Optical and Dimensional Changes Which Accompany the Freezing and Melting of Hevea Rubber

Abstract

Abstract At suitable, low temperatures, unvulcanized rubber loses its elasticity and becomes hard and opaque. Similar changes frequently occur in baled rubber which has been tightly compressed before shipment. It is said to be frozen or “boardy.” The phenomenon has been studied by many investigators who have determined changes of volume, softening temperatures, the effects of increasing time of storage at low temperatures, the influence of pressure during freezing, and changes in heat capacity and entropy. These effects have generally been ascribed to a form of crystallization, and x-ray diffraction powder patterns indicate that crystals are present in frozen rubber. When total rubber is stretched, there are changes of volume and of heat content such as attend crystallization. With x-rays a crystal fiber pattern is obtained. It and the powder pattern obtained with frozen, compact rubber have been shown to indicate similar spacings and are assumed to be caused by the same type of crystal, the differences being ascribed to conditions of orientation. Dilute solutions of rubber hydrocarbon in ethyl ether yield small crystals of the hydrocarbon when they are subjected to temperatures between −35° and − 60° C. for several hours. The optical properties and melting points of these crystals and their x-ray diffraction patterns indicate their identity with the crystals in stretched and frozen rubber. Under the best conditions the crystals appear in spherulitic groupings, the individual needles in each spherulite having optical properties that closely approach those of a uniaxial crystal with negative elongation. The crystals of sol rubber which we obtained, melted between 9.5° and 11.0° C. Crystals of gel rubber melted between −2° and 14° C., but the melting ranges within this interval were not the same for all samples. Numerous observations have repeatedly confirmed the data. About 90 per cent of the rubber in solution may be obtained as birefringent material at −65° C. Temperatures between −40° and −50° C. have been preferred, however, because better crystals are obtained in that range.