Mechanical behavior of poly(ethyleoe terephthalate) at cryogenic temperatures

Abstract

The mechanical behavior of poly(ethylene terephthalate) (PET) was studied at temperatures from 4 to 300°K with a free oscillating torsion pendulum and an Instron tester. Torsion pendulum data show the presence of relaxation maxima below 80°K. Noncrystalline specimens show a small peak at 48°K (δ) and a shoulder at 8-12° K (ϵ). A peak at 20° K is observed in crystalline samples, both oriented and unoriented. In addition drawn specimens show a pronounced peak at 48° K and the assymmetric maximum in the 100-240° K region is split into two peaks. The logarithmic decrement in the β-loss region decreases with increasing crystallinity and orientation, while the cryogenic loss peaks increase in intensity. Tensile tests with biaxially oriented and heat-set films show a twofold increase in elastic modulus and a tenfold increase in toughness at cryogenic temperatures when compared with “amorphous” PET.