C. H. Chew

Processing of hydroxyapatite via microemulsion and emulsion routes

Hydroxyapatite powders have been prepared by reacting CaCl2 and (NH4)2HPO4 in bicontinuous microemulsion, inverse microemulsion and emulsion, which have the same components as cyclohexane, non-ionic surfactant (NP5 + NP9) and aqueous solution. The characteristics of the resulting hydroxyapatite powders, such as the particle size, particle size distribution, chemical homogeneity and the degree of particle agglomeration, are strongly affected by the structure of the reaction medium. Both bicontinuous and inverse microemulsions led to the formation of much finer hydroxyapatite powders than that prepared from the emulsion composition. The two fine hydroxyapatite powders are sintered to a relative density of >95% theoretical density at 1000 degrees C, compared with a relative density of <73% theoretical density for the emulsion-derived one. The two microemulsion-derived hydroxyapatites also exhibit a higher sintered density and are more refined in grain size than that of the emulsion-derived one when sintered at 1200 degrees C for 2h.

Photoluminescence and structural characteristics of CdS nanoclusters synthesized by hydrothermal microemulsion

Spherical and uniform CdS nanoclusters were synthesized by hydrothermal microemulsion. The reaction of Cd2+ ions with S2− ions generated from the decomposition of thioacetamide proceeded in water microdroplets. The mean diameter of the CdS nanoclusters can be varied from 20 to 80 nm by increasing the reaction temperature from 30 to 120 °C. XRD results indicate that the resulting CdS nanoclusters have a reduced and distorted hexagonal lattice compared to bulk materials. Two intense luminescence bands, i.e., green and red, were observed to coexist in the CdS nanoclusters. Their peak positions and relative intensities were found to be sensitive to the size and structure of nanoclusters. These emissions are attributed to surface defects (green emission) and the Cd–Cl composite vacancies (red emission).

THE EFFECT OF ALKANES ON THE FORMATION OF ULTRAFINE SILVER BROMIDE PARTICLES IN IONIC W/O MICROEMULSIONS

ABSTRACT Ultrafine AgBr particles formed in w/o microemulsions with different chain length of alkanes were studied by UV-VIS spectrophotometry, stopped-flow spectrophotometry and transmission electron microscopy. The ultrafine particles of AgBr (5-7 nm in diameter) were obtained by mixing two separately prepared microemulsion systems both containing AOT, n-hexane and an aqueous solution of either AgNO3 or NaBr.The size distribution of the nearly spherical AgBr particles broadened from 5 to about 10 nm as chain length of alkane was increased from n-hexane to n-octane. When n-dodecane or a higher alkane was used, the microemulsion system became increasingly unstable causing precipitation of AgBr.

A novel method for the synthesis of perovskite-type mixed metal oxides by the inverse microemulsion technique

Nanoparticles of lanthanum-nickel, lanthanum-copper and barium-lead oxalates with the metal molar ratios of 1∶1, 2∶1 and 1∶1, respectively, have been successfully synthesized in inverse microemulsions. These metal oxalate particles of about 20 nm diameter were readily calcined into single-phase perovskite-type LaNiO3, La2CuO4 and BaPbO3. The calcination temperatures for these metal oxalates were generally 100–250°C lower than those for the metal oxalates prepared by the conventional aqueous solution precipitation method. The substantial reduction in the calcination temperatures is attributed to the formation of uniform, near-spherical nanoparticles of the metal oxalate precursors obtained by the unique inverse microemulsion technique.

Polymerization of methyl methacrylate in ternary oil-in-water microemulsions

Abstract Polymerization of methyl methacrylate has been studied in ternary o/w microemulsions that were stabilized by stearyltrimethylammonium chloride. The latices produced still remain transparent and stable about one year after preparation. It is calculated that each small latex particle of hydrodynamic radius (Rh) ranging from 20 to 24 nm contained two or three polymer chains of high molecular weight of 5 to 8 million. The rate of polymerization shows only two intervals for most of the systems. This corresponds to the rates of nucleation and growth of particles. But a constant rate of polymerization is also noted for the first time for microemulsions containing a lower concentration of initiator and/or at a lower temperature (T

Synthesis and Polymerization of Surface-Active Sodium Acrylamidoundecanoate

Abstract The polymerizable surfactant sodium 11-acrylamidoundecanoate (Na 11-AAU) was synthesized from acryloyl chloride and 11-aminounde-canoic acid. It had a low critical micelle concentration (CMC) of 4.3 × 10−4 mol/L. Polymerization of Na 11-AAU initiated by K2S2O8 was very fast in aqueous solution, with an activation energy of only 63.2 kJ/mol. The polymerization followed first-order kinetics with respect to Na 11-AAU and one-half order with respect to K2S2O8. The MW of poly(Na 11-AAU) was very high (1–2 million) but the MWD was rather narrow ( M w / M n = 1.45). Polymerization of Na 11-AAU in the micellar state may be responsible for the phenomena observed.

Dispersion polymerization of styrene in aqueous ethanol media using poly(ethylene oxide) macromonomer as a polymerizable stabilizer

Abstract The kinetics of dispersion polymerization of styrene with a small amount of ω-methoxy poly(ethylene oxide)40 undecyl-α-methacrylate macromonomer (PEO-R-MA-40) in the ethanol-water media was studied. A maximum and constant rate of polymerization for this system appeared at the 18%—55% conversion of styrene to polystyrene. However, only 50% of the amphiphilic PEO macromonomer was found to be grafted in the copolymer (stabilizer) after 10 h of polymerization at 65°C. But as high as 98.5% conversion of styrene to polymer was attained. The initial rate of polymerization follows the scaling relationship, (Rp)i ∞ [PEO-R-MA-40]00 [styrene]00.92 [AIBN]00.90. It increased with increasing temperature but decreased with increasing water content because of the solvency effect in the reaction medium. The molecular weight Mw of polymer increased with increasing conversion of styrene up to about 55%. It then decreased slightly towards higher conversions, leading to a broader molecular weight distribution. The activation energies for the dispersion polymerization in the initial stage (

Preparation of conducting polyaniline-coated barium sulfate nanoparticles in inverse microemulsions

Abstract Conducting polyaniline-coated BaSO4 composites have been synthesized in inverse microemulsions. The size of the composite particles ranged from about 10 to 20 nm in diameter. The formation and characterization of polyaniline coated on the surface of BaSO4 particles are discussed. The electrical conductivities (0.017–5 S cm−1) of the compacted powders of the doped polyaniline/BaSO4 composites increased almost linearly with the polyaniline content (5–22 wt.%) in the composites.

Morphology of microporous polymeric materials by polymerization of methyl methacrylate and 2-hydroxyethyl methacrylate in microemulsions

Abstract A microemulsion system consisting of methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA) and a crosslinking agent, ethylene glycol dimethacrylate, in water using sodium dodecyl sulfate as a surface-active agent was investigated. A study on the phase behaviour of the system was conducted to determine the type of microstructures that may be present. Polymerization was carried out at room temperature using dibenzyl ketone as a photoinitiator. The morphology (microstructure) of the resulting polymeric materials was found to depend strongly on the composition (water content, surfactant content, and MMA to HEMA ratio) of the precursor microemulsions.

Femtosecond Z-scan investigation of nonlinear refraction in surface modified PbS nanoparticles

Abstract The third-order optical nonlinearities of surface modified PbS nanoparticles have been measured using the Z-scan technique with femtosecond laser pulses at 780 nm wavelength. The samples studied are the nanoparticles in microemulsion with the PbS concentration ranging from 0.3×10 −3 to 1.9×10 −3 M (mol/l). A Z-scan analysis method based on the Huygens–Fresnel (H–F) propagation integral is employed to extract the nonlinear refractive index from the experimental Z-scan data with a large nonlinear phase shift. The nonlinear refractive index of the PbS nanoparticles in microemulsion is found to increase linearly with the PbS concentration. The highest concentration microemulsion gives a nonlinear refractive index of −4.7×10 −12 cm2/W, which is approximately 3 orders of magnitude higher than those of commercially available bulk semiconductors, such as ZnS and CdS. In addition, nonlinear absorption in these samples remained unmeasurable up to 0.9 GW/cm2. The mechanisms responsible for the observed large refractive nonlinearity are discussed.