Dispersion non-aqueous

Buining P A, Veidhuizen Y S J, Pathmamanoharan C and Lekkerkerker FI N W 1992 Preparation of a non-aqueous dispersion of sterioally stabilized boehmite rods Coiioid. Surf. 64 47-55... 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

Electrostatics in Non-Aqueous Media. A popular misconception in studies of non-aqueous dispersions concerns electrostatic effects. Because these are more difficult to measure than in aqueous media, there has been a general tendency to ignore them completely. However, the few investigators who have measured zeta-potentials or electrodeposition with these systems have become convinced of their importance. With the advent of modern commercial instrumentation for zeta-potentials in non-aqueous media it is to hoped that these effects will be measured rather than ignored. 

The question whether the intramolecularly crosslinked microparticles of non-aqueous polymer dispersions are really microgels is also justified, considering non-aqueous dispersions prepared from acrylic copolymers and melamine/formaldehyde crosslinker with particle sizes of about 300 nm. [45, 343]. In any case, these crosslinked polymeric microparticles are useful constituents of high-solids coatings, imparting a yield stress to those solutions which probably involves attractive forces between the microparticles. 

Biodegradable polyester-based nanoparticles have also been studied, especially in the biomedical domain. Like microelectronics, biomedical research follows the rule smaller is better . A typical example of nanoparticles based on the aliphatic polyester engineering by living ROP is provided by the poly(CL-h-GA) copolymers which form stable colloidal dispersions in organic solvents such as toluene and THF without the need of any additional surfactant [27]. The poly(CL-h-GA) particles form a new class of stable non-aqueous dispersions in... 

B. Vincent The Stability of Non-Aqueous Dispersions of Weakly Interacting Particles. Colloids Surfaces 24, 269 (1987). 

Preparation of non-aqueous dispersions of colloidal silver by phase transfer has been described [51] and advantage has been taken to form monodisperse, 7.0-nm-diameter silver particles by simultaneously reducing Ag+ and partially oxidizing Agn particles (radiolytic push-pull reduction method) [52]. The surface chemistry of nanosized silver particles has continued to receive attention [53, 54],... 

In addition to the influence on the dimer morphology, the presence of water molecules strikingly affect apparent photoreaction rate and temperature dependence of the rate (12). Since the topochemical reaction deteriorates pronouncedly at reaction temperatures close to the melting point of the starting crystal, maximal reaction rate is necessarily observed at a specific temperature for individual crystals, for example, at ca. 20"C for a -form crystal of cinnamic acid (mp 132°C) (13). In an aqueous dispersant the apparent maximal rate of photodimerization of 1 is observed about 15°C while the temperature for maximal rate in a non-aqueous dispersant is about 35 °C. The... 

Equally effective for both aqueous and non-aqueous dispersions. 

Polystyrene latices can be prepared by the dispersion polymerisation of styrene in alcohol/water mixtures containing polyelectrolytes. The experimental data obtained lend support to the hypothesis that the polymerisation mechanism which operates is analogous to that which occurs in the non aqueous dispersion polymerisation of methyl methacrylate in solutions of degraded... 

Polyacrylic acid stabilised latices have been prepared by aqueous dispersion polymerisation. The method used is analogous to the non-aqueous dispersion (NAD) polymerisation methods originally used to prepare polymethyl methacrylate particles in aliphatic hydrocarbons (1. In effect the components of a NAD polymerisation have been replaced as follows aliphatic hydrocarbon by aqueous alcohol, and degraded rubber, the stabiliser, by polyacrylic acid (PAA). The effect of various parameters on the particle size and surface charge density of the latices is described together with details of their colloidal stability in the presence of added electrolyte. 

The reaction engineering aspects of these polymerizations are similar. Excellent heat transfer makes them suitable for vinyl addition polymerizations. Free radical catalysis is mostly used, but cationic catalysis is used for non-aqueous dispersion polymerization (e.g., of isobutene). High conversions are generally possible, and the resulting polymer, either as a latex or as beads, is directly suitable for some applications (e.g., paints, gel-permeation chromatography beads, expanded polystyrene). Most of these polymerizations are run in the batch mode, but continuous emulsion polymerization is common. 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

Examples of dispersion polymerizations using macromonomers are summarized in Table 4. Non-aqueous dispersion (NAD) polymerization of polar... 

These processes must be monitored to confirm that the second polymer does not emerge as a separate particle type. Re-nucleation, producing a crop of new particles, may be detected by progressive determination of particle size and comparing actual with the theoretical size calculated on the basis of constant particle number. This is easier to do in those processes where the monomer for the second polymer is added slowly at a steady and known rate and samples can be taken at regular time intervals for particle-size determination by electron microscopy, photon correlation spectroscopy or by disc centrifuge photo-sediometry. For particles prepared by non-aqueous dispersion... 

The materials we studied are non-aqueous dispersions of polymer particles. Colloidal stability of these particles in hydrocarbon solvents is conferred by a surface covering of a highly swollen polymer (the stabilizer) on a second polymer, insoluble in the medium (the core polymer), which comprises 90 % of the material (11). These particles are prepared by dispersion polymerization polymerization of a monomer soluble in the medium to yield an insoluble polymer, carried out in the presence of a soluble polymer which becomes the stabilizer. In the examples discussed here, the core polymer is formed by free radical polymerization. Hydrogen abstraction from the soluble polymer present in the reaction medium... 

Finally, we stress that the free volume approach is only applicable to nonpolar systems. Aqueous dispersions fall outside its scope. This is vividly illustrated by the data of Evans et al. (1975), who determined experimentally that d(UCFT)/d7 = — 1 x 10 KPa for latex particles sterically stabilized by poly(oxyethylene) in aqueous 0-43 molal magnesium sulphate solutions. Both the sign and magnitude of this quantity is different from that predicted by free volume theory for the UCFT of non aqueous dispersions. Paradoxically, it falls in line with the predictions, both in sign and magnitude, published by Croucher and Hair (1979) for the pressure dependence of the LCFT of poly(a-methylstyrene) in -butyl chloride. This may be merely coincidental, but the very small pressure dependence exhibited by the UCFT of aqueous sterically stabilized dispersions emphasizes the major differences between the origins of flocculation at the UCI T for aqueous and nonaqueous dispersions. The small pressure dependence observed for aqueous systems is scarcely surprising since the UCFT of an aqueous dispersion occurs far from the critical point of water whereas that for nonaqueous dispersions is quite close to the critical point of the dispersion medium. 

Properties of Nonaqueous Dispersions. The stabilizing solvated sheath of non-aqueous dispersions is of limited dimensions. It therefore only has a significant effect on the effective volume fraction, and hence the rheology of the dispersions, when the dispersed particle size is very small and/or the volume fraction of the dispersion is very high. Dispersions of high molecular mass polymers have a much lower viscosity than their solutions at the same temperature. 

This phenomenon can be observed with many systems, for example, with aqueous electrostatically stabilized particles [126], with aqueous latexes containing particles with grafted polyoxyethylene chains [127] and with non-aqueous dispersions of coated silica particles [128] and sterically stabilized poly(methyl methacrylate) particles [129],... 

Water-in-oil dispersions are far less stable than their aqueous counterparts. This is a consequence of the low dielectric constant of organic compoimds which renders electrostatic stabilization ineffective [7]. Steric repulsive forces are therefore required to counterbalance the van der Waals attractive forces which are significant i kT) in non-aqueous dispersions [8]. This can be achieved by using block copolymers as stabilizers to prevent flocculation, or nonionic emulsifier blends which maximize the interfacial entropy of mixing by forming a condensed surfactant layer [9] (see Chapter 3). 

The emulsion polymerization technique is a heterophase polymerization technique in which three phases can be distinguished the water phase, the latex particle phase and the monomer droplet phase (the latter is usually present during part of the polymerization reaction). The product of an emulsion polymerization is a latex a submicrometer dispersion of polymer particles in water. Non-aqueous dispersions of latex particles also exist. 

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