Glass Transition
Definition and meaning of Glass Transition in chemistry.
Glass transition is the reversible change that an amorphous solid or supercooled liquid undergoes over a narrow temperature range, from a hard, brittle glassy state to a softer, rubbery or viscous state, as it is heated.
In more detail
Unlike melting, the glass transition is not a true thermodynamic (first-order) phase transition; no crystal lattice breaks down, and there is no discontinuous jump in volume or enthalpy, only a change in heat capacity and thermal expansion coefficient. Below the glass transition temperature (Tg), long-range segmental motion of polymer chains or network molecules is frozen out due to insufficient free volume; above Tg, this motion becomes possible on experimental timescales. Because it depends on molecular mobility and cooling rate rather than equilibrium thermodynamics, the measured Tg can shift slightly with how fast a sample is cooled or heated.
Key facts
| Field | Physical Chemistry |
|---|---|
| Symbol | Tg |
| Transition type | Kinetic, not first-order thermodynamic |
| Example Tg values | Polystyrene ~100 C; natural rubber ~ -70 C |
Polystyrene has a Tg near 100 degrees C; below this temperature it is rigid and glassy, as in disposable cups and CD cases, but above it the polymer softens into a moldable, rubbery material.
Frequently asked questions
Is the glass transition a true phase transition?
Not in the classical thermodynamic sense; it lacks the latent heat and sharp volume discontinuity of melting, and Tg depends somewhat on cooling or heating rate.
Does every material have a glass transition?
Only amorphous or partly amorphous materials, such as glasses, many polymers, and some pharmaceuticals, show a glass transition; fully crystalline solids melt instead.