EXPERIMENTAL EVALUATION OF THE TRANS FRACTION IN CH<sub>2</sub>-CH<sub>2</sub> BONDS OF MELT-QUENCHED POLY (ETHYLENE TEREPHTHALATE)

Abstract

In a previous paper we speculated that the molecular conformation of poly (ethylene terephthalate) (PET) quenched from the melt is not in random coil state but in frozen metastable state which is easily transformed into the stable crystalline state. This speculation should be confirmed with direct structural evidences. However, experimental methods to investigate the structure of melt-quenched PET are scarce. In this paper we have evaluated the trans fraction in CH2-CH2 bonds of PET quenched from the melt by means of infrared spectroscopy.The trans fraction was determined by two methods. The first one uses the ratio of intensities of the annealed sample and the melt-quenched one for both the trans and gauche bands (At/A°t and Ag/A°g). The trans fraction of melt-quenched PET (X°t) can be obtained from the following equation; where Ft and Fg are the ratio of extinction coefficient of the annealed sample and that of the meltquenched one for trans and gauche chain respectively. These are assumed to be unity. Scbonherr used the peak heights as the absorbance values and the 1578 cm-1 band as the internal thickness band. However, our study shows that the band widths change upon annealing and the 1578 cm-1 band is a crystallization sensitive band. In order to obtain reasonable value of the trans fraction, we used the integrated intensities as the absorbance values and the 1505 cm-1 band as the internal thickness band. Namely, a computer program was written which resolves the infrared spectar in the 760-930 cm-1 region of PET into composite vibrational bands whose envelopes are assumed to be Lorenzian. The computation was based on the principle of least squares. The integrated absorption intensity for the individual band was accurately obtained. By these procedures, we evaluated 0.2 as the value of trans fraction in melt-quenched PET.The second one is newly proposed here. If one obtains the ratio of extinction coefficient of the trans and the gauche band (et /eg ), the trans fraction (X°t) can be evaluated from the following equation; where A°t and A°g are the absorbances for the trans and gauche band in melt-quenched PET. The ratio of extinction coefficient of the trans and the gauche band was determined by using intensities of the trans and the gauche band in crystals of diol type oligomers of PET in which the CH2-CH2 bonds in both molecular ends were found to be gauche. The result was consistent with that from the improved Schonherr's method. Thus, a considerably small value of 0.2 is conclusively assigned to the trans fraction in CH2-CH2 bonds of melt-quenched PET.