[b]POLYMERS = POLY + MERS[/b]
Polymers are long chain giant molecules, are assembled from many smaller molecules called monomers.
Polymers consist of many repeating monomer units in long chains. A polymer is analogous to a necklace made from many small beads (monomers).
The process by which monomer molecules are linked to form a big polymer molecule is called Polymerization.
For polymerization the two reactive side or bifunctionality of monomer are prerequisites.
[b]Types of Polymers: On the basis of their origin[/b]
- There are many types of polymers including synthetic and natural polymers.
- Natural polymers are Proteins ,silk, keratin. Carbohydrates - cellulose, starch, glycogen DNA - RNA
- Semi synthetic Polymers- e.g Ester and ether of cellulose
- Synthetic Polymers are: PVC, Neoprene, Bakelite, PET, Nylons, PMMA, PS, Polyethylene, Silicones, epoxy resins, polyurethane.
[b]Types of Polymers:[/b]
ORGANIC: A polymer whose backbone is essentially made of carbon atoms is termed as organic polymer.
The atom attached to the side valencies of backbone carbon atom are however usually those of hydrogen , oxygen , nitrogen.
INORGANIC: these molecules generally contain no carbon atom in their chain backbone.
E.g..-, Silicones
Polymers: Thermoplastics, Thermosets, Elastomers.
Thermoplastics: Crystalline, Amorphous.
Thermoplastics - plastics that soften when heated and become firm again when cooled. This is the more popular type of plastic because the heating and cooling may be repeated.
Molecules in a thermoplastic are held together by relatively weak intermolecular forces
Most linear and slightly branched polymers are thermoplastic.
Thermoplastics have a wide range of applications such as in food packaging, insulation, automobile bumpers, and credit cards.
- Thermosets: plastics that soften when heated and can be molded, but harden permanently. They will decompose when reheated.
- Thermosets usually are three-dimensional networked polymers in which there is a high degree of cross-linking between polymer chains. The cross-linking restricts the motion of the chains and leads to a rigid material.
- For example is Bakelite, which is used in toasters, handles for pots and pans, dishes, electrical outlets and billiard balls. They are also used in construction, making toys etc..
- Elastomers: Elastomers are rubbery polymers that can be stretched easily to several times their utstretched length and which rapidly return to their original dimensions when the applied stress is released.
- Elastomers are cross-linked, but have a low cross-linking. The polymer chains still have some freedom to move.
- Rubber bands and other elastics are made of elastomers
[b]Crystalline Polymers:[/b]
Highly crystalline polymers are rigid, high melting, and less affected by solvent penetration.
Crystallinity makes a polymers strong, but also lowers their impact resistance. As an example, samples of polyethylene prepared under high pressure (5000 atm) have high crystallinities (95 - 99%) but are extremely brittle.
- Amorphous Polymers: Polymer chains with branches having irregular arrangement cannot pack together regularly enough to form crystals. These polymers are said to be amorphous.
- Amorphous regions of a polymer are made up of a randomly coiled and entangled chains.
- A two-dimensional schematic of an amorphous polymer is shown.
- Amorphous polymers are softer, have lower melting points, and are penetrated more by solvents than are their crystalline counterparts
[b]Classification of Polymers:[/b]
Homopolymers - consist of chains with identical bonding linkages to each monomer unit. This usually implies that the polymer is made from all identical monomer molecules. These may be represented as : -[A-A-A-A-A-A]-
Copolymers -consist of chains with two or more linkages usually implying two or more different types of monomer units. These may be represented as : -[A-B-A-B-A-B]
They are further classified into different types:
Statistical Copolymers: Also called random copolymers. Here the monomeric units are distributed randomly, and sometimes unevenly, in the polymer chain: ~ABBAAABAABBBABAABA
Alternating Copolymers :Here the monomeric units are distributed in a regular alternating fashion, with nearly equimolar amounts of each in the chain: ~ABABABABABABABAB
Block Copolymers: Instead of a mixed distribution of monomeric units, a long sequence or block of one monomer is joined to a block of the second monomer: ~AAAAA-BBBBBBB~AAAAAAA~BBB~.
Graft Copolymers: As the name suggests, side chains of a given monomer are attached to the main chain of the second monomer: ~AAAAAAA(BBBBBBB~)AAAAAAA(BBBB~)AAA~.
[b]Polymers classified by reaction mode:[/b]
Addition Polymers - the monomer molecules bond to each other without the loss of any other atoms. Alkenes monomers are the biggest groups of polymers in this class.
Condensation Polymers - usually two different monomer combine with the loss of a small molecule, usually water. Polyesters and polyamides (nylon) are in this class of polymers. Polyurethane Foam in graphic.
Depending on their ultimate form of use:
Plastics- e.g PVC,
Elastomers -e.g synthentic rubber
Fibres – e.g nylon
Resins – epoxy resin
[i]Contributed by Akshay Pandey[/i]