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Generic kinetic pathway of phase separation of deeply quenched polymer solutions: Transient gelation

Phase separation is a key phenomenon producing a heterogeneous structure in various kinds of material covering from liquid to soft and to hard matter. It is known that viscoelastic effects play a crucial role in phase separation of dynamically asymmetric mixtures such as polymer solutions, protein solutions, and colloidal suspensions, under a sufficiently deep quench: viscoelastic phase separation. In contrast to the common knowledge of normal phase separation that the minority phase always form isolated droplets, viscoelastic phase separation can produce a percolated network structure of a minority phase. Because of unique mechanical and electric properties of the network structure, viscoelastic phase separation is important not only scientifically, but also technologically. Here we report experimental evidence that viscoelastic phase separation of polymer solution accompanies transient gelation as a dynamic crossover, on the way to the final coexistence of the polymer-rich and -poor liquid phases. We find that dynamic phase diagrams can be scaled by the degree of polymerization N into the master one, suggesting the universality of transient gelation, i.e., dynamic crossover, in polymer solutions. This allows us to predict the type of phase separation as a function of the temperature, the polymer concentration and N of the mixture.