Study of the glass transition in the amorphous interlamellar phase of highly crystallized poly(ethylene terephthalate)

TL;DR

This study investigates how secondary crystallization affects the glass transition in highly crystallized PET, revealing reduced cooperativity and localized molecular motions with increasing annealing temperature.

Contribution

It provides new insights into the molecular dynamics of the amorphous interlamellar phase during secondary crystallization in PET using advanced thermal and dielectric measurements.

Findings

Cooperativity in the amorphous phase is significantly reduced.

Large-scale molecular motions are replaced by localized, higher-frequency modes.

Glass transition temperature decreases with higher annealing temperatures.

Abstract

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.