When the degree of crystallinity (Xwc) of semicrystalline poly(ethylene terephthalate) (PET) was within a definite range

there were dual endothermic peaks appearing in the glass transition region of differential scanning calorimetry (DSC) traces of physically aged samples. By comparing the dual endothermic peaks with the single endothermic peak of physically aged wholly amorphous samples and semicrystalline samples with a fairly high Xwc

respectively

it can be concluded that there were dual amorphous regions with quite different characteristics in the semicrystalline samples：the free amorphous region and the constrained amorphous region．The dual tanδ loss peaks in dynamic mechanical thermal analysis (DMTA) curves also confirmed the existence of the dual amorphous regions．The dual amorphous regions were found to be produced due to the different constraints that the crystallites excerted on the amorphous phase．For the semicrystalline sample crystallized from glassy state

both the lower and the upper limit of the Xcw increased with increasing crystallization temperature (Tc)．With the proceeding of physical aging

some of the free amorphous regions became constrained amorphous regions

while the gross amount of the amorphous phase kept almost unchanged. The struik’s model of “extended glass transition” was proved to be the most successfulmodel for our experimental results．