Influence of molecular weight on electro-responsive electrorheological effect of poly(ionic liquid)s: Rheology and dielectric spectroscopy analysis

Poly[methylmethacrylate trimethylamine][hexafluorophosphate] (P[MTMA][PF₆]) poly(ionic liquid)s (PILs) with different molecular weight (polymerization degree N = 6–652) were synthesized by free-radical polymerization. The influence of molecular weight on the electrorheological (ER) effect of P[MTMA][PF₆] particles dispersed in insulating carrier liquid was studied by rheological test, and the reasons behind the influence was analyzed by dielectric spectroscopy. The result shows that when the molecular weight of P[MTMA][PF₆] is less than ∼30,000 (N<∼100), the ER effect is sensitive to the molecular weight and it increases as the molecular weight increases; when the molecular weight exceeds ∼30,000 (N>∼100), the ER effect tends to saturate. Dielectric spectra analysis shows that the influence of molecular weight on the ER effect of P[MTMA][PF₆] is closely related to the interfacial polarization and the leaking conduction of P[MTMA][PF₆], which further depends on the changes of ionic conductivity of P[MTMA][PF₆] particles and dissolution ability of P[MTMA][PF₆] particles in carrier liquid with molecular weight. By activation energy calculation and XRD analysis, it demonstrates that the molecular level reason for the influence of molecular weight on conductivities, polarization and ER effect is related to the changes of the elastic potential energy of P[MTMA][PF₆] matrix and the limiting ability of matrix for ion dissolution in carrier liquid with the molecular weight.