Long chains of molecules are called polymers. Many small molecular units (called monomers) form polymers, which depending on their arrangement are so useful in everyday life; making materials like silk, wool, or plastic bags. Most of the polymers are organic compounds; it means that they are carbon chains connected to hydrogen, Oxygen or nitrogen atoms. The electrons of these atoms are shared between two atoms that are located in the low energy bonding orbitals. This stops the polymers from making electricity and the electrons play the role of a non-conductor.
In 1997, a set of highly conductive polymers were discovered. After two carbon atoms connect and create a double bond, the symmetry and interact of 2p orbitals are similar to form a __ molecular orbital. This __ orbital includes molecules that are more likely to be around each carbon nucleus.
If there is a chain molecule that has alternating double bonds, the ___ orbitals stretch and become of a same length as the entire chains, thus the electrons are removed from the chain. This leads to the establishment of an energy band structure which has the qualities of a semi-conductor. The polymer in this form is not a great conductor, but it can be developed by increasing or reducing the number of electrons in the band and letting them move around freely in the chain. “Doping” the structure with an oxidizing or reducing species is the reason why this result is achieved.
You are going to synthesize a conducting polymer, which can be done by electropolymerization of pyrrole monomers i.e. molecules that contain nitrogen. In figure1 polypyrrole chains are shown to be forming an alternating double bond system in which electrons are being removed. To increase the conductivity, this polymer is doped with p-tuolene sulphonate.
The electropolymerization reaction bonds the monomers together to form a chain. This happens at the surface of an electrode soaked in a solution of pyrrole and pTS. The experimental setup is shown in figure 4. The voltage applied between two electrodes is what brings out this reaction. You can see this reaction in figure 2. The pyrrole monomer loses electrons i.e. oxidizes and becomes charged positively. In the process, it releases H+ ions. The combination of two such pyrroles forms a dimer, so more oxidized pyrroles can be attached to each side of the chain.
This reaction happens at the surface of anode (that is where the electrons are removed). Here the anode is the transparent ITO-coated glass slide; and the polymer will grow on it as a thin black film.
Another reaction that also happens at the anode is the doping reaction with the pTS molecules. This reaction leads to charges in the polymer film and develops the conductivity. The reaction is shown in figure 3.
This experiment is conducted in order to synthesize a film of the polypyrrole at the surface of the ITO electrode by applying a stream between the two electrodes. You can calculate how many monomers were connected to form the film and the amount deposited on the electrode, because current is transfer of charge per unit time and one electron is used for the addition of one monomer to a polypyrrole chain. By measuring the film’s resistance with a multimeter, you will be able to characterize the conductivity across and along the film.
References
Painter, P. C., & Coleman, M. M. (1997). Fundamentals of polymer science: an introductory text (2nd ed.). Lancaster, Pa.: Technomic Pub. Co..
The Basics: Polymer Definition and Properties. (n.d.). The Basics: Polymer Definition and Properties. Retrieved October 28, 2014, from http://plastics.americanchemistry.com/Education-Resources/Plastics-101/The-Basics-Polymer-Definition-and-Properties.html
What Exactly Is a Polymer?. (n.d.). About. Retrieved October 27, 2014, from http://composite.about.com/od/whatsacomposite/a/What-Is-A-Polymer.htm
