Visitors to the Polymer Corporation’s pavilion at Expo 67 left with a gift. We walked away with a set of three balls. One ball bounced back to almost the same height from which it was dropped, another was less bouncy, and the third has almost no bounce. They were designed to arouse curiosity, which was the stated theme of the Polymer exhibit. Indeed, inquisitiveness about how things work and how to make them work better is the cornerstone of science. The key to satisfying curiosity is an understanding of the fundamental nature of matter, which in turn is determined by the structure and behaviour of the molecules of which it is composed. This is illustrated in an exemplary fashion by the three balls.
Rubber is a “polymer,” meaning that its fundamental structure consists of long molecules that can be thought of as being built of small molecules, or “monomers,” that have been joined together, much like linking paper clips into a chain. Natural rubber, which is an exudate of the Hevea brasiliensis tree, is composed of “polyisoprene,” a giant molecule formed by linking small molecules of isoprene. These long molecules are coiled in a tangled mess, much like the strands in cooked spaghetti. However, when a stretching force is applied, the molecules are straightened and become more organized. Scientifically speaking, they now have less “entropy.” Nature tends to move from organized states to disorganized ones, meaning toward increased entropy. The melting of ice would be a typical example. In ice, the water molecules are in a fixed and ordered state, while in water they are free to move around in a disordered fashion.
When the molecules of rubber are stretched, the entropy of the system has decreased, and when the force is released, the molecules will tend to revert to their disorganized state. The ability of a deformed material to revert to its original shape once the deforming force is removed is known as its “elasticity.” Natural rubber is not very elastic and has limited use since it gets hard in winter and soft and tacky in summer. Back in 1839, Charles Goodyear was zealously experimenting with improving the properties of rubber and made an accidental discovery that would turn out to be monumental. He had been trying to improve rubber’s properties by mixing it with all sorts of substances ranging from soup to cream cheese. On that famously serendipitous day, it was sulphur’s turn. Nothing happened, at least not until he accidentally spilled the mixture onto a hot stove. Then the rubber hardened into an elastic mass! Goodyear patented the process, coining the term “vulcanization” after Vulcan, the Roman god of fire. Although he didn’t understand this at the time, sulphur atoms had forged links between the long chains of polyisoprene, making them more difficult to untangle and increasing their tendency to return to their original state. Goodyear’s exhibit of vulcanized rubber was a major attraction at the “Great Exhibition of the Works of Industry of All Nations” held in London in 1851. Housed in the impressive Crystal Palace built for the event, Goodyear displayed rubber boats, giant balloons, shoes, medical instruments and even furniture, all made of vulcanized rubber.
With the introduction of automobiles, vulcanized rubber took on added importance. It was invaluable for belts, gaskets and of course, tires. But rubber trees did not grow in Europe or in America, and there was concern about the availability of a product that had to be shipped from long distances, especially in case of war. Could rubber be made synthetically, scientists wondered? As early as 1860, English chemist Charles Greville Williams had subjected rubber to destructive distillation and found that it was broken down into isoprene. That began efforts to synthesize rubber from isoprene, a task that proved to be very challenging. However, in 1909, Bayer Company chemist Fritz Hofmann managed to polymerize a closely related compound, methyl isoprene, and produced the world’s first synthetic rubber. Then in the 1920s, Sergei Lebedev in Russia polymerized butadiene using sodium to form a synthetic rubber named “Buna” from “butadiene” and the chemical symbol for sodium, Na. Having learned a lesson from rubber shortages during the First World War, Germany embarked on a massive program to produce synthetic rubber, and by the 1930s, chemists at IG Farbenindustrie had developed a series of “Buna” rubbers, the most famous one being Buna-S, made from butadiene and styrene. During the Second World War, much of this was manufactured by slave labour at an IG-F factory in Auschwitz.
In Canada, the government established the Polymer Corporation, located in Sarnia, to produce synthetic rubber, essentially using the German process, with raw materials obtained from petroleum. After the war, the corporation continued to carry out research in polymerization and it was these efforts that were featured at its Expo 67 pavilion, highlighted by the giveaway of the three balls.
Now, consider what happens when a ball is dropped. The impact with the surface creates a deformation, much like stretching a rubber band. How the ball bounces, is determined by how effectively the original shape is restored, which in turn is a function of the specific monomers that have been used to create the rubber, and the extent to which the polymer chains are cross-linked, that is how they are “vulcanized.” The ball made of polymerized butadiene restores its shape quickly, and bounces high. If the butadiene is polymerized together with styrene, it becomes less bouncy, and with “butyl rubber” made of isobutene and isoprene there is virtually no bounce. It is all a matter of chemistry, which is the message that was so effectively delivered by the Polymer Corporation’s bouncing balls, first raising and then satisfying curiosity. I, for one, got that message. Those balls were instrumental in stimulating my interest in chemistry.
Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.
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