Phase Separation of Homopolymers in Binary Mixed Solvents─The Co-Nonsolvency Effect

As one of the most intriguing phenomena in polymer solutions, the co-nonsolvency effect refers to the collapse of a homopolymer chain or phase separation of homopolymer solutions in mixtures of two good solvents. Despite decades of research, the molecular mechanism underlying the co-nonsolvency effect is still under debate. In this work, by using the ternary Flory–Huggins theory, we report the first complete mean-field phase behaviors of homopolymer solutions in binary mixtures of two miscible good solvents. We construct the binodal, spinodal, and critical points for several representative systems. Our calculations show that a homopolymer solution can separate into coexisting phases either when the quality difference between the two solvents is sufficiently large or in the presence of strong attractions between the two solvents. For both cases, the binodal and spinodal curves in the concentration plane of the polymer and the better solvent exhibit closed-loop features along with two critical points. However, the slope of the tie lines has the opposite sign in these two cases: it is positive in the former and negative in the latter. This suggests a convenient and decisive means to elucidate the physical mechanism of the co-nonsolvency effect. Moreover, we find that a phase-separated solution may undergo a re-entrant phenomenon while increasing the attractive strength between the two solvents.