Strain-induced crystallization and reversible phenomena in transpolyisoprene. I. Electron microscopy studies

A detailed electron microscope study has been carried out on strain-crystallized thin films of transpolyisoprene (TPI). Variables studied include degree of elongation and annealing temperature. When stretched to elongations greater than 100% and then crystallized at room temperature, the strain-crystallized films show a distinct lamellar morphology. When heated to temperatures (up to 70°C) close to the melting temperature of TPI, the original lamellar morphology is replaced by a near-fibrillar, shish-kebab-type morphology. The morphological transformation is accompanied by increases in both crystal thickness (lc) and amorphous thickness (la) and, therefore, the periodicity (L) of the lamellae. Upon cooling back to room temperature, the near-fibrillar morphology reverts to the lamellar morphology accompanied by noticeable decreases in both the crystal and the amorphous thicknesses, indicating the reversible nature of the morphological transformation. A linear crystallinity (lc/L) is shown to be relatively constant (∼53%) for annealing temperatures between 26c and 58°C, but drops to 49% at 64°C, 45% at 70°C, and returns to about 50 to 53% upon cooling back to 26°C.