- September 01, 2003, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
One difference between metals and organic materials is that metals conduct electricity. We know biological systems work in a complex matrix of electrical responses to control various activities, yet most organic materials are considered nonconductors compared to metals, such as aluminum, copper, and silver.
Conductive organic-based coatings can be formulated with organic polymers that incorporate conductive additives such as carbon black, carbon fibers, or fine dispersed metals. In some cases, organic polymers can be made more conductive by forming ionic salts through their functional groups, for example, the neutralization of carboxylic acid functional groups on the polymer with lithium hydroxide.
In the early sixties, many labs were investigating conductive, semi-conductive, and photoconductive characteristics of various organic polymers for electronic imaging and recording applications. These investigations commercialized the use of organic photoconductors used in many photocopying applications. However, the search for higher conducting polymers continued.
Organic conductive polymers have been developed that approach metal range of conductivity, but they have found limited commercial applications to date. However, new applications in light-emitting panel displays, as well as sensors in packaging products, may open opportunities for these materials.
In 1977 Hideki Shirakawa, Alan MacDiarmid, and Alan Heeger published their discovery that, upon reaction with iodine, polyacetylene exhibits electrical conductivity. Their work led to the 2000 Nobel Prize in chemistry. Since then, a lot of progress has been made in a wide range of applications, but major commercial development still is pending.
Most organic conductive polymers have unsaturated conjugated structure that utilize their pi electrons. The conductivity also can be enhanced through the use of electron donors or electron acceptors where the increased conductivity is a result of charge transfer in the polymer matrix.
The degree of conductivity exhibited by conductive polymers depends on the degree of order on a molecular level. This is due in part to crystal lattice that allows an overlapping pathway for electrons. Among polymers that have shown conductive properties are polyacetylene, polyaniline, polypyrrole, and polythiophene when combined with appropriate doping materials. Doping materials are additives that facilitate the polymer conductivity.
The polymer molecular weights are high, and typically they are applied as dispersions. Although apparently separated from each other, particles of the conductive photopolymers can join together at critical concentrations, forming conductive chains.
The presence of certain additives in the polymer enhances the conductivity. This commonly is referred to as “doping the polymer” and provides a lower energy threshold for conductivity. Doping materials can include iodine, bromine, lithium, sodium, and salts of boron tetrafluoride.
One of the earliest applications of conductive polymers was to produce antistatic coatings, lightweight batteries, and materials for circuit boards. Today, the development of many of these polymeric conductive materials may find commercial breakthrough in cell phone displays, computer monitors, and television applications. Demand for organic conductive materials to support light-emitting diodes (LEDs) is expected to mushroom in the next few years.
Polymers formed in a “potential field” can have enhanced conductivity in a specific direction. Polymers can be “lined up” in a given orientation by orienting the monomers through the use of liquid crystals or similar “template materials.” Polymerization of the monomers produces polymers that are highly ordered with excellent conductivity.
Conductive organic polymers — which have the potential of being used in packaging materials as gas/vapor-sensitive electronic noses, chemicals, or biosensors — may be at a commercial threshold, ready to offer unique packaging product concepts.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 267/695-7717; firstname.lastname@example.org.