ANALYTICAL CHEMISTRY
888 The Pliofilm canopy on the scatter shield prevented evaporation of sample and formation of frost on it. At working temperatures above minus 40" C. no frost was observed on the Pliofilm. However, a t lower temperatures a slight amount of frost was sometimes observed on the outer edges of the canopy. This could be prevented bv blowing cold, dry air over the outer surface of the Pliofilm. Without the canopy, compounds such as benzene partly volatilized during the period of irradiation and resulted in lower sample levels and erratic diffraction patterns. The Pliofilm caused only a negligible reduction in the intensity of the radiation. ACKNOWLEDGMENT
The authors wish to express their appreciation t o Elmer H. Hicks for constructing the cooling chamber and machining the Bakelite sample holder, and to Ivan Heady and Frances Pedersen for the drawings and tracings. Grateful acknowledgment is also made to K. E. Stanfield, who reviewed this manuscript and made helpful suggestions for its preparation.
LITERATURE CITED
(1) Abrahams, S. S., Collin, R. L., Lipscomb, W. N., and Reed, I. B., Rev. Sci. Instr., 21, 396 (1950). (2) Barnes, W. H., and Hampton, W.F., Ibid.,6, 342 (1935). (3) Barrett, C. S., and Trautr, 0. R.. Trans. Am. Inst. Mining Met. Engrs., 175, 579 (1948). (4) Broom&,B., Physik. Z.,24, 124 (1923). (5) Calhoun, B. A., and Abrahams, S. C., Rev. Sci. Instr., 24, 397 (1953). (6) Cox, E. G., Nature, 122,401 (1928). (7) Cox,E. G., Proc. Roy. SOC.(London),135A,491 (1932). (8) Eastman, E. D., J . A m . Chem. SOC.,46, 917 (1924). (9) Kaufman, H. S., and Fankuchen, I., Rec. Sci. Instr., 20, 733 (1949). (10) Lonsdale, K., and Smith, H., J . Sci. Instr., 18, 133 (1941). (11) Post, B., Schwartr, R. S., and Fankuchen, I., Rea. Sci. Instr., 22, 218 (1951). (12) Tombs, S . C., J . Sci, Instr., 29, 364 (1952). (13) Vonnegut. B., and Warren, B. E., J . Am. Chem. SOC.,58, 2459 (1936). (14) Wood, E. A., Rev. Sci. Instr., 24, 325 (1953). RECEIVED for review August
9, 1954.
riccepted January 26, 1955.
Quantitative X-Ray Determination of Amorphous Phase in Wood Pulps as Related to Physical and Chemical Properties G E O R G E L. C L A R K
and
H E N R Y C. TERFORD'
Noyes Chemical Laboratory, University o f Illinois, Urbana,
The paper industry has needed a correlation between the crystallinity and texture of wood pulps derived from widely scattered sources and t h e properties of paper sheets made from these pulps, to establish pulp specifications which will assure superior physical tests for paper. The percentage of amorphous phase of cellulose in the pulps appears to be a n important variable, and has been determined experimentally by x-ray diffraction with a technique involving quantitative calibration and scattering correction standards, based upon the ratio of t h e (002) peak intensity of crystalline cellulose to t h a t of the halo a t 19" for amorphous cellulose. For each pulp t h e absolute density was measured. Although crystallinity values for individual pulps have never been reported, t h e results of this investigation were of t h e same order of magnitude as the single value (approximately 70%, amorphous phase 30%) reported for wood pulp by investigators employing chemical methods. Groundwood pulps have higher amorphous contents t h a n fibers pulped from chips, indicating t h a t mechanical grinding degrades the crystalline regions of wood fibers. Bleaching increases the crystallinity of wood fibers, probably by removal of amorphous encrustants. The tensile strength of paper sheeted from pulp slurries appears to be a function of the amorphous cellulose content of the fibers. No statistical correlation exists between the tear strength of paper and crystallinity of pulp. The density of wood pulp increases with increasing crystalline contents u p to approximately 7070, then remains essentially constant. From x-ray diffraction analysis and density measurements it is possible to classify wood pulps in terms of amorphous phase content and control tensile strength, water-holding capacity, and other practical properties of paper sheets, from the pulp slurries. Tear strength is a function of pulp properties and processing.
111.
T
HE organization of cellulose primary-valence molecular chains to form visible fibers is not so well established as the crystal unit cell. According t o one widely accepted theory, cellulose consists of a crystalline phase in a disordered or amorphous matrix. The molecular chains of cellulose are considered as flexible and capable of assuming randomly kinked forms as the molecules of paraffin, rubber, and other linear polymers. CRYSTALLINE-AMORPHOUS THEORY
.4n infinite number of possibilities for the arrangement of the molecular chains might be expected in a solid system consisting of cellulose molecules. However, it is possible t o distinguish two limiting ideal cases. The first can be thought of as a completely isotropic and amorphous aggregation of randomly oriented and randomly kinked chains, the second a state of perfect three-dimensional order and orientation in which all the chains lie parallel on a regular spatial lattice. The state of cellulose as i t exists in fibers can be visualized as lying somewhere between these two limiting states. I n some regions the arrangement of straight molecular chains results in three-dimensional repeating interatomic spacings which give rise to a coherent diffraction of x-rays. These are the crystalline regions of cellulose. Between the crystalline regions and connecting them together are regions of less order in which the chains are deformed and assume more or less randomly kinked shapes. These regions are designated as disordered or amorphous regions. An individual cellulose molecule may pass through several crystalline and amorphous areas. This subdivision of the cellulose fiber into crystalline and amorphous components must be considered a schematic one. Actually continuous transitions between the two regions might be expected rather than well defined boundaries. The amorphous areas can still contain an infinite variety of structures ranging from nearly parallel chains in a somewhat disordered arrange1
Present address. Shell Chemical Co.. Houston. Tex.
V O L U M E 27, NO. 6, J U N E 1 9 5 5 ment t o randomly distributed molecules as postulated in the first limiting ideal case. Recently Frey-Wyssling (4)proposed a more complex structure for cellulose. The macrofiber is composed of microfibrils having diameters of 150 to 250 il., which can be detected with the electron microscope. Further degradation of the microfibril by ultrasonics, hydrolj-sis, or oxidation results in elementary fibrils (or micellar strands) having dimensions in the order of 70 to 90 .1. These elementary fibrils are thought to be flat filaments, sometimes only 30 A. thick, which aggregate laterally iyith each other. The elementary fibrils are made up of a crystalline core that is flattened parallel to the (101) lattice plane. It is postulated that this shape results from faster growth of the (101) plane which is more hydrophilic than the slower groning (107) plane. The crystalline core of the microfibril is embedded in the paracr>-stallinecellulose phase. The lack of order of the chain molecules in this paracrystalline phase is thought to be caused by the escaping water released from the polymerization of glucose and the crystallization of the resulting chain molecules; its existence is consistent n i t h the fact that no truly amorphous material is observed in electron micrographs a t highest magnifications. The paracrystalline phase binds the elementary fibrils to form microfibrils. Frey-Wyssling contends that the tendency toward aggregation in the (101) plane is greater than perpendicular to it and as a result, watei may be occluded between (101) planes. Thus the microfibrils are laminated and easilv split parallel to the (101) plane, while the elementary fibrils adhere laterally to each other. Hengstenberg and .\lark ( 7 ) have shown that, besides these amicroscopic (less than 50 A ) inhomogeneities, there are coarser capillaries in cellulose fibers. accessible t o colloidal dyes, which are located betv een the microfibrils. PURPOSE OF INVESTIGATION
The purpose of this investigation was (1) to develop a method of determining cellulose crystallinity, based on x-ray diffraction, sensitive enough t o distinguish differences in crystallinity within a series of wood pulps which had been subjected t o various chemical treatments; (2) to relate the crystallinity of the pulps to their densities; and ( 3 ) to determine whether a correlation eyists between the crystallinity of the pulp and the physical properties (tear and tensile strength) of the finished paper. PREVIOUS METHODS
A number of methods of estimating the disordered and crystalline fractions of cellulose based on reaction rate studies, dye absorption, sorption of water vapor, and x-ray diffraction have been reported. The hydroxyl groups on the cellulose molecular chains are the most reactive centers; in those regions of fibers where there is a close-packed crystalline array resulting from regular parallel arrangement of chains, the hydroxyl groups are held in fixed positions and in a sense are largely covered up or inaccessible. Thus, reactions are more difficult in crystalline regions which tend t o persist through treatments which a-ould easily produce swelling, reactions, and over-all fiber disintegration in those regions where the cellulose chains are disordered. Philipp and coworkers (18)determined the degree of crystallinity of cotton fibers by hydrochloric acid hydrolysis, assuming t h a t there is one specific reaction rate for the disordered cellulose and another for the crystalline cellulose. Other chemical methods have been developed based on the assumption that chemical reactions proceed faster in the disordered regions than in the crystalline regions. Goldfinger, Mark, and Siggia (6) subjected cellulose t o oxidation by sodium periodate in excess and measured the uptake of oxygen from the reagent. By establishing graphical methods of calculation for the
889 kinetics they estimated the amount of crystalline material by extrapolation. Sickerson ( 1 7 ) and Conrad and Scroggie ( 3 )refluxed cellulose in a mixture of hydrochloric acid and ferric chloride, collected and titrated the evolved carbon dioxide in barium hydroxide, and converted the rates of carbon dioxide formation to percentages of cellulose hydrolyzed to glucose. Assaf, Haas, and Purves ( 1 )applied the experimentally difficult thallation technique. Thallous ethJ-late, a strong base dissolved in ether or benzene, converts cellulose hydroxyl groups t o the thallium alcoholates Tvithout penetrating crystallites. Thnlliwn cellulosate heated with excess methyl iodide yields partly methylated cellulose and the methoxyl substitution is accepted as proportional to the number of hydroxyl groups nhich hare come in contact with the thallous ethylate reagent. Clark and Southard ( 2 ) determined the pore size-which is an indicat,ion or function of accessibility-of cellulose fibers by absorption of Sile blue sulfate. X-ray diffraction sholi-ed that the size of dye particles in solutions of Sile blue sulfate varied with concentration from single molecules t o precipitated aggregates. This phenomenon ITas used t o measure pore sine 11y showing t,hat a t certain concentrations (and therefore particle sizes) there were sharp discontinuous changes in absorption of the dye by the fibers. The rate of exchange of cellulose with heavy water was L by Frilette, Hanle, and Mark ( 5 ) t o determine the accessibi of cellulose. This technique depends upon the fact that the hydroxyl groups of cellulose react n-ith heavy water. By microtechniques of specific gravit>- determination the extent of the exchange reaction between agitated cellulose and heavy water i p determined. Sorption of \rater vapor has long been a method of rating cellulose in terms of accessibility or amorphous content. Hermans (8) discusses the method and reports values of crystallinity from sorption isotherms for natural fibers and rayons. The application of the aforementioned methods is limited because of their lack of reproducibility and simplicity. It appears probable t h a t the amount of crystalline cellulose found is dependent upon the chemicals used in its determination. The acid hydrolysis method of Philipp, Nelson, and Ziffle ( 2 8 ) is the onljattempt a t a straightforward mathematical interpretation of the data, but is somewhat complicated by the fact that some of the glucose formed by the hydrolysis of cellulose is converted to humic substances. This necessitates a correction factor for the contribution of the insoluble humic substances to the weight of the unhydrolyzed cellulose residue. Perhaps the greatest complication inherent in the above methods arises from the observations of Hess and coworkers ( I O , 2 I ) that the wetting of cellulme fibers promotes crystallization of the amorphous component. Rough estimates of crystallinity have been based on x-ray diffraction ( I S ) . T h e appearance of the characteristic diffraction pattern of cellulose is a n indication only of the crystalline or organized portions of the sample, since a varying percentage of the disordered phase would affect the intensity, under constant conditions, of the crystal pattern, causing this to be weaker the higher the percentage of disordered material; and effect the general scattering or fogging of the film, increasing with increasing proportion of amorphous phase. Conrad and Scroggie (3)measured the “crystallinity number” from x-ray patterns a3
where I t is the average intensity of the (101) cellulose interference and I , that of the film background between the (101) and (107) interferences. Although relative, the values determined for various samples plotted against the per cent accessibility by the oxidative hydrolysis method gave a straight line. I n 1948, Hermans and Weidinger (9) reported a quantitative method for determining cellulose crystallinity based on x-ray
ANALYTICAL CHEMISTRY
890 diffraction. These investigators plotted the intensity distrihution curve from the diffraction pattern registered on film and considered the area of the background as being proportional t o the amorphous component, and the area under the peaks as proportional to the crystalline component. The percentages of crystalline material calculated in this way for native fibera and rayons were in close agreement with the values obtained from sorption isotherms. However, no attempt to distinguish between various native fibers-e.&, between wood pulps or cotton linters -was reported. RESOLUTION OF INTENSITY DISTRIBUTION CURVE O F CELLULOSE
Irradiation of an amorphous material such as glass or unstretched rubber produces an x-ray diffraction paitern characterized by a broad but well defined halo. The amorphous eomponent of some synthetic polymers, which have been investigated by x-ray diffraction-e.g., polythene (24) and Kel-F (monochloratrifluaropolyethylene) (18)-maniiests itself in the form of a halo partially overlapping the sharp crystalline rings. Reliable resolution of the amorphous halo from the crystalline rings was somewhat simplified in the aforementioned examples b y the relative ease with which the ratio of crystalline t o amorphous phase could be varied by heat treating and que
F i g u r e 2. Cameras and 3 Hayes x-ray tion scatter by the amorphou. to the quantity of disorderei lII1)unI1l y.co=L.u. AILyyvy6zA best estimate of these energies would he obtained hy measuring the total area.under the diffraction peaks and the area. under the amorphous halo, the superposition of diffraction peaks upon t h e halo makes it impossible to define these areas. From Figure 1 it is seen that the maximum of the amorphous halo occurs a t 28 19* which corresponds closely to the minimum between the (107) and (021) interferences on the experimental curve. Therefore, the ratio of the intensity of the (002) crystalline interference to the intensity of the amorphous halo was taken as a meamre of the ratio of crystalline t o amorphous material in the sample. The intensity ratio was determined from the intensity distrihution curve as the ratio of the height of the (002) peak t o the height of the curve a t 28 19'. YLlr
EXPERIMENTAL
Figure 1. Position of amorphous halo of oellitlose with respect to theoretioal i n t e n s i t y distribution curve o f native cellulose by Mark
h survey of the literature failed to disclose any attempt to resolve the intensity distribution curve of cellulose beyond the generalization that the exceptionally high background results from incoherent scattering of the x-radiation by a disordered phase. Mark (IS) constructed the theoretical intensity distrihution curve for cellulose (Figure 1). The theoretical curve differs significantly from the experimental curve only in the relative background intensity between the (101) and (002) interferences. Hess (f0)produced amorphous cellulase by mechanical degrrtdation of fibers in a vibratory ball mill. The x-ray diffraction pattern obtained from fibers which had been subjected t o this treatment showed only a broad halo with no evidence of crystalline diffraction rings. Although no intensity data were reported, it was possible to calculate the position of the halo from the specimen-film distance, inside and outside diameters of the halo, and the type of radiation used. The halo was found t o extend from 28 (twice the Bragg angle) 17.4' t o 21.0". Figure 1 show8 the position of the amorphous halo (broken curve) in relation to the theoxtical intensity distribution curve of Mark. This location of the amorphous halo of cellulose explains satisfactorily the high background observed between the (101) and (002) peaks of the experimental curve. The total energy of the diffracted radiation from the crystalline component may be considcred aa proportional to the quantity of crystalline material present while the energy of the radia-
Apparatus. The x-ray diffraction patterns were obtained using a Hayes x-ray diffraction unit equipped with a Machlett A-2 x-ray tube with a copper target. The tube was operated a t 30 kv. and 25 ma. A monochromatic x-ray beam was obtained by collimating, with a 0.010-inch-diameter pinhole, the copper Kalpha radiation diffracted from the cleavage plane of a pentaerythritol crystal mounted in a General Electric XRD Monoebromator. Camera Arrangement. A Picker circular camera, the cassette holder of which was supplied with a motor to rotate the cassette and film, was modified by the addition of a small secondary camera. The secondary camera consisted of a brass cone, the base of which was cemented to the center of the double 45' sector plate supplied with the Picker camera. The inside dimensions of the cone were as iollows: diameter of base, 2.0 inches; diam-
F Figure 3.
Sohematie drawing of cilrnera
arrangement
891
V O L U M E 27, NO. 6, J U N E 1 9 5 5 eter of top, 0.4 inch; height, 0.9 inoh. The height of the cone plus the thickness of the sector plate resulted in a reference specimen-film distance of 1.0 inch. A nickel foil 0.009 inch thick was cemented to the top of the eone to ~ e r v eas an external standard. The over-all geometry of the secondary camera was such that only the (111)interference of nickel was recorded on the film within the area defined by the base of the cone. The entire
throughout the l&hour exposurees. FIgure 2 shows the monochromator and cameras positioned on the diffraction unit.
the oven. The bottles and contents were allowed to cool in a desiccator over anhydrous magnesium perchlorate. Samples of
0.17hm. in diamzter and 0.30 crd. deep, which had been drilled into a brass plate. Excess gum arabic-solution was removed by pressing the wet Bbers level with the surface of the brass plate, using blotter paper backed by a microscope slide. The remaining water was evaporated by heating the brass plate on a low temperature hot plate. When dried, the fiber pellet wasremovedfrom the brass plate, and the diameter was checked with an acculsr micrometer. The pellet diameter (or the thickness of the specimens irradiated) varied hetween0.168and0.161 om. The pellet was cemented to the top end of a '/Isinchdiameter wire with Duco cement. The bottom end of the wire w&s embedded in a cork, which in turn, was supported by a universal clamp and kingstand. When positioned before the pinhole system, the cylindrical axis of the pellet was perpendicular to the x-ray beam. Figure 4 shows the x-ray diffraction patterns of two specimens registered on a single Blm. The intensity distribution curves were obtained by mounting the xray film, secured between two clear photographic plates, on a Leeds & Northrup recording microphatometer and scanning one quadrant of the pattern .. ^. fr . re
. ".
-.
r
7.6 -
.5 -
T
.4Figure 4. X-ray diffraction patterns of t w o cellulose pellets and n i c l d standard registered on film
3-
Relationship between Cellulose and Nickel Intensities. The
.2thickness d c&.
Thewdiffractedtadiation of intensity
IC' isre-
on the Blm, while t h e emerging primary beam,' further reduced in intensity to I', is ahsorbed by the lead beam-stop, L . The intensity of the primary beam is decreased from I to I' owing to absorption by the nickel fail and e m be expressed by
where p and d Bse the linear absorption coefficient and thickness of the nickel foil, respectively. Since these quantities remain constant throughout the investigation, Equation 1 may be written
I
= constant
(2)
The intensity of a given cellulose reflection, I,, is proportional to I and the intensity of a given nickel reflection, In, ia proportional to 2'. Therefore, from Equation 2,
Thus. if the thickness and density of the cellulose specimen as
intensity IOdurina'the exposure.
.
F i g u r e 5. I n t e n s i t y distribution curve obtained fmm x-ray diffraotion p a t t e r n s as shown i n Figure 4
Background Correction for Air, Thermal, and Compton Scattering. A fraction of the background intensity of the diffraction pattern of cellulose results from radiation scattered by the air in the camera, thermal vibration of the electrons in the specimen, and the Compton effeot. To correct the background of the cellulose intensity distribution curve for these components, the diffraction pattern of a sugar cry&] was obtained. A sugar crystal was selected because sugar is considered t o be entirely crystalline, and because of the similarity of atomic composition of cellulose, (C&oOJn, and sugar, CnHnO,,. To determine the thickness of sugar crystal necessary to absorb the same quantity of radiation as a cellulose pellet, a pellet was irradiated for 4 hours using the camera arrangement de-
ANALYTICAL CHEMISTRY
892 scribed above. The 45' sector plate was rotated 90" and another 4hour exposure was made without the cellulose pellet in the x-ray beam path. The pattern obtained is shown in Figure 6. The intensities of t h e nickel interferences [ I R ( ~ intensity ~~), without cellulase and intensity with cellulose pellet in beam path! were determined from the photometer curves of the respective quadrants (Figure 7). From the equation -IRbir)
-
elcd
(4)
Iateelluioa.,
the product of the linear absorption coefficient and the thickness of the cellulose (wd), wvas found to be 0.955. The linear absorption coefficient of sugar was cdculated to he 12.5 cm.-'by averaging the mass absorption coefficients of carbon and oxygen and multiplying this value b y the density of sugar. Thus, a sugar crystal 0.955/12.5 om.-' = 0.076 em. thick nhould abmrb the same quantity of radiation a8 the cellulose pellets. A sugar cry8td 0.073 cm. thick was selected after measuring several crystals with the ocular micromFigure 6 . X-ray diffraction patterns obtained with and without eter. The x-ray diffraction pattern resulting from cellulose pellet in heam path an l a h o u r exnosure of this crvstd was ohotometered to obtain the intensity distribution curve (Figure 8). The background of this curve was assumed to result merging a specific gravity bottle and its contents in distilled only from air, tbermal, and Compton scattering. The intensity water contained in a vacuum desiccator, and evacuating the of this background was estimated to be 0.102 unit at 28 19" and system. After 30 minutes the gases were removed, as indicated 0.092 units a t 28 23", corresponding to the poxitions of the by relatively slow formation of water vapor bubbles as compared maxima of the amorphous halo and the (002) diffraction peak of to the vigorous evolution of bubbles observed initially. When cellulose, respectively. The net intensity above background of the system was restored to atmospheric pressure, the pulp was the reference nickel interference. In, eorresmndine- t o the above found to be thoroughly wetted. background intensity was 0.544 unit. That a 30-minute evacuation time wtu sufficient is shown by Calculation of Per Cent Crystallinity. The intensity I , of the the following three determinations made on one pulp: (002) peak (from the crystalline phase), the intensity Is a t 1. Ev&cu&ted30 minutes. density 1.590 28 19' (from the amorphous phase), and the intensity I B of the 1.590 2. Evacuated 1 hour, density 3. Evsounted 3 hours. denetu 1.589 nickel peak were determined from the photometer trace of each pulp sample. All intensity values were uniformly reduced to Although the pulp appeared to be saturated upon removal correspond to a reference intensity In = 0.1 unit to eliminate from the desiccator, three density determinations were made on a variations in film emulsions, developing conditions, and photompulp sample varying the equilibration time after 3O-minute eter response. The crystalline intensity I, was corrected for evacuation. Typical results were as follows: the air, thermal, and Compton scattering by the formula.
I
