August 1949
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
curves were obtained at 4oo, so", and 59.5" C., as shown in Figure 7. Examination of these three curves reveals that, the ratio of the abscissas at any given ordinate is remarkably constant. I n the range of 30 t o 90% conversion, the ratios were 3.89 * 0.05 to 2.06 * 0.02 to 1 at 59.5" C., 50" C., and 40" C., respectively. These temperature coefficients correspond to an over-all activation energy of 14,500 calories per mole. ACKNO W LEDGMEKT
The author expresses thanks to George C. Harris for furnishing the pure resin acids used in this work; to Victoria Gage for aid in carrying out the experimental part of this work; and to John T. Hays and George E. Hulse for suggestions and criticisms helpful in thc preparation of this paper. LITERATURE CITED
(1) Amberg, L. O., IND. ENG.CHEM.,40,487 (1948). ( 2 ) Coe, W.S.,Brady, J. L., and Cuthbertson, G. R., Ibid ,38,975-6 (1946). (3) Corrin, M. L., Klevens, H. B., and Harkins, Wm. D., J . Chem, P h y s . , 14, 480 (1946).
1629
(4) Harris, G. C., Wood Resins in "Wood Components," TliPPI Monograph No. 6, pp. 167-77, New York, Tech. As~oc. Pulp and Paper Ind., 1948. (5) Harris, G. C., and Sanderson, T., J . Am. Chem. Soc., 70, 334, 339, 2079, 2081 (1948). (6) Hays, J. T., Drake, A . E., and Pratt, Y . T., IND.ENG.CHEM., 39, 1129 (1947). (7) Houston, B. J. (to €5. F'. Goodrich Co.), private cnmtnunicatioii (Jan. 11, 1945). (8) Kluchesky, E. F. (to Firestone Tire and Rubber Co.), private communication (June 15, 1945). (9) communica~, Kolthoff. I. M.. et al. (Univ. of Minnesota), , orivate tion (May 25, 1944). (10) Kolthoff, I. M.. and Harris, W. E., J . Polwmer Sci., 2, 41 (194i). (11) Littmann, E. R. (to Hercules Powder Co.), U. S. 2,154,629 (1939). (12) Starkweather, H. W., et al., IRD.ENG.CHEM.,39, 210 (1947). (13) Vinograd, J. R., and Sawyer, W. M., presented before the Division of Colloid Chemistry at the 108th Meeting of the AMERICAN
CHEMICAL SOCIETY, New York, N. Y.
RBCEIVED January 6 , 1949. Presented as a part of the High Polymer Forum before the Division of Cellulose Chemistry a t the 112th Meeting of t h e AMERICAN CHEMICAL SOCIETS,New York, S . Y.
Dilatometric Measurement of Molecular Regularity in Polymers V. E. LUCAS, P. H. JOHNSON, L. B. WAKEFIELD, AND B. L. JOHNSON The Firestone Tire & Rubber Company, Akron, Ohio T h e molecular regularity of various synthetic elastomers has been determined by measurement of the cry stallizability of the raw, unstretched polymers. The relative crystallizability of the polymers was measured dilatometrically in terms of the isothermal volume decrease characteristic of the ordering process of crystallization. The molecular regularity of polybutadiene polymerized in two different activated emulsion systems has been shown to be dependent on the polymerization temperature; the crystallizability of the polybutadiene in-
creases markedly with decreasing polymerization temperature. A similar relation between polymerization temperature and molecular regularity has been found for a series of polychloroprenes polymerized in an emulsion system employing sodium rainate, sulfur, and ammonium persulfate. The stress-strain properties of gum compound vulcaniistes of the polychloroprenes showed a striking improvement which coincided with increasing crystallizability of the polymers prepared at successively lower polymerization temperatures.
R
method, to measure the amount of crystalline material in natural rubber. However, there is still some discrepancy as to the amount of rubber in the crystalline state under various conditions. These techniques have not yet been applied to synthetic polymers and their application may be difficult because of the less intense and less complete x-ray diffraction patterns obtained for many of these polymers. The measurement of volume change upon crystallization has been used successfully to compare relative amounts of crystallization in natural rubber under various conditions ( 1 , $1). .However, this method cannot be used to determine absolute amounts of crystallinity until a more definite value for the density of the crystallites has been established (6, 16). In the case of synthetic polymers, it was thought that determination of volume change might be of value as a measure of the relative degree of regularity resulting under differing conditions of polymerization. The method might be expected to be more sensitive to small differences in regularity of polymers than are x-ray techniques because volume change would be a measure of the total orientation in a polymer chain, even that portion of the 'chain which is not regular enough for x-ray diffractioni.e., imperfect crystallites.
ECEST improvements in the properties of the synthetic rubbers of the GR-S type, have increased the interest in the structure of these polymers; it was felt that the better properties "ere a result of greater molecular regularity of the polymers. An x-ray technique had been developed and appljed by Hanson and Halverson (8) to reveal a limited degree of molecular order in an elongated polybutadiene sample. More recently, Beu and his associates ( 3 )have used the x-ray method to show that the crystallizability of polybutadiene and butadienestyrene copolymers is greatly increased by a decrease in polymerization temperature. It is therefore desirable to investigate all methods which might be of value in measuring the relative degree of crystallizability of these polymers produced under diffei ing conditions of polymerization. A knowledge of the degree of crystallinity present in a polymer is also important on two other considerations. The crystalline regions have been considered to act as centers of reinforcement in gum stocks of crystallizable ,polymers. A more quantitative measure of their effect on tensile strength would, therefore, be desirable. On the debit side of the ledger, the polymers that crystallize readily become stiff in low temperature usage and the crystalline phase should be avoided. X-ray techniques have been used (4, 7 , 80) as an absolute
Vol. 41, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
1630
0
-
1st CRYSTALLIZATION
0- 2nd CRYSTALLIZATION I
A
L
-20
-10
TEMPERATURE IN %. 0 10 80
I
30
40
Figure 1. Cry-stallization of Polybutadiene Polymerized in p-Methoxyphenyl Diazothio(2-naphthyl) Ether-Potassium Ferricyanide System a t -20' C.
was usually made about 5 minutes after the relaxed polymer was immersed in the cold bath, was taken as the starting point for the volume decrease on crystallization. In some cases, crystallization was so rapid that a large portion of the crystallization had occurred before the first mercury height reading could be made. It was then necessary to obtain the true zero point for the crystallization from the behavior of the dilatometer a t higher temperatures. Sequence C-E in Figure 1 shows an isothermal crystallization. The first reading which could be obtained corresponded to point D. Obviously, the true starting point of the crystallization is given by point C, which is obtained by extrapolation of the straight-line portion of curve A-B. Similarly, the curves obtained by plotting the mercury height as a function of temperature may be extrapolated to give the mercury height in the dilatometer capillary for the start of an isothermal crystallization. The crystallizability, as measured by the isothermal volume decrease, was determined for a number of polymers. The volume decrease in cubic centimeters per gram was plotted as a function of the time in hours. The results have been grouped to show the effect of certain variables in the polymerization on the crystallizability of the polymer. EFFECT O F POLYMER COiMPOSITION
METHOD OF MEASURING VOLUME CHANGES
The volume changes were measured by use of Pyrex dilatometers of the type described by Bekkedahl and Wood ( I , 2 ) . A calibrated capillary tube, 50 cm. long, with an outside diameter of 6 mm. and a bore of about 0.5 mm. was sealed to a length of tubing of 12-mm. outside diameter. I n some cases, where it was desired to eliminate the effect of variation in the nonpolymeric content, the crude polymer was extracted twice with refluxing ethanol-toluene azeotrope, once with acetone, and then vacuum-dried to constant weight. This purification method has been shown to be suitable for refractive index measurements ( 8 2 ) . About 2 grams of polymer were accurately weighed, cut into small pieces, and placed in the open end of the large tube. A close-fitting, hollow glass plug was then inserted, and the dilatometer bulb \vas completed by drawing off the excess tubing a t a point just behind the hollow plug. The dilatometer was evacuated and filled with mercury through the capillary. A scale was attached to the capillary tube, and the amount of mercury was adjusted to permit observation of the mercury height in the capillary. Any centers of crystallization which were present were removed by immersing the dilatometer bulbs in a hot bath. Sample! of polybutadiene and polychloroprene were preheated at 70 to 7 5 " C., which was considered adequate because no variation in behavior was observed when temperatures as high as 130" C. were used. After 10 minutes of preheating, the dilatometers were immersed in a stirred bath of trichloroethylene contained in a Dewar flask. Preliininary measurements were made in which the height of the mercury column in the capillary was recorded as a function of the bath temperature while the bath was slowly cooled by increment additions of solid carbon dioxide. Readings were made at 4-minute intervals and were graphed with mercury height readings as ordinates and temperatures as abscissas. I n all cases, an initial linear dependence of the height of the mercury column on temperature was found. Departure of the curve from linearity was evidence that the polymer was crystallizing. Such curves provided qualitative information as to the temperature range and rate of crystallization. The type of curve obtained for a sample of polybutadiene polymerized at -20" C. in an emulsion system activated by potassium ferricyanide, diazothio ether, and mercaptan is given by sequence A-B in Figure 1. This type of system has been described by Kolthoff and Dale (11)- I n this case, mercury height readings have been converted to specific volumes. After the curves of mercury height vs. temperature had been obtained, the samples were heated again and then subjected to isothermal conditions. Curves showing the decrease in volume in cubic centimeters per gram of polymer versus time in hours were drawn from the data. I n those cases where the rate of crystallization was at least initially low, the h s t reading, which
The crystallizability of a series of polymers ranging in composition from polybutadiene to 70/30 parts by weight of butadiene styrene nas determined at -25' C. and the results are plotted in Figure 2. These polymers were prepared on a pilot plant scale in a p methoxyphenyl diazothio-(2-naphthyI) ether-potassium ferricyanide [MDK;-IZ3Fe(CN)6] activated system at -10" C. (16). The 70/30 butadiene-styrene copolymer showed only a small volume change after more than 26 hours a t -25" C. Polybutadiene, on the other hand, showed a very rapid initial volume decrease, eventually amounting to 23,2 X cc. per gram. The 9 O / l O butadiene-styrene copolymer occupied an intermediate position with a volume change of 13 X 10-8 cc. per gram after almost 31 hours a t -25" C. Thus, the introduction of styrene into the polybutadiene polymer chain interrupts the structural regularity and inhibits crystallization. Gum tensile data obtained from compounds of these polymers were poor in all cases. However, break occurred a t such low elongations that there was no opportunity for crystallization to occur during tests at room temperature, and no conclusions may be drawn concerning the effect of crystallizability on tensile properties. Physical test data for black-loaded compounds made from this series of polymers have been reported elsewhere (IS, 14, 16).
P A
70/30 BD:S
90/10 BD:S
--
POLYBUTADIENE
0
5
IO
TIME IN HOURS AT-25%. IS e0 25
30
Figure 2. Crystallization of Polybutadiene and Butadiene- Styrene Copolymers
3
'
August 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
I;'------
1631
The crystallizability of the crude polymers prepared a t 15", 35", and 50" C. was determined a t 5 " C., and the results are shown in Figure 5. A smooth increase in the
crystallizability of the polychloroprenes with decreasing temperature of polymerization was found for this series. The determination on the polymer prepared a t 35" C. was rerun for a longer period of time than the 200 hours shown IO in Figure 5. A total observed volume decrease of 11.0 X 10-8 cc. per gram was obtained after 624 hours. Physical properties of vulcanizates of the polymers were determined -~ 5 in a gum stock developed for neoprene (Table I). The physical test data are given in Table 11, in which the crystallizability data are included for correlation of the n degree of crystallinity with physical properties. The data in Table I1 show a striking improvement in the stress-strain properties of the vulcanizates with decreasing polymerization temperature. Modulus decreases and tensile strength and elongation increase for polymers prepared a t successively lower temperatures. The bending modulus ( l a ) ,which is given in terms of the temperature Figure 3. Crystallization of Polybutadiene Polymerized at in degrees centigrade a t which Young's modulus is 10,000 various Temperatures in p-Methoxyphenyl Diazothjo-(2pounds per square inch, shows little dependence on polynaphthyl) Ether-Potassium Ferricyanide Systems merization temperature with the exception of the sample prepared by polymerization at 15" C. The relatively higher value obtained in this case is due in part to crystallization Compared with the usual range of physical properties characterisof the vulcanizate during the test. tic of compounds of the corresponding polymers prepared a t The effect of polymerization temperature on the crystallizhigher temperatures, the properties of the compounds of these polychloroprene has been noted by Whitby (18). The ability more crystallizable polymers appeared to be generally much . of effect on polymer properties has also been the subject of a recent improved.
a
-
4
EFFECT O F POLYMERIZATION TEMPERATURE
I
I
POLYBUTADIENE IN AN MDN-K3Fe(CN)6 ACTIVATED SYSTEM. The results obtained for a series of polybutadienes prepared in p-methoxyphenyl diazothio-(2-naphthyl) ether-potassium ferricyanide activated systems a t various temperatures are given in Figure 3. The polymers prepared at lo", -20°, and -33" C. were bottle polymerizations from a single source (9). The polymer prepared a t -10' C. is the same pilot plant polybutadiene (8) as is shown in Figure 2. Extraction with ethanol-toluene azeotrope was used to purify these polymers. The increased crystallizability of the polymers as the temperature of polymerization is lowered is evident from the greater volume decrease indicated by their curves (Figure 3). POLYBUTADIENE IN A REDOXSYSTEM. The effect of polymerization temperature on the crystallizability of polybutadiene was also studied in a redox system (IO). Figure 4 shows the results obtained with azeotropeextracted samples of polymers prepared in bottles a t lo", -lo", and -20" C. Included in the figure are results for smoked sheet and for polybutadiene prepared in the GR-S system. Figure 4 shows the rapid increase in the crystallizability of the polymer which results from a decrease in its polymerization temperature. The curve for smoked sheet has been included t o emphasize the relatively high rate of crystallization of polybutadiene prepared a t low temperatures. POLYCHLOROPRENES. Polychloroprene crystallizes readily upon stretching a t room temperature and therefore is a convenient polymer on which to study the effect of crystallization upon physical properties. Chloroprene (2-chloro-1,3-butadiene)was received from E. I. du Pont de Nemours & Company as a 50% solution in xylene. It was distilled through an 8-inch column packed with Raschig rings immediately before use. For polymerization at various temperatures, an emulsion system employing sodium rosinate, sulfur, and ammonium persulfate was used (19).
-1.
IX AT 10%.
1
REDOX AT -2O.C.
--%SMOKED. SHEET -25
Q
TIME IN HOURS AT -1OOC. qo s,o IQO I ~ O
4,o
2p
'go
140
igo
'
I
20 ZO
Figure 5.
40
TIME IN HOURS AT 5.C
80
BO
100
120
140
SO
180
200
Crystallization of Polychloroprene Polymerized a t Various Temperatures in Rosin Soap System
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 41, No. 8
peratures l o m r than -33" C. ivould result in even greater strucTABLE I. G m i STOCKFORMULA FOR POLYCHLOROPRENE tural regularity of t'he polybutadiene. There appears t o be 110 Ingredient
Parts by Weight 100.0 0.3 2.0
15,s
10.5
0
125
176 325
300
350 350 325
... 400 400
200 250 300
difference between the p-methoxyphenyl diazothi0-(2-naphthy.I) ether-pot.assiuni ferricyanide and redox systems with regard to the crystallizability of the resulting polybutadienes. The curve obtained for the pol~~hlorogrenes indicat>esthat maximum polymer regularity has almost been reached at the lomest pol?^ nierizat,ion temperat'ure of 15' C. . The technique of measurement of isothermal volume decre;tae is indicated, by the examples listed, to be a simple and very useful tool for comparison of the regularity of synthetic polymers. Continued studies on the influence of polymerizatioii condit>ions,composition, and polymer structure on the regularit?. of t.he polymer chain should prove of value. A comparison OF the efficiencies of specific types of systems in producing po1ymei.s of high structural regularity is an important' aspect of the proh]em which could be investigakd by this technique,
2125 2150 1950
875 975 950
425 525
2750 2550 2978
ACKNOW-LEDGMENT
0.5
5.0 4.0 ~
111.8 ~
TAB1E
11. EFFECTO F P O L Y X E R I Z A T I O X TEMPERATLRE 01 PHYSICAL PROPERTIES OF POLYCHLOROPRENE
Observed volume decrease, cc./g. x 103 3Iodulus at 3007,, 280' F. 20 min. 40 min.
Pol) rn~riiationTemperatuie, C Neoixene 35 50 60 GN
15
200 200
4025 4325 3900
. .
400
260 820 980 840 200 290 750 900 540 730 40 min. 290 760 540 880 720 80 min. Bending modulus" (ternperature, ' C., at which Young's modulus is 10- 19 - 38 -38 - 36 -38 Ib./sq. inch) a Ail samples, except the one obtained fiotn polymer prepared at 60°, held at O0 C . for 16 hours before normal testing sequenco (II) was started.
The authors wish t.0 express their appreciation to F. ]I-. Stavely for his continued interest and to The Firestone Tire & Rubber Company for permission t o publish these results. LITEHATLXE CITED
(1) Bekkedahl, N., J . Besearch AVatl. Bur. S t a n d a d s , 13, 411-31 (1934) : Rubbe? Chem. and Technol., 8, 5-22 (1935). Hekkedahl, N., and K o o d , L. A . , IKD.EXG.CHEM.,33, 381-4 (1941); Rubber Chem. and Technol., 14, 347-55 (1941). Beu, E. K., Reynolds, W.E., Fryling, C . F., and hfoMurry, H. L., J . PoZUmer Sei., 3, 465-80 (1948). Field, J. E., J . Applied Phys., 12, 23-34 (1941): Rubbe, Chem. and Technol.. 14, 656-71 (1941). Gee, G., J . Polvmer Sci., 2, 451-62 (1947). Gehman, S. D . , Chem. Rex., 26, 203-26 (1940). Goppel, J. M., A p p l i e d Scientific Research, 1, 3-26 (1947) ; in preparation, "Proceedings of t h e Rubber TechnologJConference," London, 1948. Hanson, E. E., and Halverson, G., J . Am. Chem. Soc., 70, 77983 (1948). Hiauchi, T., arid Dean, D. J., Univcrsity of -4kron Government Laboratories t,o Office of Rubber Reserve, private cornniuriication, Oct. 1, 1947. Johnson, P. H., and Bebb, E. L., J . Polymer Sei., 3, 389-99
(1948). Kolthoff, I. M., and Dale, W.J., Ibid., 3, 400-9 (1948). IND.ENG.CHmf., 36, 40-6 (1944). Liska, J. Schulze, 1%'. A , , Reynolds, W ,B., Fryling, C . F., Sperberg. L. R., and Troyan, J. E., I n d i a Rubber W o r l d , 117, 739-42 (1948). Shearon, W. H . , Jr., McKenaie, J. P., and Samuels, hl. E..
w.,
i x
-40
!
I
I
-20 0 20 40 POLYMERIZATION TEMPERATURE IN 'C.
I
60
Figure 6 . Effect of Polymerization Temperature on Crystallizability of Polybutadiene and Polychloroprene
study by Walker and ;LIochel ( 1 7 ) . By use of the dilatometer, a more quantitative evaluation of the degree of crystallization has been obtained which correlates very well with these changes in physical properties. On the basis of these preliminary results it may be said that the tensile properties of the vulcanizates are closely related t o the crystallizability of the polymers. A similar conclusion has been reported by Gee and others (6). The results in Figures 3 to 5 are summarized in Figure 6. The h a 1 observed decrease in volume in cubic centimeters per gram has been plotted against polymerization temperature, It is most significant that the curve for the polybutadiene systems shows no tendency to flatten out a t the lowest polymerization temperatures used. It is evident that polymerization tem-
Smith, W. H., and H a n n a , N. P., J . Research Natl. Bur. Stan~lards, 27, 229-36 (1941): Rubber Chem. and Technol., 15, 266-71 (1942). Troyan, J. E., and Tucker, C. M., Hydrocarbon Chemicals Co. t o Office of Rubber Reserve, private communication, March 29, 1948. Walker, H. W., and Mochel, W. E., in preparation, "Prooeedings of t h e Rubber Technology Conference," London, 1948. Whitby, G. S., University of Akron t o Office of Rubber Reserve. private communications, Feb. 27, 1947, and April 10, 1947. Wilder, F. N. (to E. I. d u Pont de Nemours & Co.), British Patent 587,804 (May 6, 1947). Wildachut, A. J., "Technological and Physical Investigations on Natural and Synthetic Rubbers," Kew York, Elsevier Publishing Co., 1946. Wood, L. A., and Bekkeciahl, N . , J . Applied Phys., 17, 36275 (1946): J . Reseawh .VutZ. Bur. Standards, 36, 489-510 (1946); Rubbei Chem. and T e c h r d . , 19, 1145-62 (1946). Wood, L. A., and Madorsky, I., Xational Bureau of Standards t o Office of Rubber Keserve, private communication, Nov. 30, 1944. RECEIVED November 16, 1948. Presented before the Division of Rubber Chemistry, . 4 M E R I o A a CHEMICAL SOCIETY,at Detroit, Mioh., Kovember 8 to 10, 1948. Investigation carried out under the sponporsbip of the Office of Rubber Reserve, Reconstruction Finance Corporation, in connection with the government synthetic rubber program.
