Glass transition temperature

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The liquid-glass 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 state into a molten or rubber-like state.[1]

An amorphous solid that exhibits a glass transition is called a glass. Supercooling a viscous liquid into the glass state is called vitrification.

The liquid-glass transition is an atypical phase transition in that it is not connected with a discontinuous change in structure, and in that it does not have a sharp transition temperature. There is rather a glass transformation range that extends over several Kelvin or more. Upon cooling or heating through this range, one observes a smooth step in the thermal expansion coefficient and in the specific heat. The exact temperature where these effects are observed depends however on the temperature variation rate and, more generally, on the preparation protocol.[2][3]

Therefore, the glass transition temperature, Tg, must be defined by convention. One approach is to agree on a standard heating rate of 20 K/min.[1] Another approach is by requiring a viscosity of 1012 Pa·s. Since vitrification is an extreme form of supercooling, Tg is always lower than the melting temperature, Tm, of the concurrent crystalline state.



The glassy or vitreous state of matter is typically formed by rapid cooling and solidification from the molten (or liquid) state. If the liquid were allowed to crystallize on cooling, then according to the Ehrenfest classification of first-order phase transitions, there would be a discontinuous change in volume (and thus a discontinuity in the slope or first derivative with respect to temperature, dV/dT) at the melting point. Below the transition temperature range, the glassy structure does not relax in accordance with the cooling rate used. The expansion coefficient for the glassy state is roughly equivalent to that of the crystalline solid. If slower cooling rates are used, the increased time for structural relaxation (or intermolecular rearrangement) to occur may result in a higher density glass product. Similarly, by annealing (and thus allowing for slow structural relaxation) the glass structure in time approaches an equilibrium density corresponding to the supercooled liquid at this same temperature. Tg is located at the intersection between the cooling curve (volume versus temperature) for the glassy state and the supercooled liquid.[4] [5] [6] [7] [8]

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