Glass transition temperature
The glass transition temperature Tg (also known as the glass temperature or glass transition temperature) is characterized by the conversion of an amorphous, glass-like or semi-crystalline polymer specimen, with a greater or lesser degree of hardness, into a rubber-like or viscous phase. The transition to this phase does not take place spontaneously. Instead, the polymer has a so-called principal softening range of around 50 K. This phenomenon of the glass transition temperature is brought about by the freezing or thawing of longish chain segments (20 to 50 chain atoms) in the macromolecules. The macroconformation, by contrast, does not undergo any change when the glass transition temperature is attained.