Reversible Silver Electroplating and Electrostripping in the Gel Electrolyte for Smart Mirror
Authors: Furukawa Tomoki , Aoki Atsushi 
Published: December 2020

The smart glass which can be switched between the colored state and the transparent state has been widely investigated for control of light intensity and thermal energy saving. However, it has a drawback that the colored portion absorbs light and radiates heat inside. Recently, the smart silver mirror has been reported by Kobayashi [1] and our group [2], which can switch between the mirror state and the transparent state reversibly. the electroplated silver metal mirror state has an advantage to reflect light without radiating heat to the inside. From this point, the smart silver mirror would be expected as an alternative to the smart glass. Since the electrolyte solution has been used in the smart silver mirror devices reported so far, the gelation of the electrolyte solution is needed in order to eliminate the possibility of liquid leakage and evaporation. We reported that addition of polymethyl methacrylate (PMMA) into the plating electrolyte solution didn’t affect the device performance even when the diffusion rate of silver ions became slow due to increase in viscosity. In this report, crosslink points were created by modifying both ends of PMMA with carboxyl groups as a functional group. The crosslinkage of the plating electrolyte solution three-dimensionally has been performed by oxazoline.The plating electrolyte solution used in this experiment was prepared as follows. First, citric acid monohydrate and silver nitrate were dissolved in the solvent acetonitrile and 1-ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide in the same amount as acetonitrile. The telechelic polymer having functional groups at both ends was synthesized by the following method. [3] Methyl methacrylate (MMA) as a monomer, 4-cyano-4-(thiobenzoylthio) pentanoic acid as a raft agent and 4,4'-azobis (4-cyanovaleric acid) (ACVA) as an initiator were reacted in dry DMF solvent at 80 ℃. After reacting for 6 hours, a product having a carboxyl group at one end was obtained. The resulting polymer was reacted with ACVA in dry DMF solvent at 80 ℃. After reacting for 24 hours, a telechelic polymer having a carboxyl group at both ends was obtained. The crosslinking reaction was tried by adding a compound having an epoxy group or an oxazoline group into the plating electrolyte solution including the telechelic polymer. The polymer with carboxyl groups at both ends was reacted with triglycidyl isocyanurate at room temperature for 24 hours and at 70℃ for 4 hours, but the reaction did not proceed. Instead of epoxy group, the coupling reaction between the oxazoline group and the carboxyl group was carried out. 1,3-bis(4,5-dihydro-2-oxazolyl) benzene was reacted at 75℃ for 4 hours. The state of the gelled solution after the reaction was confirmed by upsetting the sample bottle. Comparing with the states before and after the coupling, the viscosity increased significantly after the coupling. It shows that the coupling reaction between the oxazoline group and the carboxyl group occurred. However, after a while, the solution had flowed through the wall of the sample bottle and was not completely gelled. This is because there are only two crosslinking points with oxazoline. If the amount of the polymer addition further increases, it becomes a highly viscous liquid before the reaction. The high viscosity makes it difficult when the solution is injected into the device using a needle. Therefore, the viscosity before the reaction is required to keep low. In the present, we will try to synthesize trioxazoline having three oxazoline groups and perform electrochemical measurement in the plating bath after crosslinking.

Reference

[1] S. Araki, et al, Adv. Mater., 24, OP122(2012).

[2] A. Aoki et al, Sol. Energy Mater. Sol. Cells, 200, 109922(2019).

[3] N. Greving, et al, Macromol. Chem. Phys., 213, 1465(2012).