Solutions of different polymers in the same solvent are incompatible as a rule and show phase separation when they are mixed. H incompatibility is also to be observed in systems where one of the polymer components is replaced by colloidal particles, sterically stabilized by a cover of polymer chains, will be discussed in this Iecture.After a discussion of the applicability of statistical thermodynamical criteria for colloid stability we focus attention on the potential of average force between two particles, V(r), and the second virial coefficient, B2• First it is shown from general arguments that V(r) and B, always decrease in magnitude upon addition of particles identjcal to the particle pair considered. The decrease is particularly large for high molecular weight polymers.Subsequently, the analysis is extended, with the help of simple models, to mixtures of polymer colloid and polymer. It is predicted that B2 should decrease and may hecome negative when the molecular weight and concentration of the polymer are sufficiently )arge. For high molecular weight polymer this is of the order of aper cent or less. More polymer is needed for low molecular weights.The destabilization is intimately connected with the expulsion of polymer from the interstitial spaces between approaching particles hecause of "volume restriction"-and "osmotic" etiects.The predictions are in accordance with some experiments that were available. Finally the applicability of light scattering as an experimental tool in these stability problems is stressed. Results are also given of the incompatibility of two polymers in a sing)e solvent in which one of the polymers is masked i.e. does not scatter light.

The stability of a free, thin liquid film against small, spontaneous thickness fluctuations is explored. The film is unstable with respect to fluctuations with wavelengths larger than a critical wavelength A, = [-2-.2y/(d2V/dh2)]4, where y is the interfacial tension and V(h) the free energy of interaction as a function of the f i l m thickness h. V(h) may include van der Waals attraction and double-layer repulsion. The kinetics of the growing fluctuations is obtained by assuming a laminar liquid flow between rigid fiLm surfaces at a constant viscosity. There are stable fluctuation-modes, which grow exponentially with time, each with a characteristic time constant T, and modes with certain wavelengths grow faster than all others (T = T~). If the van der Waals forces predominate A, and T , are given by eqn. (4.2) and (4.3) respectively. For A = 10-14-10-12 erg, y = 30 dynelcm and Iz = 100-lo00 A, A, ranges from 0.6-600 p and T , from a fraction of a second to several hours. The life-time and critical thickness h, of an unstable film are also calculated ; they depend on the time constant rrn and on the time of draining. The critical thickness is calculated for microscopic, circular films and compared with measurements of Scheludko and Exerowa. For water and aniline films, the calculated h, are 410 and 750 A respectively, whereas the experimental values are 270 and 410 A.

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