The glass transition temperature (Tg) is the temperature at which a thermodynamic pseudotransition occurs in glassy materials, which is why it is found in glasses, polymers and other amorphous inorganic materials. This means that, thermodynamically speaking, it is not properly a transition. The Tg can be understood quite simply when it is understood that at that temperature the polymer decreases its density, hardness and rigidity, and its elongation percentage decreases drastically.

It is understood that it is an intermediate temperature point between the molten state and the rigid state of the material. The study of Tg is more complex in the case of polymers than in any other small molecule material.

Above Tg, the secondary bonds of the molecules are much weaker than their thermal movement, which is why the polymer becomes rubbery and acquires a certain elasticity and capacity for plastic deformation without fracture. This behavior is specific to thermoplastic polymers and does not occur in thermostable polymers.

This type of transition is observed in glassy materials, between a liquid state (the material flows) and another apparently solid state, this apparently solid state is a state of non-thermodynamic equilibrium, the material is considered a subcooled liquid (flowing at extremely slow speeds), with movements in its practically frozen segments.

All thermoplastic polymers have a Tg, whether they are amorphous or semicrystalline. When heated, amorphous polymers present only one transition, the Tg. Semi-crystalline polymers have two, the Tg and the melting temperature of the crystals (Tm).

Tg is a value of extreme importance in polymer engineering, as it indicates the working temperature of the plastic and therefore determines whether a specific plastic can be used for a given application.

The Tg is not a temperature at which the change of state specifically occurs, there are different experimental methods to determine it, but due to the thermodynamic nature of this pseudotransition, each method provides a different Tg value, which is why scientific articles always provide the method by which its value was determined. For practical purposes, the exact value of Tg is not so important, since it is understandable that around this temperature, the polymer acquires a certain plasticity.