|MadSci Network: Physics|
Rubber band is made of what are called polymer molecules. These are long chains of atoms connected to each other through covalent bonds. At room temperature and in the unstretched condition the molecules inside the rubber band are coiled up and "floopy". They are disorganized. This state is called a high entropy state i.e. the level of disorder in the rubber band is high. When you stretch a rubber band some of the energy you provide is taken up by the rubber band in organizing the molecules in an ordered state - i.e. the molecules more or less line up in straight lines - this is also referred to as crystallization. This process gives off heat so a stretched rubber band is actually warm to touch. In the same way if you heat the rubber band the molecules in it will organize themselves into crystals and this will cause the band to contract.
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