Effects of Block Ionomer Morphologies on the Electromechanical Performance of Ionomeric Polymer-Metal Composite Actuators
Authors: Zheng Wenjian , Cornelius Chris 
Published: February 2020

Ionomeric polymer-metal composites (IPMCs), which is a type of electroactive polymers, can generate a large and fast bending actuation under a low operating voltage (1~5V) (1). This superior electromechanical performance, combined with the inherent nature of polymer such as lightweight and softness, makes them attractive for biomimetic devices such as artificial muscles and soft robots (2). IPMCs have a tri-layer structure that consists of an ionomer film incorporating ions and water (or ionic liquids), and electrodes plated on both film surfaces (1-4). Upon applying a voltage, water and ions move toward the oppositely charged side leading to unsymmetrical polymer swelling. This change is restrained within the polymer’s nonionic matrix resulting in asymmetric stress that drives the bending motion. Advanced electromechanical performance of IPMCs relies on the ion and water transport ability and mechanical integrity of the used ionomer (3-5).

In this work, the electromechanical performance of water-based IPMC actuators was examined as a function of the phase-separated morphology of a sulfonated penta-block ionomer (SBI). The SBI films---cast from a range of nonpolar/polar solvent blends---had a series of morphologies varied from poorly-connected ionic domains to well-connected lamellar/hexagonal structures. The poorly-connected structure led to a slow and small actuating response due to the small through-plane ion conductivity (1.5 mS/cm) and water permeability (1.3 × 10-6 mm2/(s MPa)). The SBI film with the lamellar/hexagonal structure however had an excessive water uptake (46wt. %) that greatly impaired the mechanical strength (53 MPa). The IPMC made from this film showed a faster and larger bending over the poorly-connected one but lost the advantage in terms of bending strain energy density. Interestingly, the best-performing IPMC is a SBI film possessing a lamella-like structure with its ionic domains perforated by nonionic polymer matrix. Its superior performance in terms of both bending displacement and strain energy density was attributed to the simultaneous improvements of ion and water transport (19.6 mS/cm and 3.1 × 10-5 mm2/(s MPa), respectively), and mechanical properties (125 MPa). Consequently, an optimal morphology exists for ideal electromechanical performance that is linked to molecule transport and ionomer mechanical properties.

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Figure 1