Viscoelastic Behavior and Ion Transport of Weakly Ordered Polymerized Ionic Liquid Block Copolymers

A series of poly(dimethylsiloxane)-block-polymerized ionic liquid (PDMS-b-PIL) diblock copolymers with varying polymerized ionic liquid (PIL) lengths and cationic structures were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization and ionization reaction. All PDMS-b-PILs samples presented a weakly ordered morphology, and the corresponding domain spacing increases with increasing PIL length and cation size. Their linear and nonlinear viscoelastic behaviors were investigated by rheology. All samples exhibit a pronounced long-term relaxation feature with a typical low-frequency scaling relation G′ ≈ ωn (n < 0.5) correlating to the ordered morphology. The exponent n increases with increasing PIL length, irrespective of the alkyl length of the cation, except for the cation with a larger phenyl group, where the additional cation–π and π–π interactions could enhance the long-term relaxation behavior. In the intermediate frequency region, an interesting entanglement-like behavior emerges, which could be attributed to the network formation induced by ion associations. Large amplitude oscillatory shear (LAOS) and extensional rheological results demonstrate that nonlinear deformation and strain hardening behavior become more pronounced with a longer PIL chain, while stronger interionic electrostatic interactions lead to a weaker nonlinear deformation but more prominent strain hardening attributing to the influence of ion association structures. The direct current (DC) conductivity of each PDMS-b-PIL sample changes from an Arrhenius to a Vogel–Fulcher–Tammann (VFT) temperature dependence as the temperature increases, but the transition temperature is insensitive to the PIL length or the cationic alkyl length except for the larger phenyl group. In addition, the dissociation relaxation time of ion pair exhibits a two-stage temperature dependence, where the weakly microphase-separated interface within the system plays a critical role.