A traffic jam at the atomic scale
When a liquid cools, its atoms usually arrange themselves into an orderly, crystalline lattice, like water freezing into ice. Some liquids, however, take a different path. Upon cooling, their molecules simply slow down, remaining in a disordered, jumbled state until they are effectively frozen in place. This process is called vitrification, and the result is an amorphous solid, or a glass.
The change from a flowing liquid to a rigid glass happens over a specific temperature range known as the glass transition temperature, or Tg. This is not a sharp freezing point. Instead, it is a gradual seizure of molecular motion. As a liquid approaches its Tg, its viscosity, a measure of its resistance to flow, increases dramatically. Within a narrow temperature range, the viscosity increases by as much as 17 orders of magnitude. Hard plastics like polystyrene have a Tg around 100 °C (212 °F), keeping them solid and rigid at room temperature. By contrast, the Tg of silicone is around -60°C, which is why it remains soft and rubbery.
The Kauzmann paradox
In 1948, chemist Walter Kauzmann identified a thermodynamic puzzle. As a supercooled liquid's temperature drops, its entropy (a measure of molecular disorder) decreases much faster than the entropy of its crystalline counterpart. Kauzmann noted that if you could continue to cool the liquid past its Tg without it turning into a glass, its entropy would eventually fall below that of a perfect crystal. This outcome is paradoxical, as a disordered liquid should not have less entropy than a perfectly ordered crystal.
The glass transition always seems to occur just in time to prevent this "entropy catastrophe". This observation has led to a long-standing debate: is the glass transition a true, fundamental thermodynamic phase transition, or is it merely a kinetic phenomenon; a result of molecules just not moving fast enough to keep up with the cooling rate? One proposed solution is the existence of an "ideal glass" state, a unique and perfectly disordered state that the liquid would transition into at the Kauzmann temperature, but creating such a state would take an impossibly long time to form. The early work of physicists like Gustav Tammann in Göttingen laid the groundwork for these modern investigations.
Measuring the slowdown
Scientists cannot see individual molecules freezing in place, so they measure the glass transition by observing changes in a material's bulk properties. One of the most common methods is Differential Scanning Calorimetry (DSC). A DSC instrument carefully heats a small sample and measures the amount of heat flow required to raise its temperature. At the glass transition, there is a distinct step-like change in the material's heat capacity, which the DSC device detects. The midpoint of this step is typically reported as the material's Tg.
This measurement is important for countless applications. In the food industry, the Tg of sugars and starches determines whether a cookie is crisp or stale. Water absorbed from the air can lower the Tg, turning a glassy and crisp texture into a rubbery, soft one. In pharmaceuticals, drugs are often stored in an amorphous state below their Tg to ensure long-term stability. The glass transition, though conceptually abstract, has tangible effects on the materials we use every day.