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.

[Nanosized Insertion Materials for Li-Ion Batteries] Discussion about size effects

Direct evidence of the impact of particle size on the thermodynamics of nanoinsertion materials is the change in the solubility limits

• In LiFePO₄ the reduction of the miscibility gap appears to result from the interface between the two end members, being the consequence either of strain, of interface energy or of the diffuse interface. The diffuse interface additionally explains the varying solubility limits that are observed with varying overall composition x in nano-Li_xFePO₄
• the presence of coexisting phases during (dis)charge: although the interfaces are directly observed in chemically lithiated materials, they are claimed to be absent under electrochemical conditions, keeping the two-phase transition mechanism

1. Because the constant voltage is indicative of the firstorder phase transition, the reduction of the composition domain where the voltage is constant, is associated with a reduction of the miscibility gap. However, this does not explain the curved shape of the voltage curve indicating a different distribution of chemical potentials in
2. Another eligible explanation is the distribution in particle sizes resulting in a distribution of voltages. Often the relative width in the particle size distribution is larger for smaller particle size.
3. A fundamental thermodynamic origin of the curved voltage profile is the smearing of the first-order phase transition as the result of configurational entropy. However, this effect can only be expected to become significant for systems smaller than ∼1000 atoms, which, considering 1000 Li atoms in LiFePO4, corresponds to systems smaller than∼4 nm. This appears consistent with the reported very small (approximately millivolt) hysteresis in equilibrium voltage curves due to this configurational entropy
4. A final factor is revealed by LTO in which the chemical potential, and hence the insertion voltage, is suggested to be different at the surface. Depending specifically on the orientation of the surface, the voltage can be expected to change gradually toward the bulk voltage over a distance of nanometers, as strengthened by our recent calculations of Li-ion storage at the oxygen-terminated surface of LTO

 

[Nanosized Insertion Materials for Li-Ion Batteries]Spinel Li4Ti5O12

The disadvantage of a high voltage of∼1.55 V versus Li metal compared with anode materials like graphite is compensated by the material's safe operation, high rate capability, low cost, and excellent recyclability

 in micrometer sized Li_4+xTi₅O₁₂ two-phase separation is unstable above 80 K and domains of 16c occupation and 8a occupation intimately mix at a nanometer length scale. This appears as a solid solution for diffraction and the open circuit potential

 - The very low interface and strain energy imposed by the coexisting phases facilitates mixing of the two phases on a small scale. This leads to a solid solution electrochemical response at relative low temperatures (above 80 K)

(c) Li occupancy of the 8a (closed symbols) and 16c (open symbols) sublattices in spinel Li₄þxTi₅O₁₂. 

  - additional lithium incorporation was predicted to lead to a negative and therefore impossible to achieve intercalation potential - directly observation indicates an increased capacity at positive potential with decreasing particle size, exceeding Li7Ti5O12. Neutron diffraction proved simultaneous occupation of both 8a and 16c, which explains the additional capacity.Furthermore, the additional capacity was suggested to reside mainly near the surface, explaining the increasing capacity with decreasing particle size

 oxygen-terminated surface (oxygen-rich surfaces) would explain the relative high voltages of the first inserted capacity as well as the additional capacity at low potential that scales with the particle surface. However, too high surface lithium storage was found to result in irreversible capacity loss, most likely due to surface reconstruction, creating a thin layer of inactive material

[Nanosized Insertion Materials for Li-Ion Batteries]LixTiO2(Anatase, Rutile, TiO2(B), and Brookite)

Advantage: inherent safety and stability of titanium oxides  working at potentials around 1.5 V

• Nanosize
• Increased reaction areas
• Shortened Li diffusion paths
• Enhanced Li solubility and capacity

Voltage profiles of different particle sizes 

(b) Solubility limits in anatase LixTiO2 where α,β, and γ represent anatase, lithium-titanate, and LiTiO2 respectively. (α)+(β) and (α+β) refer to the situation that each particle either has phase α or β and that both phases coexist within one particle, respectively.

 * the thermodynamics of insertion in anatase is strongly affected by the crystal particle size
 * The increase of the surface area has profound impact on the storage properties

a particle size of 7 nm can completely be transformed toward tetragonal LiTiO₂. Down to ∼3 nm deep, the surface allows lithium storage exceeding the orthorhombic Li_∼0.5TiO₂ composition, which is responsible for the larger reversible (dis)charge capacities observed

the storage capacity increases with decreasing particle size, suggesting similar surface environment enhanced Li storage
 - The region where the voltage is constant reflects the first-order phase transition from Li-poor anatase Lix_a≈0.025TiO₂ to Li-rich lithium-titanate Lix_b≈0.5TiO₂
 - A remarkable observation is that the Li-ion solubility in the various phases depends systematically
on the crystal particle size, shifting the miscibility gap rather than decreasing it

• The 120 nm anatase crystals can host approximately Li/Ti = 0.03
• the 7 nm particles are able to host up to Li/Ti = 0.21 while maintaining the anatase structure.

The disappearance of the voltage plateau for smaller particle sizes has been related to these changing solubility limits

Another interesting observation is the different phase behavior in particle sizes above and below ∼80 nm referred to as (α +β) and (α) + (β)

•  Large particles appear to be able to host both phases within one crystallite
• small particles have either the Li-poor anatase or lithium-titanate phase

 - The origin of this was suggested to be the prevention of intraparticle coexisting phases and the associated phase boundary, that is, preventing the resulting energy penalty due to interface energy and strain
 - The absence of the interface rules out the interface energy effects on the solubility limits as discussed for LiFePO4

Upon nanosizing, all TiO2 polymorphs suffer from a substantial irreversible capacity loss on the first cycle that appears to scale with the surface area, compromising the use of nanostructured materials. Generally, the irreversible capacity loss is attributed to trapped lithium in the host structure or decomposition of the electrolyte and SEI formation. However, titanium oxide surfaces are wellknown for H2O and OH physisorption and chemisorption, forming strong Ti -O- H type bonds. We have shown that this explains the irreversible capacity loss by the formation of Ti -O- Li at the surface of amorphous TiO2 where Li+ exchanges with H+, which reduces the electrolyte