Glass Transition Temperature (Tg)
Definition - What does Glass Transition Temperature (Tg) mean?
A glass transition temperature (Tg) is the temperature at which a polymer turns from a ductile material to a hard, brittle material. Each polymer with an amorphous structure has its own unique glass transition temperature, which makes a given polymer better suited for some applications over others.
Corrosionpedia explains Glass Transition Temperature (Tg)
Pure crystalline polymers do not have a glass transition temperature; the glass transition temperature is only applicable to amorphous polymers. Pure amorphous polymers do not have a melting temperature; they only have a glass transition temperature. However, many polymers are composed of both amorphous and crystalline structures. This means that many polymers have both a glass transition temperature and a melting temperature. The glass transition temperature is lower than the melting temperature.
The different glass transition temperatures for different polymers allow some polymers to be better suited for some applications than others. For instance, a rubber tire for a normal automobile is soft and ductile because at normal operating temperatures it is well above its glass transition temperature. If its glass transition temperature was greater than its operating temperature, it would not have the flexibility needed to grip the pavement.
Other polymers are designed to operate below their glass transition temperature. An example of this would be a stiff plastic handle. If the plastic handle were to have a glass transition temperature below its operating temperature, it would be too flexible to allow one to grab it and immediately open whatever object it is attached to.