Tensile Deformation of Polyethylene and Directionality of Its Molecules

Abstract

The specimens of polyethylene were deformed by tensile force at the various temperatures between 8°C and 80°C. The cross sectional area of the specimens is 6.8×3.3mm² and the length of their parallel portions is 50mm. The tensile stress-strain curves are shown in Fig. 2, where the logarithmic strain is taken along the abscissa. The characteristic feature of these curves is rendered by three straight lines and a curve, the former consisting of three portions of the strain ranges of 0% to 5%, 15% to 65% and over 65%, and the latter being the portion of the strain range of 5% to 15%. These strain ranges are independent on the test temperatures, but the flow stress corresponding to a certain strain decreases with the increment in the temperatures. The orientation of the chain axes of the molecules in the plastically deformed specimens was also observed from the X-ray diffraction photographs in Fig. 4. The chain axis direction of most molecules in the crystalline region can be calculated from the angle ∅ (in Fig. 4) between the equatorial direction and the radius to the point of maximum intensity on the (110) ring, with the result that the chain axis direction approaches more and more to the tensile direction. As shown in Fig. 5, the angle ε between the tensile direction and the chain axis decreases in proportion to the logarithmic tensile strain. It is evident from the X-ray diffraction patterns in Fig. 4 that the chain axes of the molecules in the amorphous region are made also oriented towards the tensile direction, because the intensity is not uniform on the halo, but stronger near the equatorial direction.