Latex dispersion interaction potentials

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

Latex dispersions display a rich range of rheological properties. The exact behavior of dispersions is dependent on both the volume fraction of particles and the interaction potential. 

Shear thinning refers to the observation of a decrease in dispersion viscosity as the applied shear rate is increased. It is because of the breakdown of structure in dispersions as the shear rate is increased. For hard sphere latex dispersions, where the interaction potential is zero except at contact, there is found to be a shear thinning behavior for volume fractions above 50%.P ... 

Practically, the addition of a nonadsorbing polymer to a dispersion can induce flocculation of dispersed particles due to the depletion attraction. This was first observed by Cowell, Lin-In-On, and Vincent [1434]. When large amounts of poly (ethylene oxide) are added to an aqueous dispersion of hydrophilized polystyrene latex particles, the particles start to flocculate. For an organic dispersion, namely, hydrophobized silica particles in cyclohexane, de Hek and Vrij [1435] observed depletion-induced flocculation when dissolved polystyrene was added. Other combinations of particles and polymers followed [1436]. Phase diagrams for different particle-solvent-polymer systems were successfully drawn using the depletion potential of Asakura as interaction potential between dispersed spheres [1437] and for dissolved polymers using statistical mechanics [1438]. 

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