No abstract

ABSTRACT

Total, sol, and gel rubber were prepared under conditions which were selected to avoid oxidation and structural changes. Total rubber when stretched and exposed to an X-ray beam produced the characteristic crystal fiber pattern. Stretched sol rubber produced no evidence whatever of this pattern even at 1,000 percent elongation. With stretched gel rubber, the pattern was formed above 100 percent elongation, and at 200 percent was sharp and intense. A large interplanar spacing of 54 A, found in the unstretched gel, was absent in the sol. The patterns obtained with stretched gel, with frozen sol, gel, and total rubber, and with gel crystals produced from an ethereal solution were measured and analyzed.

The opacity of "frozen" rubber results from crystallization of the rubber hydrocarbons. At temperatures between-15 0 and-35 0 C rubber is known to freeze in a few hours. The rubber of Hevea brasiliensis and its sol and gel fractions were frozen at about-25 0 C and maintained at that temperature for periods of 2 weeks to 3 months. Microtome sections were prepared and examined at controlled temperatures under a polarizing microscope. When unstrained rubber was studied in this way, each section appeared to be composed entirely of irregular, strongly birefringent, crystalline particles, exhibiting no systematic orientation with respect to each other. The crystalline rubber Crystallinity of frozen and stretched rubber _______________ ______ ___ 267 References __________________________ ____ _______ ________________ 268

When unvulcanized rubber is stretched quickly, an orientation of structure occurs parallel to the direction of extension. This orientation is usually regarded as a form of crystallization, because of the optical and other phenomena which accompany it. It is also accompanied by a change of volume. When unstretched rubber is subjected to suitable temperatures below about 10° C, it too becomes crystalline. The crystallization of rubber is accompanied by an increase of density, and the simultaneous production of an x-ray diffraction pattern is usually interpreted as evidence that crystallization has occurred. A diffraction pattern of this kind results from regular repetition, within the crystal structure, of groups of atoms or molecules in a definite spatial arrangement. The smallest group from which the structure can be built up in this way is called the unit cell; and the dimensions can be computed from measurements of the pattern, provided it is possible to determine the crystal form. The pattern of stretched rubber consists of interference spots, and that of frozen rubber consists of rings, but both of their measurements have been shown to indicate the same unit cell. If, in addition, the number of atoms in the unit cell is known, the density of the material can be computed. If the computed density agrees with that found by direct measurement, the fact is a valuable indication of the correctness of the assumed structure on which the calculation of density was based. Accurate knowledge of the density of completely crystallized rubber would therefore serve both as a guide in studies of structure and as a measure of the degree of crystallization of any material under consideration. In the experimental work described in this paper, the rubber was crystallized as completely as possible. Its density was determined by recognized methods, and compared with the densities calculated from x-ray measurements.