Classification of Polymer

The word polymer is derived from two Greek words, poly, meaning many (numerous) and mer, meaning units.  Basically, a polymer is a long-chain molecule made up of a large number of repeating units  of the same structure. These identical structures, which we think of as a unit made up of two or more molecules, join together to form a long chain polymer.To put it simply, a polymer is a long-chain molecule made up of a large number of repeating units of the identical structure. These monomers can be as simple as one atom, or two or three atoms, or they can be complex ring structures containing a dozen or more atoms. Organic polymers play a critical role in living things, offering simple structural substances and collaborating in essential life processes. For example, the solid parts of all flora are made of polymers. These consist of cellulose, lignin, and many resins. Cellulose is a polysaccharide, a polymer which is composed of sugar molecules. Lignin includes a complex 3-dimensional network of polymers.Another acquainted isoprene polymer is rubber.

Classification of  conducting polymers 

Conducting polymers are classified into two categories:- 

(1)Intrinsically conducting polymers 

(2) Extrinsically conducting polymers. 

Intrinsically conducting polymers

This is a polymer whose structural frame has a delocalized electron pair (conjugated pi bond) or residual charge. In an electric field, delocalized π electrons get excited and  transported through the polymer material. The valence bands and conduction bands are formed by the overlapping of orbitals conjugated π  bonds. 

Doped Conductive Polymer 

 In the doping process the polymer is either oxidized (removed electrons) or reduced (addition of electrons) so that the polymers chain carries  resonating charge. This can be further classified as follows. 

 P-doping i.e.oxidative doping is performed by an oxidizing agent such as iodine etc.

N-doping i.e.reduction by doping is performed by treating intrinsically  conductive polymer with a Lewis base, resulting in a reduction process. The very commonly used N-type dopants are Na metal, FeCl3 etc.

Extrinsically conducting polymers

The conductivity of these  polymers is due to the presence of externally added components. They may further be classified as-

• The polymer filler with conducting elements like carbon black or metallic fibres are known as conductive polymers.
• Blended polymers are the result of blending conventional polymers by combining a metal storm with a polydentate ligand.

Classification of polymers based on structure 

Polymers can be classified into three types based on their structure. 

• Linear polymer: 

 These polymers resemble long straight chains in structure, with identical links linked to each other. The monomers combine with each other to form a long chain. These polymers have a high melting point and high density. A common example of this is PVC (polyvinyl chloride). This polymer is mainly used in the manufacture of electric wires and pipes. 

• Polymers with branched chains: 

 As the name suggests, the structure of these polymers is like branches emanating from a single linear chain at random points. The monomer binds to  a long straight chain with several branched chains of various lengths. Due to these branches, the polymer is not tightly packed. They have low density and low melting point. Low density polyethylene (LDPE) used in plastic bags and general purpose containers is a common example of branched chain polymers.  

• Cross-linked or cross-linked polymer: 

 In this type of polymer, monomers combine with each other to form a three-dimensional network. Monomers are  bifunctional and trifunctional in nature and therefore contain strong covalent bonds. These polymers are brittle and hard. Examples: Bakelite (used for electrical insulators), melamine, etc.

Classification based on molecular forces 

(i) Elastomer: 

 Elastomers are rubbery solid polymers that are elastic in nature. Basically, elasticity means that the polymer is easy to stretch under force.  The most common example of this is a rubber band (or hair band). The band will stretch with a little stress. The polymer chain is held by the weakest intermolecular force and can stretch the polymer. However, as you may have noticed, when this stress is removed, the rubber band also returns to its original shape. When we do this ,it helps to introduce crosslinks between the polymer chains  to restore their original position and shape. Our car tires are made of vulcanized rubber. This is when sulfur is introduced to crosslink the polymer chains. 

ii) Thermoplastics: 

 Thermoplastic polymers are long-chain polymers in which an intermolecular force (Van der Waals force) holds polymer chains together. These polymers soften when heated (like a thick liquid) and harden when cooled to form a hard mass. It does not contain  cross bonds and can be easily molded by heating and using a mold. Common examples are polystyrene or PVC (used to make pipes). 

 iii) Thermosetting: 

 Thermosetting polymers are  low molecular weight semi-liquid polymers. When heated, they begin to crosslink between the polymer chains, making them hard and injectable. When exposed to heat, they form three-dimensional structures. This reaction is essentially irreversible. The best-known example of  thermosetting polymers is bakelite, which is used to make electrical insulation. 

iv) Fiber: 

 In the polymer classification, it is essentially a thread and is a class of polymers that can be easily  woven. They have strong intermolecular forces between the chains, giving them low elasticity and high tensile strength. Intermolecular forces can be hydrogen bonds or dipole-dipole interactions. The fibers are sharp and have a high melting point. A common example is nylon 66, which is used for carpets and clothing.

Conclusion

Today, polymeric materials are used in nearly all areas of daily life and their production and fabrication are major worldwide industries. In 2012, the total U.S. production of synthetic fibers (principally non-cellulosic) and plastics was 2.81 and 34.1 million metric tons, respectively. Among plastics, the largest shares of the total production in 2012 were polyethylenes, followed by polypropylene, poly(vinyl chloride) (PVC), and  polystyrene. Thermosetting resins (principally phenolic, urea, and melamine resins) typically represent around 10% of the total plastics production while synthetic rubbers, such as styrene–butadiene rubber (SBR) and polybutadiene, represent only about 6% of the total production in recent years.