Influence of Chain Entanglement on Rheological and Mechanical Behaviors of Polymerized Ionic Liquids

Polymerized ionic liquids (PILs) poly[1-(4-vinylbenzyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide] (P[VBMIM][TFSI]) covering a wide range of molecular weights are synthesized using reversible addition–fragmentation chain-transfer (RAFT) polymerization. The dependence of zero-shear viscosity on weight-average molecular weight (η0 ∼ Mwα) shows two power-law regimes corresponding to disentangled and entangled regimes with α = 1.1 ± 0.1 and 3.6 ± 0.4, as is typical for neutral entangled polymers. The entanglement molecular weight (Me,calc = 1.8 × 105 g/mol) of P[VBMIM][TFSI] estimated from the packing length p = 8.6 Å and Kuhn length b = 20 Å is consistent with the experimental result (Me,rheo = 1.6 × 105 g/mol) and is much higher than the most conventional uncharged polymers due to the bulkiness of the monomers. For entangled PIL samples, reversible strain-induced disentanglement is observed under large-amplitude oscillatory shear (LAOS) and strong strain hardening in extensional flow. The chain entanglement in high-molecular-weight P[VBMIM][TFSI] brings significant improvement in the mechanical strength and robust recoverability under cyclic stretch over that of the lower-molecular-weight, unentangled PILs, making high-molecular-weight P[VBMIM][TFSI] processable for high-performance electrolytes.