1.3.1 General goal of the work
presenting a general description of the more or less application-independent properties of the materials in order to estimate their potential for different applications with emphasis on EC characterization methods;
comparison with alternative approaches and results in scope of two distinctively different application areas – CP actuators and large surface area CP electrodes;
pointing out the novelty, the advantages and the disadvantages of the resulting materials and synthesis methods;
proposing and (re)introducing less traditional (in CP research) charac-terization methods and metrics.
1.3.2 General considerations and limitations
Where not stated otherwise, only p-doped organic CPs are considered in chapter
“Introduction” and if not specified otherwise, PPy is considered as an example.
CP synthesis methods used in the current thesis are described for PPy and PEDOT only.
CP electrodes and CP-modified electrodes (not considered in this work) are distinguished based on their role (e.g. chemical synthesis on non-conductive surfaces for the following EC deposition), and on the extent of impact to characteristics (e.g. PPy hydrogels). Metallic or non-CP conductors are considered as terminals to the main CP material. Only in situ chemically synthesized electrodes are considered, excluding e.g. electrodes spin-coated from commercial PEDOT/PSS dispersions.
Although the main emphasis of the work is on the EC methods, charac-terization methods and characterized properties are for consistency not limited to traditional EC ones, and due to the EC origin of the mechanical properties of CPs, ECMD characterization is considered as a subset of EC characterization.
Synthesis and preparation conditions were not fully optimized for any specific application or device. The phrase “novel applications” in the topic should be interpreted as reference to the application areas, where the chemical-EC synthesis method and PPy hydrogels have the most obvious advantages – CP actuators and energy storage devices.
1.3.3 Conducting polymer actuators
The studied actuators are metal-free ionic EAP-based air-operated tri-layer bending actuators and electrolyte-operated tri-layer linear actuators with PPy, PEDOT or combined electrodes. EC modification was studied as the EC synthesis on chemically synthesized CP electrodes.
ECMD characterization of the actuators does not pretend to be all-inclusive nor specific for some practical application to avoid the increase the bulk of the thesis without increasing the readability or clarity of the original material. There
are many performance characteristics important for actuators: absolute strain, reversible strain, strain rate, stability, force, energy consumption, power to weight ratio, reaction speed, mechanical properties, creep behavior, etc. The choice of the investigated characteristics depends usually on the application field considered. The focus of the current work was not the development (including optimization) of actuators for specific application or a comparison with a large number of published actuator descriptions, but rather a qualitative comparison with similar alternatives and the presentation of novel techniques for actuator preparation. The main goal of the characterization was presenting a metal-free chemical-EC synthesis routes and their implications. EC and actuation properties for different electrolytes were neither comprehensively analyzed nor optimized in the current work. Other combinations and materials not included in this work have been studied as well, but the data is in many ways incomparable to those considered in the current work. A minimal amount of unpublished data is included for the sake of integrity.
However, the most common characteristics (with emphasis on reversible strain and strain difference) have been presented, in order to give an idea about the magnitude of the characterized values and to allow to some degree the comparison with freely chosen actuator types from the literature and between separately published original articles.
For the actuators, one of the most important stability criterions in context of the current work is stability against delamination. It is not possible to claim absolute elimination of the delamination problem. Actuators were considered delamination-proof in basis that no delamination was observed during characterization of the other properties to the extent of at least several thousand actuation cycles.
1.3.4 Polypyrrole hydrogels and derived aerogels
Although in some literature sources even electrodeposited CP films, operating in liquid environment, have been regarded as dense gels [3, 4], in the current work, only sparse, jelly-like hydrogels were considered, which are sparse enough to collapse due to capillary forces on conventional drying. Four PPy hydrogels of different densities were characterized in detail and the evaluation of their EC modification was limited to additional doping, redoping and EC synthesis inside of the hydrogel.
For brevity’s sake and for better readability, “large surface area” in context of the current work means “large specific surface area at commercial level mass loadings”.
By conductivity (σ) or band gap between valence and conduction band, all materials can be roughly divided into four groups: insulators (σ < 10–7 S cm–1, band gap usually > 3 eV), semiconductors (10–4 < σ < 10 S cm–1, small band gap), metals/conductors (σ > 103 S cm–1, very small or no band gap) and super-conductors. For a long time, organic polymers were used as insulators. After the discovery of organic CPs and the development of the CP-based functionalized materials, CPs rapidly became a popular research field. New CPs are synthesized continuously and CPs find usage in an increasing number of functions. The electrical and optical properties of CPs are approaching those of metals and inorganic semiconductors, retaining many properties of the traditional organic polymers, such as the versatility of synthesis methods and processing options. Among many distinctive properties, from the perspective of the current thesis, some CP properties are more important than others. These are the conductivity, the volumetric changes on reversible reduction/oxidation, the structure, the ion mobility (charge compensation mechanism and rate) and the charge storage ability.
1.4.1 Historic background
Despite an early report about the EC synthesis of doped PAn by H. Letheby in 1862 [5], the chemical composition of the obtained material and its conductivity remained undiscovered. The first chemical oxidative synthesis of low molecular weight Py oligomers was reported in 1888 [6], and the first chemical synthesis of EC active PPy dates back to 1916 [7]. Nevertheless, for a long time organic polymers were generally considered as dielectrics, until the (re)discovery of the conductivity of chemically synthesized PPy (1 S cm–1) in 1963 [8] and of polyacetylenes by the end of 1964 [9]. The first EC synthesis of PPy film on Pt (8 S cm–1) was reported in 1968 [10]. Only in 1977 Hideki Shirakawa, Alan G.
MacDiarmid and Alan J. Heeger gained broader attention with the high conductivity (103 S cm–1) of iodine-doped polyacetylene [11] and were awarded the Nobel Prize in chemistry in 2000 “for the discovery and development of conductive polymers” [12]. Important steps in the research of CPs were the first EC synthesized freestanding, environmentally more stable PPy film [13] and the exchange of dopant anion [14]. Since then, many other characteristic properties, such as optical, magnetic, controllable hydrophobicity, mechanical, ionic etc. have been revealed and the CP landscape is rapidly developing. The distinguishable development directions are the synthesis of new CP types and composites with other materials, CP usage in different functions utilizing the different properties of the CPs, and the development of synthesis methods.