August 1953
1'779
INDUSTRIAL AND ENGINEERING CHEMISTRY
(4)Glavis, F. J., Ibid., 42, 2441 (1950). (5) Griin, Ad., and Waldenberg, M., J . Am. Chem. SOC.,31, 490 (1909). (6) Larsen, R. G., and Bondi, H., IND. ENG.CHEM., 42,2421 (1950). (7) Morgan, J. D., U. S. Patent 2,379,850(July 3, 1945). (8)Ibid., 2,383,147(Aug. 21,1945). (9) Murphy, C. M., and Zisman, W. A., IND.ENG.CHEM.,42, 2415 (1950). (10) Roe, E. T.,Schaeffer, B. B., Dixon, J. A,, and Ault, W. c., J . Am. Oil Chemists' Soc., 24, 45 (1947).
(11) Russell, C.R., Smith, H. E., and Schniepp, L. E., private com-
munication.
(12) Zisman, W. A., private communication. RECEIVED for review December 3, 1952. ACCEPTED May 13, 1953. Presented before the Division of Industrial and Engineering Chemistry at the 122nd Meeting, AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J. The mention in this article of firm names or commercial products under proprietary names or names of their manufacturers does not constitute endorsement by the U. S. Department of Agriculture of such firms or products.
Fractionation of Chlorotrif luorof
ethylene Polymer J
. I
H. S. KAUFMAN AND ERNEST SOLOMON M . W , Kellogg Co., Jersey City, N . J.
T
HE high polymer of chlorotrifluoroethylene, known commercially as KEL-F (trade-mark for M. W. Kellogg Co. chlorotrifluoroethylene polymer) is of interest because its chemical inertness, high softening temperature, and satisfactory mechanical and electrical properties make i t an excellent material for industrial applications. As the mechanical properties of a given high polymer depend t o a large extent on its molecular weight and its molecular weight distribution, a technique for the fractionation of this polymer was developed. Because of the very low solubility of this material at normal ambient temperatures, the fractionations were performed a t 150" C. and the intrinsic viscosity measurements at 99" C. EXPERIMENTAL PROCEDURE
CHOICEOF FRACTIONATION SOLVENT.Fractionation of high polymers by precipitation requires that the polymer-rich phase precipitated from solution be amorphous. This condition can be ensured by using a solvent that forms a eutectic phase with the polymer and working in the polymer-lean side of the phase-composition diagram. It has been shown by Richards (6) working
with polyethylene and by McHenry et al. ( 4 ) working with polytrifluorochloroethylene, t h a t a poor solvent-polymer system shows a maximum in its composition-solution temperature relationship, whereas in a good solvent-polymer system there is am almost linear relation. The shape of the composition-solution temperature relationship was therefore used as a criterion in t h e selection of solvents for use in fractionation. The solution temperature of poor solvents for KEL-F is usually above 200' C., which was considered high for practical manipulation. Lower solution temperatures can be achieved by adding good solvents t o poor solvents. The data for three solventKel-F systems, plotted in Figure 1, were obtained by measuring the solution temperatures of various polymer-solvent compoaitions using mixtures of diethyl phthalate (poor solvent) and dichlorobenaotrifluoride (good solvent) and KEL-F of medium molecular weight. This mixed solvent has the desirable attribute that its solvent power can be decreased by removing some of the lower boiling, good solvent by evaporation. The good solvent alone gave a n almost linear solubility, while the two mediocre solvent mixtures had relatively low temperature maxima. The 25% diethyl phthalate mixture, showing a solution temperature of about 135" C . in the low concentration region, was selected the initial solvent for the fractionations. FRACTIONATION APPARATUS.The fractionation apparatus was designed so t h a t all required operations could be carried out at 150"C., thereby minimizing the possibility of precipitating t h e polymer during transfer operations. A schematic representation of the apparatus is shown j~ Figure 2. STOPGPGK A
1,x 0
I
I I
SOLVENTS
DICHLOR~ENZOTRIFLUORIDE
f"DIETHYLPHTHALATE 85% OICHLOROB~NZOTRIFLUORlDE
5
IO
IS
PO
t-
25
Figure 1. Solution Temperature-Concentration :Curves of KEL-F in Various Solvents
FLASK I
Figure 2.
FLASK 2
Fractionation Apparatus
Two 500-ml. three-necked flasks (1 and 2 ) were arranged with their central openings provided with motor-driven glass stirrers. The bottoms of the flasks were bulged to form a depression to hold the polymer preci itates. A siphon tube, equipped with a greaseless, large-bore &orosilicate glass stopcock, A , connected the two flasks. One arm of the siphon tube was sufficiently long t o permit adjustment t o within about 1 cm. from the bottom of flask 1; the other arm was relatively short, and extended about 2 to 3 cm. into flask 2. Each flask was attached t o a
1780
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
vacuum source through a cold trap and stopcock. The flasks were immersed to the indicated level in a thermostated bath of silicone oil. All operations were performed a t a bath temperature of 150" =k 0.05' C. FRACTIONATION PRocmum. A solution of the polymer was prepared by agitating 3 to 4 grams of the KEL-F in 225 ml. of dichlorobenzotrifluoride a t 150" C. for 2 t o 3 hours. The solution was transferred to flask 1 and, after temperature equilibrium had been attained, 75 ml. of preheated (150" C.) diethyl phthalate was added. After a brief period of stirring, about 25 ml. of the solution was removed and stored separately in the thermostatically controlled bath for the duration of the fractionation. This sample was used as a control on the extent of polymer degradation : the polymer subsequently recovered from it by cooling to room temperature is referred to as the "zero" fraction. With stopcock A closed, suction was carefully applied to flask 1. The relatively low boiling dichlorobenzotrifluoride slowly evaporated, decreasing the solvent power of the remaining mixture, until the first fraction came out of solution. At this point the vacuum was broken and the bath temperature raised to about 155" t o 160" C. in order t o redissolve the fraction. On subsequent slow cooling back to 150" C. t h e f r a c t i o n reprecipitated. The polymer was usually in a rather sticky, gelatinous form that settled rapidly (15 minutes). The external portion of the glass siphon, including stopcock A , was heated and the solution was then transferred to flask 2. The polymer from flask 1 was removed and the entire procedure repeated, working from flask 2 Figure 3. Modified into flask 1. Viscometer The fractions, including the zero fraction, were purifieb by dissolving them in hot dichlorobenxotrifluoride, cooling to effectprecipitation, and then washing repeatedly with chloroform. Final drying was accomplished under vacuum a t 150" C. The dried fractions were weighed.
Vol. 45, No. 8
The measurements were made in the customary manner, with efflux times ranging upward from about 85 seconds for pure solvent. Kinetic energy corrections were applied and intrinsic viscosities, [77] , were obtained by extrapolation using the semilog plot required by the Martin equation:
where
?lap
-
= specific viscosity defined as
>
77sol"Pnt
[?I
=
intrinsic viscosity, c = concentration in qrams per 100 ml., and IC = constant. In order to reduce the time required t o obtain the intrinsic viscosities, a correlation (Figure 4) was developed between intrinsic viscosity and the viscosity of a solution with a concentration of 0.5 weight (equivalent to 0.816 gram per 100 ml.).
4 5'o .5 4.0
\
1
3.5
4
'
I
I
I
I I
I
I
I
I
I
Y
I
I
INTRINSIC VISCOSITY MEASUREMENTS
A dilution-type Ubbelohde viscometer was designed for ease in manipulation at the elevated temperatures required for the measurement of the solution viscosities. The filling arm of the viscometer was modified by the introduction of four bulbs of known volume with calibration marks on the constrictions between them (Figure 3). A sintered-glasq filter was inserted into the upper portion of this arm to provide a convenient means for filtering the hot solutions. In use, the viscometer was maintained a t 98.9' C. in a thermostatically controlled bath. The solvent employed was 1,1,3trifluoropentachloropropane. A solution of the polymer, prepared at 150" C., was introduced into the viscometer through the sintered-glass disk to the level of the first calibration mark. Excess solution was removed by suction through a long, narrow glass tube inserted into the filling arm. Dilution was accomplished by adding to the viscometer slightly more hot solvent than was necessary t o reach the next calibrated level. Preheated nitrogen, bubbled through the solution to effect mixing, lowered the level of the solution t o the calibration mark by the slight loss of solvent due t o evaporation. The sizes of the bulbs were such as to yield the following sequence of concentration ratios: 1.00:0.61:0.42: 0.25.
Figure 4.
Relationship between
770.555
and [?I] for KEL-F
DATA AND DISCUSSION
The procedure outlined above was applied to three samples of commercial polymer representing low, medium, and high molecular weight grades. The commercial characterization of the polymer is based on a no-strength-temperature (ATST)test ( 3 )in which a notched bar of polymer is heated under standardized conditions of load and rate of temperature increase until the sample breaks. The temperature, in degrees centigrade, a t which the bar breaks is referred t o as the NST. The test determines the temperature a t which various samples have about the same melt viscosities; NST is therefore related to molecular weight. The samples fractionated had KST's of 235, 273, and 322, respectively. The molecular weights of the 235 and 273 NST samples had previously been determined, by osmotic pressure measurements ($), to be 56,000 and 76,000, respectively. The fractionation data are presented in Table I . Polymer
August 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLEI. FRACTIONATION DATA
Sample A, 322 NST
Fraction
Wt. % Recovered,
w
Original sample ZW[nl 0
,..
... ...
Ls 1 1.80 1.83 1.83
1
21.0 15.8 11.4 12.7 16.5 21.8
3.24 2.73 1.98 1.63 1.01 0.51
2
3 4 5 6
~
B. 273 NST
Original sample ZWl?l
... ...
...
1.13 1.20 1.18
1
7.1 14.1 8.9 11.7 7.7 10.8 13.8 23.9
2.48 1.92 1.58 1.32 1.18 1.02 0.85 0.60
0 2
molecular weight of the polymer is linearly proportional to the intrinsic viscosity; the value of 01 suggests t h a t the polymer chain is rather rigid. On the assumption t h a t the above relationship is valid, it is found that the molecular weights of the fractions range from about 20,000 t o about 200,000.
eo 60
45
40
99.2 a
1781
eo
0
IO
20
In1
3.0
LO 0
10
P.0
30
0
IO
20
30
rn1
In1
~
98.0
C , 235 NST
... ...
...
0.93 0.97 1.00
12.1 13.2 9.0 7.1 18.3 21.6 7.6 7.4 2.0
1.71 1.37 1.17 1.05 0.90 0.71 0.60 0.42 0.18
Original sample
zwlnl 0
~
98.3
D, mechanically degraded A, 278 N8T
Original sample r1;Inl
...
1.28 1.32
14.8 17.1 12.6 12.1 33.8 9.4
2.75 1.98 1.40 1.08 0.66 0.44
... ...
__
...
99.8
E, thermally degraded A,
Original sample
280 NST
...
27LnI
...
...
1.28 1.32 1.35
1 2 3 4 5 6
11.4 10.9 13.5 17.0 19.4 27.2
2.7.2 2.24 1.79 1.33 0.86 0.45
99.4
recoveries in excess of 98% were achieved. Satisfactory agreement is shown between the intrinsic viscosities of the whole polymer, the zero fraction, and the calculated value, Z W [ q ] , obtained from the sum of the product of the weight fractions of the individual fractions and their intrinsic viscosities. This indicates that little or no degradation occurred during the course of the fractionation. Differential distribution curves (Figure 5, A , B, and C) were constructed from the integral curves obtained from the above data. On the basis of these curves it appears that these samples have rather broad distributions which tend to become increasingly sharp with decreasing NST. I n order to obtain a n indication of the molecular weight range covered in these fractions, the constants of the Mark equation were evaluated from molecular weight-intrinsic viscosity data on whole polymers. The equation is: [q] =
2
x
Figure 5.
Differential Distribution Curves of KEL-F Samples
The rather broad distributions encountered in these samples may explain, at least in part, the reported discrepancy between the weight average molecular weight and the number average value. The former was reported by Hall ( I ) , from light scattering measurements, t o be above 350,000 for a low NST sample; this is t o be compared with the number average value of 56,000 reported from osmotic pressure measurements ( 4 )for a sample of similar NST. An additional margin of uncertainty is introduced, as the two samples were not taken from the same batch of commercial preparation. In another application of this method two portions of the high molecular weight polymer (322 NST) were degraded t o the same intrinsic viscosity and NST; one sample was degraded by thermal treatment a t 300" C. and the other mechanically by dry grinding with pebbles in a ball mill. The differential distribution curves for these samples are given in Figure 5, D and E. It is apparent t h a t the very broad distribution of the original material has been shifted toward the lower molecular weight end. It is of interest t h a t both degraded samples gave almost identical distribution curves and t h a t these curves are similar t o t h a t of the 273 NST sample ( B ) . It is inferred from these data t h a t degradation occurs, not by a zipper mechanism, but rather by random scission. ACKNOWLEDGMENT
Grateful acknowledgment is made t o Edward Felten and Eugene Walsh for the skill with which they performed the experimental work. LITERATURE CITED
( 1 ) Hall, H. T., J . Polymer Sci., 7, 443 (1952). ( 2 ) Kaufman, H. S., and Muthana, M. S., Ibid., 6, 251 (1951). (3) M. W. Kellogg Co., New York, N. Y., Bull. 2-10-50. (4) McHenry, R. E., Frey, E. S., Gibson, D. J., and Lafferty, R. H., Jr., IND.ENG.CHEM.,42, 2317 (1950). (5) Richards, R. B., Trans. Faraday Soc., 42, 10 (1946). '
10-6M1.0
As the value of the exponent, a,is approximately equal t o 1, the
RECEIVBD for review November 6, 1952. ACCEPTED April 29, 1953. Presented before the Division of Polymer Chemistry, Symposium on Chlorotrifluoroethylene, at the 122nd Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J.
