All polymers (amorphous or semi-crystalline) go through glass transitions. The glass-transition temperature (T) is a crucial physical parameter for polymer manufacturing, processing, and use. Below T, molecular motions are frozen and polymers are brittle and glassy. Above T, molecular motions are activated and polymers are rubbery and viscous. The glass-transition temperature may be engineered by altering the degree of branching or crosslinking in the polymer or by the addition of plasticizers.
Whereas crystallization and melting are first-order phase transitions, the glass transition is not. The glass transition shares features of second-order phase transitions (such as discontinuity in the heat capacity, as shown in the figure), but it is generally not considered a thermodynamic transition between equilibrium states.