Steric stabilization theta temperature

Influence of Addition of Electrolyte and Increase of Temperature Addition of electrolyte or increase of temperature at a given electrolyte concentration to a sterically stabilized dispersion may result in its flocculation at a critical concentration or temperature, which in many cases coincides with the theta point for the stabilizing chain. At the theta point the mixing term in the steric interaction is zero and any yield value measured should correspond to the residual van der Waals attraction. The energy arising from van der Waals attraction may be calculated from the following approximate relationship,... 

Conditions that cause the extended polymer segments to become insoluble in the medium will also, in general, give rise to flocculation and even coalescence of the emulsion. Flory (3) defined theta conditions as conditions of temperature and solvent composition under which the free energy of interaction between polymer segments equals the free energy of polymer-solvent interaction. Under theta conditions, the soluble segments of a steric stabilizer would collapse, the repulsion between droplets would diminish, and flocculation of the system would be expected to ensue. This prediction has been demonstrated experimentally (20, 21). 

A key difference between elecfrostatic and steric stabilization is the significant influence that the solvent quality and the temperature can have in steric stabilization. As we saw above, interpenetration of the polymer chains gives rise to a mixing effect. At a certain temperature, referred to as the (theta) temperature, the interpenetration of the polymer chains does not lead to a change in the free energy of mixing (AGmix = 0), and a system of the polymer dissolved in the solvent behaves like an ideal solution. The solvent is referred to as a 0 solvent. 

Table 4.5 Comparison of the Theta (0) Temperature of Solutions of Steric Stabilizers with the Critical Flocculation Temperatures (CFT) of Suspensions...

A series of well-defined A-B block copolymers of polystyrene-block-polyethylene oxide (PS-PEO) were synthesized [11] and used for emulsion polymerization of styrene. These molecules are ideal since the polystyrene block is compatible with the polystyrene formed and thus it forms the best anchor chain. The PEG chain (the stabilizing chain) is strongly hydrated with water molecules and it extends into the aqueous phase forming the steric layer necessary for stabilization. However, the PEG chain can become dehydrated at high temperature (due to the breaking of hydrogen bonds) thus reducing the effective steric stabilization. Thus the emulsion polymerization should be carried out at temperatures well below the theta (0) temperature of PEG. 

---