INDUSTRIAL AND ENGINEERING CHEMISTRY
1570 Table 11.
Laboratory Physical Data' C o m p o u n d i n g Recipe
Parts Compound Ingredient Polymer EPC black Zinc oxide Sulfur Altax Stearic acid
h 100 40
Conipoun4 B 100
40 5 2 3
5
2 3 1.5
..
Test Conditions
Mooney viscosity, 4-minute reading large rotor, 212O F. Stress-strain tests, O R R specificatibn procedure, January 1, 1940, a t 77' and 212' F. Temperature rise, Goodrich flexometer; 143 lb./sq. inch load; 0.175-inch stroke; 1800r.p.m.: 30-minute test a t 212' F. Flex life DeRlattia flexometer; pierced specimen; d a t a reported as number of flex'uros to produce 0.8-inch crack growth a t 312 cycles per minute and 212" F. Resilience Goodyear-Healy pendulum method; angle of inertia, 16'; Shore hArdness, 5-second reading with Shore Type A durometer. Quality index, ratio of flexures of test sample to flexures of GR-S a t equal hysterimeter values. High-Sugar Formula 41 Reaction temp., ' F. Type of polymeriza- Contintion uous Batch Raw Xooney viscosity 66 1 I L 4 fiO h l L 4 300% modulus, Ib./ 1,200 sq. inch 1,200 Tensile strength, lb./ 4,010 sq. inch 4,240 Elongation, % a t 670 break 640 Set (10 min. after 10 11 break), % 12,000 Flex life, flexures 10,000 Temperature rise, 55 F. 54 4.9 4.3 Quality index 61 Resilience, 7c 63 66 64 Shoi e hardness a
Low-Sugar 41 ' Continuous Batch
GR-S-10
59 h l L 4 60 AIL 4
87 M L 4
-.
117
Continuous
Vol. 41, No. 8
die test thanis GR-$10. The stress-strain properties of low teniperature rubbers made in accordance with Formulas A and B by batchwise and continuous processes are summarized and compared with GR-S-10 in Table 11. The :tocks were compounded according to the standard recipe, specified by the Rubber Reserve for rosin rubbers. S o stearic acid was used in compounding the polymers prepared mith the mixed emulsifier. Except for the resilience values, which were determined on stocks cured a t 292" F. for 10 minutes more than the optimum, values are presented for stocks a t their optimum cures. The tensile strengths of bot'h the batchwise and continuous low- temperature polymers are similar; they are about 30% superior to that for GR-SI0 and are generally equivalent t o that, of nat,ural rubber. For the most part, the moduli of the low ternperature rubbers, polymerized either batchwise or continuously, were equal and only slightly higher than that exhibited by GR-S-10. The over-all flex life-heat rise balance, as expressed by the quality indexes, and t,he resilience a t 77 ' F. of the batchwise and continuously polymerized low temperature polymers are about equal but considerably superior to corresponding values for GR-S-10. The resilience at 212" F. was similar to that of GR-S-10. The low temperature stocks exhibited slightly higher Shore durometer hardness values than did GR-S-10.
1480
1290
1140
Literatnre Cited
4190
4150
3100
530
600
580
9 8000
10 9000
10 4000
(1) Goodrich Chemical Co., B. F., grivate communication. (2) I n d i a RubberWorld, 119 ( 2 ) ,228 (1948). (3) Office of Rubber Reserve, unpublished work. (4) Schulze, W. A . , Reynolds, W. B., Fryling, C. F., Sperberg, L. I ? . , arid Troyan, J. E., I n d i a Rubber W o r l d , 117 (6), 739 (1948).
54 3 4 63 64
59 3.2 61 63
__
60 1.4 JD
5g
( 5 ) Shearon, W. H.. Jr.,McKenzie, J. P., and Samuels, M . E., 1x1). ENG.CHEM.,40, 769-77 (1948), (6) Troyan, J. E., Rubber Age, 63 ( 5 ) , 585 (1948).
RECEIVED M a y 26. 1940. Investigations carried out under t h e s1)onsorsliig of the Reconstruction Finance Corporation, Office of Rubber Reserve, in connection with the government synthetic rubber program.
Values presented for stress-strain tests conducted a t 77' F.
EFFECTS OF POL IZ TEMPEIEtATIJRE ON STRUC A. W.Meyer United States Rubber C o m p a n y , Pussaic, A-. J .
T
HE polymerization of butadiene may be carried out by various mechanisms under a variety of conditions. I n this paper, primary attention is directed toward the emulsion polymerization of butadiene and of butadiene with styrene using free radical catalysts. The effect of temperature of polymerization on the structures of such polymers and copolymers is reviewed. Various investigators have established that emulsion polymerization takes place by a free radical mechanism (9, 28, %), which includes initiation, propagation, chain transfer, and termination steps. Polymerization ternperature influences these reactions and therefore the structure of the polymers. Butadiene polymerization leads to various structural configurations. I n addition to the investigation of configurations for polybutadiene shown in Figure 1, this paper describes the occurrence of branches or side chains as well as cross links which lead to gel. Furthermore, differences in average molecular weight and molecular weight distribution are considered. I n the case of copolymers of butadiene and styrene, consideration is given to differences in the ratios of butadiene and styrene in various samples of copolymer and in the distribution of butadiene and styrene in various molecules in a given sample of copolymer, as well as differences in the airangement of butadiene
and styrene unit,s with relation t o one another in a given polymer molecule.
E-nsaturation I n all the structures indicated in Figure 1, there is one carbonto-carbon double bond in each butadiene residue. Various investigations have been made of the unsaturation of butadiene polymers and copolymers. The reaction of iodine monochloride 15-ith polymers waj used by Lee, Kolthoff, and hIairs (PS), whose evperiments were based on the procedure of Kemp and Peters (PO). Rehner (50)sho\ved that difficulties such as substitution or "splitting out" of acid may occur, but KoIthoff and his con-orlrers modified the method to give accurate deterininations oi' unsaturation of polybutadiene, polyisoprene, and their copolymers Jvith styrene. They found that emulsion polybutadiene made a t 50" C. had 97 to 98% of the theoretical unsaturation; emulsion polyisoprene 977,. The unsaturation of GR-Sa t various conversions corresponded to the spectrophotometrically determined styrene content; thus, it was concluded that GR-S a t various conversions had the theoretical unsaturation. Change in the temperature of polymerization from 10" t o
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1949
T h i s is a review paper relating changes i n polymerization temperature to influence on the structure of butadiene polymers. The per cent unsaturation is near the theoretical expected for diene polymers and is little influenced b y shifting the polymerization temperature or bound styrene content. The percentage of 1,2 units in polybutadiene made a t 50" C. is 23% and decreases about 3 to 4 % when the polymerization temperature is dropped to -20' C. The addition of styrene as a comonomer does not influence the ratio of 1,2 to 1,4 addition. Per cent trans-1,4 content of hutadiene polymers increases greatly w i t h decreasing polymerization temperature (50% a t 100' C. us. 80q0 a t -20' C.). The presence of styrene as comonomer has little effect on per cent trans-1,4 addition. Reduction in polymerization temperature increases the X-ray structural regularity of butadiene polymers. diffraction patterns of stretched polyhutadiene polymerized a t 20" C. and below indicate a repeat distance of 5.1 A. along the stretched polymer chains. This corresponds to a fully extended trans-butadiene unit. When styrene content of copolymers reaches 209& even a -20' C. copolymer becomes amorphons. Polybutadiene prepared a t 5" C. has T-50 values proportional to the amount of combined sulfur indicating tendency t o crystallize. Polybutadiene made a t 50" C. or copolymers with 20 parts of styrene do not give significant T-50 values because of irregular structure. A smaller proportion of low molecular weight polymer and less branching occur with decreasing temperature of polymerization. Copolymers prepared a t 50" 6. show a greater variation with conversion in the bound styrene content than those made a t -20' C. Arrangement of butadiene-styrene units in 50" C. copolymers appears to be i n random sequence.
100" C. did not alter the per cent unsaturation of butadienestyrene copolymers, according to Kolthoff and his co-workers (2%').
Ratio of 1,2 to 1,4 Addition I n butadiene emulsion polymerization, free radical propagation reactions proceed through the formation of intermediate allylic free radicals. These radicals are resonance hybrids which may lead t o 1,4 addition or t o 1,2 addition. Furthermore, cis and trans configurations of the allylic free radicals may lead, respectively, to cis-1,4 addition or trans-1,4 addition (Figure 2). A variety of methods have been used t o determine the proportion of external double bonds (1,2 addition) to internal double bonds (1,4 addition) in diene polymers and copolymers. Ozone degradation by scission a t the double bonds has been
H H H H C'C-C'C H H BUTAD I E N € I
I
1511
used by various investigators t o study the structure. Harries (14) was the first to describe this method. Alekseeva and Belitskaya (3) applied it t o a 1-1 butadiene-styrene copolymer; Hill, Lewib, and Simonsen (16) applied it to polybutadiene and to a butadiene-methyl methacrylate copolymer. Later, Alekseeva ( 2 ) investigated a copolymer of butadiene and acrylonitrile, and recently Rabjohn et al. (99) worked on polybutadiene and butadiene-styrene copolymers. Rabjohn and co-workers concluded that ozonolysis gives an approximate idea of the structures of butadiene polymers and copolymers. The so-called "ozonization number" or apparent per cent 1,2 polymerization is based on the formic acid and formaldehyde obtained by ozonolysis of t h e terminal vinyl groups. It was considered to give results indicating only the relative proportions of 1,2 groups of the various polymers.
Table I. Ozonization of Polymers (29) % Diene Ozonization
Description Natural rubber, crepe GR-6, 75/25, 71% conversion GR-9, 65/35, 81% conversion GR-9, a t 110-130° GR-9, high cetyl mercaptan GR-S, ferricyanide initiator GR-9. latex treated with D D M GR-S: acid side Standird GR-S GR-5 cold milled 45 minutes Polybbtadiene, emulsjon 50' Polybutadiene, emulsion llOD Polybutadiene, Na sand, loo
I
22.2 22.6
18.3 22.2 20.7
19.4 12.0 22.8
L E A D S T O Trans I ,4- A D D I T I O N
"2 L E A D S TO 1,2ADDITION
H
H R
H2.
Figure 1. Polybutadiene Structures
18.0
16.1 8.1 14.3
I
1,2
8.6
19.1
19.1 10.1 13.4
"2
1
C i s 1,4
18.7
No.
11.8
22.2
The various factors in the polymerization process did not greatly influence the manner in which the butadiene molecule entered the growing polymer chain (Table I). The ozonization number for natural rubber is not construed to indicate the presence of terminal vinyl groups. No conclusion was drawn regarding distribution of styrene and butadiene units in the polymer molecule. Copolymers of butadiene and styrene made a t 110 O to 130 O C, had ozonization numbers of 17.1 and 18.0 compared with 19.1 and 19.3 for 50' C. GR-8. GR-S polymerized on the acid side gave values of only 13.4 and 14.3. Perbenzoic acid titration was applied by Kolthoff, Lee, and Mairs (8%')and by Saffer and Johnson (31) to polymers and 00polymers of butadiene. The method depends on the fact that perbenzoic acid adds much more rapidly to internal than to external double bonds. It is seen in Table I1 that 1,2 addition in emulsion polymerization of butadiene is 22 to 23% a t 50" C. and is independent of conversion change from 44 t o 75%.
H
T r a n s 1,4
11.8 19.3 20.9 17.1
100 80 81.4 80 80 80 75 7s 72 72 100 100 100
"
LEADS T O Cis I,'+- A D D I T I O N
H C-C
"' C *c
L E A D S TO 1,2 ADDITION
Figure 2. Leads to cis and trans Addition
INDUSTRIAL AND ENGINEERING CHEMISTRY
1572
Table 11. External Double Bonds in Butadiene Polymers (22) Polymerization Temp.,
Type of Polymer Emulsion Sodium
a
Cqnversion,
70
Intrinsic Viscosity,
OC.
50 50 50 50 30 30 10
75 44
...
111 1
Solubilityn,
...
..
3.6 3.8 3.6 5.1 5.2
.. .. .. ..
%
% 1,2
100 100 98 95 100 100 95
23 22 58.5 59 67 61
73.5
Before purification.
Table 111. Polymerization Temperature and Catalyst us. External Double Bonds in Butadiene-Styrene (73/25) (22) ~
Type of Polymer Emulsion
~-
Polvmerization Temp.,
Emulsion
Sodium
Standard Recipe 100 Butadiene 5 0 Ivory soap flakes (SE'flakes) 30 Water 30 Potassium persulfate 10 n-Dodeoyl mercaptan
Con- Unsaturaversion, tion,
70
OC.
100 .5 180 0.3 0.5
MDN
5 30 30 50 50 50 30 30
Redox I Redox I1 Phillips t e r t - D D X Sodium
70
70 1,2
75 80
74 78
19.9 20.5
84
76 77
19.5 19.0 18.5 18.5 20 24 22 58 56.6 59 55
60 GO 68 61 61 65
77 76
78.5 78.5 80 72 72 22 (2.5
Sodium-catalyzed polybutadiene made at 50 O C. has 58.5% 1,2 units and when polymerized a t 10' C., 73.501, 1,2 units, Butadiene-styrene copolymers contained about the same percentage of external double bonds (of total double bonds in sample) as polybutadiene. Thus, polymerization temperature, type of catalyst, percentage conversion, microgel formation, and varying styrene content did not greatly affect the percentage of external double bonds in the emulsion polymers. The results of Saffer and Johnson agreed well with those of Kolthoff et al. However, they had a unique result on an acid system copolymer made a t 10" C. showing only 14 to 15% external double bonds. This may be worth investigating further, as ozonization numbers were also low. Saffer and Johnson reported that heat softening and modifier concentration had no effect on per cent 1,2 addition and that polyisoprene had only 11 to 14% 1,2 addition.
Table IV. Type of Polymera Emulsion S R
Conversion us. External Double Bonds in Butadiene- Styrene Copolymers (22) Conversion,
Intrinsic Viscosity,
[n I
Cnsaturation,
% % 1,2 20 0.9 80 21.1 37 1.1 80 21 . 6 GO 1.2 78.5 20.6 80 1.5 78 20.6 Polymerization temperature, 50' C. Solubility, 10070, before and after purification.
Table V.
%
1,2 in Butadiene-Styrene Copolymers ( 3 1 )
GR-8 control GR-6 control German Buna S-3 unsoftened German Buna 5-3 heat softened 21 To conversion copolymer 42.6% conversion copolymer 60.@% conversion copolymer 50' C. acid system copolymer p H 4 . 3 10' C. acid system copolymer pH 4.5
%12 22-23 22 22 22 22 24 22 16-18 14-15
Vol. 41, No, 8
Infrared spectra were applied by White and Flory ( 3 6 ) to the study of the structure of synthetic rubbers. They concluded that emulsion polybutadiene contains 1,2 units but fewer than are found in sodium polybutadiene. Copolymerization with styrene or acrylonitrile was reported to reduce further the proportion of the 1,2 units. The presence of modifiers was reported ( 3 4 ) t'o have no effect on the mode of butadiene addition. Furthermore, no difference was found in the 1,2 content of sol and gel portions of elastomers. "1,2 Index" values which were assigned to polymers were believed to be accurate to 1 0 . 5 unit of 1,2 index: 10.0, 1 0 . 4 , 1 0 . 3 10.7, 1 0 . 6 9.7, 10.5, 1 0 . 0 , 9 . 6
GR-S Unmodified Buna S German Buna S
There was believed to be a slight decrease in 1,2 content for copolymer made a t 15" C. It was believed that the 1,2 index was not an exact measure of 4 2 concentration but this was considered to be below 15% for all these polymers. The 990 om.-' band was taken to indicate terminal and the 970 cm.-1 band internal double bonds. The 91 0 cni.-' band was considered t o occur with any double bond. It was reported by Field, Woodford, and Gehman (7) that infrared spectra of sodium polymers show a high fraction of vinyl side groups. The above investigators endeavored to measure the per cent 1,2 addition by means of a calibration curve from known mixtures of pure 1-octene and 2-octene.
25
I
I.
J -20
20
40 60 80 TEMPERATURE OF P O L Y M E R I Z A T I O N ( ' C , )
0
100
Figure 3. Effect of Polymerization Temperature on 1,2 Addition Polybutadiene and 71/29 butadiene-styrene copolymers
Recent work by H a r t and Meyer (f6)utilized 1-octene for the quantitative estimation of per cent 1,2 units in butadiene polymers based on the absorption at the 910 em.-' band. It was shown t h a t the temperature of polymerization had a relatively small effect on 1,2 addition in the polymerization of butadiene and butadiene-styrene copolymerization. A maximum of 23.2% 1,2 addition was found a t 50" C., which is in excellent agreement with the value of 2370 found (22) by the perbenzoic acid method. There was some indication that the amount of 1,2 addition decreased continuously with polymerization temperature to a value of 19.6% at - 19" C. (Figure 3). The styrene unit was shown to have no influence on the ratios of 1,2 and 1,4 addition a t the various temperatures investigated (Figure 4). This also is in agreement with the results of Kolthoff, Lee, and Mairs ( 2 2 )by the perbenzoic acid method. The reaction of bromotrichloromethane with butadiene was found to yield 85% l,l,l-trichloro-5-bromo-3-pentene (21 ) (Table VI). This indicates that free radicals attacked the butadiene a t a terminal carbon and that chain transfer takes place predominantly with the 1,4 rather than the 1,2 configuration.
August 1949
+ +
Table VI. Butadiene Reactions
CClaBr H&=CH-CH=CHr CIHBSH HzC=HC-CH-CHz
22
----f ClsC-CHI-CH=CH-CHzBr
(81)
CaHoS-CHz-CH=CH-CHs
(19)
4
Sivertz (19) and co-workers a t the University of Western Ontario showed that butadiene reacts with butyl mercaptan (butanethiol) in a persulfate-catalyzed emulsion system a t 50 O C. to give 100% 1,4 addition product-namely, crotyl n-butyl thioether. Thus, the butyl mercaptan free radical attacks the terminal or most hydrogenated carbons of butadiene exclusively, and chain transfer takes place with the 1,4 rather than the 4 2 configuration. Inasmuch as higher butadiene polymers disclose the existence of side vinyl groups, i t must be presumed that chain transfer a t nonterminal carbons becomes possible when the attacking radical derives from butadiene. This probably depends on the negativity of the radical. The appearance of 1,2addition products might be expected t o occur even in the first propagation step. Sivertz is planning t o analyze the product of this step. It would be interesting also to analyze products for 1,4 cis and 1,4 trans content in the case of the initiation and first propagation steps. The nature of the products and the yield indicates that the overwhelming number of initiations are made by butyl mercaptan radicals and termination is primarily made by mercaptan.
0
0
5
Figure 4.
-
-
%
trans-1,4 51.4 56 6 62 0 71 5 76 3 79 6
% 1,2 21.0 22 1 23 2 20 8 20 2 19 6
1,4/1,2 3 03 3.20 3 31 3.80 3 85 4 10
Est.
trans/&
2.16 2 89 4.2 9.3 24 6 100 0
Effect of Styrene on 1,2 Addition
Butadiene-styrene copolymers prepared at 5' C.
POLYBUTADIENE 71/29 B / S COPOLYMERS
\4 \
20 90 60 TEMPERATURE OF POLYMER12ATlOt
I
I
Figure 5. Effect of Temperature on Polymerization on trans-1,4 Addition
on the molar extinction coefficient of the trans configuration (Figure 7). The refractive indexes for polybutadiene polymerized a t various temperatures ranging from 97' to 19 O C. are given in Table VI1 along with data on trans-1,4 and 1,2 units in the polymers. The values for refractive index are shown to decrease with decreasing temperature of polymerization. Thus, there is a drop of 0.016 in ng6 as polymerization temperature decreases from 97 to - 19 O C. At the same time there is an increase in trans-1,4 units from 51.4 to 79.6% or a total of 18.2%. Measurement of refractive index may afford a rapid indication of per cent trans-1,4 units or of polymerization temperature of polybutadiene. I n using refractive index for determining per cent bound styrene, there is need for a small correction caused by the change due t o variation in polymerization temperature. Thus, the increase of 0.0016 in ng when polymerization temperature increases from 19 O t o 97' C. corresponds to about 2% estimated bound styrene. D a t a on refractive index for polybutadiene X-453 ( 5 ,. C.) show a decrease of 0.0013 compared to 50" C. polybutadiene (d7).
-
-
Structural Regularity X-Ray Studies. X-ray studies by Beu et al. (4)of low temperature butadiene polymers have given information regarding structural regularity. The repeat distance of 5.1 * 0.1 A. was obtained for stretched, chilled
PolybutadienesPolymerized at Various Temperatures (15)
Polymerization Temp., C. 1L ?? 97 1.5159 65 1 5156 50 1.5156 5 1 5148 10 1 5144 19 1 5143
25
20
15
10
PER C E N T C O M B I N E D S T Y R E N E
0
Hart and Meyer (16) used the 967 cm.-' infrared band to measure the per cent trans-1,4 addition in butadiene polymers. Pure trans-4-octene obtained from the National Bureau of Standards was employed for calibration purposes. A study of the National Bureau of Standard spectra of cis-2-butene, trans-2butene, cis-2-pentene, and trans-Zpentene revealed the fact that the 967 cm.-l band is more strongly absorbed by the trans isomer than by the cis isomer. The spectra of trans-Poctene and polybutadiene are shown to have absorption peaks a t 967 cm.-'. Therefore, it was concluded that the 967 cm.-l band may be used to measure trans-1,4 addition in butadiene polymers. The molar extinction coefficient of the 967 cm.-l band was determined by means of trans-4-octene for quantitative measurements on butadiene polymers. It was found that trans-1,4 addition increases continuously from a value of 51.4y0for the 97" C. polybutadiene t o 79.6y0 for the - 19 O C. polymer. These results indicate that a t - 19 O C. substantially all of the 1,4 addition product is in the trans-1,4 form (Figure 5). When results are plotted t o show the change in per cent trans1,4 in total 1,4 with polymerization temperature, a portion of an S-curve is obtained which is typical of such competitive reactions (Figure 6). I n butadiene 71-styrene 29 copolymer, the per cent trans1,4 also is shown to increase with lowering of polymerization temperature. The per cent trans-1,4 is greater than that for polybutadiene. The correction for absorption a t 967 cm.-l by styrene residues is -0.030/, per per cent combined styrene. This is not sufficient to account for differences in polybutadiene, possibly because of an effect of the adjacent styrene residues
UNCORRECTED
P
Trans-l,4 Addition
Table VII.
1573
INDUSTRIAL AND ENGINEERING CHEMISTRY
Calcd. % Styrene 0 74 0 37 0 37 -0 60 -1 09 -1 22
Of at temperatures no higher than 30 O C. This corresponds closely to the length (5.05 A.) of a fullv extended planar butadiene unit with normal bond angles and lengths. The fact that there is only one butadiene unit per repeating unit indicates that chains in the crystalline portion of the- polymer are in the trans configura-
Vol. 41, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
1574
0
9
CORRECTED FOR
P E H C E N T STYRENE
1
20
-100
0
-50
IO0
50
150
200
250
TEMPERATURE OF POLYMERIZATION ( ' C . )
Figure 6.
Effect of Polymerization Temperature
'
s.0,
01
'
'
"
-40
'
"
.
'
0
-20
'
'
'
20
'
"
1
'
3.0,
-I
"
UO
60
0.0;
TEMPERATURE OF P O L Y M E R I Z A T I O N ('C.)
IO
20
30
40
PER CENT STYRENE I H FEED
Figure 8. Effect of Polymerization Temperature on Extent of Polybutadiene Crystallization
Figure 9. Effect of Styrene Content and Polyrrierization Temperature on Crystallization
tion, The cis configuration would require a t least two butadiene units to complete a single geornetiic repeating unit. On cooling and stretching polymers made at -20" C. the amorphous halo present a t room temperature disappears almost completely. The major portion of the polymer is undoubtedly crystalline and most of the double bonds are therefore in the trans configuration. According to Beu et al. (4)purified polymers contain 14% vinyl side groups resulting from 1,2 addition which are presumably distributed at random along the chain. These groups are probably not included in the crystalline regions, for they would prevent periodicity in the structure. Their presence, therefore, limits the amount of polymer that can crystallize. I n the work of Beu et al. the presence of styrene in butadiene-styrene copolymer also reduced the tendency to crpstaliize (Figures 4a and 6, pages 472 and 475, of 4). Hanson and Halvelson ( I S ) found a certain amount of ordeiing in '.noncrystalline rubbers." Polybutadiene and emulsion polydimethylbutadiene displayed a greater degree of ordering than sodium polybutadiene, possibly because of the presence of a high per cent 1,2 units in the sodium polymer. GR-S and emulsion polybutadiene showed the presence of small amounts of crystallites presumably in the 1,4-trans form.
Intermolecular distances bet'ween extended chains increased with increasing amounts of side groups. The technique was to cool the raw polymer before elongation and freeze immediately after stretching to prevent relaxation. Chemical Methods. Pioneering work using chemical methods for determining (27) structure of diene polymers involves the react,ion of iodine with olefinic compounds; thus the dissociation constant of the addition product wit,h cis-2-pentene a t 0" C. is 6.68 X The iodine adduct with GR-S made by the Mutual recipe a t 50" has a dissociation constant of 94 X 10-8. Redos GR-S made at, 0" C. had a dissociation constant of 143 x 10-8, thus indicating a higher trans configuration in the 0 " C.
Table VIII. Correlation of Side Groups with Intermolecular Spacing ( 1 3 ) Amorphous Halo Nonomer,
Polymer,
Polymer Eniulsion butadiene Sodium butadiene Emulsion isoprene Sodium isoprene Emulsion dimethyl butadiene a
% l..J-n A.
A.
Equatorial Arc, A. 4.06
4.6 5.9 5.1 6.6
4.6
90 45
4.9
...
5.85 4.86 6.53
85
5.7
5.3
5.30
76 40
By perbenzoic acid method.
...
GR-S. Dilatometer Investigations. By measuring with a dilatometer the change in volume with decrease in t,emperatureit has been possible to gain inforniation regarding crystallization and regularity of butadiene polymers (24,96). As the temperature of polymerization of polybutadiene is decreased from 60" to -20" C. there is a marked percentage decrease in volume on storage a t -45" C.; t,his indicates great,er regularity and crystallization ( 2 8 )(Figure 8). It vias shonn that as the styrene content of butadiene-styrene copolymers is increased the tendency t o crystallize decreases. This variable is superimposed upon the copolymerization temperature. These results (26) are interesting parallels to the findings from x-ray diffraction w o ~ k(Figure 9). Investigations ( 2 7 )of the changes in volume using a dilatometer showed that -10" C. redox polybutadiene had a faster rate of volume decrease a t - 8" and - 20' C. than Hevea rubber. This is expected, as the trans configuration of polybutadiene is a smaller repeating unit than the cis configuration of I-Ievea. Furthermore, methyl side groups which possibly slon up crystallization in Hevea are absent in polybutadiene. T-50Data. Polybutadiene made a t 5" C. has been found (25) to give significant change in T-50 value in proportion to the
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1949
Table IX. T-50 Data on Synthetic and Natural Rubbers (25) 50"
c.
Polybutadiene or Regular GR-S, OC. (-39) Uncured (-39) 1% combined sulfur
50
c.
Poly butadiene, +6 -4.6
C.
Katural Rubber, O C. +18 + 5
amount of combined sulfur. The 50" C. polybutadiene has a low T-SO even without any combined sulfur (Table IX). GR-S prepared a t 5 " C. containing about 25% bound styrene has too much irregularity to give a significant T-50 value. The T-50 data for polybutadiene made a t 5 O C. are another indication that low polymerization temperature increases the structural regularity and tendency of the polymer to crystallize.
Molecular Weight Distribution The physical properties of rubber are determined partly by their chemical nature and partly by their average molecular weight and molecular weight distribution. Flory's work on polyisobutylene is one of the best examples of fractionation of polymers employing mixtures of solvent and nonsolvent (IO). He also correlated the molecular weight of fractions with physical properties of Butyl rubber (8). The problem is much more difficult with polybutadiene or GR-S because the structure deviates much more from linearity and, furthermore, during compounding and processing operations changes occur in molecular weight distribution and even in the formation of gel. Yanko (89) fractionated GR-S made a t 50" C. in order to characterize the physical properties of the fractions and their vuicanizates by studying the molecular weight before and after a standard milling procedure. He showed that GR-S changes greatly during milling; even cold milling greatly reduces molecular weight especially for the higher fractions. A fraction with 1,650,000 %?n(number average molecular weight) dropped to 330,000, whereas a fraction with 193,000 M n dropped only to 1&1,000. He found that elongation and tensile increased linearly with molecular weight up to 400,000 on the unmilled sample. The f i s t five fractions had higher tensile strength than the whole GR-S. By intrinsic viscosity relations to osmotic molecular weight, Yanko (38)found that [TI" = KM" where K = 5.4 X 10-4 and a = 0.66. This is identical with the value of French and Ewart ( 1 2 ) . Yanko also found that a 30 Mooney fraction had [ q ]of 1.37 and g n of 144,000. 5 Rlooney rubber had values of 0.80 and 63,000, respectively. Yanko also found that there was about 1% higher styrene content in the fractions of highest molecular weight. It is not known how significant this result, is. Very interesting work also was carried out by Johnson (17) on the effect of molecular weight distribution on the physical properties of natural and synthetic polymers. It seems rather well established that the quality of synthetic rubbers is influenced by average molecular weight and molecular weight distribution. Early investigations by Swaney and Baldwin (33) indicated that regular GR-S made a t 50" C., modified with dodecyl mercaptan (2,2'-methylenebis-4-chlorophenol, DDM), contains 30 to 40% of nonrubbery soft material below 70,000 viscosity average molecular weight, and the whole polymer has a viscosity average molecular weight of 400,000. GR-S made a t 15" C. without modifier had less than 10% low molecular weight fraction and a viscosity average molecular weight of greater than 700,000. The low molecular weight polymer was determined by solvent precipitation from benzene solution using isopropyl alcohol. The 15" GR-S was reported to have a tread stock tensile of 3800 pounds per square inch compared with 2600 for the 50"
GR-S. Data (27) are given in Table X correlating molecular weight distribution with tread wear. It is seen that polymers containing
1575
the least low molecular weight i'raction had the best tread wear. These polymers included one made a t - 10' C. Another report (87) correlates the molecular weight distribution with polymerization temperature. Reduction in polymerization temperature was also accompanied by reduction in low molecular weight fraction. These results are somewhat complicated by the fact that the low molecular weight polymers in general were not modified and Mooney viscosity data were not available. It is further reported that the increased from 200,000 to 500,000 osmotic molecular weight as the polymerization temperature decreased from 10"to -40 O C, It was reported also that polybutadiene and 70 butadiene-30 styrene copolymer (37) made a t -10" C. to 40 t o 60 ML-4 in a p-methoxyphenyl diazothi0-(2-naphthyl) ether (MDK) recipe, showed less low molecular weight material than standard GR-S. This was in line with improved wear qualities of the -10" C. copolymers. In both laboratory and factory polymerization, less mercaptan is required toproducegel-free rubber of a given Mooney viscosityfor example, 50 ML-4-as the polymerization temperature is decreased. This is true not only with DDM but also with tertiary mercaptans such a8 M T M (mixed tertiary mercaptans). Such results indicate that the number of molecules in a given weight of material is greater for the high temperature polymer and, therefore, the molecular weight distribution is broader. Various polymerization data (27') have been averaged to show that GR-S 10 of 60 ML-4 requires 0.33% DDM a t 122' F. and only about 0.25% a t 41 F. in 5-gallon reactors. At 41 O F. only 0.19% R4TM was required. I n the pilot plant 43 ML rubber was obtained at 41 O F. with 0.27% MTM, whereas a t 14' F. 49 M L rubber was obtained with 0.135% MTM. In both cases conversion was 60%.
an
Branching and Cross Linking The conversion a t which gel first appears depends upon the relative rates of the cross-linking and the chain-transfer reactions in polymerization (5). Under similar conditions of modifier consumption, the conversion a t which gel first appears will depend on the rate of the cross-linking reaction. Experimentally, i t has been found very difficult to reproduce exactly conditions of modifier consumption for polymerization a t different temperatures. However, the conditions may be considered t o be roughly comparable when identical amounts of DDM are used in the formula. Vistex solution viscosities in relation to conversions were
Table X. Molecular Weight Distribution 70/30 B/S TDN" -loo C. mill-
broken Bottle redox mill-broken unmodified Pilot plant S-3, heat softened Modified redox, untreated GR-S, untreated
5-3
DS.
Tread Wear
Tread Wear
0-1
158
10.2
41.3
41.3
7.2
14 1
7.8
19.4
29.4
30.0
7.8
140 107 106 100 81
16.0 39.9 29.7 25.2 26.0
18.7 30.8 19.7 11.8 23.2
10.6 20.2 26.5 6.8 16.6
5.4 9.1 19.4 5.3 10.0
... 16.9
Heat softened X-343
Intrinsic Viscosity Ranges 1-2 2-3 3-4 4-5
... 2.0
7.0
Gel
... ... 45.8 .. .. .. 34.5 ...
p-Tolyl diasothio-(2 naphthyl) ether.
Table XI.
Polymerization Temperature Weight Distribution
Viscosity Range GR-S Redox Redox Redox Redox Redox
+loo C. modified
+loo
C. - l o o C.
-25" C . -40° C.
0-1 29.8 39.9 10.1 11.3 3.5 5.0
1-2 19.7 30.8 19.8 18.9 28.2 13.5
US.
Molecular Over 6
Av.
.,. ...
..*
2-3
3-4
4-5
26.5 20.2 21.0 25.9 24.8 17.4
14.4 9.1 18.3 24.1 24.0 17.4
15.9 1 3 . 8 19.8 . . . 19.0 ... 17.4 29.0
...
[VI ...
1.1
... ...
0.3
INDUSTRIAL AND ENGINEERING CHEMISTRY
1576
determined for polymerization of butadiene and styrene. The Vistex peaks in Figure 10, which indicate the conversion a t which gel begins to form, occurred a t 35% conversion for the 70" polymerization and at 80% conversion for 50 O C. Using 0.10 part of DDhl in the recipe, the peak of the Vistex curve appeared at 35% conversion for a 30" C. polymerization and a t 22% conversion for a 50" C. polymerization. Similar relations between temperatures of polymerization and Vistex The Vistex have been observed in factory polymerization ($7'). peaks were somewhat higher also for the lower polymerization temperatures. Furthermore, the mercaptan consumption curves showed that a slightly larger amount of the initial mercaptan remained a t the gel point for 70" than for 50" polymerization. These data indicate that the cross-linking reaction proceeds at a slower rate relative to the chain-growth reaction a t the lower polymerization temperature. Consequently, polymers prepared at the lower temperature are expected to have less cross linking per unit of weight. v -10 PTS.DDM 5 0 ' C . ~7-10
PTS.DDM 30 C.
8 0 5 0 PTS.DDM
70'C.
0 e 5 0 P T S . D D M 50'C. 0
3
Vol. 41, No. 8
made a t -20' C. is judged to be more extended, more regular, and less tightly coiled in solution. Quantitative data on branching and cross linking are not extensive enough to allow estimation of the relative iniportance of the various structural changes which are due to decrease in polymerization temperature.
Table XII. Microstructure (15) and Viscosity-Molecular Weight Exponent of PoIybutadiene (18) Polvmerization Tbmp.,
C.
External Double Bonds, %
% trans
% cis
23.2 21.1 19.6
62 71.6 79.6
14.8 7.3
5: 5
- 19
Configuration
Of
134
...
Exponent a for E?] = K M 0.46 0.55 0.63
Ewart states that the value of a depends on polymer structure, solvent interaction, and possibly branching ( 6 ) . Thus, in the case of GR-S the styrene conionomer has changed the relationship to [TI = 5.4 X lov4 X but it is not known whether the copolymer is less branched than is polybutadiene. The amount of GR-S (Mutual recipe except 0.35% sulfole as modifier) soibed on Graphon decieased as the polymer conversion increased from 76.8 to 87.0%. T o r k is in progress to determine the gel content of the rubbers and the sorption characteristics of the nongel portion ( 2 7 ) .
s s 0
IO
20 30
PER
50 60 70 CENT CONVERSION
110
80
90
100
Figure 10. Vistex z's. Conversion and Polymerization Temperature Diffusion rate of mercaptan will be influenced by temperature and this affects the mercaptan consumption rate. This also should influence average molecular weight and molecular myeight distiibution of the polymers. Flory ( 1 1 ) has considered the possible origin of branches and cross links during polymerization of dienes. Branching occurs by chain transfer of a growing polymer radical with allylic hydrogen on polymer molecules. This reaction alone cannot lead to cross links. However, cross linking may occur by addition of a growing polymer radical a t the internal or external double bonds of a polymer molecule. The rate of free radical addition to polymer double bonds relative to monomer addition can be deduced from the average chain length and conversion a t which gelation occurs. Sivertz ( 1 9 ) states that the kinetic possibility of attack by a butyl mercaptan radical is much greater on the terminal carbons than on an unconjugated vinyl and the attack on a side vinyl in turn is more possible than upon an internal double bond. As was indicated above ( 5 ) , the gel point occurs a t a lower conversion when polymerization temperatures are increased. Flory states that the number of flam or "terminal chains" in vulcanizates increases directly with the amount of modifier and results in lower quality. Modifiers do not suppress cross linking but meiely suppress gel formation by reducing molecular weight. The relationship between intrinsic viscosity and number average molecular weight has been determined for polybutadiene polymerized a t three temperatures ( 1 8 ) .
[v] = 72.5 X lo-'
X
[v] =
x i110.55 for polybutadiene made a t 5' C. x M 0 . 6 3 for polybutadiene made at -20" C.
26.4 X 10-4
[o] = 10.6 x
J i 0 . 4 5for
polybutadiene made a t 50" C.
The increases in the exponent in the above equations are considered by Johnson and Wolfangel to be indicative of an increase of the average dimensions of the polybutadiene molecule as the polymerization temperature is decreased. Because of less branching and cross linking and more trans-1,4 units, the polymer
Bound Styrene Content us. Polymerization Temperature
Data have been obtained shon ing the variation in bound styrene for butadiene-styrene copolymer with conversion (27).
Table XIII. Variation of Styrene Content with Conversion % St,yrene in Copolymer Conversion, % 10 20 30 40 50 60 70 80 90 100
c.
500 21.4 21.9 22.3 22.9 23.5 24.2 24.9 25.9 27.2 30.0
-200 c. 24.8 24.4 24.0 23.7 23.4 23.4 23.7 24.3 25.4
30.0
Data indicate that there is greater variation with conversion for copolymers prepared a t 50" C. than for copolymers made a t -20" C. It is difficult to interpret the reason for the values a t the higher conversions for the -20" C. copolymer, as it seems unlikely that styrene content would lag behind the values for 50 polymer after being greater initially. St 90% Conversion there are 2.57, butadiene and 7.5% styrene monomer remaining. Furthermore, the minimum in the curve for the -20" C. copolymers is not in accord nith any of the present copolymerization theories. Alekseeva ( I ) carried out ozonolysis experiments on mass copolymers of butadiene with 47y0 acrylonitrile and 50y0 styrene. The nitrile polymer showed a slight preference for alternation of the butadiene and acrylonitrile groups. I n the case of the butadiene-styrene copolymer, arrangement was in random sequence. The small incleaue in tensile of GR-S polymerized at 41" F. in nonblack compounds is puzzling. It has been suggested (55) that the high tensile strength of vulcanized natural rubber may be attributed to crystallization or fibering under stress which increases tear across the direction of strain. I n carbon black stocks the pigment particles behave as rigid units and inhibit the extension of rupture. It may be that a higher degree of regularity is required t o develop high-gum tensiles with chemical rubber. However, there are indications that substantial im-
August 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
provements in quality of nonblack stocks may be obtained in the sub-freexing temperatures of polymerization. Furthermore, latex gum tensiles of more than 3000 pounds per square inch have been obtained with low temperature GR-S. It is concluded that further knowledge of interaction between elastomer and carbon black and the effects of polymer, processing, compounding, and vulcanization steps must be obtained arid integrated with the studies of properties of the final vulcanizates.
Acknowledgments The author wishes t o thank the Office of Rubber Reserve for permission to publish these results. The assistance and helpful suggestions of F. R. Mayo and J. A. Davison are gratefully acknowledged.
Literature Cited
f
Alekseeva, E. N., J . Gen. Chem. U.S.S.R., 9, 1426 (1939). Alekseeva, E. N., Rubber Chem. and Technol., 15, 698, (1942); J. Gen. Chem. U.S.S.R., 9, 1426 (1939); 11, 353 (1941). Alekseeva, E. N., and Belitskaya, R. M., Rubber Chem. and Technol., 15, 693 (1942); J. Gen. Chem. U.S.S.R., 11, 358 (1941). Beu, K. E., Reynolds, W.E., Fryling, C. F., and McMurry, H. L., J . Polymer Sci., 3, 465 (1948). Brooks, M. C., North, R. M., and Meyer, A. W., private communication to Office of Rubber Reserve. Ewart, R. H., “Advances in Colloid Science,” Val. 2, p. 233, New York, Interscience Publishers, 1946. Field, J. E., Woodford, D. E., and Gehman, S. D., J . Applied Phys., 17, 386 (1946). Flory, P. J., IND. ENG.CHEW.,38, 417 (1946). Flory, P. J., J . Am. Chem. Soc., 59, 241 (1937). Ibid.,65,375 (1943). Ibid., 69, 2893 (1947). French, D. M., and Ewart, R. H., IND. ENG.CHEM.,ANAL. ED.,19,189 (1947). Hanson, E. E., and Halverson, G., J . Am. Chem. Soc., 70, 779 (1948). Harries, C., “Untersuchungen tlber die nattlrlichen und konstlichen Kautschukarten,” Berlin, Julius Springer, 1919. Hart, E. J., and Meyer, A. W., private communication to Office of Rubber Reserve, June 10, 1948; J. Am. Chem. Soc., 71, 1980 (1949).
1517
(16) Hill, R., Lewis, J. R., and Simonsen, J. L., Trans. Faraday Soc., 35, 1067, 1073 (1939). (17) Johnson, B. L., IND. ENG.CHEM.,40,351 (1948). (18) Johnson, B. L., and Wolfangel, R. D., private communication to Office of Rubber Reserve. Jones, R., Batzold, J., Blades, A., and Sivertz, C., private communication, 1948. Kemp, A. R . , and Peters, H., IND.ENG.CHEM.,AXAL.ED., 15, 453 (1943). Kharasch, M. S., private communication, 1949. Kolthoff, I. M., Lee, T. S., and Mairs, M. A , , J . Polymer Sci. 2, 220 (1947). Lee, T . S., Kolthoff, I. M., and Mairs, M. A., Ibid.,3, 66 (1948). Lucas, V. E., Johnson, P. H., Wakefield, L. B., and Johnson, E. L., IND.ENG.CHEM.,41,1631 (1949). Meyer, A. W., Haxo, H. E., and Hermonat, W. A., unpublished work. Meyer, A. W., and Hermonat, W. A., unpublished work. Office of Rubber Reserve, private communications from organizations cooperating in government synthetic rubber program. Individual references &re not given in order t o avoid disclosure of information which may be considered confidential by participating organizations. Price, C. C., Kell, R. W., and Krebs, E., J . Am. Chem. Soc., 64, 1103 (1942). Rabjohn, J., Bryan, C. E., Inskeep, G. E., Johnson, H. W., and Lawson, J. K., Ibid., 69, 314 (1947). Rehner, J., Jr., IND. ENG.CHEM.,36, 118 (1944). Saffer, A., and Johnson, B. L., Ibid., 40, 538 (1948). Staudinger, H., Trans. Faradav Soc., 32,323 (1936). Swaney, M. W., and Baldwin, F. P., private communication to Office of Rubber Reserve. Swaney, M. W., and White, J. U., private communication to Office of Rubber Reserve. Tuley, W.F., Rubber A g e ( N . Y . ) ,64, 193 (1948). White, J. U., and Flory, P. J., private communication t o Office of Rubber Reserve. Worrell, W., Poulos, T., and Tierney, M.J., private communication t o Office of Rubber Reserve. ENG.CHEM.,ANAL.ED.,19, 165 (1947). Yanko, J. A , , IND. Yanko, J. A,, J . Polymer Sci., 3, 576 (1948). RECEIVEDM a y 26, 1949. Contribution 91 from General Laboratories, United States Rubber Company. Survey carried out under the sponsorship of the Offioe of Rubber Reserve, Reoonstruction Finance Corporation, in connection with the government synthetic rubber program.
EFFECT OF POLYMERIZATION TEMPERATURE ON PROPERTIES AND STRUCTURE OF POLYDIENES Paul E. Johnson and R. L. Bebb The Firestone Tire 6% Rubber Company, Akron, Ohio Low temperature polymerization techniques developed with butadiene-styrene copolymers have been applied to butadiene and butadiene-isoprene copolymers, and the properties of polybutadiene and polyisoprene made in several systems at 50” C. are described. The physical properties are improved with lower polymerization temperatures, and evidence of some linearity of polymer chains with increased crystallizability was observed. The low temperature resistance of the polymers, however, became poorer when the temperature of polymerization was lowered. By introducing a small amount of a diene comonomer having a side group, such as isoprene, the low temperature resistance was improved and equaled that of the polymers made at higher temperatures. It is concluded that at least part of the advantage gained by
low temperature polymerization can be attributed to the increased linearity of the molecule.
T
HE polymerization of dienes in the absence of a vinyl monomer is an attractive field of research in spite of the fact
that only poor quality vulcanizates of polydienes have thus far been reported. First is the fact that natural rubber is a pure polydiene possessing a combination of many qualities so far unattainable in any one synthetic polymer. The polydiene field is one in which little concentrated effort has been expended, even though some of the properties possessed by synthetic polydienes, such a8 favorable rebound and low temperature buildup, have been shown t o be better than those for normal GR-S(6). Furthermore, the low temperature resistance of some polydienes
