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Glass transition and melting
The so-called glass transition temperature plays an important role in the properties of polymeric materials TG. It is one of the most important parameters of polymers and provides an indication of the dimensional stability of the plastic when exposed to heat.
The glass transition temperature is the temperature at which completely or partially amorphous polymers change from the highly viscous or rubber-elastic, flexible state to the glass-like or hard-elastic, brittle state. Hence it is also called the softening temperature. Every plastic has a specific glass transition temperature that can be used to characterize it.
The transition from the glass state to the liquid state is not the same as melting. Strictly speaking, melts can only be substances that have a crystalline structure. When heat is supplied, translational and rotational movements increase, the ordered crystal lattice dissolves, and a disordered liquid is obtained. In the case of substances whose molecules are already in a disordered state in the solid state, the crystal lattice does not first have to be dissolved when energy is supplied. It is true that translational and rotational movements also increase here. However, only small parts of the molecule are free to move in translation (approx. 100 C atoms): the softening of the polymer in parts of the molecule is visible to the outside. Therefore one speaks of a glass transition area and not of a fixed glass transition temperature. Some plastics are used below their glass transition temperature, i.e. in a hard and brittle state. Examples of this are polystyrene, polymethyl methacrylate or polyethylene terephthalate. Polyisoprene, polybutadiene or polyethylene, on the other hand, are used above their glass transition temperature, where they are elastic and flexible.
The difference between heating a crystal and an amorphous solid is shown by the temperature profile. When a crystal begins to melt, its temperature remains constant until it is completely melted. The heat supplied is only used to destroy the crystal lattice. (latent heat of fusion; from lat. Latency = hidden). Imamorphic solids use all of their energy to raise the temperature of the substance.
Most plastics consist of an amorphous and a crystalline area. The presence of crystalline and amorphous areas has been shown by X-ray examinations. Crystalline polymers are almost always understood to mean partially crystalline polymers. This also explains the fragility of plastics below the glass transition temperature. In the elastic state, the molecular chains can slide past one another without problems in order to evade pressure from outside or another external force. However, if the molecules are in the glass state, there are two possibilities: Either they withstand the pressure and remain in their previous position, or they are separated by the pressure and the workpiece breaks apart.