INDUSTRIAL AND ENGINEERING CHEMISTRY
2166
The fraction of barium precipitated a t each fractionation step is accurately controlled in the chromate method by the amount of alkali chromate or dichromate added to the reaction mixtures. Control of the fraction crystallized is more difficult in the fractional crystallization procedures, since it depends, in part, on the cooling conditions. Failure to control accurately the crystallization conditions partially disrupts the system and results in a less efficient system. In a countercurrent batch fractionation scheme, the more frequently fractions of equal compositions recur, the simpler the fractionation system. The chromate procedure described in this paper is the simplest countercurrent batch fractionation system possible. The fractional crystallization schemes used for the halides are more complicated, since fractions of equal composition recur a t less frequent intervals (6). Enrichment factors of somewhat less than 1.6 are usually assigned to the fractional chloride system, while a factor of 2.0 to 2.2 is more frequently used for the acid-bromide systems (1). An enrichment factor of 3.55 per fractionation step (Table I) is obtained by the chromate procedure. It must be emphasized that the greater the enrichment per fractionation step, the fewer the total number of steps required for the separation of radium. The separation of radium from barium can be made with greater rapidity by the fractional chromate procedure. Fractional crystallization methods usually require that each fraction be crystallized no more than once a day. Xo such restrictions are involved in fractional precipitation. Precipitations may be made consecutively, each precipitation requiring about 1 hour. The principal advantage of fractional crystallization is that high salt concentrations are used, whereas relatively dilute solutions are used in the chromate procedure. The halides are much more soluble than the nitrate, the salt which limits the barium concentration in solution in the chromate procedure. Therefore, considerably smaller containers may be used for fractional crystallization, and the initial container costs would be correspondingly less. However, for radium-poor mixtures, barium : radium > 100,000: 1, the concentration of barium-radium salts in the chromate method can be greatly increased by raising the temperature to 100” C., since the nitrate is considerably more soluble in hot solutions than in cold solutions. For radium-rich mixtures, barium:radium < 100: 1, the small volume resulting from the high salt concentrations used in fractional crystallization is a disadvantage. A decrease in enrichment factor per fractionation step has been reported for the fractional crystallization of radium-rich mixt,ures due to the
Vol. 47, No. 10
formation of double salts of barium and radium halides (11, 16), such as RaBr2.2BaBr2.6HZO. No such decrease in enrichment has been found for chromate mixtures (IO). The only reagent required for fractional crystallization is hydrochloric or hydrobromic acid, most of which can be recovered for re-use. For the chromate precipitation methods, the reagents required are nitric acid, acetic acid, a source of chromate ions, and urea or a cyanate. The internal reactant is, of course, destroyed. Most of the other reagents may be recovered, if their recovery is economically practical. The principal advantages of the chromate method over fractional crystallization are greater radium enrichment per fractionation step, better control of precipitation conditions, and greater simplicity and speed of operation. LITERATURE CITED (1) Barker, H.H., and Schlundt, H., Uniu. Missouri Bull., 24, No. 26 (1923). (2) Can. Chem. Met., 17, 251 (1933). (3) Greene, C. H., J . Am. Chem. SOC.,59, 1186 (1937). (4) Henderson, L. M., and Kracek, F. C., Ibid., 49, 738 (1927). (5) Hopkins, B. S., “Chemistry of the Less Familiar Elements,” vol. I, chap. 4, p. 6 , Stipes Publishing Co., Champaign, Ill., 1938. (6) Joy, E. F., and Payne, J. H., IND. ENG.CHEM., 47, 2157 (1955). (7) MacTaggart, E. F., Trans. Inst. Chem. Bngrs. (London), 20, 65 (1942). (8) Merkulova, M. S., Trav. inst. elat. radium (U.S.S.R.), 3 , 141 (1937). (9) Parsons, C. L., Moore, R. E., Lind, S. C., and Schaefer, 0. C., U. S Bur. Mines, Bull. 104 (1915). (10) Salutsky, M. L., Stites, J. G., and illartin, A. W., Anal. Chem., 25, 1677 (1953). (11) Scholl, C. E., J. Am. Chem. Soc., 42, 889 (1920). (12) Schwind, S. B., and Croxton, F. E., “Radium, a Bibliography of
Unclassified Literature,” U S. Atomic Energy Commission, TID-363, July 1950. (13) Strong, R. K., J . Am. Chem. SOC.,43, 440 (1921). (14) Tipson, R. S., Anal. Chem., 22, 628 (1950). (15) Tolmachev, P. I., Trau. inst. etat. radium (U.S.S.R.), 3, 239
(1937). (16) Tompkins, P. C., Norris, W. P., Wish, L., Finkle, R. D., and Evans, H. P., “Methods for the Quantification of Radium,” U. S. Atomic Energy Commission, MDDC-699 (1946). (17) Walter, 2. T., and Schlundt, H., J . Am. Chem. Soc., 50, 3266 (1928). (18) Willard, H. H., and Goodspeed, E. W., IND.ENG. CHEM., ANAL. ED., 8, 414 (1936). RECEIVED for review December 10, 1953. ACCEPTED May 23, 1965, Presented before Division of Analytical Chemistry, 124th Meeting, ACS. Chicago, Ill., September 1953. Mound Laboratory is operated by Monsanto Chemical Co. for the U. S. Atomic Energy Commission under contract AT-
33-1-GEN-53.
Chain Length and ChainALength Distribution of Untreated Cotton, Flax, and Ramie Celluloses T. E. TIMELL Division of Industrial and Cellulose Chemistry, McGill University, and Wood Chemistry Division, P u l p and Paper Research Institute of Canada, Montreal, Quebec, Canada
T
HE degree of polymerization and the molecular weight dis-
tribution of high polymers have been much studied in recent years, and a number of different methods have been developed for their determination. It is evident from reviews (4, 6), however, that relatively little attention has been devoted t o materials such as textile cellulose fibers and wood celluloses, and of the former only cotton aeems to have been extensively examined. Many of the results do not agree with each ot,her, and this applies both t o the chain length distribution and, especially, t o the molecular weight of the cellulose component of these fibers.
I n Table I data for the degree of polymerization of cotton, flax, and ramie celluloses are compared as found in one earlier (3%’)and in another more recent investigation (9). The lack of agreement is great, and, while it now appears probablethatstaudinger’s values were too low, it seems almost certain that those of G r a l h , obtained with the ultracentrifuge, were much too high, particularly the value of 36,000 for the degree of polymerization of flax. Some data for the degree of polymerization of untreated cotton cellulose ohtained in recent studies by viscosity or sedimentation-
October 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
diffusion methods are summarized in Table 11, from which it is apparent that, here too, little agreement has been achieved. Generally, molecular weight data from the earlier ultracentrifuge studies (9) tended t o be improbably high for various reasons, one of them being the difficulty of estimating the diffusion constants (IO,16). D a t a based on viscosity determinations, on the other hand, tended t o be too low, partly because of failure to correct for the influence of the rate of shear on the intrinsic viscosity, and partly because of the uncertainties involved when intrinsic viscosity was converted to the corresponding degree of polymerization. Where nitrates were used, the influence of the degree of substitution on the intrinsic viscosity was also usually neglected, and it is now clear that this alone can result in serious errors (18). The most reliable value for the degree of polymerization of a ran- cotton cellulose a t present available seem8 to t o be that reported by Newman, Loeb, and Conrad (24)-5200.
Table
I. Average Degrees of Polymerization for Cotton, Flax, and Ramie Celluloses
Degree of Polymerization Material Staudinger Gralen (9)b 2020 10,800 Cotton 2330 36,000 Flax 2660 12,400 Ramie a Viscosity measurement8 in cuprammonium. b Sedimentation-diffuaion measurements in cuprammonium.
2167
A NEW TECHHIQUE
for obtaining information on comparative molecular weights and distributions of molecular weight in cotton, ramie, and flax
Schurz and Immergut (l?), although provided with five instead of four efflux bulbs, and had a capillary radius of 0.0234em. Further details concerning this instrument have been given ( 3 3 ) . METHODS
Nitration. Cotton, flax, and ramie fibers, previously dried a t room temperature, were nitrated with the standard nitrating mixture for 5 hours a t f 4 " C. After most of the acid had been removed b y filtration, the material was added rapidly and in small portions t o a 1 to 1 mixture of acetic acid and u-ater, previously cooled to -20" C. Prolonged washing was carried out with 50% aqueous ethyl alcohol, cooled t o the same temperature, after which the nitrate was boiled three times for 5 minutes each with the same solvent mixture. The raw product was dissolved in acetone, insoluble material removed by centrifuging, and the solution precipitated into water. After a final washing with anhydrous methanol, the recovered nitrate was dried for 3 hours a t 60" C. in vacuo. Data for yield and solubility in acetone are given in Table 111.
Table 11. Average Degrees of Polymerization for Cotton Celluloses Investigator Staudinger (SI) Kraemer (17 ) Jurisoh (16) Gralen (9) Gral6n (0)
Method of Measurement Viscosity Sedimentation-diffusion Viscosity, nitrate Sedimentation-diffusion,nitrate Sedimentation;diffusion, cuprammonium Jorgensen ( 1 % ) Viscosity, nitrate Conrad and others (94) Sedimentation-diffusion, nitrate Golowa and Iwanow (8) Viscosity, cuprammonium Meyerhoff (90) Sedimentation-diffusion, nitrate a Cotton from unopened bolls.
D.P. 2,000-3,000 >3,500 8,000" 2,820 10,800 4,000a 6,200 14,120 8 ,470a
I n the present study a n attempt was made t o establish the molecular weight and the chain-length distribution of the cellulose as it originally existed in three textile fibers-cotton, flax, and ramie. Nitration of these materials yielded cellulose nitrates with little or no degradation, the molecular weight of which was estimated viscometrically on the basis of data recently published by Conrad and others ( S i ) and by Lindsley and Frank (18). Application of a two-stage fractional precipitation technique resulted in t h e isolation of a number of fractions of different degrees of polymerization, from which, after analysis, the curves of integral and differential weight distribution were obtained, MATERIALS
The raw cotton was a Coastland variety, kindly supplied b y the National Cotton Council of America, Memphis, Tenn., and had been grown in Georgia in 1952. Textile flax straw was obtained through the courtesy of Coop6rative FBder6e de Quhbec, Montreal, and had been subjected only to deseeding and partial decortication. Raw ramie ribbons, which had not been subjected to any treatment, were generously supplied by Newport Industries, Inc., West Palm Beach, Fla. All three samples were extracted a t room temperature with a 2 t o 1 mixture of benzene and ethyl alcohol prior t o nitration. The nitrating mixture was that of Alexander and Mitchell ( I ) , consisting of nitric acid, phosphoric acid, and phosphorus pentoxide in the weight proportions of 64:26: 10, and was prepared 2 days before being used. The solvents were all of reagent grade quality. The Cannon-Fenske viscometer ( 5 ) was a standard instrument with a capillary bore of 0.045 cm. The viscometer permitting variation of the shear rate was similar t o that developed by
Fractionation. Cotton nitrate (5.0 grams) was dissolved in 4000 ml. of 98% aqueous acetone as rapidly as possible. To the clear solution, which was kept at 25' C. in a water bath, enough water was added with vigorous stirring t o precipitate a first fraction, which was dissolved again b y addition of a minor amount of acetone. With the temperature kept constant, solvent was slowly removed with a gentle current of air until the solution became opalescent, a t which point the evaporation of the acetone was discontinued. After stirring for an additional 15 minutes t o ensure equilibrium, the precipitated fraction was isolated by centrifugation and the remaining clear solution subjected to another fractionation. T h e fraction obtained was dried immediately a t 40" C. in a current of air and subsequently in vacuo a t 60' C. for 5 hours, after which it was weighed and analyzed. A total of six primary fractions was collected, each of which was divided into five t o eight subfractions in a similar manner. The flax and ramie nitrates were fractionated in the same way, the total number of fractions obtained in the three cases varying between 30 and 35. The total time of contact between the nitrates and the solvent varied between 20 and 30 hours under these conditions.
Table 111. Data for Nitration of Cotton, Flax, and Ramie Celluloses Material Cotton Flax Ramie
Nitrate Yield,
Solubility in Acetone,
176.0 123,5 170.0
07.7 67.5 91.1
%
%
Estimation of Degree of Polymerization. The determination of the intrinsic viscosity of the nitrates in ethyl acetate or, in three cases, in acetone solution, was carried out as described by Conrad and others ( 2 4 ) , although with some modifications. Reduced viscosities were measured a t five different concentrations, ranging from 0.1 t o 0.01 gram per dl., and corrected for kinetic energy losses, after which t h e logarithms of ns,/C were plotted against the logarithm of the rate of shear as described previously (33). B y interpolation and, t o a certain extent, extrapolation, viscosity values were obtained for a number of shear rates, usually ranging from 200 to 2000 see-'. Plotting of the
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2168
2.4
values of a series of cellulose nitrates as determined in ethyl acetate at a constant rate of shear of 500 sec.-l, with t h e corresponding degrees of polymerization as determined b y sedimentation-diffusion measurements. The following relationship was found t o be valid over a wide range of molecular weights:
1
D.P.
1.8
-
1.6 2.0
Vol. 47, No. 10
2.5
3.0
3.4
LOG 0
Figure 1. Relation of logarithm of reduced viscosity to logarithm of rate of shear at five different concentrations
reduced viscosities against concentration and extrapolating to zero gave the corresponding intrinsic viscosities. The degree of polymerization was calculated from the value valid a t a rate of shear of 500 set.-* according t o Newman, Loeb, and Conrad (g4), assuming the constant K t o equal 80. I n the case of the unfractionated nitrates, the intrinsic viscosities had previously been adjusted to a common nitrogen content of 13.60%, which was the average nitrogen substitution of the nitrates used by the authors mentioned above. For acetone solutions the corresponding value of K was assumed t o be 100 (21). Determination of Degree of Substitution. The nitrogen content of all unfractionated nitrates and some of the fractions was estimated by a n adaptation of a semimicro-Kjeldahl procedure (56). The amount of material necessary for analysis was 15 to 25 mg. and methyl purple was used as indicator in the final titration. All G=200rcc'~ values were referred to results obtained with a 160 highly purified sample of potassium nitrate.
=
K X
[771600
n here [7I60o was the intrinsic viscosity as measured a t a shear rate of 500 see.-' The constant K was, somewhat unexpectedly, found t o be approximately the same throughout and have a value of 80, which was the value used previously by Mitchell ( I ) and others. As attempts to extrapolate the viscosity values found in the present study of zero rate of shear did not seem very promising (SS), it was decided t o use the data presented b y Conrad and coworkers for the determination of degree of polymerization, although it was realized that such a treatment involved certain approximations ( 3 3 ) . T h e relationship between the logarithm of the reduced Viscosity and the logarithm of the rate of shear was found to be linear, as is seen in Figure 1. Reduced viscosities, when plotted according to Kuggins (IS)against concentrations at constant rates of shear as is done in Figure 2, gave linear relations which could easily be extrapolated graphically t o zero concentration. Application of Martin's equation (19)resulted, under the same conditions, in downward curvatures which were more pronounced the lower the rate of shear, as is seen in Figure 3. When the data were plotted, as is usually done, a t constant shearing stress instead of rate of shear, nonlinear plots were obtained according to both Huggins and Martin. Table I V shows the great influence of t h e velocity gradient on the intrinsic viscosity of the three nitrated celluloses, the value valid for the ramie nitrate at a shear rate of 2000 sec-1 being only 68% of that noted at 200 set.-' T h e viscosity slope constant, k', in Huggins' equation
% P l C = [TI
+ k' [TI*C
( I S ) was also found t o vary considerably, decreasing both with increasing intrinsic viscosity and with rate of shear (33). An example of this is given in Table V for three ramie nitrates of different, molecular weights. The intrinsic viscosity values valid for a rate of shear of 500 sec.-1 were adjusted t o a nitrogen content of 13.60% according to Lindsley and Frank (18) in order t o eliminate the influence of t h e
7
DEGREE OF POLYMERIZATION
It was found in separate experiments t h a t the intrinsic viscosity of cellulose nitrates in ethyl acetate became independe n t of the rate of shear only a t degrees of polymerization lower than approximately 2200 (SS), and t h a t the influence of the velocity gradient increased with increasing intrinsic viscosity. This implied that almost all values for [?I obtained here had to be referred t o either zero or some standard, finite rate of shear. Newman, Loeb, and Conrad ( 8 4 ) recently compared intrinsic viscosity
140
2.20 120
"p C
2.00
100 LOG
80
v z C
1.80
60
0
0.02
0.04
0.06
C,G/DI
Figure 2. Variation of reduced viscosity with concentration at seven rates of shear
0
0.02
0.04
0.06
0.08
C, G / D I
Figure 3. Relation between logarithm of reduced viscosity and concentration according to Martin (19) at seven rates of shear
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1955
degree of substitution, and then finally converted t o degrees of polymerization. D a t a for two different samples each of cotton, flax, and ramie nitrates in ethyl acetate and acetone solutions are given in Table VI. Considering the very high molecular weights involved, the degree of polymerization values checked fairly well in all three cases, and especially so in the first and the third case, where the same sample was used. The average degree of polymerization of the raw cotton, flax, and ramie fibers studied here thus appears t o be approximately 4700, 4650, and 5750, respectively, all viscosity averages which should be very close t o the weight average values. Comparison with the data in Tables J and I1 indicates little agreement between these values and those found by Staudinger and GralBn, whereas those reported by Jurisch and Jorgensen for cotton cellulose are within the same range. However, their samples were taken from unopened bolls, where the degree of polymerization is known t o be higher than after the bolls have opened. The value closest t o t h a t found here for cotton cellulose is that reported by Conrad and others (dC)-namely, 5200-and it is possible t h a t the agreement would have been even better if their value had been adjusted, as the present one, t o a n exact nitrogen content of 13.60%. Variations in degree of polymerization for celluloses of different varieties or even of the same variety are, naturally, also to be expected. It is, finally, of interest t o note t h a t the degree of polymerization values of these textile fibers were only slightly higher than the highest ones so far obtained for wood celluloses-namely, 4000 to 4500, found recently by direct nitration of two coniferous wood species (35).
Table IV. Variation of Intrinsic Viscosity with Rate of Shear for Nitrated Cotton, Flax, and Ramie Celluloses '
Table V. Variation of Intrinsic Viscosity, [ q ] , and Viscosity Slope Constant, k', with Rate of Shear for Three Ramie Nitrates in Ethyl Acetate Solution Shear Rate, [vl, DI./G. A' ~
See. -1 100 150 200 300 500 750 1000 1500 2000
A 87.8 85.6 83.9 81.8 77.5 73.0 69.4 64.6 61.4
Flax Ramie
Table VII.
Solvent Ethyl acetate Ethyl acetate Acetone Ethyl acetate Ethyl acetate Acetone Ethyl acetate Ethyl acetate Acetone
...
...
27.0 27.0 27.0 27.0 27.0 27.0 27.0
A 0,436 0.301 0.256 0.194 0.130 0.094 0.070 0.040 0.025
B 0.534 0.438 0,389 0.315 0.220 0.175 0.152 0.114 0.103
C
0:4Q4 0.434 0.357 0,280 0.236 0.184 0.168
% 13.03 13.94 13.93 13.70 13.75 13.70 13.64 13.79 13.64
___[VI600 Uncorr. 65.4 67.8 52.7 60.0 63.0 48 0 72.5 77.5 57.9
Corr. 58.8 60.7 47.4 33 2 60.0 46 6 71.7 72.8 57.3
D.P. 4700 4860 4740 4650 4800 4660 5740 5820 5730
Fractional Precipitation in One Stage of Nitrated Ramie Cellulose %
6.0
5.5
W 0
C
Nitrogen Content, Material Cotton
6.3 6.0 9.0 8.3 6.4 15.8 7.6 6.0 6.3 16.8
2
B 46.0 44.8 43.5 42.2 41.0 39.6 37.7 35.6 32.0
Table VI. Nitrogen Content, Intrinsic Viscosities, and Degree of Polymerization of Nitrated Cotton, Flax, and Ramie Celluloses
Weight,
I-
Intrinsic Viscosity, Dl./G. Cotton Flax Ramie 77.0 71.5 83.2 73.0 67.2 77.5 67.8 63.0 72.5 63.8 58.5 67.0 59.7 55.8 63.0 56.0 52.7 .59.5 53.7 ... 57.0
Shear Rate, Seo. - 1 200 300 500 750 1000 1500 2000
CHAIN-LEKGTH DISTRIBUTION
Several preliminary experiments, described in detail elsewhere (S4), were carried out t o ascertain the best conditions for the fractionations. Both theory (23, $8)and practice have shown conclusively t h a t a good separation according t o chain length is more difficult t o achieve with material of very high than of low molecular weight (S4, S 7 ) . When a sample of nitrated ramie was fractionated by a precipitation method similar t o t h a t sug-
2169
Sitrogrn Content,
%
13:is 13.70 13.76 13.70 13.73 13: 71 13.75 13.77 13.73
D.P. 4710 5520 6360 6430 6230 5840 5400 4880 4610 4430 3600 3200
a W
n W
L I4
-.1 3
3 0
2000
3000
4000
5000
DEGREE O F POLYMERIZATION
Figure 4. Integral weight distribution curves for Ramie nitrate, obtained in one-stage
fractionation A.
Intrinsic viscosities determined a t uncontrolled rates of shear and adjusted to nitrogen content of 13.60% B. Intrinsic viacositiea determined at rate of shear of 500 aec. -1 b u t not corrected for nitrogen substitution C . Intrinsic viscosities determined as for B and adjusted to 13.60% nitrogen
gested by hlitchell @?a), the results summarized in Table VI1 and Figure 4 were obtained. The variation in nitrogen content between the fractions was obviously not great, and a similar result was noted in another experiment, where 168 fractions were isolated by a three- to four-stage fractionation procedure. Although a slight change in the distribution curve was evident from Figure 4 when the data on degree of polymerization were adjusted t o a common nitrogen content of lZ.BOOj~, the difference between the two plots was not considered significant enough t o necessitate such a correction in the subsequent experiments. The necessity of repeated refractionation is clearly evident from theoretical treatments (23) and has recently been emphasized by Doty and Spurlin ( 6 ) , who pointed out that 10 refractionations might be necessary to obtain an efficient separation. These authors also have drawn attention to the fact t h a t a slight change in the integral distribution curve will influence the differential or frequency curve considerably, and recommended
INDUSTRIAL AND ENGINEERING CHEMISTRY
2170
Vol. 47, No. 10
7
- 7
6
- 6
5
- 5
+
+
-cr:
-
4=.
d
-
3 r
4 ?
3 -
0
-
Lr
- 2
2
I
I
0 2000
3000
4000
5000
6000
7000
DEGREE O F P O L Y M E R I Z A T I O N
2000
3000 DEGREE
4000
5000
6000
O F POLYMERIZATION
Figure 5. Integral and differential weight distribution curves for nitrated ramie cellulose
Figure 6. Integral and differential weight distribution curves for nitrated cotton cellulose
t h a t any inflection in the integral curve should be supported by at least 10 experimental points. It is therefore clear t h a t no definite conclusions can be drawn from the results of the present one-stage fractionation; indeed, the presence of the first inflection in the curve was not corroborated in a duplicate experiment. A striking example of the result of neglecting the influence of the rate of shear when determining the degree of polymerization of the fractions viscometrically is given in Figure 4. Curve A was based on viscosities measured in a Cannon-Fenske viscometer under conditions of uncontrolled velocity gradients, whereas for curve C intrinsic viscosities were determined a t the standard rate of shear of 500 set.-' by the procedure referred t o above, both curves being adjusted t o a common nitrogen content of 13.60%. Since the shear rate dependence of the intrinsic viscosity was more pronounced for the fractions of high degree of polymerization than for the lower ones, the apparent chainlength distribution shown in A was narrower than the actual one (plot C). When a n attempt was made t o subject the same ramie nitrate t o repeated refractionations, difficulties were encountered because of the instability of cellulose nitrates in acetone and other solutions. This shortening of the chain length has been discussed b y Spurlin (SO) and others (2, 11) and will, naturally, be more severe for a material of high than of low molecular weight. It was found t o amount t o approximately 1% per day in the present case. As a result, in several experiments, the weight average degrees of polymerization were 35% lower than the original viscosity (weight) averages, and false inflections in the integral curves were also observed. I n order to minimize this effect, it was decided t o keep the time of contact between the nitrate and the acetone a t a minimum and t o refractionate the primary fractions only once. In three final fractionations with nitrated cotton, flax, and ramie celluloses, the subdivision was accordingly limited to two stages and 30 to 35 fractions were isolated and analyzed. No attempt was made t o isolate as many fractions i n each stage as possible, as it has been shown t h a t little separation is achieved by taking more than five t o six fractions each time (66). After combining fractions with similar average degree of
polymerization, the data summarized in Table I’III were obtained. The curves of integral and differential weight distribution are given in Figures 5, 6, and 7 . The chain-length distribution curves of all three fibers evidently contained only one peak. The frequency curve for the ramie cellulose was almost symmetrical, exhibiting only a slight lefthand skewness. The lower limit was approximately 3000 and the higher 6500 D.P., but most of the material was present within the rather narrow range of 4500 t o 6000. Fractional solubility methods have a tendency t o overestimate the amount of material within the middle degree of polymerization range. The frequency curves for cotton and flax resembled each other and indicated a considerable left-hand skewness. I n both cases approximately 50% of the total number of chains was present within a lower degree of polymeriaation range of 2500 to 4500. This fact accounted for the slightly lower average degree of polymerization of these two materials as compared t o the ramie, although their peaks were located a t somewhat higher values. With one exception, a comparison can be made with previous results only in the case of cotton. Emery and Cohen’s ( 7 ) fractionation of a cotton cellulose resulted in a frequency distribution curve containing several peaks. As the reality of none of the peaks was supported by more than a few experimental points, it seems doubtful whether any significance should be attributed t o them. A very careful fractionation was carried out b y Jorgensen (la,14), who studied a sample of cotton cellulose from unopened bolls. The differential weight distribution curve obtained was similar to t h a t noted here for cotton cellulose, including an appreciable left-hand skewness. The degree of polymerization range-1000 t o 4500-was, however, not the same as t h a t found in the present study. As the relationship between the two integral distribution curves in Figure 8 was very similar t o t h a t shown in Figure 4, the difference might be due to the fact t h a t in the earlier investigation viscosity measurements mere carried out a t uncontrolled rates of shear. Some time after this study was completed, an article was published by Schulz and Marx (,%), dealing with the molecular weight and molecular-weight distribution of various native celluloses.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cctober 1955
‘80
0
°
2171
8
7 6
-
5>
d 4d 3 k 2 I
1000
2000
3000
4000
6000
5000
1000
2000
A.
Table VIII. Fraction NO.
Cumulative Weight,
%
Result obtained by Jijrgensen (12,1 4 ) Present study
the molecular weight of the cellulose nitrate. While a complete explanation of this phenomenon i s still lacking, it could, a t least partly, be due t o the inevitable shortening of the chains taking place during a n y fractionation involving contact between cellulose nitrate and a solvent such a s acetone. The number-average degrees of polymerization wkre, as could be expected, throughout lower than the weight-average values. The relative nonuniformity, U [D.P.,/(D.P., - l)] was approximately the same for cotton and flax celluloses, and much higher than t h a t of the ramie cellulose, which exhibited a very low nonuniformity. Although the value for cotton, 0.19, was twice as large as t h a t noted b y Jorgensen for the same material, the dis-
Fractional Precipitation of Nitrated Cellulose Fraction D.P.
NO.
Cotton Cellulose
7 8
1 2
9
3
10 11
4 5 6 7 8
12
13 14
9
10 11 12 13 14 15
1 2 3
4 5 6 7 8
16
17 18 19 Flax Cellulose
7000
Integral weight distribution curves for nitrated cotton celluloses
Figure 8. B.
Their degree of polymerization values were within a range of 6500 t o 9000, a s determined from viscosity values, using ultracentrifuge measurements for calibration. T h e frequency distribution curves all exhibited more than one peak, even for cotton and ramie. A comparison with the present results is difficult, however, as only a few primary fractions were isolated in each case. Table .IX summarizes the various degree of polymerization values found for the three materials investigated. The viscosity average degree of polymerization should theoretically be equivalent to the weight average degree of polymerization for cellulose nitrates. This was evidently not the case here, the weight average values calculated from the fractionation data being lower than the viscosity averages. A similar difference was noticed b y Jorgensen (141, and was more pronounced the higher
6000
1000
D E G R E E OF P O L Y M E R I Z A T I O N
DEGREE O F POLYMERIZATION
Figure 7. Integral and differential weight distribution curves for nitrated flax cellulose
4000
3000
9 10 11 12 13 14 15 16 17 18
Cumulative Weight,
%
Flax Cellulose (Contd .) 41.0 54.9 58.7 63.8 68.5 79.0 92.3 100.0 Ramie Cellulose
D.P. 4070 4440 4800 5090 5170 5270 5540 5730
2172
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table IX. Viscosity, Weight, and Number Average Degrees of Polymerization and Relative Nonuniformity of Nitrated Cotton, Flax, and Ramie Celluloses Material
Cotton
Flax Ra+
D.P.u 4720 4060 5740
D.P.w 4320 4050 4790
D.P.n 3630 3310 4620
U
0.19
0.23 0.04
Vol. 47, No. 10
(4) Conrad, C. M., IND. ENG.CHEM.,45, 211 (1953). (5) Cragg, L. H., and Hammerschlag, H., Chem. Revs., 39, 79 (1946). (6) Doty, P., and Spurlin, H. M.,in Ott, E., and Spurlin, H. &I., “Cellulose and Cellulose Derivatives,” 2nd ed., part 111, X-D, Interscience, New York, 1955. (7) Emery, C., and Cohen, W. E., Australian J. Appl. Sci., 2 , 473 (1951). (8) Golowa, 0. P., and Iwanow, W. J., “Ueber das Molekulargewicht der Cellulose,’’Akademie Verlag, Berlin, 1953. (9) G r a l h , N., “Sedimentation and Diffusion Measurements on
Cellulose and Cellulose Derivatives,” dissertation, Uppsala, crepancy was not entirely unexpected, as different methods were used for determining the intrinsic viscosities of the fractions. CONCLUSIONS
While it is believed t h a t the data presented give a reasonably correct over-all picture of the chain-length distribution of the three celluloses studied, the results should not be looked upon as anything but approximate. Some of the disturbing factors y b a b l y inherent in similar previous studies were minimized. hese factors included the effect of degradation of the nitrates in solution and the great influence exerted on the intrinsic viscosity of the fractions both by the rate of shear existing during the viscosity measurement, and by the degree of substitution. I t Neems evident, however, t h a t a more comprehensive fractionation than that applied here, involving a series of refractionations such as that outlined by Doty and Spurlin (6),would give much more reliable results. A fractionation of this kind wa8 carried out a considerable time ago by Spurlin (29) with a cellulose nitrate of low molecular weight. It seems probable that application of a similar fractionation technique, with due consideration of the difficulties mentioned above, would be able to furnish more detailed information concerning the chain-length distribution of celluloses of very high molecular weight than has so far been forthcoming. ACKNOWLEDGMENT
The author wishes to express his sincere gratitude to C. B. Purves, head of this division, for his kind interest in the present study. LITERATURE CITED
(1) .4lexander, W. A,, and Mitchell, R. L., Anal. Chem., 21, 1497 (1949). (2) Campbell, H., and Johnson, P., J . Polymer Sci., 5 , 443 (1950). (3) Cannon, M. R., and Fenske, 11. P., IND.ENG.CHEM.,ANAL. E D . ,10,297 (1938).
1944. (10) Hermans, P. H., “Physics and Chemistry of Cellulose Fibres,” pp. 117-20, Elsevier, New York, 1949. (11) Herrent, P., and Govaerts, R., J . Polymer Sci., 4, 289 (1949). (12) Heuser, E., and Jorgensen, L., T a p p i , 34, 57 (1951). (13) Huggins, M. L., J . A m . Chem. Sac., 6 4 , 2716 (1942). (14) Jorgensen, L., “Studies on the Partial Hydrolysis of Cellulose,” dissertation, Oslo, 1950. (15) Jullander, I., Arkiv Kemi, Mineral, Geol., 21A, No. 8 (1945). (16) Jurisch, I., Chem.-Ztg., 64, 269 (1940). (17) Kraemer, E. O., IND. ENG.CHEM.,30, 1200 (1938). (18) Lindsley, C. H., and Frank, M.B., Ibid., 45, 2491 (1953). (19) Martin, A. F., Division of Cellulose Chemistry, 103rd Meeting, Memphis, Tenn., 1942; T a p p i , 34,363 (1951). (20) Meyerhoff, G., Naturwissenschaffen,41, 13 (1954). (21) Mitchell, R. L., IND.ENG.CHEM.,38, 843 (1946). (22) Ibid., 45,2526 (1953). (23) Morey, D. R., and Tamblyn, J. W., J . Phys. Colloid Chem., 51,721 (1947). (24) Newman, S., Loeb, L., and Conrad, C. M., J . Polymer Sei., 10, 463 (1953). (25) Sayre, E. V., Ibid., 10,175 11953). (26) Schulz, G. V., and Marx, M., Makromol. Chem., 14, 52 (1954). (27) Schurz, J., and Immergut, E. H., J. Polymer Sei., 9, 281 (1952). (28) Scott, R. L., IND.ENG.CHEM.,45, 3532 (1953). (29) Spurlin, H. M., Ibid., 30, 538 (1938). (30) Spurlin, H. M., in Ott, E., “Cellulose and Cellulose Derivatives,” pp. 886-90, Interscience, New York, 1943. (31) Staudinger, H., Papier-Fabr., 36, 474 (1938). (32) Staudinger, H., and Feuerstein, K., Ann., 526, 72 (1936). (33) Timell, T. E., Szensk Papperstidn., 57, 777 (1954). (34) Ibid., 5 8 , l (1955). (35) Timell, T. E., unpublished results. (36) Timell, T. E., and Purves, C. B., Srenslc Papperstidn., 54, 303 (1951). (37) Wannow, H. A., and Thormann, F., Rolloid-Z., 112, 94 (1949). RECEIVED for review January 17, 1965. ACCEPTEDApril 1, 1955. Division of Cellulose Chemistry, 127th Meeting ACS, Cincinnati, Ohio, 1955.
Effect of Swelling and Supermolecular Structure on Reaction of Cellulose with Nitrogen Dioxide W. E. ROSEVEARE’ AND D. W. SPAULDING Textile Fibers Department, E. Z. d u Pont de Nemours & Co., Znc., Richmond, Vu.
UCH work has been done to show how the-crystalline and amorphous portions of cellulose affect the hydrolytic reactivity of cellulose, but there is little information about how the supermolecular structure affects oxidation. Davidson (1) has shown that as oxidation with chromic acid progresses there is scarcely any effect on the x-ray pattern, but oxidation with periodic acid makes the pattern diffuse. The chromic acid acts very largely on the amorphous regions, while periodic acid acts on the Crystalline regions as well. Completely dry cellulose, the amorphous regions of which are in the glassy state ( 8 ) , is impermeable to small molecules like Preeent address, Research Laboratory, E. I. du Pont de Nemours & Co., Inc., Kinaton, N. C . 1
oxygen and acetic anhydride. I n bhis dry state these materials react initially with only surfaces of the fiber and the reacting layer moves very slowly into the fiber. Swelling or removal of the reacted material even without swelling the unreacted cellulose greatly accelerates the movement of the reacting layer through the fiber. I n contrast t o this topochemical behavior, cellulose in solution reacts homogeneously. I n between the dry and solution states the reaction behavior should depend on the degree t o which the oxidizing agent can penetrate into the amorphous and crystalline regions. Kenyon and others ( 2 ) showed t h a t high concentrations of nitrogen dioxide with various other liquids dissolved cellulose ; therefore, it appeared probable that the swelling and degree of penetration of nitrogen dioxide into cellulose and the resulting
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