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.

中文翻译：

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二元混合溶剂中均聚物的相分离─共非偿付效应

作为聚合物溶液中最有趣的现象之一，共非溶解效应是指均聚物链的崩溃或均聚物溶液在两种良溶剂的混合物中的相分离。尽管经过了数十年的研究，共同破产效应背后的分子机制仍然存在争议。在这项工作中，通过使用三元弗洛里-哈金斯理论，我们首次报道了两种可混溶良溶剂的二元混合物中均聚物溶液的完整平均场相行为。我们为几个代表性系统构建了双节点、旋节线和临界点。我们的计算表明，当两种溶剂之间的质量差异足够大或两种溶剂之间存在强吸引力时，均聚物溶液可以分离成共存相。对于这两种情况，聚合物和更好的溶剂的浓度平面中的双节线和旋节线曲线表现出闭环特征以及两个临界点。然而，在这两种情况下，联络线的斜率具有相反的符号：前者为正，后者为负。这为阐明共同破产效应的物理机制提供了一种方便而决定性的方法。此外，我们发现相分离的溶液可能会经历重入现象，同时增加两种溶剂之间的吸引力强度。