Self-Assembly of Block Copolymers in an Ionic Liquid and Properties of Resulting Ion Gels
Authors: Mizuno Haruna , Hashimoto Kei , Tamate Ryota , Kokubo Hisashi , Ueno Kazuhide , Watanabe Masayoshi 
Published: December 2020

Ionic liquids (ILs), which are salts with low melting points, show unique properties including non-volatility and high ionic conductivity. By combining an ionic liquid (IL) with an amphiphilic ABA-type triblock copolymer, polystyrene (PSt)-b-poly(methyl methacrylate) (PMMA)-b-PSt (SMS), insoluble A blocks aggregate and form cross-linking points, whereas soluble B blocks form a gel network swollen with the IL.[1,2] As a result, a self-standing and highly ion-conductive ion gels can be obtained, and they have been applied to electrochemical devices such as polymer actuators. In these devices, the mechanical toughness and ionic conductivity are important parameters. Thus, we have investigated the correlation between microstructures formed by the block copolymer and the properties of ion gels such as ionic conductivity and mechanical property. In particular, we focused on a gyroid structure in which both insoluble and soluble segments form continuous phases, because this unique microstructure is expected to exhibit high mechanical strength and favorable ionic conductivity, simultaneously. In this study, we directly observed microstructures using AFM to investigate how microstructures affect properties of the ion gel. Herein, we used poly(ethylene oxide) (PEO), which has a lower glass transition temperature (T
g) than PMMA, as a middle block (PSt-PEO-PSt, SOS). Consequently, SOS showed various microstructures including the gyroid depending on block ratio and content of IL. It was found that ionic conductivity and mechanical property strongly depended on the connectivity of microstructures. Namely, the gyroid structure showed the enhanced mechanical property and good ionic conductivity.

In addition, we observed in-situ change in the microstructure for a stimuli responsive ion gel containing temperature-responsive poly(N-isopropylacrylamide) (PNIPAm) blocks: PNIPAm is known to show lower critical solution temperature (LCST)-type phase behavior in water while showing upper critical solution temperature (UCST)-type phase behavior in ILs.[3] PNIPAm-b-PEO-b-PNIPAm (NON) exhibits gel-to-sol transition in IL with increasing temperature due to the UCST-type phase transition of PNIPAm blocks. We have also reported photo-responsive gel-to-sol transition of ion gels by introducing a photo-responsive azobenzene moiety to either IL or polymer.[2] These transition phenomena have been studied in terms of a macroscopic property such as rheological property. However, little is known about their microstructure during the photo-responsive change. Thus, we observed the photo-responsive change in the microstructure using AFM and investigated how the microstructure change is synchronized with the rheological response. We found that UV irradiation induced the change in the microstructures from a microphase-separated gel to a sol with a vague structure, and it was correlated with the photo-responsive change in the mechanical property and ionic conductivity.

Acknowledgements

This work was financially supported by the Grant-in-Aid for Scientific Research for Basic Research S (15H05758) from Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

References

[1] Imaizumi S, Kokubo H, Watanabe M. Macromolecules, 2012, 45, 401–409.

[2] Tamate R, Hashimoto K, Ueki T, Watanabe M. Phys. Chem. Chem. Phys., 2018,
20, 25123–25139.

[3] Ueki T, Watanabe M. Chem. Lett., 2006, 35, 964–965.