Polymer electrodes classes

PPV-based devices are commonly constructed in a multilayer structure in which the polymer film is sandwiched between charge injection electrodes. For this reason, the polymer/electrode interface plays an important role on the performance of the devices and will be discussed for different classes of materials in the next sections. 

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

It was also observed that, with the exception of polyacetylene, all important conducting polymers can be electrochemically produced by anodic oxidation moreover, in contrast to chemical methoconducting films are formed directly on the electrode. This stimulated research teams in the field of electrochemistry to study the electrosynthesis of these materials. Most recently, new fields of application, ranging from anti-corrosives through modified electrodes to microelectronic devices, have aroused electrochemists interest in this class of compounds... 

This volume combines chapters oriented towards new materials with chapters on experimental progress in the study of electrochemical processes. G. E Evans reviews the electrochemical properties of conducting polymers, materials which are most interesting from a theoretical point of view and promise to open up new fields of application. His approach gives a survey of the main classes of such polymers, describing their synthesis, structure, electronic and electrochemical properties and, briefly, their use as electrodes. 

The desire to realise technological goals has spurred the discovery of many new solid electrolytes and intercalation compounds based on crystalline and amorphous inorganic solids. In addition an entirely new class of ionic conductors has been discovered by P. V. Wright (1973) and M. B. Armand, J. M. Chabagno and M. Duclot (1978). These polymer electrolytes can be fabricated as soft films of only a few microns, and their flexibility permits interfaces with solid electrodes to be formed which remain intact when the cells are charged and discharged. This makes possible the development of all-solid-state electrochemical devices. 

The temperature dependence of the conductivity of the various classes of polymer electrolyte discussed above is summarized in the Arrhenius plots in Fig. 7.23. While a wide choice of materials is now available, it is important to note that improvements in conductivity are generally accompanied by losses in chemical stability and by increases in reactivity towards the lithium metal electrode. Successful development of rechargeable LPBs is therefore likely to be linked to the use of the so-called dry polymer electrolytes, namely pure PEO-LiX systems. This necessarily confines the operation of LPBs to above ambient temperatures. This restriction does not apply to lithium ion cells. 

The third class of polymers used to prepare chemically modified electrodes is the electronically conductive polymers [25]. The polymer chains in this family of materials are themselves electroactive. For example, the polymer redox reaction for polypyrrole (Table 13.2) can be written as follows ... 

Research into chemically modified electrodes has led to a number of new ways to build chemical selectivity into films that can be coated onto electrode surfaces. Perhaps the simplest example is the use of the polymer Nafion (see Table 13.2) to make selective electrodes for basic research in neurophysiology [88]. Starting with the pioneering investigations by Ralph Adams, electrochemists have become interested in the electrochemical detection of a class of amine-based neurotransmitters in living organisms. The quintessential example of this class of neurotransmitters is the molecule dopamine, which can be electrochemically oxidized via the following redox reaction ... 

The size, cost, and accuracy requirements for successful sensors place a substantial burden on the processes used to manufacture them. Typically, today s sensors are manufactured by processes that are specific to one sensor or, at best, to a limited class of sensors. This process specificity has occurred because each type of sensor is usually made of unique materials in unique configurations that best convert the quantities to be measured into electrical signals. For example, pressure transducers may use piezoelectric or piezoresistive materials on thin diaphragms, whereas ion-selective electrodes use ion-conductive glasses or polymers around electrodes. Unfortunately, this situation implies large developmental costs. 

Figure 3.9 illustrates the electrochemical and mass transport events that can occur at an electrode modified with a interfacial supramolecular assembly [9]. For monolayers in contact with a supporting electrolyte, the principal process is heterogeneous electron transfer across the electrode/monolayer interface. However, as discussed later in Chapter 5, thin films of polymers [10] represent an important class of interfacial supramolecular assembly (ISA) in which the properties of the redox center are affected by the physico-chemical properties of the polymer backbone. To address the properties of these thin films, mass transfer and reaction kinetics have to be considered. In this section, the properties of an ideally responding ISA are considered. 

---