TONBEWS-Synthetic Rubber(42) (43) (44) (45) (46) (47) (48) (49)
Kolthoff, I. M., and Nedalia, A. I., Ibid., 5 , 391 (1950). Konrad, E., A n g e w . Chenr., 62, 491 (1950). Larson. hl. W., Chem,. E n y . P m g r e s s , 47, 270 (1951). Laundrie, R. IT.,Rowland, E. E . , Snyder, A. D., Taft, W. K., and Tiger, G. *I., IND.ENO.CHEM.,42, 1439 (1950). Livingston, J. PI-.,Chem. Eng. A’ews, 27, 2444 (1949). Livingston, J. W.,[T. S. Dept. Commerce, OTS, PB 517 (1945). Marvel, C. S., Ibid., PB 11193. hfarvel, C. S.,Bailey, W.,T., and Inskeep, G. E., J . Polymer
Schateel, R. A , and White, W.L., U. 8.Dept. Commerce, OTS, PB 214 (1945). Schulze, SIT. A., and Crouch, W. W., J . Am. Chem. Sac., 70, 3891 (1948).
Schulre, W.A., Tucker, C. hI., and Crouch, W. W., IND.EXG. CHBM.,41, 1599 (1949).
Semon, W. L., Chem. Eng. News, 24, 2900 (1946). Shearon, W. H., Jr., McKenaie, J. P., and Samuels, M. E., IND.ENG.CHEM.,40, 769-777 (1948). Smith, W . V., J . Am. Chem. SOC.,68, 2059, 2064, 2069 (1946). Snyder, H. R . , Steward, J. M., Allen, R. E., and Dearborn, R. J., Ibid., 68, 1422 (1946). Soday, Frank J., Trans. Am. Innl. Chem. Engrs., 42, 647 (1946). StBcklin, P., editors, T. R. Dawson and J. R. Scott, “Proceedings Rubber Technology Conference,” p. 434, Cambridge, W. Heffer and Sons, Ltd., 1938. Talalay, A, and Magat, hI., “Synthetic Rubber from Alcohol,’’ Xew York, Interscience Publishers, Inc., 1945. Tech. Inds. Intelligence Comm., Rubber Subcomm., U. S.Dept. Commerce, OTS, PB 13358 (1948). U. S. Patent 1,973,000 (1934). Vandenberg, E. J., and Hulse, G. E., IND.ENG. CHEM.,40,
Sci.. 1. 275 119461.
Marvel, C. S., Deanin, R., Claus, C J , Wyld, W. B., and Seite, R. L., Ibad., 3, 350 (1948). (51) Meehan, E. J., Ibzd., 1, 318 (1946). (52) Mitchell, J. M . , Spolsky, R., and Williams, H. L., IND.ENG. CHEM.,41, 1592 (1949). (53) Morton, A. A , et al., J . Am. Chem. Soc., 68, 93 (1946); 69, (50)
160, 161, 167, 172, 950, 1675 (1947); 481, 487 (1949). (64)
70, 3132 (1948);
71,
Naunton, W. J. S.,et al., U. S. Dept. Commerce, OTS, PB 32161 (1945).
Neklutin, V. C., Westerhoff, C. B., and Howland, L. H., IND. ENG.CHEM.,43, 1246 (1951). (56) Nelson, J. F., and Vanderbilt, B., Rubber Technology Conf., London, Preprint KO. 15 (1948). (57) Newton, R. G., and Scott, J. R., J . Rubber Research, 13, 1-19 (55)
(1944). (68) Owen, J. J., Steele, C. T., Parker, P. T., and Carrier, E. W., IND.ENG.CHEM.,39, 110 (1947). (59) Rabjohn, Norman, Dearborn, R. J., Blackburn, W. E., Inskeep, G. E., Snyder, H. R., and Marvel, C. S.,J . Polymer Sci., 2, 488 (1947). (60) Rubber Reserve Co., U. S. Dept. Commerce, OTS, PB 13523 (1945). (61) Saffer, A,, and Johnson, B. L., IND. ENG. CHEX., 40, 538 (1948).
932 (1945).
Wall, F. T., J . Am. Chem. Soc., 67, 1929 (1945). Wall, F. T., Banes, F. W.,and Sands, G. D., J . Am. Chem. Soc., 68, 1429 (1946).
Whitby, 0.S.,Wellman, N., Floutr, V. W.,and Stephens, H. L., IND.ENG.CHEM.,42, 445 (1950). (78) White, L. M., Ebers, E. S.,Shriver. G. E., and Breck, S., Ibid., (77)
37, 770 (1945). (79)
Wicklate, J. E., Kennedy, T. J., and Reynolds, W. B., J . Polymer Sei., 6, 45 (1951).
RECEIVED for review September 17. 1951.
ACCEPTED January 25, 1952.
Relationship between Molecular Structure and Physical Properties S. D. GEHMAN The Goodyear Tire a n d Rubber Co.,Akron 16, Ohio
This paper brings together information on relationships between t h e most i m p o r t a n t features of molecular s t r u c t u r e a n d t h e physical properties of synthetic rubbers. T h e molecular characteristics of greatest significance for the physical properties are considered t o be t h e n a t u r e of t h e monomer units, molecular weight, cross linking, details of chain s t r u c t u r e s u c h as cis-trans isomerism a n d side vinyl groups, a n d chain branching. Observed variations in properties may be dominated by t h e detail of s t r u c t u r e being investigated, b u t other uncontrolled deviations i n s t r u c t u r e occur, especially during processing a n d vulcanization. T h u s quantitative correlations of s t r u c t u r e a n d properties are usually possible only under ideal circumstances a n d very carefully controlled conditions. In general, each feature of s t r u c t u r e exerts its most conspicuous influence o n a r a t h e r limited group of physical properties. The chemical n a t u r e of t h e monomer u n i t s determines t h e intermolecular forces a n d influences especially t h e t e m p e r a t u r e range in which rubber elastici t y is exhibited, t h e swelling i n organic liquids, and t h e permeability t o gases. Molecular weight distribution is most significant for processability. Tensile s t r e n g t h and modulus are sensitive t o low molecular weights b u t become insensitive t o higher molecular weights. Cross linking in t h e raw polymer gives rise t o gel a n d affects t h e processability. T h e cross-linked network formed upon vulcanization is controlling for t h e tensile properties a n d for s t a 730
bility u n d e r stress. Regularity in t h e geometrical form of t h e chain molecules appears t o be favorable for low hysteresis a n d conducive t o crystallization u n d e r stress. This leads t o improved tensile s t r e n g t h a n d flex life. Effects of chain branching are r a t h e r obscure b u t explain variations i n properties not otherwise accounted for.
D
EFINITE features of molecular structure are required for a material that exhibits rubberlike elasticity. These involve
both geometrical characteristics of the molecules and favorable intermolecular forces. The molecules must be long-chain or threadlike molecules which, owing to thermal agitation, assume random configurations. Theory also requires chemical bonds or cross linkages at intervals along the chains, so that they are connected at these points and form a network structure. The intermolecular forces, or forces between neighboring molecules or groups of atoms, are variously designated as secondary valence forces, van der Waals forces, or cohesive forces. They must be sufficiently weak to permit configurational changes of the molecules and deformation of the network structure by relatively low stresses and yet strong enough to provide adequate tensile strength. The retractive force of rubber arises from the tendency of the chain molecules to resume their random configurations when these are altered by stretching. Judging from the wide range of rubberlike properties observed, these conditions are complied with to various degrees and in
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Synthetic various combinations, with corresponding variations in rubberlike quality. Each elastomer has a characteristic temperature range. in which the intermolecular forces come into proper balance for displaying high elasticity. At the lower end of this range, the secondary valence forces are too strong relative to the geometrical configurational forces arising from the network structure; a t the upper end, the intermolecular cohesion becomes inadequate to develop useful tensile strength. But even when compared in their optimum temperature ranges, elastomers still exhibit wide differences in properties which can be ascribed in a general way to a different balance in the necessary structural features. It is not possible to set up specific limits or magnitudes for these structural features regarded as necessary for rubberlike elasticity. There is a continuous gradation from viscous polymeric liquids to elastomers and from elastomers t o fibers and hard plastics. The intermolecular forces in some materials generally considered to be plastics may be weakened by addition of a plasticizer, a rise in temperature, or both, t o a point where the physical characteristics resemble those of an elastomer. Such a change in properties can be effected without change in the structure of the lonschain molecules. Notable progress has been made in the development of methods for measuring and defining the molecular structural features of synthetic rubber, I n many cwes, relationships to physical properties can be clearly perceived, although in other cases the effect of a particular feature such as branching of the main chain molecules on a particular property such as tensile strength may still be very obscure. This review is a discussion of some of the most significant work which is available to associate various detailed features of molecular structure with effects on physical properties of synthetic rubbers. It summarizes the important results that have been obtained and the principles that have been established. EFFECT OF CHEMICAL NATURE OF MONOMER UNITS
In describing the effects of variations in any one structural feature of a polymer, assurance that all other details of the structure remain the same would be desirable. This situation can be achieved only by very careful control of the experiments and by selection of suitable polymers for such studies. In most experiments, variations observed in properties may be dominated by the particular detail of the structure being investigated, but other uncontrolled deviations in structure frequently occur. In comparing polymers in which the long-chain molecules are made up of different monomer units, it is improbable that the molecular length distribution, the chain form, branching, and especially the system of cross linkages upon vulcanization should be the same unless special measures are taken. I n extensive technical comparisons of polymers and copolymers prepared with different monomers, all the structural variations that happen to occur usually enter into the comparisons and are superimposed on effects that are strictly due to the different chemical nature of the units in the polymer. The principle of securing diverse useful properties by the employment of different monomers is exemplified by the multiplicity of special purpose rubbers such as neoprene, Butyl, Thiokol, Lactoprene, Silastic, polyester rubbers, and Vulcollans ( 7 , 9, 11, 19, 20, 41, 68). Copolymers of butadiene and acrylonitrile furnish examples of the choice of a monomer to secure the specific, technically important property of oil resistance (57). In contrast, there has been a continuous intensive search for a better “general purpose” rubber than GR-S. Copolymers of butadiene and a very large number of monomers have been evaluated in this work (4, IS, $1, S7, 43, 51). These examinations in many cases have lacked thoroughness in the physical evaluation of the elastomers and in the control of such important factors as molecular weight distribution and combined monomer ratio. The cost and limited availability of promising monomers have fre-
April 1952 .
Rubber-
quently been discouraging factors in this work. No monomer has been found to displace styrene. The general molecular requirements for rubberlike propertiesthat is, the proper relationship between intermolecular forces and forces arising from configurational changes of the network of longchain molecules-can be met to a reasonable degree with a virtually unlimited variety of monomers. No particular chemical groupings, elements, or types of chemical bonds need to be specified for the monomers as a necessary condition for rubberlike elasticity.
A B C D E
TREAD STOCKS CHEMIGUM N-3 NEOPRENE GN GR-S BUTYL EMULSION POLY BUTADIENE
I
-60 -40 20 0 20 40°C Figure 1. Cold Stiffening of Various Elastomers There are, however, certain fundamental properties which the polymer inherits from the monomer and which can furnish a basis for interesting and useful generalizations in regard to the relationship between the monomer and the physical properties of the polymer. The intermolecular forces in polymers arise in the same way as those in liquids-Le., by dipole or induced dipole attractions or by electronic interaction. The magnitude of these forces on any one monomer unit in a polymer will be expected to be about the same as in the liquid monomer, but with necessary modifications due to rigidities and orienting effects introduced by the chain structure. The most general estimates of the intermolecular forces in polymers are to be secured from the values determined for liquids, usually by thermodynamic methods (3, 66). The energy of vaporization, usually expressed as calories per mole, is frequently taken as a measure of the cohesive molecular energy and characteristic cohesive energies have been calculated and assigned for a number of chemical groups that occur commonly in polymers. In many cases, the over-all intermolecular energy in polymers can be estimated fairly well by such methods, but the very important effect of temperature on the intermolecular forces has not been well evaluated for polymers. I n general, the intermolecular forces are weakened as the temperature is raised and strengthened as it is lowered. The configurational retractive force, on the other hand, is directly proportional to the absolute temperature. It follows that the temperature range for rubberlike elasticity will depend on the monomer. Polar groups with strong mutual forces will be especially effective in raising the temperature range. Structural features introduced into the chain molecules may also affect the low temperature stiffening. The inferior low temperature flexibility of mass poly-
INDUSTRIAL AND ENGINEERING CHEMISTRY
731
ELASTOMERS-Synthetic
Rubber-
butadiene as compared to emulsion polybutadiene is attributed to side vinyl groups on the chain molecules which interfere with configurational changes. In the case of GR-S, side groups from the styrene molecules may act in the same 'my, in addition to introducing higher intermolecular forces, For either or both reasons, low temperature stiffness increases systematicallv with increased styrene content. Figure 1 illustrates Lhe relative low temperature stiffening of a variety of elastomers (25).
r PERBUNAN
HEVEA d
.
,a'A
'x--s
6
-? \
I
Z
b-
I
g
s
_I
.2
\ \
'A
a/-
I
0
A GOO GO
2
3
4
5
GO GO OH 50 OH 7
8
Figure 2. Influence of Polar Groups on Swelling (G. Salamon and G. J. van Amerongen) 1 . n-Hexane 2. Isopropyl ether 3. Ethyl ether 4. Ethyl acetate 5. Methyl ethyl ketone 6. Acetylacetone 7. l-Methyl-2-hydroxy-3-butanone 8. n-Propyl alcohol
Physical properties such as modulus, tensile strength, and resilience measured at room temperature vary widely for polymers made with different monomers, owing to a number of structural features. The effect of the intermolecular forces on these properties is especially evident in their temperature dependence. This temperature relationship is influenced strongly by the brittle point, or, perhaps more precisely, the second-order transition temperature. Borders and Juve (10) studied the brittle pointtensile strength correlation for a large number of elastomers compounded as tread stocks. The significanca of their results is that the tensile strength tends to have the same value for the synthetic rubbers tested, provided the temperature of measurement is a t the same interval above the brittle point. Natural rubber, on the other hand, gives higher tensile strengths on this plot than does polyisoprene or the other synthetics. This deviation from the synthetic rubbers is an illustration of the effect of unique structural features not associated with the monomer unit itself. The implication from these data is that all of the polymerization systems available fail to realize the ultimate in rubber quality from the monomers. Whether or not natural rubber is actually the best rubber that can ever be obtained from the isoprene unit of structure is left open. Crystallite formation upon stretching is undoubtedly an important factor in the high tensile strengths observed for natural rubber, but this also occurs with Butyl rubber and neoprene. There is reason to believe that the presence of the double bonds in the chain structure of natural rubber in a very regular fashion favors the unhindered rotation of the molecular groups. This unusual degree of chain flexibility may help to account for the good physical properties. The effect on physical properties of varying the monomer ratio in the butadiene-styrene system has been important technically and has been thoroughly studied. Juve (31) has reported, for tread stocks, improved tensile strength and an improved balance between flex life and temperature rise in the Goodrich flexometer with increased styrene content. Storey and Williams ( 6 2 ) found
732
tear resistance to improve with higher styiene content. The tread stocks used by Borders and Juve (10) showed an increase 111 tensile strength from 1800 pounds per square inch for polybutadiene to 2920 pounds per square inch for 50-50 butadiene-styrene copolymer. These advantages are secured a t the expense of a higher brittle point, the increase being from -73" to -30" C. Thus there is a satisfactory agreement on the principal effects of increased styrene content. Strength properties are improved, but hysteresis and low- temperature stiffening suffer, The intermolecular forces determine the important technical property of swelling in organic liquids. I n the process of si+-elling, the cohesive forces of the liquid as well as those of the rubber must be overcome and replaced by cohesive forces between rubbpr and liquid molecules. I n addition, the swelling is resisted and limited by the strength of the network structure. Principles which apply for the mutual solubility of liquids (56)are pertincrit for the snrelling of polymers. Gee (22) has shoir-n that, a t lea>t to a first approximation, the swelling of a particular type of rubber by different organic liquids would be expected to be a maximum when the cohesive energy density-that is, the cohesive energy per milliliter-is equal t o that of the swelling liquid. Values for the cohesive energy densities for various types of rubber were secured from observations of the maximum swelling with a series of liquids of known cohesive energy densities. In this way, Gee secured the cohesive energy densities in Table I. The first three values in the table are so similar that some doubt may be entertained as to the adequacy of the method, although its principles are very instructive, Salomon and van Amerongen (46)carried out extensive studics of the swelling of various types of rubbers in liquids with a wide variety of chemical groups, As shown in Figure 2, rubbers IT it11 polar groups are swelled more by polar liquids than the hydiocarbon rubbers, illustrating the effectiveness of matched cohesive energy densities for maximum s-cvelling.
Table I.
Cohesive Energy Densities from Swelling Measurements
Type of Rubber
C.E.D., Calories/Ml. 63.7 65.5 67
81 88
I n many practical applications, the fact that strong intermolecular forces are essential to resist swelling by hydrocarbons introduces the necessity of a compromise when both oil resistance and low temperature flexibility are required. A great deal of work has been done with experimental copolymers of butadiene and polar vinyl compounds to determine the possibilities in securing both good oil resistance and low temperature flexibility. Apparently acrylonitrile is as satisfactory a monomer for this purpose as any that was tried (36). The permeability of rubber to gases also furnishes an instructive and important example of a property which can be shown experimentally to depend very directly on the intermolecular forces. As with swelling by liquids, the introduction of polar groups in the rubber structure reduces the solubility of nonpolar gases and vice versa, I n comparing the permeabilities of different elastomers to the same gas, however, the ratings are usually controlled by the diffusivities rather than the solubilities. Table I1 gives relative permeabilities measured by van Amerongen (2). The cohesive energy per monomer unit appears to control the diffusion of gas through the different types of rubber. The cohesive energy per monomer unit is increased by both polar groups and methyl groups, The calculated cohesive energies were correlated with the activation energies of diffusion, deter-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
Rubber-
-ELASTOMERS-Synthetic Table 11. Relative Permeabilities at 25" C. Hevea Polybutadiene Buna S Polydimethylbutadiene Perbunan Neoprene Type G Polyisobutene Thiokol B
Hz 100 82 78 33 29 26 12 3
0 2
100
80 72
I)
18 17 5 1.2
Nz 100 74 73 5.5 14 14 3.3
..
COz 100 103 92 5.6 23 19 3.7 2.4
CHI 100
..
73 2.7 11 11 2.5
..
.He 100
..76
48 38
..
24
..
Table 111. Cohesive Energy and Activation Energy (Cal./mole)
Polybutadiene Hevea Polydimethylbutadiene
Cohesive Energy 3960 4370 4780
Aotivation Energy
(He Diffusion) 5100 5900 7500
tion curve for natural rubber was more symmetrical and narrower. Neoprene Type W was found to have a more uniform molecular weight distribution than Neoprene Type GN. To this narrower distribution were attributed, a t least partially, its improved processing characteristics and lower compression set. Some of these molecular weight distribution curves as determined by fractionation and psmotic pressure and intrinsic viscosity techniques are reproduced in Figure 4. The nonuniformity in molecular weights was judged both by the ratio of the viscosity-average molecular weight to the number-average molecular weight and by the nonuniformity coefficient, p, of Lansing and Kraemer (56). The values for P are given in Table IV. Smaller values of p indicate more uniform molecular weight distribution.
Table IV. Nonuniformity Coefficients
EFFECT OF MOLECULAR
WEIGHT DISTRIBUTION
The importance of molecular weight for characterizing the molecular structure of polymers was recognized simultaneously with the growth of the concept of macromolecules. The effect of molecular weight on the viscosity of polymer solutions was of especial interest in the early work. In the case of synthetic 'rubber, the average molecular weight and the molecular weight distribution have been found to be of particular significance for the processing properties. Zapp and Gessler (63)showed that as the molecular weight distribution was widened for a given average molecular weight, the plasticity and recovery of Butyl polymers tended to become less sensitive to temperature. The recovery-temperature characteristics Were considered to be particularly important for determining the appearance or smoothness of extruded stock. The increase in plasticity resulting from a wider distribution in molecular weights was more than offset, from a processing standpoint, by the greater recovery at elevated temperatures. The ratio of recovery to plasticity appeared to be characteristic of molecular weight distribution and to become larger as the distribution was widened. This ratio determined the amount of swell for a given type of Butyl polymer after an extrusion operation. These conclusions were verified and extended in a later investigation with Butyl polymer fractions and artificial molecular weight distributions composed of the fractions (68). Fractions .of progressively lower molecular weight required progressively lower temperatures to be transformed from a highly elastic to a 'highly plastic state. For distributions having the same viscosityaverage molecular weight, a narrow distribution of molecular weight was found to be most advantageous for processing qualities, because wider distributions tended to give softer but more elastic polymers. The viscous component of deformation, how#ever,was found to be independent of molecular weight distribution and to depend only on the viscosity-average molecular weight. Figure 3 is a graphical presentation of some of these relations. The molecular weight distributions of Neoprene Type GN and Neoprene Type W as determined by fractionation (89,40) have also been connected with processability but not in so much detail as studies with Butyl. I n comparing the distribution curve for Neoprene Type GN with that for other elastomers, it was conscluded that the distribution curve for Neoprene Type GN was similar in shape to that for GR-S but with the peak for Neoprene Type GN at a somewhat higher molecular weight. The distribu-
.April 1952
B
Elastomer Neoprene Type W Neoprene Type GN Natural rubber sol GR-S X-478 GR-S: x - 5 5
mined by the temperature dependence of diffusion, for three of the rubbers as shown in Table 111. The process of gas diffusion through rubber evidently involves the separation of the structural units to create holes through which the gas molecules can pass.
0.97 1.27 0.70 1.50 1.70
All the synthetic elastomers seemed to show greater skewness in their molecular weight distributions and to have the maximum in the distribution curve a t lower molecular weight values than Hevea. It was also characteristic of the synthetic rubbers to show a long extension of the distribution curve to very high molecular weights, reflecting, perhaps, the presence of microgel. %, BLEND -I -AVE. M.W. 285,000
BLEND -4 ,AVE.M.W. 275,000
200
b
400
600 800
IO" M.W. -VISCOSITY
-t - in
BLEND-4
1000
AVERAGE
/
Figure 3. Effect of Molecular Speed on Processing The practical importance of understanding the processing difficulties and physical deficiencies of GR-S has led to a great deal of work to analyze the connections of such shortcomings with molecular weight distribution. The determination of the relationships between molecular weight and physical properties for GR-S is complicated because of the changes in molecular weight and structure which may occur during processing. I n an early study with Buna 8, Kemp and Straitiff (53)recognized the presence of an objectionable broad range in the molecular species present. High molecular weight fractions were tough, dry, and difficult to mill. The low molecular weight fractions were soft and sticky. The nerve, shrinkage, and poor processing of Buna S were attributed to the presence of a high molecular weight fraction associated with a considerable portion of soft, low polymers. Later work has coniirmed this situation as a fundamental problem
INDUSTRIAL AND ENGINEERING CHEMISTRY
133
-ELASTOMERS-Synthetic
Rubber-
in securing a t the same time good processing properties and good physical properties with GR-S. One of the conclusions from an investigation by Johnson (28)was that for best physical properties from GR-S, the molecular weight should be kept as high as could be tolerated from a processability standpoint. 4.0,
. 2
4
MOLECULAR
6
WEIGHT, M
8
IO
x IO-^
For the pure gum vulcanizates of the Butyl fractions, the molecular weight required for appreciable tensile strength (100 pounds per square inch) depended upon the unsaturation but was about 80,000. The sharp rise in the curve occurred up to a molecular weight of about 200,000, after which the improvement became more gradual. The technical importance of ascertaining the effect of molecular weight on the physical properties of GR-S has led to several investigations utilizing fractions large enough for evaluations with conventional physical tests or actual road tests with tires (28, 61). The necessity of using compounds containing carbon black and the changes in structure which the necessary processing may introduce preclude relating, quantitatively, the physical properties to the molecular weights existing in the compounds. But because the molecular weight range between the fractions was so large, there can be little question that the use of the original molecular weights before processing is adequate to disclose significant trends and relationships. Johnson (28) carried out a large scale fractionation of GI%-Sto secure fractions with the characteristics shown in Table V.
Figure 4. Molecular Weight Distribution Curves Table V. The determination of the effect of molecular weight on the physical properties of synthetic rubber vulcanizates is beset with difficulties because of changes in molecular weight and structures which may ensue during the processing preliminary to vulcanization and during vulcanization. This is especially true if the physical properties are to be measured for compounds containing carbon black. Butyl rubber is probably the most suitable type of synthetic rubber for determination of the principles involved in the relationship of molecular weight to physical properties (17, 62). However, the relatively lorn unsaturation makes it doubtful if all of the conclusions are directly applicable to a polymer such as GR-S where, for instance, the effects of lower molecular weight may be somewhat offset by introducing a larger number of cross links in the cure. Flory has carried out a careful, systematic study of effects of molecular weight with Butyl rubber fractions (17). The modulus of rubber, as ordinarily determined, may be expected to depend upon the cross-linked structure of the vulcanizate as well as the intermolecular forces and the molecular weight before vulcanization. The dependence on the original molecular weight has been considered by Flory to arise from the occurrence of terminal chains or chain ends which are not effectively linked in the network and do not contribute to its retractive force. It is apparent that this effect will become relatively more important as the molecular weight decreases and these chain ends are relatively more numerous. It was shown that there was a linear relationship between the modulus and the reciprocal of the number-average molecular weight which could be accounted for on the basis of the effectiveness of cross linking in relation to the terminal chains. The elucidation of the dependence of tensile strength on molecular weight for Butyl rubber is complicated by the fact that crystallization occurs a t higher elongations. The extent of this crystallization undoubtedly has a profound effect on the tensile strength. Flory was able to show a relationship between the fraction of the vulcanizate occurring in an orienting network exclusive of terminal chains (which were considered not to orient permanently on stretching) and the tensile strength. This concept made possible the formulation of a linear dependence of tensile strength on the reciprocal of the number-average molecular weight, or, more precisely, the reciprocal of the sum of the original number-average molecular weight and the molecular weight per cross-linked unit. It is characteristic that with increasing molecular weight there should be a sharp rise in tensile strength a t relatively low molecular weights and a subsequent flattening of the curve at higher molecular weights.
734
Sol-Gel Characteristics of t h e Unmilled Fractions of GR-S % of Whole Polymer
Fraction Whole GR-S High mol. wt. Low mol. wt.
100
Gel
Sfooney
4/212 50
34.7 17.4
Too
131
37.4
50
.
sticky
Ge, % 30.0 34.5 0
..
Swell- Inherent ing VisoosIndex ity 82 2.12 107 2.78 0.80
.. ..
I
.
Table VI. Tensile Data for GR-S Fractions 400% I-lodulus at Max.
Cure at 280" F., hlin. Whole GR-S Eigh mol. wt. fraction Wholemol. Low GR-S, wt. frao. 50% Gel
}
Tensile Strength 1325 1775 1525 1375
Elongation at
?ensile
40 2400 3525 1900
2875
Strength 80 160 3125 2300 3375 2925 2025 3125 3425 3425
k%ie Strength 660
590 610 670
The tensile data for these materials when compounded in tread stocks are given in Table VI. The low molecular weight polymer was definitely slower curing than the high molecular weight fraction. The high molecular weight fraction was superior in laboratory crack growth tests and in dynamic properties. In tread wear tests, the high molecular weight fraction showed 13% better tread wear than the unfractionated control. The blend of equal parts of a low molecular weight fraction and whole GR-S was 10% inferior to the control. A greater rate of crack growth was indicated for the low molecular weight material. It was concluded that the low molecular weight portion of the polymer was more of a detriment to quality than any other range of the molecular weight distribution. The requirements for processability limited the range and proportion of high molecular veight material which could be regarded as acceptable, so that the full advantage of higher molecular weight material could be utilized only if there were structural changes in the molecules to improve the processability. Although such changes in structure have not been accomplished, an alternative means for realizing some of the advantages of high molecular weight polymers has appeared in recent developments involving the addition of relatively large amounts of petroleum oils to improve the processability (16,64). The different rates of cure of the low and high molecular weight fractions undoubtedly aggravate the problem of securing good
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Synthetic
?
physical properties for a polymer with a broad distribution of molecular weights. The optimum state of cure for the mixture must be a compromise dictated by the rates of cure and proportions of the various fractions that are present. Yanko (61) has made the most thorough study available of the processing characteristics of fractions of GR-S together with a determination of the physical properties of the fractions after compounding and curing. A graphical presentation of the molecular weight distribution arrived a t is given in Figure 5.
I
I
-
NO. AVE. MOL. WEIGHT FOR GR-S
X55
Figure 5. Molecular Weight Distribution i n
GR-S X 55
The physical properties of the fractions were determined when compounded in a Santocure tread-type recipe. Sulfur ratios were varied to secure a modulus value of 1000 pounds per square inch a t 300% elongation for all the fractions to secure comparable flex life ratings. The curves for tensile strength us. molecular weight rise characteristically for low molecular weights and then become relatively insensitive to molecular weight. In general, stress-strain and other physical properties tended to improve up to about a value of 400,000 for k,and then to level off. This is shown by the curves in Figure 6. For the compounds with equivalent 300y0 modulus values, Goodrich hysteresis and Durometer values decreased and the rebound, tensile strength, elongation a t break, favorable balance between hysteresis and crack growth, and flex rating values improved with increasing molecular weight of the fractions. Evidence was obtained that the stress-strain properties of unfractionated GR-S could be calculated from the number average contributions of the stress-strain properties of the first six fractions ranging in 2, from 1,650,000 to 65,800. Cold milling was more effective in reducing the molecular weight of the higher molecular weight fractions than the lower molecular weight fractions. Fractions 1 and 4 were reduced from original values of 1,650,000 and 193,000, respectively, to 330,000 and 184,000 by the milling preliminary to the addition of carbon black. Processing became poorer with increasing molecular weight and larger shrinkage values were observed. The information on the effects of molecular we-ight and molecular weight distribution on physical properties from all these investigations gives a fairly satisfactory and consistent picture. EFFECT OF CROSS LINKING
The development of useful properties for elastomers generally requires that the long-chain flexible molecules be connected into a network structure by a system of cross links-that is, primary valence bonds. The effect is to give the structure stability toward stress. This is particularly evident in the effectiveness of vulcanization for suppressing plastic properties and producing elasticity and tensile strength. The concept of vulcanization for synthetic rubbers has been broadened to include the controlled introduction of any type of cross linking in the last steps of procApril 1952
Rubber-
essing t o control the physical properties, analogous to the sulfur vulcanization of natural rubber. There are no specific requirements for the chemical nature of the cross links. The chemical agents employed may vary widely, depending upon the type of rubber. Thus, in the case of Silastic and Thiokol, oxidizing reactions are .used (11, 12). For polyester rubbers, a free radical mechanism employing benzoyl peroxide was effective (9). Whenever the synthetic rubbers contain unsaturation, they can usually be cross linked by sulfur and accelerator systems which are also effective for natural rubber. For synthetic rubbers, variable amounts of cross linking may occur during polymerization or in processing, giving rise both to unique problems and possibilities for utilizing these effects. The occurrence of cross linking is soon made evident by accompanying insolubility or gel formation. The cross links limit the swelling in solvents, so that the determination of equilibrium swelling volumes is one of the best methods for characterizing cross-linked structures. There are, therefore, three important aspects to the recognized effects of cross linking on the physical properties of synth'etic rubbers. These involve the effects of cross links introduced during polymerization or processing on the processing itself and on the resultant physical properties, the effect of cross linking on swelling, and k a l l y the effect of vulcanization cross links on the physical properties. In general, cross linking is usually recognized by its effects on physical properties or swelling. The amount of cross linking required to affect physical properties seriously is generally too small t o be determined by chemical methods. The theory of cross-linking reactions during polymerization has been developed by Flory (18). The density of cross linkages increases a t higher conversions and gelation may be expected to occur when there is an average of one cross-linked unit per primary molecule. Modifiers suppress gel formation by reducing the primary molecular weight rather than the actual cross linking.
p 6oot? c
0
0
20.r
a
0
GR-S TREAD STOCK
I
J.A.YANK0
Figure 6. Stress-Strain Properties us.
Grn
In a mass polymer, a network structure of cross-linked molecules may extend throughout the entire mass. But the nature of emulsion polymerization limits the extent of a gel unit formed during polymerization to a latex particle. This condition has led to the concept of microgel, a sort of macromolecule, as a structural element of synthetic rubber (6). The occurrence of such localized gel structures might be expected to have considerable effect both on processing properties and physical properties. Cross linking of GR-S during polymer breakdown on a hot mill was found to improve tubing properties (48). A general improvement in processability was found also with increased gel content secured by blending latices of high and low gel contents. Processability was judged by shrinkage and roughness after calendering, tubing rate, swelling a t the die during tubing, and the surface appearance of extruded samples.
INDUSTRIAL AND ENGINEERING CHEMISTRY
735
-ELASTOMERS-Synthetic
Rubber
-
As a more practical method of securing this type of polymer with improved processability, a cross-linking agent, divinylbenzene, was introduced into the polymerization formula. One-half part of divinylbenzene was sufficient to give a polymer of high gel content and excellent processability. GR-S is such a polymer. A blend of this polymer with standard GR-S was found to result in better physical properties for the same processability than a polymer prepared with less divinylbenzene. The cross-linked stocks had somewhat poorer unaged stressstrain properties than standard GR-S but were about equivalent after oven aging. They were definitely inferior in regard to cut growth and flex life, even with a compounding adjustment of lower sulfur level to compensate for the cross linking already present. As far as processing is concerned, particles of tight microgel seem to act in somewhat the Pame way as a pigment does to improve processing. But the complete picture as to the effects of gels on processing is by no means simple. Loose gels which can be easily broken donm by milling are not much of a detriment for‘processing, but there appears to be an intermediate range of gel content and tightness which may make satisfactory processing difficult. The effects are especially complicated because these gel structures may be either broken dorm or formed to various extents during processing, depending upon the conditions. Thus, Sweitzer (66) has shown that certain types of carbon black have a specific action in encouraging gel formation a t higher temperatures and that this gel formation is an important factor in the reinforcing action of the blacks. Gel formed during storage of inadequately “short stopped” polymers is especially detrimental for processing. The equilibrium swelling of a gel or vulcanized polymer in a Bolvent is limited by the network structure of cross-linked molecules which resist the expansion. Gels may be compared as to the tightness of cross linking by measuring their swelling volumes or ratios of volume of swollen gel to volume of polymer. As is shown later, a quantitative relationship exists between the modulus of a polymer and its swelling volume. In the course of vulcanization, the long, flexible polymer molecules are bonded a t random reactive points, so that a network structure of indefinite extent is built up. According to the kinetic theory of rubberlike elasticity, it is this network which imparts to the vulcanizate all its elastic properties and strength. Actually there is increasing evidence that this theory represents an ideal approximation. Chain entanglements of very long molecules, for instance, may act like cross links. Secondary valence forces, especially a t lower temperatures, may add strength to the network. If crystallization occurs, the crystallites act as fixed points in the network. For compounds with carbon black loading, the carbon black particles undoubtedly serve as points of anchorage for the chainq. I n addition to these complications, in the case of technical vulcanizates the exact nature of the cross linkages introduced is, in general, still very obscure. It is therefore difficult to define the network structure and to relate the properties of technical compounds to the characteristics of the network structure. The most definite information for understanding the effect of cross linking as such on physical properties has been secured in investigations employing more closely controlled systems for introducing cross linkages. These experimental studies were designed to test the statistical-mechanical theories of rubber elasticity. These theories attribute the strength and retractive force of rubber to the existence of an infinite cross-linked network of the chain molecules in random configurations. In a study with Butyl polymers, Flory ( 1 7 )carried out a quantitative evaluation of the degree of cross linking. This involved the determination of M , the number-average molecular weight before any cross linking occurred, and No,the number-average molecular weight between points of cross linking. As no direct methods were available for measuring M,, it was deduced from a
736
determination of incipient gel points for a series of fractions wit11 decreasing M but the same unsaturation, vulcanized in the same way. M , was then considered to be the same in all the fractions and to be equal to M a t the critical point for incipient gelation, because theory indicates that an infinite network occurs when there is an average of one crossed-linked unit per primary molecule. The conclusions as to the values for M , were also verified by deductions from the theory of random cross linking in regard to the weight fraction of sol after vulcanization of the series of fractions. The network statistical theory of rubber elasticity for an ideal network of indefinitely long molecules predicts a proportionality between the equilibrium modulus and the number of chains (between cross links) per unit volume-Le., an inverse proportionality to M,. Flory extended the theory so that the modulus could be expressed in terms of both 31 and M,, thus taking into account the ineffective terminal chains, two for each molecule, which ale due to the finite value for M. In the experimental tests with the Butyl polymers, the observed modulus was found to be tyr-o or three times larger than anticipated from the theory. In a subsequent investigation, Flory, Rabjohn, and Shaffer ( 1 9 ) employed special cross-linking agents (disazodicarboxylates) with Hevea rubber and GR-S. Here the amount of cross linking was controlled by the proportion of cross-linking agent. An improved preswelling technique was used to secure the equiIibriuni modulus values required for comparison with theory. Rather good agreement R as obtained between theory and experiment for GR-S. For Hevea, the agreement was not so good, the modulus being higher than theory for low degrees of cross linking and lower than theory for high degrees of cross linking. Both of these investigations were augmented by determinations of equilibrium swelling in relation to cross linking and modulus. Bardwell and Winkler ( 6 ) made a study of the effect of controlled cross linking on the modulus and swelling volume of GR-S polymers. The cross linking was brought about by treatment of the GR-S latex from bottle polymerizations with potassium persulfate solution. The amount of cross linking was deduced from the amount of the gel fraction according to the statistical theory of random cross linking as applied t o polymerization reactions. Measurements of equilibriuni SN elling volume were used to estimate the network activity or density of effective cross-linking chains using the approximate relation :
where Y = number of network chains in volume V , v2 = ratio of volume of dry to swollen gel, Vi = molar volume of solvent, and pQ = solvent-polymer int.eract,ion.
A large dependence of the YTX-elling on the primary niolecular v-eight as well as on the degree of cross linking was found. Hut when correction for the inactive terminal chains was made, nccording to the procedure of Flory, an excellent correlation was found between the network activity deduced from the swelling observations and t’he effective degree of cross linking. The GR-S gum vulcanizates also fell into line with the polymers cross linked by the reaction with persulfate. It was apparent, hov-ever, that for higher concentrations of cross links, the rigidity increased more rapidly than the degree of cross linking, tending to confirm the existence of restraints along the chains in addition to cross links. A discussion of the dependence of physical properties on the state of cure in relation t o the cross-linked structure in technical compounds is not yet feasible. EFFECTS OF ARRANGEMENT OF UNITS IN CHAIN MOLECULES
The determination of the way in which the monomer units are linked together to form the long polymer molecules in various elastomers is a subject of considerable interest which has been
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Synthetic attacked by a variety of methods (1, 84, 34, 42, 44, 60). Variations in the details of chain structure must be invoked to explain differences in the properties of polymers made from the same monomers, if these differences cannot be explained by such things as variations in molecular weight distribution or cross linking. The aspects of chain structure upon which information has been obtained include: Head-to-tail, head-to-head, etc., sequence of unsymmetrical monomer units Alternation of comonomer units Diolefin (1,4), (3,4),and (1,2) addition cis-trans structure at double bonds Chain branching For purposes of illustration, these structural features may be represented as follows: Head-to-tail arrangement in polyisoprene:
r
-CH2-CH=
x""
-CHr-CHZ-CH=
-CH*-
Head-to-head arrangement in polyisoprene:
XHa
-cH2-cH=
CH3
-cii2-cH2-A=cH-cH2-
Alternation of comonomer units in GR-S:
-CH~-CH=CH-CH~-CH~-~H-CH~-CH=CH-CH~Butadiene
Styrene
Butadiene
Units in polyisoprene chain for various types of polymerization : (1,4)
-CH*-
(L2)
(3,4)
CH,
s""
4H-CH2-
-CHZ-C-
1
-CHZ-CH-
I CH
I
bHz Units of cis and trans structure in polybutadiene: Trans CHz
Cis
CH
%&CH!
CH=CH
Chain branching in polybutadiene: By addition -CHZ-CH-CH-CH2-
By substitution -CHz-CH=CH-CH($32
I
CH
I
CH
II
CH 1
CHz
I
6Hz I
Most of these structural variations cannot be adequately controlled during polymerization to a sufficient extent to secure a series of polymers in which differences in properties could be attributed t o one variation alone. Hence correlations with physical properties are rather indirect and conclusions are fairly well restricted to clear-cut cases involving different types of polymers. I n these comparisons, one particular feature of structure may be most strikingly different, but i t is difficult to be certain that a number of structural variations are not affecting the results.
April 1952
Rubber-
Evidence from ozonolysis and other methods indicates that the head-to-tail feature is the predominant arrangement in such polyh e r s as natural rubber, polychloroprene, and polyisoprene. For the symmetrical butadiene molecule, the question arises only in the case of successive 1,2 addition, which is a rather small proportion in emulsion polymers. There are no polymers available for comparison of the influence of this head-to-tail feature of structure alone on the physical properties. As for the alternation of comonomer units in the chain molecules, ozonolysis results indicate that in butadiene copolymers, the monomer units are not distributed regularly in the chain molecules, and a random distribution might be expected in most cases. Lacking definite correlations with physical properties of these features of chain structure, i t may be inferred that the greatest effect of such irregularities as their uncontrolled nature may introduce will arise from interference with or complete inhibition of crystalhzation. Hence the effects of such irregularities in the extreme will be represented by the contrasting properties of polymers that crystallize in comparison with those that do not, although other structural features may also affect crystallization. Information as to the proportion of (1,4), (1,2), and (3,4) addition in polymers is available from methods of ozonolysis, perbenzoic acid titration, and infrared absorption. Roughly about 20% of 1,2units occur in emulsion polybutadiene and polybutadiene copolymers. This is remarkably invariant for different emulsion polymerization conditions including polymerization temperature (M), although a trend toward lower proportions of 1,2 addition was noticeable as the polymerization temperature was lowered. The principal effect of this amount of 1,2 polymerization may be as a deterrent to crystallization. The effect of a large proportion of 1,2 units is undoubtedly evident in some of the properties of sodium or potassium mass polymers. But again, there is no assurance that some of the differences between such polymers and emulsion polymers may not be associated with differences in molecu!ar weight distribution, branching, and other structural differences as well as the proportion of 1,2 units. The occurrence of cis or trans structures has been deduced from x-ray diffraction patterns for polymers that crystallize. Apparently, either type of structure may give a high quality elastomer, since neoprene has a trans structure and natural rubber a cis structure. The effect of a random mixture of the two structures, however, is detrimental for properties that have an intimate connection p i t h crystallization. Of all these aspects. of chain structure, chain branching is probably the most difficult to detect and evaluate, and hence its effects on physical properties are most uncertain. I n general, if none of the other features of molecular structure seem to account adequately for differences in polymer properties, the differences may be attributed to chain branching. This general lack of definite evidence for branching might also be taken as an indication that it does not occur very extensively. The effects of these possible arrangements of the chain units may probably be best appreciated by considering three cases of especial interest, for which they furnish reasonable explanations of pronounced differences in physical properties The differences between natural rubber and synthetic polyisoprene, between emulsion polymers and sodium polymers, and between regular GR-S and cold rubber have been largely accounted for, in a general way, by such variations in chain structure. Natural rubber has long been regarded, from many lines of evidence, aa consisting of long-chain unbranched molecules of isoprene units in regular (1,4),head-to-tail, cis configurations (14). These firm ideas about the molecular structure of natural rubber, although still regarded as essentially correct, are now being questioned. Small proportions of vinyl groups, doublebond shifts, and trans structure are suspected from the most re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
131
-ELASTOMERS-Synthetic
Rubber-
cent infrared studies (41, 55). Of course, the existence of a gel fraction in natural rubber has been known for a long time. For synthetic polyisoprene, the chain unit arrangements are not controlled during polymerization and there is undoubtedly a great deal of irregularity. Both infrared analysis and perbenzoic acid titration indicate that about 10% of vinyl side groups occur. From measurements of unsaturation, it was concluded that there was probably some cyclization and cross linking, in which case there would probably also be chain branching. Although the infrared absorption spectrum of polyisoprene was originally interpreted as indicating a cis structure, there is now some question about this interpretation. A4bsenceof crystallization makes it probable that there is some randomness in the cis-trans arrangements also. These chain irregularities may be held responsible for a reduction in the t,ensile strength of a tread stock from 3500 pounds per square inch for natural rubber to 1550 pounds per square inch for a typical emulsion polyisoprene (14). The rebound was reduced from 62.5 to 52.0%. These values reflect n general inferiority in physical properties of polyisoprene as compared to natural rubber, which emphasizes the importance of chain unit structure. The second case selected for discussion, which involves these types of structural variations, is the comparison of sodium polybutadiene arid sodium butadiene-styrene copolymers and the corresponding emulsion polymers. Here the most pronounced structural difference is the much larger proportion of 1,2 units in the sodium polymers. Sodium polybut,adiene has been reported to have a 1,2 content of 73.5'% for a polymerization reaction at 10' C. This decreased with increasing polymerization temperatures to 47.5% for a 60" C. reaction (58, 49). Emulsion polybutadiene contains about 20% of 1,2 units. It has been pointed out ( 4 6 ) that the principal effect' of 1,2 addition on physical properties should be similar to the incorporation of a vinyl copolymer. One of the most pronounced effects is, therefore, a decrease in flexibility a t low temperatures and an increase in t'he brittle point. For polybutadiene vulcanizates this increase is from about -85" t o -62" C. (14, 33) and for t8he75-25 butadiene-styrene copolymer from about -57" to -27" C. This is undoubtedly due to the 1,2addition. For other differences, other features of niolecular structure may also be involved. The sodium polymers presumably are composed of straight-chain molecules without cross linking or branching (46). This, as well as molecular weight distribut,ion, may be responsible for the improved processing properties which have been reported for sodium polymers (32, 50): The sodium polymers were also found to be superior in flex crack growth (60) and flex crack growth-hysteresis balance (3%). The absence of cross linking in the raw polymer may also contribute to this result. But the analogous improvement in this respect of emulsion polymers with higher styrene contents makes it very probable that this improvement is directly connected with the high I ,2 content. The last example to be considered has tJo do with the explanation, on a molecular structure basis, of the improvements in emulsion polymer properties which are secured by reducing the polymerization temperature, or, more specifically, t,he reasons for the superiority of cold rubber over GR-S. The improved physical properties of cold rubber tread stocks have been analyzed in a number of interesting studies (16, 67, 50). ?larked improvements are evident in properties which are very important for tire treads, such as abrasion, flex life, and t'ear and tensile strength. The improvement in resilience is not very great. Processability has been reported to be somewhat poorer than that of GR-S (60). The low temperature flexibility of polybutadienes reacted a t lower polymerization temperatures is adversely affected by a definite tendency to crystallize (8, 65, 30). There is some question as to the extent to which these differences in properties are to be attributed to differences in chain regularity and in molecular weigh't distribution. The enhanced tendencies of the 1017 temperature polymers to crystallize are ,
738
ample proof of improved regularity of chain st,ructure. Infrared analysis (%4,26,58)has shown that the 1,2 content decreases only a few per cent with lower polymerization temperatures, so that this type of irregularity still persists in cold rubber essentially unchanged in character. On the other hand, considerable improvement in regularity is brought about by increased proportions of trans structure a t lower polymerization temperatures. The amount, of trans 1,4 structure was found t o increase continuously with IoJvered reaction temperatures from about 51% for a 97" C. polybutadiene t o nearly 80% for one reacted a t -19" C. At this lower temperature, practically all of the 1,4units were in the trans form (26). This improved regularity of molecular structure undoubtedly contributes t o the improved properties of cold rubber. Rut there is some question as to whether the molecular weight distribution and decreased chain branching may not be as important. The reduction in polymerization temperature has been recognized to be accompanied by a reduction in the low molecular weight fraction (58),so that the molecular Tveight distribution is narrower. This may be expected to give better physical properties, as the molecular species present will vulcanize a t a more uniform rate. The absence of low molecular weight material may account for poorer processing properties. There is also evidence t,hat lower polymerization temperatures tend to discourage chain branching and cross linking (58). Viscosity studies on a series of polybutadienes prepared at different reaction temperatures also led to the conclusion that lorver polymerization teniperatures gave molecules with less branching (29). This was deduced from the systematic trend of the exponent of the molecular weight in the equation connecting the intrinsic viscosity with the molecular weight. The exponent increased with decreased conversion or with decreased polymerization ternperature a t the same conversion. This was regarded as indicative of a more extended form for t h e rnolecules for the ]OK temperature polyniers such as Tvould result from less branching and a greater proportion of trans 1,4 structure. Thus, the improvement in the properties oi low temperature polymers is probably the result of several favorable featureE of molecular structure, among which the arrangement of the units in the chain molecules in a predominantly trans form is undoubtedly impor tan t. ACKNOWLEDGMENT
The author wishes to express his thanks to the Goodyear Tire and Rubber Co. and H. J. Osterhof for permiseion t o publish this paper. LITERATURE CITED i l ) Alekseeva. E . N.. and 13elitzkava 12. AI R u b h w Cham and ~. Technol:, 15, 693 (1942). (2) Amerongen, 0. J. van, J . BpplierZ P h y s . , 27, 972 (1946). (3) Amerongen, G. J. van, in R. Houvvink's "Elastomers and Plastomers," p. 194, New York, Elsevier Publishing Co., 1950. (4) Bachman, G . B , e t a l . , ISD. EKG.C r m M . , 43, 997 (1951). (5) Baker, W. O., Ibid., 41, 511 (1949). (6) Bardwell, J., and Winkler, C. A,, Indiu Riibber World, 118, BO9 (1948). (7) Bayer, O., Muller, E.. Petersen, S., Piepenbrink, 13. F.,and Windemuth, E., Anyew. Chem., 6 2 , 57 (1950); Rubber Cliem. and Teciinol., 23,812 (1950). (8) Beu, K. E . , Reynolds, TV. B., Fryling, C. F., and SIcMurry, H . L., J . Polumer Sci., 3 , 4 6 5 (1948). (9) Biggs, B. S., Erickson, R. H., and Fuller. C. S., IND.ENG, CHEM.,39,1090 (1947). (10) Borders, -4. M.,and Juve, R. D., Ibid., 38, 1066 (1946). (11) Boswell, W , E., and Jorcsak, J. S., I n d i a Rtcbber World, 120, 334 (1949). (12) Chinman. A. D.. Schmidt. H. F..and Konkle. G . M.. Rubber A g e ( N : Y . ) ,65,545 (1949). (13) Dunbrook, R. F., I n d i a Rubber World, 117, 745 (1948). (14) D'Ianni, J. D., IND.ENG.CHEM.,40, 253 (1948). (1.5) D'Ianni, J. D., Hoesly, J. J . , and Grew, P. S., Rubber A g e , 69, 317 (1951) \ - ,
%
I
/
~I
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Synthetic Fielding, J. H., IND. ENG.CHEM.,41, 1560 (1949). Flory, P.J., Ibid., 38,417 (1946). Flory, P.J., J. Am. Chem. SOC.,69, 2893 (1947). Flory, P. J., Rabjohn, N., and Shaffer, M., J. Polymer Sci., 4, 226 (1949). Forman, D. B., Radcliff, R. R., and Mayo, L. R., IND.ENG. CHEM.,42,686 (1950). Frank, R. L., et al., Zbid., 40,420,879 (1948). Gee, G., Trans. Insl. Rubber Ind., 18, 266 (1943). Gehman, S. D., Woodford, D. E., and Wilkinson, C. S., Jr., IND.ENG.CHEM.,39,1108 (1947). Hampton, R. R., A n a l . Chem., 21, 923 (1949). Hanson, E. E., and Halverson, G., J. Am. Chem. Soc., 70, 779 (1948). Hart, E, J., and Meyer, A. W., Zbid., 71, 1980 (1949). Howland, L. H., Messer, W. E., Neklutin, V. C., and Chambers, V. S., Rubber A g e , 64,459 (1949). Johnson, B. L., IND. ENG.CHEM.,40, 351 (1948). Johnson, B. L., and Wolfangel, R. D., Ibid., 41, 1581 (1949). Johnson, P.H., and Bebb, R. L., Ibid., 41, 1677 (1949), Juve. A.E..Ibid.. 39. 1494 (1947). Jive: A. E.: et al.,’Zbid., 39,i490 i1947). Kemp, A. R., and Straitiff, W. G., Ibid., 26,707 (1944). (34)Kolthoff, I. M.,Lee, T. S., and Mairs, M. A., J . Polymer Sci., 2, 199 (1947). (35)Lansing, W. D., and Kraemer, E. O., J. Am. Chem. SOC.,57, 1369 (1935). (36) Laundrie, R. W.,Feldon, N., and Rodde, A. L., I n d i a Rubber WorZd, 122,683 (1950). (37) Marvel, C. S.,Inskeep, G. E., Deanin, R., Juve, A. E., Sohroeder, C. H., and Goff, M. M., IND.ENG. CHEM.,39, 1486 (1947), (38) Meyer, A.W., Zbid., 41,1570 (1949). (39) Mochel, W. E., and Nichols, J. B., Zbid., 43, 154 (1951). (40) Mochel, W.E., Nichols, J. B.. and Mighton, C. J., J. Am. Chem. SOC.,70,2185(1948). (41) Owen, H.P.,Rubber Age (N.Y.),66, 544 (1950). (42)Rabjohn, N.,Bryan, C. E., Inskeep, G. E., Johnson, H. W., and Lawson, J. K., J . Am. Chem. SOC.,69,314 (1947).
Rubber-
(43) Rinne, W. W., and Rose, J. E., IND.ENQ. CHEM.,40, 1437 (1948). (44) Saffer, A., and Johnson, B. L., Zbid., 40, 538 (1948). (45) Salomon, G., and Amerongen, G. J. van, J . Polymer Sci.. 2, 365 (1947). (46) Salomon, G., and Koningsberger, C. J., Ibid., 2, 522 (1947). (47) Salomon, G., van der Schee, A. C., Ketelaar, J. A. A,, and van Eyk, B. J., Discussions Faraday Soc., No. 9, 281 (1950). (48) Schoene, D. L., Green, A. J., Burns, E. R., and Vila, G. R., IND. ENG.CHEM.,38,1246 (1946). (49) Schulre, W. A., and Crouch, W. W., J . Am. Chem. SOC.,70,3891 (1948). (50) Schulze, W.A,, Crouch, W. W., and Lynch, C. S., IND.ENQ. CHEM.,41,414 (1949). (51)‘Starkweather, H. W.,et al., Ibid., 39, 210 (1947). (52) Storey, E. B., and Williams, H. P., Rubber Age ( N . Y.), 68,671 (1951). (53)Sutherland, G. B. B. M., and Jones, A. V., Discussions Faradau Soc.. No. 9.281 (1950). (54) Swart,’G. H.; Pfau, E. S., and Weinstock, K. V., I n d i a Rubber World, 124,309 (1951). (55) Sweitzer, C. W., Goodrich, W. C., and Burgess, K. A.,Rubber Age (N.Y.), 65,651 (1949). (56) Syrkin. Y. K., and Dyatkina, M. E., “Structure of Molecules.” p. 282,New York, Interscience Publishers, 1950. (57) Vanderbilt Rubber Handbook, R. T. Vanderbilt Co., New York, N. Y., p. 51,1948. (58) Welch, L. M.,Nelson, J. F., and Wilson, H. L., IND.ENQ. CHEM.,41,2835 (1949). (59)White, L. M., Zbid., 41,1554 (1949). (60)Yakubchik, A. I., Vasiliev, A. A., and Zhabina, V. M., R u b b n . Chem. and Technol., 18,780 (1945). (61) Yanko, J. A , , J . Polymer Sci., 3, 576 (1948). (62)Zapp, R. L., and Baldwin, F. P., I N D . ENQ. CHEM.,38, 948 (1946). (63) Zapp, R. L.,and Gessler, A. M., Zbid., 36, 656 (1944). RECXIVE~D for review September 17, 1951. ACCBPTED January 25, 1962. Contribution 187,Research Laboratory, Goodyear Tire and Rubber Co.
POLYMERIZATION RATES IN EMULSION SYSTEMS MAURICE MORTON, P. P. SALATIELLO, AND HAROLD LANDFIELD University of Akron, Akron, Ohio
T
HE mechanism of emulsion polymerization, as
proposed by Harkins and his collaborators (1, 2 ) explains the unique characteristics of this system. I n the case of relatively insoluble monomers, the initiation of polymer nuclei supposedly occurs almost entirely in the monomer which is solubilized in the interior of the soap micelles, rather than in the individual monomer droplets or in the aqueous solution. One of the obvious reasons for this is the relatively high number of micelles with diameters of approximately 50 A., as compared with the monomer droplets with diameters of approximately 10,000 A. Since the free radicals generally arise from an initiator in the aqueous phase, the micelles would capture far more of these radicals than either of the other two phases. However, the formation of new polymer nuclei does not continue indefinitely, because of the gradual disappearance of the soap micelles which are depleted by the adsorption of soap molecules on the surface of the polymer nuclei.
April 1952
Hence, at a relatively early stage of the polymerization new particles cease to b e formed, more or less, and the p o l y m e r i z a t i o n proceeds largely by a process of growth of t h e p a r t i c l e s a l r e a d y formed, which are also very small, about 200 A., compared to the monomer droplets. It is this process of polymerization in a large and constant number of isolated loci-about 10’6 per ml.which lends to the emulsion system its special and unique characteristics. These particles imbibe monomer, which diffuses from the free droplets, and thus become polymer-monomer particles in which most of the polymerization occurs. On this basis it has been possible for Smith and Ewart (8) to develop an interesting kinetic treatment of emulsion polymerization. The most striking conclusion of their treatment is that in the ideal case the number of particles in the latex represents twice the number of growing radicals during the steady state. Hence, from a knowledge of the polymer-
T h e emulsion polymerization of .butadiene and styrene was studied with several different initiator systems. Styrene behaved according to the predictions of the theory of emulsion polymerization, in that the rate of growth of the latex particles was independent of variables other than temperature. Hence, at a given temperature, the rate can be related to the particle size. For butadiene, however, the various initiator systems showed varying degrees of efficiency, the only one which behaved according to theory being the peroxamine type. Hence the rate versus particle size relation is different for each system. The only reliable method for evaluating the relative efficiency of an initiator system is to determine the rate of polymerization per particle of latex formed.
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
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