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== Significance ==
The glass transition temperature (Tg) of a polymer is critical for defining its operating temperature limits. At temperatures above Tg, a polymer experiences a sudden drop in its mechanical stiffness. 

Typically, when mechanical stiffness is desired, a polymer’s service temperature should be below its Tg. However, in applications where flexibility is desired, the polymer should be in service about its Tg temperature. An example of this latter case is utilizing the flexibility provided by rubbers and elastomers.