4. Natural Rubber Natural rubber is a coiled linear 1, 4-polymer of isoprene.
In the polymer chain of natural rubber, the residual double bonds are located between C2 and C3 of the isoprene unit. All these double bonds have cis configuration, and thus natural rubber is cis-l, 4-polyisoprene.
In the natural rubber, there is no polar substituent. The only intermolecular forces are van der Waals’ type. The cis-configuration gives the polymeric chain of natural rubber a coiled structure. As a result, it can be stretched by the application of a force. When the force is removed, the chain returns back to its original coiled shape.
Natural rubber is soft and sticky. It can be used only in the temperature range 10 ° C-50 ° C. At higher temperature, it becomes soft and at low temperature, it becomes brittle. It has higb water absorption capacity. It is attacked by oxidising agents and organic solvents. As such, it cannot be used very extensively for commercial puposes.
Vulcanisation of Rubber
The properties of natural rubber can be modified by introducing-S-Spolysulphide crosslinks in its structure. This process of introducing-S-Scrosslnks in the structure of natural rubber by heating with sulphur at 11O ° C is called vulcanlsation of rubber.