Perished Rubber Bands

These rubber bands are made from natural rubber. To make natural rubber, a milky white sap, called latex, is tapped from a rubber tree. An acid, such as formic acid, is added to the latex to make it coagulate, and then it is passed through rollers to remove excess water, and the whole thing is dried. This purified natural rubber is polyisoprene, which is both an elastomer and a thermoplastic. To make the rubber bands, this purified rubber is drawn out into a long tube, which is then cut into slices. Over time, rubber loses its ability to stretch, becoming hard and brittle. This process of perishing occurs because exposure of the rubber to light and air induces chemical changes in the polyisoprene over time. The inevitable deterioration of natural rubber can be halted by a process called vulcanization (see Vulcanized Rubber Cube).

Rubber as a material, and rubber bands in particular, are often used to explain entropy (a measure of disorder used in physics) and the second law of thermodynamics, which states that entropy in the universe will either stay constant or increase. When it is tapped from the tree as a liquid, the major constituent of liquid rubber is a small carbon-based molecule called isoprene. As the liquid latex dries, the isoprene molecules begin to bind to one another and form long chains called polyisoprene. When a rubber band is relaxed (i.e. not stretched), these long chains are intertwined and entangled like a plate of cooked spaghetti. They are quite happy to sit coiled up together inside the rubber band due to an electrostatic attraction between them. When the rubber band is stretched, the individual polymer chains are untangled and line up (like a pack on uncooked spaghetti). The molecules inside the rubber therefore become much more ordered, and the entropy of the system is decreased. Because entropy always tends to increase, as soon as you release the rubber band, all the polyisoprene chains pull themselves back into their more disordered, tangled, plate-of-spaghetti formation, and the rubber band pings back to its original shape.