Scaling Relation and Rheological Behaviors of Polymerized Ionic Liquids in Ionic Liquid/Dimethylformamide Solutions

As a special class of polyelectrolytes, polymerized ionic liquids (PILs) possess large counterions in their size, and the complicated electrostatic interactions with added salts, especially ILs, have attracted lots of attention. In this work, the conformational change and scaling relation of an imidazolium-based polycation (poly[1-(4-vinylbenzyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide] (P[VBMIM][TFSI])) in a nonionic solvent [dimethylformamide (DMF)] are investigated in detail by combining rheology and light scattering under the influence of an added salt [IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][TFSI])] with different concentrations csalt. The hydrodynamic radius RH of PILs and the specific viscosity ηsp show two different trends on the dependence of added salt concentration: chain collapsing due to charge screening at low csalt and chain "reswelling" due to strong ion correlations at high csalt (>3 M). Rheological viscosities show strong dependence on csalt, presenting a transition from salt-free polyelectrolytes (scaling exponent ν = 1) to good solvents (ν = 0.588), and the numerical coefficient B at different csalt regimes is also estimated. For clear quantification, the Flory–Huggins interaction parameter χ and chain dynamics for PIL molecules at various csalt are calculated by the molecular dynamics simulations, which further validate results of light scattering and rheological measurements. This study could help to clearly quantify the chain conformation and rheological scaling of PIL solutions and give useful guidance to their processing and applications.