Classification of Polymers
There are several ways of classification of polymers based on some special considerations.
The following are some of the common classifications of polymers:
- Mode of polymerization
- Molecular Force
- Under this type of classification, there are three sub categories.
- Natural polymers
- These polymers are found in plants and animals. Examples are proteins, cellulose, starch, some resins and rubber.
- Semi-synthetic polymers
- b) Cellulose derivatives as cellulose acetate (rayon) and cellulose nitrate, etc. are the some examples.
- Synthetic Polymers
- A variety of synthetic polymers as plastic (polythene), synthetic fibres. Examples: - (nylon 6, 6) and synthetic rubbers (Buna - S) are polymers extensively used in daily life as well as in industry.
- There are three different types based on the structure of the polymers.
- Linear Polymers: -
- These polymers consist of long and straight chains. The examples are high density polythene, polyvinyl chloride, etc. These are represented as:
- Branched chain polymers:-
- b) These polymers contain linear chains having some branches, e.g. low density polythene. These are depicted as follows:
- Cross linked or Network polymers:-
- c) These are usually formed from bi-functional and tri-functional monomers and contain strong covalent bonds between various linear polymer chains, e.g. Bakelite, melamine, etc. These polymers are depicted as follows:
- Mode of Polymerization
Polymers can also be classified on the basis of mode of polymerisation into two sub groups.
- Addition polymers: -
- The addition polymers are formed by the repeated addition of monomer molecules possessing double or triple bonds.
- For example: - The formation of polythene from Ethene and polypropene from propene.
- However, the addition polymers formed by the polymerisation of a single monomeric species are known as homopolymers.
- For example: - polythene.
n CH2=CH2 --> (CH2-CH2-)n Homopolymer
- The polymers made by addition polymerisation from two different monomers are termed as copolymers, e.g., Buna-S, Buna-N, etc.
nCH2 = CH- CH =CH2 + n C6H5CH=CH2 à (-CH2-CH=CH-C6H5-CH2-CH-CH2-) n
1,3 – Butadiene Styrene Butadiene-styrene copolymer
- Condensation polymers
- The condensation polymers are formed by repeated condensation reaction between two different bi-functional or tri-functional monomeric units.
- In these polymerisation reactions, the elimination of small molecules such as water, alcohol, hydrogen chloride, etc. take place.
- The examples are Terylene (Dacron), nylon 6, 6, nylon 6, etc.
- For example, nylon 6, 6 is formed by the condensation of hexamethylene diamine with adipic acid.
- D. Molecular Forces
- A large number of polymers can be used in different fields because of their mechanical properties like tensile strength, elasticity, toughness etc.
- These mechanical properties are governed by intermolecular forces, e.g., van der Waals forces and hydrogen bonds, present in the polymer. These forces also bind the polymer chains.
- The polymers are classified into the following four sub groups on the basis of magnitude of intermolecular forces present in them.
- a) Elastomers: -
- These are rubber – like solids with elastic properties.
- The polymer chains are held together by the weakest intermolecular forces. These weak binding forces permit the polymer to be stretched.
- A few ‘crosslinks’ are introduced in between the chains, which help the polymer to retract to its original position after the force is released as in vulcanised rubber. The examples are Buna-S, Buna-N, neoprene, etc.
- b) Fibres:-
- Fibres are the thread forming solids which possess high tensile strength and high modulus.
- These characteristics can be attributed to the strong intermolecular forces like hydrogen bonding.
- These strong forces also lead to close packing of chains and thus impart crystalline nature. The examples are polyamides (nylon 6, 6), polyesters (terylene), etc.
- c) Thermoplastic Polymers:-
- These are the linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling.
- These polymers possess intermolecular forces of attraction intermediate between elastomers and fibres.
- Some common thermoplastics are polythene, polystyrene, polyvinyl, etc.
- d) Thermosetting Polymers:-
- These polymers are cross linked or heavily branched molecules, which on heating undergo extensive cross linking in moulds and again become infusible.
- These cannot be reused. Some common examples are Bakelite, urea-formaldehyde resins, etc.