Glass Transition Temperature Calculator
Estimate glass transition temperature Tg from monomer structure or Fox equation for copolymers.
What Happens at Tg
The glass transition is not melting. Below Tg polymer chains are frozen in place and the material is rigid and brittle; above it, segments gain enough thermal energy to move past one another and the material becomes rubbery. It is a kinetic transition, not a thermodynamic phase change, which is why the measured value depends on how fast you heat or cool.
| Polymer | Tg | State at room temperature |
|---|---|---|
| Polybutadiene | −100°C | Rubbery — elastomer |
| Poly(butyl acrylate) | −54°C | Rubbery — adhesives |
| PVC | 80°C | Glassy — rigid pipe |
| Polystyrene | 100°C | Glassy — brittle |
| Polycarbonate | 150°C | Glassy — tough |
Whether a polymer is rubbery or rigid at room temperature is simply whether room temperature sits above or below its Tg. PVC pipe is rigid because 20°C is below its 80°C transition — add plasticiser and the same polymer becomes flexible flooring.
Two Ways to Shift Tg
The Fox equation covers copolymers and plasticisers — both are mixtures, and a plasticiser is simply a very low-Tg component. Note it works on reciprocal temperatures in kelvin, so the result is not a simple weighted average.
The Flory–Fox equation covers molecular weight. Chain ends have more free volume than mid-chain segments, so short chains depress Tg. The effect matters below roughly 20,000 g/mol and becomes negligible above it.
Worked Examples
Common Mistakes
Tg is a kinetic transition in the amorphous regions. Tm is a true thermodynamic transition of crystalline regions. Semi-crystalline polymers have both.
The Fox equation uses reciprocals in kelvin. A 50:50 blend of 373 K and 219 K polymers gives 276 K, not the arithmetic mean of 296 K.
Because the transition is kinetic, faster heating shifts the measured Tg upward by several degrees. Comparisons require matched conditions.
Small plasticiser molecules migrate out over time, so Tg rises and the material embrittles. This is why old vinyl becomes stiff and cracks.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This formula connects molecular properties, material structure or environmental transport to a macroscopic behavior or exposure estimate. Assumption: Use parameters measured for the same material, solvent, temperature and environment. Empirical correlations may not transfer outside their calibration range.