Preparation of Monodisperse Bio-Based Polymer Particles via Dispersion Polymerization.

Monodisperse bio-based polymer particles were successfully prepared through the dispersion polymerization of tulip-derived α-methylene-γ-butyrolactone (MBL) in N,N-dimethylformamide/ethanol (7/3, w/w) at 65 °C with poly(vinylpyrrolidone) (PVP) as a colloidal stabilizer. The diameter of the polymer particles was well controlled by changing the composition of the reaction medium or PVP concentration. Furthermore, 100% bio-based poly(MBL) (PMBL) particles were prepared via the dispersion polymerization of MBL in water using hydrolyzed PMBL as a colloidal stabilizer, which was synthesized by hydrolysis of PMBL.

Preparation of Salt-Responsive Hollow Hydrophilic Polymer Particles by Inverse Suspension Polymerization.

Hydrophilic polymer particles with a hollow structure have potential applications such as carriers for hydrophilic drugs. However, there are few reports on preparation and morphology control of such particles via a simple method. In this study, hollow hydrophilic polymer particles were prepared by inverse suspension polymerization for water droplets containing 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) anions, 1-vinylimidazole (VIm) cations, oligo(ethylene glycol) diacrylate (OEGDA), dextran, and an initiator via the self-assembling phase-separated polymer (SaPSeP) method developed in our lab. The inner morphology of the particle could be controlled (as single- or multi-hollow structures) by changing the concentrations of the OEGDA and the dextran. The obtained hollow particles could encapsulate a hydrophilic fluorescent substance in their hollow region when the substance was added to the primary droplets before polymerization. In addition, the poly(AMPS-co-VIm-co-OEGDA) shell of the particles exhibited an ionic cross-linked structure, which could be stimulated by salt. The poly(AMPS-co-VIm-co-OEGDA) hollow particles with the encapsulated substance released the substance when salt was added to the dispersion. These results indicated that the applicability of the SaPSeP method can be broadened for morphology control of the hydrophilic polymer particles encapsulating water-soluble materials.

Preparation of Polypropylene-Composite Particles by Dispersion Polymerization.

Polypropylene (PP)/poly(benzyl methacrylate) (PBzMA) composite particles were prepared by dispersion polymerization of benzyl methacrylate (BzMA) in the presence of PP particles without a conventional dispersant. The polymerization process yielded a stable emulsion of composite particles with a "currant bun"-like morphology consisting of a PBzMA core and PP bumps, indicating that the PP particles operate as colloidal stabilizers. Conversely, when BzMA was replaced with styrene as the monomer, dispersion polymerization yielded a large amount of aggregates. Finally, a stable emulsion was formed by copolymerizing a small amount of methyl methacrylate (MMA) with styrene. This result suggested that PP must interact with a second polymer to prepare stable composite particles. The surfaces of the PP particles, which are highly hydrophilic due to their carboxyl groups, were involved in the attachment and stabilization of the polymer precipitated in the medium. A film prepared from the obtained PP/PBzMA composite particles was highly hydrophobic and strongly adhesive to a PP sheet.

Monodispersed Nitrogen-Containing Carbon Capsules Fabricated from Conjugated Polymer-Coated Particles via Light Irradiation.

Near-infrared (NIR) light irradiation induced the transformation of polypyrrole (PPy) to nitrogen-containing carbon (NCC) material due to its light-to-heat photothermal property. The temperature of the PPy increased over 700 °C within a few seconds by the NIR laser irradiation, and elemental microanalysis confirmed the decreases of hydrogen and chloride contents and increases of carbon and nitrogen contents. Monodispersed polystyrene (PS)-core/PPy shell particles (PS/PPy particles) synthesized by aqueous chemical oxidative seeded polymerization were utilized as a precursor toward monodispersed NCC capsules. When the NIR laser was irradiated to the PS/PPy particles, the temperature rose to approximately 300 °C and smoke was generated, indicating that the PS component forming the core was thermally decomposed and vaporized. Scanning electron microscopy studies revealed the successful formation of spherical and highly monodispersed capsules, and Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy studies confirmed the capsules consisted of NCC materials. Furthermore, sunlight was also demonstrated to work as a light source to fabricate NCC capsules. The size and thickness of the capsules can be controlled between 1 and 80 µm and 146 and 231 nm, respectively, by tuning the size of the original PS/PPy particles and PPy shell thickness.

Preparation of Cellulose Particles with a Hollow Structure.

In this study, we report the preparation of hollow cellulose particles via a solvent-releasing method with the ionic liquid 1-ethyl-3-methylimidazolium acetate ([Emim]Ac). A dispersion comprising [Emim]Ac droplets with dissolved cellulose and a hexane medium containing a stabilizer was poured into a large amount of acetone (precipitant), resulting in the precipitation of cellulose and the formation of solid cellulose particles with a hollow structure. We found that the formation of the hollow structure resulted from the equilibrium phase separation. Porous structures were also obtained using ethanol or t-butanol as a precipitant, where cellulose immediately precipitated (i.e., exhibited non-equilibrium phase separation). In the case where acetone was used as the precipitant, the diffusion rate of [Emim]Ac from the droplets into the precipitant was relatively low; that is, the precipitation of cellulose was delayed, which allowed the cellulose to be phase-separated into a thermodynamically stable structure (equilibrium phase separation), resulting in the formation of the hollow structure.

Incorporation Behavior of Nonionic Emulsifiers inside Particles and Secondary Particle Nucleation during Emulsion Polymerization of Styrene.

In previous studies, it was found that a large amount of the polyoxyethylene nonylphenyl ether nonionic emulsifier, Emulgen 911 (E911), was incorporated inside polymer particles obtained at the completion of a conventional emulsion polymerization with potassium persulfate as a typical water-soluble initiator. In this study, to understand the incorporation phenomenon in more detail, the incorporation behavior during a batch emulsion polymerization of styrene was investigated. The percentage of E911 incorporated inside polystyrene (PS) particles relative to the total weight of E911 used in the polymerization increased to 18% until 50% conversion and then decreased and levelled off to 13% at 70-80% conversion. A similar incorporation behavior was observed in a seeded emulsion polymerization of styrene, in which E911 was added to an emulsifier-free PS seed emulsion to set the same E911 concentration as the batch emulsion polymerization before swelling of the PS seed particles with styrene at 70 °C for 24 h. These indicate that E911 molecules absorbed once into styrene-swollen PS particles partially exited therefrom into the aqueous medium during the polymerizations. When the E911 concentration in the aqueous medium is kept above the critical micelle concentration for a long period by the repartitioning process of E911 from styrene droplets into styrene-swollen particles via aqueous medium and/or exiting from the styrene-swollen particles into the aqueous medium, secondary particle nucleation continues during the polymerization, resulting in PS particles having a broad size distribution.

Preparation of Poly(Ionic Liquid) Particles with Anionic Side Chain by Dispersion Polymerization.

Micrometer-sized poly(ionic liquid) (PIL) particles with an anionic side chain, poly(1-butyl-3-methylimidazolium 3-sulfopropyl methacrylate) (poly([Bmim][SPMT])), are successfully prepared by dispersion polymerization at 60 °C in ethanol/ethyl acetate (2/8, w/w) with poly(vinylpyrrolidone) as a stabilizer. However, the obtained particles do not maintain the particulate state during drying at room temperature due to poly([Bmim][SPMT])'s low glass-transition temperature, below room temperature. In order to prevent coalescence and maintain particle shape, a cross-linking monomer is added after the nucleation stage of dispersion polymerization. Moreover, Fourier-transform infrared spectroscopy (FT-IR) demonstrates cation exchange between the obtained particles and metal cations (Ag+ ).

Formal preparation of regioregular and alternating thiophene-thiophene copolymers bearing different substituents.

Differently substituted thiophene-thiophene-alternating copolymers were formally synthesized employing a halo-bithiophene as a monomer. Nickel-catalyzed polymerization of bithiophene with substituents at the 3-position, including alkyl-, fluoroalkyl-, or oligosiloxane-containing groups, afforded the corresponding copolymers in good to excellent yield. The solubility test in organic solvents was performed to reveal that several copolymers showed a superior solubility. X-ray diffraction analysis of the thin film of the alternating copolymers composed of methyl and branched oligosiloxane substituents was also performed, and the results suggested the formation of a dual-layered film structure.

Unprecedented Regioregular Poly(1,4-arylene)s Prepared by Nickel(II)-Catalyzed Cross-Coupling Polymerization of 2,5-Disubstituted Bromo(chloro)arylene.

The unprecedented synthesis of regioregular head-to-tail-type poly(1,4-arylene)s bearing different substituents at the 2- and 5-positions is described. They were prepared by the polymerization of 2,5-disubstituted bromo(chloro)arylenes by selective halogen-metal exchange with a Grignard reagent and subsequent cross-coupling polymerization with a nickel catalyst [NiCl2 (dppp)]. Formation of the regioregular poly(1,4-arylene)s were confirmed by NMR spectroscopy, and showed remarkable differences to those polymers having uncontrolled regioregularity. Polymerization of bromo(chloro)arylenes with a chiral alkoxy substituent also led to the regioregular head-to-tail-type polyarylene, which demonstrated circular dichroism, thus suggesting formation of a structure with higher-order regularity.

Morphology Control of Porous Cellulose Particles by Tuning the Surface Tension of Media during Drying.

We previously reported the preparation of cellulose particles by the solvent releasing method (SRM). The obtained cellulose particles had a porous structure filled with a surrounding medium. However, the structure was fragile and easily collapsed because of the capillary pressure as the medium evaporated, resulting in dense cellulose particles. To control the morphology of the cellulose particles in a dry state, we focused our study on the influence of the surface tension of the surrounding medium on the structure of cellulose particles because the capillary pressure is proportional to the surface tension. Different media such as toluene, acetone, and pentane were investigated. The morphologies of the resulting cellulose particles were estimated by volume changes, specific surface areas, and compressive strengths. From these results, as the surface tension of the media filling the particles was lowered, the particle's specific surface area increased, resulting in the formation of softer particles.

Compound droplets derived from a cholesteric suspension of cellulose nanocrystals.

This study reports microfluidic generation of Janus droplets that consist of a liquid crystal component (a cholesteric aqueous suspension of cellulose nanocrystals, Ch-CNC) and a mineral oil (MO) component. The composition of the droplets was controlled by varying the relative flow rates of MO and Ch-CNC suspension. The shape of the Ch-CNC component of the droplets was changed from a truncated sphere to a hemisphere to a crescent moon. For these three Ch-CNC phase shapes, the Ch packing of the CNC pseudolayers was preserved, however the Ch pitch reduced, which was ascribed to the change in CNC orientation at the Ch-CNC/MO interface from perpendicular to parallel. The shape of the compound droplets was tuned from a dumbbell to a sphere by reducing interfacial tension between the Ch-CNC suspension and MO phases. Photopolymerization of the monomer mixture introduced in the Ch-CNC phase of the droplets and the removal of the sacrificial MO phase enabled the generation of Ch microgels. The results of this work can be used for exploring new applications of Janus colloids and the fabrication of programmable active soft matter.

A Facile Method for Preparation of Polymer Particles Having a "Cylindrical" Shape.

A facile and novel approach to prepare monodisperse polystyrene (PS) particles having a "cylindrical" shape was discovered. Spherical PS particles prepared by dispersion polymerization were stirred in a polyvinylpyrrolidone (PVP) aqueous solution for several hours using a magnetic stirrer at room temperature. In the presence of PVP, the spherical PS particles deformed into cylindrical shapes following stirring; however, the particles did not deform in the absence of PVP. The deformation rate of the particles was affected by the molecular weight of the dissolved PVP. This stirring method is not only highly efficient and high yielding, but also applicable to other materials such as polymethyl methacrylate. Moreover, the cylindrical particles were successfully applied as particulate surfactants in a Pickering emulsion system, which exhibited excellent stability in comparison to a system using spherical particles as a surfactant. In the first case, the emulsion was left standing for more than 4 months.

Preparation of Flattened Cross-Linked Hollow Particles by Suspension Polymerization in a Solid Dispersion Medium.

Flattened cross-linked hollow poly(divinylbenzene) (PDVB) particles with encapsulated n-hexadecane (HD) were successfully prepared through suspension polymerization using the self-assembling of phase-separated polymer (SaPSeP) method, in which the solid dispersion medium was gelled by gellan gum and compressed. The solid phase induced by gellan gum can be easily changed to a liquid state by heating, allowing the obtained particles to be easily recovered after polymerization. When the polymerization was conducted in the solid dispersion medium without compression, spherical hollow PDVB/HD composite particles were obtained. In contrast, when the polymerization was conducted with the compression of the solid dispersion medium, flattened hollow PDVB/HD composite particles were obtained. The shape of the flattened hollow polymer particles was controlled by changing the compression ratio of the solid phase, and the size could also be controlled by changing the DVB/HD droplet size using the Shirasu porous glass membrane-emulsification technique. Furthermore, flattened hollow particles larger than 20 µm in size were obtained, but it was difficult to obtain spherical hollow particles of such large size using the SaPSeP method.

Water Absorption Behavior of Polystyrene Particles Prepared by Emulsion Polymerization with Nonionic Emulsifiers and Innovative Easy Synthesis of Hollow Particles.

Submicrometer-sized raspberry-like polystyrene (PS) particles, which were prepared by emulsion polymerization with polyoxyethylene nonylphenyl ether nonionic emulsifier (Emulgen 910, HLB 12.2) 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. The water absorption decreased the glass transition temperature of the PS particles dispersed in an aqueous medium. The wt % (relative to PS) of the incorporated Emulgen 910 increased with increasing initial Emulgen 910 concentration in the emulsion polymerization, but the wt % (relative to the total Emulgen 910 used) of the incorporated Emulgen 910 was constant at approximately 50% independent of the initial concentration. The vol % (relative to particle) of water increased to 46% by heat treatment at 90 °C for 24 h, which was based on further water absorption, and resulted in spherical hollow particles, where the amount of the incorporated Emulgen 910 remarkably decreased in a short treatment and then remained almost constant during the heat treatment. After another 24 h treatment, the percentage of nonhollow particles increased gradually, which was based on the escape of the water domain together with Emulgen 910 from the inside of the particles. On the other hand, spherical PS particles prepared by emulsifier-free emulsion polymerization did not contain water in the inside and were not changed to hollow ones by a similar heat treatment. From these results, an innovative easy method to synthesize hydrophobic hollow PS particles is proposed.

Emulsion Polymerization with a Biosurfactant.

Emulsion polymerization of styrene was conducted using a biosurfactant (i.e., sodium surfactin, hereinafter called just "surfactin") having very low critical micelle concentration (CMC, 2.9 × 10-3 mmol/L) and biodegradability characteristics. The nucleation mechanism was investigated by comparing with a conventional surfactant (i.e., sodium dodecyl sulfate) system. Unlike the emulsion polymerization systems using conventional surfactants, nucleation mechanisms changed above CMC in the presence of a biosurfactant. At low concentrations of surfactin (above CMC), the polystyrene (PS) particles are likely generated via a soap-free emulsion polymerization mechanism. In contrast, at high surfactin concentrations, the PS particles would be synthesized by following a micellar nucleation mechanism. However, the slope (0.23) of the log Np versus log Cs plot (Np: number of particles; Cs: concentration of surfactin) did not obey the Smith-Ewart theory (0.6), this probably being produced by the high adsorbability of surfactin.

Preparation of Poly(ionic liquid) Hollow Particles with Switchable Permeability.

Poly(ionic liquid) (PIL) particles with a single-hollow structure are prepared by suspension polymerization from monomer droplets consisting of the hydrophobic ionic liquid monomer [2-(methacryloyloxy)ethyl]trimethylammonium bis(trifluoromethanesulfonyl)amide, ethylene glycol dimethacrylate, and n-butyl acetate containing dissolved poly(n-butyl methacrylate). The obtained PIL hollow particles' shells can be changed from hydrophobic to hydrophilic by anion exchange using a LiBr/ethanol solution. In the case of hydrophilic PIL hollow particles, the water-soluble fluorescent materials can penetrate into the hollow structure, whereas in the case of hydrophobic PIL hollow particles, penetration of the fluorescent materials is restricted. In addition, the encapsulated water-soluble materials can be preserved into the hollow part by changing the shell property of the PIL particle encapsulated with the water-soluble materials from hydrophilic to hydrophobic.

Preparation of janus particles with different stabilizers and formation of one-dimensional particle arrays.

Janus particles with two hemispheres having different stabilizers, a polystyrene (PS) phase stabilized by poly(acrylic acid) (PAA) (PS(PAA)) and a poly(methyl methacrylate) (PMMA) phase stabilized by poly(vinylpyrrolidone) (PVP) (PMMA(PVP)), were synthesized by the solvent-absorbing/releasing method of PS(PAA)/PMMA(PVP) composite particles with a core-shell structure. The PS(PAA)/PMMA(PVP) composite particles were prepared by seeded dispersion polymerization of MMA using PVP as stabilizer in the presence of PS seed particles stabilized by PAA. We also demonstrated the facile formation of the colloidal chains via hydrogen bonding interaction between different stabilizers.

Preparation of submicrometer-sized quaternary ammonium-based poly(ionic liquid) particles via emulsion polymerization and switchable responsiveness of emulsion film.

Emulsion polymerization of ionic liquid monomer [2-(methacryloyloxy)ethyl]trimethylammonium bis(trifluoromethanesulfonyl)amide ([MTMA][TFSA]) was performed. In the presence of potassium persulfate and sodium dodecyl sulfate as the ionic initiator and emulsifier, respectively, the system was colloidally unstable and coagulated, owing to anion exchange between the ionic liquid monomer and ionic emulsifier. When a nonionic initiator and emulsifier were used (2,2'-azobis 2-methyl-[1,1-bis(hydroxymethyl)-2-hydroxyethylpropionamide]; VA-080, polyoxyethylenesorbitan monooleate; Tween 80, respectively), a stable emulsion was obtained without coagulation, and the zeta potential of the prepared particles exhibited a high positive charge. Utilizing this surface charge, poly([MTMA][TFSA]) (PIL) particles were then successfully prepared via emulsifier-free emulsion polymerization without an ionic initiator. Moreover, the contact angle of water on the PIL emulsion film (emulsifier-free emulsion polymerization) was found to be ∼70° (static measurement), indicating that the surface of the PIL film was relatively hydrophobic. The retreating contact angle (∼28°) also indicated water wettability. However, when water was dropped on the PIL film just after the water already on the surface had completely retreated, the PIL film was instantaneously hydrophobic again. This result suggests that responsiveness of the PIL emulsion film was switchable between hydrophobic (in air) and hydrophilic (in water).

Preparation of raspberry-like polymer particles by a heterocoagulation technique utilizing hydrogen bonding interactions between steric stabilizers.

Large polystyrene particles stabilized by poly(acrylic acid) (PAA) (L-PS(PAA)) (as the core) and small polystyrene particles stabilized by poly(vinyl pyrrolidone) (PVP) (S-PS(PVP)) (as the corona) were successfully used to prepare raspberry-like particles by a heterocoagulation technique utilizing the hydrogen bonding interaction between PAA and PVP. The coverage of L-PS(PAA) by S-PS(PVP) could be controlled by adding PVP homopolymer to the L-PS(PAA) dispersion and by changing the molecular weight of the stabilizers. Moreover, the heterocoagulation of large poly(methyl methacrylate) particles stabilized by PAA (L-PMMA(PAA)) and S-PS(PVP) particles was also accomplished, resulting in the formation of L-PMMA(PAA)-core/S-PS(PVP)-corona raspberry-like composite particles. These results suggested that the raspberry-like particles composed of various polymer particles could be formed by the heterocoagulation technique utilizing the hydrogen bonding interaction.

Synthesis of Polypyrrole and Its Derivatives as a Liquid Marble Stabilizer via a Solvent-Free Chemical Oxidative Polymerization Protocol.

Solvent-free chemical oxidative polymerizations of pyrrole and its derivatives, namely N-methylpyrrole and N-ethylpyrrole, were conducted by mechanical mixing of monomer and solid FeCl3 oxidant under nitrogen atmosphere. Polymerizations occurred at the surface of the oxidant, and optical and scanning electron microscopy studies confirmed production of atypical grains with diameters of a few tens of micrometers. Fourier transform infrared spectroscopy studies indicated the presence of hydroxy and carbonyl groups which were introduced during the polymerization due to overoxidation. The polymer grains were doped with chloride ions, and the chloride ion dopant could be removed by dedoping using an aqueous solution of sodium hydroxide, which was confirmed by elemental microanalysis and X-ray photoelectron spectroscopy studies. Water contact angle measurements confirmed that the larger the alkyl group on the nitrogen of pyrrole ring the higher the hydrophobicity and that the contact angles increased after dedoping in all cases. The grains before and after dedoping exhibited photothermal properties: the near-infrared laser irradiation induced a rapid temperature increase to greater than 430 °C. Furthermore, dedoped poly(N-ethylpyrrole) grains adsorbed to the air-water interface and could work as an effective liquid marble stabilizer. The resulting liquid marble could move on a planar water surface due to near-infrared laser-induced Marangoni flow and could disintegrate by exposure to acid vapor via redoping of the poly(N-ethylpyrrole) grains.