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Another interesting application is the use of ultraviolet radiation as a source of illumination; the lens system through which the light passes before impinging on the preparation must be made of quartz or glass which does not absorb ultraviolet rays. This technique will prove valuable in studying the distribution of compounding ingredients. Preliminary experiments have shown that it will be possible t o study the progress of vulcanization, because most of the commonly used accelerators exhibit pronounced fluorescence. It will also be possible to ascertain whether the entire high molecular substance fluoresces, whether this phenomenon is characteristic only of the low-molecular-weight fractions, or whether it results only from the fluorescence of retained solvent, It is known that a solvent can incite fluorescence by increasing the distances between the moleculcs of the solute so that they no longer interfere with their own oscillations ( 7 ) . This is proved by the fact that the intensity of fluorescence can be drcrcascd by increasing the concentration of the solute. HIGH POLYMERS AND FILM FORMATION
When certain synthetic poll mers are deposited-for example, GR-S or methyl methacrylate-the breaking of the original film (Figures 5 and 6) differs radically from that obtained from natural rubber. It has also been observed that aging of the preparation has a marked influence on the distribution and size of the globules in the case of natural rubber. The number of blobs definitely increases with time. The presence of globules in soap was shown in a previous publication ( 3 ) . However, if such a preparation is aged, the picture changes, the globules disappear, and structures identical with those reported by electron microscopy (1, 6) result (Figures 7 and 8). Great difficulties have been encountered in making preparations of certain GR-S samples, particularly those characterized by high gel content, and also of balata; both of these materials show no globules and very ragged film contours (Figure 9). These experiments, although not conclusive, may be considered the first visual proof of the difference in molecular structure between natural rubber and balata, as well as that of high-gel-content GR-S. Balata represents the trans configuration of polyisoprene, whereas rubber must be considered its cis configuration. The average chain length of balata does riot differ appreciably therefore, the difference in from that of guayule rubber (3,4, configuration of the molecules must be considered responsible for their different properties and also for the difficulty with
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which threads are formed by balata. Present knowledge of the molecular coilfiguration of GR-S is not yet advanced enough to offer a similar explanation. However, a decided difference can be noticed in the morphology of low- and high-gel-content GR-S samples. The former show some very small globules but are mainly characterized by bandlike filaments (Figure IO). Hycar OS-10 (Figure 11) shows bands, filaments, and globules, although the latter are rather rare; Hycar OR-25 (Figure 12) shows only strong and netted bands. Neoprene GN (about 2 years old) reveals only a network of bands (Figure 13). Styrene (polymerized a t 90” C. for 48 hours without catalyst) shows extraordinarily stiff and breakable threads without actual netvork formation (Figure 14). Two observations should be recorded. If a string of droplets is obtained by depositing a solution of crepe rubber between two or three micronianipulator needles and then nioving them apart (Figure l5), the droplets elongate but rcappear upon release of the tension. This might indicate that elongation has caused the short molecules to align. That the droplets are composed of matter in a liquid state but of fairly high surface tension, or that the surface layer of the drops is composed of matter in an oriented coherent condition, is shown by some drops which have been pulled out of their alignment in the thread by being attached to another thread under tension (Figure 16). If that thread is destroyed, the drop immediately aligns itself in the remaining thread. The foregoing results indicate that a more detailed study of other plastics by this new technique will reveal some valuable facts pertaining to their structure; it is hoped that the detailed discussion of the preparation of samples will aid others in using this technique for their special problems. LITERATURE CITED
(1) .4nderson, T. F., in “Advances in Colloid Science”, 1’01. I, p. 353, New York, Interscience Publishers, 1942. (2) Hall, C. E., Hauser, E. A., le Beau, D. S., Schmitt, F. O., and
Talalay, P., IND.ENQ.CHmr., 36, 634 (1944).
(3) Hauser, E. A,, and le Beau, D. S., Ibid., 37, 786 (1945). (4) Hauser, E. A., and le Beau, D. S., India Rubber W o r l d , 107, 68 (1943); 108, 37 (1943). ( 5 ) Marton, L., MMcBain, J. W., and Vold, R. D., J. A m . Chem. Soc.,
63, 1990 (1941).
(6) Prebus, A. F., in Alexander’s “Colloid Chemistry”, Vol. V. p. 187, New York, Reinhold Pub. Carp., 1944. (7)
Pringsheim, P., “Fluoreszenz und Phosphoreszenz im Lichte der neueren Atomtheorie”, Berlin, Julius Springer, 1921.
Boiling Points of Three Isomeric Heptanes FRANK S . FAWCETT Socony-Vacuum Oil Company, Znc., Paulsboro, N. J . Boiling points of 2,2-dimethylpentaneY 2,e-dimethylpentane, and 2,2,3-trimethylbutane have been determined by the comparative dynamic method over the range 1-15 atmospheres, using n-heptane as the reference liquid.
I
N A CONSIDERATION of the separation by distillation a t
pressures above atmospheric of three paraffin hydrocarbons boiling within a narrow temperature range, vapor pressure data were obtained by the comparative dynamic method (5, 6), using metal Cottrell ebulliometers with glass pumps. Successive measurements were made of the boiling point of the substance
under investigation and of the boiling point of n-heptane in ebulliometers connected to the same reservoir in which the pressure could be varied. The data were observed as a series of corresponding boiling points of the substance and of n-heptane a t the same pressure. The Washburn-Read modification ( 7 ) of the Cottrell ebylliometer (1) was used (Figure 1). The body of the ebulliometer consisted of a section of ll/c-inch standard seamless steel pipe; the diameter was reduced a t the ends, and one end was sealed with a disk welded in place. The skirt surrounding glass pump D was of 1-inch seamless steel pipe turned down to l/,rinch wall thick-
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.
ness, with a '/,-inch hole as shown at C. The seat portion of a light steel union, B, was welded in place, and the wall thickness was built up on the outside t o permit the drilling of a n opening for passage of pump D. Condenser A was 12 inches long. E, a cross section of pump D, shows the circular brace for holding the pump' in position around the stainless steel thermocouple well. The two duplicate units were connected by '/,-inch piping from condensgr A t o a 22-liter thermally insulated reservoir, equipped with a pressure gage (for approximate values only) and connections for evacuation and for admission of nitrogen from a cylinder. Tests for gas tightness were satisfactory up t o 20 atmospheres pressure. An iron-constantan thermocouple was made by silver soldering the wires and inserting the two junctions (hot and cold) in glass sheaths. The couple was calibrated a t fourteen points by comparison with several mercury-in-glass thermometers which had previously been standardized by the National Bureau of Standards; a Leeds & Northrup potentiometer No. 8662, readable t o 0.003 millivolt, was used. The 2,2-dimethylpentane was prepared through the olefin (4,4dimethyl-l-pentene) which was prepared by the procedure of Whitmore and Homeyer (8) from allyl bromide and tert-butyl magnesium chloride. The olefin was fractionally distilled and hydrogenated with Raney nickel catalyst t o give the paraffin, which was fractionally distilled. The 2,&dimethylpentane was prepared by the low-temperature alkylation of isobutane with propylene in the presence of aluminum chloride; the product of a preliminary distillation was filtered through silica gel for the removal of halogen-containing compounds and then fractionally distilled. The 2,2,3-trimethylbutane was obtained from the Ethyl Corporation, and n-heptane came from the California Chemical Company. Each of the four hydrocarbons was filtered through silica gel and distilled with a Ewell-Lecky rectifying column equivalent t o thirty theoretical plates using a reflux ratio of 25-30 t o 1; in each case only a central portion of the distillate boiling at a temperature constant t o 0.2" C. waa retained for use. Table I shows the physical properties of the four parafhs. Examination of the final 2,4dimethylpentane and of the 2,2,3trimethylbutane by infrared aborption spectroscopy showed the purity of the former t o be approximately 95 mole % ' and of the latter, 97 mole 7,.
AFTER the apparatus had been evacuated and filled with nitrogen, the union connecting the thermocouple well t o the body of the ebulliometer was opened, the upper well portion was removed, find the glass pump was lowered in place by a small wire. Twentyfive cubic centimeters of the liquid were placed in the ebulliometer, and the connection was remade; care wab taken to be sure that the thermocouple well rested between the prongs of the pump. I n this manner one unit was charged with the liquid under examination and the other with the reference liquid, nheptane. For the first comparison the apparatus was opened t o the atmosphere through the vacuum connection. Each ebulliometer was heated by a Variac-regulated GOO-watt Goldfisch-type electric heater, the opening of which was reduced by a transite plate t o a size which just received the base of the ebulliometer. A small amount of mercury was placed in each thermocouple well, and the
II
u 1 INCH
Figure 1. Construction of Ebulliometers
glass-sheathed junction was inserted. The reference junction was maintained at the ice point. As the liquids were heated, the changes in electromotive force were followed by transferring the hot junction from one well t o the other, allowing the short time necessary for a constant reading and several additional minutes. When several successive alternate returns t o each unit showed temperatures differing by less than 0.1 O C. the pressure was increased by admission of compressed nitrbgen directly from a cylinder, and the readings were repeated. I n this manner the pressure was varied over the range of interest with as small increments as desired. After the completion of a determination the apparatus and its contents were allowed to cool t o room temperature. The nitrogen was relemed through the vacuum connection until atmospheric pressure was reached, the union connection was opened and the pump was removed with a wire. The liquid was pipetted from the apparatus, and the ebulliometer was cleaned with acetone; air and (later) nitrogen were passed through the chamber. Using a fresh supply of the reference liquid the next determination was started. This procedure was carried out with each of the three isomers against the same reference liquid, n-heptane. The following considerations indicated proper functioning of
TABLE I. PHYSICAL PROPERTIES OF HYDROCARBONS 2,2-Dimethylpentane 2,4-Dimethylpentane 2,2,3-Trimethylbutane Obsvd. Lit.' Obsvd. Lit." ' Obevd. Lit.Q 79.2b 79.2 80.76 80.7 80.9b 80.9 0.W36 0.6739 0.6738 0.6730 0.6896 0.8900 1.3824 1.3824 1.3821 1.3820 1.3894 1.3895 ~
B.P., d:"
."n"
C. at 780 mm.
339
Abs. viscosity, centipoiaaat 20 C. 0.389 0.385 0.381 0.361 0.579 a Literature data from Francis ($1, except visaosity data from Edgar and Calingaert (8). b Determined with glaen apparatus similar to the metal apparatue shown.
0.585
n-Heptane Obsvg. Lit.Q 98.46 98.4 0.6835 0.883 1.3877 1.3876 0.411
0.4097
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data, and agreement of the data for 2,2,3trimethylbutane*-heptane w i t h published data (4) over the short mutual range. Operation of glass units of similar dimensions indicated that small variations of quantity of charge, vertical position of thermocouple well, or heat input did not affect the temperature within the limits of experimental uncertainty. t
=
i
T H E observed corresponding boiling points of each of the isomers and n-heptane are given in $ -22 I I I I 1 I I 1 Table 11. These temperatures were constant 100 I10 140 I60 I80 zoo reo 240 t o 0.1' C. during the measurements and are BOILING TEMPERATURE OF ll*HE?TANE ,'c. believed to be reliable to *0.lo. When plotted Figure 2. Relative Boiling Points of Three Heptanes as a Function of according to the Dtihring relation, nearly the Boiling Point of n-Heptane straight lines result which have slightly differ0 2,2,3-Trimethylbutane 9 2,4-Dimethylpentane 0 2,2-Dimethylpentane ent slopes in each case. For each aiven pressure, the difference between the boiling point of an isomer and that of n-heptane, AT, is plotted against the boiling point of n-heptane in Figure 2; some points are omitted to avoid crowding. From literature OF THE THREE PARAFFIXS AXD TABLE 11. BOILINGPOINTS WHEPTANE AT EQUALPRESSURES data (4)for 2,2,3-trimethylbutane and n-heptane and the Duhring 2,2-Di2,4-Di2,2,3-Triplots, a comparison of the present data with the data of Smith methylmethylmethyl(4) over the range 760-1600 mm. is made in Table 111. From pentane n-Heptane pentane n-Heptane butane n-Heptane Young's data (9) for n-heptane and the Diihring plots, the pres81.2' C. 98.7' C. 80.7' C. 98.4' C. C. 99 0 81.5 98.9 81.1 sures corresponding to certain boiling temperatures of the iso114 0 96.5 108.3 90.3 125.4 108.0 135.1 116.5 mers were calculated (Table IV). For comparative purposes 136.4 119.0 151.8 133.0 137.4 among the three isomers, any uncertainty in the vapor pressure 120,o 183,6 164.5 152.0 134.8 195.5 176.1 data for n-heptane would not have a great effect, since the same 152.1 134.8 204,5 184.9 163.3 146.3 222.3 202.6 reference liquid was used for all three. 176.0 159.0 The boiling points of the three isomers-2,2-dimethylpentane, 183.1 166.2 184.0 167.2 2,4-dimethylpentane1 and 2,2,3-trimethylbutaneat atmospheric 204.8 188.1 223.0 206.8 pressure are 79.2", 80.7', and 80.9" C., and at approximately 15 2 3 6 , 9 220.9 atmospheres, are 206.5', 207.2', and 210.9", respectively. The 185.0 205.5 value of CY, the vapor pressure ratio, for the pair 2,2-dimethyl186.2 206.7 201.4 221.9 pentane-2,2,3-trimethylbutane increases from 1.051 a t 80" to 205.5 226.0 1.068 a t 200" C.; for the pair 2,4-dimethylpentane-2,2,3-trimethylbutane CY increases from 1.006 a t 80' to 1.055 a t 200" C.; TABLE111. COMPARISON O F OBSERVED WITH PUBLISHED DATA but for the pair 2,2-dimethylpentane-2,4-dimethylpentaneJa FOR 2,2,3-TRIMETHYLBUTANEAXD n-HEPTAXE OVER THE RANGE decreases from 1.045 a t 80' to 1.013 a t 200" C. 760-1600 A f l f . Pressure, blm.
760 800 900 1000 1200 1400 1600 5
,
n-Heptane (Smith, 4) 98.4 100.2 104.3 108.1 114.8 120.7 126.0
Boiling Temp. C. 2,2,3-Thmethylbutane (Smith, 4) This work" 80.9 80.9 82.6 82.6 86.8 86.7 90.5 ' 90.6 97.3 97.3 103.2 103.2 108.5 108.6 ~
From large-scale Diihring plot.
TABLE IV. TEMPERATURE-VAPOR PRESSURE DATAFOR THREE HEPTANES
Tpp.,
C.
Vapor Pressure, Atm. 2,2-di2,4-di2,2,3-trimethylpentane methylpentane methylbutane
the apparatus: the original centering of the well in the pump, constant e.m.f. values over a considerable period of time even with small variations of the heat input (Variac setting) to the ebulliometers, reproducibility of certain points both with the same and with different charges of the liquids, regularity of the
ACKNOWLEDGMENT
The author expresses his appreciation for the assistance given by J. G. Ehlers of this laboratory who made the infrared examinations of two of the hydrocarbons. LITERATURE C I T E D
Cottrell, F. G., J . Am. Chem. SOC.,41, 721 (1919). Edgar, G., and Calingaert, G., Ibid.,51, 1540 (1929). Francis, A. W., IND. ENG.CHEM.,33, 554 (1941). Smith, E. R., J . Research Natl. Bur. Standards, 24, 234 (1940); 26, 134 (1941). Swietoslawski, W., "Ebulliometric Measurements", New York, Reinhold Pub. Corp., 1945. Swietoslawski, W., "Ebulliornetry", Chemical Pub. Co. of N. Y., 1937. Washburn, E. W., and Read, J. W., J . Am. Chem. SOC.,41, 729 (1919). Whitmore, F. C., and Homeyer, A . H., Ibid.,55, 4555 (1933). Young, S., J . Chem. SOC.,73,675 (1898); Sci. PTOC. Roy. Dublin Soc., 12, 374443 (1910) : International Critical Tables, Vol. 111,p. 245, New York, McGraw-Hill Book Co.
Modifying the Viscosity of SulfurCorrection On page 43 of this article by Rocco Fanelli in the January, 1946, issue, the patentee of Literature Citation (12) is H. L. Reed instead of H. L. Read as printed.
