Q. Due to the process called vulcanization, the sticky natural rubber become stiff and more durable. Why?
Answer: Cross Links are formed between the polymer chains
Notes: Natural rubber is composed of long polymer chains. These chains are independent from each other and this can move freely. For example, 2-methyl-1,3-butadiene which is called Isoprene make long chains to give rise to a natural rubber. Chemical Formula is CH2=C(CH3)CH=CH2. The chains of the natural rubber also called poly-isoprene are as follows:
The chemical formula of Sulphur (S8) is
The result of vulcanization is shown in the below diagram:

In the above chemical reaction, we see that the sulfur molecule get arranged in rubber in such as way that Cross Links are formed between the polymer chains. These cross links make sure that the chains don’t move independently. The result is that the sticky natural rubber become stiff and more durable.
Devulcanization is the reverse process. Devulcanization involves the delinking of the sulfur molecules from the rubber molecules.