What is the glass transition temperature of a polymer and what factors affect it?

In one line

The glass transition temperature (T_g) is the temperature below which an amorphous polymer is hard, rigid and glassy, and above which it becomes soft, flexible and rubbery.

Explained simply

Put a rubber ball in liquid nitrogen and hit it: it shatters like glass. At room temperature it bounces. Somewhere in between, the ball changes from glassy to rubbery; that temperature is T_g. Below T_g the chain segments are "frozen"; above it they have enough energy to wriggle and rotate, so the material can bend.

How it works

1. Below T_g: segmental motion of chains is frozen; only atoms vibrate. Polymer is hard and brittle (glassy state).
2. At T_g: segments of 20–50 chain atoms gain enough energy to rotate about single bonds. Properties such as volume (specific volume), heat capacity and modulus change sharply in slope.
3. Above T_g: rubbery/leathery state; flexible. On further heating a crystalline polymer melts at T_m; an amorphous one becomes a viscous liquid gradually.
4. T_g is lower than T_m (often T_g is roughly one-half to two-thirds of T_m in kelvin).

Factors affecting T_g:

• Chain flexibility: flexible backbones (–Si–O–, –C–O–) give low T_g; rigid groups (benzene rings in the main chain) raise it.
• Bulky side groups hinder rotation and raise T_g (polystyrene > polypropylene > polyethylene).
• Intermolecular forces: polar groups and H-bonding raise T_g (PVC higher than PE).
• Molecular weight: T_g increases with molecular weight up to a limit (fewer chain ends).
• Cross-linking raises T_g; plasticizers (e.g., phthalates in PVC) lower it by separating chains.
• Branching: more chain ends add free volume and lower T_g.

Key formula

No formula needed. Typical values: natural rubber ≈ −70 °C, PVC ≈ 80 °C, polystyrene ≈ 100 °C, PMMA ≈ 105 °C.

Quick example

Car tyres must stay rubbery in winter, so rubbers with T_g far below room temperature are used. Polystyrene cups (T_g ≈ 100 °C) are rigid at room temperature. Rigid PVC pipe becomes flexible garden-hose PVC when plasticizer lowers its T_g below room temperature.

Common mistakes

• Calling T_g the melting point. Melting (T_m) is for crystalline regions; T_g is for amorphous regions.
• Saying plasticizers increase T_g.
• Stating that elastomers must be used below T_g; they are used above it.

How MU asks it

"Define glass transition temperature. Explain the factors affecting it" (5 marks) or "Write a note on T_g and its significance" (3 marks). For full marks: definition, glassy vs rubbery state, a volume–temperature sketch showing slope change at T_g, 4–5 factors with examples, and significance (choosing a polymer for an application temperature).