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 $T_gT_g,$ 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.
Factors affecting glass transition temperature are-
1) Chain Stiffness-Stiffening groups: in the polymer chain reduce the flexibility of the chain and raise the value of $T_g$ .
poly(ethylene terephthalate) $T_g = 69^oC $
2) Intermolecular Forces-
Stronger intermolecular forces lead to a higher Tg. PVC has stronger intermolecular forces than polypropylene because of the dipole-dipole forces from the C-Cl bond.
Atactic Polypropylene $T_g = -20^oC $ 
3) Pendant Groups-
The influence of pendant groups on the glass transition temperature is somewhat more complicated.
- Bulky pendant groups, such as a benzene ring, can catch on neighboring chains like a "fish hook" and restrict rotational $\text {freedom}^{34}$. This increases $T_g$.
Atactic Polypropylene $T_g = -20^oC$ 
4) Cross-Linking-
The presence of cross-links between chains restricts rotational motion and raises Tg.
5) Plasticizers-
Plasticizers are low molecular weight compounds added to plastics to increase their flexibility and workability. They weaken the intermolecular forces between the polymer chains and decrease Tg.
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
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