THE STRUCTURAL CHANGE ON ANNEALING POLYETHYLENE TEREPHTHTALATE

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Abstract
The change in the texture of polyethylene terephthalate film on annealing was followed by electron microscopy and differential thermal analysis (DTA). In the qnenched film the ball-like strtctures named by Geil et al. could be observed. On annealing the film at the temperatures below 170°C, the fibrillar ribbons were formed through the fibrillar alignment of the ball-like structure. From the results of the density measnrements of the annealed samples quenched at various periods of times, it became apparent that the crystallization occured rather rapidly (within a day) even at 90°C in water and practically ceased at a lower crystallinity. At the temperatures below 135°C the crystallinity attained for a given time increased rapidly with the annealing temperature. These facts suggest that the crystallization at these low temperatures (cold crystallization) takes place through the inter-molecular nucleation which results in, at least partially, the fringed micell an structure. DTA revealed that the annealed samples show two melting peaks.The peak at the lower temperature may be attributable to the melting of the bundle-like crystals postulated above. The peak at the higher temperature (Ca. 260°C) would be due to the folded chain crystals formed at the lower temperature and rearranged, and newly crystallized in the course of the heating during the DTA measurement. This is confirmed by the morphological studies and the measurements of the crystallinities and the melting behavior for the samples heated in the DTA apparatus up to various temperatures (the heating rate: 10°C/min.) and quenched.The similar measurements were also made for the crystals extracted from these samples by treating them in ethyl amine, in which the rearrangement of the crystals in the course of the DTA measurement could be avoided. It became apparent that the crystals (including the bundle-like crystals mentioned above) formed at the lower temperatures began to melt at 170°C and transformed into the thicker chain-folded lamellae. This transformation was completed nearly at 210°C. These results were also consistent with the electron microscopic observation which revealed that the transformation form the ball-like structures to the lamellar structures occured at 170°C.

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