Synthetic Metals
Synthetic metals are organic or polymeric materials that can exhibit electrical conductivity similar to that of metals. They are often referred to as conducting polymers or synthetic metals. Unlike ordinary polymers, which are generally electrical insulators, these materials contain a conjugated system of alternating single and double bonds that allows electrons to become delocalized along the polymer chain.
Why Are They Called Synthetic Metals?
Metals conduct electricity because they possess mobile charge carriers. Some conjugated polymers can also conduct electricity when their electronic structure is modified by a process called doping. Therefore, although they are made mainly from non-metallic elements such as carbon, hydrogen, nitrogen, and sulphur, their electrical behaviour can resemble that of metals.
Structural Basis of Conductivity
The main requirement for electrical conductivity in a synthetic metal is an extended π-conjugated backbone. In such a structure, the p-orbitals of adjacent atoms overlap, allowing the π-electrons to become delocalized along the polymer chain.
In the undoped state, many conducting polymers have relatively low electrical conductivity. Their conductivity can be increased greatly by doping, which creates charge carriers that can move along the conjugated polymer backbone.
Important Examples
| Synthetic Metal / Conducting Polymer | Important Feature |
|---|---|
| Polyacetylene | One of the earliest and most extensively studied conducting polymers |
| Polyaniline (PANI) | Good environmental stability and useful electrical properties |
| Polypyrrole (PPy) | Good electrical conductivity and useful electrochemical properties |
| Polythiophene | Conjugated sulphur-containing polymer with useful electronic properties |
Doping of Synthetic Metals
Doping is the process of introducing charge carriers into a conducting polymer to increase its electrical conductivity. In conducting polymers, doping generally involves oxidation or reduction of the polymer backbone.
p-Type Doping
In p-type doping, electrons are removed from the π-conjugated polymer chain. This oxidation process produces positive charge carriers, such as polarons and, at higher doping levels, bipolarons. These charge carriers can move along the conjugated backbone and greatly increase electrical conductivity.
Polyacetylene can be used as a simple example. Iodine acts as an electron acceptor and oxidizes the polymer:
P + xI2 → Px+ + 2xI−
Here, P represents the conjugated polymer. The negative iodide species balance the positive charge produced on the polymer backbone. The equation is a simplified representation of the doping process.
For polyacetylene, the polymer backbone can be represented in simplified form as:
(CH=CH)n &xrightarrow; p-doping [(CH=CH)n]+
n-Type Doping
In n-type doping, electrons are added to the π-conjugated polymer chain. This reduction process produces negative charge carriers that contribute to electrical conduction.
The process can be represented generally as:
P + xD → Px− + xD+
Here, D represents an electron-donor species. The donor transfers electrons to the conjugated polymer, producing negatively charged sites on the polymer backbone.
An alkali metal can be represented as an electron donor in a simplified equation:
P + xNa → Px−(Na+)x
The exact chemical species present in a doped polymer depend on the polymer, dopant, and degree of doping. The equations above are therefore simplified representations of the charge-transfer process.
Mechanism of Increase in Conductivity
Doping changes the electronic structure of the conjugated polymer and creates mobile charge carriers. These charge carriers can move along the polymer backbone and between neighbouring polymer chains.
Conjugated polymer → Doping → Charge carriers → Charge transport → Increased electrical conductivity
The following diagram shows how p-type and n-type doping modify the electronic structure of conjugated polymers:
Right: n-type doping adds electrons to the π-conjugated backbone, creating negative charge carriers. Both processes enhance conductivity.
Key point: p-type doping involves removal of electrons and produces positive charge carriers, whereas n-type doping involves addition of electrons and produces negative charge carriers. The resulting mobile charge carriers are responsible for the substantial increase in electrical conductivity.
Properties of Synthetic Metals
- They contain an extended conjugated π-electron system.
- Their electrical conductivity can range from very low values to relatively high values depending on the polymer and its doping state.
- Their conductivity can often be controlled by doping.
- They are generally lighter than conventional metallic conductors.
- Many can be processed as thin films, coatings, or flexible materials.
- Their electrical and optical properties can be modified by chemical changes in the polymer.
Applications
- Antistatic coatings and materials
- Electrodes in electrochemical devices
- Sensors
- Organic electronic devices
- Electromagnetic interference shielding
- Rechargeable batteries and supercapacitor-related materials
- Electrochromic devices
- Flexible and lightweight electronic components
Advantages
- Low density compared with conventional metals.
- Electrical properties can be modified by doping.
- Some can be processed into thin films and flexible forms.
- They combine useful polymeric mechanical properties with electrical functionality.
Limitations
- The conductivity of many conducting polymers is lower than that of highly conducting metals such as copper.
- Some materials have limited stability under environmental or thermal conditions.
- Their electrical properties depend strongly on structure, doping level, processing, and morphology.
Conclusion
Synthetic metals are conjugated organic polymers whose electrical conductivity can be greatly enhanced by doping. Their delocalized π-electron systems and tunable electrical properties make them important materials for sensors, coatings, electrochemical devices, and modern electronic applications. Important examples include polyacetylene, polyaniline, polypyrrole, and polythiophene.