Transport and Mechanical Properties of ABA-type Triblock Copolymer Ion Gels Correlated with Their Microstructures

The microstructures of ion gels formed by the self-assembly of an amphiphilic ABA-type triblock copolymer in an ionic liquid (IL) were explored by using atomic force microscopy (AFM) along with small-angle X-ray scattering (SAXS) to correlate it with the ion transport and mechanical properties. Polystyrene-b-poly(methyl methacrylate)-b-polystyrene (PSt-b-PMMA-b-PSt) triblock copolymers with different PSt volume fractions were synthesized, and their self-assembled ion gels in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C₂mim][NTf₂]) were prepared with various IL contents. From the AFM images, disordered micelle (DM), hexagonally packed cylinder (HEX), and lamellar (LAM) microstructures composed of hard phase-separated PSt blocks and soft PMMA block swollen by the IL were clearly observed, and the structures were consistent with the SAXS results. In cylinder and lamellar structures, polydomain structures were predominant, in which directionality and regularity are maintained in a small domain (approximately several hundred nanometers), although the domains were randomly packed on a larger scale. With respect to the mechanical properties, reversed HEX and LAM structures (where hard PSt can form a continuous phase) exhibited significantly higher storage moduli below the glass transition temperature (Tg) of PSt than those above Tg of PSt. Conversely, the effect of Tg for DM and HEX microstructures was relatively low and can be ascribed to the absence of the continuous phase of PSt. The ionic conductivity measurements indicated that DM microstructures exhibited a small tortuosity (τ ≈ 1), that is, less obstacles in the diffusion path formed by the soft domain (PMMA with dissolved IL). Meanwhile, HEX and LAM microstructures exhibited significantly higher τ values although they display a continuous phase of the soft domain, which was ascribed to the polydomain structures formed in the ion gel. The results demonstrated a significant effect of microstructures as well as the important effect of polydomain structures on the transport and mechanical properties of ion gels.