Liquid-liquid phase separation in concentrated solutions of non-crystallizable polymers by spinodal decomposition

In the past, investigations of phase separation in concentrated polymer solutions centered first of all on the crystallization kinetics (1) and morphology (2) of polymer crystals growing from solutions and, secondly, on the equilibrium thermodynamic properties of liquid-liquid phase separation phenomena (3). Papkov and Yefimova (4) have recently given a classification of different types of phase separation for polymer solutions, essentially the two types mentioned above, in which they also pay some attention to the difference in final state of separation (i. e. "gel" or two liquid layers) and to the mechanism of separation itself. For liquidliquid phase separation of solutions of noncrystallizable polymers the difference in final state is ascribed to a combined effect of the thermodynamics of the system (the width of the miscibility gap in the temperature-composition diagram) and the flow properties of the concentrated polymer phase. In their own words: "For amorphous or barely crystallizing polymers, gel formation is only to be expected if the flow temperature of the polymer is high enough and the low molecular weight component does not reduce this point too much". The authors' view on the mechanism of separation can be summarized as follows: the gel is formed by separation and growth of microsections of the dilute polymer phase, leaving the concentrated polymer phase as a gradually stiffening matrix, until its final equilibrium concentration is reached. This is the well-known mechanism of nucleation and growth of one of the equilibrium phases. Recently, Cahn and others (5, 6) have described a second type of mechanism of phase separation: the spinodal decomposition of a homogeneous solution. This mechanism may lead to typical network structures in isotropic media, as has been shown for inorganic glasses (7). (Received J u n e 15, 1970)