Role of Cation Size and Dipolar Motion Restriction in the Viscosity of Polymerized Ionic Liquids

We employ coarse-grained molecular dynamics simulations to investigate how cation size and restrictions in dipolar motion affect the viscosity of a polymerized ionic liquids (PILs). As a model system, we consider poly(1-ethyl-3-vinylimidazolium) bis(trifluoro­methane­sulfo­nyli­mide) (PC2VITFSI), where cationic side chains are directly tethered to the polymer backbone. The side chains and free anions are represented as charged, Stockmayer-type rotating dipolar spheres. Our results show that increasing cation size reduces viscosity by weakening ion association and increasing free volume. Conversely, restricting dipolar motion in the side chains increases viscosity due to limited local charge and dipole screening and enhanced ion correlations. Finally, by quantifying the excess entropy, we establish a link between local microstructural order and viscosity in PILs.