Which of the following would you expect to be elastomers and which thermosetting

Question

Which of the following would you expect to be elastomers and which thermosetting polymers at room temperature? Justify each choice. Epoxy having a network structure Lightly crosslinked poly(styrene-butadiene) random copolymer that has a glass transition temperature of-50 degree C Lightly branched and semicrystalline polytetrafluoroethylene that has a glass transition temperature of-100 degree C Heavily crosslinked poly(ethylene-propylene) random copolymer that has a glass transition temperature of-0 degree C Thermoplastic elastomer that has a glass transition temperature of-75 degree C

Explanation / Answer

They can be differentiated by the degree of plastisicity. Elastomers have very long, tangled polymer chains, thermosets have shorter less tangled chains and thermoplasts are somewhere in between. Also this will help you, as this tells everything with the examples... Depending upon the intermolecular forces, the polymers have been classified into four type. (i) Elastomers (ii) Fibers (iii) Thermoplastics (iv) Thermosetting polymers (i) Elastomers : The polymers that have elastic character like rubber (a material that can return to its original shape after stretching is said to be elastic) are called elastomers. In elastomers the polymers chains are held together by weak intermolecular forces. Because of the presence of weak forces, the polymers can be easily stretched by applying small stress and regains their original shape when the stress is removed. The most important example of elastomers is natural rubber. (ii) Fibers : These are the polymers which have strong intermolecular forces between the chain. These forces are either hydrogen bonds or dipole-dipole interaction. Because of strong forces, the chains are closely packed giving them high tensil strength and less elasticity. Therefore, these polymers have sharp melting points. These polymers are long, thin and thread like and can be woven in fabric. Therefore, these are used for making fibers. Example: Nylon 66, dacron, silk etc. (iii) Thermoplastics : These are the polymers which can be easily softened repeatedly when heated and hardened when cooled with little change in their properties. The intermolecular forces in these polymers are intermediate between those of elastomers and fibres. There is no cross linking between the chain. The softening occurs as the polymer chain move more and more freely because of absence of cross link. When heated, they melt and form a fluid which can be moulded into any desired shapes and then cooled to get the desired product. Example : Polythene, polystyrene, PVC, teflon etc. (iv) Thermosetting polymers : These are the polymers which undergo permanent change on heating. They become hard and infusible on heating. They are generally prepared from low molecular mass semifluid substances. When heated they get highly cross linked to form hard infusible and insoluble products. The cross links hold the molecule in place so that heating does not allow them to move freely. Therefore a thermosetting plastic is cross linked and is permanently rigid.

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