The relaxation strength at the glass transition for semi-crystalline polymers observed by different experimental methods shows
significant deviations from a simple two-phase model. Introduction of a rigid amorphous fraction, which is non-crystalline
but does not participate in the glass transition, allows a description of the relaxation behavior of such systems. The question
arises when does this amorphous material vitrify. Our measurements on PET identify no separate glass transition and no devitrification
over a broad temperature range. Measurements on a low molecular weight compound which partly crystallizes supports the idea
that vitrification of the rigid amorphous material occurs during formation of crystallites. The reason for vitrification is
the immobilization of co-operative motions due to the fixation of parts of the molecules in the crystallites. Local movements
(Β-relaxation) are only slightly influenced by the crystallites and occur in the whole non-crystalline fraction.