Cellulose
ic
(18) Flory, P. J., and Fox, T. G., Jr., J . Am. Chem. SOC.,73, 1904 (1951). (19) Fox, T.G., Jr., and Flory, P. J., J . Phys. & Colloid Chem., 53, 197 (1949). (20) Fox, T.G.,Jr., Fox, J. C., and Flory, P. J., J . Am. Chsm. SOC., 73, 1901 (1951). (21) Guth, E., and Gold, O.,Phys. Revs., 53, 322 (1938). (22) Guth, E.,and Simha, R., Kolloid Z., 74, 266 (1936). 73,265(1951). (23) Hall, H. T.,and Fuoss, R. M., J . Am. Chem. SOC., (24) Haller, W.,Kolloid Z.,61,26 (1932). (25) Huggins, M. L., J . Am. Chem. SOC.,64, 2716 (1942). (26) Husemann, E.,and Schulz, G. V., Z. physik. Chem., B52, 1 (1942). (27) Immergut, E. H., and Mark, H., IND.ENO.CHEM.,45, ? (1953). (28) Immergut, E. H.,Schurz, J., and Mark, H., Monatsh., 84, 219 (1953). (29) Jeffery, G. B.,Proc. Roy. SOC.(London), A102, 163 (1923). (30)Kirkwood, J. G., and Auer, P. L., J . C h m . Phys., 19,281 (1951). (31) Kirkwood, J. G.,and Riseman, J., Ibid., 16, 565 (1948). ENO.CHEM.,30, 1200 (1938). (32) Kraemer, E. O.,IND. (33) Kuhn, H., and Kuhn, W., Helv. Chim. Acta, 28, 1533 (1945). (34) Kuhn, H.,and Kuhn, W., S. Polymer Sci., 5, 519 (1950). (35) Kuhn, H.,and Kuhn, W., Z. physik. Chem., A161, 1 (1932). (36) Kuhn, H., Moning, F., and Kuhn, W., Helv. Chim. Acta, 36, 731 (1953). (37) Kuhn, W.,HeZv. Chim. Acta, 28, 97 (1945). (38) Kuhn, W.,KoZloid Z., 62, 269 (1933); 68, 2 (1934). (39) Lindsley, C. H., J . Polymer Sci., 7, 635 (1951). (40)Mandelkern, L., and Flory, P. J., J . Am. Chem. SOC.,74, 2517 (1952). (41)Mark, H., “Der Feste Korper,” p. 103,Leipzig, Hirzel, 1938. (42) Mark, H., Monatsh., 81, 140 (1950). (43) Mark, H., and Tobolsky, A. V., “Physical Chemistry of High Polymers,” New York, Interscience Publishers, 1950.
Martin, A F., 103rd Meeting, AM. CHEM.SOC., Memphis, Tenn., April 1942. Mooney, M., J . Colloid Sci., 6, 162 (1951). Moore, W.R., J . Polymer Sci., 7, 175 (1951). Mosimann, H., Helv. Chim. Acta, 26, 369 (1943). Mlinster, A., J.Polymer Sci., 8, 633 (1952). Munster, A,, 2.physik. Chem., A197, 17 (1951). Newman, S.,J.Phys. & Colloid Chem., 54, 964 (1950). Newman, S.,and Flory, P. J., J. Polymer Sei., 10, 121 (1953). Newman, S.,Loeb, L., and Conrad, C. M.,Ibid., 10,463 (1953). Oth, A,, Bull. SOC. chim. Belges, 58, 285 (1949). Peterlin, A,, J . Polymer Sci., 5, 473 (1950). Ibid., 6, 621 (1952). Ibid., 8, 173 (1953). Peterlin, A.,and Stuart, H. A., 2. Physik, 112, 1 (1939). Riseman, J., and Kirkwood, J. G., J . Chem. Phys., 18, 527 (1950). Riseman, J., and Ullman, R., Ibid., 19, 578 (1951). Sadron, C., J . Polymer Sci., 3, 812 (1948). Schurz, J., Ibid., 10, 123 (1953). Schurz, J., and Immergut, E. H., Ibid., 9,279 (1952). Simha, R.,J. Appl. Phys., 23, 1020 (1952). Simha, R.,J. Phys. Chem., 44, 25 (1940). Simha, R., J. Research Natl. Bur. Standards, 42, 409 (1949). Singer, 5.J., J . Chem. Phys., 15, 341 (1947). Sookne, A. M., and Harris, M , IHD. ENG.CHEM.,37,475 (1945). Staudinger, H.,and Sorkin, M., Ber., 70, 1993 (1937). Tamblyn, J. W.,Morey, D. R., and Wagner, R. H., IND. ENG. CHEM.,37, 573 (1945). Vand, V., J . Phys. & Colloid Chem., 52,277 (1948). Weissberg, S. G.,Simha, R., and Rothrnan, S., J . Research. Natl. Bur. Standards, 47, 298 (1951). Wilson, J. N., J . Chem. Phys., 17,217 (1949). RECEIVED for review March 30, 1953. ACCEPTED September 17, 1953.
Molecular Weight Uniformity of Cellulose and Cellulose Derivatives RECENT DEVELOPMENT IN MEASUREMENT . There has beea continuous development in methods for measuring molecular weight uniformity. Fractionation methods continue to hold the center of the stage. Recent studies have shown superiority of precipitation fractionation over solution techniques and have emphasized the importance of proper choice of solvent and precipitant. The relative width of the sedimentation diagram and its change with change of distance from the center of rotation are used as a measure of polydispersity, which must be adjusted to zero concentration. New methods include electron microscopy, chromatographic adsorption, turbidity, streaming birefringence, coacervation fractionation, resinographie replica, and dielectric dispersion. Recent methods are concerned with quick approximations for securing the distribution curve, application of automatic or semiautomatic devices for tracing the curve, and improved mathematical techniques for expressing results.
. W
CARL M. CONRAD Southern Regional Research Laboratory, New Orleans, La.
P
OLYMOLECULARITY is a term referring t o molecular weight uniformity of polymeric substances-i.e., the diversity of molecular sizes. The molecular sizes vary more or less greatly, according to the number of “monomer,” or repeat, units contained in each species. Polymolecularity can be described by a distribution function which gives the frequency of each molecular size present in a sample. Although now well established, it was only comparatively recently that polymolecularity of high polymers was first recognized. According to Cragg and Hammerschlag (14), the idea of fractionation was first applied to rubber in about 1887, when it
November 1953
was found that part of the substance was soluble, but the remainder insoluble, in organic solvents. Apparently cellulose in the form of its nitrate was first fractionated in 1920 by Duclau.; and Wollman (19). Since then the subject has developed rapidly and i t has now become recognized that practically all polymers are variable in degree of polymerization. The techniques f i s t devised to deal with polymolecularity depended on solubility characteristics. However, these were gradually extended as new and more powerful methods were devised. until, a t present a dozen or more different methods have been suggested.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2511
POLYMOLECULARITY METHODS
SOLUTION FRACTIONATION. Jlitchell ( 3 3 ) in 1946 nitrated the cellulose of wood pulp and applied a solution fractionation technique. H e expressed the view that his methods of nitration and fractionation yielded a less degraded cellulose than had been obtained previously from wood pulp. I n the same year Hon-lett and Urquhart ( 2 7 ) studied the solution fractionation of secondary cellulose acetate ivith a mixture of acetone and n-butyl acetate. The cumulative curve of the cellulose fractions removed agreed well with that deduced by using different ratios of mixtures of the same solvents on different portions of the same cellulose acetat,e sample. Desreux ( 1 6 ) in 1948 described a technique and in 1949 ( 1 6 ) a fully automatic device for systematically dissolving the polymer by solution fract.ionation. He applied the technique to a mixture of 1 part of dry crushed cellulose nitrate, cellulose acetate, or polythene, and 5 to 10 parts of Celite w-ith a series of extractants containing increasingly greater ratioe of solvent to precipitant. For cellulose nitrate the curves from solution fractionation and precipitation fractionation I-,-ere almost. superimposed, but for the acetate they diverged considerably. Fuchs ( 2 1 )more recently applied a similar technique using very thin films of polyvinyl alcohol, polyvinyl acetate, and polyvinyl chloride. Because of the thinness of the film the fractionation could be carried out rapidly. In 1949 Stockmayer (69) investigated the proposition that the ~ r i t ~ i cor a l consolute point for solution of a heterogeneous polymer, conforming to the Flory-Huggins theory, should be determined by its number molecular weight. He came to the conclusion that on the basis of theory t'he critical point depended on both the weight- and z-average molecular weights. I n 1951 Heuser and Jorgensen (26) reported on their studies of solution fractionation of cellulose nit'rate with a solvent of mixtures of ethyl acetate and ethyl alcohol. The proportion of ethyl acet,ate was gradually increased as separation of fractions proceeded. It was found that resolution of the fractions was very poor in the upper half of the dist'ributions as compared with precipitation fractionation. Data on solution fractionation are summarized in Table I. PRECIPITATION FRACTIOXATIOS. d greater amount of study has been devoted to precipitation fractionation than to any other method for determining polymolecularity. Much of this has been directed boward evaluating the effect of various conditions and influences.
TABLEI. SUXX4RY
O F RECESTLITER.4TURE FXlCTION.4TION
Year of
Report 1946
Polymer Cellulose nitrate
1948
Cellulose nitrate
1951
Cellulose nitrate
1946
see-Cellulose acetate Cellulose acetate
1949 1951
Solvent E t h y l alcohol ethyl acetate Kater niethano1 acetone E t h y l acetate
+
-++ Acetone + n-but y l acetate Acetone + methanol
Polyvinyl alcohol 1949 Any polymer Theoretical a Taken or computed from authors' data.
ON
SOLUTION
Mean D P 425-2400
Reference 3litchell (35')
40-165
Desreux ( 1 5 )
548-1880 381 ?
1
Heuser a n d Jorgensen (26') Howlett arid Crquhart ( 2 7 ) Desreilx ( 1 6 ) Fuchs ( 2 2 ) Stockmayer (69)
The principal existing theories for precipitation fractionation were reviewed by A'Iorey and Taniblyn (58)in 1947: that of G. V. Schulz, based on the relative potential energy of a chain molecule in the solution and precipitate phase, and a thermodynamic treatment developed more or less jointly by Flory, Gee, and Huggins, in which a n activity is computed and used to predict the conditions of phase separation. The authors outlined a revised theory
2512
of their own in which precipitation is represented as a C O I ~ S R quence of opposing rates of solution and aggregation. There has long been a belief that in precipitation fractionation satisfactory resolution can be secured only in very dilute solution. However, Morey and h i associates have taken exception to this view. I n 1945 T a m b l p , llorey, and Wagner ( 6 1 ) reported that cellulose acetate butyrate in acetone did not fractionate any better in 0.05 than in 2.0% solution. Later Morey and TambIyn ( 3 7 ) reported that the effect of initial concentration on efficiency of fractionation is minor and that the use of very dilute solutions brings no advantages but offers serious difficulties. S s early as 1941 Mark ( 3 2 ) pointed out that in precipitation fractionation the choice of solvent and precipitant is consideled paramount. This was again emphasized by Blorey and Taniblyn ( 3 7 ) in 1946. Battista and Sisson ( 2 ) studied different precipitants for cellulose dissolved in cuprammonium hydroxide at about 0" C. Almost no reaolution occurred when a 5% solution of Rochelle salt was used. Acetone gave relatively good separation, but the weight fractions were not evenly spaced. n-Propyl alcohol gave the best resolution and fairly uniform spacing of fractions. Steele and Pacsu (68) found chloroform-hexane and pyridine-water t o be unsuitable systems for fractionation of methylcellulose. Scherer and Lou (60) found that heptane \vas a poor precipitant for nitrocellulose in ethyl acetate. They concluded that the concentration of cellulose nitrate should be low enough t o give a sufficiently low solution viscosity as too high a viscosity interferes n-ith settling of the precipitate and gives an unsatisfactory gel, and a n unsatisfactory sharpness of di-tiibution in a fraction A problem sometimes encountered is that of degradation of the polymer in the solvent-precipitant system. Thus, Steele and Pacsu (68) observed serious degradation of methylcelluloses in mixtures of pyridine and m-ater. Herrent and Govaerts (261 observed that nitrocellulose in acetone degraded from a degree of polymerization of 810 to 410 in 29 days. Other systems are not affected in this way. Scherer and 1IcNeer (51)found that ethylcellulose in ethyl acetate at room temperature and stored in brova bottles was unchanged after 47 days. Precipitation fractionation may be accomplished in a t least three ways: by addition of nonsolvent; by removal of solvent; and by the addition of a solvent and nonsolvent mixture. Recent literature provides examples of all three types. By addition of the nonsolvent, n-heptane, Scherer and Rouse ( 5 2 ) fractionated relatively large amounts of cellulose nitrate froin ethyl acetate. The precipitant was added t o the solution a t 25" almost to the point of precipitation. The solution v a s then cooled to 15' for 45 hours. The process was repeated, the period of standing being gradually reduced as the longer chains were removed. Highly reproducible results were obtained. n'annow and Thormann (66) in 1949 studied the fractionation of acetone solutions of nitrocellulose prepared from American upland cotton by kiering, bleaching, and then degrading with alkaline peroxide, using water as nonsolvent. No difficulties R ere experienced. Ideally, average degree of polymerization would have no effect on fractionation a t one degree which is qualitatively different from that at any other degree. However, there are exceptionr. The case of "reverse order" is one of these. Emery and Coheii (20) report that the separation of cellulose nitrate fractions proceeds according t o the manner predicted by the Schulz ( 5 4 ) equations only when the polymer is of low molecular weight. When the molecular weight is high and the degree of substitution uncertain, the separation of the fractions does not proceed on the basis of either nitrogen content or viscosity, but in a coniplex way dependent on both of these and possibly other unknowi factors. They call for caution in the interpretation of the results from precipitation fractionation when applied to polymers of high molecular meight, especially if the substitution is uncertain.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
Cellulose
1
a
li
F
Dobry (18) has shown that the volume percentages of different paraffin hydrocarbons required to precipitate polystyrene from chloroform and methyl methacrylate and vinyl acetate from acetone decrease markedly as the molecular weight of the paraffin nonsolvent increases. This finding confirmed earlier work of Sheppard and Sweet (65),who precipitated cellulose acetate from acetone with hydrocarbons having from 6 to 10 carbons. A rather complicating situation encountered in some cases is that of ‘5-everse order” precipitation. Morey and Tamblyn (37, 38) encountered this phenomenon and explained it as being the consequence of the effect of end groups which promote insolubility. Thus, insolubility increases toward the short end of the polymer range. This effect was found to be independent of temperature, but dependent on polymer concentration. An alternative or additional explanation is that very short chain components are capable of aggregating into crystalline units TT hich may precipitate out of solution more readily than components having a longer average chain length and less tendency to crystallize due to kinking, etc. Kales and Swanson (63) studied the effects of the presence of metallic ions on the fractional precipitation of cellulose acetate from acetone solution with ethyl alcohol. They concluded t h a t the presence or absence of calcium ions in the fractions can cause measurable changes in molecular weight and in its distribution. DeBrouckere, Bidaine, and van der Heyden (10) stated in 1940 that high polymers which are precipitated by fractionation are difficult to dry, owing to imbibed solvent. By a system of counterflow of solution and nonsolvent, which they claimed could be applied to other high polymer systems, the authors obtained pure high polymer free from the solvent. Morey and Tamblyn (36) in 1945 proposed a procedure based on progressive increase in turbidity in a solution of high polymer TI hile a suitable precipitant was slowly added. The increasing opacity t o a beam of light is registered by a suitable means, giving a n integral curve of the molecular weight distribution. Satisfactory agreement with gravimetric methods was found when the method was applied to cellulose acetate butyrate solutions. The method was further confirmed by 0 t h (41), who applied it t o nitrocellulose solutions. T h e method was calibrated with the aid of polymer fractions of known molecular weight in solutions of known composition. The operation was carried out on 0.1% nitrocellulose in acetone at 25’ C. by adding 1 ml. per minute of a precipitant consisting of 90% methanol and 10% water. Reproducible results were obtained if the temperature and speed of agitation were held constant. The results were in good agreement with results b y gravimetric methods. hforey, Taylor, and Waugh (39) applied the method to widely dispersed polyvinyl acetates in acetone solution, using water as precipitant. The expected linear relation was found between log of water concentration at precipitation and log of critical polymer concentration, over the concentration range 0.01 to 1.0 gram per liter of polymer; however, this did not hold for the lower concentrations necessary to precipitate the very short chain fractions. AIorey (35) has recently shown the kinetic basis of polymer fractionation; the “gamma” molecular weight derived from the turbidity measurements does not correspond to the number, weight, or even the z-average, but more nearly in order of magnitude t o the latter. Morey discusses conditions of nucleation, and growth of particles, and the variable relation of particle size to wave length of iight which affect the results. An example of precipitation fractionation by removal of solvent is given by Badgley, Frilette, and Mark ( 1 ) . They prepared a 2.5% solution of cellulose acetate in acetone and added ethyl alcohol just to the point of precipitation. Then rapid stirring was begun and vacuum was applied. After 30 t o 40 minutes the vacuum was turned off and stirring continued for 30 to 45 minutes. When stirring was then discontinued, fluffy flakes settled to the bottom of the container as a gelatinous mass, which was easily separated b y decantation. The treatment was applied
November 1953
repeatedly t o the solution, thus obtaining a series of fractions. The results were in good agreement with those obtained by a more conventional method. Recent studies have shown advantages in using a mixture of solvent and nonsolvent in precipitation fractionation. Thus Tamblyn, Morey, and Wagner (61) prepared a 2% acetone solution of cellulose acetate butyrate (13.1% acetyl and 36.5y0 butyryl) and precipitated fractions with a half-and-half mixture of water and acetone, containing 2% sodium chloride. When a slight turbidity persisted, the mixture was heated slightly t o redissolve the precipitate and then allowed t o stand overnight at 22” to settle. The later fractions were separated by adding water containing the same concentration of salt. Scherer and McNeer (51) successfully fractionated ethylcellulose from ethyl acetate with a mixture of 1 part of acetone and 3 parts of water as precipitant. Considerable study has been devoted to the effect of degree of substitution on precipitation fractionation of cellulose esters Tamblyn, Morey, and Wagner (61) did not observe any fractionation to acetyl or butyryl content when cellulose acetate butyrate was precipitated from acetone solution with 50: 50 water-acetone containing 2% sodium chloride. Scherer and Rouse ( 5 2 ) did not observe any difference in nitrogen contents of fractions when cellulose nitrate in ethyl acetate was fractionated with n-heptane. Likewise, Scherer and McNeer ( 5 1 ) found that ethylcellulose in ethyl acetate precipitated without varying ethyl content in the different fractions when the precipitant was 1 to 3 acetone-water mixture. On the other hand, Howlett and Urquhart ( 2 7 ) observed that the acetic acid yield from secondary cellulose acetate decreased from 55.6 to 52.oyOas the degree of polymerization of the polymer fraction increased from 106 to 622. Emery and Cohen (20)consider that disregard of the variation in degree of esterification can cause very great errors in viscosity distribution curves. Scherer and Lou (50) found for cellulose nitrate and Scherer and Icaoviello (49) for ethylcellulose t h a t no problem exists in precipitation of polymers containing different degrees of substitution, providing the composition of solvent is varied to correspond t o differences in polar nature of the ester with changing substitution. In precipitation fractionation the fractions may be removed after each increment of nonsolvent has been added (differential fractionation) or the original polymer solution may be divided into a number of portions to which are added progressively greater portions of nonsolvent (integral fractionation). Most of the preceding examples have been of the first type. However, Spencer (57) in 1948 undertook to expedite the work of obtaining a distribution curve by employing a n integral method. H e added progressively greater amounts of precipitant covering the entire precipitation range to a fixed amount of polymer solution in a series of tared weighing bottles and thus obtained an integral curve. The precipitate first formed could be reheated if desired and cooled slowly t o assure equilibrium. The supernatant liquid was then drawn off and the precipitate dried to constant weight under reduced pressure and gentle heat. The evaluation of each fraction could then be made by light-scattering or other means. Billmeyer and Stockmayer ( 4 ) in 1950 extended Spencer’s technique. They determined the weight average molecular weight and mass of each fraction. From these data distribution breadth was computed in terms of a parameter, H , which represents the departure of the distribution from t h a t of a single molecular species. The parameter, H , could be related t o other parameters of distribution breadth, such as M,/Mn. Boyer ( 5 ) believes a method which he uses for polystyrene is applicable to any soluble homologous polymer which is susceptible t o the usual type of fractional precipitation. It consists in observing the cumulative volume of precipitate corresponding to the addition of successive portions of nonsolvent to a dilute solution of the polymer. Coppick, Battista, and Lytton (13) discussed the advantages
INDUSTRIAL AND ENGINEERING CHEMISTRY
2513
I-ear of Reporl 1950 1946 1948 1949
TABLE 11.
R E C E S T LITER.4TCRE O S PRECIPITATION FR.4CTIONATIOS
Pol\ ent 8% S a O H Cuprammonium E t h y l acetate llcetone
1952
Cellulose nitrate
1949
Cellulose nitiate
E t h s l acetate acetone .%retone
I951 1949 1949
Cellulose nitrate Cellulose nitrate Cellulose nitrate
Acetone Acetone Acetone
1945 1941 1946
Cellulose acetate Cellulose acetate Cellulose acetate
Acetone -4cetone (Various)
Light petroleum Water Methanol-HZ0 (9: 1 ) E t h y l alrohol Water (Various)
1946
Cellulose acetate
Acetone
Butyl acetate
1951
1945 1946
Cellulose acetate Acetone Cellulose acetate butyrate Cellulose acetate butyrate Acetone Cellulose acetate butyrate (Various)
1945
Cellulose acetate butyrate Acetone
1949
Ethylcellulose
E t h y l acetate
1951
Ethylcellulose
1949 1949 1951 1949 1950
Trimethvl cellnlose Trimethj.1 cellulose Polyrinyl acetate Polyvinyl chloride Polymethyl methacrylate
Benzene-mc~ha- Heptane no1 Chloroform Pyridine dcetone Cyclohexanone -4cetone
1947 1947 1952 1947
Polymethyl methacrylate Vinyl acetate Polystyrene Polystyrene
Acetone Acetone Benzene Chloroform
1947
+
(Theoretical) Seven from lit (Theoretical) Any polymer 1948 (Theoretical) Any polymer 1949 a Taken or computed from authors’ data. 1951
and disadvantages of a summative, or integral, method for fractional analysis of cellulose. They gave a mathematical interpretation, together with illustrations of the conversion of sunin a t i v e distribution data into the more common integral and differential distribution plots. I n several cases studied a relatively small number of fractions have proved sufficient t o yield a comparatively accurate curve. Thus, Beall (3)obtained reasonably good representation with only 5 fractions when the binominal equation lvas used to derive the distribution curve. Wannow (66) found 8 fractions sufficient to yield exact distribution diagrams from cellulose nitrates prepared from oxidatively degraded cottons. A summary of certain data concerning precipitation fractionation is presented in Table 11. CLTRACEKTRIFUGE MEASUREMENTS. Itelatively little advance has been made since the work of Kraemer (31), Mosimann (do), and Gralen (25) in 19-11-44 in the application of the ultracentrifuge to the analytical evaluation of polymolecularity. He divided the area under the sedimentation curve, which may be assumed to remain constant duriiig a run, b y its maximum height above the base, thus obtaining a measure of the width, Fhich he called B. The change of B with change in distance, z, from the center of rotation, is a preliminary measure of polymolecularity. However, this must be adjusted to zero concentration for final use. Used under these conditions the results appear t o give reasonable values \Then applied to various polymer solutions. Jullander (29) pointed out that \Then weight averages of sedimentation and of diffusion are substituted in Svedberg’s velocity equation the result is not a weight average molecular weight. Rather the average is considered to depend both on the size distributions and on the shape of the distribution. Kinell (30)examined the relation of the width, B, of the sedimentation curve t o the distance, z,from the center of rotation, 2514
Precipitant Acetone HzO (Various) n-Ileptane Water
Polymer Cellnlose (regenerated) Cellulose Cellulose nitrate Cellulose nitrate
+
+ acetone
Water Water
Mean DPa 500-1000 480 420 83-567 120-380 810 100-2000 3 t0 Ca. 200 265 47
Reference Battista Coppick ae2nal. d Sisson (13) (2) Srherer a n d Rouse (52) Wannow a n d Thormann (66) Scherer and Lou (60) Herrent and Govaerts (85)
Emery and Cohen (20) Wannow (66) 0 t h (41) Radgley et al. ( I ) Mark ( 5 2 ) l I o r e y a n d Tamblyn
380
Ethyl alcohol Isopropyl ether
(Low vise.) 140-500
Water-acetone (1: 1) Ca. 500 (Various) 50-470
Tainblyn et al. ( 6 1 ) XIorey a n d Tamblyn
Ethyl alcohol-1380 (1:3) Acetone-IIzO (1 : 3 )
115-635
3Iorey
220
Scherer
220
Scherer a n d Icaoviello
(Various) (Various) .4lcohol (’i’arious)
(37)
(96)
and and
Tamblyn hIcNeer
(71)
(A8
?
375-4000 1
(various)
Steel a n d Pacsu (58) Steel a n d Pacsu (88) SIorcy et a7 (39) de Broiickere e t 51. (10) Rillnieg er and Storkm y e r (4) nohry (18)
Dobry ( 18 ) Royer ( 5 ) Dobry (18) Mol ey (56) ‘nemer ( 5 7 ) Beall ( 3 )
and thoned t h a t it was not linear. However, the relation could be made linear by corrections for increase in centrifugal field with distance from the center of rotation, dilution due t o the sector shape of the cell, and displacements in the maximum point of the d i m e n t a t i o n curve. Ranby (42) and Gralen and Lagermalm ( 2 4 ) studied with the aid of the ultracentrifuge the polymolecularity of cellulose and polystyrene fractions obtained in precipitation techniques. They found the fractions themselves to be rather disperse and advocated the use of their mean values for purposes of summation. Wales ( 6 2 ) and Wales, Williams, Thompson, and Ewart ( 6 4 ) have developed equations suitable for evaluation of polymolecularity when sedimentation equilibrium methods are applied. ilny moment of the molecular weight distribution can be coinputed. Information concerning recent literature dealing with ultracentrifuge studies is given in Table 111.
LITERATURE O N USE OF ULTRACENTRIFTTGE TABLE 111. RECENT FOR POLYMOLECULARITY MEASUREMENTS Year of Report 1944 1944 1947 1944 1947 1947 1948 1952
Polymer Cellulose Cellulose nitrate Cellulose nitrate Cellulose xanthate Cellulose derivatives Polymethyl methacrylate Polystyrene Polystyrene
Mean Solvent DPa Cuprammonium 170-36000 Acetone 780-2700 Acetone 60-1350 NaOH KaCl 150-530 or CSa 600-1600
+
Reference Gralen ($9) Gralen ($5’) Jullander (28) Gralen(89) Ranby (42)
Acetone
(low)
Kinell (SO)
Butanone Butanone
510-3000 860-53000
Wales et aE. ( 6 4 ) Gralen and Lagermalm
1948 Bny polymer Theoretioal a Taken or computed from authors’ data.
INDUSTRIAL AND ENGINEERING CHEMISTRY
(94)
Wales (68)
Vol. 45, No. 11
Cellulose lute, as it is necessary t o determine the relation between specific retention volume and molecular weight by some other method. Brooks and Badger (8)studied the adsorption of nitrocellulose on cellulose triacetate in methyl acetate and cosolvent. The extent of adsorption of the nitrocellulose increased with molecular weight and with degree of nitration. Practical application would require a uniform degree of nitration. The concenD, = mz/2At and D A = A2/4?rH2t tration of nitrocellulose in solution per gram of acetate was where m2 is the second moment of the optical refraction curve .without effect up to about 1%, and then deviation increased about the vertical axis through the arithmetical mean, A is the gradually. The rate of adsorption, requiring from 2 to 24 hours area under the curve and above the z-axis, H is the maximum for equilibrium, seemed to be controlled by diffusion, as reduction height, and t is the time from beginning of diffusion. of acetate particle size hastened the rate of adsorption. In studies with starch Brooks and Badger (9) found that the amount of nitrocellulose adsorbed increased with increasing molecTABLEIV. RECENTLITERATURE ON USE OF DIFFUSION ular weight, in direct contrast to Claesson's observations on MEASUREMENTS FOR EVALUATION OF POLYMOLECULARITY carbon. Only 10 minutes was required for equilibrium t o be Year established and elution could be accomdished. The extent of nf Mean -.--IReport Polymer Solvent DPQ Reference adsorntion decreased as the hvdrozen bonding character of the * Cuurammonium 170-36000 Gralen (23) 1944 Cellulose solvent increased. Gralen IS) 780-2700 1944 Cellulose nitrate Acetone Gralen [as) Cellulose 150-530 87' NaOH 1944 Recent literature in this field is listed in Table V. xanthate &aCl or C S ~ FLOW BIREFRINGENCE OR VELOCITY GRADIENT.Breazeale ( 7 ) Rosenberg and Water 20 Amylopectin 1948 Beckmann (44) in 1948 suggested use of the differential behavior of molecules of Scheibling (47) Polystyrene 1951 860-53000 Gralen and Polystyrene Butanone 1962 different length in a velocitv svstem as a means of evalu" gradient Lagermalm ating polymolecularity. He did not suggest just how this prin("4, 1951 Albumin Water ~~~~t~~~~ (($$ ciple might be used. However, as early as 1938 Sadron (45) 1951 Blood serum Water and Sadron and Mosimann ( 4 6 ) had dea Taken or computed from authors' data. veloped and applied an equation based on velocity gradient for use with flow bireLITERATURE ON EVALUATION OF POLYMOLECULARITY BY TABLE V. RECENT CHROMATOQRAPHIC FRACTIONATION fringence. Only partial success was obYear tained with cellulose acetate in cycloof Mean Report Polymer Substrate Developer DP'L Reference hexanone and with methylcellulose and 1950 Cellulose nitrate Cellulose acetate Methyl acetate + Hz0 80-575 Brooks and Badger sodium thymonucleate in dilute sodium (8) chloride solution. 1950 Cellulose nitrate Corn or potato Cyclohexane-acetone Brooks and Badger starch (9) A very limited application of birefringBrooks and Badger 1950 Cellulose nitrate Corn or potato Methanol-acetone starch ence for polymolecularity evaluations was (9) 1949 Cellulose nitrate Carbon Methanol-acetone Claesson I 1 it) undertaken by Goldstein (22) using reClaesson 1.11: id) 1949 Polyvinyl acetate Carboraffin Claesson (ii, 18) 1949 Polymethyl Carboraffin cent calculations of Scheraga, Edsall, and methacrylate Gadd (48) and the distribution function Taken or computed from authors' data. of Montroll and Simha (34). It was pointed out that flow birefringence is sensitive not merely to the presence of polymolecularity, biIt to the form of the distribution curve a t the Rosenberg and Beckmann (44) constructed a diffusion cell of steel membranes made by the sintering of metal powder which high molecular weight end. was much more rapid than previous glass cells, and therefore Joly (M), by plotting a series of curves of apparent particle suitable for quantity fractionation. length against velocity gradient for a large number of size disScheibling ( 4 7 )in 1951 described an interferometric method for tributions, was able to find empirical relations giving the particle measuring diffusion coefficients and adapted this to the measurelength of maximum frequency. From the values of the extincment of polydispersity of a number of diverse substances. Cellution angle a t two given velocity gradients he could then obtain lose derivatives were not included. the polymolecularity. When interaction was present i t was Table IV summarizes certain information about the use of necessary to extrapolate the extinction angles to zero concentration. diffusion in the measurement of polymolecularity. CHROMATOGRAPHIC FRACTIONATION. Recent studies were Table VI summarizes some work done with flow birefringence. reported by Claesson (11, 18) in 1949 and by Brooks and Badger ELECTRON MICROMETRY.Undoubtedly, the most direct as (8, 9) in 1950. Claesson dissolved nitrocellulose and polywell as spectacular method of evaluating polymolecularity is by methylmethacrylate a t various concentrations in acetone and in electron microscope observation and classification. As early as acetone-methanol mixtures and adsorbed them on a carbon column (Carboraffin-Super-Cel). He reported results showing molecular weight disTABLEVI. RECENTLITERATURE O N USE OF FLOWBIREFRINGENCE tributions in good agreement with the results of FOR EVALUATION OF POLYMOLECULARITY other methods. He concluded that frontal Year - --of Mean analysis diagrams extrapolated to zero concenReport Polymer Solvent DPB Reference tration give an upsidedown picture of the weight 1938 Cellulose acetate Cyclohexanone Sadron and Mosimann 4 6 ) 1938 Methyl cellulose 0 125% NaCl Sadron and Mosimann {AB) distribution. The selectivity of the method was 1938 Sodium thymonucleate 0 125y0 NaCl Sadron and Mosimann A 6 ) high, excelling that of the ultracentrifuge on the 1938 Serum globulin Water-glycerol Sadron and Mosimann / 4 6 ) same material. The rate of adsorption and amount 1952 Tobacco mosaic virus Water C ~ . , ~ ~ l O -Joly ($8) Sadron L5) adsorbed decreased as the molecular weight in1948 Any polymer 1938 Breaseaie (7) creased. The method was considered valuable as a 1952 Any polymer Goldstein (sa) means of judging the homogeneity of high polymer Taken or computed from authors' data. fractions. Unfortunately, the method is not abso-
EVALUATION BY DIFFUSION. Relatively little study has been devoted in recent years to evaluation of polymolecularity by another analytical method-diff usion measurements. Gralen (23) used the ratio D,/D1, where D is the diffusion coefficient and the subscripts m and A stand for the moment and area methods of derivation, respectively. The equations are
a
-
+
-
Q
*
LDlY"
Q
November 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
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1945 Boyer and Heidenreich (6) precipitated polychlorostyrene from very dilute (1 p.p.m.) benzene solutions with a n agent such as propanol. Drops of the resulting mixture, apparently containing coiled-up polymer molecules, were evaporated on a silica screen and examined under the electron microscope. Circular particles ranging in diameter from about 15 t o 500 A., were counted, according t o size classes. Molecular weight distribution curves of the expected shape and extent were obtained, though the average molecular weights were four to five times greater than values obtained with an independent method.
rather than “inferred.” The measurements were considered t o be direct, the calculations simple, and the assumptions minor. Zimm (68) has studied the possibilities of measuring polymolecularity b y light scattering T h e method appears still t o be in a n exploratory stage. Scherer and Testerman (63)studied the dielectric dispersion of cellulose nitrate solutions in acetone over a frequency range of 100 to 500 kc. They were able t o show a relatively high degree of correlation between a cumulative weight per cent curve based on dielectric dispersion of a cellulose nitrate of 443 DP and a corresponding curve based on fractional precipitation. These methods are listed in Table VIII.
TABLErrII. RECENTLITER.4TURE ON EVALUATION OF POLYMOLECULARITY WITH ELECTROX R~ICROSCOPE
COiMPARISON OF METHODS
In spite of a rather extensive accumulation of Mean Particle literature on the subject, it is still not possible t o Solvent Size Reference Cyclohexane 4 8-25 X IO5 &/mole Siegel et at. (66) form an adequate opinion of the relative merits of Benzene 15-500 A. Boyer and Heidenthe different polymolecularity methods. Kraenier reich (6) Water 9 4 x 10flg./mole Williams and Backus (51)was one of the eailiest to develop the ultra(67) centrifuge technique to this end. Wales, TVilliams, Thompson, and Ervart ( 6 4 ) suggest that sedimentation gives a better approximation to the true distribution curve than does fractionation in the conIn 1949 Williams and Backus ( 6 7 ) experimented with what they ventional manner. Gralen and Lagerniahn (@) found that fraccalled a “direct particle count’’ method. It consisted essentially tions obtained by precipitation we1 e rather polydisperse, 3%hen of counting on electron micrographs the numbers of macromoleexamined by sedimentation analysis. However, as Mark ( 3 2 ) cules per unit of solution and obtaining the dry weight of a n alihas said, while offering perhaps the best insight into the polymer quot of the unit volume. They used Dow latex particles of polydistribution, it is probably too expensire for routine use. styrene (2800 A. diameter) as reference particles t o determine the Gralen (25) also studied the diffusion method for measuiing volumes of the spray droplets which also contained the macropolymolecularity, and compared it with the sedimentation molecules. Using this technique they determined the molecular method. Results with cellulose in cupranimonium hydroxide XTeight of bushy stunt virus to be 9.4 It 0.7 X lo6. solvent were not satisfactory, but on cellulose esters in organic I n 1950 Siegel, Johnson, and Mark (66) employed a similar solvents he concluded that the two methods were of comparable technique t o that of Williams and Backus on four fractions of validity. In more recent results on polystyrene Gralen and polystyrene in cyclohexane. The molecular weights of the fracLagermalm ( 9 4 ) obtained very erratic results. Svedberg (60) tions determined by osmotic pressure and viscosity were 4.8, comments that the diffusion method requires very accurate 11, 18, and 25 X 105. The results from the electron micrograph measurements. A great deal of development would appear to estimate were 6.0, 9.9, 19, and 25 X 105, which were considered be required before a practical diffusion technique, which is comto be in satisfactory agreement. The method was thus conparable in reliability with those based on solubility, is available sidered to have merit for macromolecules, whose molecular weight for polymolecularity determinations. exceeds 1,000,000. Certainly, the ordinary fractionation methods, solution and Some references dealing with the electron microscope for polyprecipitation, offer the most concrete evidence of the inhomomolecularity are shown in Table VII. geneity of chain length and of the nature of the distribution curve. OTHERMETHODS.Dobry ( 1 7 ) described in 1945 a method of They have come to be more or less routine selections where exfractionating high polymers which is based on coacervation. ploratory work is not justified or practicable. Solution fractionaBriefly, coacervation has been defined as “the reversible collection has often been placed on a comparable footing with precipitation of emulsoid particles into liquid droplets preceding flocculation fractionation, but Heuser and Jorgensen (26) have rather tion.” Dobry found t h a t the coacervate formed by polyvinyl convincingly shown that in paper pulp and cotton linters solution acetal and benzene could be fractionated by extraction with fractionation is unable to resolve the higher ranges of molecular benzene, even after precipitation fractionation would yield no weight. On the other hand, precipitation fractionation was further precipitate. Extraction permitted the separation of the equally effective throughout the range. It is possible that the acetal into two products, one consisting of linear, the other of wide acceptance of solution fractionation has resulted from the branched molecules. fact that it has been generally applied to materials of low niolecRochow and Rochow (43)have described a method which they ular weight. However, this same criticism applies for the most designate as LLresinographic” and consider t o be applicable t o all part to precipitation fractionation also, as Tables I and I1 show. high polymers, provided they can be rendered brittle for fracCaution in the interpretation of much of the present polyturing and will remain rigid until a replica of the fractured surmolecularity data has been urged by Emery and Cohen (201, face is made. The method was applied t o silicon rubbers in which particularly for this reason. the sizes and shapes of the macromolecules were “pictured” Year of Report Polymer 1950 Polystyrene 1945 Polychlorostyrene 1949 Bushy stunt virus
TABLE17111. Year of Report Method I945 Coacervation 1950 Resinographic 1951 Dielectric dispersion
OTHER METHODS
EVALUATION O F POLYRIOLECULARITY BY
Polymer Solvent Polyvinylacetal Benzene Anyb Cellulose nitrate Acetone
Extractant Benzene
Mean DPa
.. , . ..... ,
, ,
,
443 ’
..
’
Reference Dobry ( 1 7 ) Roohowand Rorhow (4.3) Scherer and Testerman (Z)
1948 Light scattering Polystyrene Butanone . . . . 7400-34400 Zimm (68) a Taken or computed from authors’ data. b Any polymer that can be rendered brittle and that will remain rigid until replica of fractured surface is made
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
-Cellulose
i
1
Claesson (11, 12) and Brooks and Badger (8, 9) have obtained rather promising results with chromatographic techniques. The selectivity appeared t o b e very high when the proper systems were used, excelling that of the ultracentrifuge on the same material. T h e main drawbacks a t present are the low capacity of adsorption, the low rates in some cases, and the fact that the method is not absolute but must be referred t o some other method. The method is very new and these difficulties may be soon overcome. The application of methods based on flow birefringence are impracticable at present, and a great deal of improvement must be made before this technique can come into any general use. Electron microscopy offers strong attraction, but its application would seem to be confined entirely to exceptionally high molecular substances. Coacervation would seem t o be suitable only for certain exceptional cases. This would appear to be true too of the “resinographic” technique. The dielectric dispersion method of Scherer arid Testerman (63)would appear to be worthy of further exploration. The combination of osmotic pressure and light-scattering measurements was found by Billmeyer and Stockmayer ( 4 )to be a very favorable substitute for osmotic pressure and viscosity measurements, to evaluate the ratio of weight-to-number-frequency average molecular weights. This would be the situation, however, only where the exponent, a, in the equation, 1171 = KM4 had a value of unity. LITERATURE CITED
Badgley, W., Frilette, V. J., and Mark, H., IND.ENG.CHEM., 37,227-32 (1945). Battista, 0. A , , and Sisson, W. A., J . Am. Chem. Soc., 68, 915 (1946). Beall, Geoffrey, J . Polymer Sei., 4, 483-513 (1949). Billmeyer, F. W., Jr., and Stockmayer, W. H., Ibid., 5, 121-36 (1950). Boyer, R. F., Ibid., 8, 73-89 (1952); Battelle Tech. Rev., 1 (a), 1889 (1952). Boyer, R. F.: and Heidenreich, R. D., J . Appl. Phys., 16 (lo), 621-39 (1945). Breazeale, Francis, J . Polymer Sci., 3, 141-2 (1948). Brooks, AT. C., and Badger, R. M., J . Am. Chem. SOC., 72, 1705-9 (1950). Ibid., pp, 4384-8. Brouckere, L. de, Bidaine, E., and Heyden, A. van der, Bull. SOC. chim. belges, 58,418-19 (1949). Claesson, S.,Arlciv Kemi. Mineral. Geol., 26A, No. 24 (1949). Claesson, S.,Discussions Faraday SOC.,No. 7, 321-5 (1949). Coppick, S., Battista, 0. A., and Lytton, M. R., IND.ENQ. CHEM., 42,2533-8 (1950). Cragg, L. H., and Hammerschlag, H., Chem. Reos., 39, 79-135 (1946). Desreux, V., Bull. SOC. chim. belges, 57, 416-18 (1948). Desreux, V.,Rec. trav. chim., 68, 789-806 (1949). Dobry, A., J . chim. phys., 42, 109-13 (1945). Dobry, A., J . Polymer Sci., 2, 623-5 (1947). Duclaux, J., and Wollman, E., Bull. SOC. chim., 27, 414-20 (1920). Emery, C . , and Cohen, W. E., Australian J . Appl. Sci., 2 (41, 473-87 (1951). Fnchs, O., Malzromol. Chem., 5 (31, 245-56 (1951).
November 1953
(22) Goldstein, M., J. Chem. Phys., 20, 677-82 (1952). (23) Gralen, Nils, inaugural dissertation, Uppsala, 1944. (24) Gralen, Nils, and Lagermalm, G., J . Phys. Chem., 56, 514-23 (1952). (25) Herrent, P., and Govaerts, R., J . Polymer Sci., 4, 289-307 (1949). (26) Heuser, E., and Jorgensen, L., T a p p i , 34 (lo), 443-50 (1951). (27) Howlett, F., and Urquhart, A. R., J . Teztile Inst., 37, T89-112 (1946); Natural & Synthetic Fibers, 4,359-63 (1947). (28) Joly, M., Trans. Faraday SOC.,48,279-86 (1952). (29) Jullander, Ingvar, J.Polymer Sci., 2,329-45 (1947). (30) Kinell, P. O., Acta Chem. Scand., 1,335-50 (1947). (31) Kraemer, E. O., J . FranklinInst., 231, 1-21 (1941). (32) Mark, H., Paper Trade J., 113, 34-40 (July 17, 1941). (33) Mitchell, R. L., IND. ENO.CHEM., 38,843-50 (1946). (34) Montroll, E. W., and Simha, R., J . Chem. Phys., 8, 721 (1940). (35) Morey, D. R., J. Colloid Sci., 6,406-15 (1951). (36) Morey, D. R., and Tamblyn, J. W., J . Appl. Phys., 16, 419-24 (1945). (37) IMorey, D. R., and Tamblyn, J. W., J . Phys. Chem., 50, 12-22 (1946). (38) Morey, D. R., and Tamblyn, J. W., J . Phys. & Colloid Chem., 51,721-46 (1947). (39) Morey, D. R., Taylor, E. W., and Waugh, G. P., J . Colloid Sei., 6 , 470-80 (1951). (40) Mosimann, H., Helv. Chim. Acta, 26, 369-98 (1943). (41) Oth, A,, Bull. 8oc. chim. belges, 58, 285-300 (1949). (42) Ranby, B. G., Norsk. Skog.sind., 1 (12), 295-301 (1947). (43) Rochow, T. G., and Rochow, E. G., Science, 111,271-5 (1950). (44) Rosenberg, J. L., and Beckmann, C. O . , J . Colloid Sci., 3, 483504 (1948). (45) Sadron, C., J. phys. radium, 9, 381-3 (1938). (46) Sadron, C., and Mosimann, H., Ibid., 9,384-6 (1938). (47) Scheibling, G., J. chim. phys., 48, 559-62 (1951). (48) Scheraga, H. A., Edsall, J. T., and Gadd, J. O., J . Chem. Phys., 19,1101 (1951). (49) Scherer, P. C., and Icaoviello, J. G., Rayon & Synthetic Textiles, 32, 47-50, 52 (1951); Natura2 & Synthetic Fibers, 9 (21, 95-7 (1952). (50) Scherer, P. C., and Lou, L. H., Rayon & Synthetic Textiles, 33 (51, 41-4 (1952). (51) Scherer, P. C., and McNeer, R. D., Ibid., 30, 56-9 (1949). (52) Scherer, P. C., and Rouse, B. P., Rayon Textile Monthly, 29, 55-7 (1948). (53) Scherer, B. C.. and Testerman, M. K., J . Polymer Sei., 7, 54965 (1951). (54) Schulz, G. V., J . makromol. Chem., 3d Ser., 1, 131-46 (1943). (55) Sheppard, S. E., and Sweet, 9. S., J . Phys. Chem., 36, 819-29 (1932). (56) Siegel, B. M., Johnson, D. H., and LMark, H., J . Polymer Sci., 5 , 111-19 (1950). (67) Spencer, R. S., Ibid., 3, 606-7 (1948). (58) Steele, R., and Pacsu, E., Textile Research J., 19, 784-90 (1949). (59) Stockmayer, W. H., J . Chem. Phys., 17,588 (1949). (60) Svedberg, The, J . Phys. & Colloid Chem., 51, 1-18 (1947). R. H., IND. ENG. (61) Tamblvn. J. W.. Morey. D. R.. and Wagner. CHEM.,37,573-7 (1945). (62) Wales, M., J.Phys. & Colloid Chem., 52, 235-48 (1948). (63) Wales, M., and Swanson, D. L., Ibid., 55,203-10 (1951). (64) Wales, M., Williams, J. W., Thompson, J. O., and Ewart, R. H., Ibid., 52,983-98 (1948). (65) Wannow, H. A., MeEliand Teztilber., 30 ( l l ) , 519-22 (1949). (66) Wannow, H. A., and Thormann, Fr., Kolloid-Z., 112, 94-110 (1949). (67) Williams, R. C., and Backus, R. C., J. Am. Chem. SOC.,71, 4062-7 (1949). (68) Zimm, B. H., J . Chem. Phys., 16, 1099-116 (1948). RECEIVED for review March 30, 1953.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCIWTEDAugust 7, 1953.
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