The Initial Molecular Interactions in the Course of Enthalpy Relaxation and Nucleation in Polyethylene Terephthalate (PET) as Monitored by Combined Nanocalorimetry and FTIR Spectroscopy

Abstract

Fast scanning calorimetry (FSC) and Fourier transform infrared (FTIR) spectroscopy are combined to trace the evolution of the calorimetric properties and of the molecular interactions in a strongly crystallizing polymer, polyethylene terephthalate (PET). The minute sample amount (≈5 ng) required for FSC allows for the realization of cooling rates up to 50 000 K s −1 and to quench the sample into the amorphous state. By subsequent annealing at varying temperatures T a and times t a enthalpy relaxation, (homogeneous) nucleation and crystallization can be monitored with high precision. Determining the difference IR spectra between a quenched and an annealed sample unravels the development of the intra-and inter-molecular interactions in detail; i) as first response taking place during enthalpy relaxation, the far reaching Coulomb interactions between the polar carbonyl (C ═ O) moieties are active; ii) in contrast, the methylene unit (CH 2 ) and the aromatic ring show a response only, if homogeneous nucleation sets in, while the ester (COC) moiety remains uninfluenced; iii) a hierarchy is observed in of the response of the different molecular moieties; iv) if crystallization comes to play, all molecular units are involved. The results are compared with recently published findings for polyamide 66 demonstrating the high molecular specificity of homogeneous nucleation and crystallization.