We've learned that elastic stretches and returns to its original shape because it contains fibers of rubber or rubber-like material. But why does rubber behave that way in the first place? Scientists now know that rubber belongs to a group of compounds known as elastomers, a mash-up of "elastic" and "polymers." Polymer molecules look like long chains of repeating units. These units, or monomers, are identical along the entire length of the molecule and stay connected by strong covalent bonds. In most polymers, carbon atoms form the backbone of the chain.
Here's where it gets interesting. Many polymers contain big, bulky monomers, which means the individual building blocks of the molecule crowd together. This makes the polymer stiff and inflexible at room temperature. Other polymers have subunits that fit together so well that they have a crystalline arrangement at normal temperatures. These kinds of polymers give us plastics and resins. Then there are elastomers, which have an altogether different arrangement. Under normal conditions, the long molecules of an elastomer are irregularly coiled like a slithering mass of snakes. When a force is applied to these molecules, they straighten out in the direction in which they're being pulled. Imagine the mass of snakes suddenly aligning and stretching out to their full length, from end to end. As soon as the force is released, the molecules spontaneously return to their normal, coiled-up arrangement.
The dividing line between plastic polymers and elastic polymers is a temperature known as the glass transition temperature, or Tg. Extremely flexible molecules have glass transition temperatures in the range of negative 195 degrees Fahrenheit (negative 125 degrees Celsius). Rigid or crystalline molecules can have glass transition temperatures as high as 419 degrees Fahrenheit (215 degrees Celsius). For example, Tg for polystyrene (used in Styrofoam) and polymethyl methacrylate (used in Lucite) is 212 degrees Fahrenheit (100 degrees Celsius).
Natural rubber is a polymer known as polyisoprene. It's built from repeating units of isoprene, a hydrocarbon consisting of five carbon atoms and eight hydrogen atoms. In its long-chain form, polyisoprene can take one of four three-dimensional shapes, which chemists refer to as isomers. Natural rubber consists almost entirely of one of these isomers, cis-1, 4 polyisoprene.
As we've already noted, natural rubber is stable across a fairly narrow range of temperatures. As the isoprene molecule begins to cool, it crystallizes. As it heats up, it starts to lose its elasticity. And if it reacts with oxygen and ozone in the atmosphere, the molecule starts to fall apart at the carbon-to-carbon double bonds. During vulcanization, polyisoprene is heated with sulfur, which causes adjacent polymer chains to link together via sulfur atoms, forming a loose molecular network that overcomes all of the disadvantages of untreated rubber.
Not that you need to know any of this to enjoy your elastic-infused clothing and accessories. Then again, it's kind of cool to think that the elastic in your waistband might have hailed from the jungles of South America or that your Spanx body shaper owes its existence to two pocket-protector-toting, polymer-slinging chemists from DuPont.