How Do Ionic Liquids “Fold” Ionenes? Computational and Experimental Analysis of Imidazolium Polymers Based on Ether and Alkyl Chain Variations Dissolved in an Ionic Liquid

Molecular dynamics simulations and complementary experiments are used to understand the inter- and intramolecular structures and conformational properties of ionenes (i.e., cationic polymers formed from condensation reactions) dissolved in an ionic liquid, 1-ethyl-3-methylimidazolium bistriflimide [C2mim+][Tf2N–]. The simulated structural properties are benchmarked against experimental analyses of these same polymers, mainly using dynamic light scattering experiments. Four different imidazolium ionenes are considered, corresponding to variations in the chemistry and structure of the repeat units including: (1) poly(decylimidazolium) (PD10); (2) poly(tetraethyleneglycolimidazolium) (PE10); (3) alternating copolymer (P(ED)5); and (4) a block copolymer (PE5D5). Detailed computational analyses of the polymer structure and conformational properties were performed, including the radius of gyration, end-to-end distance, torsional distributions, and site–site and spatial distribution functions. Overall, there is a competition between intramolecular associations between the imidazolium groups and the ether sites in the polymers versus the intermolecular interactions of the polymer imidazolium groups with the surrounding anion molecules. The polymer with only ether linkages (PE10) results in the most intramolecular interactions, leading to significant coiling behavior and chain contraction. These strong interactions reduce chain flexibility but they also result in much more linear chain configurations and alignment of the imidazolium groups. The polymer structure is strongly affected as the concentration of alkyl groups is increased, and we find that the polymer architecture (alternating vs block copolymer) also has an important influence. These detailed observations are important for understanding the connection between the molecular design of imidazolium polymers and their emergent structural properties, which could lead to unique opportunities for creating polymer composites.