X-Ray Diffraction Studies of the Stretching and Relaxing of Polyethylene

Abstract

The orientation of the crystallites when polyethylene is stretched at 96°C is normal in that, from the onset of stretching, the preferred orientation is that wherein the long chain axis is parallel to the direction of stretch. At low extensions the preference for this orientation is weak, but it progressively becomes stronger as stretching proceeds. When polyethylene is stretched at room temperature, the crystallite behavior is more complicated. The first 20 percent extension produces no preferred orientation of the crystallites. From 30 percent to 200 percent extension the preferred orientation is that wherein the 011 ``axis'' is parallel to the direction of stretch, and the long chain axis therefore inclined to this direction at an angle of 64°. From 200 percent extension to the break (about 600 percent extension), the tilt of the crystallites in the preferred orientation progressively lessens and almost vanishes at the break. The stress‐strain diagram of polyethylene is interpreted in the light of these findings. The preferred orientation of the crystallites in the region of the shoulder in necked‐down polyethylene is also that wherein the 011 ``axis'' aligns with the stretching direction. When stretched polyethylene is relaxed by shrinking at elevated temperatures, the crystallites become disoriented but in a non‐random manner. This behavior is described qualitatively.