Effect of Molecular Weight on Viscosity Scaling and Ion Transport in Linear Polymerized Ionic Liquids

A series of acrylic polymerized ionic liquids (PILs) with imidazolium cations and bis(trifluoromethylsulfonyl)imide (TFSI) anions were synthesized via reversible addition-fragmentation chain-transfer polymerization. The absolute molecular weights (MWs) of PILs were determined from size exclusion chromatography with light scattering. The degree of polymerization (N) ranged from 15 to 254, and steady rotational rheology indicated the zero-shear viscosity (η0) measured at a constant distance above the glass transition scales as η0 ∼N1.0 for N < 92, in agreement with the theory for unentangled polymer melts. In the range from N = 92-254, we measured η0 ∼N2.3 which is interpreted as a transition region. The N1.0 scaling in the unentangled regime is in contrast to the prior report of η0 ∼N1.7 in polyethylene-based PILs (Macromolecules, 2011, 44, 7719) but in agreement with a calculated η0 ∼N1.1 of acrylic ammonium TFSI PILs (Macromolecules, 2016, 49, 4557). Oscillatory shear rheology revealed that electrostatic interactions in this system were weak enough to have no impact on delaying the onset of flow, which was supported by a lack of ion aggregation in wide-angle X-ray scattering. The polymer nanostructure was also found to be minimally influenced by the degree of polymerization. Ionic conductivity slightly decreased as MW increased but overlapped when normalized to the calorimetric glass transition temperature.