The infrared spectrum of polyethylene terephthalate. I The effect of crystallization

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Abstract
Films of amorphous polyethylene terephthala were drawn and heated to various extents. Intensity change of the infrared spectral bands resulting from these treatments were followed. There are more than seven bands which change their intensities by drawing, and sixteen bands affected by heat treatment. Exactly the same changes are also observed in several aromatic esters of ethylene glycol. The bands that intensify in the crystalline state of these model compounds correspond to the crystalline bands of polyethylene terephthalate, and the bands that appear in the melt of model compounds correspond to the amorphous bands of polyethylene terephthalate. Main crystalline bands and all of the amorphous bands are associated with ethylene glycol residue. A crystalline band makes a couple with one of the amorphous bands and the frequencies of this couple have reasonable values as the characteristic vibration of the ethylene glycol residue. Thus, the spectral change can be interpreted by the rotational isomerism of the ethylene glycol residue, the gauche form in the amorphous part changing into the trans form by crystallization. The crystalline band at 973 cm. −1 corresponds to the Bu type CO stretching vibration of the trans form, and the amorphous bands at 1099 and 1042 cm.−1 correspond to A and B type CO stretching vibrations of the gauche form. The crystalline bands at 1475, 1387, and 988 cm. −1 are considered to be manifestations of the perturbation of the crystal field.

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