Multiphase separation is a ubiquitous phenomenon in multicomponent mixtures such as biomolecular condensates, yet the understanding of the underlying mechanism remains far from complete. In this work, we revisit the phase behaviors of symmetric ternary polymer mixtures by using the Flory–Huggins (FH) theory, with a focus on multiphase coexistence. The phase behavior of this system is uniquely determined by χN, where χ is the pairwise FH parameter between any two of the three species, and N is the chain length. By performing numerical calculations and analytical derivations, we identify six χN regions, each having different phase behaviors in the composition triangle. In particular, three separate three-phase regions emerge when χN > 18/7 and merge into a single three-phase region when χN > 4 ln 2. Phase evolution by varying either χN or the overall composition exhibits substantially more complex behaviors than in symmetric binary mixtures. The four-phase coexistence, as suggested by phase equilibrium condition, is found to be unobservable in ternary mixtures. One intriguing implication of our phase diagram is that adding a third polymer into an immiscible binary blend may make the system homogeneous, even though any two of the three components are immiscible with each other. Our study not only presents the first complete phase behaviors of symmetric ternary polymer mixtures but also serves as a foundation for studying multiphase coexistence in asymmetric multicomponent systems.
Xin et al. (Wed,) studied this question.