Glass transition temperature

The glass transition temperature is a term applied to polymers with predominately amorphous domains. If a polymer drops below this temperature, or T_g, the compound becomes a glass, or a hard and brittle material. T_g is important in elastomers, since in order for an elastomer to retain its elastic properties, it must remain above its glass transition temperature. A famous example of this is the Challenger explosion, caused by the leaking of an oxidizing agent into the fuel tank. The rubber seals between the two tanks dropped below their T_g, becoming glass-like and losing their sealing capability, allowing the two compounds to mix, and subsequently explode.