“Important things for easy life—through polymers”.
In modern world, polymers are an integral part of an individual’s life. They have the most diverse structure and applications ranging from domestic articles to highly sophisticated instruments. Polymers are used in almost all fields such as medicine, industry, agriculture, construction and so on. In recent days, these materials are used to prepare nanomaterials.
The human body is built up and functions with polymers such as DNA, RNA, hormones, enzymes, proteins, lipids, phosphonitrilic acids and so on. Most of the food materials that we eat are polymers such as carbohydrates, starch and so on. In view of their importance, a proper understanding of polymeric materials is very essential.
The word polymer is derived from Greek word poly, which means many and meros which means units (or) parts. Polymers are macromolecules of high molecular masses built up by the linking together of a large number of small, repeated units by a covalent bond. The repeating unit present in the formation of a polymer is known as polymerisation.
For example,
The size of the polymer molecule is decided by the number of repeating units present in it. The number of repeating units (n) in a chain formed in a polymer is known as the “degree of polymerisation”.
In the figure, n is the degree of polymerisation. It is different from polymer and can be 104 or more.
Molecular weight of polymer = Molecular weight of repeating unit × degree of polymerisation
Polymeric materials can be classified into several ways:
For example, cotton, silk, wool, nucleic acids, proteins, starch, cellulose, natural rubber, etc.
For example, polyethylene, polyvinyl chloride, nylon, terylene, etc.
For example, polyethylene, polystyrene, polyvinyl chloride, etc.
For example,
Copolymers are classified into four categories depending upon the nature of the distribution of different monomers in the polymer chain.
For example, polyethylene, polystyrene, polyvinyl chloride, etc.
For example, polysilanes, polygermanes, etc.
For example, nylons, polyester, polyvinyl chloride, high-density polythene, etc.
For example, glycogen, amylopectin, low-density polythene, etc.
For example, elastomers like rubber.
For example, Bakelite, urea formaldehyde resin, silicones, etc.
For example, polyethylene, polypropylene, polyvinyl chloride, etc.
For example, polyester, nylon, polyamide, etc.
For example, polythene, polypropylene, polyvinyl chloride, etc.
For example, polyester, Bakelite, urea formaldehyde, etc.
For example, nylon, polyester, etc.
The stereo chemical arrangement of the monomer units in the main chain of a polymer is known as tacticity. The orientation of monomeric units in a polymer can take an orderly or disorderly twist with respect to main chain polymers that are mainly classified into isotactic, syndiotactic and atactic polymers.
Polymerisation is mainly of two types:
Condensation is brought about by monomers containing two or more reactive functional groups condensing with each other to form large condensed polymer and also loss of small molecules such as H2O, NH3, HCl and so on.
The process is continuous and forms polymers.
The aforementioned reaction is continued and forms a large polymer.
n molecules of ethylene glycol react with n molecules of terephthalic acid.
The aforementioned reaction proceeds that n molecules of hexamethylenediamine react with n molecules of adipic acid to form nylon 66.
n molecules of diamine and n molecules of dicarboxylic acid react to form polyamide.
Addition polymerisation takes place in compounds containing reactive double bonds. Chain polymerisation is characterised by a self-addition of the monomer molecules to each other very rapidly through a chain reaction. No by-product, such as HCl, NH3, H2O and so on, is formed. This polymerisation occurs in the presence of catalyst, light, or heat.
In the addition polymerisation, free radical, carbonium ion, or carbanium ions, act as active centres. Hence, polymerisation may occur in the following:
In addition, polymerisation, consists of the following steps:
The initiation of the polymer chain is brought about by free radicals produced by the decomposition of monomers; thus, this reaction is called free radical polymerisation.
The decomposition of the initiation to form free radicals can be induced by heat energy, light energy or catalysts.
Three steps included in free radical polymerisation are as follows:
Normally, H2O2, benzoyl peroxide, hydroperoxide, tertiary butyl peroxide and azobis-isobutyl nitriles (AIBN) act as initiators.
Propagation reaction is a very fast reaction; in this reaction, no middle product is formed.
In this step, chain propagate polymer radical deactivates with coupling or disproportionation reaction to stop chain propagation and forms dead polymer.
For example, polymerisation of acrylonitrile in the presence of benzoyl peroxide.
For example, polymerisation of methyl methacrylate in the presence of azobis-isobutyl nitrile.
The following are the important points in reaction mechanism:
For example, cationic polymerisation of isobutylene in the presence of BF3:
The following are the important points:
Acrylonitrile, styrene and methyl methacrylate participate in anionic polymerisation. For example, Anionic polymerisation of styrene in the presence of alkali metal amide:
Coordination polymerisation is invented by two Italian scientists, Ziegler and Natta. They shared the Nobel Prize for Chemistry in 1963 using the Ziegler–Natta catalyst to polymerise non-polar monomers.
The mixture of titanium halides and trialkyl aluminium is known as Ziegler–Natta catalyst.
The reaction between titanium chloride and trialkyl aluminium forms Ziegler–Natta catalyst.
In this process, trialkyl aluminium adsorbs on the surface of titanium chloride, and forms electron deficient bridge structure.
In this structure, titanium chloride acts as catalyst and trialkyl aluminium acts as the co-catalyst.
Coordination polymerisation is a form of addition polymerisation in which monomer adds to a growing macro molecule through an organometallic active centre.
The coordination polymerisation of alkene can be proceeded by monometallic or bimetallic mechanism depending on the catalyst. Alkene or substituted alkene is polymerised by Ziegler–Natta catalyst.
The double bond of alkene will undergo cis addition with the empty orbital of Titanium catalyst to form a four-membered ring coordinate intermediate.
The Ziegler–Natta catalyst can control the linearity and tacticity of the polymer. Free radical polymerisation of ethylene produces a low density branched chain soft, rubbery polymer. However, the Ziegler–Natta catalyst produces more linear, rigid, high density, high tensile strength, harder and tougher isotactic polymer.
In the presence of the Ziegler–Natta catalyst, coordination polymerisation occurs and gives isotactic polymer of olefin.
For example, propylene undergoes coordination polymerisation in the presence of Ziegler–Natta catalyst at 50°C and gives isotactic polymer of polypropylene.
Polymers are a mixture of different monomers with different molecular weights or masses. Hence, three kinds of molecular masses have been identified, which are as follows:
Where Ni = the number of molecules of mass Mi
The number average molecular mass is a good index for tensile strength, but not for flow.
where wi = weight fraction of molecules of Mi
This is a measure of the distribution of molecular mass of a polymer. This can be calculated using the weight average molecular weight divided by the number average molecular weight.
In a monodisperse system, w = N. However, the polydispersity index (PDI) value is always greater than one, that is, the weight of the average molecular mass is always greater than the number average molecular mass.
where a = constant.
When a = unity, the viscosity and weight average molecular masses are equal. v is almost less than w, a polydispersive polymer is represented as
Plastics are mainly of two types:
For example, PVC, nylon, polystyrene and polyethylene
For example, Bakelite
Polythene is the most widely used plastic. Polythene is obtained by high-pressure polymerisation of ethylene, making use of oxygen as initiator. The reaction takes place at 1,500 atmospheric pressure and 180°C–250°C temperature range. Ethylene polymerised into a waxy solid known as polyethylene.
By using free radical initiator, low density polythene (LDPE) is obtained, while by using ionic catalysts, high density polythene is obtained.
The properties of polythene are as follows:
The uses of LDPE are as follows:
It is obtained by heating a water emulsion of vinyl chloride in the presence of a small amount of benzoyl peroxide or hydrogen peroxide in an autoclave under pressure.
Vinyl chloride so needed is generally prepared by treating acetylene at 1–1.5 atmospheres with hydrogen chloride at 60–80°C, in the presence of metal chloride as catalyst.
PVC is colourless, odourless, non-inflammable and chemically inert powder, resistant to light, atmospheric oxygen, inorganic acids and alkalis but soluble in hot chlorinated hydrocarbons such as ethyl chloride. Pure resin possesses high softening point and a greater stiffness and rigidity, but is brittle.
The uses of PVC are as follows:
It is prepared by the polymerisation of styrene in the presence of benzoyl peroxide catalyst.
Polystyrene is a transparent, light-stable and moisture-resistant material. It is highly electric insulating and highly resistant to acids, and is a good chemical resistant. However, it has less softening and is brittle. It has the unique property of transmitting light through curved sections.
It is used in moulding of articles such as toys, combs, buttons, buckles, radio and television parts, refrigerator parts, battery cases, high-frequency electric insulators, lenses, indoor lighting panels, food containers, food packaging, umbrella handles and so on.
The presence of benzoyl peroxide catalyst and high pressure polymerisation of tetrafluoroethylene gives Teflon.
Teflon has a twisted, zigzag structure with fluorine atoms, packed tightly in a spiral around the carbon-carbon skeleton. Due to the presence of highly electronegative fluorine atoms, there are very strong attractive forces between different chains. These strong attractive forces give the material extreme toughness, high softening point, exceptionally high chemical resistance towards all chemicals, high density, waxy touch, very low coefficient of friction and extremely good electrical and mechanical properties. It can be machined, punched and drilled. The material cannot be dissolved and cannot exist in a true molten state. Around 350°C, it sinters to form a very viscous, opaque mass, which can be moulded by applying high pressure.
The uses of Teflon are as follows:
These are polyamides. The word nylon is now accepted as a generic term for synthetic polyamides, which are characterised by a repeating acids linkage (—NHCO—). Nylon is formed with dicarboxylic acids and diamide under condensation process. It has been named on the basis of number of carbon atoms present in that two monomer units.
For example, nylon 6,6, nylon 6,10, nylon 6,11, etc.
Nylon 6,6 is formed with the condensation reaction of hexamethylenediamine and adipic acid.
They are translucent, whitish, horny and high melting polymers. They possess stability up to high temperature and good abrasion resistance. They are insoluble in common organic solvents and soluble in phenol and formic acid.
The properties of nylon fibres are as follows:
These are condensation polymerisation products of phenolic derivatives with aldehydes, prepared by condensing phenol with formaldehyde in presence of acidic or alkaline catalyst. Depending upon catalyst and reactants mainly three kinds of resins are formed, they are classified as follows:
Phenol react with carbonium ion to form ortho- and para-methylol phenol.
Ortho-methylol phenol condenses to form novalac resin.
The polymethylol phenol condenses and forms resol resin.
Novalac resin is soluble and fusible solid. Resol resin is a hard and brittle solid. Bakelite is a rigid, hard, infusible, water resistant, and insoluble solid. It resist to non-oxidising acids, salts and organic solvents, but are attacked by alkali due to presence of free hydroxyl groups. All phenol formaldehyde resins possess excellent electrical insulating character.
The uses of resol resin are as follows:
Diisocyanate and diol give polyurethanes.
For example, a reaction between 1,4-butane diol and 1,6-hexane diisocyanate gives “Perlon - U”, a crystalline polymer.
The properties of polyurethanes are as follows:
These are used as coatings, films, foams, adhesives and elastomers.
Rubbers are high polymers, which have elastic properties. Thus, the rubber band can be stretched to four to 10 times its original length, and as soon as the stretching force is released, it returns to its original length. The elastic deformation in an elastomer arises from the fact that in the unstressed condition, an elastomer molecule is not straight chained, but in the form of a coil, it can be stretched like a spring consequently. The unstretched rubber is amorphous.
Isoprene is the basic molecule present in natural rubber. Dispersive form of isoprene units are known as latex. In the processing of natural rubber isoprene molecules polymerise and form long, coiled chains of cis-polyisoprene.
By making small incisions on the bark of rubber trees, like having a brasiliensis and guayule, the rubber latex can be collected into small vessels, as it oozes out. It contains 25%–45% of rubber in the form of milky colloidal emulsion, the remainder of which is made mainly of water and small amounts of protein and resinous material with time, the flow of latex from the incision made start decreases. Thus, at regular intervals, tapping is necessary throughout the life of the tree.
Latex is diluted to make 15%–20% of rubber and is filtered to eliminate any dirt present in it. It is then coagulated in a tank, fitted with irregular partitions by adding about one kg of acetic acid or formic acid per 200 kg of rubber, to a soft white mass. After washing and drying, the coagulated residue is treated as follows:
It is a trans-form of natural rubber. (In natural rubber, isoprene units are linked with cis-form). It is obtained from the matured leaves of Dichopsis gutta and Palaquium gutta trees, grown mostly in Malaya and Sumatra. Gutta–percha can be recovered by solvent extraction process, when insoluble resins and gums are separated. Alternatively, the matured leaves are grounded carefully and is treated with water at about 70°C for half an hour and then poured into cold water, when Gutta–percha floats on water surface it is removed.
The properties of gutta percha are as follows:
It is used in the manufacturing of golf ball covers, submarine cables, adhesives and tissues for surgical purposes.
The drawbacks of raw (natural) rubber are as follows:
To improve the properties of rubber, it is compounded with some chemicals such as sulphur, hydrogen sulphide, benzoyl chloride and the rubber mix is prepared for vulcanisation. The addition of compounding agents is facilitated by the process of mastication. Mastication of rubber means that it is subjected to severe mechanical working. Oxidative degradation accompanied by a marked decrease in the molecular weight of the rubber occurs and converts rubber into a soft and gummy mass.
Heating of raw rubber with sulphur at around 100°C –140°C is known as vulcanisation. Sulphur combines chemically at the double bonds of rubber chains and forms cross links. With these crosslinks, the rubber becomes stiff and the percentage of sulphur determines the stiffness of rubber.
For example, a rubber tyre contains 3–5% of sulphur.
The advantages of vulcanisation are as follows:
or example, ebonite is a better insulator.
The superior properties of vulcanised rubber compared to raw rubber are summarised in Table 2.1.
Table 2.1 Raw rubber vs. vulcanised rubber
Compounding is “mixing of the raw rubber with other chemicals so as to impart the product-specific properties suitable for particular job”. The following substances are generally mixed with raw rubber:
For example, for white products, titanium dioxide (TiO2) is the usual pigment. For colour products, the following pigments are used:
For example, addition of carbon black in the elastomer is used in the manufacture of automobile tyres.
The landmark discovery of rubber is the greatest achievement in polymer industry and with the efforts of scientists and technologists the first useful synthetic rubber Buna-S was prepared.
Due to better performance properties of synthetic elastomers, natural rubber failed to give stiff competition.
It is a copolymer of styrene (25% by weight) and butadiene (75% of weight). The monomers are emulsified in water using soap or detergent. The reaction is initiated by peroxide initiators. Polymerisation is carried out at 5°C, and therefore, the product is known as cold SBR.
Styrene rubber is slightly inferior to natural rubber in its physical properties. It possesses high abrasion resistance, high load-bearing capacity and resilience. However, it gets readily oxidised, especially in the presence of traces of ozone present in the atmosphere. It swells in oils and solvents. It can be vulcanised in the same way as natural rubber, but it requires less sulphur and more accelerators for vulcanisation.
The uses of styrene rubber are as follows:
It is a copolymer of a 1,3-butadiene and acrylonitrile. They are also prepared in emulsion systems. They are noted for their oil resistance but not suitable for tyres.
It possesses excellent resistance to heat, sunlight, oils, acids and salts, but it is less resistant to alkalis than natural rubber because of the presence of cyano groups. As the proportion of acrylonitrile is increased, the resilience to acids, salts, oils, solvents, etc., increases, but the low temperature resilience suffers. Vulcanised rubber is more resistant to heat and ageing than natural rubbers and may be exposed to high temperature.
The uses are as follows:
Thiokols are those elastomers in which sulphur forms a part of the polymer chain. It is a copolymer of sodium poly sulphide (Na2S4) and ethylene dichloride.
Thiokols have outstanding resistance to swelling and disintegration by organic solvents, mineral oils, fuels, solvents, oxygen, ozone, gasoline and sunlight. Thiokol films have low permeability to gases. They have the following limitations:
The uses of Thiokols are as follows:
These polymers are formed by the reaction between diisocyanates and polyalcohols.
Polyurethane elastomers have outstanding abrasion resistance and hardness combined with good elasticity and resistance to oils, greases, chemical, weathering and solvents.
They are used in applications where extreme abrasion resistance is required such as in heel lifts, surface coatings, manufacture of foams, spandex fibres and small industrial wheels.
Silicones are organic silicone polymers. They have alternate Si-O-bonds.
Preparation: For preparation of silicones, dialkyl-substituted silanes are used as raw materials.
They undergo hydrolysis and condensation polymerisation to form silicone polymers.
Silicones are of mainly two types:
Alkyl chlorosilanes are prepared by this process.
The properties of silicones are as follows:
The uses of silicones are as follows:
Reclaimed rubber is rubber obtained from waste rubber articles such as worn-out tyres, tables, gaskets, hoses, foot wear and so on. The process of reclamation of rubber is carried out as follows:
The waste is cut to small pieces and powdered by using a cracker, which exerts powerful grinding and tearing action. The ferrous impurities, if any are removed by the electromagnetic separator. The purified waste powdered rubber is then digested with caustic soda solution at about 200°C under pressure for 8–15 hours in a “steam-jacketed autoclave”. By this process, the fibres are hydrolysed. After the removal of fibres, reclaimed agents (like petroleum and coal tar based oils) and softeners are added and sulphur gets removed as sodium sulphide and rubber becomes devulcanised. The rubber is then thoroughly washed with water sprays and dried in hot air driers. Finally, the reclaimed rubber is mixed with small proportion of reinforcing agents (like clay, carbon black, etc.).
The reclaimed rubber is of less tensile strength lower in elasticity and possesses lesser wear-resistance than natural rubber. However, it is much cheaper, uniform in composition and has better ageing properties. Moreover, it is quite easy for fabrication.
For manufacturing tyres, tubes, automobile floor mats, belts, hoses, battery containers, mountings, shoes and heals, etc.
Polymers, generally of low strengths and moduli of elasticity, are needed for structural purposes. For these reasons, polymers are combined with fillers (which are primary silicates) to get better products. The fillers are solid additives, which modify the physical properties, particularly, the mechanical properties of basic polymeric materials. For example, they improve: (i) thermal stability, (ii) mechanical strength, (iii) insulating characteristics, (iv) water resistance, (v) external appearance, (vi) rigidity, (vii) finish, (viii) hardness, (ix) opacity and (x) workability, besides reducing (xi) cost and (xii) shrinkage on setting and brittleness.
Usually, specific fillers are added to a polymeric compound to impart special characters to the final products. Some examples are as follows:
The combination of polymeric substance with solid fillers, is called filled or reinforced plastics. The filler acts as a reinforcing material while the polymer acts as binder, which links the filler particles. The polymer serves as stress transforming agent from filler to filler particles.
Most commonly used fillers are as follows:
(i) wood flour, (ii) saw dust, (iii) ground cork, (iv) asbestos, (v) marble flour, (vi) china clay, (vii) paper pulp, (viii) corn husk, (ix) mica, (x) pumice powder, (xi) carbon, (xii) cotton fibres, (xiii) boron fibres, (xiv) silicon carbide, (xv) silicon nitride, (xvi) graphite, (xvii) alumina, (xviii) glass fibres, (xix) Kevlar fibres, (xx) cotton fibres, (xxi) metallic oxides such as ZnO, PbO and so on, and (xxii) metallic powder such as Al, Cu, Pb and so on.
Fillers are usually employed in a sizable weight percentage. The percentage of filler can be up to 50% of the total moulding mix.
Polymers used are thermoplastics, thermosetting polymers as well as rubber (elastomers) such as polyethylene, polypropylene, nylon-6, PET, polystyrene, melamine, silicone, natural and synthetic rubbers, epoxy among others.
Some examples of filled plastics are as follows:
Filled or reinforced plastics find numerous applications. Some examples are as follows:
Biopolymers are macromolecules that occur in nature from plants, trees, bacteria, algae or other sources that are long chains of molecules linked together through a chemical bond. They are often degradable through microbial processes such as composting. For example, cellulose, proteins, starch, collagen, casein, polyesters, etc.
“Sustainable biopolymers” are sourced from sustainably grown and harvested cropland or forests, manufactured without hazardous inputs and impacts, healthy and safe for the environment during use and designed to be reutilised at the end of their intended use through recycling or composting. The potential for using these materials to make synthetic polymers was identified in the early 1900s, but they have only recently emerged as a viable material for large-scale commercial use.
While biopolymers can be made from an almost unlimited range of bio-based materials, most of the currently marketed biopolymers are made from starch. Corn is currently the primary feedstock, with potatoes and other starch crops also used in lower amounts.
At present, either renewable or synthetic starting materials may be used to produce biodegradable polymers. Two main strategies may be followed in synthesising a polymer. One strategy is to build up the polymer structure from a monomer by a process of chemical polymerisation. The alternative is to take a naturally occurring polymer and chemically modifying it to give it the desired properties. A disadvantage of chemical modification is that the biodegradability of the polymer may be adversely affected. Therefore, it is often necessary to seek a compromise between the desired material properties and biodegradability.
Biodegradable polymers can be easily degradable by biological activities of decomposers. The natural raw materials are abundant, renewable and biodegradable, making them attractive feedstock for a new generation of environmentally friendly bioplastics. Even if only a small percentage of the biopolymers already being produced were used in the production of plastics, it would significantly decrease our dependence on non-renewable resources.
Poly (lactic acid) has become a significant commercial polymer, useful for recyclable and biodegradable packaging, such as bottles, yogurt cups and candy wrappers. It has also been used for food service ware, lawn and food waste bags, coatings for paper and cardboard and fibres for clothing, carpets, sheets and towels and wall coverings. In biomedical applications, it is used for prosthetic materials, drug encapsulation, biodegradable medical devices and materials for drug delivery.
Starch-protein compositions have the interesting characteristic of meeting nutritional requirements for farm animals. Hog feed, for example, is recommended to contain 13%–24% protein, and complemented with starch. If starch-protein plastics were commercialised, used food containers and service ware collected from fast food restaurants could be pasteurised and turned into animal feed.
There are enormous societal benefits that result from a shift to bio-based plastics. Bio-based materials have the potential to produce fewer greenhouse gases, require less energy and produce fewer toxic pollutants over their lifecycle than products made from fossil fuels. They may also be recyclable or composted (depending on the biomaterial and how it is produced), reducing waste streams to already crowded landfills or to incinerators.
As the cost of petroleum increases, making products with bio-based materials is increasingly attractive. Increased demand for agricultural and forest-based feedstock also offers new resource-based economic development opportunities for farmers, struggling rural communities and manufacturing sectors.
However, many of these advantages are not inherent in the material. They depend on ensuring that bio-based products meet minimal standards for the safe production, use and end-of-life disposition.
Making the transition from a petroleum-based to a bio-based economy also gives us an opportunity to ensure that the impact of product standards on the environment, health and society are included.
The widespread use of these new plastics will depend on developing technologies that can be successful in the marketplace. This, in turn, will partly depend on how strongly society is committed to the concepts of resource conservation, environmental preservation and sustainable technologies. There are growing signs that people indeed want to live in greater harmony with nature and leave a healthy planet to the future generation. If so, bioplastics will find a place in the current age of plastics.
Due to non-availability of free electrons, most normal polymers are insulators. Scientists have taken this property as an advantage, and with their curiosity and challenging nature, they prepared conducting polymers as promising materials.
“Conducting polymer is an organic polymer having highly delocalised π-electron system and electrical conductance”.
Conducting polymers are broadly classified into two categories as intrinsically conducting polymers and extrinsically conducting polymers.
Polymers which contain conjugated π-electron backbone or delocalised electron pairs act as intrinsic conducting polymers.
For example,
If the conductivity of intrinsically conducting polymers is less, they are doped with positive or negative charges and this process is known as doping.
It is mainly oxidative or p-doping, reductive or n-doping and protonic acid doping.
Mechanism of conduction: The removal of an electron from the polymer П-back bone using a suitable oxidising agent leads to the formation of delocalised radical ion called polarion.
A second oxidation of a chain containing polarion, followed by radical recommendation, yields two charge carriers on each chain. The positive charges sites on the polymer chains are compensate by anions formed by the oxidising agent.
The delocalised positive charges on the polymer chain are mobile, not the dopant anions.
Thus, these delocalised positive charges are current carriers for conduction. These charges must move from chain to chain as well as along the chain for bulk conduction.
Mechanism of conduction: The addition of an electron to the polymer П-back bone by using a reducing agent generates a polarion. A second reduction of chain containing polarion, followed by the recombination of radicals, yields two negative (-ve) carriers on each chain. These charge sites on the polymer chains are compensated by cations formed by the reducing agent.
Polyaniline is partially oxidised first, with a suitable oxidising agent, into a base form of aniline, which contains alternating reduced and oxidised forms of aniline polymer backbone. This base form of aniline when treated with aqueous HCl (IM) undergoes protonation of imine nitrogen atom, creating current due to +ve sites in the polymer backbone. These charges are compensated by the anions (Cl−) of the doping agent, giving the corresponding salt. This doping results increase conductivity up to 9–10 orders of magnitude.
Conducting polymers are the most important materials to be used in electric and electronic applications. Some of the uses are as follows:
In 1985, Alan MacDiarmid investigated polyaniline as an electrically conducting polymer.
The properties of polyaniline are as follows:
For example, under reducing condition, it becomes yellow and under oxidising or basic condition, it becomes blue.
The advantages are as follows:
The disadvantages are as follows:
The uses of intrinsically conducting polymers are as follows:
Polymers whose conductivity is due to externally added ingredient are known as extrinsically conducting polymers. They are conductive element filled polymers and blended conducting polymers.
Conducting polymers have many uses because they are light weight, easy to process and have good mechanical properties. They are used in the following:
Polyphosphazenes are hybrid inorganic organic polymers with a number of different skeletal structures that contain a backbone of alternating phosphorous and nitrogen atoms, and are interesting, commercially promising materials. A variety of substituents can substitute the basic backbone and hence we can get a variety of products. The basic backbone of polyphosphazene is as follows:
Preparation: The most popularly used method for preparing polyphosphazenes is ring opening and substitution method. Allcock and co-workers discovered that cyclic trimer (hexachlorocyclo triphosphazene) can be thermally ring opened and can give high molecular weight soluble poly (dichlorophosphazene). After the replacement of the chlorine atoms in poly (dichlorophosphazene) by reaction with organic/organometallic nucleophiles, they give a variety of polyphosphazenes.
polyphosphazenes have many important properties; biocompatibility, high dipole moment, flexibility, chemical inertness, broad range of glass transition temperature (Tg), elastomeric property and impermeability are the most important of them.
Based on their wide range of unique properties, polyphosphazenes have countless and advanced applications. They have potential for formation of new compounds. The applications include in challenging areas of biomedical research such as tissue generation, macromolecules and so on. These are also used as ion-conductive membranes for rechargeable lithium batteries and fuel cell membranes. These are advanced materials of elastomers for aerospace engineering. Polyphosphazenes are good photonic materials and fire-resistant polymers.
Composites are multiphase materials that exhibit a significant proportion of the properties of both the constituent materials.
Composite materials composed of at least two distinctly dissimilar materials act in harmony. A judicious combination of two or more distinct materials can provide better combination of properties or an artificially prepared multiphase material in which the chemically dissimilar phases are separated by a distinct interface.
For example, wood is the composite of cellulose and lignin, bone is the composite of a soft, strong protein collagen, and brittle, hard apatite material.
Packing paper impregnated with bitumen or wax, rain-proof cloth (cloth impregnated with waterproof material), insulating tape, reinforced concrete, etc.
Composite material mainly comprises of the following:
Hence, a good matrix phase should be ductile, having corrosion resistant and possess high binding strength.
Composites are broadly classified into three categories:
Discontinuous composites are further divided into (a) aligned (b) randomly oriented.
Among these, fibre-reinforced polymer composites are widely used.
These are prepared by reinforcing a plastic matrix with a high-strength fibre material.
Fibre-reinforced composites involve three components, namely filament, a polymer matrix and an encapsulating agent (which ties fibre filaments to polymer). Glass fibres and metallic fibres are commonly employed for this purpose. The fibres can be employed either in the form of continuous lengths, staples or whiskers. Such composites possess high specific strength (tensile strength/specific gravity) and high specific modulus (elastic modulus/specific gravity), stiffness and lower overall density.
The fibre-reinforced composites possess superior properties such as higher yield strength, facture strength and fatigue life. The fibres prevent slip and crack propagation and inhibit it, thereby increasing mechanical properties. When a load is applied, there is a localised plastic flow in the matrix, which transfers the load to the fibres embedded in it. When a soft phase is present in hard matrix, the shock resistance of the composite is increased. On the other hand, if hard-reinforcing fibres are present in a soft matrix, the strength and modulus of the composite are increased. To obtain composites having the maximum strength and modulus, it is essential that there should be maximum number of fibres per unit volume, so that each fibre takes its full share of the load. The fibre-reinforced composites are, generally, anisotropic (i.e., having different directions), and the maximum strength is in the direction of alignment of fibres. For getting isotropic properties, the fibres are oriented randomly within the matrix, for example, ordinary fibre glass. It may be pointed here that the cost of laying fibres aligned in a particular direction is much higher than that for random orientation. For preparing fibre-reinforced composites, the following are essential:
Some important reinforced composites are described here.
Limitations: The limitations are as follows:
Applications: They are used in automobile parts, storage tanks, floorings (industrial), transportation industries, plastic, pipes, etc.
Applications: They are used as structural components (like wing, body and stabiliser) of aircrafts (military and commercial) and helicopter’s recreational equipment (fishing rod), sport materials (golf clubs), etc.
Composites have the following advantages over conventional materials such as metals, polymers, ceramics and so on.
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Ans.: The repeating unit present in the formation of a polymer is known as a monomer.
Ans.: Polymers are macromolecules of high molecular masses built up by the linking together a large number of small, repeated units by a covalent bond.
Ans.: The chemical process leading to the formation of a polymer is known as polymerisation.
Ans.: The number of monomer units in a polymer is known as the degree of polymerisation.
Ans.: Homopolymers are formed with same monomer units.
For example, PE, PS, PVC, etc.
Copolymers are formed with two or more different monomers.
For example, styrene butadiene rubber (styrene + butadiene)
Ans.: Addition polymerisation is the process of polymerisation by the addition of monomer units which have unsaturated double or triple bonds.
For example, polyethylene, polyvinyl chloride, etc.
Condensation polymerisation takes place where the monomer units have two or more reactive functional groups.
For example, polyester, nylon, polyamide, etc.
Ans.: Linear, long chain polymers which can be softened on heating and hardened on cooling are known as thermoplastics.
For example, polythene, polyvinyl chloride, etc.
Ans.: Hexamethylenediamine and adipic acid.
Ans.: Vulcanisation is heating of the raw rubber at 100–140°C with sulphur.
Ans.: Cis-polyisoprene is a natural rubber.
Ans.: Trans-polyisoprene is gutta percha.
Ans.: Rubber with sodium bisulphite is passed through a creping machine and the coagulum is rolled into sheets. The sheet is hence having the surface like crepe paper; hence, it is known as crepe rubber.
Ans.: An elastomer is vulcanisable rubber like polymer, which can be stretched to at least twice its length and returned to its original shape and dimensions as soon as stretching force is released.
Ans.: In vulcanisation, sulphur combines chemically at the double bonds of different rubber spring and provides cross-linking between the chains. Hence, for stiffening the rubber needs vulcanisation.
Ans.: Cotton, silk, wool, nucleic acid, proteins, starch, cellulose, etc.
Ans.: Polyphosphazenes, polysilanes, polygermanes, etc.
Ans.: Antioxidants, colouring agents, vulcanising agents, accelerators, plasticisers and inert fillers are adding in the compounding of raw rubber.
Ans.: Bio-polymers are macromolecules that occur in nature from plants, tress, bacteria, algae or others source that are long chains linked together through a chemical bond. They are often degradable through microbial process such as compositing.
For example, cellulose, proteins, starch, collagen, casein and polyesters
Ans.: (a) Lactic acids: It is obtained from fermentation of sugar feed stocks conversion of starch from corn, potato peels, etc.
(b) Triglycerides: It is produced from soya bean, fax and rape seed.
(c) Starch: It is found in corn, potatoes, wheat, tapioca and some other plants.
(d) Collagen: It is found in mammals. It is used in capsules for drug.
Bio-based materials have the potential to produce fewer greenhouses gases, require less energy and produce fewer toxic pollutants.
Q.1 (a) Write a note on the properties and uses of Teflon.
(b) Differentiate between natural polymer and synthetic polymer.
(c) Write a note on silicone rubbers.
Q.2 (a) Distinguish between addition and condensation polymerisation.
(b) Explain the differences between thermoplastics and thermosetting plastics with examples.
(c) What is meant by degree of polymerisation?
Q.3 Write the structures of four addition polymers and four condensation polymers with their respective monomers.
Q.4 (a) Describe the preparation, properties and engineering uses of polyethylene.
(b) What is meant by fabrication of plastics? Mention the different fabrication techniques.
Q.5 (a) What are elastomers? Give examples.
(b) What are the ingredients used in the compounding of plastics? What are their functions?
Q.6 (a) What is a plastic?
(b) Write the merits and demerits of using plastics in the place of metals.
Q.7 (a) Identify the thermosets and thermoplastics among the following:
(b) What is Bakelite? How is it manufactured? Mention its uses.
Q.8 (a) Explain the process of extrusion moulding with a neat diagram.
(b) How are the following polymers prepared? Mention their properties and uses.
Q.9 (a) What are elastomers? Give the preparation, properties and uses of Buna-S.
(b) Describe a method for moulding of thermoplastic resin.
Q.10 (a) Explain the preparation, properties and uses of Bakelite.
(b) Describe the process of compression moulding with a neat sketch.
Q.11 (a) Why are silicones called inorganic polymers? Discuss the synthesis of linear chain silicones.
(b) Why can Bakelite not be remoulded? Write its repeating unit.
(c) Describe condensation polymerisation with an example.
Q.12 (a) What is a homochain polymer? Give examples.
(b) What is polymerisation? Explain the different types of polymerisation with examples.
Q.13 (a) How is HDPE prepared? Give its properties and uses.
(b) Explain the injection moulding process with a neat diagram. Mention its advantages.
Q.14 Write informative notes on the following:
Q.15 What are the common constituents of plastics and what are their functions?
Q.16 Write short notes on the following:
Q.17 Write an essay on the preparation, properties and uses of the following:
Q.18 Discuss the various polymers related to natural rubber with emphasis on their preparation, properties and uses.
Q.19 Write an account of application of polymers in bio-medical electronic fields.
Q.20 Write an essay on the various types of synthetic rubber with brief description of the preparation, properties and uses of any three of them.
Q.21 Write an essay on fibre-reinforced plastics.
Q.22 What type of rubber would you recommend for the following?
Q.23 Write short notes on the following:
Q.24 Define the following terms and give examples: monomer, polymer, polymerisation and degree of polymerisation.
Q.25 Distinguish clearly between the following:
Q.26 Discuss the addition polymerisation reaction mechanism.
Q.27 Explain condensation polymerisation with an example.
Q.28 Write informative notes on the following:
Q.29 Give the preparation, properties and engineering uses of the following:
Q.30 Discuss the various polymers related to natural rubber with emphasis on their preparation, properties and uses.
Q.31 Write an essay on the various types of synthetic rubbers.
Q.32 What are elastomers? Write the structure for natural rubber and gutta percha.
Q.33 What is vulcanisation of rubber? Mention its uses. Explain why natural rubber needs vulcanisation. How is it carried out?
Q.34 How is crepe rubber obtained from latex?
Q.35 What is latex? How is natural rubber isolated from it?
Q.36 Write short notes on silicones. How are they prepared?
Q.37 Write a short note on conducting polymers.
Q.38 Give the structural unit of gutta percha.
Q.39 Give detailed notes bio-polymers and its importance.
Q.40 Explain the preparation methods of bio-polymer and the importance of bio-degradable polymers.
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