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(10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
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FIALKOV:Acta physicochim. U.R.S.S. 3, 711 (1935). FOOTE:.4m. Chem. J. 29, 203 (1903). FOOTE, BRADLEY, AND FLEISCHER: J. Phys. Chem. 37, 21 (1933). GRACE:J. Phys. Chem. 37,347 (1933). KRACEK:J. Phys. Chem. 36, 117 (1931). LASKUSO:Z. physik. Chem. Al70, 134 (1934). FAJANS AXD KARAGUNIS, from MEYERA N D DUSKEL:Z. physik. Chem., Bodenstein Festband, p. 556 (1931). RAE: J. Phys. Chem. 36, 1800 (1931). RAE: J. Chem. SOC.1831, 1578. SETTERBERG, from Gmelin's Handbuch der anorganischen Chemie, System S r . 25 (Caesium), Val. 2, p. 200. Verlag Chemie, G.m.b.H., Berlin (1938). WELLSAND PEXFIELD:Am. J. Sci. (31 43, 17 (1892). WELLSA N D WHEELER:Am. J. Sci. [31 44,43 (1892).
THE ASALOGY BETWEEK T H E MECHANISM OF DEFOR,MATIOS OF CELLULOSE AND THAT O F RUBBER P. H. HERMSNS Breda, Holland Received October 90, 1940
When we review the well-founded experimental results (2, 4, 6 to 17, 18, 24, 34,35, 36) concerning the extension of cellulose and of rubber, we must conclude that there is a striking analogy between the behavior of regenerated cellulose and that of rubber upon deformation. It seems to us important to emphasize this analogy particularly, quite apart from the fact that we have not yet succeeded in drawing up a theoretical picture of the fine structure and mechanism of molecular deformation which is capable of accounting for all the phenomena observed (1,2,4,34). The analogy in question can scarcely be accidental or merely superficial, and we are convinced that it is intimately connected with a similarity in the intrinsic structure of the two substances which has until now received insufficient attention. Like all similar analogies, this one of course has its limitations, which can best be understood when the following general features are kept in mind: (a) The lateral cohesive forces between the carbohydrate chains of the cellulose molecules are, a t a given temperature, very much stronger than those between the hydrocarbon chains of the rubber molecules. (b) The influence of temperature on the behavior of rubber can to a certain extent be compared to the influence of the degree of swelling in the case of cellulose.
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P. H. HERMANS
We shall now attempt to give a resume of the corresponding phenomena observed upon the stretching of isotropic cellulose filaments and isotropic rubber. The long-range extensibility of rubber-like substances has always been considered to be one of the most striking and characteristic of their properties. Since we know that in the case of isotropic cellulose filaments the characteristic stretch st must be considered as the only true measure of the deformation by stretching, the property of long-range extensibility must also be ascribed to cellulose, a t least as far as the freshly coagulated state is concerned. In special cases (filaments of swollen viscose) , values of st as high as 7 have been observed, which indicates an extensibility of 600 per cent (16). I n the case of filaments of normal viscose a degree of stretch as high as 300 per cent is no exception. Under certain circumstances,
I
10
20
I
KG.md
30
FIG. 1. Comparison of the stress-strain relation of vulcanized natural rubber, Duprene, and Thiokol with that of swollen isotropic cellulose. Strain in kg. mm.-2 corrected for the actual area.
therefore, the property of long-range extensibility may also be ascribed to cellulose. The explanation of this property presents problems similar to those in the case of rubber. The stressstrain diagrams of isotropic cellulose filaments, especially those of swollen filaments, exhibit a striking similarity in shape and character to those of rubber. This is illustrated by figure 1, in which several stressstrain curves of vulcanized natural rubber, Xeoprene, and Thiokol, reproduced from Houwink’s book (20), are compared with that of a swollen isotropic cellulose filament. (In the case of cellulose the stress given is that acting on the actual cross section of the filament considered dry, and the elongation is the characteristic one.) In the case of cellulose filaments spun from more dilute viscose, which exhibit a higher percentage of stretch, the similarity is even greater. It would serve no useful purpose to compare diagrams for oriented cellulose fibers with those for non-oriented rubber, as is often done in the literature, in order to demonstrate that the two substances behave quite
DEFORM.iTION OF CELLULOSE h N D OF RUBBER
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differently in this respect, There are, on the other hand, many indications that the stress-strain relations of all isotropic substances consisting of linear polymers of high average molecular weight are essentially alike in general appearance under suitable conditions of temperature and (or) swelling, If we now consider the changes which the physical properties of cellulose undergo upon stretching, it is found that in this respect also there exists considerable correspondence with the behavior of rubber. In the non-extended state rubber is isotropic and for the most part amorphous. During the process of extension, however, a sort of crystallization takes place which is manifested in the x-ray diagram. This phenomenon was first discovered and described by Katz (21). We have seen that an entirely comparable alteration in the x-ray diffraction picture also takes place upon the stretching of swollen cellulose filaments. When fresh isotropic cellulose filaments are dried, crystalline particles are also formed. The same phenomenon can be observed when raw rubber is cooled under suitable conditions. The majority of investigators have reported that the crystalline x-ray pattern which appears when rubber is stretched shows definite sharp spots from the very beginning. Upon further extension these spots merely increase in intensity, but they do not change in position or in sharpness. This means that the crystallites are completely oriented in the direction of stretch as soon as they are formed. Only their number increases when stretching is continued. Recently Clark has shown, however, that under somewhat modified conditions moderately extended rubber may show sickle-shaped interferences, just as cellulose does. Apparently there is no essential difference between the two substances in this respect (2). A detailed study of the optical behavior of lightly vulcanized rubber in polarized light has recently been carried out by Thiessen and Wittstadt (35,36), in which a t the same time the behavior of the same samples with respect to x-rays was investigated. These investigations confirm the fact that crystallization phenomena occur in rubber when it is sufficiently stretched. The formation of crystalline particles only begins, however, after a certain degree of extension has been reached, which degree is dependent on the temperature. In the case of Thiessen and Wittstadt’s samples this point lay a t about 300 per cent stretch a t a temperature of 20°C. Beyond this point the crystallization increases with continued extension. The formation of crystalline matter is accompanied by an additional great increase in the optical anisotropy. When the rubber was suddenly stretched more than 200 per cent and then kept a t a constant length, the value of the double refraction continued to rise slowly for a time and finally approached a limiting value. Thiessen and Wittstadt concluded from this that crystallization is a time reaction and continues to increase after extension has ceased.
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P. H. HERMANS
The optical behavior of cellulose exhibits very great similarity with the above, with this difference, that, as we have seen above, the crystallization in this case begins much earlier; in fact, it begins as soon as the process of extension itself. Swollen cellulose filaments also exhibit the phenomenon that the optical anisotropy continues to increase for Pome time when a rapidly stretched filament is kept a t a constant length. An unbiased consideration of all these facts leads to the conviction that the intrinsic deformation mechanisms of regenerated cellulose and of rubber must show very great similarity. It would be interesting to investigate, conversely, whether or not a relation between the experimental and the characteristic stretch can be found upon stretching vulcanized rubber in the swollen state, similar to that found in the case of cellulose. Tentative observations seem indeed to point in this direction. We hope to be able to return to this point a t a later opportunity. A detailed discussion of the various theories of the molecular structure of rubber which have been proposed would be out of place in this paper. According to a concept which is a t present widely accepted, isotropic amorphous rubber is composed of a tangled snarl of long flexible molecular chains with random orientation, During elongation the chains, or a t least the greater part of the chain sections, become oriented and extended in the direction of elongation and may arrange themselves into parallel alignment in a number of small regions. A corresponding picture of the structure of freshly coagulated isotropic cellulose gel must therefore be imagined. There remains, however, the question as to why the stretching process is entirely or partially reversible with rubber, while this is not the case with cellulose, or at least to a much smaller degree. In the first place it must be taken into consideration that the high reversible elasticity of rubber is subject to certain conditions and may, for example, disappear completely upon cooling to a sufficiently low temperature. For raw rubber this temperature still lies above O T . , but for vulcanized rubber it is much lower. The work of Wohlisch, Xeyer, von Susich, I-alk6, Mark, Guth, and Kuhn (5, 27, 28, 29, 37) has shoT5-n that the elasticity of rubber must be conceived of as a kinetic phenomenon. The straightened-out configuration of the molecular chains returns, upon release of tension, to the more probable crumpled or rolled-up configuration, owing to thermal vibrations (micro Brownian movement, according to Kuhn ( 2 7 ) ) . This tendency is so strong that it is able to neutralize the lateral cohesive forces by which the oriented chains are bound together in the crystalline regions formed during stretching. Upon lowering the temperature, hon ever, the cohesive forces increase relative to the contractile tendency to such a degree that the latter no longer dominates, or a t least only to a smaller extent. In the case of cellulose the kinetic contractile tendency is also undoubt-
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edly present (13, 28, 30). In this case, however, the lateral cohesive forces between the chains are so strong from the very beginning that all the points of contact between the chains newly formed during the process of elongation are irreversibly fixed. This will be true particularly for crystalline regions when once they are formed. The new molecular constellations which occur as a result of the deformation are thus successively stabilized. The cellulose crystallites, unlike those of rubber, cannot be “fused” by increasing the temperature, because their melting point is so high that it cannot be reached without destroying the material, The fact that the contractile tendency is also present in the case of cellulose is
FORCE gh00 Denier
FIG.2. Stress-strain diagram of isotropic cellulose filaments of different degrees oi swelling, q . Stretch, s, in per cent of elongation is plotted against strain, corrected for actual denier.
shown by the fact that stretched filaments still exhibit considerable partial contraction vhen they are allowed to smell. Owing to this swelling, a neakening of the cohesive forces in the amorphous regions takes place, whereby a t certain suitable spots points of contact can still be broken. The filament then becomes shorter and thicker. We have called this phenomenon swelling retraction (11); it is also familiar in the rayon industry. Since in cellulose the swelling never extends to the actual crystallites, the retraction is always only a partial one, which (unlike the case of rubber) never entirely brings the substance back into the isotropic state. The influence of a diminution of the degree of swelling on the stress-strain diagram of cellulose filaments is demonstrated in figure 2. Speaking in terms of rubber technology, this influence corresponds to a “stiffening”
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H. HERMANS
of the material, and is similar to the effect of cooling on vulcanized rubber, as demonstrated in figure 3 (compare reference 20). At a sufficiently low temperature, shortly before the long-range extensibility disappears, the stress-strain curve of rubber shows a marked yield point, which is comparable with that of curve I11 in figure 2, which refers to air-dried isotropic cellulose filaments. Since the content of crystalline matter increases not only upon the cooling of rubber but also upon the reversal of cellulose filaments, the observed change in the mechanical properties may presumably be ascribed to this fact. When rubber is frozen a t the temperature of liquid air it becomes brittle and hard as glass, and can then be pulverized by hammering. When the original sample is isotropic, Hock found that particles of irregular shapes are obtained as when glass is pulverized. When, however, the rubber is frozen while it is in a highly stretched state, the particles exhibit a marked fibrous appearance. Rubber in this condition has a typical fiber structure,
KG.mm.-'
FIG.3. The influence of cooling on vulcanized rubber (to be compared with figure 2)
and, as Polanyi and Schob (31) found, its tensile strength is then many times that in the isotropic state. Exactly the same phenomena may also be observed with cellulose filaments which have been rigorously dried for several days over phosphorus pentoxide. The cellulose is then also inelastic: and hard as glass, and it can be pulverized in a mortar. Figures 4a and 4b are photomicrographs of the cellulose particles obtained when this experiment is carried out with isotropic and previously stretched fibers. I n the absence of a certain amount of water, cellulose apparently possesses no plasticity. Similar observations have been reported for other linear polymers. Wohlisch and du Mesnil de Rochemont pointed out that the great extensibility of elastin and collagen is not a property of these materials as such, but is dependent on the presence of a sufficient amount of water of swelling, The dried material is horny and only slightly extensible. The form of the stress-strain curves in this case is also a similar function of the water content, as in the case of cellulose. The new synthetic fiber Nylon can be colddrawn only ih the presence of small amounts
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of water or other hydroxylic suhstances. I t lies outside the scope of this paper to examine more deeply into these phenomena, which belong to the general subject of plasticizing agents, and we mention them here solely as interesting examples. When the striking analogy between the behavior of regenerated cellulose and that of rubber has once become apparent, many suggestiuns ran be oh:ained from the large amount of impartant work published on the subject of rubber, which are of great interest in the investigation of ccllulose. We should likc to
