Showing papers by "Guanghui Ma published in 2003"

Preparation of W/O (water-in-oil) emulsions using a PTFE (polytetrafluoroethylene) membrane - A new emulsification device

Abstract: The potential of polytetrafluoroethylene (PTFE) membranes as water-in-oil (W/O) emulsification devices was investigated to obtain uniformly sized droplets and to convert them into microcapsules and polymer particles via subsequent treatments. Uniform W/O emulsion droplets have not been achieved using glass membranes unless the membrane was rendered hydrophobic by treatment with silanes. If a PTFE membrane is capable of providing uniform droplets for a W/O emulsion, a coordinated membrane emulsification system can be established since glass membranes have been so successful for O/W (oil-in-water) emulsification. In order to examine the feasibility of PTFE membrane emulsification, O/W and W/O emulsion characteristics prepared using PTFE membranes were compared with those prepared by the conventional SPG (Shirasu porous glass) membrane emulsification method. A 3 wt.% sodium chloride solution was dispersed in kerosene using a low HLB surfactant. Effects of the membrane pore size, permeation pressure, and the type of emulsifiers and concentration on the droplet size and on the size distribution (CV, coefficient of variation) were investigated. The CV of the droplets was fairly low, and the average droplet size was correlated with the critical permeation pressure of the dispersed phase, revealing that the PTFE membrane could be used as a one-pass membrane emulsification device. Low CV values were maintained with a Span 85 (HLB = 1.8) concentration, 0.2-5.0 wt.% and a range of HLB from 1.8-5.0. For a brief demonstration of practical applications, nylon-6,10 microcapsules prepared by interfacial polycondensation and poly(acrylamide) hydrogels from inverse suspension polymerization are illustrated.

Preparation of Uniform Hollow Polystyrene Particles with Large Voids by a Glass-Membrane Emulsification Technique and a Subsequent Suspension Polymerization

Abstract: Hollow polymer particles with large voids were prepared with styrene (St) as the main component and in the presence of a small amount of N,N-dimethylamino- ethyl methacrylate (DMAEMA) via a glass-membrane emul- sification technique and a subsequent suspension polymer- ization. A mixture of the monomer, hexadecane (HD), and N,N-azobis(2,4-dimethylvaleronitrile) as an initiator was used as a dispersed phase (oil phase). By the careful pushing of the dispersed phase through the pores of the glass mem- brane into the aqueous phase, an emulsion of fairly mono- disperse monomer droplets was formed. Then, the polymer- ization was performed by temperature being elevated to 70°C. The aqueous phase (continuous phase) contained poly(N-vinyl pyrrolidone) as a stabilizer, sodium lauryl sul- fate as a surfactant, Na2SO4 as an electrolyte, and sodium nitrite (NaNO2) as a water-soluble inhibitor. Results related to the effects of the HD content, DMAEMA, and the com- position of the comonomer, including the crosslinker and flexible segment, on the features of the hollow particles were investigated. When the content of DMAEMA was higher than 1.0 wt % based on the total monomer, small, secondary particles were generated in the aqueous phase, but the gen- eration was effectively prevented when DMAEMA was lim- ited to 0.5 wt %. Hollow particles, with an average diameter of around 7 m, were obtained with an St-DMAEMA sys- tem. The void size of the hollow particles was controlled by the HD content. When the HD content was lower (25 wt % based on the oil phase), unbroken hollow particles were easily obtained. However, they tended to break into halves after drying when the HD content was increased to 50 wt %. A soft segment, lauryl acrylate, and a crosslinker, ethylene glycol dimethacrylate, were added to overcome this prob- lem. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 244 -251, 2003

Preparation of uniform titanium dioxide (TiO2) polystyrene-based composite particles using the glass membrane emulsification process with a subsequent suspension polymerization.

Abstract: Uniform titanium dioxide (TiO2)-polystyrene-based composite particles were prepared using the glass membrane emulsification process followed by a subsequent suspension polymerization. The oil phase, consisting of anatase TiO2 ne powder, monomers, methyl laurate as the hydrophobic additive, Disperbyk-180 and the poly(styrene-co-2-ethyl hexylacrylate) were emulsified through the membrane pores into the aqueous phase containing stabilizers to form a (solid-in-oil)-in-water (S/O/W) emulsion of monomer droplets. The suspension polymerization was carried out at 343 K for 24 h under a nitrogen atmosphere. An SPG membrane with a pore size of 5.25 mum was employed and 20-25 mum TiO2-polystyrene based composite particles were obtained depending on the composition of polymerizing oil phase. The effects of the co-monomer, 2-ethylhexyl acrylate and the cross-linking agent, divinyl benzene on the dispersion stability of TiO2 in the oil phase, the surface feature of the particle and the encapsulation loading were investigated in this study. The membrane emulsification process was capable of preparing the composite particles with similar to5 wt% of TiO2 encapsulated, which accounts for with at least 85 wt% of TiO2 in the oil phase.

Synthesis and characterization of polyurethaneurea–vinyl polymer (PUU–VP) uniform hybrid microspheres by SPG emulsification technique and subsequent suspension polymerization

Abstract: Uniform polyurethaneurea-vinyl polymer (PUU-VP) hybrid microspheres of about 20 mum were prepared by a Shirasu porous glass (SPG) membrane emulsification technique and a subsequent radical suspension polymerization process, that is, a mixture of a 40 wt% urethane prepolymer (UP) solution of xylene and a vinyl monomer (VP) containing an initiator was permeated through the uniform pores of the SPG membrane into a continuous phase containing a stabilizer to form uniform droplets. Then, the droplets were allowed to stand for chain extension at room temperature with di- or triamines for 2 h in the absence or presence of ethyl acetate (EA), followed by suspension polymerization at 70degreesC for 24 h. The effect of the type and the amount of the monomer composition, crosslinker, and chain extender on the monomer conversion, particle morphology, and stability of the dispersion was investigated. A strong relationship was found between the compatibility of PUU with VP and the stability of the dispersion. The monomer conversion increased and the stability of the dispersion was improved as the amount of the crosslinker was increased. Solid and spherical PUU-VA hybrid particles with a smooth surface and a higher destructive strength were obtained when a trifunctional crosslinker and a chain extender were used together. (C) 2003 Wiley Periodicals, Inc.

Synthesis and characterization of crosslinked uniform polymeric microspheres containing a polyimide prepolymer by a new emulsification process

Abstract: A highly crosslinked poly(styrene-divinyl benzene-trimethyolpropane trimethacrylate) microsphere containing a polyimide prepolymer (PIP) was prepared by a new emulsification method, Shirasu porous glass (SPG) membrane emulsification, and a subsequent radical suspension polymerization. That is, a mixture of styrene, divinyl benzene, trimethyol propane trimethacrylate (TMPTMA), and PIP containing an initiator was permeated through the uniform pores of the SPG membrane into a continuous phase containing a stabilizer to form uniform droplets. Then, the suspension polymerization was carried out at 65 degreesC for 24 h. The trifunctional crosslinker TMPTMA was added to enhance the destructive strength of the microsphere, and PIP was incorporated into the microsphere to provide an adhesion force by a known self-curing reaction at 220 degreesC. The effects of the feed amounts of TMPTMA and PIP on the monomer conversion, size distribution, and destructive strength of the microsphere were investigated. The monomer conversion increased with an increase in TMPTMA. The particle size distribution became narrower as the TMPTMA feed amount increased, but it became broader with an increase in PIP. The destructive strength increased with increases in TMPTMA and PIP. After a heat treatment at 220 degreesC, the destructive strength decreased because of the heat degradation of the polymer. The addition of TMPTMA suppressed the heat degradation, and PIP could undergo self-curing at 220 'C. The obtained highly crosslinked uniform microsphere containing PIP has potential applications in liquid-crystal displays. (C) 2003 Wiley Periodicals, Inc.

The Preparation and Characterization of Monomethoxypoly(ethylene glycol)‐b‐poly‐DL‐lactide Microcapsules Containing Bovine Hemoglobin

Abstract: Methoxypoly(ethylene glycol)-b-poly-DL-lactide (PELA) microcapsules containing bovine hemoglobin (bHb) were prepared by a W/O/W double emulsion-solvent diffusion process. bHb solution was used as the internal aqueous phase, PELA/organic solvent as the oil phase, and polyvinyl alcohol (PVA) solution as the external aqueous phase. This W/O/W double emulsion was added into a large volume of water (solidification solution) to allow organic solvent to diffuse into water. The optimum preparative condition for PELA microcapsules loaded with bovine hemoglobin was investigated. It was found that homogenization rate, type of organic solvent, and volume of the solidification solution influenced the activity of bovine hemoglobin encapsulated. When the homogenization rate was lower than 9000 rpm and ethyl acetate was used as the organic solvent, there was no significant influence on the activity of hemoglobin. High homogenization rate as 12 000 rpm decreased the P-50 and Hill coefficient. Increasing the volume of solidification solution had an effect of improving the activity of microencapsulated hemoglobin. The composition of the PELA had the most important influence on the success of encapsulation. Microcapsules fabricated by PELA with MPEG2k block (molecular weight of MPEG block: 2000) achieved a high entrapment efficiency of 90%, better than PLA homopolymer and PELA with MPEG5k blocks. Hemoglobin microcapsules with native loading oxygen activity (P-50 = 26.0 mmHg, Hill coefficient = 2.4), mean size of about 10 pm, and high entrapment efficiency (ca. 93%) were obtained at the optimum condition.

Preparation of hemispherical poly(4-vinylpyridine-co-butyl acrylate)/poly(styrene-co-butyl acrylate) composite microspheres by seeded preswelling emulsion polymerization

Abstract: Seeded preswelling emulsion polymerization was carried out by using monodispersed poly(4-vinylpyridine-co-butyl acrylate) [P(4VP-BA)] particles as the seed, and styrene and butyl acrylate as the second-stage monomers under different polymerization conditions, to obtain hemispherical polystyrene (PST)-rich-P4VP-rich microspheres. Prior to polymerization, toluene was added into the preswelling system together with the second-stage monomers. It was found that, with the increase of the amount of toluene, the particle morphology showed a tendency toward desirable hemispherical structure, and the colloidal stability of composite latex was improved. When the weight ratio of toluene/seed latex was increased up to 7.5/40 (g/g), the stable hemispherical latex could be obtained. However, when toluene was not added, the coagulum formed on the wall of the reactor during polymerization, and the composite particles with multiple surface domains (such as sandwich-like, popcorn-like) were formed. In addition, the final morphology of composite particles was influenced by the polarity of the seed crosslinker and the hydrophilicity of the second-stage initiator, which could affect the mobility of poly(styrene-co-butyl acrylate) [P(ST-BA)] chains. The morphology development during the polymerization was investigated in detail, and a schematic model was derived to depict the formation mechanism of hemispherical P(4VPBA)/P(ST-BA) composite microspheres. The results revealed that the mobility of the P(ST-BA) chains influenced the diffusion of the P(ST-BA) domains on the surface of the P(4VP-BA) matrix. When the mobility of the P(ST-BA) chains allowed small-size P(ST-BA) domains to coalesce into one larger domain, complete phase-separated morphology (hemisphere) could be achieved. (C) 2003 Wiley Periodicals, Inc.

Performance of titanium dioxide microcapsules as a photo-oxidation catalyst for decolourization of methylene blue.

Abstract: The performance of anatase type titanium dioxide (TiO2) encapsulated with styrenic copolymers via the solvent evaporation or suspension polymerization process was investigated as a photocatalyst for the decolourization of methylene blue (MB, 3,4-bis(dimethylamino)-phenothiazine-5-thionium chloride) in the aqueous phase. The TiO2 microcapsules, loaded with 4-8 wt% TiO2, were dispersed in a B aqueous solution containing an adequate amount of hydroperoxide as an oxygen source, and the mixture was exposed to 365 nm UV light. The decolourization of MB proceeded according to the first order of the MB concentration. The apparent rate constant, defined based on the unit weight of loaded TiO2, depended on the initial concentration of MB. The capsule walls, composed of cross-linked and/or uncross-linked poly(styrene-co-2-ethylhexyl acrylate), favoured the adsorption of MB on the capsule surface and promoted the decomposition. The observations of the cross-section of microcapsules by transmission electron microscopy (TEM) showed similar morphology of microcapsules regardless of preparation method; a thin layer of hydrophilic TiO2 particles being localized near the particle surface. The sustainability of the microcapsules was also proved by doing experiments successively for 7 days. Smaller size microcapsules were favoured for the decomposition of MB, and the rate constant increased with the surface area of microcapsules in unit volume of the reaction mixture. Although large size microcapsules with uncross-linked polymer wall had a disadvantage of breaking under high shear agitation, those prepared with cross-linked polymer wall by suspension polymerization still remained effective after the sustainability test for 7 days.

Synthesis of functional graft copolymers by solution copolymerization and their evaluation as dispersants in nonaqueous phase dispersion polymerization

Abstract: The poly(HEMA-co-MMA-g-PMMA) graft copolymer was prepared with a poly(methyl methacrylate) (PMMA) macromonomer, 2-hydroxyethyl methacrylate (HEMA), and methyl methacrylate (MMA), and its application as a dispersant for the nonaqueous phase dispersion polymerization of polystyrene (PST) was investigated. Monodisperse PST particles were obtained with two-dimensionally tailored graft copol- ymers, with the number of grafted chains controlled and the polar component (HEMA) in the backbone chains balanced. As for the reactor, a stirred vessel with moderate agitation yielded uniform polymer particles, whereas sealed glass ampules with an overturning motion yielded broader size distributions. Increasing the polarity of the solvent in the continuous phase yielded smaller polymer particles with a gradual deterioration of monodispersity. Uniform polymer particles with a coefficient of varia- tion of less than 6% were obtained up to 30 wt % solid contents. © 2003 Wiley Periodicals,