When a plastic or rubber is cooled up to certain temperature, it becomes so hard and brittle that it breaks into pieces like glass, on application of stress.

The temperature below which the polymer becomes hard, brittle and glassy and above which it is softer and flexible, is known as glass transition temperature ( Tg ).

Significance:

A polymer below its Tg, is hard, stiff and brittle and therefore it is not preferred material for use at temperature below its Tg. In general, a polymer material should have much lower Tg than the operating temperature. Example- polymers to be used for refrigerators, air conditioners or used in cold countries, should have lower Tg, so that they will not break during use.