Thermoreversible Ion Gel with Tunable Modulus Self-Assembled by a Liquid Crystalline Triblock Copolymer in Ionic Liquid

Ion gels with tunable storage moduli are prepared through the self-assembly of an ABA triblock copolymer AOA-12 comprised of a thermotropic liquid crystalline (LC) polymer as the end-block A and poly­(ethylene oxide) (PEO) as the mid-block B in a room-temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium bis­(trifluoro­methyl­sufonyl)­imide ([EMIM]­[TFSI]). The PEO mid-block is well-solvated by this ionic liquid, whereas the LC polymer end-blocks aggregate and serve as the physical cross-linkers. For comparison, a triblock copolymer AOA-0 containing a non-LC side-chain polymer with the same mesogen is also synthesized, and its corresponding ion gel is prepared. The ion gels with relatively high concentrations of the LC triblock copolymer have hierarchical structures with different microphase-separated nanostructures and the LC arrangement of the LC blocks. By incorporating the azobenzene mesogen in the side chains, transparent AOA-<i>n</i>/[EMIM]­[TFSI] (where <i>n</i> is the number of carbon atoms in the spacer between the azobenzene mesogen and the polymethacrylate backbone, and <i>n</i> = 0, 12), ion gels are obtained with concentrations of the polymer as low as around 2 wt %. The ion gel obtained has a storage modulus as high as ∼10 kPa, while its conductivity is close to that of the pure IL mainly because of the high IL concentration of the ion gel. Furthermore, the storage modulus of the AOA-12/IL ion gel can be tuned by temperature because of the thermotropic phase behavior of the LC block. These ion gels are potentially useful as high-temperature ionic membranes or thermal-responsive soft actuators