Coarse-Grained Models for Simulations of Conformational Properties of Three-Dimensional Ladder Polymers in Solutions

The newly developed three-dimensional ladder polymers are very promising membrane materials for gas separations. The conformational properties of such specific polymers in solutions that dictate the excellent properties are hard to resolve by experiments and pose great challenges and opportunities for simulations as well. To facilitate the molecular dynamics (MD) simulations, the coarse-grained (CG) models for such a polymer and its solvent (chloroform) have been parametrized using the bottom-up and top-down schemes, respectively. In particular, equivalent high temperatures are adopted for the CG MD simulations of the all-syn and all-anti configurations. It is confirmed that such CG models exhibit very good structural representability and temperature transferability. As revealed by the extensive simulations, for a higher degree of polymerization, the syn chain exhibits a spring-like shape, whereas the antichain shows an open, straight shape. For the antichain in vacuum, the scaling relations with n are found for both the end-to-end distance (R_e) and radius of gyration (R_g). For the syn chain, no similar scaling relations with n are identified for the R_e and R_g in vacuum, but only for the R_e in solution, revealing the key roles of the solvent. These observations are closely associated with the unique chain rigidity, self-similarity, and shape, which provide valuable guides for tuning membrane separations.