2364
ELIZABETH WEIR TOOR AND P. W. SELWOOD
Vol. 74
Preparation of essentially pure CrOOH was achieved in several of the experiments reported, for example 25, 26 and 28. The thermal analysis curve (Fig. IC) is for material of almost theoretical composition. The average density of CrOOH as determined by the displacement method using a 1-cc. pycnometer on samples containing 61.0-61.2% Cr !vas 4.12 g./cc.
tern of CrOOH is somewhat similar to the arrangement of lines in the patterns of lepidocrocite or boehmite, members of the y-series. An elucidation of the structural relationships among these oxyhydroxides should prove to be a most interesting and important contribution to inorganic chemistry. Although the data presented do not define stabDiscussion ility relations in the system chromium(II1) oxideX-Ray data for CrOOH indicate that i t is not water, discussion of several experimental facts CrOOH was isomorphous with any of the isoformular aluminum, suggests probable relationships. manganese or iron compounds. Iron and alumi- formed in the hydrothermal bomb a t teniperanum oxyhydroxides are isodimorphous, and grou- tures below 419-424°, and is probably the stable tite (a-MnOOH) also is isomorphous with diaspore phase below this temperature since i t decomposed arid goethiteI2 members of the alpha series. How- endothermically a t 420430' in the thermal anever, a second manganese oxyhydroxide, mangan- alysis apparatus to give rhombohedral Cx-203. ite, does not have the same unit cell as boehmite The cubic oxide may perhaps be regarded as inonoand lepidocrocite which are members of the gamma tropic, as is the cubic gamma iron oxide, since series13; also a third iron oxyhydroxide, P-FeOOH, differential thermal analysis curves of mixtures of has been reported.14 ,1mineral having the com- CrOOH and cubic oxide did not show the second position CrOOH has been found'j but from avail- minimum which would be expected for a stable able datal6 it appears to give an X-ray diffraction cubic oxide-rhombohedral Crs03transition. Phase relationships in which CrOOH is the pattern different from synthetic CrOOH. The arrangement of lines on the X-ray diffraction pat- stable phase below a transition temperature of 4111-424', rhombohedral Cr203is stable above that (12) J. !\'. Gruner, A M . A I i n ~ r d 32, . ~ 6.54 (1947). temperature, and the cubic oxide is a metastable 113) hf. J. Buerger, 2. Krisi., 95, 163 (1936). phase, are consistent with the data presented i 1 4 ) IT. €3. Weiser a n d IV 0. hIilligan. THIS JOCRNAL, 57, 238 ( 193.5). except for the serious difficulty that the rhombo(1;) Abstract in A m . X i n c v o l , 34, 339 11049). hedral oxide could not be converted to CrOOH. (16) hf.Fleisher and J. M. Axelrod, U. S. Geological Survey, private I m . u x , YEWTORK
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RECEIVED I)ECEMDER 7 , 19,51
' C O l I K I B I l l O X FROM I H E CHEMICAL LABORATORY OF N O R T H \ \ ESI'ERN UNIVERSITY]
The Diamagnetic Anisotropy of Natural and Synthetic Rubbers] BY ELIZABETH V-EIK TO OR^
AND
P.LY.SELWOOD~
The change in anisotropy with elongation has been found for natural rubber and for wveral synthetic rubbers. IJnsatuated rubbers have a large principal susceptibility perpendicular to the direction of stretching, because of the presence of olefinic double bonds. The differences between natural rubber and polybutadiene are attributed to the presence of unsaturated side-groups caused by 1,2-addition in polybutadiene. It is probable that the magnetic anisotropy of these rubbers depends not upon the actual degree of crystallinity of the rubbers, but upon the ability of the long-chain molecules to align themselves parallel to the direction of stretching. Therefore the changes in anisotropy with stretching will be large when there is no cross-linking, and small when cross-linking occurs to any large extent. Saturated rubbers have an anisotropy opposite in sign to that of unsaturated rubbers This must be caused by the broadening of electronic orbits perpendicular to the direction of stretching. Apparently methyl side-groups cause such a broadening of electronic orbits in polyisobutylenes, a n effect much greater than the similar effect in polyethylene.
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Introduction The magnetic anisotropy of rubbers has previously been studied by Mine. Cotton-Feytis.j She first observed that vulcanized commercial rubber showed some anisotropy and studied the effects of compression, stretching, and hot and cold working upon crude rubber. By Krishnan's oscillatioii method5 she measured the anisotropy of stretched rubbers, using rubber bands stretched around a glass or Plexiglas disc. The anisotropy was found (1) This is the third paper froin this Laboratory on the diainagiictic anisotropy of high polymers. T h e second. by tVeir and Selrvoori, .AI)peared in THISJOVRKAL, 73, 3484 (10.51) (2) Award of t h e d u P o n t Company Postgraduate Fellowship in Chemistry is gratefully acknowledged. 13) Inquiries concerning this p a p e r should he addressed to P. \Y. Selwood. ( 4 ) E. Cotloii-Feytis, c . i'
