Abstract
The effect of the addition of free polymer to a sterically stabilized nonaqueous colloidal dispersion is examined theoretically by considering that the interparticle potential is a sum of contributions from (i) the forces due to the presence of adsorbed polymers, (ii) the forces due to the presence of the free polymer, and (iii) the forces due to the van der Waals attraction. After suitable division of the pair potential into reference and perturbation parts, the second-order perturbation theory is used to determine the conditions for phase separation. Separation into a disordered dilute dispersion and an ordered concentrated dispersion occurs when the concentration of the free polymer exceeds a certain limiting value. This limiting free polymer concentration is shown to decrease with increasing molecular weight of the free polymer, and with increasing particle concentration. This is in agreement with experimental results. The calculation of the forces due to the presence of the free polymer in solution has been carried out for two limiting cases; one in which the free polymer can penetrate into the adsorbed layer and the second in which the polymer sheath around the particle is impenetrable to the free polymer. Calculations for the low-molecular-weight free polymer indicate that the actual situation might correspond to one intermediate between the two limiting cases.
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