Influence of Chain Entanglement on Rheological and Mechanical Behaviors of Polymerized Ionic Liquids
Gang Liu, Ronald G. Larson, Lei Li, Huan Luo, Xi He, Yanhua Niu, Guangxian Li
Abstract
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 (η 0 ∼ M w α ) 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 ( M e,calc = 1.8 × 10 5 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 ( M e,rheo = 1.6 × 10 5 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.