Explain the term 'glass transition temperature' with its significance. (asked in MU Dec 2024)

Answer: \(T_g\) is the temperature below which an amorphous polymer is hard and glassy and above which it becomes soft, flexible and rubbery; it decides whether a polymer is used as a plastic or a rubber.

Definition: below \(T_g\) the polymer chains cannot move (only atoms vibrate), so the polymer is hard, rigid and brittle like glass. When it is heated above \(T_g\), segments of the chains gain enough energy to rotate and slide, so the polymer becomes soft, flexible and rubbery. (It is different from the melting point \(T_m\), where crystalline regions melt.)

Examples: polystyrene \(T_g\approx100\) °C (rigid at room temperature); natural rubber \(T_g\approx-70\) °C (rubbery at room temperature).

Significance:

• A polymer used as a rigid plastic must have \(T_g\) above its service temperature; an elastomer must have \(T_g\) well below it.
• It sets the processing temperature for moulding and shaping thermoplastics.
• It helps in choosing polymers for low-temperature or high-temperature use.
• \(T_g\) can be lowered by adding plasticisers (e.g. PVC made flexible) and is raised by bulky side groups, cross-linking and strong intermolecular forces.