Crystalline morphology in thin films of natural rubber II. Crystallization under strain

Abstract

The morphology of thin films of natural rubber crystallized under unidirectional strain has been examined in the electron microscope. As strain increases the spherulitic morphology of unstrained films gives way gradually to a fibrillar morphology with crystalline filaments ($\alpha$ filaments) measuring 60 by 250 $\overset{\circ}{\mathrm A}$, growing perpendicular to the strain axis. The nucleation of these filaments is governed by the amount of strain but their rate of growth depends only on temperature and time. At very high strains (> 300%) crystallization occurs rapidly allowing no time for filament growth, so that the resulting morphology consists of chains of nuclei ($\gamma$ filaments) running in the direction of extension. The high strain transition from $\alpha$ to $\gamma$ filaments is accompanied by no change in the electron diffraction pattern, but the earlier transition from spherulitic to filamentous growth reveals a rotation of the molecular axis into the direction of strain. This appears to correspond to a physical rotation of the $\alpha$ filaments about their direction of growth.