Recent Work on
Molecular Weight of Cellulose Osmotic molecular weights and intrinsic viscosity of cellulose nitrates and cellulose nitrate fractions in several solvents have been systematically studied. By combination of these results with data on the intrinsic viscosity of the primary celluloses in cupriethylenediamine, a relation is given which permits calculation of the degree of polymerization of cellulose from intrinsic viscosity in cupriethylenediamine. Direct osmotic pressure measurements of solutions of cellulose in cupriethylenediamine were attempted, using polyvinylbutyral and sintered polytrifluorochloroethylene membranes. The shear dependence of the intrinsic viscosity of cellulose nitrates in acetone was studied and the results are presented and discussed.
E. H. IMMERGUT, B. G. R ~ N B Y AND ~ , H. F. MARK Institute of Polymer Research, Polytechnic Institute of Brooklyn, Brooklyn, N . Y .
T"
work reported here was undertaken in order to provide a rapid means of classifying commercial cellulose samples by establishing a relationship between intrinsic viscosity and molecular weight. Fifty different cellulose sources were used, such as textile rayon, tire cord, wood pulp, hydrolyzed cotton linters, and some special chemical cottons of high degree of polymerization. The molecular weights ranged from 27,000 to 900,000, corresponding to a degree of polymerization range from 90 to 3000. With most polymers direct determination of molecular weight by osmotic pressure, light scattering, or sedimentation entails no great difficulties With cellulose such direct measurements have not yet been possible, because the osmotic membranes which usually consist of cellulosic materials are too easily attacked by the solvent used in making up the cellulose solutions; light scattering, sedimentation, and diffusion are not easy to apply with cellulose solutions and require expensive equipment. For many industrial purposes it is desirable to have a rapid method for determining the molecular weight of cellulose materials or determining some property closely related to it. Such a property is the intrinsic viscosity, [?I, which is related t o the molecular weight by a rather simple formula, first suggested by Staudinger in the form
[?I
=
KmM
(1)
and later amended by Houwink, Kuhn, and Mark to
i*
[?I = K M M ~ (2) Intrinsic viscosities can be determined, relatively easily, but must be calibrated with known molecular weights with theaidof a soluble cellulose derivative. ii convenient derivative is cellulose nitrate, which in acetone readily lends itself to determinations of osmotic molecular weight. It has been shown (416)that under mild conditions there is little or no degradation during nitration and the degree of polymerization of the nitrate is essentially the same as that of the original cellulose. This, then, permits the establishment of a relation between nitrate degrees of polymerization as determined by osmotic pressure and intrinsic viscosities of the primary celluloses as determined by viscosity measurements in cupriethylenediamine (CED). In this report such a relationship is established over a degree of polymerization range from 90 to 3000. Viscosities of the primary celluloses were measured in cupriethylenediamine and those of I Present address, Institute of Physical Chemistry, Uppsala Univ., Uppsala, Sweden.
November 1953
the nitrates in acetone, ethyl acetate, and ethyl lactate; the OSmotic pressure measurements with the nitrates were carried out in acetone. It was found that for all samples investigated the relationship between intrinsic viscosity and molecular weight may be expressed by Equation 2, where constants KMand a depend on the solvent used for the nitrates. The exponent, a, is rather close t o unity for all three solvents and therefore the Staudinger relationship may be used without great error. The viscosities of the nitrates are highest in ethyl acetate and the k' values are highest in acetone. Exponent a, which is related t o solvent power, was found to be highest for ethyl acetate and lowest for acetone. The slopes of the curves of reduced osmotic pressure us. concentration are approximately inversely proportional to the molecular weight, as was found also by Munster ( 1 8 ) for cellulose nitrates and by Bawn ( 4 ) for polystyrene. For approximate calculation the Staudinger constants listed in Table I are recommended.
TABLE I. STAUDINOER CONSTANTS Solvent D P Range kn 8.07 x 10-3 CED Up t o 300 6 . 4 x 10-3 CED 300-3000 11.4 X 10-8 Acetone Up t o 600 9.1 x 10-3 Acetone 600-3000 kn = constant in Staudinger equation, [ q ] = k m ( D P ) . Kq = I/km: D P Kq[qI.
Krl 124
156 88
110
NITRATION
All samples were nitrated by R. L. Mitchell and his group according to the method originally developed by Staudinger and Mohr (16)and later improved by Mitchell and coworkers (1). The nitrogen content of the individual samples was between 13.74 and 13.98%. OSMOTIC MEASUREMENTS OF NITRATES
Zimm-Myerson osmometers ( 1 8 ) were used with gel cellophane membranes supplied by the Sylvania Co., Fredericksburg, Va. The membranes were conditioned for acetone by immersing them for about 2 hours, first in a 50:50 mixture of water and ethyl alcohol, then in pure ethyl alcohol, then in an ethyl alcohol-acetone mixture, and finally in pure acetone. It is advisable to leave the membranes in the final solvent for a t least a week before using them or else keep them clamped in the osmom-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2483
Nitrate Sample
-
-
?E ! us. c slope
25.00 c. Du Pout
%
...
Mn osm, 25.0' C. 27,000
90
0.33
25.00 c. 1.22
0.2
K' 0.16
Unfract.
...
34,000
115
0.28
1.4;
1.oo
0.48
Fraction (ppt.)
... ...
35,000
120
0.31
1.29
0.98
0.59
...
54,000
180
0.28
2.32
2.7
0.50
2.19
AIS 42
Recov. aged alkali cellulose cotton linters Recov. aged alkali cellulose cotton linters Tire cord Rayon
Unfract. Unfract.
13.92
78,000 80,000
260 265
0:31
2.95 3.00
3.33 7.0
0.38 0.78
...
AIS 37 &IS 63 hlS 65 &IS 83 RIF 66
Tire cord Rayon Rayon Tire cord Tire cord
Fraction (soln.) Unfract. Unfract. Fraction (ppt.) Unfract.
13:74 13.88
90,000 90,000
300 300 335 460
0.12
3.15 3.58 3.65 4.70 5,63
7.7 8 01 7.7 11.15 12.45
AIS 47
Recov. aged alkali cellulose cotton linters Viscose cotton linters K o o d pulp Wood pulp 91.5a wood pulp Wood D U ~ D 94.5a-wdod pulp Cotton linters Chemical cotton
P a r t . fract.
0.28
6.35
23.0
0.57
6.81 7.10 7.14 8.04 8.65 9.00 8.89 9.6
li.2 27.8 22.8 33.0 46.0 51.0
0.37 0.55 0.45 0.51
7.51 11.2 9.06
56 1
0.61
9.25 9.93
9.4 10.4 9.78
54.5 63.0 53.2
0.62
SO.
AIS 45 116 6 1
11s 82 RIS 46
31s 54
LIS 52 ars 84 AIS AIS LIS LIS hIS
67
51 38 50 39 AIS 43
Cellulose Source Recov. aged alkali cellulose wood pulp Rayon
Fractionation Part. frac,t.
P a r t . fract.
...
Unfract. Fraction (ppt.) Unfract. Unfract. Fraction (soln.) Unfract. Fraction (soln.) Unfract.
100,000
D P osni
13:92
137,000 140,000
..,
165,000
550
13.98
190,000 194,000 207,000 240,000 260,000 270,000 270,000 290,000
635 690 800 865 900 900 965
13:90
13.88 13:93
...
...
465
650
191,
P
0.18
..
0:25
0:is 0.19 0126
0:34 0.19
50.0
[VI
1.04
1.42 Herc. (1.54)
...
0.78 3.44 0.63 4.04 0.58 4.23 0.51 0.39 Herc.5.82 (5.62)
...
0.62 0.63 0.63
6.59
.. .. .. ...
.. .. ..
RIS 49 AIS 68
99 a wood pulp
Fraction (ppt.) Unf rac t . Unf ract
.
13:96
...
292,000 300,000 300,000
975 1000
0.32
Wood pulp
1000
0:29
ars 89
High D P wood pulp Acetylation cotton linters Acetylation linters
Fraction (soln.) Unfract. Fraction (ppt.)
13.96 13.98 13.95
375,000 430,000 523,000
1220 1430 1745
0:33
12.7 12.9 16.3
92.9 112 199
0.67 0.75
... ... ...
Cotton linters Cotton lintera Chemical cotton Chemical cotton
Fraction (soin.) Fraction (soh.) Unfract. Unfract.
13:9s 13:85
530,000 546,000 650,000 700,000
1765 1830 1940 2330
0.42 0.31 0.18 0.23
15.9 16.1 20.4 21 .o
161 175.4 261 .o 285
0.64 0.68 0.60
17.6
A I S 85
X I S 53
XIS 86
AIS 40 AIS 90
115 44 hI8 91
Cotton linters
Nitrogen,
0.28
0.58 0.56
0.68
0.65
AIS 87
Cotton linters
Fraction (ppt.)
13.90
710,000
2365
0.30
21.2
299
0.67
118 88 J I S 48
Unbleached cotton linters Unbleached cotton linters
Fraction (ppt.) Cnfract.
...
810,000
2700 3335
0.31
25.3 30.0
349
0.55
...
1,000,000
..
..
..
10.2 Herc. (10.4)
16.5
... ... ... .... ..
eter submeiged in acetone and with acetone inside the cell until there is hardly any difference (=t0.03em.) between the menisci in the solution and reference capillary. In order to be certain that this difference is actually caused by the membranes, due to strains or perhaps even slight impurities in the membrane, it is necessary to check not only whether the two capillaries are of equal bore, but also whether the bore is the same a t different heights of the capillaries. Before the cell is filled with the polymer solution, a reading of the correction factor due to the membranes should be taken, and this correction should be added to, or subtracted from, the equilibrium osmotic height. During the measurement, the filling capillary of the osmometer is sealed u ith a stainless steel rod, which should fit tightly. The rod also permits adjustment of the meniscus in the solution capillary; by moving up or down, the solution meniscus is lowered or raised correspondingly. I n this nay, if the approximate equilibrium height is known, one can approach the final value in a rapid manner by adjusting the meniscus above and below the approximate osmotic height, thus approaching the actual value from each side. This method is especially valuable where a relatively large amount of diffusible material is present and it is necessary to obtain the equilibrium value as quickly as possible. Solutions having concentrations of 0.1, 0.2, 0.3, and 0.4 gram of polymer per 100 ml. of solution were used for all samples except MS 87,88, 89,90, and 91 where the viscosity of 0.3 or 0.4 gram per 100 ml. was so high that it became impossible to fill and empty the osmometer, thus necessitating the use of lower concentrations. I n Figure 1 a few typical curves of T / C us. c are shown for cellulose nitrates of different molecular neights.
+
0
2484
0.1
0.9 0.3 e. GRAM/IOO ML,
0.4
Figure 1. Relation between Reduced Osmotic Pressure and Concentration of Cellulose Nitrate i n Acetone
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 19
Cellulose TABLE 11 (Continued) Ethyl Acetate Ethyl Lactate C. [ V I 25.0' C. B.P.I. du Pont
[VI 25.0'
4)
-
-
-
CED
118. C
1.35
D P Ray0 ( K v 75j( 101
1.65
1.62
212
...
...
1.56
117
2.86
2.96
3.26
3.22
242
3.97 3.55
... 4.04
4.47
...
4.50 4.5
338 335
1:25
0:is
...
5.07
380 375 377 455 545
1.96
0.37
1164
0:33
2.32
2:41
0:33
..
..
.. ..
...
3:29
0:'34
5:20
0 : 38
4:31 4.95
0:34 0.33
..
..
..
4:4l
o:i2
5:64
..
..
..
1.19
1.27
BPI 1.38
1.59
...
4.39 4.44 4.70
Hero. (4.22)
4.46
C., Rayonier
191 20.0:
...
... .... .
5.10 5.32
...
...
5.02 6.07 7.28
8.15
7.80
...
9.27
695
(13.1) 9.49 9.77 14.6 15.50 15.6 11.55 15.0
980 712 735 1090 1160 1170 1165 1125
13.67 16.1 13.75
1025 1205 1030
25.00 25.3 25.53
1875 1895 1915
25.85 28.10 31.40 33.45
1940 2105 2355 2510
33.73
2530
46.40 50.00
3480 3750
6.31
c
[ V I 25.0"C.,
8.13 ... 9.52
...
11.0 12.4 11.25 10.85
...
12.9 11.3
...
... ... ... ... 11.7 ...
12.3 11.80 Hem. (12.05)
..
ii:i
...
... ... ...
14.0
19.7
...
22.0
... 20.0
... ... ...
...
... ... ...
(13.1)
...
10.2 14.7 15.8 15.65
... ...
...
16.1 18.6 26:35 22.0-22.1 Du Pout
...
...
22.iL22.7 Du Pont
27.0
Du Pont
... ...
5.0
slope
..
November 1953
Cellulose
Mn
DPosm
0.82
14,600
1.12
1.01
18,600
1.07
.. ..
1 .oo
..
..
1.05
19,500
0.98
..
1.68
29,000
1.34
2:is
2.18 2.24
42,000 43,000
1.30 1.25
3:ZO
2.41 2.34 2.30 2.85 3.16
49 ,000 49,000 54,000 74,000 76,000
1.27 1.25 1.13 0.99 1.17
..
4.02
89 ,000
1.26
5.48 3.90 3.89 6.13 6.64 6.34 6.67 6.17
103,000 105,000 112,000 130,000 140,000 146,000 146,000 156,000
1.55 1.10 1.07 1.36 1.34 1.30 1.30 1.16
5.70 6.62 5.57
158,000 162,000 162,000
1 .05
10.75 9.68 10.95
203 ,000 232,000 283,000
1.49 1.33 1.10
12.35 11.77 13.45 14.35
286,000 296,000 314,000 378,000
1.10 1.16 1.21 1.08
14.50
384,000
1.07
19.90 21.4
438,000 541 ,000
1.29 1.13
.. ..
.. 8175
Viseosity measurements were carried out in acetone, ethyl ace-
Dilution Viscometer
..
171
Rayodier
0.35
tate, and ethyl lactate using dilution viscometers as directed by Alfrey, Goldberg, and Price ( 2 , 7).
Figure 2.
..
Du'knt
0.555
VISCOSITY DETERMINATIONS
r'
k'
*.
.. ....
..
.. .. *.
0:34
.. ..
..
2:is
3:94
.. .. ..
6:28
.. .. 12.0
..
.... .. .. ..
1.20 1.03
The open ends of these viscometers should be covered by a cotton-packed calcium chloride tube or closed b y stopcocks as shown in Figure 2. After each addition of solvent, a gentle current of nitrogen is bubbled through the mixture to assure coniplete mixing. With a volatile solvent, such as acetone, care must be exercised during the mixing operation as well as in dran-ing up the solution into the upper bulb of the viscometer, in order t o avoid concentration changes due t o evaporation. The reduced viscosity, ~ ~ ~was / cplotted , against c on semilog paper, as shown in Figure 3. Straight lines were obtained in all cases, which permitted a reasonable determination of the intrinsic viscosities. The values obtained for the various samples in acetone, ethyl acetate, and ethyl lactate are shown in Table 11. The k' values for all samples were calculated and are also given in Table 11; they are fairly constant, but a slight variation in nitrogen content causes a considerable change i n k ' without noticeably affecting the intrinsic viscosity. For example, MS 86 rrith 13.95% nitrogen has a k' value of 0.75, whereas MS 87 with 13.90oJ,nitrogen shows a value of 0.670. I n the course of this investigation, a few observations were made on the decomposition of the samples under various experimental conditions. It is well known that cellulose nitrate slowly degrades if kept dissolved in various solvents (6). This is explained by assuming that nitric acid is generated in small quantities, which causes a slow hydrolysis of the glucosidic bonds and leads t o a decrease of the average degree of polymerization. When the solutions were kept meticulously dry, in the dark, under nitrogen, and at low temperatures, this degradation was, in general, so slow that it did not interfere with the measurements reported in this investigation. I n a few cases, however, distinct deterioration of the solutions was observed after several weeks or months. It appears t h a t certain nitrates were more stable than others; some solutions after having been stored for several weeks showed no measurable changes in their osmotic and viscometric
INDUSTRIAL AND ENGINEERING CHEMISTRY
2485
by Equation 2, which correlates [?I]and JG,,for all samples of this investigation. Some samples degraded when kept in the dry state for a sufficiently long time, and [77] and iv, decreased , in such a manner t h a t the new values remained on the [v] = K,wM~line. On the basis of experience it seems best to store cellulose nitrates in polyethylene bottles, wetted out with absolute alcohol under nitrogen in the absence of moisture a t temperatures around 0" C.
50
COB.lBINATION O F OSMOTIC AND VISCOMETRIC MEA SUR EM ENT S
PO
Plotting the logarithms of the intrinsic viscosity (essentially a weight average) in a given solvent against the logarithms of the osmotic (number average) molecular weight there are obtained the graphs shown in Figures 4, 5, and 6. Such a procedure is justified by assuming that the ratio between weight-average and number-average molecular weight is fairly constant for all samples. Considering first the viscosity measurements in acetone, there are three different series of data:
tu 10
1. I
5
Measurements carried out i n the Research Laboratories, Rayonier Corp., Shelton, Wash. hleasurements carried out in the Viscose Rayon Research Laboratories] E. I. du Pont de Nemours & Co., Richmond] Va. Measurements carried out a t the Polytechnic Institute of Brooklyn.
I
0
Figure 3.
0.1 25
0.25 c, GRAM/I00 ML.
0.5
Typical Viscosity Curves for Cellulose Nitrates i n Acetone
behavior; others, which were kept in the same manner for the same time, showed marked changes in viscosity and osmotic pressure. This may be due to the leaching out of metal ions from the glass bottles by the cellulose nitrate solutions. Although, the authors did not attempt to study this phenomenon in a systematic manner, they remeasured both viscosity and osmotic pressure of the degraded sample and found that both decreased in such a manner that the new [v] --Zn point remained on the curve given
Table I1 contains the intrinsic viscosities, determined independently by these cooperating institutions, and demonstrates that the agreement is, in general, satisfactory, although in a few cases, such as MS 47 and &IS 49, the values deviate more than legitimate experimental errors would permit. Yet, if all available intrinsic viscosities are plotted against the osmotic molecular weights on a log-log scale, the points of all three sources are arranged on a relatively narrow straight band, which does not show any noticeable significant curvature over the whole molecular weight range and, therefore, permits with fair accuracy the determination of the numerical values of K,Mand a in Equation 2. They are for viscosity measurements in acetone a t 23' C.
K.,~= 1.1 x 10-4
(2a)
a = 0.91
and can be used to convert intrinsic viscosities of a cellulose nitrate of 13.74 t o 13.98% nitrogen into number-average molecular weights over the range of 23,000 to 1,000,000. Khereas the viscosities are contributed by three independent
-
ivn
-
.un
LOG
13
IO
Figure 4. Relation between Intrinsic Viscosity a n d Osmotic Molecular Weight 2486
Figure 5 .
Relation between Intrinsic Viscosity and Osmotic Molecular Weight
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
Cellulose series of measurements, the osmotic molecular weights were all carried out a t the Polytechnic Institute of Brooklyn. It therefore appeared imperative to have independent checks of these data, in order to eliminate the possible errors of a single observer. Samples were therefore selected and submitted to other laboratories in order t o obtain independent checks. J. B. Nichols, Chemical Department, E. I. du Pont de Nemours & Co., J. H. Elliott, Hercules Powder Co., and P. H. Frank, University of Vienna, kindly agreed to carry out these checks, each with three different samples, selected so that all nine check samples covered in a reasonable manner the whole molecular weight range. The check values and the original data were satisfactorily within the limits of experimental error of osmotic measurements of this type and showed no conspicuous systematic deviation. As a consequence, the osmotic data were taken as a good first approximation for the establishment of the scale on the abscissas of the log-log plots versus the viscosities in the different solvents. Table I11 contains some other [SI and M , data from the literature (3, IO,11, 19, 16,16)for cellulose nitrates in acetone and Figure 4 shows how these independent measurements compare with the authors' eamples. Figure 5 shows the logarithms of the intrinsic viscosity in ethyl acetate plotted against the logarithms of the osmotic molecular weights. The data on the abscissa are identical with those of Figure 4,whereas the viscosities consist essentially of two series of measurements, one carried out in the Research Laboratory, Rayoitier Corp., Shelton, Wash., the other a t the Polytechnic Institute of Brooklyn. Table I1 contains the values of the intrinsic viscosities for all samples as determined by these two laboratories and others which were determined in the Viscose Rayon Research Laboratory of E. I. du Pont de Nemours & Co. The values show reasonable agreement with each other, although in a few cases the data deviate somewhat more than would be expected. Particuhrly the s:Lmples of highest molecular weight show definitely more scattering than the corresponding figures in acetone. Nevertheless, the log-log plot of Figure 5 represents a fair linear band ove? the whole range of molecular weights and permits a reasonable determination of K M and a for ethyl acetate as solvent. The values obtained are:
K~~ = 0.38 x 10-4 a = 1.03
(2b)
A series of corresponding viscosity measurements was also carried out in ethyl lactate; one in the Du Pont laboratories and the other a t the Polytechnic Institute of Brooklyn. Again a few independent checks were kindly carried out by A. F. Martin, Her-
E. 0. Kraemera ( I f ) I
I1 I11
Badger 5-3, 4 8-1, 1-4 P-3 2 P-4:2
34,000 46,700 53,300
1.40 2.00 2.20
and Blaker (8) 40,900 1.32 64,700 2.24 128.200 4.37 216,200 6.84
Staudinger and coworkers (16, 16)
a
Recalculated values.
Hut Lnt FK 6 FK 9
I. Jullander (10) 2.41 60,000 2.82 86,000 5.07 133,000 444,000 15.0
Husemann and Sohulz (IS) 30,000 1.2 37,000 1.6 65,000 2.3 100,000 3.9 142,000 5.5 160,000 6.0 215,000 8.6 295,000 11.3 380,000 13.7 420,000 11.6 415,000 15.0 415,000 14.8
cules Powder Co., and the same osmotic data were used as previously. The intrinsic viscosities are given in Table 11; the loglog plot is shown in Figure 6. With the exception of a few high ,molecular weight samples, all points are satisfactorily arranged along a straight band, which shows some random scattering but no significant systematic deviation. The values for K.w and a in ethyl lactate a t 25' C. are:
K , =~ 1.22 x 10-4 a = 0.92 All three exponents a are relatively close to unity and the viscosity-average molecular weights of cellulose nitrates in the three solvents are therefore essentially weight averages. CORRELATION WITH VISCOSITIES OF PRIMARY CELLULOSES
The cellulose nitrates studied in this investigation were with a few exceptions directly prepared by mild nitration from cellulose samples, the intrinsic viscosities of which in cupriethylenediamine were well known. Plotting then the logarithms of these intrinsic viscosities against the logarithms of the molecular weights of the corresponding cellulose, one obtains a relationship which makes i t possible to convert cupriethylenediamine intrinsic viscosities into cellulose degree of polymerization over the range between DP = about 100 and DP = about 3000 (Figure 7). The cupriethylenediamine viscosities were measured independently in the Rayonier and Du Pont laboratories and are shown
I
05
I
LEGEND
m
Figure 6.
TABLE 111. OSMOTIC MOLECULAR WEIGHT-VISCOSITYVALUES OF OTHERINVESTIQATORS Sample 2% In1 Sample 2% h1
HERCULES DUPONT
Relation between Intrinsic Viscosity and Osmotic Molecular Weight
November 1953
Figure 7.
Cupriethylenediamine [q] us. Cellulose
INDUSTRIAL AND ENGINEERING CHEMISTRY
li?, 2487
in Table 11. The agreement is excellent; a few check values determined at the Polytechnic Institute of Brooklyn fall very close to the virtually identical values of D u Pont and Rayonier. The log-log plot shown in Figure 7 represents over the whole range a fair straight line and leads to K,v = 1.33 X 10-4
(2d)
a = 0.905 These values are recommended in converting cupriethylenediamine intrinsic viscosities into cellulose ils a is rather close t o unity, the relation
an.
K.wMa
[q] =
can be interpolated with reasonable accuracy by the Staudinger relation [ a ] = KmM 1771
=
k,(DP)
or (3)
I
