Unraveling the versatility of rubber
Chemical structure of polyisoprene, the main constituent of natural rubber.
Dating back to 1,000 BCE, the Olmecs discovered that a stretchy, bouncy substance could be extracted from the sap of certain trees, such as Hevea brasiliensis (also known as the rubber tree). The sap from this tree contains polyisoprene, which can then be processed into natural rubber or alternatively dried and used as latex like those found in rubber gloves. These polyisoprene strings are very flexible, explaining why rubber is highly elastic. However, natural rubber like those obtained from tree sap become rigid in cold temperatures. This is because long strings of polyisoprene are able to flow past each other inside the rubber, but in colder conditions, the rubber contracts and the polyisoprene are less elastic.
Figure demonstrating the vulcanization of polyisoprene rubber. Notice how the two polyisoprene chains (green and blue) are connected by the sulfur bridges (black).
In 1839, Charles Goodyear found that adding sulfur to rubber and then heating this mixture resulted in sulfur bridges being formed between the polyisoprene chains, thereby linking them together. This chemical process is called vulcanization. A balance is then formed between the elasticity of the polyisoprene chains and the rigidity of the sulfur bridges giving rubber the unique properties that allow it to deform and return to its original shape. This invention was very useful in giving rubber its versatility to be used in many various manners, particularly in the automobile industry.
Now in the 21st century, most rubber is not derived from trees. There are so many kinds of synthetic rubber made in the industry today and even more elastic materials made from petroleum. For example, polysulfide rubber is a type of synthetic rubber that is used to bind rocket fuel together. Most sports balls, like soccer balls and footballs, have some rubber in them. Footballs, for instance, contain an air bladder inside them made of polyurethane, which itself is surrounded by leather made from cows. The amount of rubber present in the ball significantly affects its functionality. A tennis ball is filled with air that allows it to be easily compressed when it bounces. The rubber inside this ball allows it to return back to its original shape without getting deformed. Rubber, bouncy balls that we would play with as kids have a significant amount of vulcanized polybutadiene, another synthetic rubber. The sulfur bridges in these bouncy balls prevent it from losing its shape and contribute to how bouncy it is. Baseballs have a rubber core that is wrapped in string to somewhat decrease its bounciness so that not every hit is a home run. Clearly, rubber has contributed greatly to many different disciplines ranging from sports to rocket science, and the significance of its chemical properties should not be underestimated or overlooked.
