glass transition temperature

A stick of cold caramel is hard and brittle; warm it slightly and it turns soft and pliable, without ever melting into a liquid. The glass transition temperature, written Tg, is the temperature at which an amorphous solid makes that kind of change — passing from a rigid, brittle glassy state to a soft, rubbery state.

This is not melting. There is no crystal lattice to break and no sharp heat absorption; instead molecular mobility suddenly increases as the material crosses Tg, seen on differential scanning calorimetry as a step change in baseline rather than a peak. Below Tg molecules are essentially frozen in place; above Tg they can move, relax and rearrange.

Tg is central to the stability of amorphous drugs and freeze-dried biologics. Below Tg the disordered molecules lack the mobility to recrystallize, so the high-solubility amorphous form is kinetically locked and stable; above Tg they regain freedom and tend to crystallize or aggregate, destroying that advantage. Formulators therefore store amorphous products well below Tg and add high-Tg polymers or lyoprotectants to raise it. Absorbed water is a powerful plasticizer that lowers Tg, which is one more reason moisture control matters.