Density of Amorphous Polyethylene Terephthalate

Abstract

POLYETHYLENE terephthalate can be melt-spun and afterwards hot-drawn to form continuous filament yarn¹ marketed under the trade name of ‘Terylene’. In the quenched or spun form, it possesses a very low degree of molecular orientation, and the X-ray diagram shows a characteristic amorphous halo with no evidence of crystalline reflexions. The density of this amorphous form is 1.335 gm./c.c., whereas the calculated crystallite unit-cell density is 1.455 gm./c.c.². With the onset of crystallinity, indicated by an increase in density, the growth of crystallite reflexions in the X-ray diagram, and an increase in the brittleness of the sample, the amorphous halo in the X-ray diagram disappears and hence no X-ray estimate of the degree of crystallinity is possible. It can, however, be estimated from the ratio of the infra-red absorptions in the 898 cm.⁻¹ and the 875 cm.⁻¹ bands³, and from the density.