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.

Effect of temperature on polymer;

The hard and brittle state is the glassy state and soft-flexible state is the viscoelastic state. If viscoelastic state of polymer is heated further, the polymer becomes a viscous liquid and can flow. This state is known as viscofluid state.

Below Tg the molecules in polymer do not move apart and do not have movements within the molecular chains. Therefore, a stress transfer property is lost and polymer below Tg is brittle.

Above Tg the polymers in the viscoelastic state have flexibility or stress transfer properties.

Above Tm, the kinetic energy of the polymer molecules is high enough to cause movement with the chain segments as well to move from one position to another. Therefore, this state has flow character like liquids.

FACTORS AFFECTING ON Tg:

1) Chain Stiffness-Stiffening groups: in the polymer chain reduce the flexibility of the chain and raise the value of Tg.

poly(ethylene terephthalate) Tg=69oCTg=69oC

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 Tg=−20oCTg=−20oC

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 freedom34freedom34. This increases TgTg.

Atactic Polypropylene Tg=−20oCTg=−20oC

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