Microscopic Studies of Lyogels ULTRA-ILLUMINATION BY INCIDENT LIGHT ERNST A. HAUSER Massachusetts Institute of Technology, Cambridge, Mass.
D. S. LE BEAU Midwest Rubber Reclaiming Company, East S t . Louis, Mo.
A new technique is described which permits the microscopic examination of lyogels, particularly of natural and synthetic rubber,with ultra-illumination by incident light. The morphology of the original lyogel and that of its fractions, separated by solvent extraction, can be studied, as well as the changes it undergoes when subjected to chemi-
cal reactions or physical forces. Results so far obtained by this technique in the study of natural and synthetic rubbers, soap, and other lyogels are discussed. Several; photomicrographsare reproduced as examplesof the applicability and versatility of this simple technique for studying the morphology of lyogels.
T
HE introduction of dark-field illumination in the microscopic studies of colloidal systems usually called ultramicroscopy must still be considered one of the most important advanaes toward a better understanding of matter present in the colloidal range of dimensions. However, the ultramicroscope has its limitations. The slit ultramicroscope, for example, will not reveal the presence of fibrillar particles or aggregates if they are stationary and if their longitudinal axes lie parallel t o the beam of light entering the preparation through the slit
a t right angles to the optical axis of the microscope. The modern dark-field condensers, on the other hand, illuminate the preparation from all angles and therefore will not permit one to ascertain with accuracy if any preferential orientation of matter is present in the preparation unless the condenser is fitted with an h i m u t h stop (7). Even then another drawback of the customary dark-field condensers limits their use. They are applicable only if the preparation is present in an extremely thin layer and if the continuous phase of the preparation is transparent. These limitations are primarily responsible for the fact that the use of the ultramicroscope has been largely limited to the study of dispersions of lyophobic colloids (lyophobic sols or emulsions), or to those lyophilic systems which have become optically heterogeneous due either to low temperature (for example, soap gels, 19, I@, to desolvation of the colloid ( I 4 ) , or to the use of higher concentrations than those which permit complete solvation (9,8, 4, Id, -16,16). All attempts to apply ultra-microscopy t o solvated lyogels have, therefore, met with little success. Recently the electron microscope with its high resolving power and the technique developed for the study of lyophilic colloids, such as soaps (I, 16) and natural and synthetic rubber (6),has offered a new approach to the morphology of lyogels. Ordinary microscopes or ultramicroscopes using white or even ultraviolet light, in accordance with the fundamental laws of optics, can never approach the resolving power of the electron microscope; nevertheless, it seemed of interest to ascertain whether the method used in preparing samples of lyogels s u i t able for electron microscopy could also be successfully applied to ultramicroscopic studies. Success in such an attempt would make available a method based on equipment within more reasonable financial reach than an electron microscope, not to mention the complexity of installation and difficulties encountered in operating it. Such a technique would also make it possible t o follow visually, under an ultramicroscope, the changes gels undergo if exposed to the influence of various chemical or physical reactions, such aa desolvation, aging, tension, or pressure. All this is not possible with the electron microscope. In working with lyogels, one algo may not overlook the fact that exposure to the electron beam can cause certain changes in the condition of some specimens, such as embrittlement of rubber samples (4.
t I/ #/RROR lV//A
/
Figure 1. Path of Light through Lens System of Ultropak Microscope for Ultra-illumination with Incident Light Len-
through whioh light is focused onto objelaat ere movable.
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Figure 2.
Filwrr from Ilevra I'alr Crepe Rubber ( X 1750)
Figure 3.
Fibers from Sol Fraction of Hevea Pals Crepe ' Rubber ( X 1750)
Figure 4.
Fibers from Gel Fraction of HerRubber ( X 1750)
Figure 5.
Fiber fmm Milled Hevea Smoked Sbsot Rubber ( X 2000)
Figure 6.
Fiber Network from Sol Fcaetion of Vulcanized Hevea Pale Crepe ( X 1750)
Figure 7.
Fibers from Milled Red Inner-Tube R e i a i m ( X 1750)
Figure 8. Fibers from Butyl Rubber (X 1750)
Pals %pa
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Figure 9. Fibers fmm Palmulive Soap ( X 3000) Figure 10. Fiber from Metbylsilieon, “Bouncing Putty” ( X 900) Figure 11. Fibers froni California Bentunite ( X 2100) Figure 12. Iseveit Rubber l a t e r Coagulated with Acetic Acid and Then Stretched ( X 1750)
‘THEULTBOPAK
Figure 1.
RaGanglrd oane 01 illuminating light. By this arairgeprent diffusionor scattering of light caused hy reflections from uneven surfaces or from o p q u e spseimons e m he avoided; the resulting clear ultra,illuminetian can be adjusted to suit the condition of the investigated sample. The preparations studied were made by divpersiug or dimolving the colloid to be invetigeted in an appropriste mlventether, petrolcum ether, benzene, hexane, xylene, toluene, etc. for ruhher (6)and plastics, and water for proteins, soap, clay, etc.-and spreading this solution on e. nonmisoihle liquid. Then e. small piece of very fine wire gauze (18 mierona permesh) which h a been inserted below the solution is lifted upward through it. The coated wire gause is then placed on s miormcow slide. and the preparation is ready for observation
The lens system through which the light pagses before it i U w mates the preparstion om be lowered or raised independently of the ohjeotive which has been brought into the comot food distance from the prepamtion. Thus, one e m form a steep or
Figures 2 to 9 show results obtained with natural rubber as well as its eo1 and gel fraction, the effectof vulcanization, and
01 all known ultramicrascope construction, only one seemed to
fulfill the desired requirements. This is the Ultropak, originnally designed by Heine (IOz If). Although the importance of thisinstrument in the scientifio and technical studies has frequently been pointed out (6, 7, S),it has not yet found the recognition it deserves. The Ultmpak diflors from the standard type of darkfield optics by making use of uitrs.tllumination with indirect light. This means circular illurninstion of the preparation fmm above; it cause8 a dsrk-field effect,but the tight does not pa4s through the mssnirying lens system hefore it is refleeted by the specimen through it into the obsemer’e eye or onto the photographic film. The path the light takes is shown schematically in
PHIYMIMICROGRAPNS
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1945
the morphology of Butyl rubber and soap. Considering that these photomicrographs were taken with dark-field illumination and remembering the difference in resolving power of the instrument, the eimilarity of the illustrations to those obtained with the electron microecope (I,6,14)is evident. In the case of a puttylike dilatant material such as some of the recently developed silicon resins, a small piece of the substance can be placed on the wire gauze and be expanded until fine fibers form (Figure 10). I n the study of clay gels the thixotropic gel is merely spread on a glass slide (9) and allowed to dry. One can therek" observe the formation of a coherent film consisting of a network (Figure 11) caused by filamentous aggregation of the clay particles. This type of ultra-illumination by incident light makes possible various manipulations under the microscope without enhancing the clearness of the observation by blurring the picture with undesired reflections or the like. For example, it is possible to study elastic deformations by forming a blob of elastomer on the points of two micromanipulator needles and then carefully moving them apart; or a coagulum of rubber latex can be formed on the ends of a micropincette, and the pincette is then allowed to open, as Figure 12 shows. The resulta so far obtained with this technique substantiate the assumption that the morphology of lyogels of quite different chemical composition is very similar (1'7, 18); this finding offers an explanation for the analogy of some of their properties. In conclusion, the authors would like to stress that this contribution offers only a condensed discussion of their finding8 and
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a limited number of illustrations. They hope to add more detailed results as soon as possible, but believe that the information presented here should not be withheld any longer. LITERATURE CITED (1) Anderson, T. F., "Advances in Colloid Science", Vol. 1, p. 363 ff., New York,Interscience Publiahers, 1942. (2 Baohmann, W., KoU&Z., 9,312 (1911);23, 86 (1918). (3{ Baahmann, W., I. unorg. Chm., 73,No. 2 (1911) (4) Darke, W. F., MoBain, J. W., and Salmon, C. S., Proc. Roy. Soc., A98,396 (1921). (6) Hall, C. E., Hauaer, E. A.,le Beau, D. S., Schmitt, F. O., and Talalay, P., IND. ENO.CHIOM., 36,834 (1944). (6) Hauser, E. A,, Chm. Fabrik, 4,277 (1931). (7) Hauser, E. A., "Colloidal Phenomena", New York, McGrawHill Book Co., 1939. (8) Hauser. E. A., Ku&chuk, 7 , 188 (1931). (9) Hauser, E. A., and le Beau, D. S., J . Phye. C h m . , 42, 981 (1938);43,1037 (1939). (10) Heine, H., U. S. Patents 1,840,448(1932)and 1,936,444(1933). (11) Heine, H.,I . Wias. Mihmkop., 48,460(1931). (12) Lawrence, A. C. S., "Soap Films", London, C.Bell and Sons, 1929. (13) Maclennan, K.,J . Soe. C h m . Id.,42,393T (1823). (14) Marton, L., MoBain, J. W., and Vold, R. D.. J . Am. C h m . SOC.,63,1990 (1941). (15) Seifria, W., Colloid Symposium Monograph, 3, 286 (1926). (18) Vold, R. D., and Ferpuson, R. H.,KoUoid-I., 11, 146 (1912). (17)Weimam, P. P. von, in J. Alexander's "Colloid Chemistry", Vol. 111, p. 89,New York,Chemical Catalog Co., 1931. (18) Weimam, P. P.von, Rubber Chem. Tech.,2,108 (1929). (19) Zsigmondy, R.,and Bachmann, W., KoU&Z., 11, 146 (1912).
MINIMUM WORK IN MULTISTAGE COMPRESSION HAROLD G. ELROD, JR. U. S. Nom1 Academy, Annapolis, Md.
A
PERUSAL of the literature indicates that the general con-
dition for minimum work in multistage comprmion is not widely known. Since very high pressures are now being used in chemical synthesis processes, it is believed that the following derivation has practical value. Consider first the simple two-stage process shown in Figure 1. Isentropic compression with intercooling to a fixed temperature, T,,is assumed. The work of compression is given by
W
hi
- ha + h2 - hi
5
dhr
- dha + d h
(2)
= T
9
Equation 7 then results: (7)
(1)
Now consider variations of W produced by changes of the intermediate pressure pa: dW
(5)
(2)
where h4 varies at constant pressure, p4; ha varies at constant temperature, Ta; h varies a t constant entropy, sz = Si; h~ is k e d . When W is a minimum, dW = 0. Or
The subscripts in Equation 3 refer to the locations of the derivatives. We can simplify Equation 3 with the following general thermodynamic relations, (4)
The validity of Equation 7 does not depend upon the assumption of isentropic compression; a constant compression efficiency, the same for both stages, might have Seen assumed instead. Even if there are more than two stages of compression, Equation 7 must, nevertheless, apply to every intermediate pressure with respect to adjacent pressures. Otherwise, variations of the total work would not be zero for all possible variations of the intermediate pressures. CONVENIENT RULE FOR DESIGN
If in Figure 1 the isobar joining points 3 and 2 is a straight line,
