Layer thickness, polymeric stabilization

IV. These and similar reactions produce polymer structures (not brushes), which are, in fact, bonded silicone oils known as polysiloxanes. Any cross- linked polymer layer thickness can be chosen. Multifarious methods of synthesis are known. Very advantageous is the shielding of the silica backbone by the polymeric layer, leading to a good pH stability. 

Figure 14 shows the variation of the steady shear relative viscosity at the high shear limit with the effective volume fraction as defined by equation 46 for poly(methyl methacrylate) (PMMA) suspensions of different sizes in decalin sterically stabilized by means of grafted poly (12-hydroxy stearic acid) chains with a degree of polymerization of 5. The stabilizing polymer layer thickness is 9 1 nm, in particular, A = 9 nm... 

Although the actual diameter of a polymeric particle can be measured by microscopic or other methods, the effective diameter for hydrodynamic puiposes, and hence the effective volume fraction, may be considerably larger. Surface layers can significantly increase the effective volume of latex particles. Such layers may be due to adsrxbed surfactants, adsrabed or reacted polymeric stabilizers such as poly(vinyl alcohol), hydroxyethyl cellulose or poly(ethylene oxide), and surface charges on the polymer particle. The smaller the particle size, the greater will be the contribution a surface layer (rf given thickness to the effective volume of flie particle. 

In nature as well as in technology, polymeric emulsifiers and stabiUzers play a major role in the preparation and stabiUzation of emulsions. Natural materials such as proteins, starches, gums, cellulosics, and their modifications, as well as synthetic materials such as polyvinyl alcohol, polyacryhc add, and polyvinylpyrrolidone, have several characteristics that make them extremely useful in emulsion technology. By the proper choice of chemical composition, such materials can be made to adsorb strongly at the interface between the continuous and dispersed phases. By their presence, they can reduce interfacial tension and/or form a barrier (electrostatic and/or steric) between drops. In addition, their solvation properties serve to increase the effective adsorbed layer thickness, increase interfacial viscosity, and introduce other factors that tend to favor the stabilization of the system. 

The efficiency of membrane separation increases with the permeability and the selectivity. Thin membranes are economic, since according to Equation (2.1) the gas flow is inverse proportional to the layer thickness. However thin polymeric films, which have favorable permeability and selectivity, are too weak to withstand the high pressure difference between permeate and retentate side. The economic breakthrough set in with the production of ultrathin compound polymeric membranes. These are designed as hollow fibres with a thick porous back-up layer for mechanical stability and a thin dense non porous membrane layer for gas separation. The porous layer only has a slight influence on gas separation. These hollow fibres are combined in a bundle, which is arranged in a cylindrical container [2.13]. Several of these bundles, also called modules, can be added to... 

The inherently high colloid stability of nanoemulsions when using polymeric surfactants is due to their steric stabilization. The mechanism of steric stabilization was discussed above. As shown in Fig. 1.3 (a), the energy-distance curve shows a shallow attractive minimum at separation distance comparable to twice the adsorbed layer thickness 28. This minimum decreases in magnitude as the ratio between adsorbed layer thickness to droplet size increases. With nanoemulsions the ratio of adsorbed layer thickness to droplet radius (8/R) is relatively large (0.1 0.2) when compared with macroemulsions. This is schematically illustrated in Fig. 1.28 which shows the reduction in with increasing 8/R. 

Other steric repulsions can arise and stabilize the film at higher thickness in the case where the interfaces are covered by thick polymeric layers (either flattened on the surface or in a brush-like structure). 

The interaction parameter should be smaller than 1/2 to provide a repulsion. This requires the medium to be a good solvent for the free dangling polymer. Polymeric repulsion occurs only when polymeric stabilizer layers overlap. The thickness of these layers is often of the order of 10 nm. In contrast, electrostatic double layers can be much thicker if the ion concentration of the medium is low (eq. 10.4.4). Also, the polymer repulsion potential is quite steep. As a result, the total potential for polymerically stabilized systems (Figure 10.4.3) shows no deep primary minimum. A shallow minimum, similar to the secondary minimum for electrostatically stabilized systems, is possible. 

Film stability is a primary concern for applications. LB films of photopoly-merizable polymeric amphiphiles can be made to crosslink under UV radiation to greatly enhance their thermal stability while retaining the ordered layered structure [178]. Low-molecular-weight perfluoropolyethers are important industrial lubricants for computer disk heads. These small polymers attached to a polar head form continuous films of uniform thickness on LB deposi-... 

Paine et al. [99] tried different stabilizers [i.e., hydroxy propylcellulose, poly(N-vinylpyrollidone), and poly(acrylic acid)] in the dispersion polymerization of styrene initiated with AIBN in the ethanol medium. The direct observation of the stained thin sections of the particles by transmission electron microscopy showed the existence of stabilizer layer in 10-20 nm thickness on the surface of the polystyrene particles. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results supported the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents. 

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