Molecular Weights of Celluloses ELMER 0. KRAEMER E. I. d u Pont de Nemours & Company, Inc., Wilmington, Del.
to whether the particles or kinetic units are single molecules or not. The first phase can be satisfactorily elucidated with the ultracentrifuge; the second phase requires information of another sort and is admittedly much more difficult to handle in an entirely rigorous manner.
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N SPITE of a great deal of attention which the subject
has received, there is little agreement among investigators as to the molecular weights of cellulose and cellulose derivatives or the state of dispersion of these materials in the solution. Hess and his school (9, 17) still maintain that cellulosic materials dissociate in suitable solvents and concentrations into small units, even so far as units of one, two, or four glucose residues. Haworth and his co-workers ( 1 , 7 )defend the end group method of determining the molecular weights of polysaccharides (recently criticized by Hess, S), and explain the discrepancy between their values and the much higher ones obtained with several other methods by the assumption that association into large aggregates occurs in solution. Unfortunately, neither of these schools has made a practice of reporting viscosities, so that other workers are left in the dark as to the degree of degradation of their products. The McBains (14) also feel that cellulosic materials are generally aggregated in solution, and they consider the exceptionally high viscosity of dilute solutions t o be due largely t o the loose, bulky character of the aggregates. Staudinger, on the other hand, has continued to defend his viscosity method for determining molecular weights, but recently (16)he changed his constant of proportionality and increased his molecular weights of cellulose in cuprammonium by 100 per cent, thus removing a large part of the discrepancy that formerly existed between his conclusions and those of the writer. Unfortunately a detailed quantitative comparison of Staudinger’s results and those of the writer cannot be made, owing to the unsuitability of his type of cuprammonium (a solution of precipitated and dried basic copper sulfate in ammonium hydroxide) for ultracentrifugal work. Parr and her co-workers (2, 6) go so far as t o suggest that cellulose is practically insoluble in cuprammonium, dispersing in this solvent only to particles 1-1.5 microns in size, and imply that these particles retain their identity in solutions of cellulose derivatives. By means of the ultracentrifuge the conclusion has been .reached that the molecular weights of celluloses and certain derivatives may be very large, even up t o 3000-3500 glucose residues per molecule. Results have also been obtained supporting qualitatively, but not quantitatively, Staudinger’s conclusions relating viscosity characteristics and molecular weights. I n view of this situation, therefore, i t is not expected that this paper will remove all discrepancies or will be universally accepted. Neither will i t be possible to correlate the present results in detail with those of other workers. The principal purpose of the paper is to outline the experimental findings and to describe briefly the conclusion to which they lead. The method for investigating molecular weights, like most methods with a satisfactory theoretical foundation, depends upon the use of solutions. Therefore, two principal aspects are to be considered: (a) The determination of the sizes of the particles or kinetic units in solutions of celluloses or cellulose derivatives, and (b) the consideration of the question as
Determination of Kinetic Unit Size in Cellulosic Solutions For solutions containing large particles, as cellulosic solutions undoubtedly do, the most satisfactory general means for determining particle size experimentally is by the sedimentation-equilibrium method in the ultracentrifuge. This method possesses the following advantages: 1. It has the same thermodynamic foundation as osmotic pressure or vapor pressure methods (16). 2. It is accordingly not influenced by particle shape. 3. In general it is not affected by solvation (13). 4. Its sensitivity increases with increase in particle size. 5. It can be used with complex solvents like cuprammonium, with which osmotic pressure measurements would be very difficult. 6. It avoids difficulties associated with the use of semipermeable membranes. 7. It permits recognition of the uniformity or nonuniformity of particle size, and it can give a quantitative rating of the degree of nonuniformity (12). 8. For solutes containing relatively small molecular weight contaminants, it is much less adversely affected than osmotic pressure and other methods.
1200
The comparison of the intrinsic viscosities and molecular weights of celluloses and cellulose derivatives, determined with the Svedberg ultracentrifuge, demonstrates that a simple relation exists between the two when suitable solvents are used. The different viscosity characteristics of various technical cellulosic products represent different degrees of degradation of the native cellulose chain, the range in chain length being about 1 to 10. Observations of sedimentation velocity in the high-speed ultracentrifuge show that cellulosic materials are not homogeneous in molecular weight and that the heterogeneity is not of the stepwise or discontinuous character as believed by some investigators. Cyclic conversion of cellulose to cellulose acetate and measurement of the molecular weights and intrinsic viscosities of the products provide very definite evidence that the units in solution are truly molecules in the chemist’s sense of the term.
and Cellulose Derivatives - - ments. As we have pointed out, these constants are pro portional to the reciprocal specific volumes ( 1 1 ) . The following table gives representative values:
With the slow-speed Svedberg ultracentrifuge the particle sizes in solutions of various cellulosic materials have been determined b y the sedimentation-equilibrium method; the results obtained are outlined in Table I. As will be shown below, the particle sizes in these particular cases are identical with the molecular weights. I n addition to the molecular weights, the tables also present data Gn the intrinsic viscosities [v] of the solutions. The intrinsic viscosity is defined by the equation:
where qr c
= =
Native cellulose Purified cotton linters
Wood o d n s
Commercral regenerated celluloses Farr and Eckerson hydrocellulose Beta-celluloses Gamma-cekluloses Dynamite nitrocellulose Plastics nitrocellulose I/s-sec. nitrocellulose Commercial cellulose acetates
viscosity of the‘solution, relative to that of the solvent concentration, grams solute per 100 cc. of solution
measured a t a concentration which gives it a value not greater than 1.2-1.3. TABLEI. MOLECULAR WEIGHTS AND INTRINSIC VISCOSITIES
The consistency of the above results, especially of the values for D. P./[q], may not appearverygood, but they should be considered in the light of the accuracy and consistency attainable by osmotic pressure and other methods. The inconsistencies are partially due to the fact that in most cases the complete calculations of true weight-average or Z-average molecular weights were not carried out, owing to their laborious character. With materials as nonuniform as most cellulose derivatives, the short-cut method of averaging, which was employed, is not entirely free from objection. The above data support Staudinger’s conclusions that an amroximatelv linear relation exists between molecular weight a6d the intriisic viscosity [v]. Using the following valueslfor
D. P./hl, Cellulose in cupsammonium sec-Cellulose acetate i n acetone Nitrocellulose in acetone
260 230 270
the average molecular weights (strictly speaking, the weight-average molecular weights) of numerous cellulosic materials were estimated from intrinsic viscosity measure-
>3500 1000-3000 600-1000
>570,000 150,000-500,000 90,000-150,000 30,000-90,000 40,000 3,000-15,000
