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).

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.

Role of Osmotic Pressure for the Formation of Sub-micrometer-Sized, Hollow Polystyrene Particles by Heat Treatment in Aqueous Dispersed Systems †.

In a previous article, it was reported that a rather amount of sulfate end-groups as initiator fragment were buried in the inside of polystyrene (PS) particles, which were synthesized by emulsion polymerization with potassium persulfate initiator and nonionic emulsifier and operated to absorb water from the aqueous medium, resulting in hollow particles, when the PS emulsion was treated at higher temperature than the glass transition temperature of PS. In this article, it was clarified that the water absorption is based on the osmotic pressure between the outside (aqueous medium) and inside of the PS particles due to the buried sulfate end-groups.

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.

Do encapsulated heat storage materials really retain their original thermal properties?

The encapsulation of Rubitherm®27 (RT27), which is one of the most common commercially supplied heat storage materials, by polystyrene (PS), polydivinyl benzene (PDVB) and polymethyl methacrylate (PMMA) was carried out using conventional radical microsuspension polymerization. The products were purified to remove free RT27 and free polymer particles without RT27. In the cases of PS and PDVB microcapsules, the latent heats of melting and crystallization for RT27 ( and , J/g-RT27) were clearly decreased by the encapsulation. On the other hand, those of the PMMA microcapsules were the same as pure RT27. A supercooling phenomenon was observed not only for PS and PDVB but also for the PMMA microcapsules. These results indicate that the thermal properties of the heat storage materials encapsulated depend on the type of polymer shells, i.e., encapsulation by polymer shell changes the thermal properties of RT27. This is quite different from the idea of other groups in the world, in which they discussed the thermal properties based on the ΔHm and ΔHc values expressed in J/g-capsule, assuming that the thermal properties of the heat storage materials are not changed by the encapsulation. Hereafter, this report should raise an alarm concerning the "wrong" common knowledge behind developing the encapsulation technology of heat storage materials.

Preparation of stimuli-responsive "mushroom-like" janus polymer particles as particulate surfactant by site-selective surface-initiated AGET ATRP in aqueous dispersed systems.

Micrometer-sized, monodisperse, "mushroom-like" Janus poly(methyl methacrylate)/poly(styrene-2-(2-bromoisobutyryloxy)ethyl methacrylate)-graft-poly(2-(dimethyl amino)ethyl methacrylate) (PMMA/P(S-BIEM)-g-PDM) particles were successfully synthesized by site-selective surface-initiated activator generated by electron transfer for atom transfer radical polymerization in aqueous dispersed systems with spherical PMMA/P(S-BIEM) composite particles having controlled morphologies prepared using the solvent evaporation method. The anisotropic nonspherical shape of the obtained particles was controlled by changing the percentage of the surface area occupied by localized initiation sites (bromine group) at the surface of the PMMA/P(S-BIEM) composite particles with different P(S-BIEM) contents. Grafted PDM layer formed at the surface (contacting with water) of the P(S-BIEM) phase reversibly exhibited the volume phase transition in response to temperature and pH, which gave different nonspherical shapes ("open" or "closed" mushroom-cap). On the basis of such dual stimuli-responsive properties, the nonspherical particles effectively operated as particulate surfactant for Pickering emulsion, resulting in a stable 1-octanol-in-water emulsion at optimum temperature and pH value, and the Pickering emulsion could be easily unstabilized quickly by controlling them.

Preparation of hemispherical polymer particles by cleavage of a Janus poly(methyl methacrylate)/polystyrene composite particle.

Micrometer-sized, hemispherical particles were successfully prepared as a result of the cleavage of Janus PMMA/PS composite particles by dispersion into acetone/water (9/1-10/0 v/v) media or a THF/water (8/2 v/v) medium. The spherical composite particles having a Janus structure were prepared by the slow evaporation of toluene from homogeneous PMMA/PS/toluene droplets dispersed in an aqueous medium in advance. It was clarified that the difference in affinity between PMMA and PS phases with respect to the media caused the cleavage of the composite particles. This method is expected to be a novel approach to the preparation of nonspherical polymer particles.

Preparations of polystyrene/aluminum hydroxide and polystyrene/alumina composite particles in an ionic liquid.

Polystyrene (PS)/aluminum hydroxide (Al(OH)(3)) composite particles were successfully prepared by the sol-gel process of aluminum isopropoxide (Al(OPr(i))(3)) in a hydrophilic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) using ammonium hydroxide (NH(4)OH) as a catalyst in the presence of PS seed. Transmission electron microscopy observation of ultrathin cross-sections of the composite particles revealed that the composite particles had a core-shell morphology consisting of a PS core and a Al(OH)(3) shell having high crystallinity. The amount of secondary nucleated Al(OH)(3) could be reduced by dropwise addition of NH(4)OH. Moreover, PS/η-Al(2)O(3) composite particles were successfully prepared by heat treatment of PS/Al(OH)(3) at 300 °C in N(2) atmosphere, which is below the decomposition temperature of PS.

Dual stimuli-responsive "mushroom-like" Janus polymer particles as particulate surfactants.

"Mushroom-like" Janus poly(methyl methacrylate) (PMMA)/poly(styrene-2-(2-bromoisobutyryloxy)ethyl methacrylate)-graft-poly(2-(dimethylamino)ethyl methacrylate) (PDM) particles synthesized in our previous report were applied as particulate surfactants. The PDM moiety at the one side of the Janus particles reversibly exhibited the volume phase transition in response to pH and temperature in an aqueous medium; that is, the surface property of the Janus particles comprising both PDM and PMMA reversibly changed between amphiphilic and hydrophobic based on the nature of PDM. Consequently, 1-octanol-in-water emulsion droplets stabilized by the amphiphilic Janus particles coalesced in the alkaline region and at 60 degrees C around neutral pH because of desorption of the hydrophobized Janus particles from the interface to the oil phase.

Preparation of micrometer-sized, onionlike multilayered block copolymer particles by two-step AGET ATRP in aqueous dispersed systems: effect of the second-step polymerization temperature.

The polymerization rate, control/livingness, and particle morphology in seeded activators generated by electron transfer for the atom-transfer radical polymerization of styrene with PiBMA-Br macroinitiator particles were investigated at 70, 90, and 110 degrees C. At 110 degrees C, the polymerization proceeded quickly until 60% conversion was reached, but control/livingness was not observed. This seems to be the reason for the high activation rate and spontaneous initiation of styrene, which significantly increased the radical concentration, resulting in a number of radical terminations. As a result, the block copolymer was not sufficiently formed, leading to a sea-island structure. However, at 70 and 90 degrees C, the polymerizations were almost complete in 14 and 7 h, respectively. Control/livingness was maintained, resulting in PiBMA-b-PS. As a result, onionlike multilayered particles were successfully synthesized. These polymerization behaviors were discussed from the viewpoint of the radical concentration and propagation rate coefficient at various temperatures.

Preparation of "mushroom-like" Janus particles by site-selective surface-initiated atom transfer radical polymerization in aqueous dispersed systems.

A versatile approach for the preparation of micrometer-sized, monodisperse, "mushroom-like" Janus polymer particles in aqueous dispersed systems is proposed. The synthetic methodology of the Janus particles consists of the following two steps. The first step is the preparation of spherical poly(methyl methacrylate) (PMMA)/poly(styrene-2-(2-bromoisobutyryloxy)ethyl methacrylate) (P(S-BIEM)) Janus particles based on the internal phase separation induced by solvent evaporation from the solvent droplets dissolving the polymers. The second step is surface-initiated atom transfer radical polymerization (ATRP) of 2-(dimethylamino)ethyl methacrylate (DM) using the Janus particles with ATRP initiator groups at one side of the surface as macroinitiator. As a consequence, mushroom-like PMMA/P(S-BIEM)-graft-poly(DM) Janus particles were prepared, which had pH-responsive property.

Preparation of poly(acrylic acid) particles by dispersion polymerization in an ionic liquid.

Poly(acrylic acid) (PAA) particles were successfully prepared by dispersion polymerization of acrylic acid in ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoro-methanesulfonyl)amide ([DEME][TFSA]) at 70 degrees C with low hydrolysis grade (35.4%) poly(vinyl alcohol) as stabilizer. Interestingly, the PAA particles were easily extracted as particle state with water. Thus, the PAA particles had a cross-linked structure during the polymerization without cross-linker. Moreover, it was also noted that the cross-linking density of the PAA particles could be controlled by thermal treatment at various temperatures in [DEME][TFSA] utilizing the advantages of nonvolatility and high thermal stability of the ionic liquid.

A novel approach for preparation of micrometer-sized, monodisperse dimple and hemispherical polystyrene particles.

Micrometer-sized, monodisperse dimple and hemispherical polystyrene (PS) particles were successfully prepared by heating (55-70 degrees C) of spherical PS particles dispersed in methanol/water media (40/60 to 80/20, w/w) in the presence of decane droplets, and subsequent cooling down to room temperature. Decane was absorbed by the PS particles during the heating process. Decane-absorbed PS particles phase-separated into PS and decane phases in the inside during the cooling process, and eventually dimple and/or hemispherical particles were formed by removal of the decane phase from phase-separated PS/decane particles by evaporation. The size of the dimple, which is determined by the volume of decane phase-separated from decane-absorbed PS particles during the cooling process, increased with increases in the heating temperature and the methanol content.

Incorporation of nonionic emulsifier inside carboxylated polymer particles during emulsion copolymerization: influence of methacrylic acid content.

The influence of the methacrylic acid (MAA) content (0-10 mol %) on the incorporation of polyoxyethylene nonylphenyl ether (Emulgen 911, HLB 13.7) nonionic emulsifier inside polymer particles during emulsion copolymerization of styrene (S) and MAA was investigated. The amount of incorporated emulsifier after centrifugal washing with 2-propanol to remove adsorbed emulsifier from the surfaces was directly measured by gel permeation chromatography (GPC) and (1)H NMR. The level of incorporation increased with increasing MAA content and reached 74% of the total amount of emulsifier at 10 mol % MAA. At lower MAA contents (0-3 mol %), the particle size distribution was bimodal because of formation of new particles by secondary nucleation. However, only limited secondary nucleation occurred at higher MAA contents (6-10 mol %), and monodisperse particles were thus obtained.

Effect of molecular weight on the morphology of polystyrene/poly(methyl methacrylate) composite particles prepared by the solvent evaporation method.

The effect of molecular weight on the morphology of polystyrene (PS)/poly(methyl methacrylate) (PMMA) composite particles was investigated. PS/PMMA composite particles with different molecular weights (M*=MwPS+MwPMMA)/2 approximately 2x10(4)-1x10(6) g.mol(-1)) were prepared by the release of toluene (T) from PS/PMMA/T (1/1/24, w/w/w) droplets dispersed in an aqueous solution of polyoxyethylene nonylphenyl ether nonionic surfactant (Emulgen 911). As T evaporated, the spherical droplets phase separated, resulting in snowmanlike composite particles with Janus morphology. The nonspherical shape was closely related to the morphology, which depended on M*. The interfacial tension between the phase-separated PS and PMMA phases increased with an increase in M*, and this would allow the formation of the snowmanlike shape to decrease the interfacial area between the PS and the PMMA phases.

Preparation of microcapsules containing a curing agent for epoxy resin by polyaddition reaction with the self-assembly of phase-separated polymer method in an aqueous dispersed system.

To prepare cured epoxy resin particles encapsulating a curing agent (diamine), the self-assembly of phase-separated polymer (SaPSeP) method was developed to be applicable to polyaddition reaction of a stoichiometrically imbalanced system. The SaPSeP method was developed by the authors for preparation of micrometer-sized, hollow cross-linked polymer particles by radical polymerization based on the self-assembly of phase-separated polymer at the inner interface of particles. Although a polyaddition reaction, in general, requires that the reactants are in stoichiometric balance for the cure reaction to proceed well, diamine was successfully encapsulated within a cured epoxy resin shell by utilizing the SaPSeP method regardless of stoichiometric imbalance. The results provide further support of the previously proposed SaPSeP mechanism for the formation of hollow particles. Moreover, such diamine capsules can be employed in one-component epoxy adhesives.

Preparation of micrometer-sized, monodisperse "janus" composite polymer particles having temperature-sensitive polymer brushes at half of the surface by seeded atom transfer radical polymerization.

Micrometer-sized, monodisperse polystyrene (PS)/poly[methyl methacrylate-(chloromethyl)styrene] [P(MMA-CMS)] composite particles having hemispherical structure were prepared by solvent evaporation from toluene droplets containing dissolved PS and P(MMA-CMS) dispersed in aqueous solution, which had been prepared using the membrane method. The formation of hemispherical ("Janus") morphology by phase separation between the PS and the P(MMA-CMS) was confirmed by both optical and electron microscopy. Atom transfer radical polymerization (ATRP) of 2-(dimethylamino)ethyl methacrylate (DM) was subsequently carried out in the presence of hemispherical PS/P(MMA-CMS) composite particles in an aqueous dispersed system. After polymerization, the morphology of the particles changed from spherical to "mushroom" shape as observed by scanning electron microscopy, indicating that DM polymerized inside or on the surface of half [P(MMA-CMS) phase] of the particles. 1H NMR spectra were consistent with chloromethyl functional groups in P(MMA-CMS) operating as ATRP initiators in the DM polymerization.

Formation of "snowmanlike" polystyrene/poly(methyl methacrylate)/toluene droplets dispersed in an aqueous solution of a nonionic surfactant at thermodynamic equilibrium.

"Snowmanlike" polystyrene (PS)/poly(methyl methacrylate) (PMMA) composite particles were prepared by evaporation of toluene from PS/PMMA/toluene droplets dispersed in an aqueous solution of polyoxyethylene nonylphenyl ether surfactant (Emulgen 911). Partitioning experiments revealed that the Emulgen 911 concentration was higher in the droplets than in the aqueous solution during toluene evaporation. As a consequence, the interfacial tensions between the polymer phases (PS and PMMA) and the aqueous phase (gammaP-T/W) were extraordinarily low (approximately 10(-1) mN/m). The interfacial tension between the PS and PMMA phases containing toluene (gammaPS-T/PMMA-T) measured by the spinning drop method was not affected by the presence of Emulgen 911. Based on minimization of the total interfacial free energy at a polymer weight fraction in the toluene droplet of 0.17, the formation of spherical droplets is expected, in agreement with experiment. The subsequent morphology change of the PS/PMMA/toluene droplets from spherical to snowmanlike during toluene evaporation under thermodynamic equilibrium is attributed to (i) the low values of gammaP-T/W, which explains the increase in the interfacial area between the droplets and the aqueous phase, and (ii) the increase in gammaPS-T/PMMA-T with increasing polymer weight fraction.

Preparation of multihollow polymer particles by seeded emulsion polymerization using seed particles with incorporated nonionic emulsifier.

Emulsion polymerizations of styrene using poly(oxyethylene) nonylphenyl ether nonionic emulsifier were carried out at different emulsifier and initiator (potassium persulfate, KPS) concentrations to prepare polystyrene (PS) seed particles with incorporated nonionic emulsifier. Seeded emulsion polymerizations of styrene using the PS seed particles with different amounts of incorporated emulsifier were carried out to develop a novel method for the preparation of multihollow particles. When seed particles with a small amount of incorporated emulsifier were used, non-hollow spherical particles were prepared. However, multihollow particles were obtained in the case of seed particles with a large amount of incorporated emulsifier. Moreover, the higher the initiator concentration in the preparation of seed particles, the more effectively were hollow particles prepared. On the basis of the above results, a mechanism for the formation of multihollow structure was suggested.