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Crystallization of a rubber band by stretching

When a piece of rubber is pulled it extends and when it is released it shrinks back to its original length. The macroscopic ability of rubber to stretch under a small force (elasticity) originates from a change in the microscopic molecular structure. An orientational difference in the alignment of the molecules arises between the pulling direction and the direction perpendicular to that in which the rubber is being stretched. A two-dimensional diffraction image collected on an R-AXIS RAPID II shows these effects simultaneously. If the rubber is an amorphous substance, a broad ring is observed. If it is a crystalline substance, a narrow ring—or a part of the ring that becomes more like a spot—is observed. Figure 1 shows the two-dimensional diffraction images of a piece of rubber in its relaxed state (right, top) and after it has been stretched (right, bottom). Before it is stretched, the width of the diffraction ring is broad and uniform, revealing that the rubber band in this state is nearly amorphous.

Rubber band
Figure 1: Debye rings and diffraction profiles of a rubber band before and after it is pulled

 

On the other hand, after the rubber band is pulled, spots appear only in the horizontal direction, revealing that the molecules in the rubber band are crystallized by pulling the rubber band, and that they are oriented in the same direction. When the two-dimensional images are converted into diffraction angle/intensity profiles and the profiles are compared, diffraction peaks that correspond to constituent components, (C5H8)n and (C3H4)n, appear, confirming that the molecules in the elastic band have crystallized.