(723h) Tuning Ionene Conformational Structures with Different Ionic Liquid Solvents

Ion-containing polymers have been used as novel gas sorbents and as electrolytes in many advanced devices, such as electrochemical membranes for capacitors, lithium-ion batteries, and fuel cells. Most of these ion-containing polymers are polyelectrolytes, wherein the cation is pendant from the polymer backbone. On the other hand, in ionene-based polymers, the charge lies within the polymer backbone, which imparts a high charge density and unique electrostatic interactions.^1,2

It has been found that by grafting different functional groups to these polymers, their separation performance can be significantly enhanced in gas separation membrane applications.³ Since ionenes are condensation polymers, they offer a much more tailorable platform to accommodate very sophisticated functionalities associated with high-performance polymers, such as polyamides, polyesters, and polyimides. In many studies, it has been shown that the performance of the ionenes can be tuned by incorporating alkyl spacers and counterions.⁴ However, there is not much known at the molecular level about changes in the structural behavior of ionenes upon inclusion of various functionalities and counterions. This information is crucial for synthesizing new ionenes for a specific application.

In this regard, we have used molecular dynamics (MD) simulations to study the structural and conformational properties of alkyl and ether substituted imidazolium-based ionenes. These ionenes are solvated in 1-ethyl-3-methylimidazolium bistriflimide [EMIM⁺] [Tf₂N^-]. We have performed both experiments and MD simulations to understand the inter- and intramolecular structural properties of four different ionenes. We have considered four imidazolium-based ionenes: poly(decylimidazolium) (PD₁₀), poly(tetraethyleneglycolimidazolium) (PE₁₀), alternating copolymer (P(ED)₅), and a block copolymer (PE₅D₅). The results show that the type of polymer linkage and the polymer architecture significantly affect the inter- and intramolecular interactions. The ether-based linkages show more intramolecular interactions, resulting in a coiled structure (low radius of gyration), whereas a pure alkyl-containing polymer has a more open structure.⁵ In addition, a strong interaction between the anion and imidazolium groups of the polymer chain is observed, which plays an important role in changing the structure and conformation of the polymer chain. We have now extended this work to examine the effect of different sizes and basicity of anions on the structure and conformation of these ionenes. Our results further explain the molecular-level behavior of different ionenes when immersed in different solvents, and this provides critical information for tuning the structures and resultant properties of ionene-based polymers

References:

1. Jason E. Bara, E. O’Harra, Recent Advances in the Design of Ionenes: Toward Convergence with High‐Performance Polymers. Macromol. Chem., Phys. 220, 1900078, 2019.
2. Asem I. Abdulahad, Chainika Jangu, Sean T. Hemp, Timothy E. Long, Influence of Counterion on Thermal, Viscoelastic, and Ion Conductive Properties of Phosphonium Ionenes. Symp., 342, 56–66, 2014.
3. GabrielZarca, W. Jeffrey Horne, Inmaculada Ortiza, AneUrtiaga, Jason E.Bara, Synthesis and gas separation properties of poly(ionic liquid)-ionic liquid composite membranes containing a copper salt. J. Membr. Sci., 515, 109-114, 2016.
4. Mana Tamami, David Salas‐de la Cruz, Karen I. Winey, Timothy E. Long, Structure–Property Relationships of Water‐Soluble Ammonium–Ionene Copolymers. Chem. Phys., 213, 965, 2012.
5. Praveenkumar Sappidi, Xiaoyang Liu, Kathryn E. O’Harra, Jason E. Bara, and C. Heath Turner, 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. Macromolecules, 54, 4, 1611–1622, 2021.