Glass transition

The temperature-dependent change in a material’s behavior from liquid-like relaxation to rigid glassy response, without the abrupt structural change of ordinary crystallization.

Structural relaxation: The gradual return of a disordered material toward equilibrium after a change in temperature or stress. Its timescale grows dramatically as a glass-forming material approaches its glass transition.
Supercooled liquid: A liquid cooled below its equilibrium freezing point without crystallizing. Glass-forming materials usually approach the glass transition while remaining supercooled.
Glass transition in polymers: The temperature-dependent change between glassy and rubbery behavior in an amorphous polymer. Polymer stiffness and flexibility often change sharply across this transition.
Crystallization: The formation of an ordered crystal structure from a disordered phase. Unlike crystallization, the glass transition does not require a new ordered phase.
Vitrification: The formation of a glass from a liquid or solution without crystallization. It names the kinetic arrest that leaves a material in a glassy state.
Relaxation time: The characteristic time required for a system to return toward equilibrium after a disturbance. The glass transition is conventionally associated with relaxation becoming too slow on an observation timescale.
Differential scanning calorimetry: A technique that measures heat flow into or out of a sample as its temperature changes. It detects the heat-capacity step commonly used to measure the transition.
Melting: The transition of a crystalline solid into a liquid at its melting temperature. Melting is a sharp equilibrium transition, unlike the history-dependent glass transition.
Free volume theory: A model linking molecular mobility to the unoccupied space available for molecular rearrangement. It explains slowing motion by the loss of space needed for molecules to move.
Viscosity: A measure of a fluid’s resistance to flow under applied stress. Its steep rise near the transition makes a supercooled liquid appear rigid.

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