Photo-healable ion gel with improved mechanical properties using a tetra-arm diblock copolymer containing azobenzene groups

- We compared tetra-arm diblock copolymer (A(B)4) ion gels with triblock copolymer (A(B)2) ion gels.
- A(B)4 ion gels have higher mechanical properties than A(B)2 ion gels.
- A(B)4 ion gels have more uniform network structure than A(B)2 ion gels.
- A(B)4 ion gels show reversible photo-induced sol-gel transition at certain temperatures.
- Photo-healable ion gel with higher photo-healing efficiency can be realized.

Photo-healable ion gels with improved mechanical properties are demonstrated using a tetra-arm diblock copolymer (A(B)4) containing azobenzene groups. Physical ion gels were prepared by self-assembly of A(B)4 in a hydrophobic ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6), where A block is the ionic liquid-compatible tetra-PEG, and B block is the thermo-/photo-responsive random copolymer of 4-phenylazophenyl methacrylate and N-isopropylacrylamide (P(NIPAm-r-AzoMA)). The A(B)4 ion gels have a higher modulus and fewer network defects than the triblock copolymer (A(B)2) ion gel (A block is PEG; B block is P(NIPAm-r-AzoMA)). Self-standing A(B)4 ion gels were obtained at a lower polymer concentration than A(B)2 ion gels in [C4mim]PF6. The A(B)4/[C4mim]PF6 and A(B)2/[C4mim]PF6 binary systems exhibit low-temperature gel-states and high-temperature sol-states. The gel-to-sol transition temperature also depends on the photoisomerization state of the azobenzene moiety in the polymers. It is confirmed that photo-induced sol-gel transition is reversible at a suitable temperature. Photo-healable ion gels are demonstrated using the photo-induced reversible sol-gel transition, where damaged regions are UV-light-irradiated to induce the gel-to-sol transition then visible-light-irradiated to heal the gels by the sol-to-gel transition. Remarkable photo-healing efficiency in terms of the mechanical properties is realized by this procedure.

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