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Polymers that are not structured like crystals on 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 in state. Materials above this temperature are generally more flexible, and those in colder temperatures are fragile because the molecules can’t bend or readily move into different places. Glass transition is seen in only solids that do not have molecules arranged into 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 one object can be different. Crystal-based structures melt at a certain temperature, but structures with both kinds of molecules tend to flow over long periods of time. Amorphous components may be strong at one temperature, while crystalline molecules might be in a melted state if they have already undergone phase transition.
Glass transition is different from actual melting, because there is no latent heat to absorb temperature increases. Unlike a melting substance, a transitioning polymer will continue to heat up as the transition temperature is crossed. The heat capacity of the polymer increases nonetheless, so the process it undergoes is called a second order transition. Crystalline structures instead absorb heat, and do not increase in temperature, while they are melting.
If it were to be physically bent, an object would flex like a piece of rubber when it is below the point of glass transition. It could also remain solid if the molecular bonds are strong enough to withstand the force. Objects with molecules not as strong will break or shatter below the glass transition temperature. Plastic car dashboards as well as plastic pales often react in a similar way with temperature changes.
Amorphous materials require a certain amount of thermal energy to change their molecular structure. Glass transition is dependent on the energy needed to change the state of a particular material. The phenomenon is different from melting also because it is not as apparent. Materials oftentimes do not show the associated properties following 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 readily across a surface.