Sterically stabilized latex particles

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).

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).

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. 

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. 

It is noteworthy that a basic assumption made in the derivation of the free radical desorption rate constant is that the adsorbed layer of surfactant or stabilizer surrounding the particle does not act as a barrier against the molecular diffusion of free radicals out of the particle. Nevertheless, a significant reduction (one order of magnitude) in the free radical desorption rate constant can happen in the emulsion polymerization of styrene stabilized by a polymeric surfactant [42]. This can be attributed to the steric barrier established by the adsorbed polymeric surfactant molecules on the particle surface, which retards the desorption of free radicals out of the particle. Coen et al. [70] studied the reaction kinetics of the seeded emulsion polymerization of styrene. The polystyrene seed latex particles were stabilized by the anionic random copolymer of styrene and acrylic acid. For reference, the polystyrene seed latex particles stabilized by a conventional anionic surfactant were also included in this study. The electrosteric effect of the latex particle surface layer containing the polyelectrolyte is the greatly reduced rate of desorption of free radicals out of the particle as compared to the counterpart associated with a simple... 

Anotlier model system consists of polymetliylmetliacrylate (PMMA) latex, stabilized in organic solvents by a comb polymer, consisting of a PMMA backbone witli poly-12-hydroxystearic acid (PHSA) chains attached to it [10]. The PHSA chains fonn a steric stabilization layer at tire surface (see section C2.6.4). Such particles can approach tire hard-sphere model very well [111. 

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.

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

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. 

The results showed that all batch polymerizations gave a two-peaked copolymer compositional distribution, a butyl acrylate-rich fraction, which varied according to the monomer ratio, and polyvinyl acetate. All starved semi-continuous polymerizations gave a single-peaked copolymer compositional distribution which corresponded to the monomer ratio. The latex particle sizes and type and concentration of surface groups were correlated with the conditions of polymerization. The stability of the latex to added electrolyte showed that particles were stabilized by both electrostatic and steric stabilization with the steric stabilization groups provided by surface hydrolysis of vinyl acetate units in the polymer chain. The extent of this surface hydrolysis was greater for the starved semi-continuous sample than for the batch sample. 

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... 

One possibility is to provide the particles with a thin steric barrier such as the "micro hairs which could occur on the surface of a latex solvation of the polymer chain beyond the ionic end group. Indeed some evidence for this occurs with certain laticcs. For example, Smithan et al. (1973) have reported evidence of steric stabilization with polystyrene latices with a high content of carboxyl groups on the surface prepared by an essentially conventional emulsion polymerization method. Microsteric stabilization with latices could be an important factor and this is undoubtedly an area that needs more extensive investigation. 

As shown by Ono et al, (1974, 1975) decrease of the HLB of a mixed emulsifier by use of an increasing proportion of a nonionic emulsifier increases the stability of the latex to coagulation by electrolyte addition despite an increase in its average particle size. This is because purely eledrostatic stabilization by adsorbed ionic emulsifier is supplemented by steric stabilization by the adsorbed nonionic emulsifier which effectively decreases the van der Waals attractive force between the latex particles (which causes them to coalesce), thereby increasing their stalrility. 

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