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Conductive Polymers:

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Daryl Lawrence

on 3 March 2014

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Transcript of Conductive Polymers:

Conductive Polymers:
Changing the world one monomer at a time

Our Ideas & Reflections
Analyizing potential contributions to Singapore:

- Singapore places a major emphasis on defence (as seen by the $12.3 billion budget for MINDEF in 2013). A potential application for DSO would be the incorporation Conductive Polymers into stealth vehicles due to their radar-absorbent properties (depending on the Band Gap). These lightweight polymers can also increase the speeds of these vehicles.

- Singapore also faces an increasing population entering their silver years. Such lightweight and flexible technologies can potentially increase usability by elders when it comes to fixing LED televisions or other appliances which may involve polymer circuits in the future.
Introduction
Timeline
Theory
Conjugation
Conductive Polymers depend on conjugation in organic molecules.

Thus, these compounds can include aromatic rings (including Polythiophene and Polyaniline).
Let's consider a long alkyl chain (e.g.) of conjugated single and double C-C bonds for illustration.

Each of the sp2-Carbon atoms also has a remaining electron in an unhybridized p-orbital, which forms a pi bond with one of the neighbouring Carbon atoms.
This pi molecular orbital also overlaps with the adjacent two pi molecular orbitals. This overlap carries on for each pi molecular orbital, such that a cloud of delocalised pi electrons is formed that is orthogonal to the plane of the molecules.
These pi molecular orbitals form bonding and antibonding orbitals which distinguish themselves into specific bands separated by a forbidden Energy / Band Gap (Eg). The occupied band is called the Valence Band while the unfilled one is called the Conduction Band.

In order for conduction to occur, at least one of these bands has to be partially filled. These polymers are thus similar to semiconductors, which typically have Energy Gaps of <3eV.
Conduction
Valence and Conduction Bands with reference to their energies
Table of common Conductive Polymers synthesized, their structures, Band Gaps and Conductivities
However, to further increase conductivity, polymers are usually oxidized partially (through p-type redox doping) or reduced partially (through n-type redox doping) by electron acceptors of electron donors respectively. These dopants, introduce additional energy states in the Band Gap to produce partially filled bands for effective conduction, as shown below.
Dopant effects on bands of Conductive Polymers (additional energy states are contributed to the band structure)*
Synthesis and Doping
General:

Conductive Polymers can be synthesized via a variety of different routes that may involve electrochemical polymerization, metathesis polymerization, solid-state polymerization and pyrolysis. Some of the initial ground-breaking synthesis methods as well as the more common ones are included and explained.
First-discovered, Efficient Synthetic Methods:

The catalyst, AlEt3/Ti(OBun)4, is preferred catalyst system for producing linear polymers of high molecular weight, high crystallinity and high conductivity upon doping. As an example, acetylene has been successfully polymerized in the form of a thin film, which is often termed as Shirakawa’s polyacetylene for his elaborate work in the area. The acetylene is usually bubbled into a stirred catalyst solution for a homogeneous polymerization.
First methods of Polyacetylene synthesis by Shirakawa
*The Fermi Level, which also changes, indicates the level which has a 50% probability of being occupied at thermodynamic equilibrium.
1. Poly(p-phenylene vinylene), or PPV, derivatives are the most studied Conductive Polymers for electroluminescent applications. The most common method to prepare it is via the Wessling route from the Sulfonium precursor polymer. The final PPV product in the shown reaction, however, is insoluble in organic solvents although here are PPV derivates that are soluble, such as Poly[2-methoxy-5-(2’-butyl-5(2;-ethylhexyl)-1,4-phenylene vinylene], be introducing various side chains onto the phenylene units.
Wessling Synthesis Route for PPV
2. Without involving water-soluble Sulfonium salts, soluble PPVs can also be obtained from the substituted dichloro-p-xylene in organic solvents using the Gilch route.
Gilch Route for synthesis of soluble Conductive Polymers
3. Another common Conductive Polymer is Polythiophene. Poly(3-alklythiophenes), or P(3AT)s, are commonly prepared using 3-Bromothiophene as the starting material, are meltable and soluble in common organic solvents. Grignard reagents are commonly used in the synthesis of these P(3AT)s.
Chemical Doping is a common mode of increasing dopants into the Conductive Polymer. For example, trans-polyacetylene can be reacted with an oxidizing agent such as iodine (p-type dopant), resulting in an increase in conductivity of about 10^-5 to 10^2 S/cm. Most conjugated polymers can also be chemically doped with electron donors (due to n-type dopants) to gain high conductivities.
Chemical Doping
Chemical Doping Reactions

Chemical Equation (1) represents p-type doping with I2 dopants while Chemical Equation (2) represents n-doping with Na dopants
It was found that n- and p-type dopants have a countering effect when included in the same conjugated polymer. There is an observed ‘undoping’ effect by an Na-doped polyacetylene film by I2. As shown in diagram, the electrical conductivity of the sample gradually decreases until it reaches a minimum (as the effects of n- and p-type dopants cancel out) and then, increases again, as the p-type dopant dominates. This can be observed from the graph below.
INTERESTING!
Graph of Conductivity Ratio, where the initial conductivity refers to that of the solely Na-doped Polyacetylene, against Time of introduction of I2 dopant
Another method is Charge-Injection Doping, which involves the injection of charge carriers, depending on nature of Conductive Polymer, into the band gap by applying an appropriate potential. No counter ions are generated to stabilize the added charge to the polymer backbone (as no physical dopants are added). This allows the modification of the charge carrier density with a minimized distortion of the material structure, as would have been the result if counter ions had been generated. This method, when tested with a sample of Polyacetylene or Poly(3-hexylthiophene), also known as P3HT, demonstrates a metal-insulator transition with a metallic-like temperature dependence. This enables superconductivity to be reached with temperatures fall below 2.35K. This further increases the potential of Conductive Polymers.
Charge-Injection Doping
Henry Letheby first synthesized Polyaniline in
1862
by anodic oxidation of aniline in Sulphuric Acid
In the
1950s
, research reported the semi-conducting properties in Charge Transfer Complex salts with halogens.

Charge Transfer Complexes* were later found to have resistivities as low as 8 ohms-cm.
* A Charge Transfer Complex is an association involving one or more molecules. There is a transfer of a fraction of electric charge producing a stabilising attractive interaction for the complex.
In the early
1970s
, Tetrathiafulvalene is shown to have nearly metallic conductivity.

Later that decade, in
1977
, Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa submitted a research paper presenting the high conductivity of Iodine-doped Polyacetylene.
The three scientists were awarded the Nobel Prize in Chemistry in 2000 for their work on the 'discovery and development of Conductive Polymers'
Hideki Shirakawa
Hideki was born on August 20th, 1936. He recalls that he always loved polymers and pursued his interests at Tokyo Institute of Technology, where he earned his doctorate.
Alan MacDiarmid
Alan MacDiarmid (14th April 1927 - 7th Feb. 2007) was visiting the Tokyo Institute of Technology in 1975, when he met Hideki and invited him to join him at the University of Pennsylvania.
Alan J. Heeger
Alan Heeger (born on 22nd Jan. 1936) was initially a pure Condensed-Matter physicist,. He gained an interest in the metallic behaviour of the polymer (SN)x. He tested other materials, as well, with Alan MacDiarmid and Hideki, and helped in the development of the exciting field of Conductive Polymers
Advantageous Properties
- High Conductivity

- Lightweight (as compared to metals)

- Ease of modification of properties
(e.g. via doping)

- Environmentally stable

- Low absorbance in visual spectrum
(transparency is possible)

- Flexible
Disadvantageous Properties
- Toxic

- Commercial material inconsistencies

- Poor solubility in solvents (they can form polymer salts easily, diminishing solubility in organic solvents. Solubility in water is only moderate)

- Current design issues (compromising efficiency in commercial technologies)
Despite its disadvantages, which are mostly because of the commercialization issues and not the theoretical limits of this technology, intensive research is being conducted.

For now, let us delve into the wonderful world of Conductive Polymers!
Applications
Polymer Solar Cells
Research into these solar cells began in the 1990s, based on a composite of a semiconducting polymer and Fullerene (C60).

When a photon is absorbed, a delocalised pi electron is excited from the Valence Band to the Conduction Band across the Band Gap to Fullerene with very high efficiency. An interpenetrating network enables the separated charges to be collected at the electrodes. As with inorganic solar cells, this enables green power generation.
Organic Light-Emitting Diodes (OLEDs)
By 1987, researchers were making a diode by casting a polythiophene from solution onto electrodes. It was also demonstrated that polymers such as poly(phenylenevinylene) luminesce when a voltage is applied to a thin film between two metallic electrodes. This led to the first polymer light-emitting diodes.

Subsequent work demonstrated that polymer LEDs can emit light efficiently in a variety of colours. Emissive displays fabricated from polymer LEDs were introduced as products in cell phones and personal digital assistants (PDAs) in 2003. Full colour polymer displays are under active development by several companies.

The great commercial advantage of such polymer devices is that the active luminescent semiconducting material can be processed from solution. Using three different polymers with red, green and blue emission, full colour displays can be fabricated using ink-jet printing.
World 's first 16.7 million color flexible OLED by Sony
Cheap, flexible, disposable electronic circuits
These electronic circuits are based on plastic and paper. Conducting polymers are ideal for this purpose as they can be deposited on a substrate very simply by techniques such as ‘line patterning’. A conventionally printed circuit on plastic or paper is exposed to a
solution containing a conducting polymer which then deposits preferentially on the bare areas of the substrate to create the circuit. The insulating pattern can then be chemically removed.

Philips Research in the Netherlands has already developed polyaniline plastic chips for use as readable barcode labels in supermarkets.
A flexible, semitransparent plastic chip using conducting polyaniline and containing a 27mm integrated circuit which still operates when sharply bent (Philips Research)

Our Personal Thoughts:

- This beautiful field, encompassing Chemistry and Physics, displays the interdisciplinary nature of today's emerging fields and solutions to modern problems. This project has required us to put in an extra effort to understand this technology and this has enabled us to realise the potential of the technologies out there waiting to be discovered.
References
[1] Sherf U, Mullen K, Synthesis-Stuttgart,
1992,

1-2, 23-38

[2] Dai L, Intelligent Macromolecules for Smart Devices: From Materials Synthesis to Device Applications, Springer, Ohio,
2004

[3] ChemComm, Twenty-five Years of Conducting Polymers, The Royal Society of Chemistry,
2003
, Focus Article

[4] Kumar D, Sharma R C, Eur Polym J, Vol 34,
1998
, 8, 1053 - 1060

[5] Berry G C, Matyjaszewski K, Progress in Polymer Science Vol 37,
2012
, 9, 1177-1332
Numerous Plausible Applications
Although we proposed a mechanism, there is no definite one for this organometallic one.
Thank you for viewing our illustrations!
By: Park Young Joo and Daryl Jude Lawrence
M13608
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