Fractionation of Cellulose by Summative Method SYDNEY COPPICK, 0. A. BATTISTA, AND M. R. LYTTON American Viscose Corporation, Marcus Hook, Pa. Conventional stepwise precipitation or solution procedures for the measurement of the chain length distribution of cellulose are time-consuming and require the use of a polymer-solvent system that is relatively inert or stable, In the case of cellulose all known solvents effect a chemical breakdown of the cellulose chains to some extent. I t is for this reason that the fractionation of cellulose is usually performed on the cellulose nitrate derivative, using acetone as the stable solvent medium. The authors used a summative method for the fractionation of cellulose, whereby the time of polymer-solvent contact is held to a minimum. In this manner, attendant degradation is substantially reduced, and the accumulation of degradation products precluded. In practice, the summative method requires the use of a fresh aliquot of dissolved polymer to obtain each fraction. Summative cuts on these aliquots are made by means of fractional precipitation, and the fractions are recovered from the supernatant mother liquor after centrifuging. The sum-
A
THOROUGH review of the literature up t o 1945 on the fractionation of high polymers, which includes cellulose and cellulose derivatives, has been prepared by Cragg and Hammerschlag (4). This paper considers primarily the literature on the fractionation of cellulose, especially as it relates to the authors’ objectives. As reviewed by Purves (16),the main difficulties encountered in the fractionation of cellulose arise from the fact that all solvents for cellulose effect some chemical breakdown of the cellulosp chains. The fractional precipitation of cellulose from a zinc oxidesodium hydroxide solvent using sodium sulfate as the precipit a n t was carried out by Tyden (61). Strauss and Levy (19) employed cupriethylenediamine as the solvent and 8 N sulfuric acid as the precipitant. Battista and Sisson (6)fractionated cellulose on a micro scale from cuprammonium solvent a t low temperatures. Under their conditions they found that the ability t o precipitate fractions on a chain length basis depended on the solvent-precipitant combination used. Their findings supported the so-called reverse order precipitation phenomena of Morey and Tamblyn (14)with cellulose derivative systems wherein short-chain material may precipitate from solution before longer chain material. Neumann, Obogi, and Rogovin (16)used sodium hydroxide as the solvent, and recovered fractions by varying the temperature. They considered, also, fractional solution using cuprammonium with varying concentrations of copper ion. A comprehensive study was carried out by Kumichel ( 1 1 ) in which he considered in great detail the fractionation of cellulose using cuprammonium solvent with increasing copper ion concent ration. Dolmetsch and Reinecke ( 7 , 8) investigated a 10% sodium hydroxide-solvent system making temperature their fractionation variable. They, as well as Schieber (17, IS), came t o the conclusion that under the conditions used fractional solution of cellulose by means of sodium hydroxide was unsatisfactory; the long chains are too insoluble and there is always an attenda n t danger of degradation. They did not consider the possi-
mative distribution curve is obtained by plotting the per cent of material not precipitated against the corresponding weight average degree of polymerization (D.P.). The mathematical interpretation of summative data is explained, and the conversion of summative distribution data to more common integral and differential distribution curves is illustrated. Although the merits of the summative method for the determination of the chain length distribution of cellulose are emphasized in this paper, it is a procedure that lends itself to the fractional analysis of any high polymer-solvent system. The fractional analysis of the chain length distributions of a typical viscose rayon, of rayons produced from pulps of known distribution,. of rayons made from alkali cellulose crumbs aged for varying times, and of various rayon pulps have been carried out by the summative method using cold 8% sodium hydroxide as the solvent. Characteristic curves showing the weight average differential distributions for these samples are shown.
bility of solubilizing the long chains by first putting them into solution and regenerating with a minimum amount of degradation. Davidson (6, 6) made a study of the fractional solution of oxycelluloses and hydiocelluloses in sodium, lithium, potas,ium, and tetramethyl ammonium hydroxides. The fractional solution of rayons and rayon pulps using phosphoric acid (73 to 83%) was investigated by Ekenstam (9). THE SUMMATIVE METHOD
In the fractionation of cellulose it is important t h a t the solute be exposed t o the solvent for as short a time as possible. The summative method of fractional analysis allows for a minimum time of contact between solvent and solute. The summative method consists of dissolving the polymer in a solvent and adding slowly a relatively large volume (one third) of partial solvent in order t h a t a small portion of the polymer will precipitate. The mixture is centrifuged and the precipitate is discarded. The polymer is quantitatively regenerated from an aliquot of the clear supernatant liquor. From the weight of the polymer obtained the weight per cent in solution is calculated. This value is represented as F ( p ) . A degree of polymerization measurement on this fraction is represented as p , the weight average or viscosity value. The procedure is repeated with a fresh sample of polymer, but this time a stronger precipitant is used in order t o precipitate more polymer. Thus, by varying the composition of the precipitant any number of values of F(p).with its companion are obtained. The plot of F ( p )against F gives the summative distribution curve. To facilitate handling, the summative fractionations should be carried out a t constant volume. The summative method has an unusual merit in that the conditions existing a t any one cut are predetermined by the nature of the sample and not by the conditions existing after other fractions have been removed. This is of considerable importance in the reproducibility of experiments. I n the conventional
2633
INDUSTRIAL AND ENGINEERING CHEMISTRY
2534
stepwise precipitation procedure conditions of partial precipitittion exist. Thus, material above a certain molecular weight may be almost totally precipitated whereas the components below this molecular weight are distributed between the two phases. No matter what the molecular weight of a particular species is, it will be more concentrated in the precipitate than in the supernatant liquid. Furthermore, the distribution coefficient will favor the precipitated phase t o a greater extent as the niolecular weight rises, and/or the amount of material precipitated increases. The amount of material precipitated up to a given point in a regular stepwise fractionation will, therefore, be a complicated function of the number and weight of the various fractions that have been obtained. Beall (3) has developed the xiathematics of a satisfactory method for handling fractionation data subject to such limitations. The resolving power of the summative process may not be SO good as the resolving power of a stepwise fractionation because the low molecular weight components may be occluded with the precipitate. Nevertheless, fractional solution of the highly soluble short-chain components, from the precipitant into the supernatant liquor, will occur as conditions of equilibrium are approached during centrifuging Also, as Jplrgensen ( I O ) ha3 clearly shown, divergences between stepwise solution and s t e p wise precipitation procedures are not serious in the rang? of low molecular weights, but become extremely serious in the range of high molecular weights. , The summative method of fractional analysis described herehi has been applied primarily to the fractionation of cellulose in ~t sodium hydroxide-solvent system. However, the general procedures may be used for measurement of chain length distributions of other polymer systems where the solute may be dissolved in a n inert organic solvent such as cellulose nitrateacetone, or cellulose acetate-acetone systems. The application of the summative method of fractionation to the nitrate system has been applied by Tasman and Corey (20). The authors have worked for some time, also, on the use of the summative method of analysis for the fractionation of cellulose nitrates and cellulose acetates.
Vol. 42, No. 12
where f(,) is some function of P such that f b , dP denotes the number of all the molecules having a degree of polymeriznt ion between P and P dP. If p,,,:" is the weight average degree of polymerization for a heterogeneous sample of cellulose with varying degrees of polymerization such that P has intermediate values betweeti V I and 7 1 , then since P i s an additive function (12, I$'),
+
(3)
where A F ( , ) is a finite weight fuction of chain length equivalmt to P. If F ( p )is a continuous function of P , P = n
(4)
In the summative analysis fr:xtions of cellulose for which the degree of polymerization lies b(xtweeri zero and n are isolated. Substituting Equation I , Pa F , =
I=dF(,,P
(5)
1' = 0
Now, on differentiating with respect to P'(p) and converting to the general rather than the particular vslut:s of the functions involved,
MATHEMATICAL INTERPRETATION OF S U M M A T I V E DATA
For the mathematical interpretation of summative fractionation data, i t must be assumed that the average degree of polymerization of each summative fraction recovered from the supernatant liquid, after the precipitation and centrifuging steps, is the average of chains ranging from zero to n, where n is the degree of polymerization of the longest chains remaining & solution. In practice, i t s e e m improbable that this assumption is fully met. For example, even after several refractionations b y the stepwise precipitation procedure, it is unlikely that a fraction having a uniform degree of polymerization distribution will be obtained. If P is the degree of polymerization of a MATHEMATICAL. homogeneous fraction of cellulose, let Fcp) be some function of P such that d F b ) denotes the weight fraction having a degree of polymerization between P and P dP; then
so that if a finite .qalue of A F ( , ) is taken as small
p=- A(FF) AF
-
PiFi
- Fd'z
Fi - Fz
+
P = O
Mark ( I d , 1 3 ) shows that the function F ( = )is related t o the number distribution curve in the following manner
PO p = o F ( p ) = -p
I'
J =
25
30
35 x,
40
45
5
% Acetone
,.
Figure 1.
0
Fraction of pulp remaining in solution us. composition of acctoncwater precipitant
= co
Solubility Characteristics in Fractionation System
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
December 1950
2535.
80
I \
70
70
.
60
60.
2 50
5 50 -
loo w
90
b?
