Probing conducting polymers
Polymers that exhibit metallic and semiconductor-like properties are the focus of research at the University of Leicester, UK. The team wants to enhance the fundamental understanding and properties of these materials for potential use in microactuators, bio-electrochemical sensors and batteries for energy storage.
One significant finding is that incorporating single-walled carbon nanotubes improves the rheological properties of thin polyaniline films (a conducting polymer) by an order of magnitude (see diagram above).
Dr Mohamoud Mohamoud at Leicester says, ‘Conducting polymers are a special class of [materials] that have extended alternating single and double bonds in the backbone of their chains. The bond conjugation drives electron mobility and charge transport. Conventional polymers have single bonds’.
Technological application of conducting polymers depends upon the exchange of charged and neutral particles between the material and agents in the media it is exposed to – ions and solvents in liquid phase and gaseous agents. ‘For example, the conductivity of polyacethylene can be increased to up to 105 S cm-1 compared to the conductivity of copper (106 S cm-1),’ says Mohamoud. ‘Polymers can replace metals in electronics with the added advantage of their light weight.’
The researchers are exploring how the ion/ solvent content, polymer structural dynamics and material properties are interrelated. It has been found that polyaniline films are stiffest and have the largest storage and loss moduli in perchlorate medium. The values in sulphate and chloride media are similar to each other and smaller than in perchlorate. While solvents such as water lead to polymer plasticisation.
The team has also studied the mechanisms of ion and solvent transfer, which controls the material response. Mohamoud explains, ‘We found different ions are involved in charge compensations at different stages upon application of electrical voltage, depending on the nature and type of conducting polymer. For polyaniline, protons are transferred first in acidic media followed by anions as a response to polymer redox switching. Mobile species transfers are kinetically controlled rather than thermodynamically’.
Although the studies are currently restricted to the laboratory, the team is working to develop actual devices from the materials and is tight-lipped about collaborations with industry.
‘Though in the early stages of development, conducting polymers have been used in electrical capacitors, electrostatic discharge coatings, sensors and corrosion protection,’ says Mohamoud. ‘It is estimated that conducting polymers will [have] a compound annual growth of 15% [from 2008-13].’