Latex dispersion steric stabilization

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. 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

From the family of AG (P, T) curves the projection on the (P, T) plane of the critical lines corresponding to the UCFT for these latexes can be calculated and this is shown plotted in Figure 4. It can be seen that the UCFT curve is linear over the pressure range studied. The slope of the theoretical projection is 0.38 which is smaller than the experimental data line. Agreement between theory and experiment could be improved by relaxing the condition that v = it = 0 in Equation 6 and/or by allowing x to be an adjustable parameter. However, since the main features of the experimental data can be qualitatively predicted by theory, this option is not pursued here. It is apparent from the data presented that the free volume dissimilarity between the steric stabilizer and the dispersion medium plays an important role in the colloidal stabilization of sterically stabilized nonaqueous dispersions. 

The nucleation mechanism of dispersion polymerization of low molecular weight monomers in the presence of classical stabilizers was investigated in detail by several groups [2,6,7]. It was, for example, reported that the particle size increased with increasing amount of water in the continuous phase (water/eth-anol), the final latex radius in their dispersion system being inversely proportional to the solubility parameter of the medium [8]. In contrast, Paine et al.[7] reported that the final particle diameter showed a maximum when Hansen polarity and the hydrogen-bonding term in the solubility parameter were close to those of steric stabilizer. 

Dispersion polymerization is defined as a type of precipitation polymerization by which polymeric microspheres are formed in the presence of a suitable steric stabilizer from an initially homogeneous reaction mixture. Under favorable circumstances, this polymerization can yield, in a batch process, monodisperse, or nearly monodisperse, latex particles with a relatively large diameter (up to 15 pm) [103]. The solvent selected as the reaction medium is a good solvent for both the monomer and the steric stabilizer, but a non-solvent for the polymer being formed and therefore a selective solvent for the graft copolymer. This restriction on the choice of solvent means that these reactions can be carried out... 

A variety of organic colloids including emulsions and polymer latexes have been dispersed in carbon dioxide in the presence of surfactants (3,13). In most cases, owing to the lower interfacial tension of the former as explained shortly it is easier to form organic-in-C02 emulsions than water-in-C02, emulsions. Sterically stabilized colloids are stable above the critical flocculation density (CFD) and precipitate below this density. In some cases the CFD occurs at the upper critical solution density of the steric stabilizer, that is, the density at which the stabilizer phase separates from CO2, as has been shown by theory (14,15) and experiment (16). So-called ambidextrous surfactants have been designed to allow polymer latexes produced in CO2 to be transferred to an aqueous solution to form a dispersion (17,18). 

Note that in calculating A for sterically stabilized systems, allowance must be made not only for the intervening dispersion medium between the particles but also for the presence of any adsorbed layers (Vincent, 1973b). A is typically of order kT for latex particles, although it can be substantially larger for dispersions of metals or for inorganic sols (Gregory, 1970 Visser, 1972). 

If a polystyrene latex that is stabilized solely by an electrostatic mechanism is coagulated by the addition of electrolyte, that coagulation is usually irreversible to subsequent dilution. In contrast, sterically stabilized dispersions can usually be flocculated by the addition to the dispersion medium of a nonsolvent for the stabilizing moieties mere dilution of the concentration of the nonsolvent to a suitably low value is often sufficient to induce the particles to redisperse spontaneously. 

Figure 5.2 presents a similar plot for a poly(methyl methacrylate) latex sterically stabilized in n-heptane by poly(12-hydroxystearic acid). In this instance, however, the reduction in the solvency of the dispersion medium for the stabilizing moieties was achieved by adding a miscible nonsolvent (specifically ethanol) to the dispersion medium (Napper, 1968a). Flocculation was again accompanied by an abrupt increase in turbidity when a certain volume fraction of ethanol was added to the ra-heptane. In this instance, it was possible to observe the slow flocculation of the latex particles (i.e. flocculation apparently in the presence of a small repulsive potential energy barrier at a rate slower than that predicted by Smoluchowski, 1917). It is, however, usually diflicult to detect such slow flocculation because of the sharpness of the transition from stability to flocculation for stericaUy stabilized dispersions. 

Comparison of theory with experiment. It will be shown in Section 13.3.2.1 that the flat plate potentials can be used to calculate the osmotic disjoining pressures in concentrated monodisperse sterically stabilized dispersions. Evans and Napper (1977) have compared the theoretical predictions using the above equations with those measured by Homola and Robertson (1976) for polystyrene latex particles stabilized by poly(oxyethylene) of molecular weight ca 2 000 in aqueous dispersion media. The elastic repulsion in the interpenetrational-plus-compressional domain was estimated from the following expression for the constant segment density model... 

Experimental evidence for elastic steric stabilization There is a paucity of experimental studies of elastic steric stabilization. Smitham and Napper (1976a,b) have shown that it is possible to prepare polystyrene latex particles stabilized by poIy(oxyethylene) and dispersed in molten poly(oxyethylene). These experiments suggested that the maximum particle size that could be elastically stabilized was dependent upon the molecular weight of the stabilizing moieties, as would be expected intuitively. Everett and Stageman (1978a) have also reported the elastic stabilization of poly(methyl methacrylate) particles stabilized by poly(dimethylsiloxane) in liquid poly(dimethylsiloxane). 

This diagram is able to explain some puzzling observations disclosed by Cowell and Vincent (1982). They report clear differences between the stability behaviour of latex dispersions depending upon whether the particles are naked or pre-coated by terminally anchored chains. The addition of free polymer to the sterically stabilized particles resulted in the transition sequence stability- instability stability. This is precisely what would be predicted from Fig. 17.20 if bridging flocculation is absent (as it must be if the free polymer and stabilizing moieties are identical in chemical composition). 

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],... 

Another (better) option is to polymerize the OM directly on a latex (in fact, S. Jasne from Polaroid had proposed this route in the mid 1980s [55], finally without practical success, and it was again recently proposed by the Intch Company DSM [56]), or in a sterically stabilized colloidal form [57]. Both concepts are based on the idea that an OM polymer blend should be a dispersed system, but that unquestionable idea could get around the very complicate dispersion task by starting with colloidal particles. This is not basically wrong however, it was not taken into account that... 

The van der Waals attraction between two atoms is very short range, acting over a few tenths of a nanometer. For latex particles, however, each atom or molecule of one particle attracts every atom or molecule in the other particle. The net attraction is long range and of considerable strength. At practical particle concentrations, dispersions would be stable for only a few seconds if they were not stabilized. The attractive force between latex particles can be overcome by electrostatic or steric stabilization. 

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