Explain the term ‘glass transition temperature’ with significance.

Glass transition temperature (T_g) is the temperature below which an amorphous polymer (or the amorphous part of a polymer) is hard, rigid and brittle like glass (the glassy state), and above which it becomes soft, flexible and rubbery (the rubbery state).

Below T_g the chain segments cannot rotate or move, apart from vibrating. Above T_g segmental motion becomes possible. T_g is a second-order transition, unlike T_m (melting of crystalline regions), and T_g < T_m.

Examples: polystyrene T_g ≈ 100 °C (rigid at room temperature); natural rubber T_g ≈ −73 °C (rubbery at room temperature); PVC T_g ≈ 81 °C.

Significance

• It decides the service temperature range. Plastics are used below T_g (rigid) and elastomers are used above T_g (flexible).
• It tells us how the polymer will behave and how to process it (moulding, extrusion) safely.
• It helps in choosing a polymer for a given use and in adjusting properties, e.g. adding plasticisers lowers T_g.
• It gives information about chain flexibility, molecular weight, cross-linking and crystallinity.