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 exhibit desirable properties like moderate ion conductivity, and have received great attention in applications of energy storage/conversion, stimuli-responsive materials, etc. However, less is known about their dynamic properties such as viscosity and chain diffusion, which are also important in the context of 3D printing, electro-adhesion and calculating free ion transference numbers. In this work, the center-of-mass diffusion of solvent-free, fully-charged PILs were probed using fluorescent recovery after photobleaching (FRAP). 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 (RAFT) polymerization with precise control of degrees of polymerization N ranging from 40 to 236. A fluorescent acrylate monomer with the 7-nitrobenzofurazan group was copolymerized into polymer backbones at trace levels as a probe of chain motion. The diffusion coefficient (D) of TFSI-f-PIL_N was determined by fitting diffusion model into fluorescence intensity profiles obtained from post-bleaching images. Within the uncertainty of 3~20 %, a scaling relationship of D~N^-2 was observed which is consistent with the scaling of linear neutral polymers. Wide-angle X-ray scattering exhibited no peak at ~5 nm^-1 for the long-range imidazolium-TFSI ionic aggregation. The results indicated the molecular weight dependence of center-of-mass diffusion is not impacted by the electrostatic interactions of IL groups. But no transition from the Rouse regime (D~N^-1) to reptation regime (D~N^-2) was observed within the studied N range. We hypothesized that the single diffusion coefficient scaling trend is due to the increased stiffness of present PILs compared with their neutral analogues as the bulky ionic groups will impact the conformations of chains. This work provided a fundamental insight into the chain length dependence of diffusion dynamics of ionic polymer melts and also proposed a general method to study center-of-mass diffusion in dry ionomers.