what is glass transition temperature

The glass transition temperature (Tg) is the temperature range at which a material changes from a rigid, glassy state to a rubbery or viscous state. It is the temperature at which the amorphous region of a material experiences a transition from a rigid state to a flexible state. The Tg is a property of the amorphous portion of a material and is always lower than the melting temperature (Tm) of the crystalline state of the material, if one exists. The Tg is important in many industrial, technological, and scientific fields that require materials with measured, stable, and controlled Tg values. The Tg is directly related to a materials strength and capabilities in any given end-use application, including its tensile strength, impact resistance, modulus of elasticity, and its operational temperature range. Different operational definitions of the Tg are in use, and all yield different numeric results, but they agree within a few kelvins.