What is the glass transition temperature?

The glass transition temperature (Tg) refers to the specific temperature at which a polymer transitions from a hard and relatively brittle state (glass-like) to a softer and more flexible state. At this temperature, the molecular chains of the polymer gain enough thermal energy to overcome some of the intermolecular forces holding them in place, allowing for increased mobility and flexibility. This transition is crucial because it marks the point where the material begins to soften, which can significantly affect its mechanical properties and usability in various applications.

Understanding the glass transition temperature is vital for engineers and material scientists when designing materials for specific applications, as it helps in predicting how a polymer will perform under different temperature conditions. For example, using a polymer above its Tg means it will exhibit more ductility, while below that temperature, it will be more rigid.

The other options pertain to different aspects of polymer behavior: the rigidity of a polymer is not solely defined by Tg, molding temperatures are typically above the Tg and can vary widely, and thermal degradation occurs at much higher temperatures than Tg.