Effect of chain composition of poly(ethylene terephthalate) structure and properties

The effect of 2,2′-oxydiethanol (diethylene glycol, DEG) content (ranging from 2 to 15 mol-%) on unit cell dimensions, density, melting point, and small angle X-ray scattering (SAXS) of undrawn and drawn annealed (in the range from 80 to 260°C) poly(ethylene terephthalate), (PET), is studied. It is established that the unit cell parameters are almost unaffected by the DEG content. A significant decrease of the melting point and the degree of crystallinity of PET is observed, however. A decrease of the density of fully amorphous copolyester is established too, which is important for calculating the crystallinity from density measurements. At the same time a remarkable increase of both SAXS intensity and long spacing is registered. It is assumed that the effect of bonded DEG on PET structure and properties can be explained by the predominating comonomer (DEG) units distribution in the amorphous regions–only a small portion of them is involved in the crystallites as lattice defects.