Glastillståndet

Glastillståndet, or the glass transition, describes the temperature range over which amorphous materials change from a hard, brittle glass into a softer, more rubbery or viscous state as temperature increases. It is a kinetic phenomenon rather than a single sharp phase change and is central to understanding how materials behave in real-world conditions.

The transition occurs over a broad temperature interval and is characterized by a shift in molecular mobility. Below the glass transition temperature (Tg), molecular motion is highly restricted, giving the material high stiffness and low heat capacity. Above Tg, mobility increases, leading to reduced stiffness, higher free volume, and notable changes in thermal and mechanical properties. Tg is not universal; it depends on cooling and heating rates, material history, and the presence of plasticizers or additives. Polymers typically exhibit a wide range of Tg values, while inorganic glasses have higher Tg values determined by composition.

Measurement and interpretation rely on techniques such as differential scanning calorimetry (DSC), which detects changes in heat capacity, and dynamic mechanical analysis (DMA), which monitors modulus and damping. Tg is a practical guide for processing, performance, and stability, affecting molding, extrusion, and the shelf life of amorphous pharmaceuticals.

Glastillståndet is a foundational concept across polymer science, materials engineering, geology, and pharmaceuticals, influencing design choices and failure analysis where temperature-induced softening or aging is a concern.