Sulfonated PEEK-based IPMC actuators: Exploring environmental influences

Abstract

Ionomeric Polymer-Metal Composites (IPMCs) with a layered metal/polymer/metal structure are innovative biomimetic materials widely used in applications such as actuators, sensors, artificial muscles, and robotics due to their dynamic response to electrical stimuli. Nafion® is commonly used for these applications but is associated with high costs, low water retention, and poor performance at elevated temperatures. To address this, sulfonated poly(ether ether ketone) (SPEEK) is a promising and cost-effective alternative. This study focuses on evaluating the viability of SPEEK in IPMCs under varying environmental conditions, specifically a wide range of relative humidity (RH) levels (30%, 60%, and 90%) and two different counterions (H+ and Li+). FTIR tests confirmed the successful sulfonation of SPEEK, and sulfonation degrees (SD) of 44%, 72%, and 91% were determined by titration. Results indicate that increased SD leads to higher hydration levels, enhancing performance as actuators. Regarding electromechanical performance, SPEEK-based IPMCs demonstrated results comparable or superior to Nafion. Higher RH positively influenced electromechanical performance, increasing displacement and displacement rate values. Additionally, electrochemical/mechanical tests confirmed that, for the H+ counterion, SPEEK exhibited superior performance. In conclusion, SPEEK-based IPMCs, especially with H+ as the counterion, can potentially replace Nafion, offering a cost-effective alternative with favorable electromechanical performance under varying environmental conditions.