Particles, sterically-stabilized

Flomogeneous solution Growing polymer particle Sterically stabilized colloid... 

A low content of F-68 in the final product was not surprising. Uncharged chains of F-68 are merely mechanically entrapped during the nanoparticle assembly process, which involves charged molecules of the reactants. The inclusion of F-68, however, is crucial for particle steric stabilization. 

When soft colloids (such as stars, block copolymer micelles, or particles sterically stabilized with grafted chains) are suspended in solvents of intermediate quality at high concentrations, an increase of temperature leads to an increase of their effective volume fraction, which in turn can yield vitrification [26,190,199]. A representative example is depicted in Fig. 23a for a star with nominal / = 128 and... 

Oxygen Quenching in Poly(vinyl acetate) Particles. In order to study a system in which fluorescent groups could be Introduced into both the stabilizer and core polymers, we turned our attention to poly(vinyl acetate) [PVAc] particles sterically stabilized by poly(2-ethylhexyl methacrylate) [PEHMA] (14,15). Phenanthrene [Phe] was chosen as the fluorescence sensor. It was Introduced into the stabilizer by mixing a small amount (ca. 1%) of 9-phenanthrylmethyl methacrylate 1. with EHMA in the synthesis of PEHMA. It was introduced into the core polymer by mixing a trace (ca. 0.01%) of J with VAc in the presence of unlabelled PEHMA in the particle synthesis step. 

Fig. 8.3. The dependence of the UCFT and LCFT of polyfmethyl methacrylate) latex particles sterically stabilized by poly(dimethyl oxane) in n-propane upon the weight fraction of latex particles (after Everett and Stageman, 1978a).

More recently, the existence of a critical coagulation particle volume fraction has been established by Vincent et al. (1980) for polystyrene latices stabilized by poly(oxyethylene) terminally grafted to the particle surface and by Cowell and Vincent (1982a) for physically attached poly(oxyethylene). Bridget (1979) has found that a parallel situation pertains with silica particles sterically stabilized by terminally anchored polystyrene in ethyl benzene. 

Fig. 8.8. Phase diagram for 0-6 diameter polystyrene latex particles sterically stabilized by low molecular weight poly(oxyethylene). The arrow (t) indicates the onset of hexagonal close packing (after Thompson and Pryde, 1981).

Fig. 10.5. The distance dependence of the potential energy of interaction of latex particles sterically stabilized by poly(vinyl alcohol) in water. The different particle radii were (1) 500 nm, (2) 100 nm and (3) 10 nm. The left-hand ordinate corresponds to an elastic modulus of l-4x lO Nm" whereas that of the right-hand side corresponds to l-2x 10 Nm (after Sonntag, 1974).

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. 

Fig. 13.3. The distance dependence of the surface pressure of a monolayer of latex particles sterically stabilized by poly(vinyl pyrrolidone) of molecular weight 40000 in 2 M NaCl (after Garvey et al., 1979).

Fig. 13.5. The osmotic pressure of poly(methyl methacrylate) latex particles sterically stabilized by poly(12-hydroxystearic acid) in n-dodecane as a function of the volume fraction of latex (after Cairns el al., 1976).

Both of the diagrams shown in Fig. 14.1 illustrate the strong repulsive interactions that can be generated in heterosteric stabilization by incompatible polymers. Indeed it is evident in this example that the 2-3 particle interactions are stronger than either the 2-2 or 3-3 interactions. In addition. Fig. 14.1b shows the appearance of a —SkT pseudo-secondary minimum in the interactional free energy of polystyrene-coated particles at 5 K below their 0-temperature. This minimum would be sufficient to ensure 2-2 homoflocculation. The 3-3 and 2-3 interactions are clearly repulsive and so the qualitative free energy calculations confirm the possibility, foreshadowed above, of the selective flocculation of one particle type in mixtures of particles sterically stabilized by different polymers. 

The introduction of free polymer into the dispersion medium appears to make no significant difference to the number of phases accessible to the system. This can be discerned from the following simplistic considerations using the Phase. Rule, which will be assumed to be applicable. We consider a dispersion of colloidal particles sterically stabilized by terminally grafted polym chains. Free polymer is dissolved into the dispersion medium. 

Three component triangular diagrams. Both Cowell et al. (1978) and Vincent et al. (1980) presented what they termed three component phase diagrams for aqueous systems composed of water (or electrolyte solution), free poly(oxyethylene) and polystyrene latex particles sterically stabilized by poly(oxyethylene). Such a diagram is reproduced in Fig. 16.7. 

There exists only fragmentary data in the literature for the experimental dependence of V2 on the particle radius. For example. Table 16.4 shows the data of Clarke and Vincent (1981a) for the floccidation of silica particles, sterically stabilized by polystyrene in ethyl benzene, by free polystyrene of molecular weight ca 7 000. An approximate dependence of V2 on the inverse square root of the particle size is indicated. 

Table 17.1 shows a test of this predicted molecular weight dependence for polystyrene latex particles, sterically stabilized by thin layers of poly(oxyethylene) of molecular weight 750, using poly(oxyethylene) of different molecular weight as the free polymer. These experimental results were reported by Cowell et al. (1978). It is apparent that V2 decreased with molecular weight with an exponent of ca —0 5, as expected from the crude theory. 

Surfactants are employed in emulsion polymerizations to facilitate emulsification and impart electrostatic and steric stabilization to the polymer particles. Steric 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.)... 

These principles are demonstrated in experiments on PVAc particles sterically stabilized with PEHMA. Phenanthrene [Phe] is a particularly useful label for these studies. It does not form excimers singlet selfquenching is inefficient. Its emission profile virtually always decays... 

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