Two mechanisms of liquid-liquid phase separation in polymer solutions are possible: nucleation and growth of one of the equilibrium phases, and decomposition by a spinodal mechanism. The conditions for and characteristics of both mechanisms are discussed. By combining a number of observations on phase separation in concentrated solutions of poly(2,6-dimethyl-1,4-phenylene ether) and of an ethylene — vinyl acetate copolymer in caprolactam, it becomes clear that a spinodal mechanism is operative at the onset of the separation process. In later stages the flow properties of the concentrated polymer phase determine the final state of the separated mixture.
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