Dielectric study of the glass transition of PET/PEN blends

TL;DR

This study investigates the dielectric properties and glass transition behavior of PET, PEN, and their blends, revealing how copolymerization and composition influence relaxation modes, activation energies, and material cooperativity.

Contribution

It provides a detailed dielectric analysis of PET/PEN blends, linking molecular structure, copolymerization, and glass transition dynamics with experimental validation.

Findings

Activation energies around 3 eV for dominant modes.

Increased PEN content decreases polarizability.

High-frequency modes are more Arrhenius-like and less cooperative.

Abstract

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.