Correlation between the Molecular Structure and Multiscale Dynamic Relaxation in Polymerized Ionic Liquid Block Copolymers

Using poly(methyl methacrylate) (PMMA) as the neutral block and imidazolium-based polymerized ionic liquids (PILs) as the charged block, a series of neutral-charged diblock copolymers PMMA-b-PILs with different PIL chain lengths and different ion species were prepared by sequential reversible addition–fragmentation chain transfer (RAFT) radical polymerization combined with postpolymerization modification. Different from PMMA-b-PILs with the [TFSI]− anion showing a homogeneous state, those with [BF4]− and [PF6]− anions show a weakly microphase separated structure due to the strong Coulombic interactions between ion pairs. Experimental results from broadband dielectric spectroscopy and dynamic rheology further demonstrated that PMMA-b-PILs with the anion [TFSI]− exhibit five typical structural relaxations, including dipole polarization of PMMA side groups, association and dissociation between ion pairs, dipole polarization of PIL cationic side groups, motion of chain segments, and reptation relaxation of the whole chain, which could be promoted or inhibited by changing the PIL chain lengths and the substituent structures on imidazole cations. When the anion [TFSI]− is replaced by [BF4]− or [PF6]−, stronger Coulombic interactions between ion pairs and weak microphase separation significantly inhibit the multiscale relaxation behavior of PMMA-b-PILs. More specifically, the polarization relaxation of cations could be divided into fast and slow modes, corresponding to intramolecular hopping and intermolecular hopping of anions, respectively. The whole chain relaxation in the low-frequency region also displays long-time characteristics. On the other hand, longer PIL chains or weaker ion pair interactions could contribute to the increased direct current conductivity of PMMA-b-PILs. Below the glass transition temperature (Tg), the anion transport follows the Arrhenius equation dominated by the mechanism of successive hopping between cations; while above Tg, it shows Vogel–Fulcher–Tamman (VFT) behavior controlled by the coupling of segment motion. The transition from Arrhenius to VFT is more pronounced as interactions between the ion pairs become stronger.