X-Ray Studies of Crystallite Orientation in Cellulose Fibers Natural Fibers, WAYNEA. SISSON,University of Illinois, Urbana, Ill. parallel to thejiber axis will be equal; if the crystallites are limited in their rotation, the spots uti11 be unequal. Froni a study of the density distribution f o r several jibers and from the synthesis of various possible types of fiber diagrams, it is concluded that the crystallites of natural fibers have a n unlimited rotation around their “b” axes in the face of the cell wall. These results appear to be best explained by the assumption of a dejinite discontinuity of crystalline structure.
I n well-oriented fibers it is impossible io determine whether ihe crystallites are arranged with a definite crystallographic plane aluuys parallel to the face of the cell wall, or whether the crystallites are rotated at random around their o u n axes. Howecer, it is shown that, in fibers haring a wide deviation f r o m a parallel orientation or a large spiral angle, it is possible to obtain information in this connection. If the crystallites have a random rotation around their own axes, the diffraction spots from the planes
X
inner face of the cell wall, and their direct influence on the activity of the cell membrane (35) and the molecular mechanism of growth (32). The type of orientation existing in rayon fibers is also of theoretical importance, especially in connection with the micellar theory, the physical and chemical properties of the fiber, and the orienting forces operating during the spinning of the filament. The discussion includes two general types of fibers: (I) crystallites arranged parallel to the fiber axis, and (11) crystallites arranged on a spiral to the fiber axis. Each type is examined with the x-ray beam (a) parallel and (b) perpendicular to the fiber axis. The other experimental conditions are essentially the same as those described in the earlier paper
-RAY data show definitely that in most natural cellulose fibers the b axes of the crystallites (i. e., the direction of the cellulose chains) approach an orientation in the cell wall either parallel to or a t some spiral angle to the fiber axis (3, 17, 26) ; in a previous paper (28) a method was described for determining the degree of this orientation in various fibers. When studying in greater detail this orientation with reference to the fiber axis and its effect upon the physical and chemical behavior of the fiber, the question naturally arises as to whether or not the a or c axes of the crystallites niight also have a definite orientation with reference to the cell wall. Owing to the cylindrical form of the fiber, it has b e e n heretofore impossible to d e m o n s t r a t e conclusively from x-ray data the presence or absence of this type of‘ orientaFIGURE1. CROSSSECTION OF A t i o n (171, a n d , s i n c e CELL WALLWITH 6.10 A. PLANE there are no available O F THE CELLULOSE U N I T C E L L data regarding the rela(GREATLY ENLARGED) OCCUPYING tive ref r a c t i ve indices THE TANGENTIAL POSITION, AND along the a and C axes of THE b AXIS PARALLEL TO THE FIBERAXIS the unit cell, it is also difficult t o s o l v e t h e problem by optical methods, It is the purpose of the present paper to show how a special application of the technic (28) already described (i. e., the measurement of density distribution around the x-ray diffraction rings) may be used in certain cases to solve the following problem: Are certain planes of the crystallites oriented with respect to the face of the cell Tall and are the crystallites limited in their rotation around their 6 axes, or are the crystallites not oriented with respect to the cell wall and have they a random orientation around their b axes? These two types will be referred to as “selective” and “nonselective” orientation, respectively. A selective type of orientation has been assumed by Sponsler (SO) to exist in natural fibers, and he has pointed out its theoretical importance in connection with such questions as the micellar structure, the molecular configurations of the
(28) *
FIBERAXIS FIBERAXIS PARALLEL TO X-RAYBEAM. A diagrammatic CRYSTALLITES P a R A L L E L TO
sketch of the cross section of a cylindrical fiber having the cellulose chains parallel to the fiber axis and the 6.10 A. plane tangent to the face of the cell wall (selective orientation)
;.-.. b,lo--.:
A
B
FIGURE 2. THEORETICAL X-RAYPATTERNS WITH X-RAY BEAX PARALLEL ( A ) AND PERPENDICULAR ( B ) TO FIBER AXIS is shown in Figure 1. If the x-ray beam is parallel to the fiber axis, then, oIying t o the cylindrical form of the cell wall, all the planes would hare a random arrangement with reference to the x-ray beam regardless of whether the crystal lattice has a selective or nonselective orientation; and an x-ray pattern of random arrangement, such as is shown in Figure 2A, would result. I n this diagram and in the discussion to follow, only the principal planes parallel to the b 51
I N D U S T R I A 1, A N D E N G I N E E R I N G C H E M I S T R Y
52
axis of the cryst a l l i t e a r e considered - namely, the ( l o l ) , (lo?), and (002) planes (16)-and these mill be referred to as the 6.10, 5.40, and 3.98 planes. +--AWL& T5 FIBER 4x15 -+ respectively (these A B numbers are the FIGURE 3. ORIENTATION (A) AND THEO- s p a c i n g s o f t h e RETICAL DENSITY DISTRIBUTION CURVES (BI FOR FIBERSWITH S H A R P S P I R A L p l a n e s i n iingSTRUCTURE strom units). The x-ray pattern in Figure 211, of course, will be obtained only when these planes have a scattering from the ideal position parallel to the fiber axis large enough to include the angle 8 required by the equation: .
I
n A = 2d sin
e
COS
However, for both a selective and a nonselective orientation, all three planes will diffract a t the equator line as shown iii Figure 2B, since a = 90" for all three planes in both case.
A
I n view of the above difficulties, it is concluded that, when dealing with fibers having the crystallites parallel to the fiber axis, it is impossible to arrive a t a definite conclusion regarding the presence or absence of a selective orientation. However, as will be shown later, it is possible to obtain information in this connection when there is a large deviation from a parallel orientation or when the crystallites have a large spiral angle.
CRYSTALLITES WITH SPIRAL ARRANGEMENT ,Many cellulose fibers such as .flax, cotton, and wood have a spiral structure ( 2 ) . Before attempting to determine whether the crystallites have a selective or nonselective orientation in these fibers, it is desirable to consider the effect upon the x-ray pattern of numerous variables that may exist in ,piral fiber.. b : 5'
e
Although the discussion could be adapted to either the 3.98, 5.40, or 6.10 planes, selective orientation throughout this paper will be illustrated with the 6.10 plane occupying the tangential position. FIBERAXIS PERPEXDICULIR TO X-RAYBEAM. If the xray beam is perpendicular to the fiber axis and a selective orientation is assumed, diffraction mill occur when the tangent to the cell wall makes an angle * 0 t o the x-ray beam for the 6.10 plane, 90" * 0 for the 5.40, and approximately 45' * 0 for the 3.98 plane. If a nonselective orientation is assumed then the planes, instead of diffracting for the position> of the cell wall given abol-e, may be in a position to diffract a t any section of the cell n-all. The relation between the angle measured on the film between a radius drawn through the intensity maxima and the line parallel t o the fiber axis 6, ancl the angle between the normal to the set of reflecting planes and the fiber axis a and the diffracting angle 0 is given by the relation (22): cos a: cos 6 =
Vol. 27, No. 1
B
FIGURE 4. ORIENTATIOZ ( A ) AND THEORETICAL DENSITYCURVES ( B ) FOR FIBERSDEVIATING FROM A SHARP SPIRAL STRUCTURE
Using known technic, it appears that the only method of obtaining direct experimental evidence regarding selective orientation in a fiber having the crystallites parallel to the fiber axis is t o examine by a microtechnic the outer edge of a cross section of a single fiber ( 2 7 ) , or by the use of soft x-rays with the fiber axis a t small oblique angles to the x-ray beam. Either technic is difficult and the results are uncertain. Eckling and Kratky (8), using a microtechnic, were successful in photographing single ramie fibers; but they fouiitl no evidence of a selective orientation.
%
'
..
,
+ANGLE
FIGURE 5.
TO
...
FIBER A x / S - - r
T H E O R E T I C 4 L DEYSITY DISTRIBUTIOX CURVES FOR V.4RYING V.4LUES OF c$
9 diagrammatic sketch of the orientation in a section of a cell wall having a sharp spiral structure is shown in Figure 311. The distribution of the crystallites about the spiral angle C#I in a section of the back and in the edge of the cell wall i. represented by an ellipse. Since x-rays penetrate the entire fiber, the orientation as shown by x-rays, when the beam is perpendicular to the fiber axis, will be the projection uf both- th; upper and lower walls plus the two edges. I n other words, the xray pattern pictures the t h r e e - d i m e n Yional orientation of the cryqtallites projected on a plane (assuming a nonselective orientation), Thus the d e n s i t y distribution around the x-ray diffraction ring is not an exact meabure of the cryst a l l i t e orientation with reference to the fiber axis, since the density values are always shifted toward the equator line hecause of the cylindrical form of the fiber. That the tame factor causes anomalous v a 1 u e s 80 60 W LO 0 10 40 1 0 90 hen t h e orienta+ANGLE TO N B E R AXIS -+ tion is measured by FIGURE6. MEASURED DENSITY DISthe refractive index TRIBUTION CURVES FOR NATURAL FIBERS i n e t h o d h a s been
pointed out by I'redcm (2.3). lii Figure 313 the d u t t d liiic represents the true spiral orienttition and the eontinimz- lini: tlie appareut orientation OS Figure 3A as determiued irmn s-ray data. In most cell \ i l i l l s then: is coilsiderable scattering frmn ii in:rfect spiral orientation 8 s represmted in I4gure 4.,1, Figtire 4B shows illis y e of distriliution, which is t , y p i d (if most fibers, projer d on a plane. The two lower CU~Rrepresent the distri ition in the top and liottom walls, t.lie dotted line the sum of t,tiese two, arid tlre upper Ilea the total xihie (the two face.: plus the two edges) mated from the x-ray dtita. Althouglr the cross of a single filJmrarely occiirs ils a true circle, it is asi;nir~nltimt ill any bimdle of fibers approximately the same armiimt u f w ~ l l inaterial is present a t any iingle t,o the x-ray bran) (x-rw? beam pwpendieular to fiber axis) as ruoold exist f
