Introduction to conducting polymer

• How can classify the semiconducting polymers?

There are at least four major classes of semiconducting polymers that have been developed so far. They include conjugated conducting polymers, charge transfer polymers, ionically conducting polymers and conductively filled polymers. 

• What are the advantage and disadvantage of conductively filled conducting polymers?

Advantage: ease of processing, good environmental stability and wide range of electrical properties
Disadvantage: lack of homogeneity and reproducibility

• Describe the main properties of ionic polymers (polymer electrolytes) 

The ionic conduction mechanism requires the dissociation of opposite ionic charges and the subsequent ion migration between coordination sites, which are generated by the slow motion of polymer chain segments. Consequently, polymer electrolytes normally show a low conductivity and high sensitivity to humidity. They often become electrically non-conducting upon drying.

•  What are the advantage and disadvantage of charge transfer (CT) polymers?

A desired crystal structure is, essential for good conductivity in the molecular CT complexes. However, the resultant materials are often brittle and unprocessable

• Describe the doping mechanism in conducting polymer?

Since most organic polymers do not have intrinsic charge carriers, the required charge carriers may be provided by partial oxidation (p-doping) of the polymer chain with electron acceptors (e.g.I2, AsF5) or by partial reduction (n-doping) with electron donors (e.g.Na, K). Through such a doping process, charged defects (e.g.polaron, bipolaron and soliton) are introduced, which could then be available as the charge carriers

• Describe some of the most commonly used doping methods

1. Chemical Doping: Almost all conjugated polymers can be either partially oxidized (p-type redox doping) by an oxidizing agent such as iodine or partially reduced (n-type redox doping) by an reducing agent such as Na+ complex

2. Electrochemical Doping: conjugated polymers can also be easily oxidized (p-doping) or reduced (n-doping) electrochemically with the conjugated polymer acting as either an electron source or an electron sink. In particular, the doping reaction can be accomplished by applying a DC power source.

Compared with chemical doping, electrochemical doping has several distinct advantages. Firstly, a precise control of the doping level can be achieved simply by monitoring the amount of current passed. Secondly, doping-undoping is highly reversible with no chemical products requiring removal. Finally, both p- or n-type doping can be achieved even with dopant species that cannot be introduced by conventional chemical means

3. Photo-doping : The irradiation of a conjugated polymer macromolecule with a light beam of energy greater than its band gap could promote electrons from the valence band into the conduction band

4. Charge-injection Doping : Using a field-effect transistor (FET) geometry, charge carriers can be 

injected into the band gap of conjugated polymers  by applying an appropriate potential on the metal/insulator/polymer multilayer structure. Just like photo-doping, the charge-injection doping does not generate counter ions, allowing a systematic study of the electrical properties as a function of the charge carrier density with a minimized distortion of the material structure

5. Non-redox Doping : Unlike redox doping, the non-redox doping does not cause any change in the number of electrons associated with the polymer backbone, but merely a rearrangement of the energy levels. The most studied doping process of this type is the protonic doping of polyaniline emeraldine base (PANI-EB) with aqueous protonic acids, such as HCl, d,l-camphorsulfonic acid (HCSA),p-CH3-(C6H4)SO3Hand (C6H5)SO3H, to produce conducting polysemiquinone radical cations

6. Secondary Doping : The interaction of an HCSA-doped PANI-EB with m-cresol was found to cause the absorption band characteristic of the localized polarons to largely disappear at 800 nm, while a very intense free carrier tail commencing at ca.1000 nm developed. These spectroscopic changes have been attributed to the so-called “secondary doping” process, which causes a conformational transition of the polymer chain from a “compact coil” to an “expanded coil” due to molecular interactions between the HCSA-doped polyaniline and m-cresol

• Describe some of methods to synthesis of Soluble Conjugated Polymers

 1. By Substitution : soluble forms of various conjugated polymers have been prepared by grafting suitable side groups and/or side chains along their conjugated backbones.
Polyacetylene derivatives: Soluble poly(methylacetylene) and poly(phenylacetylene) have been synthesized as polyacetylene grafted with methyl or phenyl side groups.
Poly(p-phenylene vinylene) derivatives: The most common method to prepare PPV is the so-called Wessling route, from the sulfonium precursor polymer. Without involving the water-soluble sulfonium salts, soluble PPVs have also been obtained from the substituted dichloro-p-xylene in organic solvents via the so-called Gilch route. Compared with the Wessling method, the Gilch route allows easier access to a large range of substituted PPV derivatives soluble in organic solvents
2. By Copolymerization: The combination of optoelectronic properties characteristic of conjugated structures and the solubility of the soluble polymeric segments into a single copolymer chain should, in principle, lead to a material with properties characteristic of both the constituent components.

• Describe the mechanism of conducting polymer?

 As metals have high conductivity due to the free movement of electrons through their structure, in order for polymers to be electronically conductive they must possess not only charge carriers but also an orbital system that allows the charge carriers to move. The conjugated structure can meet the second requirement through a continuous overlapping of S-orbitals along the polymer backbone. Due to its simple conjugated molecular structure and fascinating electronic properties, polyacetylene has been widely studied as a prototype for other electronically conducting polymers

• What's the main difference when comparing conducting polymers to their inorganic counterparts?

(a) Unlike their inorganic counterparts, a weak intermolecular overlap of electronic orbitals combined with a greater degree of disorder in conducting polymers result in narrow electronic bands and a low mobility of charge carriers.

(b) Compared to the inorganic counterparts, conducting polymers have an advantage in achieving high sensitivity and selectivity by virtue of their chemical and structural diversity 

• Why do one can conclusion that "conducting polymers are one of the most attractive electrochromic materials"?

because of advantages such as high coloration efficiency, rapid switching ability, and diverse colors

• Describe at least 3 parameter of sensing mechanism of conducting polymers

redox reactions, ion adsorption and desorption, volume and weight changes, chain conformational changes, or charge transfer and screening.

• How can improve the important parameters of conducting polymers in display technology?

Stability, rapid response times, and efficient color changes are still critical parameters that need improvement
The high surface area facilitates enhanced interaction between the materials and analytes, which leads to high sensitivity, and the small dimensions enable fast adsorption/desorption kinetics for analytes in the material, which allows a rapid response time.