Posted by: Hourglass PJ14 MAR 2012
If, as mine sometimes does, your copy of The Pharmaceutical Journal arrives rolled up with other post inside a Royal Mail red rubber band, you might be interested to know that the first rubber band was patented in England on today’s date, 17 March, by Stephen Perry in 1845.
What is particularly interesting about rubber bands is their unusual behaviour when exposed to heat. This is due to the property in physics known as entropy, which describes the disorder in a system.
Heat causes rubber bands to contract, while cooling causes expansion. This is the opposite of the effect of heat on other substances, for example, water, which expands when heated.
Rubber, however, does not have a crystalline structure, which would allow its molecules to break apart when heated and enable expansion. Rubber is made from string-like molecules which when heated behave like strings of Christmas tree lights and get tangled, so causing a rubber band to contract.
Also counterintuitively, rubber heats when stretched. When you stretch rubber, the tangled molecules are pulled straighter into a straight line. Entropy (or disorder) is reduced and heat is given out. Stretch a rubber band and press it to your forehead and it will feel warm. Let it contract again and the rubber will feel cool as the molecules tangle, the entropy increases and the rubber sucks up heat from you.