The effect of temperature on the infrared spectra of poly(ethylene terephthalate)

Reversible and irreversible spectral changes are observed on heating of solvent‐cast films of poly(ethylene terephthalate) (PET) between 30 and 230°C. The irreversible changes are due to the gauche‐trans isomerization of the ethylene glycol segments and the corresponding changes in the symmetry and resonance characteristics of the aromatic rings. On the other hand, there are thermally reversible spectral changes. These reversible effects are primarily observed for the modes of the aromatic ring and the trans ethylene glycol segment. These reversible spectral changes include intensity variations and frequency shifts and are found to be linearly dependent on the measurement temperature. These reversible changes arise from changes in intermolecular and intramolecular forces as the temperature changes.