Classification of polymers? 2005 2006 2008
1. Origin or Source :
(1)Natural polymer : These polymers are generally obtained from nature. They are available in nature.
Examples : Rubber, Cotton, Silk, Wool, Cellulose, Starch, Proteins etc.
(2) Semiplastic polymer : They are chemically modified natural polymers. For example cellulose is naturally occurring polymers, cellulose on acetylation with acetic anhydride in the presence of sulphuric acid forms cellulose diacetate polymers. It is used in making thread and materials like films glasses etc. Vulcanized rubber is also an example of semisynthetic polymers used in making tyres etc. gun cotton which is cellulose nitrate used in making explosive.
Such as : hydrogeneted , halogeneted or hydro-halogeneted natural rubber cellophane, cellolose, rayon , leather etc.
(3) Syntethic polymer : Man made polymers prepared syntheticall from low molecular weight compounds are called synthetic.The polymers which are prepared in the laboratories are called synthetic polymers. These are also called man made polymers.
Example: PVC, polyethylene, Polystyrene, nylon etc..
2. Thermal response :
(1) Thermoplastic polymer :
- Thermoset polymers soften when heated and harden when cooled. Simultaneous application of heat and pressure is required to fabricate these materials.
- On the molecular level, when the temperature is raised, secondary bonding forces are diminished so that the relative movement of adjacent chains is facilitated when a stress is applied.
- Most Linear polymers and those having branched structures with flexible chains are thermoplastics.
- Thermoplastics are very soft and ductile.
The commercial available thermoplasts are
- Polyvinyl Chloride (PVC) and Polystyrene
- Polymethyl methacrylate
(2) Thermosetting polymer :
- Thermosetting polymers become soft during their first heating and become permanently hard when cooled. They do not soften during subsequent heating. Hence, they cannot be remolded/reshaped by subsequent heating.
- In thermosets, during the initial heating, covalent cross-links are formed between adjacent molecular chain. These bonds anchor the chains together to resist the vibration and rotational chain motions at high temperatures. Cross linking is usually extensive in that 10 to 15% of the chain mer units are cross linked. Only heating to excessive temperatures will cause severance of these crosslink bonds and polymer degradation.
- Thermoset polymers are harder, stronger, more brittle than thermoplastics and have better dimensional stability.
- They are more usable in processes requiring high temperatures
- Most of the cross linked and network polymers which include
- Vulcanized rubbers
- Polyester resins
- Thermosets cannot be recycle, do not melt, are usable at higher temperatures than thermoplastics, and are more chemically inert
3. Mode of formation :
(1)Addition polymer : An addition polymer is a polymer which is formed by an addition reaction, where many monomers bond together via rearrangement of bonds without the loss of any atom or molecule. Addition polymers have -c-c- linkage along the main chain and no other atom appears in the main chain .They are formed by olefinic diaolefinic mechanism ; known as addition or chain growth polymerization .
Example: Polyethylene , polypropylene, PVC , polystyrene etc..
(2)Condensation polymers : Condensation polymers are any kind of polymers formed through a condensation reaction, releasing small molecules as by-products such as water or methanol.Condesation polymers are formed from intermolecular reactions between bifunctional or polyfunctional monomer molecules with the elimination of small biproduct molecule.
Example: Polyamides, Polyesters, Polyethers etc.
4. Form and use :
(1) Plastic : When a polymer is shaped into hard and tough utility articles by the application of heat and pressure .It is called plastic.
Example: PVC, Polystyrene, PMMA etc.
(2)Elastomer: When a polymer is vulcanized into rubbery product exhibiting good strength and elongation it is called elastomer.
Example: Rubber, synthetic rubber , Silicon rubber etc.
(3)Fibre : When a polymer drawn into long fillament like material ; whose length atleast 100 times of its diameter it is a=called fibre.
Example : Nylon , Terylene etc.
(4)Liquid resin : When a polymer is used as adhesive, potting compounds sealants etc in liquid form it is called liquid resin.
Example: Epoxy adhesive ,Polysulphide sealants etc.
5. Tacticity : (based on the configuration)
(1)Atactic : When the arrangement of side substituents R is at random around the carbon backbone chain. Such polymer are called atatic polymer
(2)Isolatic : If the substituents R is at the same side of the carbon atom then it is called Isolatic.
(3)Syndiotactic :When the substituents R is arranged alternately around the carbon chain it is called sydiotactic.
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