By Grahame Turner Updated Mar 24, 2022
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Many natural materials expand when heated and contract when cooled, but a rubber band does the opposite. When you blow a hair dryer on it, the band shortens, and when you place it in the freezer, it stretches.
Rubber is a cross‑linked polymer whose chains are entangled like a ball of yarn. In a solid state, the chains are jammed together in a random coil configuration. Temperature changes the kinetic energy of the chains: heat increases their vibration, while cold slows it down.
At ambient conditions, the entangled chains resist extension. When you pull on a rubber band, the chains straighten, storing elastic energy. When you release it, the chains collapse back into their coiled state, and the band snaps back to its original length.
Heating the band “unwinds” the coil structure. The increased motion allows the chains to align more linearly, reducing the volume they occupy. As a result, the band shrinks, even though the individual polymer molecules are still moving faster. This counterintuitive effect is well documented in polymer physics research.
Cooling reverses the process. The chains become more rigid and are forced into a tighter, more efficient packing arrangement. The band therefore lengthens slightly and becomes more elastic, making it easier to stretch.
In summary, the apparent contradiction between rubber’s behavior and that of most materials stems from the unique entangled structure of its polymer chains and how temperature alters their motion.
