Biaxial Orientation Mechanism of Drawn Natural Rubber. IV. The Effect of Aspect Ratio

Abstract

Biaxial orientation (BO) of crystallites was observed for uniaxially elongated natural rubber (NR) films as a function of an aspect ratio (AR ¼ width/length of the original sample). The wide angle X-ray diffraction results were analyzed, showing that the crystalline c-axis was parallel to the draw-direction, while the a-axis became parallel to the film surface. The degree of BO was quantitatively evaluated by an orientation function of the (200) plane, F 200 . The value of F 200 increased with increasing AR, showing that the mechanism of microscopic crystallization is controlled by the macroscopic AR. The observed BO is consistently explained by the shish kabab-like secondary crystallization in the ellipsoidal free space surrounding the primary fibril. The shape of free space becomes highly ellipsoidal in the film having high AR.