Structural evolutions of the amorphous fraction of polyethylene terephthalate during the secondary crystallization

Abstract

We have studied the structural evolution of polyethylene terephthalate (PET) during the isothermal crystallization through the temperature-modulated differential scanning calorimetry (TMDSC) and the positron annihilation lifetime spectroscopy (PALS). By tracing the changes in the amorphous free volume size, and the content of the crystal, the mobile amorphous fraction (MAF), and the rigid amorphous fraction (RAF), we clarified how the RAF participates in the crystallization. It was found that the limiting temperature of the RAF, T* determines the behaviors of the RAF in the crystallization. For isothermal crystallization at temperatures below T*, the RAF is strictly constrained by the lamella such that it cannot participate to the crystallization. For isothermal crystallization at temperatures above T*, chain segments inside the lamella are able to move, and the restrictions exerted on the RAF by the lamella are eased. Then, the activated RAF participates in the crystallization mainly through the lamellar perfection. As a result, instead of a monotonic increase in the RAF content, a non-monotonic change and even a monotonic decrease as a function of the isothermal crystallization time were observed. Moreover, an anomalous increase in the MAF content at the late stage of isothermal crystallization was found. Our results demonstrate that the activated RAF can indeed participate to the crystallization, and reveal how the structural evolutions of the activated RAF and the MAF proceed.