BIAXIAL ORIENTATION OF POLY(ETHYLENE-TEREPHTHALATE) .1. NATURE OF THE STRESS-STRAIN CURVES

The orientation characteristics of extruded poly(ethylene terephthalate) (homopolymer) sheets are discussed with respect to: mode of extension (simultaneous versus sequential), strain rate, stretch ratio, and temperature. The nature of the stress-strain curve recorded during the process of extension, and its dependence on the parameters of extension mentioned above, are analyzed. Results show that for specimens stretched in the biaxial simultaneous mode, the stress-strain curve is concave upward, which is typical for amorphous polymeric materials. Strain-hardening occurs at high planar extensions, and is accompanied by a steep rise in the stress measured during stretching. The magnitude of this stress increases with increasing strain rate and extension ratios, but decreases with increasing temperature of stretching. During biaxial sequential extension, the stress-strain curve is concave upward at low strain rates or at high temperatures of stretching, but becomes convex upward at higher strain rates or lower temperatures. The curve also changes from concave to convex upward at high limiting extension ratios in the first direction. The convex upward nature of the stress-strain curve is typical of semi-crystalline materials, indicating the occurrence of strain-induced crystallization during stretching in the first direction. Again, the level of stress, measured during stretching, increases with increasing strain rate and limiting extension ratio in the first direction, while it decreases with increasing temperature of stretching.