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Polymers

Polymers are high molecular mass compound and their structure is composed of a large number of simple repeating units.

The repeating units are usually obtained from low molecular mass compounds called monomers. Polymers have very vast spectrum of properties and that is why they are very important for different applications.

The polymer materials are so important in our daily life that survival of human life depends on these polymers. Following are some uses of polymer materials we use in our daily life.

 

What is Polymer?

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In modern civilization, almost every part of life involves polymers. Polymers are chief products of plastics, elastomers, fibers and paints & varnishes industries.
The use of polymers in furniture, automobile parts, tyres, gear and seals are very much common in our daily life.

Polymer is a combination of two Greek words 'Poly' & 'Mer'; 'Poly' means many and 'Mer' means units or parts. Polymer is defined as giant molecules with very high molecular mass and consists of a large number of repeating units, which are known as monomers. The process of formation of polymer from various monomer units is known as polymerization.

It involves the conversion of small molecular mass units (monomers) to high molecular mass units which are called as polymer. The term polymer and macromolecules are often used interchangeably but there are some very important differences between both of them.

Polymers are made up of hundreds to thousands monomer units, but it is not necessary for macromolecules. For example, protein, carbohydrate are example of macro molecule but polythene is an example of polymer. Hence all polymers are macromolecules but all macromolecules are not polymers.

The branch of science which deals with the structure, properties, reactivity and applications of polymers is called as polymer science.

Polymerization

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The process of formation of polymer from monomer units is known as polymerization. Depending on the mode and mechanism of polymerization reaction, polymerization can be of two types,
  1. Addition polymerization
  2. Condensation polymerization

1. Addition Polymerization


Monomer units, added to grow a polymer chain caring a reactive intermediate like free radical, carbocation or carbanion, is known as addition polymerization. Since it forms a long polymer chain, it is also known as chain growth polymerization.
For example, ethene (H2C = CH2) is polymerized for polythene. On the basis of the presence of intermediates, addition polymerization can be of three types;

(a) Free Radical Addition Polymerization

When polymerization is initiated by peroxides, free radical is formed as intermediate. A variety of unsaturated compounds like alkenes, alkadienes readily undergoes polymerization in the presence of peroxide like benzoyl peroxide, acetyl peroxide with high temperature and high pressure.

For example, the polymerization of ethene to form polythene is followed by free radical polymerization. Reaction takes place at high temperature (350-570 k) and high pressure (1000-2000 atm) in the presence of benzoyl peroxide as an initiator for reaction.

Peroxide generates free radical which reacts with ethene to form another radical,
which takes part in propagation step to form polymer chain. The combination of polymeric chain through radical site terminates the reaction to form polymer chain.

(b) Cationic Addition Polymerization
  • Cationic polymerization is initiated by an acid which adds on the double bond to form a cation.
  • This cation takes part in chain propagation step to form polymer chain.
  • The polymerization of isobutylene to form butyl rubber is an example of cationic addition polymerization.
  • Common acids used are phosphoric acid and sulfuric acid, these are used as initiators.

(c) Anionic Addition Polymerization
  • Just like free radical and cationic addition polymerization, an anion can be an initiating reagent in polymerization reaction.
  • Such type of polymerization reaction is called as anionic addition polymerization.
  • Generally, vinyl compounds show this type of polymerization as their positive charge gets stabilized by anion through delocalization.
  • Styrene, dienes, methacrylate, vinyl pyridine, aldehydes, epoxide, cyclic siloxane, and lactones show this type of polymerization as there is a possibility of delocalization charge in the presence of some anion.
  • The common initiators for anionic addition polymerization are covalent or ionic metal amides, alkoxides, hydroxides, cyanides, phosphines, amines and some organometallic compounds. For example, formation of polystyrene is initiated by anion and forms polystyrene.

2. Condensation Polymerization

  • When both reactant of a chemical reaction are bi-functional, they undergo a series of condensation reactions and form a polymer chain.
  • The condensation process takes place in a stepwise manner with the loss of simple molecules like water, carbon dioxide, alcohol, etc.
  • Condensation polymerization starts from the condensation reaction of two reactant molecules and forms dimer.
  • This dimer again reacts with another molecule to form trimer and so on.
  • Since the product of condensation reaction is also bifunctional, therefore, the sequence of condensation goes on and the polymer chain keeps on growing step by step.
  • For example, condensation polymerization of terephthaloyl chloride and ethylene glycol forms a polyester known as (polyethyleneterephthalate) by the elimination of hydrochloric acid.
  • Instead of tersphthoyl chloride benzene-1,4-dicarboxylic acid can be used for the formation of PET or decron.
  • Another example of condensation polymerization is polyurethanes which is made from a dialcohol and diisocyanate monomers.

Classification of Polymers

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Polymers can be classified in a number of ways.
  1. Based upon the sources or origin.
  2. Based upon the structure of polymer.
  3. Based upon the molecular forces.
  4. Based upon the nature of monomer units.

Based upon the polymer sources or origin


On the basis of origin or polymer sources, they can be classified into three types.

1. Natural Polymer

Such type of polymer is found in nature, in animals and in plants. For example, protein, cellulose, rubber are natural polymers.

2. Semi Synthetic Polymer

The derivatives of natural polymers are included in this class. For example, cellulose acetate is a derivative of cellulose which forms due to acetylation of cellulose and used for making threads, films and glasses.

3. Synthetic Polymer

They are man-made polymers made synthetically and extensively in daily life, like, fibers, plastics, rubbers.

Based upon the Structure of Polymer


On the basis of structure of polymers, they can be classified into three types.

1. Linear Polymer


When monomer units are joined together in the form of long straight chain and these polymeric chains are stacked over one another to give a well packed structure, then such arrangement of monomer unit is known as linear polymer. For example, polythene, polyvinyl chloride, etc. Linear polymer shows high melting point, high tensile strength and density value.

2. Branched Polymer

Polymeric linear chains get branches of different length along the main chain, called as branched polymers. Due to branching along the main chain, these polymers show low melting point, densities as well as tensile strength. For example, glycogen.

3. Crossed linked or three dimensional network polymer
  • In such type of polymers, the linear polymer chains are joined together to form a three dimensional network.
  • Due to cross linking between linear chains, this polymer is also termed as cross-linked polymer.
  • They are hard, rigid and brittle in nature like Bakelite, urea-formaldehyde resin, etc.

Based upon the Molecular Forces


The mechanical and chemical properties of polymers depend on the intermolecular forces of attraction like van der wall forces, hydrogen bonds and dipole-dipole interaction between polymeric chains.

Since the magnitude of these forces depends upon the molecular size and the number of functional group present in polymers. On the basis of these intermolecular forces, polymers can be classified into four categories.

1. Elastomers

They have weakest attraction force in between polymeric chains. Hence they are amorphous with high degree of elasticity. They have the ability to starch out over ten times than the original length and get back to original position after removing the force. For example, Buna-S , Buna-N , Neoprene.

2. Fiber

These types of polymers have strongest intermolecular force of attraction between polymeric chains. Due to strong force of attraction, they have high tensile strength, and least elasticity. Here either hydrogen bond or dipole-dipole interaction gets involved between chains.

Since the molecules of theses polymers are thread like and easily packed, they are termed as fibers.

3. Thermoplastic (Polymer plastic)

They have intermediate intermolecular force of attraction between molecules.
In such type of polymers, hard linear and branched chain become soft after heating but again gets rigidness after cooling.

The process of heat softening and cooling can be repeated as many times as desired without changing the chemical composition of polymers. Polythene, polyvinyl chloride, Teflon, polystyrene are some of the example of thermoplastics.

4. Thermosetting Polymers (Polymer plastic)

They are low molecular mass substance which can be mold on heating.
But unlike thermoplastics, the chemical composition gets changed for them after heating to give a hard, in-fusible and insoluble mass.

The harding on heating is due to the cross linking of polymeric chains to give a three dimensional network polymer and hence they can be heated only once. For example, Bakelite, urea-formaldehyde resin, etc.

Based upon the Nature of Monomer Units


On the basis of nature of repeating units in polymers, they can be classified as,

1. Homo Polymers

Those polymers which are made up of same type of monomer units are known as
homo polymers.

For example, Polyvinyl chloride is an addition polymer which contains same type of monomer unit that is vinyl chloride.

nCH2=CH-Cl -[-CH2-CH-Cl-]-n
(Vinyl chloride) (Polyvinyl chloride)

2. Co Polymer
  • When two or more types of monomer units are associated in polymerization reaction to form polymer, they are called as co polymers. For example Nylon-6,6 is formed by the condensation polymerization of adipic acid and hexamethylenediamine. While Nylon-6 is formed by the polymerization of caprolactum only, hence it’s an example of homo polymers.

Polymer Concrete

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This material is made up of different types of polyester resins. These resins are light weight and stable toward the freezing-thawing cycles.

The chemical and physical properties of polymer concrete make it a good replacement of cement as a binder. Polymer concrete is generally used in new construction and repairing of old concrete. Because of adhesion properties of polymer concrete, it is used in patching for both polymer and cementitious concretes.

It shows low permeability and due to this property, it is used in swimming pools, sewer pipes, drainage channels, electrolytic cells.

Polymer concrete is also composed of aggregates that include silica, quartz, granite, limestone, and other high quality material.

Polymer Composites

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Any material which is made of more than one component is called as composite. For example, when natural rubber is embedded between two layers of cotton fibres, water proof composite material is formed because cotton is made up of cellulose which is a good material for making rain coats, gloves and other water proof fabrics while natural rubber makes it water proof.

Hence we use composite to make a material which has the properties of both its constituent components.

Nowadays, a composite is made up of two components, a fiber and matrix. Generally, fiber is glass, Kevlar, carbon fiber, or polyethylene and matrix is thermosetting polymer. The common thermosetting polymers are epoxy resin, polydicyclopentadiene, or a polyimide. In formation of composite, fiber is embedded in the matrix for making the matrix stronger. These composites are also called as fiber-reinforced composites. They are very strong infecting more than steel, but weigh much less.

The main application of composites is in making automobiles, disc brake pads, shower stalls and bathtubs, Imitation granite and cultured marble sinks and counter tops. They are widely used in manufacturing of spacecraft. In general, commercially produced composites use a polymer matrix material which are called as a resin solution.

A large number of polymers are available for making polymer composite depending upon the starting raw ingredients. Common examples of polymer matrix are polyester, vinyl ester, epoxy, phenolic polymer, polyamide and polypropylene etc.
Fiberglass is the most common fiber-reinforced composite. This matrix is formed by reacting polyester and styrene.

The fibers aren't lined up in any particular direction in fiberglass. For increasing the strength of composites all fibers are lined up in the same direction.

Lithium Polymer

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Lithium polymer is a rechargeable lithium-ion polymer battery also abbreviated as Li-poly, Li-Pol, LiPo, LIP, PLI or LiP. Lithium polymer consists of many identical secondary cells arranged in parallel manner which increase its discharge and current capability.

Lithium polymer is based on comparatively new technology with a plastic anode material and SPE (Solid Polymer Electrolyte) as the electrolyte. Some gelled conductive material is added in the solid electrolyte to promote the conductivity of lithium polymer. It is used as a portable computer, personal digital assistant, cellular phones etc.

This type of batteries show high voltage per cell and they can be made in very thin and large footprint configurations. These batteries are based on liquid Lithium-ion electrochemistry in a matrix of conductive polymers which eliminates free electrolyte within the cell.

Lithium Polymer

Polymer Plastic

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Generally the term polymer and plastic are used interchangeably. While all plastics are polymers but all polymers are not plastic. There are a large number of polymer which are studied as plastic. On the basis of their nature, they are classified as thermosetting and thermoplastic and thermosetting plastic. Some common examples of plastics are,

High Density Polyethylene (HDPE)

It is an addition polymer of ethene. It is translucent and shows resistivity for moisture, hence have wide application in our daily life.

Low Density Polyethylene (LDPE)

LDPE is similar to HDPE but it’s resistivity towards chemical is very less. Compared to HDPE, it is more translucent and is used mainly for squeeze applications.

Polypropylene (PP)


The containers made form have contact clarity and an excellent moisture barrier. They are stable at high temperatures (<200 degrees). This plastic shows excellent chemical resistance but poor impact resistance in cold temperatures.

Polystyrene (PS)

This plastic is mainly used as packaging materials due to its excellent clarity, poor impact resistance.

Polyvinyl Chloride (PVC)

This plastic is a semi rigid material with good resistance to oils . It is unstable at temperatures over 160 degrees and gets distorted.

Polyethylene Terephthalate (PET)

Generally, soda bottles and water bottles are made from PET plastic due to its excellent clarity and very good alcohol and essential oil barrier properties. The main disadvantage of PET is its heat sensitivity.

Polymer Uses

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The light duty tyres, beltings, hoses and rubber soles are made from the synthetic rubber or from Buna-S a polymer from styrene and butadiene. Fabric and magnetic recording tapes are made of Dacron or terylene, a polymer obtained from the monomers of ethylene glycol and dimethyleterepthallic acid.

The glyptol a polymer of pthallic acid and ethylene glycol is used for the manufacture of paints and liquors. Nylon-66 is polymer of hexamethylene diamine and adipic acid, it is used to prepare fabrics, tyre cords, ropes, carpets etc.

  1. Lot of electric goods , phonograph, records, fountain pen barrels, combs etc.., are made from a polymer of formaldehyde and phenol called Bakelite.
  2. Melamine-formaldehyde resin or melamine is used in the manufacture of plastic crockery.
  3. Polyurethane a polymer of ethylene glycol and ethylene di-isocynate is used in the manufacture of paints and heat insulators.
  4. Polythene is used to prepare pipes, toys bags, wire insulators, bottles etc.
  5. The polyvinyl chloride is used to prepare sheet, water pipes, hand bags etc. Radio and television cabinets are prepared from the polystyrene a polymer of styrene.
  6. Insulators and gaskets are prepared from the polymer material Teflon or poly fluoroethene.
  7. Latex paint is prepared from polymer material polyvinyl acetate.
  8. Nowadays polymers are being used in nanaotechnology and a lot of research is under progress to find the use and the properties of the polymers for use as semiconductors.
  9. Polyacrylonitrile is used to prepare Orlon fiber and acrilon films.
More topics in Polymers
Thermoplastic Polymers Liquid Polymer
Homopolymer Polyurethane
Synthetic Polymer Polypropylene
Radical Polymerization Condensation Polymerization
Free Radical Polystyrene
Polyethylene Terephthalate Acrylic Polymer
Natural Polymer Radicals
Polyvinyl Chloride Polyethylene
Thermosetting Polymer Copolymer
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