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How does the glass transition differ from the freezing of a crystalline solid?
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amorphous solid
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The primary difference between the glass transition and the freezing of a crystalline solid lies in the nature of the structural change and the thermodynamic behavior of the material during cooling.
- Crystalline Freezing: When a liquid cools to its freezing point, it undergoes a first-order phase transition characterized by an abrupt, discontinuous change in volume as atoms arrange themselves into a regular, repeating lattice pattern. This process involves nucleation and growth, resulting in a structure with long-range order.1234
- Glass Transition: If a liquid is cooled rapidly enough to bypass crystallization, it enters the glass transformation range. Unlike freezing, this transition is gradual and exhibits no abrupt discontinuity in volume. As the temperature drops toward the glass transition temperature (), the material's viscosity increases enormously until the molecular rearrangement time exceeds experimental timescales. The atoms become "frozen" into a disordered, noncrystalline state that retains short-range order but lacks long-range periodicity.1243
While the freezing point is a sharply defined temperature, is dependent on the cooling rate, shifting downward as the rate of cooling is reduced.1