Temperature-Modulated Changes in Thin Gel Layer Thickness Triggered by Electrochemical Stimuli

A series of thermoresponsive hydrogels containing positively charged groups in the polymeric network were synthesized and modified with the electroactive compound 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS). ABTS, which forms a dianion in aqueous solutions, acts as an additional physical cross-linker and strongly affects the swelling ratio of the gels. The influence of the amount of positively charged groups and ABTS oxidation state on the volume phase transition temperature was investigated. A hydrogel that possesses a relatively wide and well-defined temperature window (the temperature range where changes in the ABTS oxidation state affects the swelling ratio significantly) was found. The influence of the presence and oxidation state of ABTS on mechanical properties was investigated using a tensile machine and a rheometer. Then, a very thin layer of the gel was deposited on an Au electrochemical quartz crystal microbalance with dissipation (EQCM-D) electrode using the electrochemically induced free radical polymerization method. Next, chronoamperometry combined with quartz crystal microbalance measurements, obtained with an Au EQCM-D electrode modified by the gel, showed that the size of the thin layer could be controlled by an electrochemical trigger. Furthermore, it was found that the electrosensitivity could be modulated by the temperature. Such properties are desired from the point of view construction of electrochemical actuators.