  An analysis of the glass transition of four materials with similar chemical
structures is performed: PET, PEN and two PET/PEN blends (90/10 and 70/30 w/w).
During the melt processing of the blends transesterification reactions yield
block and random PET/PEN copolymers that act as compatibilizers. The blends
obtained in this way have been characterized by 1H-NMR and DSC. A degree of
randomness of 0.38 and 0.26 has been found for the 90/10 and 70/30 copolymers.
It is shown by DSC that this copolimerization is enough to compatibilize the
blends. The alpha relaxation, the dielectric manifestation of the glass
transition, has been studied by thermally stimulated depolarization currents
(TSDC). The relaxation has been analyzed into its elementary modes by means of
a relaxation map analysis. The activation energies of the modes of the glass
transition do not change significantly between the four materials: in all cases
the modes with a larger contribution have around 3 eV and modes with less than
1 eV are not detected. The change in the pre-exponential factor accounts
entirely for the relaxation time change from material to material, that is
larger as the PEN content increases. The compensation law is fulfilled and
compensation plots converge for high-frequency modes. The polarizability
decreases as the PEN content increases due to the increased stiffness of the
polymer backbone. An analysis of the cooperativity shows that the central modes
of the distribution are the most cooperative while high-frequency modes tend to
behave more as Arrhenius. The low-frequency modes are difficult to study due to
the asymmetry of the distribution of relaxation times. PEN turns out to be the
less cooperative material. It is demonstrated how the parameters obtained from
the dielectric study are able to reproduce calorimetric data from DSC scans and
are, therefore, a valid description of the glass transition.
