Snowman-like Particles Research Articles

Preparation of asymmetric particles by controlling the phase separation of seeded emulsion polymerization with ethanol/water mixture

Alcohols are discovered for the first time to tune the morphology of poly(vinyl benzyl chloride)-poly(3-methacryloxypropyltrimethoxysilane) (PVBC-PMPS) composite particles through seeded emulsion polymerization within the alcohol/water mixture. Here, monodispersed linear PVBC particles was synthesized through the dispersion polymerization and employed as the seeds. The as-obtained PVBC-PMPS composite particles could be dramatically tuned from core-shell structures to snowman-like particles, to dumbbell-shaped particles, to inverse snowman-like particles when the ethanol content in reaction mixtures is only adjusted within a narrow range. The morphology of fresh PMPS bulges was observed after removing the linear PVBC seeds with N,N’-dimethyl formamide, and their formation mechanism was studied by monitoring the free radical polymerization and sol–gel process of 3-methacryloxypropyltrimethoxysilane. It has been confirmed that the sol–gel kinetics were the main factor on the particles’ morphology. In addition, morphologies of PVBC-PMPS particles were also varied by the MPS feeding amount, types of the co-solvent and pH values of alcohol/water mixtures.

Rapid hemostasis accompanied by antibacterial action of calcium crosslinking tannic acid-coated mesoporous silica/silver Janus nanoparticles

It is important to control bleeding and prevent bacterial infection for the wound people. The effective way is to fabricate an asymmetric Janus matrial for realizing rapid hemostasis and promoting wound healing. Herein, mesoporous silica nanoparticles (MSN) modified by tannic acid (TA), silver nanoparticles, and calcium ions (Ca-TA-MSN@Ag) with Janus structure were prepared via redox and coordination reactions. These anisotropic snowman-like particles possess obvious chemical compartition, in which silver nanoparticles are embedding in large MSN body. During blood coagulation, TA with catechol structure acts as a vasoconstrictor. Then, Ca-TA-MSN@Ag with high specific surface area (510.62 m2·g−1) and large pore volume (0.48 m3·g−1) induces red blood cell aggregation to form three-dimensional network structure with fibrin. Additionally, calcium ions as clotting factor IV and negative charge of Ca-TA-MSN@Ag accelerate coagulation cascade reaction. These three synergistic effects on animal model showed that hemostatic time of Ca-TA-MSN@Ag was shortened by nearly 50% compared to that of MSN. Moreover, Ca-TA-MSN@Ag possessed good blood compatibility, biocompatibility and antibacterial activity (~99%) against E. coli and S. aureus. The anisotropic Janus particles of Ca-TA-MSN@Ag with hemostatic performance and antibacterial activity will be a promising biomaterial for designing wound dressings in clinical application.

A novel approach toward Snowman-like polymer/SiO2 hybrid nanoparticles via gas-driving.

For the first time, we report an interesting transition from conventional core-shell polymer/SiO2 particles to self-stable snowman-like particles, which can be achieved by adding a low-boiling point oil-soluble monomer because the volatile monomer not only plays a lubrication role, but also acts as a gas "motor" to drive the silica precursor polycondensate migration.

Self-assembled heteromorphous raspberry-like colloidal particles from Pickering-like emulsion polymerization

Heteromorphous raspberry-like poly(vinylidene fluoride)/polystyrene (PVDF/PS) complex colloidal particles (CCPs) were prepared by a facile and high-efficiency approach with one pot Pickering-like emulsion polymerization following conventional free radical polymerization mechanism. Difference of hydrophobicity between PVDF and PS drove the initial generated snowman-like particles self-assembling into colloidal particle clusters and then a Pickering-like emulsion continued polymerization till formation of the heteromorphous raspberry-like PVDF/PS CCPs. In order to achieve heteromorphous raspberry-like PVDF/PS CCPs with homogeneous size and morphology, influences of St/PVDF feed ratio, polymerization time, and polymerization temperature were systematically investigated. When the feed ratio of St/PVDF was high for synthesis of the CCPs, double crystallization peaks for PVDF were observed with a strong peak at about 132 °C and a weak peak at near 70 °C, indicating that partial PVDF seed particles were covered completely by PS. Furthermore, the static water contact angle of latex nanocoating prepared with the heteromorphous raspberry-like PVDF/PS CCPs reached 153.9°, exhibiting a superhydrophobicity. As a result, an alternative way is provided for fabrication of hierarchical colloidal particles and even superhydrophobic smart coatings.

Self-assembly of microcapsules via colloidal bond hybridization and anisotropy.

Particles with directional interactions are promising building blocks for new functional materials and may serve as models for biological structures. Mutually attractive nanoparticles that are deformable owing to flexible surface groups, for example, may spontaneously order themselves into strings, sheets and large vesicles. Furthermore, anisotropic colloids with attractive patches can self-assemble into open lattices and the colloidal equivalents of molecules and micelles. However, model systems that combine mutual attraction, anisotropy and deformability have not yet been realized. Here we synthesize colloidal particles that combine these three characteristics and obtain self-assembled microcapsules. We propose that mutual attraction and deformability induce directional interactions via colloidal bond hybridization. Our particles contain both mutually attractive and repulsive surface groups that are flexible. Analogously to the simplest chemical bond--in which two isotropic orbitals hybridize into the molecular orbital of H2--these flexible groups redistribute on binding. Via colloidal bond hybridization, isotropic spheres self-assemble into planar monolayers, whereas anisotropic snowman-shaped particles self-assemble into hollow monolayer microcapsules. A modest change in the building blocks thus results in much greater complexity of the self-assembled structures. In other words, these relatively simple building blocks self-assemble into markedly more complex structures than do similar particles that are isotropic or non-deformable.

Direct one-pot synthesis of chemically anisotropic particles with tunable morphology, dimensions, and surface roughness.

Previously, synthesis of anisotropic particles by seeded polymerizations has involved multiple process steps. In conventional one-pot dispersion polymerization (Dis.P) with a cross-linker added, only spherical particles are produced due to rapid and high cross-linking. In this Article, a straightforward one-pot preparation of monodisperse anisotropic particles with tunable morphology, dimensions, surface roughness, and asymmetrically distributed functional groups is described. With a cross-linker of divinylbenzene (DVB, 8%), ethylene glycol dimethacrylate (EGDMA, 6%), or dimethacryloyloxybenzophenone (DMABP, 5%) added at 40 min, shortly after the end of nucleation stage in Dis.P of styrene (St) in methanol and water (6/4, vol), the swollen growing particles are inhomogeneously cross-linked at first. Then, at low gel contents of 59%, 49%, and 69%, corresponding to the cases using DVB, EGDMA, and DMABP, respectively, the growing particle phase separates and snowman- or dumbbell-like particles are generated. Thermodynamic and kinetic analyses reveal that moderate cross-linking and sufficient swelling of growing particles determine the formation and growth of anisotropic particles during polymerization. Morphology, surface roughness, sizes, and cross-linking degrees of each domain of final particles are tuned continuously by varying start addition time and contents of cross-linkers. The snowman-like particles fabricated with DVB have a gradient cross-linking and asymmetrical distribution of pendant vinyl groups from their body to head. The dumbbell-like particles prepared using DMABP have only one domain cross-linked; i.e., only one domain contains photosensitive benzophenone (BP) groups. With addition of glycidyl methacrylate (GMA) or propargyl methacrylate (PMA) together with DVB or EGDMA, epoxy or alkynyl groups are asymmetrically incorporated. With the aid of these functional groups, carboxyl, amino, or thiol groups and PEG (200) are attached by thiol-ene (yne) click and photocoupling reactions.

Facile synthesis of core–shell/hollow anisotropic particles via control of cross-linking during one-pot dispersion polymerization

Preparation of anisotropic particles based on seed phase separation involves multiple processes, and asymmetrical structures and surfaces cannot be produced when anisotropic shapes emerge. In conventional one-pot dispersion polymerization (Dis.P) using cross-linker, only spherical particles are prepared due to rapid and high cross-linking. Herein, monodisperse snowman-like particles with core–shell/hollow structures and partially rough surface were synthesized straightforward by a modified one-pot Dis.P, in which ethylene glycol and water (6/4, vol.) were used as medium, and ammonium persulfate (APS) aqueous solution, vinyl acetate (VA) and/or acrylic acid (AA), divinylbenzene (DVB) and styrene (St) were added at 6h. The cross-linking of growing particles was confined to exterior (forming cross-linked shell), and gel contents were low, leading to phase separation. Asymmetrical morphologies, structures, sizes and surface roughness were flexibly tuned by varying amounts of APS, VA and/or AA, water and DVB, and DVB adding speed. At low APS contents or high DVB amounts, the inhomogeneous cross-linking of head enabled its phase to separate, producing elongated head. With addition of VA and AA, phase separations inside head and body were induced, generating hollow structure. Adding DVB very slowly, nonlinear growth of third compartment occurred, forming bowed head.

Facile synthesis of core-shell, multiple compartment anisotropic particles via control of cross-linking and continuous phase separations in one-pot dispersion polymerization

Preparation of anisotropic particles based on phase separation of prefabricated seeds in polymerizations generally involves multiple process steps. In conventional one-pot dispersion polymerization, only spherical particles are produced. Herein, anisotropic particles with asymmetrical core-shell structure, multiple compartments, and continuously tunable surface roughness and sizes were synthesized by a modified one-pot dispersion polymerization. A mixture of polar solvents, ethylene glycol (EG), and water (6/4, vol.) was used as medium for polymerization of styrene (St). In the presence of ammonium persulfate (APS) and vinyl acetate (VA), divinylbenzene (DVB, 16.1–66.4 %) and (St, 50.3–0 %) were added at 60, 180, and 360 min, respectively. Dense cross-linking was confined to exterior of swollen growing particles, resulting in phase separation and formation of snowman-like particles with coarse, core-shell structured body, and smooth, no or partially cross-linked head. The newly formed compartments were also inhomogeneously cross-linked and their phases separated, producing three and four compartment anisotropic particles with an aspect ratio up to 2.3. Gel content of final particles was less than 75 %. Due to a sequential cross-linking from growing particle to newly formed compartments, reaction stability and particle monodispersity were maintained even when high amounts of DVB were added. Asymmetrical morphologies, structures, sizes, and surface roughness of particles were continuously tuned by varying DVB amount and its start addition time. This one-pot method provides a simple, efficient route for synthesis of anisotropic particles.

Facile Preparation of Raspberry-Like Superhydrophobic Polystyrene Particles via Seeded Dispersion Polymerization

A simple and facile approach was developed to fabricate raspberry-like or snowman-like particles via seeded dispersion polymerization by just changing the ratio of second monomer styrene (St) to seeds in which poly(styrene-co-hydrolyzed-methacryloxypropyltrimethoxysilane) [P(St-co-MPS)] latex was used as seeds with hydrolyzed-MPS as a cross-linking agent. The morphologies of final products were confirmed by field-emission scanning electron microscopy and transmission electron microscopy. Interestingly, the seed part of snowman-like particles showed raspberry-like with adsorbing quantities of PS particles while the other part smooth. The formation mechanism of the raspberry-like particles was also discussed. The superhydrophobic surface with both the static contact angle of 158° and high adhesion to water could be achieved by the hydrophobization of the particulate film with octadecyltrimethoxysilane that was formed from the raspberry-like particles decorated by a thin layer of silica nanoparticles. Further, through encapsulating Ag nanoparticles within the surface, the obtained raspberry-like PS/Ag/SiO2 nanocomposite particles exhibited excellent antibacterial property simultaneously.

Synthesis of Anisotropic Polymer/Inorganic Particles via Asymmetric Swelling–Dissolving Process

In this work, an asymmetric swelling-dissolving process of original submicrometer-sized decentered sulfonated polystyrene/silica (SPS/silica) particles in a ternary mixed solvent (water/ethanol/heptane) was first reported. Actinia-like and porous snowman-like SPS/silica composite particles are fabricated through tuning the composition of the ternary mixed solvent. Actinia-like particles, with a silica core embedded in a "blooming" SPS matrix, are obtained when the composition of the mixed solvent is 5 g/5 g/0.1 g (water/ethanol/heptane). If the amount of heptane in the mixed solvent is doubled, then porous snowman-like particles are produced. The TEM and SEM images show that silica particles are exposed in these two anisotropic SPS/silica composite particles compared with the original decentered SPS/silica particles. Considering the particles morphology and the swelling and dissolving performance of SPS in different solvents, the formation of the new-shaped anisotropic SPS/silica composite particles should be attributed to an asymmetric swelling-dissolving process; that is, the swelling-dissolving rate of SPS coating around the protruding silica part is faster than the other part of the composite particles. The anisotropic swelling-dissolving property of polymer/inorganic composite particles inspires a facile way to the fabrication of new composite particles.

Synthesis and characteristics of snowman-like fluorescent PMMA microbeads

This paper reports the fabrication of fluorescent snowman-like poly(methyl methacrylate) (PMMA) colloidal microbeads using a two-step method. The fluorescent cross-linked PMMA seeds were first synthesized by dispersion polymerization and then swelled in a monomer solution containing the methyl methacrylate monomer, a secondary cross-linker and initiator, followed by heat-induced polymerization. Scanning electron microscopy, confocal laser scanning microscopy, and Fourier transform infrared spectroscopy confirmed the synthesis of fluorescent snowman-like particles.

Surfactant effect on functionalized carbon nanotube coated snowman-like particles and their electro-responsive characteristics

The core–shell structured snowman-like (SL) microparticles coated by functionalized multi-walled carbon nanotube (MWNT) were prepared in the presence of different surfactants including cationic surfactant-cetyl trimethylammonium bromide (CTAB) and anionic surfactant-sodium lauryl sulfate (SDS). The effect of surfactants on adsorption onto SL particles was characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and conductivity. The cationic surfactant is found to be more effective than anionic surfactant for helping nanotube adsorbed onto microparticle due to the presence of electrostatic interaction between the functionalized MWNT and the surfactant. Furthermore, the MWNT/SL particles dispersed in silicone oil exhibited a typical fibril structure of the electrorheological characteristics under an applied electric field observed by an optical microscope (OM), in which the state of nanotubes wrapped on the particles strongly affects their electro-responsive characteristics.

Carbon nanotube coated snowman-like particles and their electro-responsive characteristics

We report the fabrication of core-shell structured snowman-like microparticles coated with multi-walled carbon nanotubes and their electro-responsive electrorheological behavior under an applied electric field strength when dispersed in silicone oil. It is observed that they form a chain-like structure, possessing microfluidic potential applications with their solid-like property.

Core−Shell Structured Semiconducting PMMA/Polyaniline Snowman-like Anisotropic Microparticles and Their Electrorheology

Core-shell structured semiconducting snowman-like particles were synthesized, and their electrorheological (ER) characteristics under an applied electric field were examined. Monodispersed snowman-like poly(methyl methacrylate) (PMMA) particles were fabricated previously using a seed emulsion polymerization procedure. These anisotropic particle-based ER fluids, which were tested using a rotational rheometer, exhibited unusual ER properties in the flow curves at various electric field strengths when analyzed using the Cho-Choi-Jhon model. The dielectric spectra, as supporting data for the ER effect, were measured using a LCR meter. The relaxation time of the ER fluid was relatively shorter than typical ER fluids.

Hybrid Dissymmetrical Colloidal Particles

Colloidal particles with a controlled morphology combining both organic and inorganic parts were synthesized through a seeded emulsion polymerization process. Silica seed particles from 50 to 150 nm were first surface-modified by adsorption of an oxyethylene-based macromonomer or covalent grafting of a trialkoxysilane derivative. Then, emulsion polymerization of styrene was carried out in the presence of these particles, the formation of polystyrene nodules being highly favored at the silica surface in such conditions. While varying different experimental parameters, we have demonstrated that the ratio between the number of silica seeds and the number of growing nodules is a key parameter in controlling the morphology of the final hybrid nanoparticles. For instance, in the particular case when this ratio was close to 1, dumbbell-like or snowman-like particles were obtained. Further selective surface modification of their silica moiety was also tested successfully, indicating a potential application of these hybrid particles as original building blocks toward supraparticulate assemblies.