George L. Clark

X-Ray Diffraction Patterns of Hevea, Manihot and Other Rubbers

Abstract When rubber from Hevea brasiliensis is stretched quickly and exposed to a beam of x-rays, a crystal fiber diffraction pattern is obtained.” During previous work, done jointly by the National Bureau of Standards and the University of Illinois on sol and gel fractions of this rubber, no pattern was produced by the stretched sol fraction. With the stretched gel fraction, however, the pattern was sharp and intense. Rubber from Manihot glaziovii also may be separated into sol and gel fractions, and in recent work they, too, have been stretched and examined by x-rays. With the stretched Manihot sol, a few interferences were obtained, but many more resulted with the stretched gel. When crystallized by “freezing” at low temperatures for 24 hours, the interferences from the gel again outnumbered those from the sol. The x-ray measurements of Manihot rubber agree with those of Hevea rubber and indicate that the same structure exists in each. Rubbers obtained from latices of Funtumia elastica, Cryptostegia g...

X-Ray Diffraction Patterns of Crystalline Sol Rubber Prepared from Ethereal Solution

Abstract In a previous publication there was reproduced an x-ray diffraction pattern of crystals obtained from a solution, cooled to temperatures between −40° and −50° C, of an ether-gel fraction of rubber from Hevea brasiliensis. Measurements of three strong interferences indicated that the crystals were similar to those of stretched gel rubber and of frozen sol, gel and total rubber. Although many attempts were made at that time, unsatisfactory diffraction patterns were obtained with crystalline ether-sol rubber. The interferences were faint and not suitable for accurate measurement and analysis. Since the crystalline material is bulky and some specimens are more compact than others, it seemed possible that the average number of cell diameters was too small to permit sharp definition, and that more material was required for the examination by x-rays. However, it is not certain that lack of definition was caused by insufficient crystalline rubber. The possibility that parts of the long-chain molecules be...

X-ray Diffraction Patterns of Sol, Gel, and Total Rubber when Stretched, and when Crystallized by Freezing and from Solutions

Abstract Sol, gel, and total rubber were prepared by a method which avoided the effects of light and oxygen. The total rubber when stretched and exposed to an x-ray beam behaved like other specimens, and produced the characteristic crystal fiber pattern. Stretched sol rubber produced no evidence whatever of this pattern, even at 1000 per cent elongation. With stretched gel rubber, the pattern was formed above 100 per cent elongation and at 200 per cent was sharp and intense. The measurements of interplanar spacings agree excellently with the results reported recently by Lotmar and Meyer. A large, interplanar spacing of 54 A. U. was found in the unstretched gel. It was absent in the sol. Specimens of frozen sol, gel, and total rubber were examined with a cylindrical and with a flat-film camera to compare the spacings and intensities with those of stretched gel. Within experimental error, sol, gel, and total rubber produce similar patterns. The spacings are similar to those in stretched rubber, with the exc...

X-Ray Diffraction Studies of the Bureau of Standards Rubber Fractions

Abstract At the request of Dr. Washburn, a careful preliminary x-ray diffraction study has been completed on various samples of rubber fractions prepared at the Bureau of Standards by the methods already described. The results are so interesting, especially in structural differentiation between the ether-soluble (sol-rubber) and the ether-insoluble (gel-rubber) hydrocarbons, and in producing new data bearing upon the several theories of the structure of whole rubber, that it has seemed desirable to record very briefly some of the more important observations. It will be recalled that native rubber, raw, purified, or vulcanized, gives a typical liquid halo or “amorphous” type of diffraction pattern in the unstretched condition, which changes to the characteristic crystal fiber pattern when the rubber specimen is stretched. Upon the basis of these facts, several theories of the structure of the rubber hydrocarbon have been proposed: the folded or spiral molecules model; the fringe model (bundles of molecular...

The Long Spacings of Rubber and Cellulose

Abstract 1. A new method for the examination of highly polymerized compounds is proposed. This consists in the direct measurement from diffraction interferences of colloidal micelle dimensions. 2. The construction of an instrument consisting of an x-ray tube with magnesium target and vacuum camera without intervening window or air space is described. 3. The major spacing of unstretched vulcanized rubber is found to be 99.3 A˚. U. 4. The number of isoprene molecules per unit cell is estimated to be of the order of eight thousand. 5. Three measurements of the long spacings of cellulose are recorded.