Haruyuki Ishii

Directed Orientation of Asymmetric Composite Dumbbells by Electric Field Induced Assembly

Assembly and directed orientation of anisotropic particles with an external ac electric field in a range from 1 kHz to 2 MHz were studied for asymmetric composite dumbbells incorporating a silica, titania, or titania/silica (titania:silica = 75:25 vol %) sphere. The asymmetric composite dumbbells, which were composed of a polymethylmethacrylate (PMMA)-coated sphere (core-shell part) and a polystyrene (PSt) lobe, were synthesized with a soap-free emulsion polymerization to prepare PMMA-coated inorganic spheres and another soap-free emulsion polymerization to form a polystyrene (PSt) lobe from the PMMA-coated inorganic spheres. The composite dumbbells dispersed in water were directly observed with optical microscopy. The dumbbells incorporating a silica sphere oriented parallel to an electric field in the whole frequency range and they formed a pearl chain structure at a high frequency of 2 MHz. The titania-incorporated dumbbells formed chain structures, in which they contacted their core-shell parts and oriented perpendicularly to a low-frequency (kHz) field, whereas they oriented parallel to a high-frequency (MHz) field. Since the alignment of dumbbells in the chains depends not only on the interparticle forces but also on the torque that the induced dipoles in the dumbbells experience in the electric field, the orientation of dumbbells perpendicular to the electric field was the case dominated by the interparticle force, whereas the other orientation was the case dominated by the torque. The present experiments show that the incorporation of inorganic dumbbells is an effective way to control the assembled structure and orientation with an electric field.

Preparation of asymmetrically nanoparticle-supported, monodisperse composite dumbbells by protruding a smooth polymer bulge from rugged spheres.

A novel method is proposed to create asymmetrically nanoparticle-supported, monodisperse composite dumbbells. The method consists of the three steps of double soap-free emulsion polymerizations before and after a heterocoagulation. In the first step, soap-free emulsion polymerization was conducted to cover silica cores with cross-linked poly(methyl methacrylate) (PMMA) shells. Then, positively or negatively charged silica nanoparticles were heterocoagulated with the silica-PMMA core-shell particles. In the heterocoagulations, the nanoparticles surface-modified with a cationic silane coupling agent, 3-aminopropyltriethoxysilane, were used as the positively charged ones, and silica nanoparticles without any treatment were used as the negatively charged ones. In the third step, soap-free polymerizations at different pH values were performed to protrude a polystyrene (PSt) bulge from the core-shell particles supporting the charged silica nanoparticles. In the polymerization, the core-shell particles heterocoagulated with the positively charged silica nanoparticles were aggregated in an acidic condition whereas the silica nanoparticles supported on the core-shell particles were dissolved in a basic condition. For the negatively charged silica nanoparticle, a PSt bulge was successfully protruded from the core-shell particle in acidic and neutral conditions without aggregation of the core-shell particles. The protrusion of the PSt bulge became distinctive when the number of heterocoagulated silica nanoparticles per core-shell particle was increased. Additional heterocoagulation experiments, in which positively or negatively charged magnetite nanoparticles were mixed with the asymmetrically nanoparticle-supported composite dumbbells, confirmed direct exposure of silica nanoparticles to the outer solvent phase.

Colloidal polarization of yolk/shell particles by reconfiguration of inner cores responsive to an external magnetic field.

Yolk/shell particles, which were hollow silica particles containing a movable magnetic silica core (MSC), were prepared by removing a middle polystyrene layer from multilayered particles of MSC/polystyrene/silica shell with heat treatment followed by a slight etching with a basic solution. An ac electric field was applied to the suspension of the yolk/shell particles to form pearl chains (1D structure) of yolk/shell particles. Observation with an optical microscope showed that the MSCs in the silica compartment of the pearl chains had a zigzag structure under the electric field. An external magnetic field applied to the suspension could form a novel structure of doublet MSC in the shell compartment of the quasi-pearl chain structure. Application of a magnetic field was also performed for 2D hexagonally close-packed assemblies of the yolk/shell particles, which could two-dimensionally form a doublet structure of MSCs as if they were polarized in the compartment. Switching on/off the magnetic field successfully controlled the positional ordering of cores in the consolidated silica shell.

Novel Mini-Reactor of Silicone Oil Droplets for Synthesis of Morphology-Controlled Polymer Particles

Inside spaces of emulsion droplets can be used as mini-reactors for material synthesis. The novel application of sol-gel derived silicone oil droplets as mini-reactors was examined for the case of polymerization of styrene (St) and comonomers with the oil-soluble initiator 2,2-azobis(2,4-dimethylvaleronitrile). Polydimethylsiloxane (PDMS) droplets prepared from dimethylsiloxane were used as the mini-reactors, in which the polymerization of St without comonomers was first conducted. In the polymerization, the St/PDMS volume ratio was varied from 0.025 to 0.10. After the polymerization, each PDMS droplet contained a polystyrene (PSt) particle. The St/PDMS ratio of 0.05 enabled the synthesis of micrometer-sized, spherical PSt particles with low polydispsersity. Copolymerization of St with comonomers having hydrophilic groups deformed the spherical shape of particles to lens-like or disk-like morphologies that were obtained with acrylic acid or sodium 4-styrene sulfonate, respectively. In another copolymerization, with divinylbenzene used as a cross-linker, hemispherical polymer particles were formed. To diversify the particle morphologies further, the proposed mini-reactor synthesis was combined with the recently proposed silicone oil droplet templating method (Ohta et al., 2012). Around the PDMS droplets containing a polymer particle, polymeric shells with a depression were successfully formed with the proposed method. The remaining PDMS oil inside the polymeric shells was extracted with ethanol, which caused hemispherical polymeric bowl-shaped capsules having a protrusion on the inside.

Direct observation of micron-sized silica rattles to demonstrate movability of inner spheres in the silica compartment suspended in aqueous media

Micron-sized silica rattle particles were directly observed in aqueous media with an optical microscope to demonstrate the movability of inner silica spheres within the compartment of the silica shell. The rattle particles were prepared by a combined method of fabricating multilayered particles (silica sphere/polystyrene (PSt) inner shell/silica outer shell) and removing the polymer component in the multilayered particles. The polymer component of PSt was removed by heating the multilayered particles at 500 °C for 4 h, which resulted in a successful preparation of silica rattle particles without any residual polymer component. The silica rattle particles in the presence of polyvinylpyrrolidone used as a viscosity enhancer were observed with the optical microscope under an alternating electric field, revealing that less than 10% of inner spheres in the pearl chain of rattle particles could be randomly moved in the compartment of the silica shell. For increasing the percentage of inner spheres randomly moving, the silica rattle particles were slightly etched with a diluted NaOH solution to detach the inner spheres from the inside wall of the silica shell. The slight etching of the silica component led to the observation of approximately 75% inner spheres randomly moving under the applied electric field. This is the first direct observation to show that the single spheres incorporated in the shell compartment are capable to freely move without sticking to the inside wall of the shell.

Magnetoresponsive, anisotropic composite particles reversibly changing their chain lengths by a combined external field

Magnetoresponsive, anisotropic composite particles were prepared to explore a new type of building blocks reversibly changing their chain lengths by switching on an external magnetic field. The composite particles were synthesized with three-step polymerization comprising (i) polymerization to coat magnetoresponsive silica particles with crosslinked poly(methyl methacrylate) (PMMA), (ii) polymerization to form a polystyrene (PSt) lobe on the PMMA-coated particles and (iii) polymerization to form another PSt lobe on the opposite side of the former lobe. The structure of the composite particles was analyzed with scanning transmission electron microscopy showing rod-like polymer particles incorporating a magnetoresponsive particle in the middle of a rod-like particle. The composite particles suspended in aqueous solution of polyvinylpyrrolidone used as a viscosity enhancer were observed by optical microscopy under applied external fields. Application of an alternating electric field at a high frequency of 2 MHz oriented the rod-like particles parallel to the electric field and assembled them to form pearl-chain structures of the composite particles. The chain lengths of the oriented rod-like particles were extended during the application of the electric field. While applying the electric field, an additional application of magnetic field with a field strength of 100 mT changed the chain structure so as to allow the magnetoresponsive parts to come close to each other. A combined application in which the magnetic field was switched on and off intermittently under a fixed electric field could reversibly compress and extend the particle chains and control their chain lengths.

Anionic liposome template synthesis of raspberry-like hollow silica particle under ambient conditions with basic catalyst.

Hollow silica particle was obtained with a vesicle template synthesis in water under ambient conditions in the presence of ammonia. Biomimetic vesicles, liposomes were used, which consisted of a zwitterionic phospholipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and a tiny amount of charged amphiphiles, hexadecylamine (HDA) or dicetylphosphate (DCP). Aggregation of silica occurred for DPPC or cationic DPPC/HDA liposome, whereas well-dispersed hollow silica particle could be obtained for anionic DPPC/DCP liposome. The hollow particle synthesized with the anionic liposome had single-layered and raspberry-like structures. Electrostatic repulsion between anionic vesicles maintained stable dispersion of the as-synthesized particles during the reaction. Formation of the raspberry-like morphology is explained by silica particle precipitation selectively induced around the liposomes under basic conditions due to affinity of silica precursors for the liposomes. Synthesis of well-dispersed hollow silica particle with a raspberry-like morphology is the first report in vesicle template syntheses.

Miniaturization of anisotropic composite particles incorporating a silica particle smaller than 100 nm

Two-step aqueous polymerizations with a water-soluble initiator of potassium persulfate were conducted to prepare anisotropic composite particles incorporating a silica core smaller than 100xa0nm. The two-step polymerization consisted of the first polymerization to coat the silica cores with cross-linked polymethylmethacrylate (PMMA) shell and the second polymerization to protrude a polystyrene (PSt) bulge from the core–shell particles. The concentration of ionic comonomer of sodium p-styrenesulfonate (NaSS) in the first polymerization was an important factor to stabilize the core–shell particles during the second polymerization as well as the first one, and an appropriate concentration of NaSS could prepare the anisotropic composite particles incorporating a single core. Another important factor for small, anisotropic composite particles was duration time for swelling the core–shell particles with the second monomer of styrene. Extension of the duration time from 2 to 4xa0h facilitated protrusion of the PSt bulge from the particles incorporating a 44-nm silica core. The composite particles were also employed to fabricate anisotropic hollow particles. Chemical etching of silica component in the composite particles with hydrofluoric acid successfully created anisotropic hollow polymer particles with a cavity size corresponding to the silica cores.

Preparation of various Janus composite particles with two components differently combined

Janus composite particles with a combination of organic and inorganic substances were synthesized by soap-free emulsion polymerization in which an amphoteric initiator of 2,2′-azobis[N-(2-carboxyethyl)-2-2-methylpropionamidine] (VA-057) was employed to introduce a polystyrene (PSt) lobe onto silica cores surface-modified with 3-methacryloxypropyltrimethoxy silane (MPTMS). Thermogravimetric analysis and X-ray photoelectron spectroscopy were used to characterize the surface-modified silica particles and showed that a small amount of MPTMS introduced onto the surface of silica particles could successfully prepare SiO2–PSt Janus particles. The oxide part of SiO2–PSt Janus particles obtained with the polymerization was further surface-modified with 3-aminopropyltriethoxysilane (APS) to introduce positively charged amino groups on the silica surface. The silica surface modified with APS was covered with gold by electroless deposition in which a gold precursor of auric chloride was reduced with ascorbic acid in the presence of polyvinylpyrrolidone. The electroless deposition of gold successfully produced Janus particles with a combination of gold and PSt surfaces. Furthermore, dissolution of the polymer component of the Au–PSt Janus particles in tetrahydrofuran led to another Janus type of particles with an inorganic combination of Au and SiO2.

Preparation of oil-containing, polymeric particles having a single depression with various shapes

Oil-containing particles with a single depression were created by polymerization to form polymeric shells encapsulating oil droplets of polydimethylsiloxane (PDMS) that were prepared by the base-catalyzed hydrolysis and condensation of dimethyldiethoxysilane (DMDES). A reactive silane agent of 3-methacryloxypropyltrimethoxysilane (MPTMS) was added to the PDMS emulsion and used as a source of the polymeric shell. The water-soluble initiator potassium persulfate was employed to initiate the polymerization for the shell formation. Polymeric particles with a wrinkled depression were observed at low MPTMS concentrations, whereas those with a pseudo-hemispherical depression were obtained at high MPMTS concentrations. Another shell source, styrene (St), was added to the polymerization system for extending morphologies of the oil-containing particles. An increase in MPTMS concentration in the polymerization with the addition of St varied the depression shape from an ellipsoidal hemisphere to a pseudo-hemisphere. Spherical particles without any depression were also obtained at a high MPTMS concentration in the polymerization with the addition of St. The containment of PDMS oil in the polymeric particles was confirmed by thermal gravimetric analysis (TGA) and energy dispersive X-ray (EDX) measurement. Shrinkage volumes by the polymerization of MPTMS and St were calculated to examine the dominant factor for formation of the depression. The calculation of shrinkage volumes indicated that the volume shrinkage of MPTMS in the polymerization was an important factor to determine the morphology of oil-containing particles.