Stretched electric conductivity

Microfibril Reinforced Polymer-Polymer Composite via Hot Stretching Electrically Conductive Functionalization... 

A non-electrochemical technique which has been employed to alter the physical characteristics of a number of polymers is that of stress orientation [26, 27], in which the material is stressed whilst being converted to the desired form. This has the effect of aligning the polymer chains and increasing the degree of order in the material, and is obviously most applicable to materials which can be produced via a precursor polymer. With Durham polyacetylene (Section 4.2.1) increases in length in excess of a factor of twenty have been achieved, with concomitant increases in order, as shown by X-ray diffraction and by measurements of the anisotropy of the electrical conductivity perpendicular and parallel to the stretch direction. 

A number of other characteristics are required in order to ensure a viable polymeric conductor. Chain orientation is needed to enhance the conducting properties of a polymeric material, especially the intermolecular conduction (i.e., conduction of current from one polymer molecule to another). This is a problem with many of the polymers that are amorphous and show poor orientation. For moderately crystalline or oriented polymers, there is the possibility of achieving the required orientation by mechanical stretching. Liquid crystal polymers would be especially advantageous for electrical conduction because of the high degree of chain orientation that can be achieved. A problem encountered with some doped polymers is a lack of stability. These materials are either oxidants or reductants relative to other compounds, especially water and oxygen. 

The next major improvement in sample quality was the result of developments in direct synthesis. Oriented samples were obtained by the polymerisation of acetylene with the conventional titanium catalyst on a single crystal substrate and in nematic liquid crystals, see Tsukamoto (1992). Heat-treating the catalyst was found to produce polymer that could be stretched to give oriented samples. This resulted in a higher electrical conductivity along the orientation direction, up to 2xl05 fl-lm-1 being obtained. Naarman and co-workers heat treated the catalyst at 393 K and carried out the reaction in silicone oil at room temperature (Theophilou et al., 1986). Silicone oil was chosen as it has the same viscosity at room temperature as the usual solvents... 

As mentioned in the introduction, the electrical conductivity upon doping is one of the most important physical properties of conjugated polymers. The conductivity ranges from lOOOOOS/cm for iodine-doped polyacetylene [41], 1000 S/cm for doped and stretched polypyrrole [42], to 500 S/cm for doped PPP [43], 150 S/cm for hydrochloric acid doped and stretched polyaniline [44], and 100 S/cm for sulfuric acid doped PPV [45] to 50 S/cm for iodine-doped poly thiophene [46]. The above listed conductivities refer to the unsubstituted polymers other substitution patterns can lead to different film morphologies and thus to a different electrical conductivity for the same class of conjugated polymer in the doped state. 

Metals are large atoms that tend to lose electrons to form positive ions or form positive oxidation states. To emphasize their loose hold on their electrons and the fluid-like nature of their valence electrons, metals are often described as atoms in a sea of electrons. The easy movement of electrons within metals gives them their metallic character. Metallic character includes ductility (easily stretched), malleability (easily hammered into thin strips), thermal and electrical conductivity, and a characteristic luster. Metal atoms easily slide past each other allowing metals to be hammered into thin sheets or drawn into wires. Electrons move easily from one metal atom to the next transferring energy or charge in the form of heat or electricity. All metals but mercury exist as solids at room temperature. Metals typically form ionic oxides such as BaO, (BeO is one exception that is not ionic.)... 

The mechanical and electrical properties of polyacetylene (PA) were modified by blending it with polybutadiene (PB). Further enhancement of the electrical conductivity of the blends was obtained by stretch elongation of the blends prior to doping. 

As expected, the electrical conductivity of the doped blend is also a function of the polyacetylene composition of the material. (5) Furthermore, stretch induced elongation of the blends leads to a dramatic increase in conductivity subsequent to doping, further confirming that the electrical properties are also very sensitive to the arrangement of the respective phases. 

By contrast, the ECP must have conjugated rigid-rod macromolecules. Several such polymers show high electrical conductivity (usually after doping), viz. polyacetylene (PAc), polyaniline (PANI), polypyrrole (PPy), polyparaphenylenes (PPP), or poly-3-octyl thiophene (POT). The resins are expensive, difficult to process, brittle and affected by ambient moisture, thus blending is desirable. For uniaxially stretched fibers the percolation threshold is 1.8 vol%, hence low concentration of ECP (usually 5-6 vol%) provides sufficient phase co-continuity to ascertain conductivity similar to that of copper wires (see Table 1.79). 

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