What is "glass transition"

n. The transition which occurs when a liquid is cooled to an amorphous or glassy solid; occurs if the cooling rate is so fast that normal crystallization is prevented.

The glass–liquid transition or glass transition for short is the reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a molten or rubber-like state, as the temperature is increased. An amorphous solid that exhibits a glass transition is called a glass. The reverse transition, achieved by supercooling a viscous liquid into the glass state, is called vitrification.

The glass-transition temperature T of a material characterizes the range of temperatures over which this glass transition occurs. It is always lower than the melting temperature, T, of the crystalline state of the material, if one exists.

Hard plastics like polystyrene and poly(methyl methacrylate) are used well below their glass transition temperatures, that is in their glassy state. Their T values are well above room temperature, both at around . Rubber elastomers like polyisoprene and polyisobutylene are used above their T, that is, in the rubbery state, where they are soft and flexible.

Despite the massive change in the physical properties of a material through its glass transition, the transition is not itself a phase transition of any kind; rather it is a laboratory phenomenon extending over a range of temperature and defined by one of several conventions. Such conventions include a constant cooling rate and a viscosity threshold of 10 Pa·s, among others. Upon cooling or heating through this glass-transition range, the material also exhibits a smooth step in the thermal-expansion coefficient and in the specific heat, with the location of these effects again being dependent on the history of the material. However, the question of whether some phase transition underlies the glass transition is a matter of continuing research.