Science Sidetrip:

One Big Happy Molecule

Then there comes onto the scene a tinkerer named Charles Goodyear. In the winter of 1839 Goodyear was in Massachusetts trying to figure out how to make natural rubber more useful so that he could finally make a living at his until-now fruitless tinkering. He had mixed rubber latex and sulfur together when he had a little accident. He spilled the mixture on a hot stovetop. When the mixture was through frying, Goodyear couldn't believe what had happened. Wouldn't you know sulfur was just the extra ingredient he needed to make rubber work in cold weather. After mixing hot gooey rubber latex and sulfur and letting the mixture cool, he took the rubbery solid that resulted and tacked it to the outside of his door. The cold Massachusetts winter didn't make it brittle. What's more, it didn't become gooey when heated anymore, either. Goodyear was onto something here. This process for making rubber more useable became known as vulcanization. A sulfur (S 8 ) molecule. What Goodyear had done was this: he crosslinked the rubber. Let me explain. The sulfur molecules each contain eight sulfur atoms, arranged in a ring, like you see on your right. When these sulfur molecules are heated with polyisoprene molecules, something nifty happens. The sulfur rings open, and fall apart. Fragments of the sulfur rings will join with the polyisoprene, joining the chains together, as you see below:

Here's the intriguing part. Once the rubber has been crosslinked, all the molecules have joined into one big molecules. Polyisoprene molecules are big as molecules go already, having molecular weights sometimes over one million. But now they have all joined into one molecule, one so big that a person can see it, feel it, and pick it up. A piece of crosslinked rubber contains one molecule. Amazing, isn't it?

This crosslinking makes the rubber stronger. It also allows the rubber to keep its shape better when it is stretched over and over again. It keeps the rubber from getting gooey in hot climates because, think about it, a single molecule can't flow like a substance made up of many molecule. Think of the way you can pour a bucket full of gravel, but you can't really pour a boulder, and you'll get the idea.