusa, C. Poncet-Legrand, S. Ravaine, C. Mingotaud, E. Duguet and E. Bourgeat-Lami, Chem. Mater., 2002, 14, 2354–2359 CrossRef CAS .
  26. A. Perro, S. Reculusa, E. Bourgeat-Lami, E. Duguet and S. Ravaine, Colloids Surf., A, 2006, 284–285, 78–83 CrossRef .
  27. J. L. Luna-Xavier, A. Guyot and E. Bourgeat-Lami, J. Colloid Interface Sci., 2002, 250, 82–92 CrossRef CAS PubMed .
  28. J. L. Luna-Xavier, A. Guyot and E. Bourgeat-Lami, Polym. Int., 2004, 53, 609–617 CrossRef CAS .
  29. X. J. Cheng, M. Chen, S. X. Zhou and L. M. Wu, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 3807–3816 CrossRef CAS .
  30. D. M. Qi, Y. Z. Bao, Z. M. Huang and Z. X. Weng, J. Appl. Polym. Sci., 2006, 99, 3425–3432 CrossRef CAS .
  31. Q. Shang, M. Wang, H. Liu, L. Gao and G. Xiao, Polym. Compos., 2012, 34, 51–57 CrossRef .
  32. T. Forestier, M. Ferrié and S. Ravaine, Colloid Polym. Sci., 2013, 291, 187–192 CrossRef CAS .
  33. S. Shi, S. I. Kuroda and H. Kubota, Colloid Polym. Sci., 2003, 281, 331–336 CrossRef CAS .
  34. H. F. Huang and H. R. Liu, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 5198–5205 CrossRef CAS .
  35. Q. Niu, M. W. Pan, J. F. Yuan, X. Liu, X. M. Wang and H. F. Yu, Macromol. Rapid Commun., 2013, 34, 1363–1367 CrossRef CAS PubMed .
  36. X. Liu, M. W. Pan, J. F. Yuan, Q. Niu, X. M. Wang and K. C. Zhang, RSC Adv., 2014, 4, 4163–4169 RSC .
  37. J. F. Yuan, L. X. Wang, L. Zhu, M. W. Pan, W. J. Wang, Y. Liu and G. Liu, Langmuir, 2015, 31, 4087–4095 CrossRef CAS PubMed .
  38. C. Lu and M. Urban, ACS Nano, 2015, 9, 3119–3124 CrossRef CAS PubMed .
  39. X. Pei, K. Zhai, X. Liang, Y. Deng, K. Xu, Y. Tan, X. Yao and P. Wang, J. Colloid Interface Sci., 2018, 512, 600–608 CrossRef CAS PubMed .
  40. S. Xu, W.-F. Ma, L.-J. You, J.-M. Li, J. Guo, J. J. Hu and C. C. Wang, Langmuir, 2012, 28, 3271–3278 CrossRef CAS PubMed .
  41. X. G. Qiao, P.-Y. Dugas, B. Charleux, M. Lansalot and E. Bourgeat-Lami, Macromolecules, 2015, 48, 545–556 CrossRef CAS .
  42. S. W. Zhang, S. X. Zhou, Y. M. Weng and L. M. Wu, Langmuir, 2005, 21, 21248 Search PubMed .
  43. J. Zhou, S. W. Zhang, X. G. Qiao, X. Q. Li and L. M. Wu, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 3202–3209 CrossRef CAS .
  44. X. G. Qiao, M. Chen, J. Zhou and L. M. Wu, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 1028–1037 CrossRef CAS .
  45. H. Kim, E. S. Daniels, S. Li, V. K. Mokkapati and K. Kardos, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 1038–1054 CrossRef CAS .
  46. Y. Zhang, H. Chen and Q. Zou, Colloid Polym. Sci., 2009, 287, 1221–1227 CrossRef CAS .
  47. Y. F. Wu, Y. Zhang, J. X. Xu, M. Chen and L. M. Wu, J. Colloid Interface Sci., 2010, 343, 18–24 CrossRef CAS PubMed .
  48. D. Z. Xu, M. Z. Wang, X. W. Ge, H.-W. M. Lam and X. P. Ge, J. Mater. Chem., 2012, 22, 5784–5791 RSC .
  49. Y. Zhang, Z. Chen, Z. Dong, M. Zhao, S. Ning and P. X. He, Int. J. Polym. Mater., 2013, 62, 397–401 CrossRef CAS .
  50. Z. X. Chen, Y. H. Zhang, L. L. Duan, Z. G. Wang, Y. L. Li and P. X. He, J. Adhes. Sci. Technol., 2015, 29, 2117–2129 CrossRef CAS .
  51. S. Shi, L. Zhou, T. Wang, L. Bian, Y. Tang and S. Kuroda, J. Appl. Polym. Sci., 2010, 120, 501–508 CrossRef .
  52. R. K. Wang, H. R. Liu and F. W. Wang, Langmuir, 2013, 29, 11440–11448 CrossRef CAS PubMed .
  53. S. Hosseinzadeh, Y. Saadat, S. Abdolbaghi, F. Afshar-Taromi and A. Hosseinzadeh, Colloid J., 2014, 76, 104–112 CrossRef CAS .
  54. R. Chen, N. Ren, X. Jin and X. Y. Zhu, Langmuir, 2018, 34, 3420–3425 CrossRef CAS PubMed .
  55. C. Chen, L. P. Zhang, M. F. Sheng, Y. Guan, H. Dong and S. H. Fu, J. Mater. Sci., 2019, 54, 1898–1912 CrossRef CAS .
  56. J. Mrkic and B. R. Saunders, J. Colloid Interface Sci., 2000, 222, 75–82 CrossRef CAS PubMed .
  57. J. R. Zhang, T. Qiu, H. F. Yuan, W. Z. Shi and X. Y. Li, Mater. Lett., 2011, 65, 790–792 CrossRef CAS .
  58. J. Wang, F. X. Lin, J. X. Chen, M. Z. Wang and X. W. Ge, J. Mater. Chem. B, 2015, 3, 9186–9193 RSC .
  59. Y. X. Li, Z. Huang, Y. Wu, C. Yang, Y. Gao and Z. Q. Wang, Colloids Surf., A, 2012, 407, 71–76 CrossRef CAS .
  60. X. Fan, L. Niu, Y. H. Wu, J. Cheng and Z. R. Yang, Appl. Surf. Sci., 2015, 332, 393–402 CrossRef CAS .
  61. M. Kohri, Y. Nannichi, H. Kohma, D. Abe, T. Kojima, T. Taniguchi and K. Kishikawa, Colloids Surf., A, 2014, 449, 114–120 CrossRef CAS .
  62. D. Nguyen, E. Duguet, E. Bourgeat-Lami and S. Ravaine, Langmuir, 2010, 26, 6086–6090 CrossRef CAS PubMed .
  63. D. Nguyen, S. Ravaine, E. Bourgeat-lami and E. Duguet, J. Mater. Chem., 2010, 20, 9392–9400 RSC .
  64. Z. Zhang, H. Shao, X. Zhou, L. Zhao, H. Liu, X. Ji and H. Liu, Mater. Chem. Phys., 2017, 195, 105–113 CrossRef CAS .
  65. H. Zhu, Q. Zhang and S. Zhu, Dalton Trans., 2015, 44, 16752–16757 RSC .
  66. T. J. Yao, C. X. Wang, J. Wu, Q. Lin, H. Lv, K. Zhang, K. Yu and B. Yang, J. Colloid Interface Sci., 2009, 338, 573–577 CrossRef CAS PubMed .
  67. C.-J. Weng, Y.-L. Chen, C.-M. Chien, S.-C. Hsu, Y.-S. Jhuo, J.-M. Yeh and C.-F. Dai, Electrochim. Acta, 2013, 95, 162–169 CrossRef CAS .
  68. S. Shi, S. Kuroda, K. Hosoi and H. Kubota, Polymer, 2005, 46, 3567–3570 CrossRef CAS .
  69. E. Bourgeat-Lami and E. Duguet, in Functional Coatings, ed. S. K. Ghosh, Wiley-VCH, Weinheim, 2006, pp. 85–152 Search PubMed .
  70. G. Odian, Principles of Polymerization, John Wiley and Sons, Hoboken, 4th edn, 2004 Search PubMed .
  71. M. J. Percy, C. Barthet, J. C. Lobb, M. A. Khan, S. F. Lascelles, M. Vamvakaki, S. P. Armes and H. Wiese, Langmuir, 2000, 16, 6913–6920 CrossRef CAS .
  72. M. J. Percy, J. I. Amalvy, C. Barthet, S. P. Armes, S. J. Greaves, J. F. Watts and H. Wiese, J. Mater. Chem., 2002, 12, 697–702 RSC .
  73. M. Chen, L. M. Wu, S. X. Zhou and B. You, Macromolecules, 2004, 37, 9613–9619 CrossRef CAS .
  74. M. Chen, S. X. Zhou, B. You and L. M. Wu, Macromolecules, 2005, 38, 6411–6417 CrossRef CAS .
  75. A. Pich, S. Bhattacharya, A. Ghosh and H.-J. P. Adler, Polymer, 2005, 46, 4596–4603 CrossRef CAS .
  76. Z. Z. Xu, A. Xia, C. C. Wang, W. L. Yang and S. K. Fu, Mater. Chem. Phys., 2007, 103, 494–499 CrossRef CAS .
  77. H. Zhang, Z. X. Su, P. Liu and F. Z. Zhang, J. Appl. Polym. Sci., 2007, 104, 415–421 CrossRef CAS .
  78. D. M. Qi, Y. Z. Bao, Z. M. Huang and Z. X. Weng, Colloid Polym. Sci., 2008, 286, 233–241 CrossRef CAS .
  79. Y. Y. Guan, X. H. Meng and D. Qiu, Langmuir, 2014, 30, 3681–3686 CrossRef CAS PubMed .
  80. C. Wang, F. Yan, X. H. Meng, Y. Qiao and D. Qiu, Soft Matter, 2018, 14, 9336–9342 RSC .
  81. X. P. Zhang, Y. Y. Guan, Y. F. Zhao, Z. J. Zhang and D. Qiu, Polym. Int., 2015, 64, 992–998 CrossRef CAS .
  82. T. Arpornwichanop, D. Polpanich, R. Thiramanas, T. Suteewong and P. Tangboriboonrat, Int. J. Biol. Macromol., 2015, 81, 151–158 CrossRef CAS PubMed .
  83. S. Pan, M. Chen and L. M. Wu, J. Colloid Interface Sci., 2018, 522, 20–28 CrossRef CAS PubMed .
  84. X. Zhang, Y. Sun, Y. Mao, K. Chen, Z. Cao and D. M. Qi, RSC Adv., 2018, 8, 3910–3918 RSC .
  85. Q. Yu, Y. Yin, Y. Yang, Y. Han, Y. Liu, B. Li and L. Weng, J. Sol–Gel Sci. Technol., 2018, 87, 749–759 CrossRef CAS .
  86. J. N. Kim, Y. Z. Dong and H. J. Choi, ACS Omega, 2020, 5, 7675–7682 CrossRef CAS PubMed .
  87. J. He, D. Chen, K. Han, X. X. Huang, L. W. Wang, J. Y. Deng and W. T. Yang, J. Polym. Sci., Part A: Polym. Chem., 2014, 52, 2894–2898 CrossRef CAS .
  88. X. P. Pei, Y. Tan, K. Xu, C. Liu, C. G. Lu and P. X. Wang, Polym. Chem., 2016, 7, 3325–3333 RSC .
  89. K. Zhai, X. Pei, C. Wang, Y. Deng, Y. Tan, Y. Bai, B. Zhang, K. Xu and P. Wang, Int. J. Biol. Macromol., 2019, 131, 1032–1037 CrossRef CAS PubMed .
  90. J. F. Yuan, W. T. Zhao, M. W. Pan and L. Zhu, Macromol. Rapid Commun., 2016, 37, 1282–1287 CrossRef CAS PubMed .
  91. Y. J. Wang, S. F. Song, J. F. Yuan, L. Zhu, M. W. Pan and G. Liu, Ind. Eng. Chem. Res., 2018, 57, 8907–8917 CrossRef CAS .
  92. S. S. Gao, S. F. Song, J. Wang, S. X. Mei, J. F. Yuan, G. Liu and M. W. Pan, Colloids Surf., A, 2019, 577, 360–369 CrossRef CAS .
  93. S.-H. Kim, G.-R. Yi, K. H. Kim and S.-M. Yang, Langmuir, 2008, 24, 2365–2371 CrossRef CAS PubMed .
  94. F. Tiarks, K. Landfester and M. Antonietti, Langmuir, 2001, 17, 5775–5780 CrossRef CAS .
  95. Z. H. Cao, A. Schrade, K. Landfester and U. Ziener, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 2382–2394 CrossRef CAS .
  96. A. Schrade, V. Mikhalevich, K. Landfester and U. Ziener, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 4735–4746 CrossRef CAS .
  97. A. Schrade, V. Mailänder, S. Ritz, K. Landfester and U. Ziener, Macromol. Biosci., 2012, 12, 1459–1471 CrossRef CAS PubMed .
  98. Z. X. Chen, Y. H. Zhang, Y. Liu, L. L. Duan, Z. G. Wang, C. S. Liu, Y. L. Li and P. X. He, Prog. Org. Coat., 2015, 86, 79–85 CrossRef CAS .
  99. S. Samanta, S. L. Banerjee, S. K. Ghosh and N. K. Singha, ACS Appl. Mater. Interfaces, 2019, 11, 44722–44734 CrossRef CAS PubMed .
  100. Y. Zhang, Q. Zou, X. Shu, Q. Tang, M. Chen and L. M. Wu, J. Colloid Interface Sci., 2009, 336, 544–550 CrossRef CAS PubMed .
  101. H. Zou and S. P. Armes, Polym. Chem., 2012, 3, 172–181 RSC .
  102. X. G. Qiao, Z. Zhou, X. C. Pang, M. Lansalot and E. Bourgeat-Lami, Polymer, 2019, 172, 330–338 CrossRef CAS .
  103. S. A. F. Bon, S. Cauvin and P. J. Colver, Soft Matter, 2007, 3, 194–199 RSC .
  104. K. J. Cassidy, O. Z. Durham and D. A. Shipp, Macromol. React. Eng., 2019, 1800075 CrossRef .
  105. M. Gill, J. Mykytiuk, S. P. Armes, J. L. Edwards, T. Yeates, P. J. Moreland and C. Mollett, J. Chem. Soc., Chem. Commun., 1992, 108 RSC .
  106. J. Stejskal, P. Kratochvil, S. P. Armes, S. F. Lascelles, A. Riede, M. Helmstedt, J. Prokěs and I. Křivka, Macromolecules, 1996, 29, 6814 CrossRef CAS .
  107. N. Roosz, M. Euvard, B. Lakard, C. C. Buron, N. Martin and L. Viau, J. Colloid Interface Sci., 2017, 502, 184–192 CrossRef CAS PubMed .
  108. S. Maeda and S. P. Armes, J. Colloid Interface Sci., 1993, 159, 257–259 CrossRef CAS .
  109. S. Maeda and S. P. Armes, J. Mater. Chem., 1994, 4, 935–942 RSC .
  110. M. G. Han and S. P. Armes, Langmuir, 2003, 19, 4523–4526 CrossRef CAS .
  111. H. Zou, D. Wu, H. Sun, S. W. Chen and X. Wang, Appl. Surf. Sci., 2018, 436, 1083–1092 CrossRef CAS .
  112. S. Kamei, M. Okubo and T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 1986, 24, 3109–3116 CAS .
  113. Y. Y. Sun, Y. Y. Yin, M. Chen, S. X. Zhou and L. M. Wu, Polym. Chem., 2013, 4, 3020–3027 RSC .
  114. X. L. Fan, X. K. Jia, H. P. Zhang, B. L. Zhang, C. M. Li and Q. Y. Zhang, Langmuir, 2013, 29, 11730–11741 CrossRef CAS PubMed .
  115. X. L. Fan, X. K. Jia, Y. Liu, B. L. Zhang, C. M. Li, Y. L. Liu, H. P. Zhang and Q. Y. Zhang, Polym. Chem., 2015, 6, 703–713 RSC .
  116. X. L. Fan, J. Liu, X. K. Jia, Y. Liu, H. Zhang, S. Q. Wang, B. L. Zhang, H. P. Zhang and Q. Y. Zhang, Nano Res., 2017, 10, 2905–2922 CrossRef CAS .
  117. M. Okubo, H. Kobayashi, C. J. Huang, E. Miyanaga and T. Suzuki, Langmuir, 2017, 33, 3468–3475 CrossRef CAS PubMed .
  118. J. G. Tsavalas, F. J. Schork and K. Landfester, JCT Res., 2004, 1, 53–63 CAS .
  119. F. Li, Y. Y. Tu, J. W. Hu, H. L. Zou, G. J. Liu, S. D. Lin, G. H. Yang, S. Y. Hu, L. Miao and Y. M. Mo, Polym. Chem., 2015, 6, 6746–6760 RSC .
  120. C. B. Du, X. L. Hu, P. Guan, X. M. Gao, R. Y. Song, J. Li, L. W. Qian, N. Zhang and L. X. Guo, J. Mater. Chem. B, 2016, 4, 1510–1519 RSC .
  121. C. B. Du, N. Zhang, S. C. Ding, X. M. Gao, P. Guan and X. L. Hu, Polym. Chem., 2016, 7, 4531–4541 RSC .
  122. C. B. Du, X. L. Hu, Y. Cheng, J. F. Gao, Y. W. Zhang, K. H. Su, Z. J. Li, N. Zhang, N. H. Chang and K. Y. Zeng, Mater. Sci. Eng., C, 2018, 83, 169–176 CrossRef CAS PubMed .
  123. Y. Lan, A. Caciagli, G. Guidetti, Z. Y. Yu, J. Liu, V. E. Johansen, M. Kamp, C. Abell, S. Vignolini, O. A. Scherman and E. Eiser, Nat. Commun., 2018, 9, 3614 CrossRef PubMed .
  124. U. Hasegawa, T. Nishida and A. J. van der Vlies, Macromolecules, 2015, 48, 4388–4393 CrossRef CAS .
  125. P. Chambon, A. Blanazs, G. Battaglia and S. P. Armes, Macromolecules, 2012, 45, 5081–5090 CrossRef CAS .
  126. C. J. Mable, N. J. Warren, K. L. Thompson, O. O. Mykhaylyk and S. P. Armes, Chem. Sci., 2015, 6, 6179–6188 RSC .
  127. C. J. Mable, L. A. Fielding, M. J. Derry, O. O. Mykhaylyk, P. Chambon and S. P. Armes, Chem. Sci., 2018, 9, 1454–1463 RSC .
  128. C. J. Mable, I. Canton, O. O. Mykhaylyk, B. U. Gul, P. Chambon, E. Themistouk and S. P. Armes, Chem. Sci., 2019, 10, 4811–4821 RSC .
  129. J. N. Zhang, N. N. Liu, M. Z. Wang, X. W. Ge, M. Y. Wu, J. J. Yang, Q. Y. Wu and Z. L. Jin, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 3128–3134 CrossRef CAS .
  130. B. B. Yang, J. N. Zhang, J. Z. Lin, B. Wu, Q. Liu, W. L. Yang, M. Y. Wu, Q. Y. Wu and J. J. Yang, Macromol. Res., 2013, 21, 123–126 CrossRef CAS .
  131. J. W. Li, S. Sihler and U. Ziener, Langmuir, 2019, 35, 6161–6168 CrossRef CAS PubMed .
  132. H. Zou, S. S. Wu and J. Shen, Langmuir, 2008, 24, 10453–10461 CrossRef CAS PubMed .
  133. L. Kind, F. A. Plamper, R. Göbel, A. Mantion, A. H. E. Müller, U. Pieles, A. Taubert and W. Meier, Langmuir, 2009, 25, 7109–7115 CrossRef CAS PubMed .
  134. X. Du, X. M. Liu, H. M. Chen and J. H. He, J. Phys. Chem. C, 2009, 113, 9063–9070 CrossRef CAS .
  135. Z. Qian, Z. C. Zhang, L. Y. Song and H. R. Liu, J. Mater. Chem., 2009, 19, 1297–1304 RSC .
  136. Y. Cai, H. B. Xie, J. Sun, H. S. Liu, J. Wang, Y. F. Zhou, W. Y. Nie and L. Y. Song, Mater. Chem. Phys., 2013, 137, 796–801 CrossRef CAS .
  137. M. D'Acunzi, L. Mammen, M. Singh, X. Deng, M. Roth, G. K. Auernhammer, H.-J. Butt and D. Vollmer, Faraday Discuss., 2010, 146, 35–48 RSC .
  138. X. Fan, L. Zheng, C. Jiang, S. Xu, X. Wen, Z. Cai, P. Pi and Z. Yang, Surf. Coat. Technol., 2012, 213, 90–97 CrossRef CAS .
  139. X. T. Zhou, H. Shao and H. R. Liu, Colloid Polym. Sci., 2013, 291, 1181–1190 CrossRef CAS .
  140. L. Wang, L. Song, Z. Chao, P. Chen, W. Nie and Y. Zhou, Appl. Surf. Sci., 2015, 342, 92–100 CrossRef CAS .
  141. Z. N. Li, C. J. Wu, K. Zhao, B. Peng and Z. W. Deng, Colloids Surf., A, 2015, 470, 80–91 CrossRef CAS .
  142. P. B. Landon, A. H. Mo, A. D. Printz, C. Emerson, C. Zhang, W. Janetanakit, D. A. Colburn, S. Akkiraju, S. Dossou, B. X. Chong, G. Glinsky and R. Lal, Langmuir, 2015, 31, 9148–9154 CrossRef CAS PubMed .
  143. J. Chang, L. Zang, C. Wang, L. Sun and Q. Chang, Nanoscale Res. Lett., 2016, 11, 114 CrossRef PubMed .
  144. F. Zhang, Z.-Y. Xiao, S.-R. Zhai, B. Zhai, P.-F. Yang and Q.-D. An, J. Sol–Gel. Sci. Technol., 2016, 78, 228–238 CrossRef CAS .
  145. M. Yu, Q. Wang, Z. Min, Q. Deng and D. Chen, RSC Adv., 2017, 7, 39471–39479 RSC .
  146. J. F. Dechézelles, V. Malik, J. J. Crassous and P. Schurtenberger, Soft Matter, 2013, 9, 2798–2802 RSC .
  147. Z. B. Li, T. Y. Chen, J. J. Nie, J. T. Xu, Z. Q. Fan and B. Y. Du, Mater. Chem. Phys., 2013, 138, 650–657 CrossRef CAS .
  148. N.-H. Cao-Luu, Q.-T. Pham, Z.-H. Yao, F.-M. Wang and C.-S. Chern, J. Mater. Sci., 2019, 54, 7503–7516 CrossRef CAS .
  149. H. S. Hwang, S. B. Lee and I. Park, Mater. Lett., 2010, 64, 2159–2162 CrossRef CAS .
  150. M. W. Pi, T. T. Yang, J. J. Yuan, S. Fujii, Y. Kakigi, Y. Nakamura and S. Y. Cheng, Colloids Surf., B, 2010, 78, 193–199 CrossRef CAS PubMed .
  151. S. Xu, X. J. Song, J. Guo and C. C. Wang, ACS Appl. Mater. Interfaces, 2012, 4, 4764–4775 CrossRef CAS PubMed .
  152. T. Fadida and J. P. Lellouche, J. Colloid Interface Sci., 2012, 386, 167–173 CrossRef CAS PubMed .
  153. S. Z. Zhou and X. G. Qiao, Colloids Surf., A, 2018, 553, 230–236 CrossRef CAS .
  154. M. Agrawal, A. Pich, N. E. Zafeiropoulos, S. Gupta, J. Pionteck, F. Simon and M. Stamm, Chem. Mater., 2007, 19, 1845–1852 CrossRef CAS .
  155. M. Agrawal, N. E. Zafeiropoulos, S. Gupta, E. Svetushkina, J. Pionteck, A. Pich and M. Stamm, Macromol. Rapid Commun., 2010, 31, 405–410 CrossRef CAS PubMed .
  156. N. Ding, Y. J. Sun, B. H. Chen, D. D. Wang, S. N. Tao, B. T. Zhao and Y. X. Li, Colloids Surf., A, 2020, 599, 124867 CrossRef CAS .
  157. M. Agrawal, A. Pich, S. Gupta, N. E. Zafeiropoulos, P. Formanek, D. Jehnichen and M. Stamm, Langmuir, 2010, 26, 526–532 CrossRef CAS PubMed .
  158. T. Ren, J. Wang, J. Yuan, M. Pan, G. Liu, G. Zhang, G. Zhong, G.-J. Zhong and Z.-M. Li, RSC Adv., 2015, 5, 36845–36857 RSC .
  159. N. Ali, B. L. Zhang, H. P. Zhang, W. Zaman, X. J. Li, W. Li and Q. Y. Zhang, Colloids Surf., A, 2015, 472, 38–49 CrossRef CAS .
  160. T. X. Li, J. F. Zhang, X. J. Ni, L. K. Wang and C. Yang, Colloids Surf., A, 2018, 538, 818–824 CrossRef .
  161. Y. X. Li, Y. F. Pan, L. L. Zhu, Z. Q. Wang, D. M. Su and G. Xue, Macromol. Rapid Commun., 2011, 32, 1741–1747 CrossRef CAS PubMed .
  162. L. Q. Xu, B. S. M. Yap, R. Wang, K.-G. Neoh, E.-T. Kang and G. D. Fu, Ind. Eng. Chem. Res., 2014, 53, 3116–3124 CrossRef CAS .
  163. Q. Tian, X. J. Yu, L. F. Zhang and D. M. Yu, J. Colloid Interface Sci., 2017, 491, 294–304 CrossRef CAS PubMed .
  164. Z. H. Cao, C. Walter, K. Landfester, Z. Y. Wu and U. Ziener, Langmuir, 2011, 27, 9849–9859 CrossRef CAS PubMed .
  165. K. Furusawa and C. Anzai, Colloids Surf., 1992, 63, 103–111 CrossRef CAS .
  166. R. Ottewill, A. Schofield, J. Waters and N. S. J. Williams, Colloid Polym. Sci., 1997, 275, 274–283 CrossRef CAS .
  167. G. Li, X. Yang and J. Wang, Colloids Surf., A, 2008, 322, 192 CrossRef CAS .
  168. C. Wang, J. Yan, X. Cui and H. Wang, J. Colloid Interface Sci., 2011, 354, 94 CrossRef CAS PubMed .
  169. C. S. Wagner, S. Shehata, K. Henzler, J. Yuan and A. Wittemann, J. Colloid Interface Sci., 2011, 355, 115–123 CrossRef CAS PubMed .
  170. A. San-Miguel and S. H. Behrens, Langmuir, 2012, 28, 12038–12043 CrossRef CAS PubMed .
  171. M. Kanahara, M. Shimomura and H. Yabu, Soft Matter, 2014, 10, 275–280 RSC .
  172. Y. C. Liu, M. L. Li and G. F. Chen, J. Mater. Chem. A, 2013, 1, 930–937 RSC .
  173. M. L. Li and G. F. Chen, Nanoscale, 2013, 5, 11919–11927 RSC .
  174. M. L. Li, G. F. Chen and S. Bhuyain, Nanoscale, 2015, 7, 2641–2650 RSC .
  175. H. Y. Cao, L. B. Zhang, L. L. Wu and X. Z. Kong, ACS Appl. Mater. Interfaces, 2016, 8, 29136–29147 CrossRef CAS PubMed .
  176. Y. Guo, B. G. P. van Ravensteijn, C. H. J. Evers and W. K. Kegel, Langmuir, 2017, 33, 4551–4558 CrossRef CAS PubMed .
  177. G. Sun, M. Fan, L. Chen, J. Luo and R. Liu, Colloids Surf., A, 2020, 589, 124458 CrossRef .
  178. R. Li, X. L. Yang, G. L. Li, S. N. Li and W. Q. Huang, Langmuir, 2006, 22, 8127–8133 CrossRef CAS PubMed .
  179. G. L. Li, Y. Y. Song, X. L. Yang and W. Q. Huang, J. Appl. Polym. Sci., 2007, 104, 1350–1357 CrossRef CAS .
  180. J. Y. Wang and X. L. Yang, Colloid Polym. Sci., 2008, 286, 283–291 CrossRef CAS .
  181. J. Y. Wang, H. Li and X. L. Yang, Polym. Adv. Technol., 2009, 20, 965–971 CrossRef CAS .
  182. H. L. Liu, D. Wang and X. L. Yang, Colloids Surf., A, 2012, 397, 48–58 CrossRef CAS .
  183. T. Li, B. Ye, Z. W. Niu, P. Thompson, S. Seifert, B. Lee and Q. Wang, Chem. Mater., 2009, 21, 1046–1050 CrossRef CAS .
  184. H. Minami, Y. Mizuta and T. Suzuki, Langmuir, 2013, 29, 554–560 CrossRef CAS PubMed .
  185. M. Chenal, J. Rieger, A. Philippe and L. Bouteiller, Polymer, 2014, 55, 3516–3524 CrossRef CAS .
  186. G. Li, X. Yang, F. Bai and W. Huang, J. Colloid Interface Sci., 2006, 297, 705–710 CrossRef CAS PubMed .
  187. W. Yan, M. Pan, J. Yuan, G. Liu, L. Cui, G. Zhang and L. Zhu, Polymer, 2017, 122, 139–147 CrossRef CAS .
  188. Q. Wu, Z. Q. Wang, X. F. Kong, X. D. Gu and G. Xue, Langmuir, 2008, 24, 7778–7784 CrossRef CAS PubMed .
  189. Y. B. Bao, J. B. Wang, P. F. Xue, Q. Y. Li, W. H. Guo and C. F. Wu, J. Mater. Sci., 2012, 47, 1289–1295 CrossRef CAS .
  190. J. F. Huang, Q. Y. Li, Y. B. Bao and C. F. Wu, Colloid Polym. Sci., 2009, 287, 37–43 CrossRef CAS .
  191. Y. B. Bao, Q. Y. Li, P. F. Xue, J. F. Huang, J. B. Wang, W. H. Guo and C. F. Wu, Mater. Res. Bull., 2011, 46, 779–785 CrossRef CAS .
  192. Y. Lan, Y. C. Wu, A. Karas and O. A. Scherman, Angew. Chem., Int. Ed., 2014, 53, 2166–2169 CrossRef CAS PubMed .
  193. H. Zou, J. Liu, Y. Li, X. Y. Li and X. Wang, Small, 2018, 14, 1802234 CrossRef PubMed .
  194. J. Y. Wang and X. L. Yang, Langmuir, 2008, 24, 3358–3364 CrossRef CAS PubMed .
  195. B. Liu, M. Zhou, H. Liu, X. Wang and X. Yang, Colloids Surf., A, 2013, 436, 1027–1033 CrossRef CAS .
  196. W. Jiang, C. M. Grozea, Z. Shi and G. Liu, ACS Appl. Mater. Interfaces, 2014, 6, 2629–2638 CrossRef CAS PubMed .
  197. S. Mehlhase, C. G. Schäfer, J. Morsbach, L. Schmidt, R. Klein, H. Frey and M. Gallei, RSC Adv., 2014, 4, 41348–41352 RSC .
  198. T. Song, T. Liu, X. Yang and F. Bai, Colloids Surf., A, 2015, 469, 60–65 CrossRef CAS .
  199. R. Guo, X. Chen, X. Zhu, A. Dong and J. Zhang, RSC Adv., 2016, 6, 40991–41001 RSC .
  200. J. C. Gaulding, S. Saxena, D. E. Montanari and L. A. Lyon, ACS Macro Lett., 2013, 2, 337–340 CrossRef CAS .
  201. A. M. Islam, B. Z. Chowdhry and M. J. Snowden, Adv. Colloid Interface Sci., 1995, 62, 109–136 CrossRef CAS .
  202. H. Zou and X. Wang, Langmuir, 2017, 33, 1471–1477 CrossRef CAS PubMed .
  203. W. S. Choi, H. Y. Koo and W. T. S. Huck, J. Mater. Chem., 2007, 17, 4943–4946 RSC .
  204. U. Hasegawa, S.-I. Sawada, T. Shimizu, T. Kishida, E. Otsuji, O. Mazda and K. Akiyoshi, J. Controlled Release, 2009, 140, 312–317 CrossRef CAS PubMed .
  205. A. Schrade, Z. H. Cao, K. Landfester and U. Ziener, Langmuir, 2011, 27, 6689–6700 CrossRef CAS PubMed .
  206. L. N. Zhang, H. Zhang, M. Liu and B. Dong, ACS Appl. Mater. Interfaces, 2016, 8, 15654–15660 CrossRef CAS PubMed .
  207. Y. Lan, J. Liu, E. Eiser and O. A. Scherman, Polym. Chem., 2019, 10, 3772–3777 RSC .
  208. T. Kanai, H. Nakai, A. Yamada, M. Fukuyama and D. A. Weitz, Soft Matter, 2019, 15, 6934–6937 RSC .
  209. H. Zou, D. D. Miao, H. Sun and X. Wang, Langmuir, 2018, 34, 14302–14308 CrossRef CAS PubMed .
  210. S. X. Zhai, H. Sun, B. W. Qiu and H. Zou, Mater. Adv. 10.1039/d0ma00090f .
  211. M. R. Pope, S. P. Armes and P. J. Tarcha, Bioconjugate Chem., 1996, 7, 436–444 CrossRef CAS PubMed .
  212. M. J. Burchell, M. J. Willis, S. P. Armes, M. A. Khan, M. J. Percy and C. Perruchot, Planet. Space Sci., 2002, 50, 1025–1035 CrossRef CAS .

This journal is © The Royal Society of Chemistry 2020