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of condensable vapor to non-condensable gas. Furthermore, the experimental results of Figures I1 and I11 are limited to such cases. At high humidities, while the same principles apply, computation methods become more involved. Further work is being done in this laboratory to include
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cases where the initial humidity is much higher and where liquids other than water are employed, special attention being devoted to the concomitant problem of heat transmission and to securing more precise data than were possible in these preliminary experiments.
X-Ray Methods Used in Determining Structure of Cellulose Fibers’ Organomolecular Investigations 0. L. Sponsler UNIVERSITY OF CALIFORNIA AT Los ANGELES, CALIFORNIA
A
S a result of x-ray diffraction studies of plant fibers, three hydroxyl groups produces the destruction of the fibrous carried on during the past few years, a conception of nature of the material; and the substitution of larger groups, cellulose has been obtained in which glucose residues such as acetyl, tends to weaken the fiber, as such, even when are pictured as occurring in long chains in the fiber wall. This three groups or less are involved. chain structure, proposed by Sponsler2and Sponsler and Dore13 A considerable amount of work is still to be done, along the has been verified lately by hleyer and Mark4 by somewhat lines suggested above, in the correlation of x-ray data with t h o s e from chemical and different x-ray methods, but physical reactions. It is for a complete agreement in all that reason that the followdetails has not yet been The very fine capillary tube-like character of the fibers ing discussion is presented reached. T h e present introduces several complicating features into the of several methods used in status of these investigainvestigations of their molecular structure by x-ray x-ray work w i t h fibers, tions is briefly summarized methods. A n approximately parallel arrangement which are somewhat differhere. They show (1) that a of the fibers into a block which can be turned as desired ent from those used with space lattice with structural with respect to the x-ray beam gives control of the crystals or crystal powders. units of 84-glucose residues atomic planes in the fibers to a limited extent. When In general, x-ray work exists in the wall; (2) that the beam passes lengthwise through the fibers, the block with fibers i n v o l v e s t h e these residues are attached resembles a mass of crystal powder; when at right same fundamental methods by glucosidal linkages into angles to the fibers the block resembles in its reflections and principles as with ordilong chains; and (3) that the a single large orthorhombic crystal: but when at any nary crystalline materials,6 chains are parallel to one other position, on account of the cylindrical construcbut there are a few unusual another and extend lengthtion of the individual fiber, it resembles a block concomplications when working wise of the fiber. Those taining a few large crystals so oriented that their with fibers t h a t a r e d u e features of the fiber wall are b axes are parallel but otherwise in random arrangeprimarily to the characterisfairly well established, but ment. tics of the individual fiber, other points need verificathat is. to its shaDe and tion. The distance between flexibility, and to t i e way the chains and the orientation of the glucose residues in the chain, as well as the orienta- in which it is formed in the plant. I n addition, the irregularity in the shape of the constituent structural units tends tion of the chains themselves, are still in question. In the fiber wall the glucose residues, acting as links to introduce further complications into the interpretation of of the chain, are held together through oxygen bridges by the x-ray diffraction patterns. The discussion may become somewhat clearer if we recall primary valence forces; while the chains are held to one another laterally by either secondary valence forces or by some kind very briefly the principles involved in x-ray diffraction. of residual force fields which produce cohesion. The primary When a beam of x-rays is directed on to a natural face of a valence forces, being much stronger than the other, give the crystal a t a suitable very small glancing angle, the effect is fiber different properties longitudinally from those laterally. somewhat as though it were split into two beams-a strong Qualitatively, at least, that construction is in agreement one which continues through the crystal in a straight line, with such physical properties of the fiber as tensile strength, and a weaker one which is diffracted a t an angle equal to the glancing angle. This effect is produced only when a swelling phenomena, and thermal expansion-properties which have different values for the longitudinal and lateral definite relation is established between the wave length, directions. This chain structure is also consistent with cer- the glancing angle, and the distance between the atomic tain chemical properties, such as those involving the formation planes, of which there must be a considerable number lying of substitution products of cellulose, at least to the extent parallel to the crystal face and separated from one another to which ester formation may occur before the fiber structure by equal distances. The diffracted beam does not come from is destroyed, for example, the esterification of more than a single atomic layer, but is an additive composite of many weak beams, each from one of the parallel layers. The 1 Received March 27,1928. Presented before the Division of Cellulose actual production of this resultant beam depends upon the Chemistry at the 75th Meeting of the American Chemical Society, St. Louis, conditions just mentioned; the intensity of the beam, Mo., April 16 t o 19, 1928. 2
a 4
J. Gca. Physiol., 9,221 (1925); 677 (1926). Colloid Symposium Monograph, Vol. I V , p. 174 (1926). B w . , 61,593 (1928).
6
1924.
Bragg, W. H., and W. L., “X-Rays and Crystal Structure;” tondon,
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however, depends to a considerable extent upon the atomic composition of the planes, that is, the number and kind of atoms, and also upon the location of secondary planes whenever they happen to be present. The methods described in the following paragraphs are applicable to the type of cassette introduced by Hull,6 in which the use of narrow slits produces a flat beam of x-rays. The fibers are placed in the path of this flat beam, which passes through them to impinge upon a photographic a m . The film is bent into an arc a t the center of which the block of fibers is located. The diffracted beams are “reflected” to the film on both sides of the principal beam, and the resulting photograph, on which the beams are represented by images of the slit, is called the diffraction pattern. The Fiber The fibers used, ramie bast fibers, are minute capillary tubes whose walls are composed of cellulose. They are very flexible and more or less cylindrical in shape. Owing to the way in which the wall is deposited, when growing in thickness, by addition of layer upon layer from the inside, the wall appears laminated, in the micro~cope,~ as though it were built up of concentric cylinders. Irregularities in the thickness of these layers and pressure conditions associated with the neighboring cells, tend to produce flattened regions in the wall, giving the tube the appearance more nearly of an unsymmetrical many-sided prism than of a true cylinder. I n no case, however, are these flattened parts marked off by sharp angles, as are the faces of crystals. Unfortunately for x-ray interpretation, there are no constant angles from which crystallographic axes may be computed. I n Figure 1, which represents a microscopic view of a cross section through a fiber at right angles to the long axis, the flattened regions are indicated by a. The concentric lamellae, as well as the flattened regions, are purposely s I i g h t 1y over-emphasized. The glucose-residue chains of which these lamellae are composed are too small to be shown in the drawing, f, but if the magnification were sufficiently great only the end view of each chain would appear, since they run lengthwise of the fiber. If we Figure 1-Microscopic were to indicate the end views of View Of a Section Of the chains by dots, the lamellar a Ramie Bast Fiber Diagrammatic lamellae layers would appear to be made UP ,;$ :; ~ b ~ in turn~ of still thinner d layers, ~ only~ centric lines. Long a d s of one glucose residue in thickness. A fiber is perpendicular t o the page. TWO renions marked Section Of a lamella, such as that B B have planes of the one have parallel t o similar planes of the marked A in Figure’ ‘7 other. to be enormouslv enlarged to make the dots visib1e;but without regard to proper proportions the arrangement of the chains would be somewhat as shown by the dots in Figure 2. Since there would occur then concentric layers of chains, any small portion of the fiber, as A in Figure 1, would have the necessary arrangement of glucose-residue units to produce x-ray diffraction patterns, and would act as though it were a small crystal particle. The tangential distance may be designated as the a axis; the radial, the b axis; and a line perpendicular to the page, the c axis. There would be no uniformity in size of these crystal-like portions of the fiber, and there would be no boundary surface separating one from another. The length in the c direction would depend upon the distance to which the chains extend without bending from a straight line, and in these flexible fibers that would be governed to a marked , extent, when arranging fibers for making diffraction patterns,
0 ’
-4-
’ Phys. R w , 10, 661 (1917).
’ Aldaba, A m . J . Botany, 14, 16 (1927).
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by the attempts of the operator to keep the fibers straight. The length of the diffracting region in the c direction then would be determined primarily by the operator rather than by any inherent characteristic of the fiber structure. Opposed to that, the dimensions of the diffracting region in a and b directions are practically entirely out of the operator’s control. They are determined almost entirely by the conditions of pressure and growth a t the time of formation in the plant. It becomes evident, then, that each crystallike, or diffracting, portion of the fiber has tangential planes of glucose units extending lengthwise of the fiber, as well as radial and diagonal planes also extending lengthwise. These are indicated by the lines in Figure 2. Although every portion of every lamella is potentially a diffracting region, nevertheless, when a C 7 .NGCrVTI8L-C fiber is position to diffract Figure 2 - Marked ~ ~A in~ beam of x-rays from, let us say’ to Figure 1 Enormously Enlarged Show Location of Ends of the tangential planes of Figure Cellulose-Chain Molecules In2, only those regions included in ~ ~at Intersection f BB of Figure 1are effective. Representing the positions of A p i c t u r e of the fiber in a ~ ~ ~ ~ lengthwise direction may be obtained by again referring to Figure 1. When we think of the figure as representing a thin transverse slice cut from the fiber, we picture the knife in cutting such a slice as passing through chain after chain across the fiber. I n the resulting slice, perhaps 0.01 mm. thick, the chain lengths would be about 20,000 glucose residues long. We may therefore think of that slice as consisting of 20,000 sections each only one glucose unit in thickness, and each section as representing one plane of glucose units. The whole length of the fiber would consist of millions of these one-unit-thick planes, all lying parallel to one another across the fiber a t right angIes to its long axis. Arrangement of Fibers for Diffraction
I n order to obtain sufficiently strong diffraction lines it is necessary to pack many thousand fibers into a bundle only 3 or 4 mm. in thickness. If they are in a thoroughly tangled condition every possible orientation of all the diffracting regions in the fibers will occur, and the diffraction pattern will be directly comparable to that from a mass of crystal powder where all of the planes are represented. But if the fibers, 50~ to 100~ mm. long, are ~laid approximately t ~ ~ parallel in the bundle, then, obviously, nearly all of the planes which are located in similar position in the fibers will be parallel to one another; for example, if the cross-section planes, a t right angles to the long axis, of one fiber are in a position to reflect the beam then the same planes of nearly all the fibers will also be in position to produce a diffraction line. Opposed to that, a further result is that none of the planes which are parallel to the long axis of the fiber will be in position to reflect the beam. I n other words, the parallel arrangement gives the operator a certain amount of control over the position of the planes somewhat as he would have with a large single crystal. The extent to which this control of planes is possible involves the degree of parallelism attainable in the bundle, as well as such characteristics of the fiber as flexibility, cylindrical shape, and the minute diameter of the fiber. If from the end of that long bundle a section is cut in which the fiber pieces are 3 to 4 mm. long, and is attached to the protractor of the cassette (film holder inclosing slits), it may be turned to known angles as a crystal is turned, and the xray beam made to pass through it lengthwise or crosswise of the fiber as desired. Then, when the position of one set
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Composition of Planes
of planes is determined, all other planes may be referred to that by means of the protractor readings.
In determining the position of a set of planes in the fiber, a comparison is made of the density of its line from the several photographs just mentioned; but when determining On account of their flexibility and very small diameter, the composition of the plane-that is, the relative number it is impossible, in building up a bundle, to obtain absolute of atoms and their distribution in the plane-a study is made parallelism of the fibers. At best, deviations from a truly of the maximum intensities of the lines from all the different parallel arrangement will extend over a range of 10 to 15 sets of planes. Here quite different principles are involved, degrees. There will occur, therefore, a sufficient number of and although they are discussed in the books on x-ray diffracfibers so oriented as to produce a diffraction line from a given tion from crystals, there are one or two points which seem to set of planes at every angle included within that 10 to 15 need special emphasis. degrees range; and the number of fibers a t the various angles I n the simpler crystals, such as rock salt, the planes are will correspond to a normal distribution curve. As a result layers of atoms one atom in thickness; but in the organic of this distribution in numbers at different angles, photo- materials, as cellulose, the planes are layers of complex molegraphs taken with the bundle oriented to slightly different cules, or groups of atoms comparable to molecules. The positions will show the same line strong on one film and less distance between these layers is considered as the distance from strong on another, depending upon the number of fibers the center plane of atoms of one layer to the center plane which are in the proper position for reflection. By turning of the adjoining layer. Between these principal planes the bundle successively through small angles the position of secondary planes occur, which are made up of the outlying the maximum number of parallel fibers will be shown by the atoms of the large glucose nuclei. The beam reflected from density of the lines on the negatives, and with that information these thick layers is the resultant from the principal planes concerning the bundle, the position of all other planes with modified by the accompanying secondary planes. If no respect to the c axis may be obtained from a similar study secondary planes existed the reflected beam would consist of each line. By a judicious choice of angles for a given line of superimposed waves from the principal planes all meeting is, crest on crest, trough on the location of the corresponding planes may be determined in the same phase-that trough. The interposition of secondary planes, interwithin 2 or 3 degrees. I n the lattice of the fiber several sets of planes are likely leaved between the principal planes, would add an increto make nearly the same angle with the c axis; and as a re- ment to the beam in which the waves would not be in phase, sult of that there will occur in the bundle several groups of and therefore their contribution would not be so great or fibers which will reflect as many different lines, one from each they might even produce an annulling effect as the half-phase group. Several such groups will occur at any position of the is approached-that is, when trough meets crest. This annullbundle, but only a few will contain a sufficient number of ing effect may be very considerable in the complex layers of fibers to produce lines on the photographic film. That means, glucose nuclei in the cellulose fibers, and requires careful of course, that several lines will appear on every photograph, study when the x-ray patterns are interpreted. The effective atoms in the planes in cellulose are of carbon but the lines from one position of the bundle will not be the same as those produced when the protractor shows that the and of oxygen. They are so nearly alike in atomic weight bundle has been turned, let us say, 10 degrees to a new that the ability of one to reflect x-rays is very probably about position. On a given film the density of one of these lines equal to that of the other. On that account the intensity of may be a t its maximum, while that of the other lines may be a t the diffracted beam is likely to be more strongly influenced by various degrees of intensity below its maximum, correspond- the number of atoms per unit area than by the kind of atoms. Unfortunately, the intensity of a diffraction line from ing to the relative number of fibers effective in that position. A complete set of diffraction patterns from a bundle of fibers is influenced by so many factors that in most cases it fibers consists of about ten photographs-one taken with is not safe to assume that the density of one line as compared the long axis of the fibers at 90 degrees to the x-ray beam, a to another is due to the number of atoms in the corresponding second with the long axis parallel to the direction of the beam, sets of planes, For these reasons it seems that the comand the remaining a t suitable intervening angles. From these position of the reflecting planes in cellulose cannot be deterthe position is determined a t which each line appears a t its mined by the x-ray methods alone, at least by the methods which have been used up to the present time. maximum density and through that, its relation to the c axis. Position of Planes
Caustic Soda Improves Joint Strength of Certain Woods Treating with caustic soda certain species of wood which frequently produce weak or inferior joints when glued into doors, furniture, airplane propellers, and similar articles, improves the strength of these joints, experiments made by the Forest Products Laboratory, Forest Service, United States Department of Agriculture, show. A 10 per cent solution of caustic soda gave the best results as a treating material. Joints of hard maple, yellow birch, white oak, red oak, red gum, black cherry, basswood, and Osage orange wood treated with caustic soda showed a decided improvement in strength over joints of untreated wood. While caustic soda solutions weaker than 10 per cent improved the strength of joints, they were not so effective as the 10 per cent solution. In the tests made the wood surfaces to be joined were brushed with the caustic soda solution, and after a period of 10 minutes were wiped with a cloth to remove any excess solution or dissolved material, and were then allowed to dry before they were glued. The same grade of glue was used and the density of
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the wood tested was substantially the same in every case. When the blocks of wood had been conditioned for 10 days they were cut into specimens of the proper size and their strength tested by a special machine devised for the purpose. The effect of the caustic soda treatment in improving the strength of glued joints was especially pronounced in certain woods in which “starved joints,” those in which the film of glue between the wood surfaces is not continuous, are ordinarily produced. The shearing strength (measure of the capacity of wood to resist slipping of one part upon another along the grain) of a piece of untreated wood glued under favorable conditions was 3110 pounds, as compared with 1570 pounds for an untreated piece in which starved joints were manifest, and 3250 pounds for a piece treated with caustic soda solution, but glued under the same starved-joint conditions. The laboratory has experimented with other materials, but none of these has been found so satisfactory as caustic soda.
