Since the technique measured property, such
as amorphous content or fractional crystallinity directly rather than the rate of change of the properties
with time or temperature, it was not so restricted in the time scale over which measurements were made.
It also had the advantage of measuring the change in concentration of different functional groups with
temperature and determining the temperature range over which chain mobility set in and defining the
type of molecular groups involved in the configurational changes.
The change in absorbance and shift in peak position with temperature are discussed in terms of the
separation of crystalline and amorphous bands as well as defining the cis/trans ratio as a function of
temperature.
Depending on the change in absorbance or peak position with temperature of the IR bands, they have
been characterized as type I or type II behaviour. Measurements on both have been used to characterize
the glass transition, crystallization and melting behaviour of PET