greater than that of the ketone wave without affecting the latter, the electroactive material must be a n oxidation product of ethyl alcohol. At first, it was thought that biacetyl was produced (possibly through a peroxide intermediate) ; however, the polarographic behavior ( 5 ) of biacetyl, as well as that of possible peroxides and hydroperoxides, does not satisfactorily correspond with that of the prewave-producing material. Unfortunately, as no polarographic data which agree with those of the oxidation product could be found in the literature, it n a s impossible to identify the material from the information available. The E U Zof the postwave and its subsequent destruction with bisulfite indicate the nave to be undoubtedly due to some acetaldehyde which is also formed during the photochemical oxidation of ethyl alcohol. The ieaction leading to the prewax eproducing materia! shov s zero-order
kinetics. This is in agreement with the polarographic results, in that the kinetic data also suggest that the ketone must be a catalyst or sensitizer and that the ethyl alcohol must be the progenitor of the prewave-producing material. As the present authors were interested only in determining the cause of the preu-ave and the postwave, and in ascertaining whether it affected the normal ketone wave, no studies were made in solvents other than ethyl alcohol and no further attempts u-ere made to identify the prewave electroactire species. This could presumably be identified by isolating it or its reduction product and characterizing either of them by usual methods. Another possible approach might be a polarographic study of all possible oxidation products and comparison of their behavior nith that of the electroactive material. The latter method would probably be more tedious than tlie more direct approach.
ACKNOWLEDGMENT
The authors wish to thank the Atomic Energy Commission, which helped support the work described. LITERATURE CITED
( I ) hshworth, RI., Collection Czechosloc . Chem. Communs. 13, 229 (1948). (2) Barak, M., Style, D. W. G , A-atuTr 135, 307 (1935). (3) Berthoud, A , Helv.Chzm. Acta 16,592 119x3). \----,
(4) Day, R. A , , Jr., Milliken, S. R., Shults, I\'. D., J . Am. Chem. Soc. 74, 2 i 4 1 (1952). (5) Harrison, S., Collection Czechoslot . Chem. Communs. 15. 818 (1950). (6) Komyathy, J. C., Malioy, F.', Elving, P. J., ANAL. CHEM.24, 431 (1952). (7) Leone, J. T., Ph.D. thesis, University of Michigan, 1956. (8) Tokuoka, &I., Colleclzon Czechosloz.. Chem. Communs. 7 , 3 9 2 (1935). RECEIVED for review October 13, 1956. .\ccepted January 12, 1957.
Carbon RepI ica Technique for Exa mina tio n of Paint Surfaces in Electron Microscopy WILLIAM R. LASKO Research Laborafory, Titanium Division, National Lead Co., Sayreville, N. 1.
b The organic nature of paint surfaces necessitates the elimination of organic solvents for replication. The use of a water-soluble plastic as the primary replica with evaporated carbon as the final replica, followed by shadowing, has proved successful. Examples illustrating the attack of organic solvents on baked paint surfaces are presented and details of the methylcellulosecarbon replica technique are described.
R
have rpceived wide acceptance for the study of surface structure of materials. The simple plastic film technique has been particularly successful. Recent work reported by Agar and Revel1 ( I ) has demonstrated that a true plastic replica of the surface (with the top surface of the replica flat) requires a film thickness approximately twice the thickness of the structure height. This criterion is not always achieved when using Formvar replicas, but it is possible to obtain satisfactory results even if the surface structure penetrates to the top surface of the replica. However, with w r y rough surfaces, thick replicas are EPLIC'ATION TECHNIQUES
784 *
ANALYTICAL CHEMISTRY
required to prevent rupture in the clectron beam. This introduces electron transmission difficulties when attempting to study such surfaces as paint coatings because of the roughness encountered, and suggests the use of multiple step replica techniques. Multiple step replica techniques have the disadvantage of loss of detail due to repeated replication. Bradley's work (3) on the carbon replica technique, however, indicates that the primary ForinTar replica is capahle of a surface resolution of about 30 A. IThile there seemq to be some question with respect to the final resolution follov-ing multiple replication, the resolution is adequate for the study of paint surfaces reportrd here. REPLICATING SYSTEMS
In the study of surfaces of inorganic materials, the use of aromatic or aliphatic solrents for the plastic replicating medium is satisfactory. For surfaces of organic coatings however, solvents cause a n uncertainty in the reproduction process because they can attack the organic phase. When this takes place, part of the surface may be pullcd away
L
IJI
MDEPOSiTED JJJM,/GR~W~
i4) SHADOWED
CARBON FILM (5)
Figure 1. Methylcellulose-carbon replica technique
during stripping of the replica or the film surface may be distorted or softened. Kater-soluble plastics are a satisfactory substitute to eliminate this condition. On the other hand, these plasticq cannot be used as final specimen supports because they rupture easily under the electron beam, but they are satisfactory as primary replicas for multiple replicating systems. A number of multiplc replicating systems using water-
soluble plastics have heen cmploycd. Some of the more common ones include poly(viny1 alcohol)-silica, methylcellulose-Formvar, and methylcellulose-silica. As the stability of the final replica in theelectronbeam is of major significance, the development of the evaporated carbon film technique by Bradley (92) suggests an ideal material for the final replica when a water-soluble plastic is used as the primary replica.
I/#DIA. CARBON RODS
Figure 3.
Replicas of polished gloss surface
Chromium shadowing, X 25,000 Nitrocellulose replica 8. Melhyl~ellulore-carbonreplica
A.
I SPRING FILAMENT POSTS Figure 2. Carbon evaporation apparatus
The use of evaporated carbon has advantages over silica or plastic as the final replica in a two-step replica technique. Carbon evaporates with less difficulty than silica and forms a chemically inert substrate which is more stahle than silica under the electron beam. It is also more stable than a plastic film of comparable thickness. Furthermore, carbon films are sturdy enough to permit handling without the need of additional backing or special specimen screen supports.
Figure 4. A.
Replicas of same paint surface
Chromium rhodowina. .. X 8000 Nitrocellulose replica 8. Methylcellulori
METHYLCELLULOSE-CARBON REPLICA TECHNIQUE
Preparation of Primary Replica. A schematic representation of the steps used in the preparation of the carbon replica of a paint surface is given in Figure 1. True reproduction by the two-step replica technique is dependent on the characteristics of the primary replicating medium; therefore, careful control of the solution concentration is essential. Too low a concentration results in stripping difficulties which produce mechanical strains, while too high a concentration results in excessive shrinkage of the film at the suriace of the sample, resulting in the development of wrinkles. For paint surfaces a 5% aqueous methylcellulose solution (Methocel, The Dow Chemical Co.) is employed. The solution is applied uniformly with the aid of a dropper maintaining the paint surface a t an angle of approximately 20" to the horizontal. When dry, the film is stripped from the test surface with a small piece
of Scotch tape. The stripped film is then attached with Scotch tape to a microscope slide and placed in the vacuum evaporator for coating with carbon. Carbon Evaporation Procedure. The method of carbon evaporation consists of passing 60-cycle alternating current of approximately 50 to 60 amperes through '/*-inch diameter graphite rods (National Carbon, pure spectrographic grade) in a vertical position above the specimen with one rod tapered to a fine point. The carbon rods are held in close contact hy a spring, as shown in Figure 2, so that they do not separate during the evaporation process. Intense local heating occurs at the regions of contact. I n order to ensure that the vacuum is maintained at 0.1 micron of mercury or less, even while the hot regions are degassing, vacuum readings are checked closely, and a preliminary warming of the carbon rods a t a lower temperature than
that needed for evaporation is performed as an additional precaution. A satisfactory carbon thickness is maintained hy visually observing the color (light brown) of the condensed carbon developed on a piece of white porcelain through the window port of the More vacuum evaporator bell jar. elaborate methods of controlling the carbon thickness have been described (3). When the evaporation is completed, the carbon-plastic coated slide is removed, cut into 0.3-cm. squares ivith a sharp razor, and placed on the surface of a Petri dish filled with distilled water until the methylcellulose is completely dissolved. Two to 3 hours is required for complete dissolution of the mcthylcellulose with frequent replacement of the distilled water. Specimen screens (2O&mesh, woven stainless steel wire disks, inch in diameter) are placed on top of the floating carbon sections and removed with forceps. When dry, the carbon-coated screens again are placed VOL. 29, NO. 5. MAY 1957
785
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in the vacuum evaporator for shadowing with chromium. The selection of the shadowing angle for paint surfaces generally is not too critical. However, in the case of exceedingly smooth surfaces, such as high gloss paint films, an evaporation angle of approximately 5” to 7 ” with the horizontal is required to show surface irregularities of very small dimensions normal to the surface. When the shadowing process is completed, the metal-coated specimen is ready for examination in the electron microscope. Electron Microscope Technique. An RCA-type EMU electron microscope equipped with self-biased electron gun and binocular viewer was employed. A platinum objective aperture with a n opening of 40 microns in diameter was used to increase the contrast in the final image. All precautions necessary for the maintenance of a high resolution image were exercised. Kodak Medium Plates were used and developed in Kodak Versatol Developer. All electron micrographs of the paint surfaces \Yere processed t the negative print stage to make height and depressions more readily intei preted. QUALITY
APPLICATION!
.
..
”
urganic paint surtaces require a multiple replicating system as discussed above. For most work the electronic magnification is maintained at X1400 and enlarged optically to a final magnification of X8000. An example to illustrate the unsatisfactory reproduction of a baked paint surface with a nitrocellulose replica is shown in Figure 4 4 . The same paint surface by the positive - technique is methylcellulose-carbon . -. shown m Figure 4,B. Figure 4 4 , shows a roughened surface, while Figure 4,B, shows a moderakely smooth surface. -I n order to substantiate that the roughened surface was the result of attack by the replicating medium, a 786
Methylcellulore-c’orbon replicas of paint surface Chromium shrndowing,
A.
B.
Original rvrfoce
X 8000
Surface after nitroceilulore replication
OF REPRODUCTION
A comparison for quaIity of reprodnction using the methyl-cellulose-carhon technique with the negativc nitrocellulose technique was made employing a highly polished glass surface. The initial negatives were taken a t an electronic magnification of X3500 and enlarged optically to X25,OOO. The electron micrographs (positive prints) in Figure 3 illustrate approximately the same areas of the glass plate obtained by the two techniques. These electron micrographs show that the reproduction obtained by the water-soluble plastic as the primary replica in conjunction with evaporated carbon gives the same degree of film structure as is obtained with nitrocellulose.
-
Figure 5.
ANALYTICAL CHEMISTRY
Figure 6.
Methylcellulose-carbon replii Chromium rhodowing, A.
Glosiy finish
particularly smooth paint surface was ----:nerl T h a ma+hylcellulose-carbon replica technique was employed first; the electron micrograph is shown in Figure 5 4 . The negative nitrocellulose technique was then applied t o the same surface and the replica discarded. The meth~~lcellulose-carbontechnique was again applied to the same area of the paint surface exposed to the nitrocellulose replica; this electron micrograph is shown in Figure 5,B. A comparison of the micrographs shows that the nitrocellulose replicating medium attacks the organic phase of the paint surface. Further examples of the versatility of the positive carbon replica technique are shown in Figure 6, where A represents a smooth surface commonly ascribed as a “glossy” surface, while B V.,Y..ILL.
I
...,,
X 8000 8.
Mat finish
represents a rough surface commonly ascribed as a “flat” or “mat” surface. It is essential in this technique that the replicating media do .not interact with the surface to be observed and thereby introduce artifacts. A particular advantage of the technique is that the replicated surface is left undamaged and can be used for additional studies. LITERATURE CITED
(1) Agar, A. W., Revell, R. J. M., Brit. J .
A p p l . Phgs. 7, 17-25 (1956). (2) Bradley, D. E., Ibid., 5 , 65-6 (1954). (3) Bradley, D. E, J . A p p l . Phys. 27, 1399-1412 (1956).
