Morphology of strain-induced crystallization of natural rubber. I. Electron microscopy on uncrosslinked thin film

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Abstract
Pt shadowing, Au decoration, OsO4 staining, electron diffraction, and bright- and dark-field electron microscopy have been used to elucidate strained and unstrained thin-film morphology of natural rubber. Unstrained natural rubber exhibits a 100–150-Å nodular morphology in the melt at room temperature. When cooled below room temperature, the original nodular morphology is replaced by an unoriented lamellar morphology. The crystal thickness of the lamellae is about 55 Å at −28°C. When stretched to elongations greater than about 200% the strain-crystallized films show a distinct fibrillar morphology at room temperature. The fibrils appear to be composed of individual crystallites, 120 Å in diameter. When cooled to −25°C, including those films which have been highly stretched (900%), the original fibrillar morphology is replaced by an oriented lamellar morphology. The transformation to lamellar morphology involves a lateral alignment of the room-temperature crystallites and a decrease in crystal thickness. Upon heating above −25°C, the lamellar crystals thicken and lamellar periodicity increases, but eventually the structure reverts to the fibrillar form as room temperature is approached, indicating the reversible nature of the morphological transformation.

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