Effect of Molecular Weight on Self-Diffusion Dynamics and Viscoelasticity of Polymerized Ionic Liquids

A systematic series of polymerized ionic liquids (PILs) with various molecular weights (MW) was synthesized both with and without fluorescent labels. Size exclusion chromatography was applied to the unlabeled samples for absolute MW determination, and used to calibrate the labeled polymers. Fluorescence recovery after photobleaching (FRAP) was applied to investigate the self-diffusion of dry PILs. The self-diffusion coefficient (D) was extracted by analyzing the post-bleached fluorescence images. The diffusion coefficient monotonically increased with number of repeating units (N) but no entanglement regime was observed in the studied range. This was further confirmed by rheology where viscosity (η₀) scale as ~ N ¹, as unentangled polymer melts. Oscillatory measurements showed no clear entanglement or ion aggregation plateau at intermediate frequencies but the terminal region extended to lower frequencies as N increased. The lack of ion aggregation was supported by X-ray scattering data where no ion aggregation correlation peak was observed. This study provides a fundamental insight into the effect of molecular weight on dynamic properties of ionic polymer melts.