CHAPTER-II POLYMER MATERIALS

Points to be Studied 2.1 Polymers 2.2 Polymerization mechanism 2.2.1 Additional polymerization 2.2.2 Condensation polymerization 2.2.3 Homo-polymer 2.2.4 Co‐polymer 2.3 Degree of polymerization 2.4 Defects in the polymers 2.5 Mechanical properties of polymers, deformation, reinforced polymers 2.6 Applications of polymers.

HOMOPOLYMER AND COPOLYMER On the basis of origin, polymers can be classified as natural and synthetic polymers. On the basis of chemical nature, they can be classified as organic and inorganic polymers. Organic polymers have carbon atoms in the polymer backbone of chain. For example; polypropylene, poly (vinyl chloride) are organic polymers because the polymeric backbone is composed of carbon atoms. Inorganic polymers do not have carbon atoms in their polymeric chain such as Silicone rubbers.  On the basis of monomer composition, polymers can be classified as homo-polymers and co-polymers.  Homopolymers can be defined as the polymers which have one type of constituent monomer units in polymeric chain. They can be represented as; -A-A-A-A-A-A-A-A-A-A-A-A- Copolymers are composed of two or more different types of monomer units and generally represented as;  -A-B-A-B-A-B-A-B-A-B- Homopolymers can be linear, branched or cross-linked which depends on the linkage between monomer units. 

Homopolymer Examples Polyethylene: It is one of the most common examples of Homopolymer which is composed of ethylene monomer units. 

Poly (vinyl chloride): It is commonly known as PVC Poly (vinyl chloride): It is commonly known as PVC. It is a thermoplastic which can be mold in different shapes. It is supplied in powder form which shows resistance against oxidation and degradation. It is a chain growth polymer of vinyl chloride (CH2=CHCl). 

Homopolymers are composed of same types of monomers whereas copolymers are composed of different monomers. Some common examples of homopolymers are nylon 6, nylon 11, polyethylene, polypropylene, polyvinyl chloride, and poly-acrylonitrile. Polyester, Nylon-6, 6 are examples of copolymers. Copolymers are polymerized from two or more different monomers. They can be classified as alternating copolymer, block copolymer, graft copolymer and random copolymer. Some other differences are listed below.  Homopolymers   Copolymers  These polymers have same type of monomer units. This is also known as heteropolymer as they are composed of two different kinds of monomers. They can be classified as linear, branched, cross linked and network homopolymers.  They can be classified as alternating copolymer, block copolymer, graft copolymer and random copolymers. Linear homopolymers have linear long polymeric chain of same types of monomer units. For example; HDPE   In an alternating copolymer, the two monomers are arranged in an alternative way and can be represented as ABABABABAB.  Branched homopolymers have short or long branches bonded on parent polymeric chain such as LDPE. In random copolymer the monomers are arranged in any order such as AABAAABBBBAB. Cross linked and network homopolymers have braches on parent chain which are interconnected with each other to form cross linked and network polymer.  In block copolymers, two blocks of homopolymers are joined together.It can be represented as AAAAAAABBBBBBB. Short term stiffness Long term stiffness Short term impact strength Long term impact strength Example: Polyethylene, Poly(vinyl chloride), Polypropylene, Polystyrene Example: Nylon-6,6, polyester, protein, Buna-S

Mechanical Properties of Polymers Specific gravity of Polymers lies between 0.09 to 0.3 which is less than metals ( 0.3 to 12) Thermal conductivity of polymers is very low. Thermal expansion of polymers is five times higher than metals. Modulus of rigidity of polymers is low. Most of Polymer plastics becomes brittle and yellow, when exposed to sunlight for longer duration. They are good electrical insulators. The stress-strain relation relationship in a polymers is not linear like that of metals. Polymers can be deform elastically and plastically

Applications of Polymers In view of their light color, light weight, easy shapability and low cost, plastic materials occupies many of the household articles ( chairs, buckets, pens, toys etc.). Phenolics ( Bakelite), a kind of synthetic polymer has been widely used as insulating components in generators, transformers and switch gears in heavy electrical engineering. Bakelites are also used in printing circuit boards in electronics and telecommunication industries. Phenolic resigns are used as varnish in all electrical windings in all types of motors. Teflon, a good insulator, finds application both in high and low temperatures.

Applications of Polymers 6) Many of the fibrous cheap materials such as wood, silk, cotton are used as insulating materials. 7) Due to improved mechanical strength and thermal resistance, rubbers are used as insulators in cables. 8) Plastics and polyesters are used in chemical engineering, building construction, marine, road transportation, dairy and agriculture industries. 9) In medical field polymers like Dacron, Teflon, and Polyurethane are used for replacement of heart valves, blood vessels. 10) Play ground equipments, various balls, golf clubs, Swimming pools and protective helmets are produced from polymers.