High-Intensity Circular Dichroism of Head-To-Tail Regioregular Poly(1,4-Phenylene)s in the Aggregated State.

Circular dichroism (CD) studies on poly(1,4-phenylene)s bearing a chiral side chain in the aggregated conditions were carried out. Little CD was observed in a solution form, while addition of a poor solvent into the polyphenylene solution induced aggregation and a strong CD was observed, accordingly. Applying the controlled degree of polymerization (DP) of poly(1,4-phenylene) in the use of bidentate diphosphine Chiraphos as a ligand for the nickel catalyst, the relationship of DP with CD strength was studied to reveal to show the highest CD at the DP=84 (gabs=ca. 2×10-2). It was also found that the related aggregation was observed in good solvent 1,2-dichloroethane upon standing the solution at 4 °C for 3-23 days to observe gabs=ca. 10-1. Studies on the substituent effect of poly(1,4-phenylene) suggested that CD behaviors were dependent on the type of non-chiral substituent on the aromatic ring as well as the side-chain chirality.

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 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.

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.

Synthesis of Micrometer-Sized Poly(methyl acrylate) by Temperature-Step Microsuspension Polymerization with Iodoform Based on the "Radical Exit Depression" Effect§.

Previously, we have reported the successful preparation of micrometer-sized poly(methyl methacrylate) particles without submicrometer-sized byproduct particles by microsuspension iodine-transfer polymerization (ms ITP), in which the radical exit depression (RED) effect was expected, with the benzoyl peroxide initiator at 8 wt % relative to the monomer. However, it was difficult to apply it simply under a similar condition for methyl acrylate (MA), which is more hydrophilic than methyl methacrylate (MMA), because the polymerization rate in the water phase (Rpw) arising from the oligomer radicals exiting from the monomer droplets is high, resulting in a lot of submicrometer-sized byproduct particles. In this study, the problem was overcome by utilizing a two-step temperature process in the microsuspension polymerization with iodoform (ms I) of MA, which supports the proposed mechanism in the ms ITP of MMA in the previous paper. Although the control of the molecular weight (Mn) and the molecular weight distribution (Mn/Mw) was restricted, the preparation of micrometer-sized particles without byproduct particles was realized and a high conversion was reached within a practical time that meets the demands of the industry by utilizing the ms I. The optimal conditions for MA were 70 °C for 2 h, followed by 80 °C for 4 h with a high content of initiator (8 wt % relative to a monomer).

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 and Morphology Control of Poly(ionic liquid) Particles.

Poly(ionic liquid)s (PILs) are prepared by the polymerization of ionic liquid (IL) monomers that have polymerizable groups on their cationic or anionic component. PILs also share many of the characteristic properties of ILs and can be used in various materials such as CO2 sorbents, polymer electrolytes, dispersants, and microwave-absorbing materials. In this feature article, we survey our research, focusing on the preparation of PILs in the particulate state (PIL particles) and on the morphological control of the PIL particles, including (1) the preparation of PIL particles by dispersion polymerization and emulsion polymerization, (2) control of the morphology of composite particles consisting of a PIL and poly(methyl methacrylate) (PMMA), (3) the preparation of hollow particles with a PIL shell, and (4) the preparation of PIL particles containing reduced graphene oxide (rGO). The size of the obtained PIL particles could be controlled through a modification of the synthesis conditions and the mode of polymerization (i.e., dispersion polymerization or emulsion polymerization). The obtained PIL particles maintained the characteristic properties of the corresponding ILs; moreover, the solubility of the PIL particles could be easily modified by changing their counteranion. Using seeded polymerization, we prepared PMMA/PIL composite polymer particles and subsequently demonstrated that their morphology can be manipulated to yield a core-shell or Janus structure. Hollow particles consisting of a PIL shell were also prepared, and modification of the polarity and penetration behavior of the shell through anion exchange was demonstrated.

Preparation of Cross-Linked Monodisperse Poly(acrylic acid) Particles by Precipitation Polymerization.

Water-soluble polymers such as poly(acrylic acid) (PAA) are widely applied in dispersants, thickeners, flocculants, and superabsorbent polymers. Here, submicrometer-sized monodisperse PAA particles were successfully prepared by precipitation polymerization in acetonitrile at 55 °C with 2,2'-azobis(2,4-dimethylvaleronitrile) as an initiator. In a medium with a high dielectric constant, the particles were stabilized by electrostatic repulsion without a colloidal stabilizer during the polymerization. Moreover, the obtained PAA particles were spontaneously cross-linked in the absence of a cross-linker. The degree of the cross-linking was strongly dependent on the amount of water (500-10,000 ppm) in the polymerization medium. The PAA particles swelled more with acrylic acid with an increase in the amount of water. Thus, in the case of high water content, the particles would become the main polymerization loci, which in turn results in the cross-linking structures owing to the α-hydrogen abstraction of PAA chains during the polymerization.

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.

Preparation of Methacrylate Polymer/Reduced Graphene Oxide Nanocomposite Particles Stabilized by Poly(ionic liquid) Block Copolymer via Miniemulsion Polymerization.

Poly(n-butyl methacrylate) (PnBMA)/reduced graphene oxide (rGO) nanocomposite films are prepared using two different routes. The first route involves preparation of PnBMA nanoparticles containing homogeneously dispersed rGO nanosheets by miniemulsion polymerization using a block copolymer of ionic liquid (IL) monomer and nBMA. The IL units act as adsorption sites for rGO whereas BMA units provide solubility in the BMA monomer droplets. Nanocomposite films obtained from miniemulsion polymerization exhibit higher tensile modulus in comparison with the films prepared by mixing a PnBMA emulsion and aqueous graphene oxide (GO) dispersion. The second route involves preparation of PnBMA particles armored with rGO nanosheets via miniemulsion polymerization using the same poly(ionic liquid) (PIL) block copolymer. An anionic exchange reaction is conducted to obtain more hydrophilic PIL units in the block copolymer, thus providing adsorption sites of GO nanosheets at the interface of the polymer particles. Subsequent chemical reduction of GO to rGO using hydrazine monohydrate results in formation of a PnBMA/rGO nanocomposite. The resulting nanocomposite film exhibits electrical conductivity (2.0 × 10-3 S m-1).

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+ ).

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.

Mesoporous magnetic silica particles modified with stimuli-responsive P(NIPAM-DMA) valve for controlled loading and release of biologically active molecules.

Mesoporous magnetic silica particles bearing a stimuli-responsive polymer valve were prepared and their performance as a microcapsule was evaluated. In this study, first, mesoporous magnetic iron oxide (Fe3O4) particles were prepared by a solvothermal method. Then, the magnetic particles were coated with silica and functionalized with vinyl groups using 3-(trimethoxysilyl)-propyl methacrylate (MPS). Subsequently, the Fe3O4/SiO2 composite particles grafted with MPS were used to carry out the seeded precipitation copolymerization of N-isopropylacrylamide (NIPAM) and 2,2-dimethylaminoethyl methacrylate (DMA). Here N,N'-methylenebisacrylamide (MBA) was used as a cross-linker. Brunauer-Emmett-Teller (BET) surface analysis suggested that the mesoporous structure was retained in the final Fe3O4/SiO2/P(NIPAM-DMA-MBA) composite hydrogel particles. The prepared Fe3O4/SiO2/P(NIPAM-DMA-MBA) composite hydrogel microspheres exhibited a pH-dependent volume phase transition. At lower pH values (<7), the inclusion of DMA shifted the volume phase transition to higher temperature because of the protonation of the tertiary amine groups. The composite hydrogel particles possessed a high saturation magnetization (51 emu g-1) and moved under the influence of an external magnetic field. The loading-release behaviour of these biologically active molecules suggested that a portion of the encapsulated guest molecules was released at a temperature below the lower critical solution temperature, LCST (<35 °C).

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.

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.

Synthesis of polymeric nanoparticles containing reduced graphene oxide nanosheets stabilized by poly(ionic liquid) using miniemulsion polymerization.

Polymeric nanoparticles containing reduced graphene oxide (rGO) nanosheets have been prepared by aqueous miniemulsion radical polymerization of methyl methacrylate (MMA) utilizing poly(ionic liquid) (PIL) as stabilizer to effectively disperse the rGO nanosheets in the monomer phase. The PIL that gave the best results in terms of rGO dispersibility was a block copolymer of the ionic liquid monomer 1-(2-methacryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)amide ([Mbim][TFSA]) and MMA, the concept being that the MMA units impart solubility in the MMA monomer droplets whereas the IL units act as adsorption sites for rGO. The rGO dispersibility in vinyl monomer was demonstrated to be superior using the above PIL block copolymer compared to the corresponding statistical copolymer or PIL homopolymer. Overall, the approach developed demonstrates how PILs can be employed to conveniently switch (turn ON/OFF) the dispersibility of PIL/rGO via anion exchange reactions, which can be an efficient strategy for synthesis of polymer/rGO nanocomposite materials.

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).