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INDUSTRIAL AND ENGINEERING CHEMISTRY
certain end point, may be discounted on the strength of Fisher and McColm’s experience (16). The measurements of specific refraction, which were also quoted in support of the polycyclic spiro structure by Staudinger and Geiger, seem definitely too low for a monocyclic formulation. However, the specific refraction of rubber and cyclized rubber needs further investigation; pending this, not too much reliance should be placed on structural conclusions based on present values. The measured refractive index and particularly the density of rubber have been yielding continuously lower figures as purer specimens were prepared. Recent values, shown in Table V, give poor agreement with the refractivity calculated from the theoretical atomic values of Vogel ($7). The refractive index of latex cyclized rubber shown in the table was found by first redispersing the wet flocculate with the aid of 0.25% of sodium hydroxide (calculated on the polymer) and pebble-milling for 48 hours. A drop of the resulting dispersion, when deposited on the prism of an Abbe refractometer and spread out, left a uniform thin powdery deposit on drying. This was instantaneously fluxed to a transparent film by pouring over it carbon disulfide vapor from a bottle. After 3 days, it was assumed that the small a m u n t of carbon disulfide had evaporated from the film, and the reading was taken as in the normal procedure used with a liquid. A sharpedged shadow was obtained and the refractive index could be read to 0.0004 by several observers. The possibility of some solvent retention or oxidation of the film, however, cannot be excluded entirely by this method. The calculated specific refraction for cyclized rubber as shown is based on the statistical monocyclic product of 57% unsaturation. In addition to the somewhat doubtful values of refractive indices, Staudinger’s supposed polycyclo products agree rather well in sintering temperature with both Fisher’s and the writer’s values for monocyclic polymer. This temperature, though naturally not sharply measurable, is remarkably sensitive to the degree of cyclization of latex cyclized rubber, so t h a t Staudinger’s polymers are not likely to differ very greatly in cyclization degree from those of the other workers. Finally, the densities of Staudinger’s supposed mono- and polycyclic rubbers were exactly identical with each other and with the writer’s latex cyclized rubber and very nearly with Fisher’s average figure. It is therefore, proposed that cyclized rubber prepared by these vkrious authors is a well defined monocyclic product. Concerning the thermal cyclization of rubber, it may he speculated t h a t this is an ionic reaction promoted by relatively weak acids derived from protein and other contaminants. The reaction proceeds (91) in a few hours a t 300” to 320” C. It can be calculated on the assumption of a constant activation energy of 33 kg.-cal. by a wide extrapolation, that an acidity of H o = p H = +1 is approximately sufficient t o account for such rates,
h
393
Literature C i t e d Ball, R. M., and Jeppeson, M. A., J . Chem. Phgs., 3, 245 (1935). Bell, R. P., J . Chem. Soc., 1943, 629. Bennett, G.M., et al., Ibid., 1947, 474. Berg, S., KoZloid-Beihefta, 5, 1 (1941). Bloomfield, G. F., J . Chem. Soc., 1943, 289. (6) Boyer, R. F., Spencer, R. S.,and Wiley, R. M., J . Polymer Sci., 1, 249 (1946). (7) Braude. E. A,. and Stern. E. S.. J . Cheni. Soc.. 1948. 1983. (8) Bruson, H. A., Sebrell, J. B., and Cdvert, W. C.~,ISD. ENG. CHEM.,19, 1033 (1927). (9) Caliezi, A., and Schinz, H., Helv. Chim. Acta, 32, 2556 (1944). (10) deVries, O., India-Rubber J . , p. 9 (March 23, 1935). (11) D’Ianni. J. D., et al., IND.ENG.CHEM.,38, 1171 (1946). (12) Evans, A. G., and Meadows, C;. W . , Trans. Faraday SOC.,46, 327 (1950). (13) Evans, A. G., and Polanyi, M., J . Chem. Soc., 1947, 252. (14) Eyring, H., and Stearn, A. S.,Chem. Revs., 24, 25 (1939). (15) Fisher, H. L., and Fisher, H. L., and McColm, E. M . , IND. ENG. CHEM.,19, 1325, 1328 (1927). (16) Flory, P., J . Am. Chem. Soc., 61, 1518 (1939). (17) Frommandi, G., KoZZoidchem-BeiheSte, XXVII (6), 189 (1928). (18) Gordon, M. (to Dunlop Rubber), Brit. Patent 634,879 (March 29, 1950). (19) Gordon, M., London University, thesis (1949). (20) Gordon, M., Proc. Roy. SOC.( L o n d o n ) , in press. (21) Gordon, M., and Everett, R. M. (to Dunlop Rubber), Brit. Patent 647,732 (1950). (22) Gripenberg, J., Hughes, E. D., and Ingold, C. K., .Vatwe, 161, 480 (1948). (23) Hammett, L. P., “Physical Organic Chemistfy,” pp. 223, 311 New York, McGraw Hill Book Co., 1940. (24) Hinshelwood, C. N., J . Chem. Soc., 1947, 694. (25) Imperial Chemical Industries, Brit. Patent 520,985 (May 8, 1940). (26) Krause, A. H., Scott, H. E., and Susie, A. G., Rubber A g e (nr. Y . ) , 60, 189 (1948). (27) Mackenzie, H. E. M., and Winter, E. R, S., Trans. Farudag Soc., 44, 159 (1948). (28) Newton, E. B. (to B. F. Goodrich Co.), U. 8. Patent 1,906,402 (May 2, 1933). (29) Rhodes, E., India;?bbeY J . , 7, 23 (March 1935). (30) Staudinger, H.. Die Hochmolekularen Organischen Vet bindungen,” p. 382, Berlin, Springer, 1932. (31) Staudinger, H., and Geiger, E., Helu. Chim. Acta, 9, 549 (1926). (32) Staudinger, H., and Staudinger, H., Rirbber Chem. and Tecl~noZ., 17, 15 (1944). (33) Staudinger, H., and Widnier, W., Helu. Chim. .4cta., 9, 529 (1926). (34) Stevens, H. P., Rubber Chem. a n d Technol.. 17, 51 (1944). (35) van Veersen (to Rubber Stichting), Brit. Patent Application 15,088/48 (1948). (36) Ihzd., 28,350/47 (1947). (37) Vogel, A. I., J . Chem. Soc., 1948, 1809. RECEIYED September 30, 1950
Optical Examinations Anisotropy in Vulcanizates Stefan0 Oberto
Research Laboratorw, PErelli Co., Milan, Itad@
A
VULCANIZED rubber is frequently anisotropic. The dif-
ferent values of modulus, tensile and tear strength, swelling in solvents, shrinkage after vulcanization, and electrical properties, according to the direction along which the tests are carried out, are well known. Anisotropy can also be examined in other ways, and this report illustrates how interesting optical examinations of anisotropy in vulcanized rubbers can be made, completing and defining the results obtained by other means. D~~~~~and Porritt (3) dealing with grain distinguish the effect caused in rubber alone from that obtained when elongated particles are present.
The grain is formed during those processes of manufacture which deform the stock in a definite direction, such as mixing on rolls, calendering, and extruding. Hot rubber deformed in this way and then cooled unequally, on one face only, for or cooled after anchoring to a fabric liner, takes up a condition of internal strain. This grained unvulcanized stock may result in the vulcanized rubber retaining some of the grain. Further, rubber stock containing compound ingredients of a n acicular type such as clay, light magnesium carbonate, and tripoli, have the particles of the fillers aligned with their longest dimensions parallel to the direction of travel of the rubber, resulting in marked develo ment of grain. , . In general, very little of the grain due to the rugber phase survives h o t vulcanization, but that arising from fillers will aln’Ay s remain. ,
.
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Vol. 43, No. 2
T h e anisotropic behavior of vulcanizates is revealed by differences in modulus, breaking load, and tear, swelling in solvents, and electrical properties, according to the direction of test. Anisotropy is attributed both to a state of tension in the rubber phase only and to a preferential orientation of the elongated particles of certain fillers. I t has recently been recognized that anisotropy due to movement of the stock in the mold during the first phase of vulcanization is of very great importance. By submitting overflow slabs, sheets, and test pieces to examination under transmitted polarized light, it is relatively easy to obtain optical evidence of the presence of anisotropy caused by movements during molding operations. The localization of the movements may make it desirable to examine with a microscope a t low magnification microtome sections of the vulcanizates 50 microns thick. A transparent gum compound can be used, but more details can be seen if 1% of carbon black is added. The transparent compound may be used under polarized
light for the direct examination of samples a few millimeters thick or of sections of equal thickness taken from products of different dimensions. In injection-molded products, it is possible to tell from the variations of double refraction when molding conditions favor scorching before the rubber has come to rest. If talc, or some other ingredient with elongated strong double-refractive particles, is introduced into the transparent compound, it is possible to observe the orientation of the particles caused by the processing or molding flow. In a calendered sheet, the talc particles lie almost parallel to each other, but toward the center of the sheet the orientation may be less marked or may occur along slightly different directions. In injection molding the talc clearly reveals flow direction and anisotropy. Under optical obsenation, in restricted areas, the difficulty of obtaining entirely isotropic rubber products is confirmed. Almost pure gum compounds have been examined, but the anisotropy found will probably assume even greater importance in loaded mixings, especially thoseloaded withcarbon black.
Gurney and Gough (4) have shown that movements in a raw compound take place in the mold during the first stages of vulcanization, and that the consequences of these movements, so far a s anisotropy is concerned, are in fact more important than those which derive from calendering. The beginning of vulcanization fixes the rubber which haa only just stopped moving during the molding, or which is still moving owing t o the retarded thermal expansion of internal parts. In such cases, anisotropy may occur in the rubber phase alone, but when acicular particles of compounding ingredients are present t,heir orientation is an obvious consequence. Scott (7) points out that following flom*, suitably exaggerated, during the molding, “the dielectric constant and dissipat’ion factors are, for some mixtures, different in the direction of flow and in the two mutally perpendicular directions. R e s u h are consistent with the hypothesis that the filler particles are oriented in t,he rubber matrix.” Here \!*e see again underlined the effects of flow in molding, and the influence of the pigments. The photoelastic properties of rubber-that is, double refraction as a function of strain-have been very carefully studied; it would appear, however, that double refraction in the absence of external stress due to previous deformations in the unvulcenized or semivulcanized state has been examined to a lesser degree. Ames (1) briefly examined the double refraction of raw calendered rubber and its disappearance during recovery by thermal treatment. Hurry and Chalmers ( 5 ) , dealing with photoelastic applications in the technology of rubber, mentioned the examination of movements of the niataial in the process of injection molding, but did not develop the theme. Kruse (6) examined through crossed Nicols transparent>rubbers in which, when scorching had taken place, a plastic flow and successive relaxation were provoked. His examination was, hoxever, conducted with thwmal treatments rather far removed from those employed in the process of vulcanization, and concerned more with the mechanism of vulcanization than with the appearance of anisotropy in technical operat>ions. In the field of plastic materials cases have recently been cited of examinations made under polarized light for the purpose of revealing or measuring double refraction as a result of molding ( 2 )or in calendering (8). The examinations described in this report illustrate typical and current cases of anisotropy existing in vulcanized rubbers produced in the laboratory, and which can be examined optically--
anisotropy in the rubber phase alone due to flow a t the beginning of vulcanization, and that due to orientation of the pigment particles following calendering or injection molding.
Preparation of Samples Vulcanized rubbers prepared according to the following formulas have been used for general optical examinations of anisotropy, the observation of specimens 30 to 50 microns thick under the microscope, and the free observation of specimens a few millimeters thick. Compounds Smoked sheet Sulfur Captax Diphenylguanidine Zinc oxide Stearic acid MPC carbon black Talc
A
I3
100 2 0.5 0.6 1 1.5
100 2 0.j 0.5
...
...
...
1
1.5 1
c 100 2 0.6 0.6 1 1, 5
...
1-15
ri 100 2.7 0.7
‘i’ I .6
...
...
1: 100 2.7 0.7
...
1 1.5 1
...
Compound A is used preferably for macroscopic examinations, and should show the anisotropic behavlor of the rubber phase alone ; Compound B is used in microscopic examinations, as it is too opaque for thicker specimens; more detailed data can, however, be obtained v i t h it. With C, containing some talc, it is possible to observe the orientation of the particles of the fillers, and to determine the anisotropy due to it, independently of that which occurs in the rubber phase alone. With magnesium carbonate and clay in fair percentages it is not possible to make useful examinations, even if thin sections are used. Colloidal silica can, however, be adopted fool loading, as it only slightly influences the transparency. Compounds D and E differ from A and B only in the initial speed of vulcanization and risk of scorching. Whereas it is possible to make a direct examination between crossed polaroids of sheets or articles of Compound A up to 4 mm. thick, sections about 1 to 2 mm. thick can be obtained from vulcanized products of any dimensions made with this compound, instead of with the production compound. The sections are cut with a rotating blade liberally wetted with soap and water. The sections for microscopic examinations are obtained by immersing the vulcanized specimen in paraffin, cooling it with dry ice, and cutting it with a microtome. They are mounted in castor oil, which does not cause the rubber to swell and allows easy relaxation from occasional stresses due to mounting. The
February 1951
Figure 2.
Section of Overflow Produced in Transfer Molding
Compound A, 5 O p thick.
A. B. C.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Crossed Nicols.
5X
A, Showing Brilliant Traces bf OpeningsVulsanized for 60 minutes at 120° C. Crossed polaroids. Scale lto3 Vuloanizedfor 10 minutes at 151'C. Section 50p thick of slab cut aeroes Line of overflow. Crossed
Figure 3.
Nicols. 1 O X
examination is made under a not very powerful microscope, with crossed Nicols. To obtain the outlines of the photographed objecte the Nicols may be only partly crossed.
Optical Examinations The fist microscopic examinations of anisotropy in vulcanized rubbers were made on thin sections cut from rubber disks where mold overflow (spew) was also' present; the double-refractive characteristics of this latter, noticed on various occasions, made advisable a more frequent and systematic examination of the overflow and of the areas near it. A mold was built in which a slab of rubber measuring 20 x 20 x 0.4 cm. can be vulcanized; the upper wall of the mold has a t e e n openings in groups of three, in five rows, measuring 2 x 10.05 cm.; from these the rubber can flow into cavities a t the back.
A.
B.
Injection-Molded Article of Compound A
Scorched for 3 minutes at 110' C. Vulcanized for 40 minutes at 127' C. Crossed polaroids. Scale lto2 Scorching time 7 minutes at 110' C.
If a slight excess of rubber is loaded into the mold, there is a movement of the rubber when the mold is closing and when the temperature rises, which does not altogether cease, or has only just ceased, when vulcanization commences; this movement is a t its maximum near and inside the opening. Let us consider in particular what happens in the case of a transparent compound, made according to Formula A. Figure 1, A , shows a part of the slab of rubber between crossed polaroids; after 60-minute vulcanization a t the relatively low temperature of 120" C., the brilliant traces of the openings through which rubber has flowed can be seen, but the double refraction is not the same at
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
396
Figure 4. Sections from Injection-Molded Articles with Progressively Increasing Scorching Time 2-mm. sections.
Crossed polaroids.
2X
Figure 7.
Section of Slab Cut across Line of Overflow
Likc Figure 1. C , ~
U with L
tion 3 O p thick.
Figure 5.
Section of Small Injection-Molded Specimen
Compound C with 1STo talc. Crossed Nicols.
12X
Figure 6. Section of Calendered Sheet Photographed Twice with Different Orientation of Nicols Compound C with 1% talc.
Crossed Yicols.
1OX
all the openings, and is o n the whole strictly localized in t h r regions of greatest movement. If, on the other hand, vulcanization takes place a t the much higher temperature of 151O C. for 10 minutes, the specimen between the crossed polaroids shows $1 more intense and widespread double refraction. I n some cases the most intense double refraction is not localized under the opening but is slightly displaced to one side of it. Figure 1, B , shows, with respect to the preceding case, the effect of more rapid scorching in fixing the strajiied rubber.
Figure 8.
Compound H, 1Yc carbon blark. Partly crossed Nicols. 1OX
Ser-
Section of Overflow beyond Opening
Compound B, 3Ofi thick.
Figure 9.
Vol. 43, No. 2
Part13
c*ro.isrd Yicols.
1OX
Sectinii of Small Injection-IkIolded Specimen
Compound € w.ith I 1% carbon black.
crossed Nicols.
Section 309 thick. I'artly 12x
If a section is cut from this s.iiii(~slab LLC'IOSSa line of overflow, and this section is examined under a not very powerful microscopr, a relatively thin, strongly double-refractive area will he seen in the overflow (Figure 1, C), which continues for half the thickness of the specimen. The region of high molccular orientation, to which the double-refraction corresponds, has its limits sharply defined under these conditions. The rubber n-hich remains in the injection channels, in the case of injection (transfer) molding, is in a condition similar to that
s
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1951
Figure 10.
Section of Overflow of Disk for Schopper Rings
Compound E.
Partly crossed Niculs.
45X
Figure 11. Figure 12.
Section of Schopper King Disk
Compound E. Partly crossed Nicols. A . l’criphericregion of disk U . Section of ring for stress-strain test cut from disk
Section of Calendered Sheet
Compound B with 1 7 4 oarlrori 1~lac.l.. P a r t l y crushed Nicols.
d
397
15X
just examined. Figure 2 gives the appearance of a section of rubber vulcanized in the channel, with Nicols still crossed. A very strong double refraction is noted a t the center. By drawing the rubber cylinder by hand along its axis it is often possible t o obtain internal breakdown without its spreading to the surrounding area-that is, the elongation to break of the fibers internally oriented is very easily exceeded, and this, like that which occurs in the case of strongly calendered slabs vulcanized a t a low temperature, is decidedly reduced in the direction of orientation (9). The examination can be easily extended to a completed article produced by injection (transfer) molding, if this has a constant thickness of a few millimeters; it is then possible to follow the directions of flow and their regularities (Figure 3, A ) , whether the svmmetry of the mold is respected or not. Above all, i t can be seen whether scorching has prematurely taken place in the compound, which considerably alters the appearance of the product under polarized light (Figure 3, B ) , even when the moldingappears t o the naked eye to be perfectly regular. An almost complete absence of double refraction can be obtained with the same compound molded normally in the same mold, or a more plastic compound molded at a lower temperature, or again by using a compound much less liable to scorch (Formula D instead of A). If the dimensions of the object are irregular or too big, large sections having a constant thickness of 1 or 2 mm. can be taken from the article molded from a transparent mix; these sections can be examined between polaroids. As the scorching of the compound to be molded increases, the double refractions in the section of the mold product become more uneven, as can be seen in Figure 4,illustrating three different degrees of scorching, a t the last of which injection molding becomes impossible. The orientation of the acicular or flattened type particles of a
compounding ingredient is very easily ascertained by using Formula C; Figure 5 shows the section of a small injection molded specimen containing 15y0of talc, with crossed Nicols. I n normal light the section seems to be perfectly uniform, whereas under polarized light the distribution into areas of different orientation is evident. The behavior of the individual particles can be examined under normal light a t a higher magnification, but obviously this is more complicated than examining the whole section a t once under polarized light. The examination becomes less obvious with a higher concentration of the ingredients or when the particles are h e r , but there is reason t o suppose t h a t the marked orientation of the particles now observed is maintained. A n interesting examination of the orientation of the talc particles can be made by observing the section of a calendered sheet made of Compound C, with 1% of talc. The particles have a tendency to be oriented in the direction of calendering, but this tendency is not always complete or uniform throughout the t,hickness of the sheet. Figure 6 shows, side by side, the same section of this sheet photographed between crossed Nicols, with the principal directions differently oriented in each of the two cases. The majority of the brilliant particles which appear in one photograph are dark in the other, indicating a good collective orientation in the direction of calendering; but the particles in a central band are partially obscured in the first photograph, and decidedly brilliant in the second, from which it can be concluded that they are differently oriented. This lack of uniformity in orientation of elongated particles in the calendered sheet has repeatedly been observed; the general orientation of the filler particles caused by calendering has in the meantime been recorded. A much more detailed structure of the anisotropy produced by movement during vulcanization or processing appears when
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INDUSTRIAL AND ENGINEEHING CHEMISTRY
Formulas B and E are used with a slight addition of active black, but not enough to make sections 30 to 50 microns realized with the microtome definitely opaque. By making a slab similar to that of Figure 1, A and B, with such a compound, and cutting a section across a line of overflow (Figure 7), a much more complicated structure results instead of the thin luminous brush seen in Figure 1, C; the structure now completely covers the entire area of the microtome section. Still more revealing is a section taken from the overflow beyond the opening (Figure 8 ) . It is not necessary to confirm that in normal light the sections are completely uniform, apart from some parallel marks of the microtome blade. The small injection-molded specimen which, when made with Compound C (Figure 5 ) , indicated the oriented particles of talc shows a finer and more detailed structure when made with Compound B (Figure 9), giving a better idea of the complex movements of the injected mass. When tension stress-strain tests are carried out on 44.5/52.5mm. Schopper rings, they are cut from disks 57 mm. in diameter. The overflow of such a vulcanized disk, made with Compound B or E, shows in section a strong double refraction, which in fact originated these examinations (Figure 10). The flow of rubber toward the overflow is, however, easily seen in the peripheral regions of the disk (Figure 11, A ) . It also spreads inside the ring, which is punched with double circular knives and has not a very square outline; a section is shown in Figure 11, B. The tension stress-strain test is therefore made on specimens which are clearly anisotropic, in the case of the compounds examined ; but the optical criterion is perhaps excessively sensitive. All blacks give rise to this phenomenon, although to a lesser degree when the diameter of the particles is greater-for example, lampblack is less suitable than active or semiactive blacks. Red iron oxide, however, also behaves in the same way, with some dichroitic effect. When dealing with other white ingredients the actual double refraction of t,hc part.icles, or the repeated refractions of light, disturb the phenomenon. The characteristic anisotropy visible in misings containing a slight percentage of black is not confined to natural rubber. mixes. The behavior of synthetic elastomers, such as chloroprene, styrene, nitrile, and polgisobut~ylenel,ubbers, is similar.
Vol. 43, No. 2
I t is difficult to explain why such a small percentage of a h e pigment reveals a double refraction even when there are very slight movements. It can be supposed that the relaxation of the deformed rubber molecules is slower when very h e particles are dispersed between them, or that there is an orientation of the chains on which the structure of a carbon black depends. Even. if the second hypothesis is true, the practical and theoretical interest of the examination is not lessened, and it is only to be regretted that an equally simple examination of compounds having a high percentage of carbon black, where the effect should be even greater, is not possible. The introduction of talc has made it possible to note a lack of uniformity in the orientation of the particles of calendered sheets, but if 1% of carbon black is added, a much fmer and more complex structure, predominantly stratified, as in Figure 12, can be observed in calendered vulcanized sheets. The flows of rubber along the surface of the two cylinders (perhaps a t different temperatures), with a partial return of material when they join together to form the sheet, cause a complex distribution of shear rates leaving clear traces when observed under polarized light, We do not yet knoF, then, even in t,he simple case of calendered sheets, when it will be possible to find, not an isotropic, but a uniformly anisotropic material.
A ckn~w~ledgment Grateful acknowledgment is made to Luigi Emanueli, managiiig director of the Pirelli Co., for permission to publish thie paper.
Literature Cited (1) Ames, India Rubber J . , 67, 344 (1924). (2) Barwell, F. T., Index Aero, 6, No. 1. 89 (1950). ( 3 ) Dawson, T. R., and Porritt, B. D., “Rubber, Physical and
cal Prooerties.”
D.
Chemi-
328. Crovdon. Enelsnd. . Reaearch Assoo. I
”
Brit. Ribber Manufacturers, 1935.
(4) Gurney and Gough, Trans. I n s t . Rubber Ind., 22, 132 (1946). (5) Hurry and Chalmers, India Rubber W o ? l d ,119, No. 6, 717 (1949). ( 6 ) Kruse, J., Kollotd Z.,11, 100 (November 1948). (7) Scott, A. H., J . Reseurch Nntl. B u r . Standurds, 43, 355 (1949). (8) Smith, E. F., and IYangsgard, A . E’., J . Polymer Sci., 5, 168
(1950). (9) Van Rossem, India Rubber J . , 63, 343 (1921). RECEI>.I:DOctober 3, 1950
ic AND ~ ~ ~ ~ s - P B L Y I S Q P R E N FRO ES PLANT SPECIES
A SINGLE
Walter Schlesingsr and H. M. Eeeper Wm. Wrigleg dr. Co., Chicago, Ill. T h i s work was begun as part of a long-range fundamental investigation of the chemical and physical structure of chicle and other plant materials useful in the manufacture of chewing gum. The authors succeeded in demonstrating the existence of two types of hydrocarbon polymers in chicle and in identifying these with gutta (trans-) and caoutchouc (cis-) polyisoprenes from gutta-percha and Hevea rubber, respectively. Identification was accomplished by comparison of x-ray diffraction patterns and infrared absorption spectra of commercial chicle, and of a specially prepared sample of chicle obtained by tapping a single tree, with polymers isolated from gutta-percha Pahang and Hevea pale crepe.
This is the first proved example of the production of both cis- and trans-polyisoprene by a single plant. I t ie hoped that further study of polyisoprene isomerism in chicle and other substances will shed light on the important problem of the biogenesis of rubber and guttapercha. niolecular weight polyisoprenes, as found in nature,
HIoccur ”” in two distinct forms, which are believed to differ from each other in the spatial configurations of the carbon atoms around the double bonds (1, 4,6). These two polymeric unsaturated hydrocarbons are exemplified by caoutchouc (from natural rubber), considered to be the cis- form, and by gutta (from gutta-percha or balata), hfJlicved to be the trans- form
