Mechanical behaviour of polymer mixtures in the phase separation region

The liquid-liquid phase transition triggered by changes in the composition of polymer mixtures in solution or melt is often accompanied by "critical" opalescence, which signals the appearance of a microemulsion in the mixture. The viscosity of the polymer mixture in this region is characterized by a sharp minimum, observed, as a rule, over an extremely narrow range of concentration. Depending on the concentration of the solution, the type of polymer, and the solvent, the viscosity may decrease by a factor of 8-10. On transition from micro- to macro-separation, viscosity rapidly increases back to the original level. Changes in the composition of the mixture can alter the concentration at which phase separation occurs, but the minimum in viscosity invariably corresponds to the moment of phase separation. The critical opalescence region represents the formation of phase particles up to 80-100 nm in size, and this corresponds to the point of viscosity drop. This effect is due to the appearance of thermodynamically stable microemulsions in the polymer mixture, in the region between the binodal and the spinodal in the phase diagram. These emulsions are characterized by lower molecular interaction of incompatible polymers in the highly developed interfacial layer. Extremal changes at the point of phase separation are also observed for other mechanical characteristics of polymer mixtures in solutions or melts, for example, G′ and G″ dynamic moduli or complex viscosity η*. Keywords: polyblends, critical phenomena, viscosity, emulsions, phase separation.