Development of transparent and flexible actuators with ionic liquid-based electroactive polymers

Abstract

The objective of this research is to develop new ionic electroactive polymers (iEAPs) with enhanced light transmittance and electroactive performance. iEAPs actuated in the open air often have strip-like electrode-electrolyte-electrode sandwich structures and are an emerging class of actuators due to their lightweight, low actuation voltage and high output strain. Unfortunately, it has been challenging to fabricate transparent iEAPs because of the difficulties of achieving polymeric electrolyte materials with good compatibility and producing transparent electrodes with good conductivity. In this study, a transparent iEAP based on polyvinyl alcohol (PVA), a long alkyl chain ionic liquid (IL, C10MIMCl), and polythiophene electrodes (PEDOT: PSS) has been successfully developed with improved electromechanical performance. The above challenges were overcome based on three main contributions: (1) development of a novel polymeric electrolyte composed of PVA and C10MIMCl with high compatibility and transparency, (2) a deeper understanding of the anion-controlled actuation mechanism for determining suitable electrolyte material compositions, (3) a facile fabrication method for assembling thin transparent PEDOT: PSS electrodes with enhanced conductivity. To achieve the research goal, a polymeric PVA/C10MIMCl electrolyte with 96.5% light transmittance was firstly developed, and intriguing anion-controlled actuation was discovered. The actuation mechanism is ascribed to the hydrogen bonds formed between the polymer chains and the IL cations (C10MIM+), allowing the IL anions (Cl-) to migrate within the polymeric electrolyte upon receiving an applied voltage. Moreover, harnessing the inherent hygroscopicity of the C10MIMCl, electrolyte flexibility is further enhanced, and widened ion transport channels are created for faster migration, which leads to a larger volume difference between the anode and cathode sides of the electrolyte. These synergistically overcome the negative effects of ILs, i.e., high viscosity and self-assembly resulting in slower ions migration, on the electromechanical response of the iEAP. As a result, improved actuation performance can be achieved. A semiautomatic computational method based on the B-spline interpolation and the circle regression algorithm was also established to facilitate a more reliable method to evaluate the actuation performance of the strip actuators. Actuators made of the as-prepared electrolyte with gold electrodes show an angular deflection per unit length as high as 40.3 m-1. Secondly, a facile film transfer technique was developed to assemble self-supporting transparent PEDOT: PSS electrodes on the PVA/C10MIMCl electrolyte for fabricating the transparent iEAPs. This method overcomes the common processing difficulties, including swelling of PVA in aqueous solutions and the agglomeration and precipitation of PEDOT: PSS suspension in ionic solutions. The PVA/IL/PEDOT: PSS iEAPs show high optical transparency of 90.2% and improved electromechanical output at voltages as low as 0.5 V, due to the enhanced electrodes conductivity after thermal and acid treatments, and the ion insertion due to redox reactions of polymer electrodes during actuation. This study significantly contributes to the development of a new type of transparent soft actuator that can be actuated at low voltages. The new knowledge and skills discovered can facilitate future research on smart structural design of iEAPs for broadening the applications in advanced biomedical devices such as bionic eyes, endoscopes, and artificial corneas.