Polymers
1.0 Introduction
2.0 Polymers and polymerization
3.0 Free-radical vinyl polymerization
4.0 Ionic Polymerization
5.0 Natural Rubber
6.0 Synthetic Rubber
7.0 Polyvinyl Chloride (PVC)
8.0 Teflon or PTFE (Polytetrafluoro ethylene)
9.0 Nylon-66
5.1 Vulcanization: Cross-Linking of Rubber
In $1839$, Charles Goodyear accidentally dropped a mixture of natural rubber and sulfur onto a hot stove. He was surprised to find that the rubber had become strong and elastic. This discovery led to the process that Goodyear called vulcanization, after the Roman god of fire and the volcano. Vulcanized rubber has much greater toughness and elasticity than natural rubber. It withstands relatively high temperatures without softening, and it remains elastic and flexible when cold.
Vulcanization also allows the casting of complicated shapes such as rubber tires. Natural rubber is putty-like, and it is easily mixed with sulfur, formed around the tire cord, and placed into a mold. The mold is closed and heated, and the gooey mass of string and rubber is vulcanized into a strong, elastic tire carcass.
On a molecular level, vulcanization causes cross-linking of the cis$-1,4-$polyisoprene chains through disulfide $(–– S –– S ––)$ bonds, similar to the crysteine bridges that link peptides. In vulcanized rubber, the polymer chains are linked together, so they can no longer slip past each other. When the material is stressed, the chains stretch, but cross-linking prevents tearing. When the stress is released, the chains return to their shortened, kinky conformations as the rubber snaps back. Figure given below shows the structure of rubber before and after vulcanization. Vulcanization of rubber introduces disulfide cross-links between the polyisoprene chains. Cross-linking forms a stronger, elastic material that does not pull apart when it is stretched.
Rubber can be prepared with a wide range of physical properties by controlling the amount of sulphur used in vulcanization. Low-sulfur rubber, made with about $1$ to $3$$\%$ sulphur, is soft and stretchy. It is good for rubber bands and inner tubes. Medium-sulfur rubber (about $3$ to $10\%$ sulphur) is somewhat harder, but still flexible, making good tires. High-sulphur rubber ($20$ to $30\%$ sulphur) is called hard rubber and was once used as a hard synthetic plastic.
