POLYMER
Introduction:
Polymers are macromolecules having high molecular weights
and they are composed of smaller repeating units called monomers. Monomers are
small and simple molecules depend and have a capacity of forming two chemical
bonds with two other monomers. The functionality of monomers depends upon the
number of bonding sites in monomer. Let us take some day-to-day life examples
to understand polymers better. The DNA in human body, proteins, nerve fibers
etc. is all polymers. The toys of children are plastic polymers. The PVC pipes,
raincoats, carpets are again few examples which we use often in our routine
lives. Glass and concrete are inorganic polymers.
What polymers mean?
To understand what a polymer basically means, let us
break into two words; here, (poly) means many and (mers) means units. In the
structure of the polymer molecule a simple chemical unit repeats itself for a
large number of times.
For example, CH = CH (Ethylene monomers)
-CH – CH – CH – CH – (Polymer known as polyethylene)
The number of repeat units in a given polymer molecule is
termed as degree of polymerization. Molecular weight of the polymer is the
product of the molecular weight of the repeat unit and its degree of
polymerization.
Classification of Polymers
There are various ways to classify polymers based on
source, structure, mode of forming, molecular forces etc.
Classification on the bases of source:
Natural polymers: These polymers are obtained either from
plants or animal and are named as plant polymer e.g. cellulose, jute, linen,
silk, wool etc.
Semi-synthetic: the properties of natural polymers are
modified by some sort of chemical treatment. The properties which are modified
are appearance, tensile strength, luster etc. e.g. acetate rayon, viscose
rayon.
Synthetic polymers: These are man-made, produced in labs
or industries from chemical substances e.g. nylon, terylene and polythene etc.
Classification on the basis of structure of polymers:
These polymers are further classified into three types.
Liner polymers: They are made up of long continuous
chains without any excess attachments or branches. The repeating units are
joined together to form a long chain. e.g. polythene and PVC
Branched polymers: These polymers consist of chain
structures having one main chain of molecules with smaller chains as branches
of main chain. They have low density. E.g. polypropylene having methyl groups
as branches.
Cross-linked polymers: Linking of chain polymers by
strong covalent bonds which form the network like structure.
Classification on the basis of polymerization process:
Addition polymers: These polymers are formed by the
simple addition of monomers and have – C – C- linkages along the main chains.
Condensation polymers: These polymers are formed by
reactions between the molecules by the elimination of a simple byproduct and
the main chain may contain O, N, S atoms in the main chain along with – C – C-
linkages.
Classification on the basis of thermal responses:
Thermoplastic polymers: On the application of heat, they
become soft and on cooling they become hard. Hence these polymers can be
remolded and recycled. E.g. PVC, polythene
Thermosetting polymers: These do not become soft on
heating under pressure, hence cannot be remolded and recycled. E.g. Bakelite
Classification on the basis of crystallinty:
Crystallinty means the orderly or how closely the
molecular chains are packed. Different polymers have different tendencies to
crystallize at different rates. It depends upon the chemical nature of the
polymer and the physical conditions.
Tacticity: It can be defined as the spatial disposition
of the substituent attached to the asymmetric carbon atoms in the chain. They
are broadly classified under three categories:
Isotactic: Identical spatial disposition
Syndiotactic: alternating configurations
Atactic: Completely random and may be considered as a
combination of Isotactic and syndiotactic.
On the basis of application and physical properties:
Fibers: They are the strongest of the three types.
Rubbers: They are characterized by elasticity.
Plastic: They are stronger than that of rubber. They are
rigid and can be flexible and soft
Lastly, polymers should have certain properties like
strength, elongation, solubility, crystallinty and bi- refringence.
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