Rubber bands have long, "cross linked" stringy molecules called polymers, according to Vince Calder of Argonne National Laboratory. This means the strands of a rubber band are randomly coiled--like a ball of yarn--and contain a lot of entropy, or disorder, among its polymer molecules. When stretched, the strands are straightened and have more order and less entropy.
Heating a rubber band results in two types of effects: physical and thermal, according to Argonne National Laboratory. The easiest way to see the physical effect is to stretch it out and apply energy from a hair dryer.
Most materials expand when heated, but rubber bands exhibit the exact opposite effect--they shrink, according to Argonne National Laboratory. As the temperature of a rubber band increases, the polymer strands start moving around more, and as they are tangled, they become even more cross-linked and contract. Once the heat dissipates, the rubber band then stretches out to its original size.
Stretching out a rubber band causes changes to the temperature of it by increasing or decreasing entropy. Heat is absorbed as entropy increases, and vice versa, according to PhysLink. The lips are very sensitive and an excellent way to observe the heating effects of rubber bands. When a person stretches a rubber band and places it against her lips, entropy has decreased, and she feels her lips getting warmer as the rubber band gives off heat. When the rubber band contracts, entropy increases and the rubber band absorbs heat from the lips.
By far, the most used effect of heating rubber is to create a more durable, synthetic rubber with a process called "vulcanization," according to "Science and Technology of Rubber." Vulcanization essentially heats rubber by adding an agent, usually sulfur. Most modern products--such as automobile tires and rubber tubing--that use rubber contain vulcanized rubber, according to TutorVista.