Molecular Dynamics Study of Polymer Coiling in a Water-Organic Binary Solvent System

To investigate how solvents influence the intrachain coiling and interchain aggregation of polymers, we performed coarse-grained molecular dynamics simulations with varying polymer-solvent interactions. In the case of pure solvents, strong polymer-solvent interactions led to well-dispersed chains without polymer coiling, while weaker interactions resulted in spontaneous chain aggregation. In a binary solvent system composed of an aqueous solvent and an organic solvent, polymer aggregation preferentially occurred in the organic-rich domains following phase separation of these solvents. Notably, polymer aggregation was absent in the pure organic solvent with a strong polymer affinity, indicating that phase separation between organic and aqueous solvents plays a role in promoting polymer aggregation and subsequent coiling. To mimic the solvent evaporation process during polymer film formation, we gradually removed solvent molecules and analyzed changes in the coiling and aggregation of the polymer chains. When polymer-solvent interactions were strong, solvent molecules tended to remain between polymer chains during evaporation, preventing full coiling. In contrast, when interactions were weak or moderate, polymers aggregated as the number of remaining solvent molecules decreased, resulting in comparable levels of aggregation regardless of the solvent strength. These results show that polymer-solvent affinity critically affects the polymer coiling, both in solvent and during solvent removal.