The nature of a conducting polymer's intra- and intermolecular structure and the associated structural phase behavior are fundamental issues which strongly impact the physical properties manifested by this unique class of materials. Even relatively small changes in the specific chemical architecture and/or processing procedure can lead to significant variations in the resultant structural forms and in their physical properties. Ultimately a deeper understanding of the various structure-property interrelationships will, in part, form the foundation for future efforts that require even more specialized conducting polymer structures with highly specific properties.
The actual term, structure, may be used to describe the intrinsic unit construction on a myriad of length scales. Although the hierarchal organization of conjugated polymer structure (and polymers in general) is an exceedingly important and complex issue, it is beyond the limited scope of this chapter. For brevity and conciseness, the text that follows will be restricted to reviewing the structure and structural response of prototypical conducting polymer host systems at molecular length scales (ranging from approximately 2Å to 200Å). Even within this rather narrow range there is an immense diversity with respect to the structural forms and phase behavior.
Conducting polymers have some similarities to conventional polymeric materials,
but, it is clearly the extensive main chain -conjugation and its implicit
``stiffness'' with respect to chain bending and twisting that most influences
the overall physical behavior. As a direct consequence virtually all linearly
unsubstituted conducting polymers, as shown in Fig. 1, are found to be
intractable and infusible. These model systems also tend to form crystalline
phase structures with many common features. Hence these compounds may be
conveniently lumped together to form one basic ``class'' of conducting polymer
materials that have, loosely speaking, similar structural characteristics.
In the quest for improved performance, better processibility and novel applications a wealth of newer compounds with specific chemical architectures have been synthesized. The three most common approaches which have been implemented are: greater main chain flexibility (e.g., polyaniline), side chain substitution (e.g., poly(3-alkylthiophenes)), and fabrication from a soluble precursor polymer (e.g., poly(p-xylene--dimethylsufonium chloride) to yield poly(p-phenylene vinylene)). All of these modifications can produce materials with a range of structural forms and, in some instances, striking new physical behavior.
In one conducting polymer, polyaniline (PANI), the added main chain flexibility surrounding the amine/imine nitrogen linkages is ostensibly responsible for a myriad of structural effects. There is an overall reduction in the effective intrachain conjugation length and a marked decrease in the tendency to form crystalline phases. This also implies, indirectly, that there are implications for the electronic transport properties that may be achieved. However within amorphous films there is a unique ability to control the conducting polymer molecular matrix and, concomitantly, the mass transport properties so that high performance separation membranes may be fabricated.
Side chain substitutions are now routinely used to enhance solvent solubility and fusibility so that various conventional polymer processing methods may be utilized. Some of these polymeric materials, e.g. poly(3-alkylthiophenes), may contain only a small volume fraction of electrically active regions (i.e., the -conjugated main chain). Still these materials can exhibit extremely high d.c. conductivities after doping (or more precisely, intercalation) by a guest specie. This intercalation process can also provoke significant dimensional changes in the molecular unit construction. This latter property enables the fabrication of novel self-structuring devices.
Even more exotic synthesis and processing procedures have further expanded the horizons within the structure/property interrelationships and the potential for new applications. Langmuir-Blogett monolayer film deposition techniques have been applied successfully to yield thin film multilayer heterostructures for use as electronic devices. Chemical coupling of various ion selective crown ethers can create conducting polymer hosts in which the extent of main chain -conjugation is directly influenced by the solution concentration of a specific ion when immersed. In all these examples it is often the subtle interplay between the molecular level structural ordering and the electroactive nature of the conducting polymer host which gives rise to these properties. Hence an intimate knowledge of the structure and its structural phase behavior is an integral component of conducting polymer research.