  Poly(ethylene terephthalate) (PET) is a semi--crystalline polymer that can be
crystallized to different degrees heating from the amorphous state. Even when
primary crystallization has been completed, secondary crystallization can take
place with further annealing and modify the characteristics of the amorphous
interlamellar phase. In this work we study the glass transition of highly
crystallized PET and in which way it is modified by secondary crystallization.
Amorphous PET samples were annealed for 4 hours at temperatures between 140C
and 180C. The secondary crystallization process was monitored by differential
scanning calorimetry and the glass transition of the remaining interllamelar
amorphous phase was studied by Thermally Stimulated Depolarization Currents
measurements. Non--isothermal window polarization is employed to resolve the
relaxation in modes with a well--defined relaxation time that are subsequently
adjusted to several standard models. Analysis of experimental results, show
that cooperativity is reduced to a great extend in the interlamellar amorphous
regions. The evolution of the modes on crystallization temperature reveals that
large scale movements are progressively replaced by more localized ones, with
higher frequency, as crystallization takes place at higher temperatures. As a
consequence, the glass transition temperature of the amorphous interlamellar
phase tends to lower values for higher annealing temperatures. Evolution of
calorimetric scans of the glass transition are simulated from the obtained
results and show the same behaviour. The interpretation of these results in
terms of current views about secondary crystallization is discussed.
