Nonaqueous dispersions sterically stabilized

Chemical Grafting. Polymer chains which are soluble in the suspending Hquid may be grafted to the particle surface to provide steric stabilization. The most common technique is the reaction of an organic silyl chloride or an organic titanate with surface hydroxyl groups in a nonaqueous solvent. For typical interparticle potentials and a particle diameter of 10 p.m, steric stabilization can be provided by a soluble polymer layer having a thickness of - 10 nm. This can be provided by a polymer tail with a molar mass of 10 kg/mol (25) (see Dispersants). 

Stability of Sterically Stabilized Nonaqueous Dispersions at Elevated Temperatures and Pressures... 

Besides temperature and addition of non-solvent, pressure can also be expected to affect the solvency of the dispersion medium for the solvated steric stabilizer. A previous analysis (3) of the effect of an applied pressure indicated that the UCFT should increase as the applied pressure increases, while the LCFT should be relatively insensitive to applied pressure. The purpose of this communication is to examine the UCFT of a nonaqueous dispersion as a function of applied pressure. For dispersions of polymer particles stabilized by polyisobutylene (PIB) and dispersed in 2-methylbutane, it was observed that the UCFT moves to higher temperatures with increasing applied pressure. These results can qualitatively be rationalized by considering the effect of pressure on the free volume dissimilarity contribution to the free energy of close approach of the interacting particles. 

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. 

Surfactants are employed in emulsion polymerizations to facilitate emulsification and impart electrostatic and steric stabilization to the polymer particles. Sicric stabilization was described earlier in connection with nonaqueous dispersion polymerization the same mechanism applies in aqueous emulsion systems. Electrostatic stabilizers are usually anionic surfactants, i.e., salts of organic acids, which provide colloidal stability by electrostatic repulsion of charges on the particle surfaces and their associated double layers. (Cationic surfactants are not commonly used in emulsion polymerizations.)... 

Dispersion stabilization with nanoparticles is also known. A recent example of a dispersion stabilized by nanoparticles was published by Tohver et al. This group used zirconia particles to stabilize an aqueous colloidal system of larger silica particles. The dispersion was stabilized by electrostatic stabilization and thus is essentially applicable only to aqueous systems. Surface modification of the particles changes the stabilization mechanism to steric stabilization, and dispersions in both aqueous and nonaqueous systems have been demonstrated. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

One advanta us feature of steric stabilization for practical applications is that it is usually equally effective at both high and low volume fractions of the dispersed phase. This would not be true, for example, for electrostatic stabilization in nonaqueous media (Albers and Overbeek, 1959 Feat and Levine, 1976). In that case, the spatial extension of the double layers is so great, due to the low dielectric constant of the dispersion medium, that the mere act of preparing high solids dispersions forces the particles close together. The net result is that little repulsion remains between their double layers and the particles coagulate. c. 

Verwey and de Boer surmised that the particles were surrounded by an oriented layer of oleic acid molecules. These had their polar carboxylic acid groups adsorbed at the surfaces of the particles and their nonpolar tails oriented toward the nonaqueous dispersion medium. Verwey and de Boer even represented the particles (see Fig. 2.1) by a schematic diagram that would be immediately recognizable today as depicting steric stabilization. 

We note parenthetically that Koelmans and Overbeek (1954) used the term steric hindrance stabilization contemporaneously with Heller and Pugh to refer to stabilization in nonaqueous dispersion media by n-alkyl tails. 

The preparation of nonaqueous sterically stabilized polymer latices was pioneered by Osmond and coworkers at ICI Paints Division. Their work is summarized in a comprehensive monograph (Barrett, 1975) that provides actual recipes for the successful generation of nonaqueous dispersions. Only the most general considerations need therefore be mentioned here. 

Many sterically stabilized dispersions can be induced to flocculate simply by decreasing the solvency of the dispersion medium for the stabilizing moieties. This may be achieved for some systems by changing the temperature and/or pressure. Other dispersions, however, resolutely defy all such efforts to induce flocculation merely by adjusting the ambient conditions. This is especially evident with certain aqueous sterically stabilized dispersions. For example poly(vinyl acetate) latices stabilized by poly(oxyethylene) in pure water are stable at 100 C. In these instances, the addition of a substance that reduces the solvency of the dispersion medium for the stabilizing moieties usually permits flocculation to be observed. In aqueous systems, for example, the addition of electrolytes will commonly reduce the solvent power that water displays for the polymeric stabilizing moieties. In nonaqueous media, all that is usually required is the addition of a nonsolvent for the stabilizing chains that is miscible with the dispersion medium. This method is also applicable to the flocculation of aqueous sterically stabilized dispersions. 

The thermodynamic contributions to steric stabilization according to the free volume theory for nonaqueous (and some aqueous) dispersions... 

In summary, for nonaqueous dispersions, the combinatorial free energy of interpenetration favours stabilization. Both of the corresponding free energies associated with contact dissimilarity and free volume dissimilarity favour flocculation. These conclusions are represented schematically in Fig. 7.2. Since the combinatorial free energy is purely entropic in origin, it is scarcely surprising that nonaqueous sterically stabilized systems are usually found to be entropically stabilized at room temperature and pressure for it is this term that imparts stability. Anticipating the results of the next section, we stress that this does not necessarily imply that all nonaqueous dispersions are entropically stabilized at room temperature. 

Free volume theory enables the relative magnitudes of the various components of steric stabilization to be calculated for nonaqueous dispersions. The basis of these theoretical predictions is the thermodynamic factor (i - y i), where X = interaction parameter. The way by which this factor enters the steric interaction is exemplified by the interpenetrational domain for spheres... 

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. 

One possible explanation for the phase separation in both aqueous and nonaqueous systems is the very high occupancy of the space by the sterically stabilized particles. This would mean that the free polymer cannot diffuse into the dispersion media without a significant loss of configurational entropy. The exigencies created by such severe volume restrictions at high dispersed phase concentrations could be responsible for phase separation. The fact that the polymer chains cannot physically diffuse into the dispersion would prevent the chains from inducing either depletion flocculation or depletion stabilization. 

Simple homopolymers or random copolymers that are solvated by the dispersion medium are only weakly adsorbed to the surface of the polymer particles and cannot therefore provide the strongly anchored sheath of solvated polymer chains necessary for steric stabilization [3.60]. Block and graft copolymers play a key role in the steric stabilization of nonaqueous dispersions. Block and graft copolymers contain long runs of identical monomers in the polymer chain (block) or in side chains (graft) ... 

Block and Graft Copolymer Stabilizers in Dispersion Polymerization. A sterically-stabilized, nonaqueous, polymer dispersion is made simply by heating a solution of a free radical initiator (e.g., azobisisobutyronitrile), an appropriate monomer, and a suitable block or graft copolymer in an organic liquid which is a nonsolvent for the polymer product and acts as a diluent for the dispersion. The block or graft copoly-... 

During the dispersion polymerization, the polymer precipitates from an initially homogeneous reaction mixture containing monomer, initiator, steric stabilizer, and solvents. Under favorable conditions, monodisperse polymer particles stabilized by a steric barrier of dissolved polymer are formed. The early work, mainly done in nonaqueous media such as aliphatic hydrocarbons, was thoroughly reviewed by Barrett [90]. Most of the studies dealt with polymer particles in the 0.1- to 2- jun size range. 

---