TIIERECRYSTALLIZATION OF AMORPIIOUSCELLULOSE
Dec., 1946 [COMMUNICATION
No. 35 FRVM TllE LABORATVRY FOR
CELLULoSE
RESICARCH OF TllR A K U
AND
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AFFILIATED COMPANIES]
On the Recrystallization of Amorphous Cellulose BY P. H. HERMAN5 AND A. WEIDINGER 1. Preparation of Amorphous Cellulose by Grinding.-In a paper dealing with the treatment of native cellulose fibers in a vibrating ball mill Hess, Kiessig and Gundermann' reported that the X-rav diamam characteristic of the fibers soon disappeared; -the crystalline interferences faded and were replaced by a broad amorphous ring, indicating that the crystalline lattice was destroyed by the mechanical impact of the halls on the fiber. Microscopic examination reveals two characteristic types of mechanical disintegration products. First, the fibers show a marked fibrillation into finest fibrils and secondly numerous particles without clearly recognizable structure appear. The latter will be desi~natedas "cellulirse powder." Both products can be nhserved shnrtly after the beginning of the Finding operation, hut, as the latter proceeds, microscopically visible fiber fragments more and more disappear and the powder becomes prciliiminant. The fibrous fragments still show a crystalline X-ray pattern, the powder does not. Examination with the electron microscope (EM) reveals the occurrence of fine'fibrils admixed with the powder. The fineness of the fibrils extends itself far beyond the resolving power of the light, microscope. According to Hess, et al., elementary fibrils of io-130 A. diameter preformed in the original fiber appear as the final disintegration product. This statement could not he confirmed by Husemann.2 Hess, et nl., have only published one single picture of elementary fibrils from wood pulp fibers and according to Husemann this picture must he ascribed to some kind of impurity in Hess' preparations and is by no means characteristic of the wood fiber. According to Husemann no lower limit of fibril size is observed and no morphologically very characteristic products are obtained when the fibers are milled in the dry condition. As will he shown helow, we ourselvcs have not found any indication of prefnrmrd elementary fibrils in ramie and nther fibers. The diamrter o f the fibrils seems t o assume any s i x h w n to the limit of the resolving power of the instrument used. The powder particles seem t o consist of loose agglomerations of a cellulose component and porcelain grindings from the balls and the walls of the mill. It is difficult to disclose the morphological structure of the cellulnse cwnponent, hut X-ray photographs of the powder reveal that it must occur in a Don-crystalline condition. It is not the aim nf this paper to enter into much (I) K. Herr. 11.
m v , e4 l11111.
Kicssix
ixntl
1.
Giindermnnn. %. Dhysik. Chrm..
(2) E. Ilumm;lan. J . ntukruni. C h r m 1. 188 (1943).
detail on the morphology of the grinding products and we shall merely consider them as a source of amorphous cellulose. We shall confine ourselves to reproducing some characteristic micrographs of the miniline Droducts. I n Figs. 1 4 some low
-Fig. I.-Fibrillation i n raiiiic lil~ersafter milling (stained with icdjtte-zinc chloridr; mag. 64 X ) .
magnification pictures taken with an ordinary microscope are given. Figure l shows the fibrillation in ramie fibers, Fig. 2 the "powder" obtained from ramie after two hours of milling, suspended
1
I
\
I
Fig. 2.-The ''powdvi" 1 n w ~.iiiiiv lilrrs after two hours 01 milling (suspcr~~lccl in water; mag. 64 X).
in water. In Fig. 3 the grinding product of rayon after one hour of milling is reproduced, showing unchanged fragments of rayon filaments and "powder." Finally, Fig. 4 shows the "powder" from wood pulp fibers after four hours nf milling (all suspended in water). I t should be remarked that the dimensions of the powder particles depend on the manner in
1'. H. I-hkMANS AND A.
2548
Vol. 6s
WElbINCEK
b
Fig. :I. T l l C ' ~ ~ > O , v d ~ fro,,, r'' r;ryot, aftrr onc Ilrouiitl > Itouri Recryr,tallizc..rl Nativc cottoii
Viscose rayon 1.88
1.98 1 , 'io TVootl pulp 1.2; 2 . I2 1 , tid 1
Integral heat cal./g.
Ratio
21.7 30.0 20.0
11.5 15.2 11.7
11.:;
11.1
%.(I 1X.X
13.7 11.5 lU.4
10.4
Discussion In prwious work4 el-idence has been brought forward that the density and the sorptive power of cellulose preparations depend on the percentage of crystalline :substance and that the former two quantities approximately represent a measure for the latter. The percentage of the crystalline portion in native cotton, wood pulp and rayon fibers was estirnated to be about ti0, 50 and 237,. The integral heats of wetting also run parallel with the percentage of amorphous substance, though there is no strict proportionality, since, for instance, the absorption ratio of rayon is about 1.9, whereas the ratio between the integral heats of wetting of rayon and cotton is 2.1-2.2. The results obtained in the present investigation are in qualitative conformity with these earlier conc1us:ions. The X-ray data show that the ground powders practically entirely consist of amorphous cellulose. These powders also show the highest sorption ratios and the highest heats of wetting (Table 11). The difference with the original fibers is riiuch greater in wood pulp than in rayon, in rcinformity with the higher Percentage of crysta.llinc su1istanc.e i n the former. 'The t1iffcrciii.i~between the sorptivc capacity antl thc heat of wctting o f the original samples antl that of the ground powder is much greater in wood pulp than in rayon, in conformity with the fact that: the former originally contains the greatest percentage of crystalline substance. Upon recrystallization in hot water, both wood pulp and rayon p1:nvtler yield a product of approximately equal siirptiv- capacity and heat of wetting, these quanti ties, however, being markedly lower than in rayon. I t would therefore seem that the percentage i ) f crystalline substance reached on recrysta'ilization id the amorphous products is somewhat higher than the approximately constant amount present in all kinds of rayon hitherto investigated. '
50 100 30 100 relative humidity. Fig. la.----Sorption isotherms a t 20" referriiig to rayon (A) and wood pulp (13): 0, original sample; 1.1, ground sample; A,ground sample after recrystallization.
5;
It is not clear why the ratios between the heat of wetting and the sorption ratio of the amorphous powder (Table I1 last column) differ somewhat in wood pulp and rayon. For reasons outlined in previous work,4 quantitative Cetiuctions from this kind of result can only have an apptoximate character. Moreover, it is not certan tl. a t the ground products were quite free from small quantities of the original fibers, and more work with products ground for various times would be necessary to reach more precise conclusions. If, however, we venture to take the average figure of 1.68 as the sorption ratio of the recrystallized products and consider this figure as a measure of the percentage of amorphous substance contained therein, and if we further assume that native cotton contains 40% of amorphous s ~ b s t a n c ethe , ~ percentage of amorphous substance in the recrystallized powders would be 1.68 x By the same reasoning rayon would contain 1.88 X 40 = 75:;; amorphous substance. The difference between the heat o f wetting of the amorphous and that o f the recrystallized products amounts to 10 cal./g. or 1 .W kcal./mole in both cases investigated. Since recrystallization also occurs on wetting the powder with water at room temperature (though the diffraction pattern so obtained is somewhat blurred) we may tentatively assume that the fraction of crystalline substance, x, which is formed when the amorphous powder is wetted with water in the calorimeter, is equal to that present in the recrystallized powder. The heat of wetting of the amorphous powder may then he represented by (1 - .v!
rr,, , + xirrr + N ~ ~ , l i , i r
and that of the recrystallized product by (1
- x)?r&,'>+ XffLyLydr
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P. H. HERMANS AND A. WEIDINGER
Vol. 68
where W,, is the heat of wetting of amorphous Finally i t should be mentioned that K. Lauer, cellulose, H c r the heat of crystallization of cellu- et al.,5 in earlier reports on the heats of wetting of lose I1 and Hb,y& the heat of hydration of cellulose ground and recrystallized grinding products of I1 (which forrns a crystalline h ~ d r a t e ) .The ~ dif- cellulose fibers have stated that there was no ference of 1.62 kcal./mole is, hence, xHcr. Now, difference whatever with those of the original the heat of crystallization of @-glucoseis about 5.5 samples. This statement is incorrect, just as many kcal./mole and that of cellulose will be also very other items in that publication, which also omits near this value. This leads to the expression 5 . 5 ~ most of the essential experimental details of the = 1.62 or x = 0.28, which corresponds to 72y0 work. amorphous substance, a figure of the same order Acknowledgment.-We are indebted to Dr. of magnitude as that previously derived from J. J. Hermans of this Laboratory for cooperation sorption data. in the experimental work and for discussion of From the X-ray data shown in Fig. 11 we may the subject matter. The electron micrographs also venture to compute this figure. To that end were taken by the “Institute for Electron Microswe may assume that the intensity of the diffracted copy” with an instrument constructed in the Labradiation in the maximum of the broad “amor- oratory of Technical Physics of the Technical phous” band represents a measure for the per- College in Delft, Netherland. The X-ray work centage of amorphous substance. We then find was carried out by one of us (W.) as a guest in the (in arbitrary units; height in mm. of the photom- same laboratory. eter curves) 26 in the ground powder from ramie Summary and 16 in the recrystallized one. Taking the In conformity with a paper by Hess and coformer as being 100% amorphous, the percentage of amorphous substance in the latter ’would be workers cellulose fibers can be transformed into (16 : 26) X 100 = 62%. This figure is in approxi- approximately wholly amorphous products by mate agreement with the figure of 67% as derived grinding in a vibrating ball mill and these products partly recrystallize on heating with water. A from sorption data. If we take the surface of the whole area under quantitative study of the intensity distribution in the dotted curves in Fig. 11 instead of their maxi- X-ray photographs of the amorphous and recrysmum height a s a measure of the amorphous per- tallized powders from ramie fibers shows that the centage, we find 72y0instead of 62%. However, amorphous substance gives rise to a broad band for several reasons (not to be outlined here) the whose maximum intensity lies a t the site of the As interference of cellulose I1 (hydrate cellulose). former procedure seems preferable. There is, however, another, perhaps more im- Computation of the intensities leads to the conportant conclusion which may be tentatively de- clusion that the recrystallized products contain rived from this investigation, namely, that the about 60-70% amorphous substance. The integral heats of wetting and the sorptive intensity of the “background,” measured under the crystalline interference AB of cellulose 11, if capacities of the grinding products of rayon and corrected for scattering by air and for absolute wood pulp were determined before and after reintensity of the incident beam, may be taken as a crystallization. The results obtained are in fair measure of the percentage of amorphous sub- conformity with the foregoing and with previous stance. This means that this quantity must be deductions on the percentage of crystalline subapproximately equal in all regenerated cellulose stance in various cellulose fibers. THE KETHERLANDS RECEIVED JULY2, 1946 fibers provided our former deductions4 are cor- UTRECHT, rect. This subject will be dealt with in another (5) K . Lauer, R. Doderlein, C . Jackel and 0. Wilde, J . makrom. paper. Chcm., 1, 76 (1943).
