Glass transition temperature
The glass transition temperature is a term applied to polymers with predominantly amorphous domains. If a polymer drops below this temperature, or Tg, the compound becomes a glass, or a hard and brittle material. Tg 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 Tg, becoming glass-like and losing their sealing capability, allowing the two compounds to mix, and subsequently explode.