Particle submicron phases

It would often be desirable to create submicron particles containing two or more polymers. These could be in the form of a blend or in the form of a graft copolymer. In a simple blend, the two polymers may or may not be compatible. If they are compatible, the particle will be homogenous. If the polymers are not compatible, then microphase separation is likely. However, if the phase separation occurs in submicron particles, the phase domains will be small, and decent dispersion of the two polymers will occur. Homogenous, grafted, or phase-separated morphologies might conceivably be of practical value. 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

The uniform polymeric microspheres in submicron-or micron-size range can also be prepared as seed particles by the soapless emulsion or dispersion polymerization of a hydrophobic monomer like styrene. The uniform seed particles are swollen with the organic phase including functional comonomer, monomer, and oil-soluble initiator at a low temperature in an aqueous... 

It should be noted that concentration of phospholipids and AmB in methanol is limited by the solubility of these two substances in the organic phase. A high final concentration of methanol in the mixture with water is also necessary to form submicronic particles. Thus, the suspensions had to be concentrated by rotary evaporation before testing in biological systems. 

As a simple and practical alternative to coated microspheres, submicron particles of pendant amine-functionalized PPE 39 could be fabricated by phase... 

Methods for analysis of the particle size distribution in the aerosol cloud include techniques such as time of flight measurement (TOE), inertial impaction and laser diffraction. Dynamic light scattering (photon correlation spectroscopy) is confined to particles (in suspension) in the submicron range. In addition to the size distribution, the particle velocity distribution can be measured with the Phase Doppler technique. 

This is consistent with the result that Horowitz et al. have found on submicron particles obtained by a hyponitrite method they are too small to give any diamagnetic behavior. However, the heat capacity measurement shows a break at 90 K that corresponds to about 90% of superconducting phase in the powder (16). 

In a similar vein, the time scales to achieve equilibrium for inorganics have been examined by Meng and Seinfeld (1996), who show that small (submicron) particles can come to equilibrium with the gas phase in less than a few hours typically but that larger particles may not. The major factors determining the time needed to reach equilibrium are the aerosol size distribution,... 

Gas-phase reactions can also be used to produce products of low volatility that condense to give an aerosol. The reaction of gaseous NH3 with HC1 to form particles of solid ammonium chloride and the reaction of gaseous S03 with water vapor to form H2S04 are typical examples. Such methods tend to give submicron particles. 

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

The characterization of petroleum cracking catalysts, with which a third of the world s crude oil is processed, presents a formidable analytical challenge. The catalyst particles are in the form of microspheres of 60-70 micron average diameter which are themselves composites of up to five different micron and submicron sized phases. In refinery operation the catalysts are poisoned by trace concentrations of nickel, vanadium and other contaminant metals. Due to the replacement of a small portion of equilibrium catalyst each day (generally around 1% of the total reactor inventory) the catalyst particles in a reactor exist as a mixture of differing particle ages, poisoning levels and activities. 

Nd2Ni04+5 powder was prepared by nitrate-citrate route as described by Courty, et a1. (1973). Stoichiometric amounts of neodymium and nickel oxides were dissolved in diluted nitric acid. After addition of a large excess of citric acid, the solution was dehydrated and heated until self-combustion of the precipitate to obtain submicronic precursor particles (Boehm, 2005). The final annealing was performed at 1000°C for 12 hours to obtain a single crystalline phase. The particles were then ball milled to obtain an average grain size (d0 5) of about 0.8 pm. A terpineol-based slurry was prepared from this powder and this was deposited on the electrolyte by screen printing and then sintered at 1100°C for three hours in air (Lalanne, 2008). 

For colloidal systems, the homogeneous dispersed state becomes unstable when the attractive minimum in the interparticle potential exceeds a few kT. For — 5 < miJkT < — 2, dispersions of submicron particles separate on a reasonable time scale, i.e., a few days, into two coexisting equilibrium phases. With stronger attractions, equilibration becomes quite slow, requiring up to six months or a year. The nature and compositions of the phases depend on the range and magnitude of the attractive potential. 

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