Effect of Chain Rigidity on the Decoupling of Ion Motion from Segmental Relaxation in Polymerized Ionic Liquids: Ambient and Elevated Pressure Studies

Z Wojnarowska and HB Feng and Y Fu and SW Cheng and B Carroll and R Kumar and VN Novikov and AM Kisliuk and T Saito and NG Kang and JW Mays and AP Sokolov and V Bocharova, MACROMOLECULES, 50, 6710-6721 (2017).

DOI: 10.1021/acs.macromol.7b01217

Conductivity in polymer electrolytes has been generally discussed with the assumption that the segmental motions control charge transport. However, much less attention has been paid to the mechanism of ion conductivity where the motions of ions are less dependent (decoupled) on segmental dynamics. This phenomenon is observed in ionic materials as they approach their glass transition temperature and becomes essential for design and development of highly conducting solid polymer electrolytes. In this paper, we study the effect of chain rigidity on the decoupling of ion transport from segmental motion in three polymerized ionic liquids (polylLs) containing the same cation anion pair but differing in flexibility of the polymer backbones and side groups. Analysis of dielectric and rheology data reveals that decoupling is strong in vinyl-based rigid polymers while almost negligible in novel siloxane-based flexible polylLs. To explain this behavior, we investigated ion and chain dynamics at ambient and elevated pressure. Our results suggest that decoupling has a direct relationship to the frustration in chain packing and free volume. These conclusions are also supported by coarse-grained molecular dynamics simulations.

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