Polyphenylenes and Other
Intrinsically conducting polymers (ICPs) have been the subject of intense research. They are oligomeric or polymeric materials composed of phenylene rings and related units such as naphthalene, anthracene or heteroaromatic rings such as pyrrole and thiophene, which are connected to one another through carbon-carbon single bonds or through vinylene groups (-C=C-). These polymers have unique electrical and thermophysical properties. Due to the low hydrogen content and aromatic structure, they show excellent chemical, thermal, and oxidative stability and are practically insoluble in all common solvents. They are also potentially electrical conducting materials, particularly when doped.
Due to the fully aromatic ring structure and absence of freely rotating groups, the mobility of the repeat units is highly restricted which results in very high melting and softening points. This makes the synthesis and processing of these resins difficult and expensive. In fact, their melt viscosities are often so high that injection molding and similar processing methods are not feasible or practical.
Polyphenylenes are an important class of conductive polymers. The phenylene units in these polymers are connected to one another through carbon-carbon single bonds resulting in linear polymers with a backbone that is comprised of aromatic rings only. By far the largest attention received poly(para-phenylene) (PPP). This polymer is very stable up to temperatures of about 500 to 600°C with minimal and only slow oxidation. It is quite insoluble in most solvents and has a very high melting point. It exhibits unusual electronic and optical properties and can be processed into a crystalline thin film, for example by vacuum deposition, that is electrically conducting when doped. PPP is photoconducting and has the potential for electroluminescence1 (EL) applications such as light-emitting diodes.
Poly(para-phenylene vinylene) (PPVs) and its derivatives are another important class of conductive polymers that have been widely studied because of many interesting and potentially useful optical and photo-electronic properties. The phenylene units in polyphenylene vinylenes are connected to one another through carbon-carbon double bonds resulting in a rigid, rod-like linear polymer comprised solely of double bonds and aromatic rings. It can be processed into a highly ordered crystalline thin film that is electrically conducting upon doping. Likke PPP, PPV is capable of electroluminescent and can be used as the emissive layer in polymer-based organic light emitting diodes, for example, for electroluminescent displays. In fact, PPV was one of the first materials used for this purpose. PPV and its copolymers are also used as efficient acceptors in polymeric solar cells (PSCs).
Another important conductive polymer is (doped) polyaniline. This polymer is not part of the polyphenylene family because it has amine groups in the backbone. It is a very attractive conductive polymer because it is relatively inexpensive, easy to synthesize, and can be easily chemically modified. Not surprising, it is one of the most studied conductive polymers and finds many applications as a conductor and for electromagnetic shielding of electronic circuits. Polyaniline is also used as a corrosion inhibitor and for the manufacture of conducting nanofibers.
Polypyrole (PPy) is a very promising conducting polymer. It can be easily processes and has many interesting electrical properties. It is chemically and thermally stable. Like many other fully aromatic polymers, PPy is an electrical insulator, however, when oxidized it becomes an electrical conductor. The conductivity of PPy strongly depends on the preparation technique, and on the polymer additives and can be increased by about two orders of magnitude. It can be used as a gas sensor, anti-electrostatic coating, solid electrolytic capacitor and as a component in many other elctronic devices.
Poly(thiophene) and its derivatives are promising conductive materials. Undoped polythiophenes have rather low electrical conductivity. However, when dopend at even low levels of less than one percent the electrical conductivity increases many times. In particularly, regioregular poly(3-alkylthiophene)s (PATs) are of special interest because of their relative structural order which leads to a high charge–carrier mobility. These polymers are soluble and fusible and demonstrate novel characteristics such as solvatochromism2 and thermochromism3. The emission/absorption can be tuned from ultraviolet to IR by changing the substituent of the polythiophenes.
Polyacetylene or polyethyne (repeating unit C2H2) is a rigid, rod-like polymer that consists of long carbon chains with alternating single and double bonds between the carbon atoms. It is a well know conductive polymer as its discovery started the development of (doped) highly conductive organic polymers. Its electrical conductivity was discovered by Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid who received the Nobel Prize in Chemistry in 2000 for their work. They synthesized this polymer for the first time in the year 1974 when they prepared polyacetylene as a silvery film from acetylene, using a Ziegler-Natta catalyst. Despite its metallic appearance, the first attempt did not result in a conductive polymer. However, three years later, they discovered that oxidation with halogen vapor resulted in a conductive polyacetylene film, which had a much higher conductivity than any other previously known conductive polymer. Although its discovery started the development of conductive organic polymers, polyacetylene has no commercial applications.
COMMERCIAL Conductive Polymers
Today, a number of electrically conducting and electroluminescent1 polymers exist that find or might find applications as chemical sensors, electro-magnetic shielding, antistatic coatings, corrosion inhibitors, electrically conducting fibers, and in “smart” windows that can regulate the amount of light passing through it. One of the most exciting potential uses of these novel materials are compact electronic devices such as polymer-based transistors, light-emitting diodes and lasers. Some of these electronic devices might find novel applications in the electronic industry, for example in flat flexible television screens, and as acceptors in polymeric solar cells (PSCs). We may soon see many other new electroluminescent plastics.
Conductive polymers have the potential advantages of lower manufacturing cost and that they can be processed into thin films.
1Electroluminescence (EL) is an opto-electrical phenomenon in which a material emits light in response to an electrical current flowing through the material or to a strong electrical field. Electroluminescence is caused by recombination of electrons and holes in a (semi-) conducting material. The excited electrons emit their energy as photons (light) during recombination with holes. The electrons and holes may be separated either by doping the material to form p-n junctions (semiconductor electroluminescent) or by excitation when high-energy electrons, accelerated by a strong electric field, pass through the material.
2Solvatochromism is the shift of the absorption / emission band of a compound in a solution when the polarity of the solvent is changed. Thus, the compound (dye) will have different colors in different solvents of different polarity.
3Thermochromism is the reversible change of a materials color due to a shift of the absorption / emission band when the temperature changes. Thus, these materials change its color when the temperature is raised or lowered.