Glass Transition - The
reversible change in an amorphous polymer or in amorphous
regions of a partially crystalline polymer from (or
to) a viscous or rubbery condition to (or from) a
hard and relatively brittle one. NOTE - The glass
transition generally occurs over a relatively narrow
temperature region and is similar to the solidification
of a liquid to a glassy state: it is not a phase transition.
Not only do hardness and brittleness undergo rapid
changes in this temperature region but other properties,
such as thermal expansion and specific heat also change
rapidly. This phenomenon has been called second order
transition, rubber transition and rubbery transition.
The word transformation has also been used instead
of transition. Where more than one amorphous transition
occurs in a polymer, the one associated with segmental
motions of the polymer backbone chain or accompanied
by the largest change in properties is usually considered
to be the glass transition.
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