  When a polymer solution undergoes viscoelastic phase separation, the
polymer-rich phase forms a network-like structure even if it is a minor phase.
This unique feature is induced by polymer dynamics, which are constrained by
the temporal entanglement of polymer chains. The fundamental mechanisms of
viscoelastic phase separation have already been elucidated by theory and
experiments over the past few decades; however, it is not yet well understood
how viscoelastic phase separation occurs in multicomponent polymer solutions.
Here, we construct a new viscoelastic phase separation model for ternary
polymer solutions that consist of a polymer, solvent, and nonsolvent. Our
simulation results reveal that a network-like structure is formed in the
ternary bulk system through a phase separation mechanism similar to that
observed in binary polymer solutions. A difference in dynamics is also found in
that the solvent, whose affinity to the polymer is similar to that to the
nonsolvent, moves freely between the polymer-rich and water-rich phases during
phase separation. These findings are considered important for understanding the
phase separation mechanism of ternary mixtures often used in the manufacturing
of polymeric separation membranes.
