JULY, 1937
INDUSTRIAL AND ENGINEERING CHEZlISTRY
on white lead but tends to reduce moduli and tensile strengths. Pine tar gives short stiff stocks; lead oleate behaves similarly to stearic acid. Stearic acid does not improve gasoline and kerosene absorption. Stearic acid and lead oleate stocks d o not adhere to the rolls as do wood rosin and pine tar stocks. Basic-carbonate white lead is slower in setup but otherwise behaves similarly to litharge. Zinc oxide gives fast setup and moderate physical properties; it is retarded by wood rosin. Litharge confers low water absorption, magnesia high Water absorption. Mrater absorption increases the temperature, the rate of increase being dependent on the
793
formulation. Water absorption is not appreciably influenced by the softeners used.
Acknowledgment Thanks are due to John H. Baldwin and Francis E. illoseley of the Research Department for their assistance in obtaining the data published herein and to E. MT. McMullen, director of research, and J. R. MacGregor, vice president, for permission to publish the paper. RErEIVED
A4nr~l 17, 1937.
Presented before the Division of Rubber Cliern-
Istry at the 93rd Meetine of the American Chemiral Snriety. Chapel Hill c., A p l i l 12 to 1ti,1937.
s.
Colloidal Structure of Rubber in Solution S. D. GEHMAN AND J. E. FIELD The Goodyear Tire & Rubber Company, Akron, Ohio
T
HE type of colloidal behavior exhibited by rubber solutions presents the nature of the colloidal structure in solution as a complicated problem with nearly as many aspects as there have been fields of experimentation. Most of the methods of colloidal research have been applied, a t least to some extent, to rubber solutions. A satisfactory scientific understanding of the structure will eventually require a convergence of the points of view arrived a t by these different methods. Each source of independent information about the structure imposes its limitations upon the possibilities. In this work the writers have relied principally upon what is essentially a new and untried method of colloidal research as applied to rubber solutions-a study of the intensity and depolarization of the transversely scattered light. In addition to securing this new information, they have attempted to show how the results are related to those obtained from the method which has been of greatest general use in the past-namely, viscosity measurements. Light-scattering measurements have been used for many years in the study of suspensions and colloidal systems such as gelatin solutions (13, I d ) , agar-agar sol-gel transitions (j), .gum mastic (18), and numerous other colloidal suspensions. X o systematic study of the light scattered by rubber solutions seems to have been made. Optical Apparatus and Method The optical system for making measurements of the relative intensity and depolarization of the transversely scattered light is illustrated in Figure 1: The system is mounted in a darkroom. The source of light,
S,is a 1000-candle-power Pointolite tungsten arc. The lens
Measurements are reported of the intensity and depolarization of the light scattered transversely by solutions of purified rubber in various solvents. Viscosity measurements on the same solutions are included. The viscosity is shown to be a function of the dielectric'polarization of the solvent. Experimental results are compared with the expectations on the basis of various theories of light scattering. Intensity and depolarization measurements are consistent in indicating that the colloidal units responsible for the light scattering are large compared to the wave length of light, increase in size as the concentration is increased, and vary in size for the different solvents. The scattering is best explained by assuming in the solutions anisotropic scattering units similar in nature to the cybotactic groups of a liquid which may interlock sufficiently to give a continuous structure throughout the solution.
system, L, L', is focused to give a beam of approximately parallel light, the angular divergence from the axis being 2.5". The source and lenses are mounted on an optical bench in a lighttight housing, cooled by a fan. The beam is limited by diaphragms with rectangular openings, so that it has a cross section of 0.5 X 2 cm. at the light-scattering cell. F is a yellow filter (Corning No. 351) which makes the light more homogeneous and removes any blue light that might cause fluorescence. It also protects the solutions. The right-angle prism, G , reflects part of the light beam into one entrance of a Martens polarizing photometer, T. This reflected beam is reduced in intensity by a neutral glass filter, F', and then serves as a standard of comparison. This arrangement eliminates errors due to variations in the brightness of the source. P is a Polaroid disk which can be introduced in such a way as to polarize the incident beam with the direction of vibration either vertically or horizontally, as
VOL. 29, NO. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
794
LJ
~~
L J
~
~~.
~
~~
~
-i
Pummerer (87). Petroleum ether was selected FIGURE 1. D I A ~ R AOFMOPTICAL SYSTEM FOB MEASURING RELATIVE INTEXas the extracting fluid rather than ethyl ether SITY AND DEPOLARIZATION OF TRANSVERSELY ScAmmEn LIGIZT because i t seemed to preserve the gel structurc of the crepe rubber better. The apparatus used for the extraction is shown in Figure 3, whioh is self-explanatory. Rubber stoppers, protected by aluminum foil, were used for the connections. The extracted for use with unpolarized'liKht. Since the scattered light is rubber was precipitated with acetone and dried in vacuo. It was always handled with the greatest care of t.he anile of th; @icol, in this case, Rives thcratitio the to a v o i d contsminatiou with dust.
of
mtensity of the vertlral component, !he relative total intensitv can he calculated. The equi ment.tlius makes possible a plete analysis of the scatteroflight for any statto of polarization of the incident beam. The intensity of the liaht scatt,ered hv solutions of nwified rubber is so low that great care is required in matchhg the fields, and the eye must he completely dark-adapted. The sensitivity of the eve decreases for low intensities fM1. Sufficient readings wcrc taken so that the average could h d&icaied to within 5 few tenths of a degree of arc of the photometer did
corn:
Theory Before p r e s e n t i n g the results, it Will be advantageous to g i v e a brief rPsumt of the theoretical background of the method of lights c a t t e r i n g measurements as applied to the investigation of colloids. Thiswill greatly facilitate the discussion of the results and is necessary in order to understand them. Four types of light scattering should be considered in the case of tight scattering by rubber solutions:
Light-Scattering Cell and Solutions T o make the light-scattering and viscosity measurements desired, a technic was required that would fulfil the following requirements: 1. Eliminate motes and dust from the solvent. 2. Protect the solutions from air. 3. Permit the concentration of a solution to be changed by the addition of dust-free solvent without exposure to air. 4. Pmvide for viscosity measurements on the identical
solutions used for lightacattering measurements.
These needs were met by the use of the combined viscometer and light-scattering cell shown in Figure 2: It consisted of a spherical and a tubular unit joined by R connect.ing tube. The bulb of the spherical unit has a volume of shout 35 cc. The tubular unit is graduated and has a volume of about 60 ee. Light-scattering measurements were made on the solution when in the snherical bulb. the bulb heinn irnmemnrl .in the water bath prevcously described. To i i k ; viscosity measurements, an Ostwald type viscometer is attached as show. The canillarv diameter was 0.4 . mm. The . ..-viarometer . may be filled f;om {he tom without any interehaneo of fluid between the sphere and tube, by tilting'the entire h i t . The whole cell was immersed in a water thermostat at 30' C. for the viscosity measurements. When making up a solution, a weighed amount of rubber was introduced into the bulb and the solvent poured into the tube. ~
1. Scattering by colloidal articles. 2. icattering due to local density fluctuations
(6, 31).
3. Scattering due to coinposition f luc t u 8 tions ( 6 ) . 4. Scattering due to the random orientation of anisotropic molecules (89).
~~
FIGURE 2. VISCOMETERAXU LIGHT-SCATTERING CELL
By fluctuations are meant statistical varia-
INDUSTRIAL AND EN GINEERING CHEMISTRY
JULY, 1937
tions from the aversge values due to the thermal agitation of the molecules. Rayleigh worked out the procedure for calculating the light scattered by small, spherical particles in suspension. His fundamental equation is well known and has frequently been
795
1. All the scattering is due to particles having a different refractive index from the medium. 2. The particles are small compared to the wave length of light but large enough to be assigned a definite index of refraction and geometrical form. 3. The distance between the particles is large compared to the wave length of light. 4. The particles are in random orientation and distribution.
Under these limitations there can be practically no question of the validity of the theoretical work, but there is considerable question as to how closely a rubber solution fulfills these conditions. Gans defines the depolarization of the scattered light, incident light unpolarized, as J =2 JI Jz
,gl
-
where Jz = intensity of horizontally vibrating component in scattered light J1 = intensity of vertical component
PUR/f/€D ___)
# I TROGEN
The quantity p in Equation 1 is J2/J1, so that
XO T W8 TEU EA TU
FIGURE 3. RUBBER EXTRACTION APPARATUB
the basis for light-scattering studies of colloidp. A modification of this equation by Putzeys and Brosteaux (18)is given here :
where I = intensity of transversely scattered light E = intensity of incident beam T = distance at which I is observed X = wave length of light N = Avogadro’s number p1 = refractive index of the protein p~ = refractive index of the medium 4 = specific volume of the protein g = Concentration, grams per cc. M = mol. weight of large molecules assumed responsible for scattering p = ratio of horizontal t o vertical component in scattered light Equation 1 was used b y Putzeys and Brosteaux for the determination of the relative molecular weights of proteins from measurements of the intensity of the light scattered p)/ by water suspensions of the proteins. The factor 6(1 (6 - 7 p ) is a correction factor introduced by Cabannes to take care of the observed depolarization due to imperfect spherical symmetry. Putzeys and Brosteaux compared the intensity of the light scattering from a series of proteins in water solution, the molecular weights of which were available from the ultracentrifuge; they concluded that the intensity of the scattered light was directly proportional to the molecular weights and was not influenced by solvation, ion concentration, or aggregation. Rayleigh’s equation is limited to small, spherica,l particles in random arrangement. Calculations have also been made for the scattering from larger spherical particles (2, 3, 32). Gans (8) elaborated the theory to apply to differently shaped particles. H e adopts the more general assumption that the particles are ellipsoids of the revolution. Depending upon the ratio of the axes, these particles in the limiting cases can be rods, spheres, or plates. The assumptions for Gans’ theory are:
+
Gans showed that the depolarization is a function of the concentration and used Ofo to indicate the depolarization, extrapolated to zero concentration-i. e., the depolarization for a single particle. 0’0 is dependent on the particle shape. From Gans’ equations, the following values have been derived for Po:
ela
Particle Form Rods
(mz
m4
Spheres
1)2
+ 8mZ + 21 0
2 ( 1 - m2)Z 7m4 6m2 2
Plates where m
-
+
= E2 = PZ
+
refractive index of particle refractive index of medium
According to these equations it should be possible to determine the general shape of the colloidal unit from the depolarization of the scattered light. Thus, for a solution of rubber in ether, rn = 1.13 and we compute the following values for Ofo: Particle Form Rods
0 ‘0
0.00236 0
Spheres Plates
0.00734
It is apparent from the expressions for Pothat, the depolarization increases as m deviates from unity, If O is the depolarization of the scattered light for vertically polarized incident light, Gans has shown that: e’
= 28
(4)
If pu is the depolarization ratio for unpolarized incident light, p. for vertically polarized incident light, then from Equations 3 and 4, Pu
=
2Ptl 1 Pu
+
(5)
Furthermore, it follows from Gans’ theory that if the incident light is horizontally polarized, the scattered light will be unpolarized; i. e., p h = 1. Gans’ equations show that the intensity of the scattered light is proportional to V z ,if V is the volume of the particle, and is rather insensitive to particle shape.
IYDUSTRIAL AND ENGINEERING CHEMISTRY
796
For m = 1.13 as before, we calculate the following relative intensities of the light scattered by single particles of equal volume, the incident light being unpolarized : Particle Shape Rods Spheres Plates
Relative Intensity 1.02
1.00 1.03
When the particles are large compared to the wave length of light, Gans’ theory does not apply and the intensity of scattered light decreases as the particle size increases for the same concentration ( I O ) . Krishnan (15, 16, 17) developed a new method for investigating structure in liquids, glasses, etc., by means of lightscattering measurements. It surmounts to some extent the limitation of previous theory to particles small compared to the wave length of light. The method utilizes measurements of pu and p. which have already been defined. In contrast, pfi is the ratio of the vertical to the horizontal components in the scattered light for horizontally polarized incident light. Gans’ theory shows that ph = 1 and this value is actually observed for ordinary liquids and for suspensions of particles which are small compared to the wave length of light. Krishnan discovered, however, that for some systems ph is definitely less than one, since the scattering units, which he regards as molecular clusters, are large. He derived the relation pu
=
VOL. 29, KO. 7
For rubber solutions these factors must be very important in their influence upon the light scattering, and it seems probable that the ultimate explanation of the results will be in these terms rather than in the terms generally applicable to a colloidal suspension. Light-scattering measurements afford the possibility of the introduction of newer and more powerful theoretical concepts for the explanation of the properties of rubber solutions and lyophilic colloids than the comparatively simple mechanical pictures of macromolecules or discrete, solvated micelles usually advanced in connection with the colloidal description of such solutions. In the application of the theory of hindered rotation and cybotactic groups to rubber solutions, difficulties will be encountered due to the fact that the units are large compared to the wave length of light and that there is every reason to suppose that the scattering units show a size distribution rather than uniformity.
Dependence of Scattered Light Intensity on Solvent Figure 4 shows the relative intensity of the scattered light from one per cent by volume solutions of rubber in various solvents. The points are measured values. The curve is obtained by computing the factor in Equation 1 for each solution and dividing by the value of the factor for the ethyl ether solution. This gives computed values rela-
(Lc gz:)
Ph 1
l+Pv Equation 6 reduces to Equation 5 when pfi = 1. Hence the deviation of ph from unity is a n indication of the size of the p,) scattering units. To a first approximation, 2 p , / ( 1 represents the portion of pu. due to the anisotropy of the scattering units. The portion due to the finite size of the p t ) which Krishnan calls units is, then, pu - ( 2 p t ) / ( 1
+
+
AptA.
The theory of the Krishnan effect, the deviation of ph from unity, was further developed by Gans (9) and Vrkljan and Katalinib (33). Their analysis shows that the effect also depends upon the geometrical form of the scattering units. A large unit with spherical symmetry still gives pfi = 1. For rod-shaped scattering units with appropriate assumptions about the optical anisotropy, Vrkljan and Katalinib give for the length of the unit:
i
CARBON D/SULPH/D€ \
4 0.3
5
t
CARBON TETffACHLORIDf CHf OROfORM
0 / d ,. .
/.6
REFRACTWE /NDEX O f SOL VENT
FIGURE4. FROM ONE
RELATIVE INTENSITY OF SCATTERED LIGHT PER CENT BY VOLUME SOLUTIONS OF RUBBER IN VARIOUS SOLVEXTS
(7)
where k
2x
=7
x
A’ = wave
length of light in the system
Theory is still inadequate to explain completely the optical properties of liquids, but the causes of the discrepancies with classical theory are beginning to be recognized and dealt with (as). The modern view of the structure of a liquid must be taken into account. It can no longer be considered that the positions and orientations of the molecules are random. On the contrary, the molecules of a liquid form quasi-crystalline or cybotactic groups, small compared to the wave length of light, which are anisotropic for anisotropic molecules. The hindered rotation of the molecules and the anisotropy of the electric forces must be introduced into the calculations to explain the depolarization of the scattered light and the optical properties of pure liquids. Evidence for the existence of cybotactic groups is also available from other fields (4, 20, 34).
tive to the ethyl ether solution. As previously pointed out, this intensity factor is practically independent of the particle shape, so that if the scattering units were of the same size in the different solvents and the type of scattering conformed to Gans’ assumptions, the curve should coincide with the experimentally observed values. Although the intensity for the heptane solution is on the curve, the other solvents show marked departures. The scattering for the carbon disulfide, carbon tetrachloride, and chloroform solutions falls below the curve, whereas that from the benzene solution is greater than expected relative to the ether solution. The same facts are indicated even more strikingly by the following relative intensities observed for 10 per cent by volume solutions of petroleumether diffusion rubber: Solvent Ethyl ethei Heptane Benzene
Relative Intensity of Scattered Light Calculated Observed 1 000 1 000 0 643 0 507 0 130 0 016
JULY, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
Thus the relative intensity of the scattering of ether and heptane solutions might indicate a reasonable conformance t o the conditions for the application of Gans’ theory of the scattering by colloids and a reasonably identical scattering unit. For the other solvents used, either the scattering units or the mechanism of the scattering process appears to be different and the difference in the index of refraction of the solvent and the rubber is no longer the controlling factor. The intensity of the scattering for chloroform, carbon tetrachloride, benzene, and carbon disulfide tends to be the same. It seems improbable that the scattering units would be different in just the degree necessary to compensate for the differences in refractive index. Much more probable is the possibility that the light scattering arises from a structure of oriented groups which is essentially the same for these solvents, the mechanism of the scattering being analogous to that occurring for a pure liquid or liquid solution rather than to that from a colloidal suspension. We consider this a sounder point of view than to conclude from Figure 4 that the scattering units in chloroform, carbon tetrachloride, and carbon disulfide solutions are smaller and those in benzene larger than in the ether solution or vice versa, depending upon how large the units are compared to the wave length of light. More definite information about the size of the scattering units can be secured from the depolarization measurements which will be discussed later.
Dependence of Scattered-Light Intensity and Viscosity on Concentration Curves showing the relative intensity of the scattered light for various solvents used are given in Figures 5 and 6. Petroleum-ether diffusion rubber was used in both cases but was extracted from different batches of acetone-extracted crepe rubber and showed a large variation in the solution viscosity. The specific viscosity of a 0.25 per cent solution in ethyl ether of the rubber used for the data of Figure 5 was 1.12 and that for the rubber in the case of Figure 6 was 2.02, nearly twice as great.
% RUBBfU
I
1
/
2
CONCENTRATION
FIGURE 5. RELATIVE IKTENSITY OF SCATTERED LIGHTFOR VARIOUS SOLVENTS
The curves of Figure 5 illustrate the point previously made that the light-scattering properties of rubber solutions show marked differences from ordinary lyophobic colloidal sus-
797
1.2
ph RU88ZR
CONC€NrR17/ON
FIGURE 6. RELATIVE ISTENSITY O F SCATTERED LIGHTFOR VARIOUSSOLVENTS
pensions. The intensity of the light scattering for the rubber solutions is not, in general, proportional to the concentration. The writers do not believe that the flattening of the curves a t the higher concentrations in Figure 5 was influenced to any measurable extent by light absorption and secondary scattering which serve to flatten this type of curve for more opaque suspensions. The writers failed to measure any light absorption by means of a Weston photoelectric cell mounted to receive the incident beam after passage through the solutions. The curves in both Figures 5 and 6 for ethyl ether and heptane solutions show a distinct change in slope at about 0.4 per cent by volume concentration which may be indicative of a change in structure a t this concentration, somewhat corroborating the views of Staudinger. Unfortunately, the intensity of the light scattered from carbon tetrachloride and chloroform solutions is so low that for these solvents we cannot say much about the form of the curve a t low concentrations. The flattening of the intensity curves shows that the tendency of the rubber solutions to scatter light like a pure liquid is most pronounced at the higher concentrations. Another possible interpretation is that the size of the scattering unit changes with the concentration. As the concentration is increased, the structure should approximate that of solid rubber and it is conceivable that the intensity of the scattered light would actually decrease if the curves were extended far enough. The viscosity curves in Figure 7 show an entirely different character from the light-scattering intensity curves. The viscosity measurements were made on the identical solutions used for the light-scattering ineasurefnents shown in Figures 4 and 5. The viscosity curves are convex towards the concentration axis, whereas the intensity curves are concave. There is some disagreement in the literature upon the relative viscosities of benzene and carbon tetrachloride solutions of rubber, and the meagerness of the data has been pointed out by Philippoff (26). Philippoff also summarized the mathematical treatment of the viscosity-concentration curve. The explanations of McBain (19) for the viscosity
INDUSTRIAL AND ENGINEERING CHEMISTRY
798
g$
ir,
VOL. 29, NO. 7
70 60
8$
50
4s
40
F
k
% 30
E B
B
’
%RUBBER
30 40 SO D/Ef ECTR/C POL ARIZATON OF so.! VENT
% RUQBEU
SOL-
0
CONCENTRATION
“20
FIGURE8. RELATION BETWEEN DIELECTRIC MOLECULAR POLARIZATION OF VENTS AND VISCOSITY
/.
0.5
2o
FIGURE 9.
COffCENTU.4 TiON
MEASURED VALUES OF
pu
FIGURE 7. VISCOSITYCURVES
0.8
0.06
I
1
-A
I
CHLOROFORM
0.04
0.6
,
CHL OROfORM
a:
Y
Q
a02
0.4
HEPTANf
0.2 % RUBBER
I
J
/
2
COnVCCNTRAT/ON
FIGURE10. MEASURED VALUES OF Ph
%RUBBER
FIGURE11.
ANO
ETHER
/ CONCENTRAT/ON
MEASURED VALUES
2
I
I
9 OF
RUBBER
/ CONCENTRATION
FIGURE12. PLOTOF 2p,/(l
+
2
pv)
A P ~
of colloidal solutions are closely related to some of the ideas arrived at from our work. Figure 8 shows the interesting relation found to exist between the dielectric molecular polarization of the solvents and the viscosity. The dielectric molecular polarization is given by
where K
M
d
=i
dielectric constant
= molecular weight =
density of solvent
Ostwald previously pointed out the connectionof the dielectric polarization with various colloidal phenomena (24, 96),and it has been used in studies of the swelling of rubber (7)’ but the writers believe this to be the first time that such a direct relation to the viscosity of rubber solutions has been shown. The direct implication is that electrical forces are concerned in the viscosities of the solutions. An older interpretation of this same function of the dielectric constant was that it represented the actual volume occupied by the atomic nuclei of the liquid (11).
Depolarization of Light Scattered by Rubber Solutions The depolarization of the scattered light was measured for the solutions of which the intensity of scattered light is given in Figure 5. For suspensions which conform to the assumptions of Gam’ theory, definite conclusions can be drawn concerning the particle shape from the depolarization of the scattered light. It was previously shown that Gans’ theory predicts a limiting value, Ofo, for a rubber-ether solution of 0.0024 if the particles are rods, 0 if they are spheres, and 0.0073 if they are plates. Figure 9 shows the measured values of p., from which we compute from Equation 3, Ofo = 0.01, which is larger than the theoretical values. Furthermore, Gans’ theory predicts that for the same particle Po will decrease as the difference in refractive index between medium and particle diminishes, whereas we find that a carbon tetrachloride solution of rubber shows a larger depolarization than the ether solution, This is definite proof that the rubber solutions do not conform to the assumptions of Gans’ theory. To secure information about the structure from the depolarization of the scattered light, it is therefore necessary to adopt the analysis of Krishnan and measure, in addition to p., pv and PA. Measurements of p h are shown
JULY, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
in Figure 10. The fact that ph is less than unity is evidence that the scattering units are large compared to the wave length of light and helps to explain the discrepancies with Gans’ theory. For any one solvent, the smaller the values of ph, the larger are the scattering units; hence Figure 10 shows that the scattering units become smaller a t lower concentrations. This can be correlated with the form of the intensity curves (Figure 5 ) when it is realized that particles large compared to the wave length of light scatter less light as they become large. Depolarization factors for the benzene solutions are not given because the intensity for the solution was so low compared to that from the solvent that the values were influenced by the scattering from the pure solvent. A correction is difficult to apply because of the nonadditive character of the scattering (22). In Krishnan’s analysis, the size of the scattering unit is also indicated by A p u = pu - 2pw These values are 1 po’ plotted in Figure 11, and the data agam show an increase in the size of the scattering unit with the concentration. The p y ) , which is the part of pu because of the factor 2 p , J ( l anisotropy of the scattering units, is plotted in Figure 12 and also incremes with concentration. At zero concentration, the value agrees fairly well, for the ether solution, with the depolarization predicted by Gans’ theory for plateshaped particles. The anisotropy factor is a large part of pu, showing that the scattering units have a large anisotropy. Gans’ theory predicts an increase of the depolarization with the concentration but, for these comparatively dilute solutions, the writers do not consider that the increases observed are due to this cause. The principal reasons for this view are the decrease of ph with concentration and the fact that the depolarizations do not correspond to those provided for by the theory. As a check on the accuracy of the depolarization measurements, the measured values of p . and ph can be inserted in Equation 6 and values of pu can be calculated t o compare with the measured values. In general, a fair agreement is obtained. The equipment was also checked by comparing depolarizations measured for the pure solvents with those given in the International Critical Tables. Some observations on the depolarization factors for more concentrated solutions are given in the following table; the rubber used for these solutions was 10 per cent by volume petroleum-ether diffusion rubber, but of a different batch from that used for previous data:
+
+
2p.
r-pu-
Solvent Ethyl ether Heptane Benzene
Ph
p~
0,418 0,353 0.446
0,0428
0.0440 0,0949
Obsvd. 0,137 0.147 0.305
Calcd. 0.139 0.161 0.281
1
+ pv
0.0821 0.0844
0.1731
APU
0,0549 0,0626 0,1319
The depolarization factors, p a , for these solutions are as large as those observed for pure liquids such as the solvents; but for these liquids ph = 1, indicating that the cybotactic groups are small compared to the wave length of light, whereas the scattering units in the rubber solutions are large. Use of Equation 7 makes possible a calculation of the length of the scattering unit, if the assumptions involved are accepted. Substituting ph = 0.535 corresponding’to the measurement of 1 a 2 per cent solution in ether, and A’ = 5800 8. X 1 comes out as 5900 8., in somewhat better than qualitative agreement with values given by Schulz (SO) for the size of the colloidal unit from viscosity and osmosis measurements. For the ether solution a t O.lo per cent concentration, I is, by the same calculation, 3600 A. Assuming the usual spacings and angles for the rubber hydrocarbon chain molecule, this would correspond to a molecular weight gf 53,000. For the carbon tetrachloride solutions 1 is 7400 A. at 2 per cent
m5,
799
and 3400 8. a t 0.1 per cent. The curves decrease so rapidly that it is difficult to extrapolate them to zero concentration.
Conclusions Analysis of the light scattered transversely by rubber solutions indicates that the units of structure responsible for the scattering are large compared to the wave length of light and possess a high degree of anisotropy. The scattering in many respects resembles that from a pure liquid more than that from a colloidal suspension. This is especially true for solvents of refractive index nearly the same as that of rubber and for high concentrations. In view of these facts the most probable view in regard to the structure appears to be that it is a loose structure of anisotropic groups with hindered rotation analogous to the cybotactic groups in a liquid.
Acknowledgment The authors wish to express their indebtedness to H. J. Osterhof for giving them the benefit of his experience in many discussions and for numerous helpful suggestions.
Literature Cited Alexander, Jerome, “Colloid Chemistry,” Vol. 1, p. 340, New York, Chemical Catalog Co., 1926. Blumer, H., 2.Physik, 32, 119 (1925). Ibid., 38, 304 (1926). Cartwright, C. H., Phys. Em., 49, 470 (1936). Donnan, F. G., and Krishnamurti, K., Colloid Symposium Annual, 7, 1 (1930). Einstein, A., Ann. Physik, 33, 1275 (1910). Ermolenko, N., and Levina, S., Kolloid-Z., 75, 59 (1936). Gans, R., Ann. Physilc, 62, 330 (1920). Gans, R . , Physik. Z.,37, 19 (1936). Gehman, S. D., and Morris, T. C., IND.ENG.CHEM.,Anal. Ed., 4, 157 (1932). Getman, F. H., “Theoretical Chemistry,” p. 113, New York, John Wiley & Sons, 1922. Holway, A. H., J. Optical SOC.Am., 27, 120 (1937). Kraemer, E. O., and Dexter, S. T., S. Phys. Chem., 31, 764 (1927). Krishnamurti, K., Proc. Roy. SOC.(London), A129, 490 (1930). Krishnan, K. S., Proc. I n d i a n Acad. Sci., l A , 717 (1935). Ibid., IA, 915 (1935). Ibid., 3A, 211 (1936). Lange, B., Z. physik. Chem., 132, 1 (1928). McBain, J. W., S.Phys. Chem., 30, 239 (1926). Madge, E. W., Physics, 5, 39 (1934). Martin, W. H., S. Phys. Chem., 24, 478 (1920). Ibid., 26, 75 (1922). Mueller, H., Phys. Rev., 50, 547 (1936). Oatwald, Wo., Kolloid-Z., 45, 56 (1928). Ibid., 70, 96 (1935). Philippoff, W., Rubber Chem. Tech., 10, 76 (1937). Pummerer, R . , Ibid., 2, 367 (1929). Putzeys, P., and Brosteaux, J., Trans. Faraday SOC.,31, 1314 (1935). Raman, C. V., and Krishnan, K. S., PhiE. Mag., 5, 498 (1928). Schulz, G . V., Rubber Chem. Tech., 6, 101 (1933). Smoluchowski, M. V., Ann. Physik, 25, 205 (1908). Stutz, F. G . A., S. Franklin Inst., 210, 67 (1930). Vrkljan, V. S., and Katalinib, M., Physik. Z.,37, 482 (1936). Warren, B. E., Phys. Rev., 44, 969 (1933).
RECEIVED April 17, 1937. Presented before the Division of Rubber Chemistry at the 93rd Meeting of the American Chemioal Society, Chapel Hill, N. C., April 12 t o 16. 1937.
