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 it's glass transition temperature. A famous example of this issue is with 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.