Properties of PPy

It is commonly recognized by researchers in this field that the mechanical properties of PPy s vary widely from strength and tenacity to extreme brittleness. Consequently, it is necessary to understand how the mechanical properties are affected by the chemical structure, the processing conditions, and the conditions of use (service environment). The ultimate aim is to develop and understand causal relationships between the structure of the PPy and the mechanical properties. Such relationships would enable the deliberate manipulation of the structure (e.g., by controlling the processing conditions) to produce desired mechanical properties. 

The published work on the mechanical properties of PPy is reviewed below. Most of this work has been conducted on dry films, which is discussed first. Some recent work on the adhesion of PPy to electrode materials is also presented along with the results of investigations into environmental effects on mechanical properties. 

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The polymerization conditions used to prepare PPy not only determine the polymer composition but also influence the structure of the polymer from the molecular level to the microscopic level. In this section, studies characterizing the detailed structure of PPy films, coatings, particles, and colloidal dispersions are reviewed. These studies provide the foundation for understanding the properties of PPy s, as described in Chapter 3. 

To fully optimize the material properties, the relationships between the structure and the properties must be thoroughly understood. The current state of knowledge concerning the electrical, chemical, and mechanical properties of PPy structures are reviewed in this chapter. 

FIGURE 3.6 A schematic illustration of the effect of applied potential on the ion-exchange properties of PPy s containing different counteranions (DS = dodecyl sulfate). 

The most pertinent feature in Table 3.7 is the vast range of mechanical properties that have been reported for PPy. It is apparent that the composition of the polymer (e.g., counterion type) and the polymerization conditions have a significant effect on the polymer properties. However, the relationships are not straightforward. For example, Wynne and Street113 have shown that acetonitrile solvent yields PPy films with very good mechanical properties, whereas Sun and coworkers11 and others have reported the opposite. It is clear that systematic analyses are required to elucidate the determinants of the mechanical properties of PPy s. 

Most of the previous studies have been empirical in nature. However, they have been important in demonstrating those factors that influence the mechanical properties. For example, mechanical properties of PPy films have been observed to improve as the polymerization temperature decreases. Sun and coworkers11 have observed that the tensile strength of PPy/pTS increases as the synthesis temperature... 

The key to understanding the properties of PPy films is to develop an understanding of the relationship between the structure of the polymer and its mechanical properties. Unfortunately, few studies have been devoted to gaining such an understanding. This is due, in part, to the intractable nature of PPy films that makes structural characterization difficult. 

Some information is available about the environmental effects on the mechanical properties of PPy. For most intelligent polymer systems, mechanical properties need to be stable with time in the service environment. One study has tracked the changes... 

Figures 3.11a and 3.11b show the effect of different aqueous environments on the mechanical properties of PPy/pTS. It is clear that the films become more ductile in potassium chloride and potassium hydroxide solutions. Little change in properties is noted for films in air, water, sulfuric acid, or potassium sulfate. The reasons for these changes are not clear at present, although it is known that alkaline solutions have a degradative effect on PPy.128 These processes are likely to cause a reduction in the molecular weight and/or crosslink density of the polymer, and hence increase its ductility. The sensitivity of the polymer to environmental conditions demonstrates the importance of determining its behavior in the actual service environment.

The actuation force or movement generated during redox cycling is directly related to the concomitant changes in mechanical properties. Using a simple linear elastic model of the small-strain mechanical properties of PPy, it has been shown that the actuation strain (eo) at a constant applied stress (a) is accurately predicted from Equation 3.3... 

Fully understanding the mechanical, electrical, and chemical properties of a material is vital for its successful application. The literature reports confirm that, the properties of PPy s vary widely, and are related to composition and processing conditions in a complex way. Thus, the study of the basic properties must be conducted at a fundamental level, by developing structure-property relationships. This, however, requires a greater understanding of the structure of PPy s, both at the molecular and supramolecular levels. 

Use of PAn and its derivatives as ion exchange membranes [308a-h] and stationary column materials [308i-k]. Due to more favourable ion exchange properties of PPy, more applications have been made for PPy than most other polymers. 

The work presented above has shown that the nature of the counter-anion determines both the structure and physical properties of PPy. The extent of incorporation of the counter-anion is governed by the level of negative charge on this species, as well as its size. The maximum extent of incorporation occurs in PPy Au(CN)2-. ... 

Structme and physical properties of PPy, 672 Structme of doped polythiophenes, 115 Styrenesulphonic acid, 750 Substituted bithiophenes, 93 Substituted diphenylacetylenes, 811 Substituted oligomers, 93 Substituted polyacetylene, 227. 243 Substituted poly(alkylthiophenes), 833 Substituted polyanilmes, 838. 853 Substituted polypyrroles, 819. 849... 

It has been reported that the electrical conductivity strongly depends on the ordered states of the conductive polymer [86]. In other words, the structural and electronic properties of PPy can be easily modified due to the soft lattices, which are reflected in the crystalline structure of the nanomaterials. Iron oxide nanoparticles were thought to serve as a template for the subsequent PPy matrix formation. Ultrasonic energy has been reported to have a strong capabUity to alter the electronic structure of the polymer or even produce various nanoparticles. Here, the crystalline structure change with the nanocomposite formation was investigated by selected-area electron diffraction (SAED) and dark-field TEM microstructures. 

After that, the use of oxalate salts was adopted, and in spite of a slight dissolution of the metal during the electropolymerization, very adherent (11.5 MPa adherence) and homogeneous films were obtained by Beck and Michaelis [14,15]. Some time later, the study was taken up again by Su and Iroh who studied the effects of various electrochemical process parameters on the synthesis and the properties of PPy electrodeposited on iron [89-90]. They confirmed that in acidic medium the pyrrole electrochemical polymerization takes place on the passivated electrode coated with crystalhne iron(ll) oxalate, and that PPy is deposited after the desorption of oxalate. As mentioned previously, the work of Shaftinghen, Deslouis et al. [91 ] showed that the protection properties of the material were dependent on the pyrrole electropolymerization conditions. 

To determine the electrochemical properties of PPy, cyclic voltammetry was used for ACN containing 1 M tetraethylammonium tetrafluoroborate (TEABF4). The potential was scanned in the range -1 to +1 V and the scan rate was fixed at 5 mV/s. 

TABLE 8.1 Solubility in m-Cresol and Electrical Properties of PPy In Situ Doped with Different Sulfonic Acids... 

TABLE 8.4 Properties of PPy-DBSA Powders as a Function of Polymerization Time... 

When PPy-DBSA powder is added to a polar solvent. Coulomb interactions between cationic PPy chains and anionic DBSA molecules may induce the close approach of the polar solvent molecules to the doping spot. New molecular interactions between solvent and PPy chain or solvent and dopant will be developed through hydrogen bondings or dipole interactions, which lead to the solvation of the PPy-DBSA. Since PPy-DBSA is dissolved in polar solvents at the price of partial loss in polymer-dopant interaction, the electrical properties of PPy-DBSA processed from the polar solvents worsened. 

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