When plastic or rubber is cooled up to certain temperature, it becomes so hard and brittle that it breaks into pieces on application of stress.
The temperature below which the polymer becomes hard, brittle and glassy and above which it is softener and flexible, is known as glass transition temperature (TG).
The glass transition is a property of only amorphous portion of a semi-crystalline solid. The crystalline portion remains crystalline during the glass transition.
When a polymer is heated beyond $T_g$, it passes from glassy state to rubbery state. Further heating much above, causes melting of the polymer and it starts flowing and it starts flowing. The temperature below which the polymer is in rubbery state and above which it is a liquid is called melting temperature of the polymer, while the temperature at which the polymer starts flowing is known as its flow temperature.
Signification of Tg
Molecular weight- GTT (glass transition temperature) is directly proportional to molecular weight of polymer.
Cross links- GTT is directly proportional to degree of cross links
Flexibility- GTT is inversely proportional to flexibility.
Plasticizer – GTT is inversely proportional to plasticization.
Inter molecular force – GTT is directly proportional to intermolecular force.
Side groups – GTT is directly proportional to number of side groups