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The Magical Glass Transition: Do you know what the glass transition temperature is?
In our daily lives, we come into contact with a variety of glass materials, but few people think about the science behind these materials. Glass is a distinctive substance with unique transformation properties, especially as it changes from one state to another. This phenomenon is called glass transfer, or glass transition, and is a gradual and reversible process that occurs in amorphous materials from a hard and brittle "glassy" state to a viscous state as the temperature increases. Thick or rubbery state.
The transformation process of glass is a mysterious phenomenon because even in the temperature range up to 500 K, this transformation does not cause significant changes in the material structure.
Glass transition temperature Tg is an important parameter that describes the temperature range of this transition. This temperature is always lower than the melting point Tm of the crystalline state because glass is essentially a higher energy state. Many rigid plastics, such as polystyrene and polymethyl methacrylate, typically have a Tg of around 100 °C. This means that they remain solid below this temperature and become softer and more flexible above this temperature.
The application of rubber elastomers such as polyisoprene and polyisobutylene is exactly the opposite. These materials are used in a state above their Tg, in which they appear soft and flexible. Such a cross-linked structure prevents the free flow of molecules, so the rubber can maintain a fixed shape at room temperature.
Although the physical properties of the glass change, the glass transition is not considered a phase change, but rather a dynamic phenomenon that relies on thermal history.
In many materials, when the conventional freezing process is replaced by rapid cooling, the crystalline phase transition is avoided and the glassy state is directly formed. Such materials have glass-forming capabilities, the ability to remain in an amorphous state when cooled rapidly. This property is related to the composition of the material and can be predicted by rigidity theory. The next question is, can the structure of a material that remains in a glassy state further relax over time?
Glass changes structure slowly within its transformation range. Even at lower temperatures, the configuration of glass will be relatively stable, while the structure of many materials will tend to a state of thermal equilibrium after a certain amount of heating or cooling. This process demonstrates the basic principle of Gibbs free energy minimization and provides a dynamic driving force that allows the structure of the glass to change in time.
Many researchers believe that glass has a dynamically locked state in which its entropy and density depend on its thermal history, and that this state will not reach thermal equilibrium.
When it comes to glass transition, there is also Schrödinger's paradox, that is, as the liquid is supercooled, the entropy difference between the liquid phase and the solid phase decreases, and the temperature when the entropy difference is zero can be deduced. This temperature is named the Kauzmann temperature. This led to the idea that the liquid might self-crystallize before reaching this temperature. The numerous hypotheses seeking to explain Kauzmann's paradox offer different perspectives on the nature of the glass transition.
From more recent research, the definition of glass transition temperature is not uniform and is affected by different standards. The results may produce different values under different circumstances. However, when making these measurements, the rate of cooling or heating can significantly affect the measured Tg value. Exploring the indoor glass transition phenomenon makes us wonder, is there a more ingenious way to understand the underlying reasons for this phenomenon?
