The glass transition temperature is the point at which amorphous polymers change state. Materials above this temperature are more flexible, while those below are brittle. The glass transition is different from melting and occurs at different temperatures depending on the material’s heat capacity. Objects with weak molecular bonds break below the glass transition temperature, while those with strong bonds may remain solid. The glass transition requires thermal energy and is not visually apparent like melting.
Polymers that are not structured as crystals at the molecular level change state with temperature differently than crystalline substances. The glass transition temperature is the point at which the polymer undergoes a change of state. Materials above this temperature are generally more flexible, and those at cooler temperatures are brittle because the molecules can’t bend or move easily to different places. The glass transition is seen only in solids that do not have molecules arranged in crystals; these are called amorphous and include glass, gels and thin films.
Depending on the material, the glass transition point occurs at a different temperature, which is related to its heat capacity. Some materials such as rubber have both crystalline and amorphous molecules. The temperatures for each in an object can be different. Crystal-based structures melt at a certain temperature, but structures with both types of molecules tend to flow for long periods of time. Amorphous components can be strong at one temperature, while crystalline molecules can be in a molten state if they have already undergone a phase transition.
The glass transition is different from actual melting, because there is no latent heat to absorb temperature increases. Unlike a substance that melts, a transition polymer will continue to heat up when the transition temperature is exceeded. However, the heat capacity of the polymer increases, so the process it undergoes is called a second-order transition. Crystalline structures on the other hand absorb heat, and do not increase in temperature, while they are melting.
Were it to be physically bent, an object would flex like a piece of rubber when below the glass transition point. It might even remain solid if the molecular bonds are strong enough to resist the force. Objects with molecules that are not as strong break or shatter below the glass transition temperature. Plastic car dashboards as well as plastic pales often react similarly to temperature changes.
Amorphous materials require a certain amount of thermal energy to change their molecular structure. The glass transition depends on the energy required to change the state of a particular material. The phenomenon is different from the dissolution also because it is not so evident. Materials often do not exhibit the associated properties after the glass transition unless a force is applied to them. Melting, however, is visually apparent and has a more dramatic effect, such as when a crystalline ice cube melts into water that flows easily over a surface.
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