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An Introduction to Electroactive Polymers

Sobi K. Chacko 1, Raneesh Balakrishnan 2* and Visakh P.M. 3

1Department of Physics, NSS Hindu College Changanassery, Kerala, India

2Department of Physics, Catholicate College, Pathanamthitta, Kerala, India

3Natural Bioactive Materials Laboratory, Department of Bioengineering, Ege University, Bornova/Izmir, Turkey

Abstract

This chapter provides a comprehensive introduction to electroactive polymers (EAPs), a class of smart materials that exhibit electromechanical responses to external stimuli, such as electric fields, or mechanical stress. EAPs are broadly classified into two main categories: ionic and electronic EAPs, based on the mechanisms underlying their electromechanical behaviour. This chapter explores the classifications and fundamental properties of EAPs, including their electrical, mechanical, and thermal behaviors, highlighting their unique characteristics, such as flexibility, biocompatibility, and ease of processing. The chapter also briefly discusses various applications in which EAPs play a vital role, such as in sensors, actuators, artificial muscles, energy harvesting systems and flexible electronic devices. This overview aims to provide a foundational understanding of electroactive polymers, laying the groundwork for the further exploration of their diverse applications in advanced technologies.

Keywords: Electroactive polymers, smart polymers, piezoelectric polymers, dielectric elastomers, stimuli responsive, ionic polymer, electronic polymers

1.1 Introduction

Electroactive polymers (EAPs) represent a class of electroresponsive materials with a remarkable ability to experience mechanical deformation or changes in shape in response to an electric field [1]. This unique property can cause significant shape or size changes in response to an electric field, known as electroactive actuation, and distinguishes EAPs from traditional polymers, making them highly attractive for a wide range of applications in fields, such as robotics, biomedical engineering, and smart materials [1-3]. EAPs have gained much interest in recent decades because of their unique properties and advantages over conventional ceramic actuators, such as flexibility, light weight, large deformation, responsiveness to various stimuli, biocompatibility, structural simplicity, low acoustic noise, low cost, scalability, and enhanced dielectric constant [4-7]. The properties and performance of EAPs depend on various factors, such as the chemical structure, morphology, fabrication method, environmental conditions, and applied stimuli. This ability makes them unique, leading to various potential applications in different fields including soft robotics, electrolytes, sensors and actuators, supercapacitor applications, energy harvesting and storage, smart textiles, tissue engineering, and wound healing [1, 3, 8-10]. EAPs can be categorized into two main types based on their actuation mechanisms: ionic EAPs (e.g., ionic polymer-metal composites and conductive polymers) and electronic EAPs (e.g., piezoelectric polymers, electrostrictive polymers, and dielectric elastomers). Each type offers distinct advantages and challenges depending on the specific application requirements. This chapter explores the underlying principles of electroactive polymers and their classification based on mechanisms, fundamental properties, recent advancements, and promising applications [1, 3, 11]. By understanding the unique capabilities of EAPs and their potential impact on emerging technologies, we can unlock new possibilities for innovation and design in responsive materials.

Table 1.1 outlines the key milestones in the development of electroactive polymers. The journey of EAPs began in 1880, when Wilhelm Röntgen conducted an experiment to test the effect of an electrostatic field on the mechanical properties of a strip of natural rubber [12]. But it did not directly involve the discovery of electroactive polymers as we understand them today. The piezoelectric response in natural rubber is fundamentally different from the engineered electroactive behaviors seen in synthetic polymers, which typically exhibit stronger and more tunable properties due to their crystalline structure and dipole alignment. Later, in 1899, Sacerdote demonstrated that a rubber band can produce a strain response when activated by an electric field [13, 20].

The discovery of electrets, a class of dielectric materials capable of retaining quasi-permanent electric charge or polarization, is an important milestone in the field of electroactive materials. The initial discovery was credited to Motojiro Eguchi, a Japanese physicist who, in 1925, found that certain dielectric materials could hold an electric field-induced charge after being heated and cooled in the presence of a strong external electric field [14]. Eguchi's groundbreaking work, published in 1925, described the behavior of waxes and other materials that exhibited permanent electrical polarization, which he termed "electret." By subjecting these substances to high electric fields, Eguchi demonstrated that they could retain a stable electric charge even after the field was removed, and the material was cooled to room temperature. This significant milestone has expanded the scope of electroactive materials and laid the foundation for the development of various electret-based technologies [14]. While electrets exhibit permanent electrical polarization, the discovery of modern electroactive polymers, such as piezoelectric and conducting polymers emerged much later. Although Rontgen's natural rubber experiment laid the groundwork for understanding the piezoelectric effect, the discovery of electroactive polymers as synthetic materials with specific electromechanical properties occurred later. Polyvinylidene fluoride (PVDF) was discovered to exhibit piezoelectric behavior in 1969 by Tadashi Kawai [15]. PVDF is one of the earliest and most well-known electroactive polymers, owing to its piezoelectric properties [21, 22]. This discovery opens up new possibilities for the use of EAPs in various fields [1, 23, 24].

Table 1.1 Key milestones in the development of electroactive polymers.

Event Year References Wilhem Roentgen observed the electrically-induced deformation in a rubber sheet. 1880 [12] Extended study on the relation between Electrical stimuli and Strain response by Sacerdote. 1899 [13] The first piezoelectric polymer was discovered by Eguchi-Electret. 1925 [14] Discovery of piezoelectric response in PVDF by Kawai. 1969 [15] Development of conductive polymers by Alan MacDiarmid, Alan Heeger, and Hideki Shirakawa. 1977 [16-18] Introduction of dielectric elastomer actuators (DEAs) by Pelrine et al. 1998 [19] The first EAP Actuators and Devices (EAPAD) Conference by Bar-Cohen. 1999 [20]

In the 1970s, remarkable advancements occurred in chemistry with the discovery of a novel category of polymers that demonstrated significant electronic conductivity when partially oxidized. This surprising innovation was credited to the joint work of three distinguished scientists, Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa [16-18]. Their remarkable contributions led to the Nobel Prize in Chemistry being awarded to them in 2000. This pivotal research has opened up new possibilities for developing conducting polymers that hold immense potential for diverse applications across various fields.

1.2 Types of EAPs

In accordance with their operating mechanisms and working principles, electroactive polymers are fundamentally categorized as electronic/dielectric/dry polymers and ionic/wet polymers [3, 11]. Subsequently, the primary classifications were further stratified into discrete subcategories.

1.2.1 Piezoelectric Polymers

Piezoelectric polymers can generate an electrical charge when they experience mechanical stress or deformation. This fascinating process, called the direct piezoelectric effect, occurs because of the unique asymmetrical arrangement of the molecular dipoles within the polymer structure. Understanding this phenomenon opens exciting possibilities for various applications, such as sensors and energy harvesting technologies. When subjected to mechanical forces, such as compression or stretching, the dipoles become reorient, resulting in the accumulation of electric charge. Conversely, an inverse piezoelectric effect occurs when an electric field is applied to induce mechanical deformation in the polymer [22, 25]. There are several remarkable materials with diverse applications, including PVDF and its co-polymers, nonbiodegradable materials, and poly (l-lactic acid) (PLLA), which are biodegradable and often used in medical applications [21, 22, 26-28]. Other notable materials include cellulose, a natural polymer derived from plants that is eco-friendly and versatile;...