Conducting Polymers: Bioinspired Intelligent Materials and Devices

by Toribio Fernandez Otero

Overview

Conducting polymers are organic, conjugated materials that offer high electrical conductivity through doping by oxidation and a wide range of unique electromechanical and electrochromic characteristics. These properties can be reversibly tuned through electrochemical reactions, making this class of materials good biomimetic models and ideal candidates for the development of novel flexible and transparent sensing devices.

This book comprehensively summarises the current and future applications of conducting polymers, with chapters focussing on electrosynthesis strategies, theoretical models for composition dependent allosteric and structural changes, composition dependent biomimetic properties, novel biomimetic devices and future developments of zoomorphic and anthropomorphic tools.

Written by an expert researcher working within the field, this title will have broad appeal to materials scientists in industry and academia, from postgraduate level upwards.

Product Details

ISBN-13:	9781782623151
Publisher:	RSC
Publication date:	11/26/2015
Series:	Smart Materials Series , #19
Pages:	304
Product dimensions:	6.15(w) x 9.20(h) x (d)

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CHAPTER 1

Life, Bioinspiration, Chemo-Biomimesis and Intelligent Materials

1.1 Introduction

Life is chemistry, but current chemical models, developed from reactions taking place in the gas phase or dilute solutions, are unable to describe life and life functions. Scientists are concerned with the development of theoretical models for the description of life functions, predicting health and diseases, and advancing the different ways for health restoration, even before the emergence of new diseases. In parallel, the progressive development of a new technological world constituted by devices and tools working, as biological organs do, driven by the chemical reactions of the device's constitutive materials should be expected.

1.2 Basic Hypotheses

• Life and life functions originate from chemical reactions taking place in the dense gel of the intracellular matrix (ICM) in living cells.

• Most of the reactions sustaining both life and life functions (such as walking, memory, thinking or consciousness) involve complex organic molecules (enzymes, proteins, nucleic acids and so on) as reactants.

• Biochemical reactions induce specific and convoluted conformational movements (allosteric effects, folding or unfolding) of the organic reactants and, when required for charge and osmotic balance, the exchange of water and ions with the surroundings.

• Current physical or chemical kinetic models do not include any conformational or structural movements induced by reactions.

• Current chemical models were developed from reactions taking place in the gas phase at low pressure, or in solutions using very dilute reactants.

• Gas phase and dilute solutions are far away from the dense reactive gel structure constituting the ICM of living cells.

• Any attempt to describe life and life functions requires advanced chemical models attained from systems where at least one of the reactants is a complex molecule or carbonaceous structure making up part of the dense gel.

• The gel reaction must drive conformational movements of the reactant polymer or macromolecule, and the simultaneous exchange of ions and water.

1.3 Bioinspiration, Biomimesis, Chemo-Biomimesis, Intelligent Materials and Systems

Evolution may be considered the most powerful engineering tool or form of designer machinery that has worked for billions of years to create a plethora of efficient molecules, reactions, structures, cells, organs, functions, systems and beings. These evolutionary molecules have been, and will continue to be, the inspiration of the human species to develop tools, devices, structures, arts, science and technology.

In this context, terms such as bioinspiration, biomimicry or biomimesis, chemo-biomimesis, intelligent materials and intelligent systems appear with rising frequency in scientific papers. Their widespread use has resulted in different meanings when used by different authors and speakers. At this stage, the best possible clarification for some of these concepts may be given by a reference defining the characteristics of the inspiring top-level biological organs or functions and those attained by the new biomimetic material, device or structure.

Bioinspiration: learning from nature's macroscopic, microscopic or molecular structures and being inspired by them to try to adapt physical or mechanical structural efficiency using different materials and scales to solve human problems.

Biomimicry or biomimesis: construction of new tools, devices or robots mimicking some biological physical functions from the extracellular matrix (ECM) of living cells.

Chemo-biomimesis, chemo-biomimicry, electro-chemo-biomimicry or electro-chemo-biomimesis: construction of new chemical- or electrochemical-driven tools, devices or robots mimicking biological functions (from walking or proprioception to consciousness or brain memory) generated by chemical or electrochemical reactions in the ICM of living cells.

Intelligent materials: the most intelligent materials and systems come from nature and are, simultaneously, actuators, sensors and self-healing, e.g., haptic muscles. Artificial intelligent materials are as intelligent as they fit most of these biological characteristics. For those covering the same number of characteristics the most intelligent are those getting the highest efficiency.

In this context the present book is mainly concerned with the exploration of the incipient electro-chemo-biomimetic and chemo-biomimetic scientific and technological space. The emerging scenery is based on new reactive dense gel materials that can mimic, in its simplest expression, the contents of the ICM of living cells from biological organs, including reactive macromolecules, reaction-driven conformational movements, ions and water (or solvent).

A basic, important and differential point related to metals and inorganic materials is that, despite deep change of the material composition (polymer/ ion ratio) during reactions, a relatively low variation of the mechanical consistency is observed, thus maintaining the material's integrity.

Similar composition variations by several orders of magnitude can only be observed inside the functional cells of biological organs when they pass from rest to work states. Reactions involving these artificial dense gels promote variation of the gel (polymer/ion) composition. These material properties, the values of which change with the material composition, are named composition-dependent properties. Parallel variation during reactions involving the composition-dependent properties mimics biological functions. The development of a new technological field giving new chemo-biomimetic and electro-chemo-biomimetic devices and envisaging new tools and robots based on those biomimetic properties is emerging. The state of the art will be presented here.

In parallel, such reactive gels, the reactions of which drive the conformational movement of the reacting polymer chains and structural macroscopic changes, can be taken as a new system and reaction model. New theoretical tools will allow exploration of unknown fields beyond the borders of chemical kinetic models discussed in current chemical, biochemical and biological textbooks. The quantitative inclusion of these reaction-driven conformational and structural changes in present chemical models should result in more advanced structural chemical kinetic models. The theoretical description and quantification of reaction-driven conformational changes can allow the subsequent theoretical description and quantification of biochemical reactions, life, life functions, health and diseases.

1.4 Available Reactive Materials

During the last 30 years a plethora of new redox organic molecules and carbon-based structures has been discovered: conducting polymers (CPs), redox polymers, viologens, porphyrins, phthalocyanines, fullerene derivatives, carbon nanotubes, graphenes and so on. Most of them form electronic conducting films, or can be supported by polymers forming electronic conducting films. These conducting films can be used as self-supported film electrodes in liquid electrolytes for the study of their redox reactions.

Regarding a general description of the available reactive materials, this book focuses on CPs. However, concepts, reactions, properties, devices and theoretical models here presented can be translated to any of the different reactive materials mentioned above and those that have yet to be discovered, as well as biological reactions.

1.5 Intrinsic CPs

Monomeric units linked by σ bonds (insulating chain; Figure 1.1a) constituting any neutral chain (i.e., not charged) can be named an intrinsic CP. By oxidation or reduction, polaronic (radical anions or radical cations, using chemical terminology) p-conjugated structures, involving several monomeric units (Figure 1.1b), are formed along every chain. These conjugated structures allow the flow of electrons across the material under potential gradients. In partially oxidized or partially reduced films of CPs, electrons can jump between neighbor polaronic structures from the same chain (intrachain jumping) or take part in different chains (inter-chain jumping). Both individual chains and films become electronic conductors. The electronic conductivity increases under the control of the chain oxidation or reduction state, which determines the number of polarons per chain and the polaronic concentration (carrier concentration) in the film. More polarons per chain and a higher polaronic concentration mean shorter distances for electronic jumps, lower electrical resistance and higher conductivity. The electronic conductivity is an intrinsic property of the polymer chain, being controlled by its oxidized/reduced state.

Obtaining the neutral state of the polymer chains in films becomes, as will be explained in this book, a difficult task. This means that obtaining insulating films becomes almost impossible, hence the name conducting polymers.

1.5.1 Available Material Families

The literature presents thousands of CPs, which represent only a tiny fraction of the possible CPs that will be synthesized during the coming decades. To try to clarify the field of this intricate forest, a basic monomeric unit can be taken as a reference. Every monomeric unit can give a family of polymer–ion compounds of the basic CP attained by polymerization and seven derived families of intrinsically CP materials. Thus, each basic monomer can give eight different families of intrinsically conducting materials.

• Basic CP compounds may be obtained by polymerization of each of the simplest monomers in the presence of different salts: polyacetylene, polyaniline, polypyrrole, polyindole, polyfurane, polycarbazole and so on. The synthesis generates a salt polymer anion. Different polymeric compounds are generated by polymerization of the same monomer in the presence of different anions.

• Substituted CPs are any basic monomer including several hydrogen atoms, each linked to a carbon atom, with substitution of one, two or three of those hydrogen atoms (two must always remain unsubstituted in order to allow the subsequent polymerization) by an organic group.

The substituted monomer gives, by polymerization, a substituted CP. Long organic substituents induce steric polymerization limitations giving short or very short oligomers.

The substituents can be selected (electron donors, electron acceptors, long and flexible, long and rigid etc.) in order to modify, related to the basic polymer, the physical (optical, electrical, mechanical) or chemical (electrochemical potentials, chemical stabilization) properties of the resulting polymer.

• Self-doped CPs appear when the substituent is an organic salt with the organic anion covalently linked to the monomer balanced by a small cation; the resulting polymer is an intrinsically conducting polyelectrolyte, where each monomeric unit is a salt of an organic monomeric anion and a cation. By electrochemical oxidation of the polymer, the generated positive charges in the chain are compensated by its own anions, forcing the exit of the cations (cation-exchange materials).

• Copolymers can be made by a combination of monomers (substituted or not), dimers or trimers, each including two or three different monomeric units. Their polymerization gives random or alternative copolymers with some specific properties, such as faradaic electrodis-solution. Dimers or trimers attained by the combination of basic and substituted monomers can allow broad control of the polymer's physical and chemical properties.

• CP–organic macroion blends require the synthesis of CP films in the presence of large organic counterions (organic salts or organic acids, polyelectrolytes, ionic liquids) to give blend materials of the CP with the organic macroion. The electro-generated, positively charged chains of the CP are compensated by the negative charges of the organic macroion that are incorporated and trapped inside the growing polymer material. The trapped macro-anion will force the exchange of balancing cations during the subsequent oxidation/reduction of the material.

The large macroion can be selected to provide the resulting CP with some specific physical or chemical properties. When the CP is mixed with commodity polymers or other organic molecules the result is also known as a CP blend.

• CP–inorganic macroion hybrids requires the electrosynthesis of CPs in the presence of large inorganic macroions, such as polyoxometallates, giving CP–macroion hybrid materials. The macroion is selected to provide the CP with some magnetic, electrical or electrochemical properties. Most of the polyoxometallates are also electroactive.

• Composites involve mixing CPs with particles or nanoparticles of different materials (carbon, carbon nanotubes, fullerenes, graphenes, commodity plastics, semiconductors, biological materials, metals and so on) to give conducting composite materials.

• CP salts. Each of the above families can be oxidized (or reduced, see Chapter 4) in the presence of electrolytes, generating a new polymer salt:

[MATHEMATICAL EXPRESSION OMITTED], (1.1)

where Pol represents any polymeric chain in the film and A- is the anion solved in water. Being a reversible reaction, when the polymer is translated into a different electrolyte with a different anion, after some consecutive oxidation/ reduction cycles, the first anion can be completely exchanged by the new anion and the oxidized material is a new CP–anion compound.

The procedure can be repeated as many times as new compounds are required. A similar procedure can be applied with different cations for CPs exchanging cations (Section 4.3) with the electrolyte. In a similar way to metal atoms that can give different inorganic and organic salts, each constituting a different material, every CP can form hundreds of different compounds through very easy, soft and clean electrochemical procedures by substituting the charge balance anion or cation by a new one.

Figure 1.2 shows the basic molecular structure corresponding to seven of the eight polypyrrole families.

1.6 Biomimetic Reactive Gels

Films of CPs can be used as freestanding electrodes or as film coatings for metal electrodes in liquid or solid electrolytes. The flow of anodic or cathodic currents promotes the oxidation or reduction, respectively, of the film [eqn (1.1)]. By oxidation/reduction, electrons are extracted or injected from or towards, respectively, each polymer chain constituting the film, generating positive or negative charges. The reaction of the material only occurs under charge balance. Emerging positive (oxidation) or negative (reduction) charges on any film chain forces the simultaneous entrance of ions from the electrolyte to keep the film's charge balance: the material becomes a polymer–ion (polyelectrolyte) compound (Figure 1.3). The presence of charged chains and balancing counterions in the film forces the exchange of water for osmotic balance: the film becomes a dense gel. In order to lodge balancing ions and solvent, the volume of the film increases (swelling), driven by the reaction.

In summary, under the reaction conditions the film becomes a swelling/ shrinking dense electroactive gel (Figure 1.4), the of composition of which mimics, in its simplest expression (reactive macromolecules, ions and water), the ICM of living cells. These dense gels usually include low water (or solvent) percentages (5–20%) compared with the usual polyacrylamide gels, which can have water contents of greater than 95%.

CHAPTER 2

Electrochemical Methods

2.1 Introduction

This book treats different aspects of the unexplored scientific borders between chemistry, electrochemistry, polymer science, biochemistry, biology, the behavioral sciences and mechanics. Conducting polymers (CPs) may be synthesized by electrochemical methods, which will be discussed here. The chemo-biomimetic properties, functions and devices, as well as the mimicked biological functions and organs, studied and described here include electrical (electronic or ionic) pulses linked to chemical reactions in CPs. Electrochemical textbooks cover links between electrical signals and chemical reactions.

In this context, basic electrochemical methodologies are expected to play a central role to attain our aims. The most common methodologies used for the study of the electrosynthesis, the material's oxidation and reduction reactions, and for the actuation of the final electrochemical devices are linear potential sweeps or linear potential cycles, potential steps and consecutive square potential waves, the flow of constant currents and consecutive square current steps. A brief description of both methods and the electrochemical cells used will be presented here. For a deeper study, some excellent electrochemical textbooks (on methods and kinetics) may be consulted.

(Continues…)

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Excerpted from "Conducting Polymers"
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Copyright © 2016 Toribio Fernández Otero.
Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents

Life, Bioinspiration, Chemo-Biomemesis and Intelligent Materials;
Electrochemical Methods;
Electrosynthesis of Conducting Polymers;
Gel Membrane Electrodes: Electrochemical Reactions;
Membrane Composition-Dependent Electrochemical Properties;
Reaction-Driven Conformational, Allosteric and Structural Changes;
Conformational. Allosteric and Structural Chemistry: Theoretical Description;
Electro-Chemo-Biomimetic Devices;
Multi-Tool Devices Mimicking Brain-Organ Intercommunication;
Final Comments and Challenges