Molecular-Weight Dependence of Center-of-Mass Chain Diffusion in Polymerized Ionic Liquid Melts

Polymerized ionic liquids (PILs) with flexible polymer chains and weakly interacting ionic liquid (IL) groups have received great attention for their desirable properties in electrochemical applications such as ionic conductivity. Less is known about their dynamic properties such as center-of-mass chain diffusion and how it depends on molecular weight in the presence of IL groups. In this work, a series of acrylic PILs with imidazolium cations and bis(trifluoromethanesulfonyl)imide (TFSI) anions (TFSI-f-PIL_N) were synthesized via reversible addition-fragmentation chain-transfer polymerization with degrees of polymerization N ranging from 40 to 236. A fluorescent acrylic monomer with the 7-nitrobenzofurazan group was copolymerized at trace levels as a probe of chain motion, and the diffusion coefficient (D) of TFSI-f-PIL_N was determined by fluorescence recovery after photo bleaching at T_g + 45 K. Within the uncertainty of 3-20%, a scaling relationship of D ∼ N^-2 was observed which is the same as the scaling of linear neutral polymers. Wide-angle X-ray scattering exhibited no peak at ∼5 nm^-1, indicating no long-range imidazolium-TFSI ionic correlations. Our results indicate that the molecular weight dependence of center-of-mass diffusion is not affected by electrostatic interactions of IL groups. No transition from a Rouse regime (D ∼ N^-1) to reptation regime (D ∼ N^-2) was observed within the studied N range.