28SCIENCE REPORTER, DECEMBER 2015

Polymers in everyday life

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“POLYMER Science” has facilitated a multitude of opportunities for

the betterment of society and mankind. Take a look around you and you will fi nd that EVERYTHING, yes EVERYTHING is a polymer.

Let’s begin with the bristles of a toothbrush. The history of polymer bristles started at Du Pont in 1938 where for the fi rst time synthetic nylon bristles were made which found great acceptability. Disposable crockery used in homes is made up of polystyrene. Melamine is another polymer used to manufacture reusable crockery as it is hard, thick and dish-wash safe. Tefl on cover in non-stick pans is a coating of polymer polytetrafl uoroethylene. The building unit of credit cards, debit cards and other plastic cards is polyvinyl chloride acetate which is a dense and water resistant material. The clothes that

we wear are made up of polymers viz., cotton, nylon etc.

The term polymer stems from the Greek lineage: polus (many) and meros (parts). Thus, polymer means “many parts” and designates an entity made up of repeating structural units called monomers. Today, this fi eld of science embodies a versatile class of synthetic

and natural polymers. With a wide range of properties like strength, elasticity, plasticity, toughness, frictional resistance, lighter weight, greater workability, durability, chemical inertness, ease of fabrication, and structural versatility, polymers have emerged as highly desirable building blocks for various day-to-day utility components.

The structural versatility and electronic communication in Electrically Conducting Polymers have made these materials an ideal choice in a number of applications such as opto-electronic devices, electronic skins, nano-electronics, conducting textiles, bio-electronics, cellular phones, laptop computers, sensors, batteries, point of care diagnostic devices and so forth.

A.K. BAKHSHI & PRIYANKA THAKRAL

29 SCIENCE REPORTER, DECEMBER 2015

Conduction in PolymersTraditionally, polymers have been considered insulators (offer resistance to the fl ow of electricity) with no free charge carriers to assist the fl ow of electric current. Until the 1970s, therefore, the quest to produce polymers exhibiting conductivity similar to that of metals was considered an oxymoron. However, the accidental invention of conduction in organic π-conjugated polymers opened new frontiers in the world of electronics.

The roots of this historic discovery were laid in the laboratory of Hideki Shirakawa where unconscious excess addition of Ziegler Natta catalyst to acetylene gas in a reactor at -78°C resulted in a shiny metallic fi lm of polyacetylene (PA) instead of black powdery PA. Shirakawa collaborated with Alan G. MacDiarmid and Alan J. Heegerat of the University of Pennsylvania, where the trio made the crucial breakthrough

when they found that oxidizing or reducing PA resulted in a polymer with conductivity 10 million times greater than the pristine insulating PA. By varying the concentration of oxidizing or reducing agents, they could make their PA an insulator, a semi-conductor, or a conductor to rival metals such as silver and copper.

This invention provoked extensive research interest worldwide in a new class of polymers called Electrically Conducting Polymers (ECPs). ECPs being lighter in weight, cheap and functionally versatile, were soon visualized as attractive alternatives to metals where mining, shipping and processing required huge cost and energy. This spectacular development was appreciated with a Nobel Prize in Chemistry in 2000.

The emergence of conduction in the otherwise insulating PA triggered extensive application based research using this material. However, PA had some serious disabilities; it was unstable in air,

insoluble in solvents and so was unprocessable. With this began the urge to explore the possibility of conduction in other classes of conjugated polymers and to develop them to a level of maturity of commercialization. Today, a vast number of ECPs are known

which include polyaniline (PANI), polypyrrole (PPY), polythiophene (PTP), polyparaphenylene (PPP), polyisothianaphthene (PITN), polyparaphenylenevinylene (PPV), polyethylene dioxythiophene (PEDOT) and their derivatives. Applications of Conducting PolymersIn 1990, the discovery of polymer light-emitting diodes by Richard Friend and colleagues at Cambridge opened a new chapter in the evolution of ECPs as commercial devices. The emerging trends witnessed companies such as American Dye Source, H. W. Sands, Sigma-Aldrich, Panipol, Plextronics, Ormecon, PolyOne, Bayer AG, etc., targeting large scale commercial markets for ECPs.

Two commercially available conducting materials reported in early 2000 were BAYTRON by Bayer (PEDOT) and PANI under the name of ORMECON by Zipperling Kesseler and Co. (Ormecon International). A great deal of R&D into conductive polymers has resulted from the continuing proliferation of sensitive electronic devices, the need to protect them and exciting new technologies. The Global Industry Analysts Inc. predicts US Conductive polymer market to reach US $1.6 billion by 2017.Electrically conducting textiles: A conducting textile is a fabric that conducts

In 1990, the discovery of polymer light-emitting diodes by Richard Friend and colleagues at Cambridge opened a new chapter in the evolution of ECPs as commercial devices.

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BatteriesActuators

ConductiveDevices

Applications Conductivepaints

Sensors

Bio-electronics

Nano-electronics

Conductiveinks

Lightemittingdiodes

Lightemittingdiodes

SolarCars

Anti-Corrosivecoatings

Textile based electronics by Fibretonic

ECPs based fabrics for decorative/aesthetic purposes

30SCIENCE REPORTER, DECEMBER 2015

electricity. Imagine that the clothes you wear could record your surroundings, hear your heart beat, or create energy from movement. ECPs provide such a lucrative opportunity.

Right from the entertaining conducting-textiles clad performers in movies and television to health care monitoring, conducting textiles do cover

a reasonably good range of applications which include static dissipation, EMI shielding, heating elements and military applications. Some companies providing conducting yarns and textiles include Novonic, Smartex, Ohmatex, and Swicofi l. Bombay Textile Research Association designed PPY- and PANI-doped conducting textiles for developing smart

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mats (for intruder detection, security and commercial establishments) and textile-based heating systems (incorporation in jackets, inner wears, gloves for warmth).Batteries: With increasing numbers of cellular phones, laptop computers, cordless drills and other electronic gadgets, the importance of batteries that can withstand many deep cycles with high energy density and high stability has become increasingly apparent. The most passable application of ECPs is their serviceability as electrodes in light weight and rechargeable batteries.

In general, the electrode materials are required to be conductive or porous to allow the interchange of ions. The use of ECPs as electrode materials in batteries relies on their electrochemical redox processes. Mac Diarmid and Heeger were the fi rst to develop a PA-based battery system, which was found to have longer life, higher energy and power densities in comparison to lead acid batteries. Another advantageous aspect of these polymer batteries is their non-toxic nature that minimizes the disposal problems. Polymers based on PA, PTP, PPY, PANI and PEDOT are some of the viable options explored for construction of electrodes in batteries. Sensors: In any sensor, the sensing process is divided into two parts, recognition which results in selectivity (HEART of design), and amplifi cation which increases the intensity of the weak signals to the level detectable by electronics. Compared to the inorganic counterparts, ECPs are more sensitive and selective by virtue of their chemical and structural diversity. ECPs have been employed as alcohol sensors, methane sensors, humidity sensors and even taste sensors.

Nobel Prizes in Polymer ChemistryHermann Staudinger (1953)

“for his discoveries in the fi eld of macromolecu-lar chemistry”

Karl Ziegler & Giulio Natta (1963)

“for their discoveries in the fi eld of chemistry and technology of higher

polymers”

Paul J. Flory (1974)

Alan J. Heeger, Alan G. MacDiarmid, Hideki Shirakawa (2000)

“for his fundamental achievements, both theoretical and experimental, in the physical

chemistry of the macromolecules”

“for the discovery and development of conductive

polymers”

Solar battery developed by European research-ers integrating thin-fi lm organic solar cells with

a fl exible polymer battery for application in smart cards and mobile phones.

In general, the electrode materials are required to be conductive or porous to allow the interchange of ions. The use of ECPs as electrode materials in batteries relies on their electrochemical redox processes.

31 SCIENCE REPORTER, DECEMBER 2015

PPY and PANI based materials being biocompatible have been used extensively for detecting food-borne pathogens, enzymes, glucose levels, etc. in the body. ECPs based sensors play an important role in the improvement of environmental monitoring and clinical diagnostics because of rapid detection, high sensitivity, small size, and specifi city achievable.Opto-electronic devices: Opto-electronic devices are electrical to optical or optical to electrical transducers, or instruments that use such devices in their operations. Electrochromic displays, solar cells and light emitting diodes are some of the attention grabbing applications of ECPs.Electrochromism: It refers to the persistent change in the optical properties of a material induced by reversible redox processes. ECPs based electrochromic devices have found applications in information display and storage, automotive industry (in rear view mirrors and visors), architecture (as smart windows), sunglasses, electronic paper, military camoufl age and so forth.

PEDOT and its derivatives have become a subject of considerable

interest as electrochromic materials due to their chemical stability, structural versatility, enhanced optical contrast and faster redox switching. Several ECPs based electrochromic applications are approaching commercial markets as a result of high degree of color tailorability, good UV stability, rapid switching ability, large temperature range of operation and low cost production.Electroluminescence: It is an opto-electrical phenomenon in which the material emits light in response to the passage of an electric current. The discovery of electroluminescence in conjugated polymers has provided a new impetus for the development of polymer light-emitting diodes (PLEDs). Electroluminescence in conjugated polymers was fi rst reported in 1990, using PPV as the single semiconductor layer sandwiched between metallic electrodes.

PANI and PEDOT based materials are being commercially used in fabricating LEDs. Recently, Kodak together with Heraeus conducting polymers, GSI technologies and Azoteq announced a breakthrough in capacitive touch screen technology that incorporates new two-sided transparent conductive fi lms for

Conducting polymer fabric based sensor

use in a variety of printed electronic applications.Solar cells: Polymer solar cells (PSCs) have emerged as a promising photovoltaic technology to address the ever-growing energy needs and environmental concerns as they offer cost-effective production of large area fl exible devices with low environmental impact and versatility in organic material design. Various PTP based structures have been successfully exploited for photovoltaic properties offering low band gap and higher charge carrier mobility. Nano-electronics: Nano structurization of ECPs has emerged as a unique interdisciplinary research area, as they combine the advantage of organic conductors with low dimensional materials. PANI, PPY and PEDOT nanofi bres/tubes have found prospects in PLEDs. PEDOT nanotubes have been proposed for biosensing brain tissues. Some other areas where nano-structured ECPs have found utility include catalysis, super-capacitors, nano-conducting textiles and biomedical applications.

Bio-analytical and biomedical applications: ECPs present a number of

Flexible electronics based on ECPs

Solar cells based on ECPs

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“POLYMER Science” has facilitated a multitude of opportunities for the betterment of society and mankind. Take a look around you and you will fi nd that EVERYTHING, yes EVERYTHING is a polymer.

32SCIENCE REPORTER, DECEMBER 2015

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Traditionally, polymers have been considered insulators (offer resistance to the fl ow of electricity) with no free charge carriers to assist the fl ow of electric current. Until the 1970s, therefore, the quest to produce polymers exhibiting conductivity similar to that of metals was considered an oxymoron. However, the accidental invention of conduction in organic π-conjugated polymers opened new frontiers in the world of electronics.

advantages for biomedical applications which include biocompatibility, ability to entrap and controllably release biomolecules, ability to transfer charge from a biochemical reaction, and the potential to easily alter the electrical, chemical, physical, and other properties. These unique characteristics are useful in many biomedical applications, such as biosensors, tissue-engineering scaffolds, neural probes, drug-delivery devices, and bio-actuators. PPY was one of the fi rst CPs studied for its effect on mammalian cells. Electrical stimulation of ECPs has been

Fuel cell cathode based on PEDOTPolymer LEDs

Roll to roll fl exible thin solar fi lms by Konarka technologies

smart stretchable devices as it is imperative to improve the operation and mechanical capabilities of devices. In the close future, these materials will be used in niche applications edging over the shortcomings (such as stability, performance, effi ciency), and revolutionizing the world of electronics.

Prof. A.K. Bakhshi is Sir Shankar Lal Professor of Chemistry at Delhi University. He has been Vice-Chancellor of UP Rajarshi Tandon Open University (UPRTOU), Allahabad; Executive Director of Tertiary Education Commission (TEC), Mauritius and Head, Department of Chemistry, Delhi University. He has also been the Director of the Institute of Lifelong Learning (ILLL), University of Delhi and the Centre for Professional Development in Higher Education (CPDHE), a UGC Academic Staff College of the University of Delhi. Address: Department of Chemistry, University of Delhi, Delhi-110007; Email: akbakhshi2000@yahoo.comMs. Priyanka Thakral (pthakral16@yahoo.com) is presently a Research Scholar (UGC-SRF) at the Department of Chemistry, University of Delhi, Delhi-110007. She is a recipient of Indian Science Congress Association Best Poster Presentation Award 2015.

used to release a number of therapeutic proteins and drugs.

ECPs have several other potential applications in multidisciplinary fi elds, such as electrostatic materials, conductive adhesives, artifi cial muscles, printed circuit boards, antistatic coatings, conductive paints, conductive inks, radar systems, acid base indicator, fi eld effect transistors, anti-corrosive coatings, smart windows, capacitors, robotics and many more.

However, there are still challenges in translating high performance laboratory-scale devices into

large-scale merchant market devices. Full-fl edged commercial realization of organic electronics is still hampered by the understanding of how best to couple the chemical structure and properties of the polymers to obtain stable, processable ECPs.

Along with good properties, efforts are being made to develop

Ultrathin TV Screens