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S, E. SHEPPARD, R. H. LAMBERT, AND R. D. WALKER
REFERENCES (1) BAKER,G . L., A N D GOODWIK, &I.: Univ. Del. Agr. Exp. Bull. No. 216 (1939). (2) BRICCS,D. R., AND HANIG, M.: J. Phys. Chem. 48, 1 (1944). T. W., A N D MAES,L. A. F.: U. S. patent 1,385,525 (July 26,1921). (3) DOELL, (4) HILLS,C. H., WHITE,J. D., JR.,AND BAKER, 6. L.: Proc. Inst. Food Tech. 1942,47. (5) HUNT,C. H. : Science 48, 201 (1912). (6) JAMESON, E., TAYLOR, F. N., A N D WILSON,C. P.: U. S. patent 1,497,884 (June 17,1924). (7) KORTSCHAK, H. P.: J. Am. Chem. Soc. 61,681,2312 (1939). (8) MAGOON, C. A., A N D CALDWELL, J. S.: Science 47,592 (1918). (9) MERRILL, R. C., AND PETERSON, M.: Unpublished data. (10) MYERS,P. B.: U. S. patent 2,163,621 (June 27, 1939). (11) MYERS,P. B.: U. S.patent 2,165,902 (July 11, 1939). R., FEHLBERG, E. R., A N D BEACH,W. >I.: Ind. Eng. Chem. (12) OLSEN,A . G . , STUEWER, 31, 1015 (1939). (13) OWENS,H. s.,LOTZKAR, H., ERRIL ILL, R. C., AND PETERSON, M.: J . Am. Chem. S O C . 66,1178 (1944). (14) SCHULTZ, T. H., LOTZKAR, H., OWENS,H. S., AND MACLAY, W. D.: J. Phys. Chem. 49, 554 (1945). (15) THOMAS, A. W.: Colloid Chemistry, p. 180. McGraw-Hill Book Company, Inc., New York (1934). (16) WALLERSTEIN, L.: U. S. patent 2,008,999 (July 23, 1935).
DESENSITIZING BY DYES I N RELATION TO OPTICAL SENSITIZING OF THE SILVER HALIDES’ S. E. SHEPPARD, R. H. LAMBERT,
AND
R. D. WALKERa
Eastman Kodak Company, Rochester, New York Received December 91, 1946
Historical and practical aspects of the problem of desensitizing photographic materials by dyes may be found in articles by Luppo-Cramer (18), Hamer (15), and Brooker (9). These articles also discuss in greater or less degree the theoretical question of the physicochemical mechanism involved. Luppo-Cramer himself, who introduced as the first practical desensitizer in the developing bath the dye phenosafranin (9)) has proposed two different explanations: firstly, an oxidation hypothesis; secondly, what he terms one of “insulation of the latent image,” by which is intended a sort of “poisoning” of the latent image as a catalyst for development or, in another terminology, as an impenetrable potential barrier. Modificatjons of the oxidation theory have been proposed, as that involving the participation of atmospheric oxygen (7), or the dualistic electrochemical hypothesis of Baur (4),but not much seems to have been attempted in the direction of systematic experimentation on the redox potentials of the systems involved; nor as to whether, as in the related process of photographic Communication No. 1059 from the Kodak Research Laboratories. Most of the experimental work was done by Mr. Walker, who has now joined the University of Florida. 1
2
211
DESENSITIZING BY DYES
development, the deciding factor may be kinetic rather than dynamic (16). In a preliminary exploration of the field, a number of dyes, principally ones of notable desensitizing character or well known as optical sensitizers, were exposed t o hydrogen gas in the presence of colloidal platinum and at a pH of 8. It was observed that the desensitizing dyes were rapidly reduced, whereas most of the optically sensitizing dyes (including all of the cyanine dyes tried) were not. On testing the systems potentiometrically (12) it was soon observed that the cyanine (basic) sensitizing dyes did not appear t o be reversibly reduced, whereas such explicit desensitizers as methylene blue are definitely and reversibly reduced a t platinum electrodes, and well-poised potentials can be obtained. In the least complicated cases, when no semiquinone formation is involved, the general formulation of the potential, t o include the influence of hydrion, has been given by Michaelis (20) as Oxp Eo = E RT -In nF Rep where Oxp = 4 (ox), Rep = $ (red), and and # symbolize proper functions of the participant stages and the pH,--explicitly, equations for the dissociation a t different stages of proton addition. Usually, if the potential is plotted as a function of pH, the relation will be linear over a certain range-corresponding t o a single proton uptake-and will pass over t o another linear segment, corresponding t o a further association dissociation stage.
-
+
-
DEFINITIONS AND CONDITIONS OF DESENSITIZING
The terms “desensitizing” and “desensitizers” have both a general and a specific reference in respect to dyes and the photographic process. The general reference does not require limitation t o latent-image formation and t o the use and presence of developing agents, whereas the specific reference is, or should be, to just such limitations and conditions. It is this latter reference, corresponding to the practical application, which is usually intended by such investigators as Luppo-Cramer, Seyewetz, Hamer, and Brooker, although they frequently ignore the distinction and allude to the more general, theoretical problem. In terms of a simplified theory of oxidation-reduction potentials, the behavior of dyes in relation t o silver-silver halide should be regulated by equations of the type discussed by Abegg (l), Sheppard and Mees (25), Nietz (24), Reinders and Beukers (6),and Evans and Hanson (14) for the development of the photographic latent image. Thus it may be assumed that the single potential of a silver halfcell or silver electrode is defined by N
E’ = Eo
4- RTln*
C,
where C, is an assumed concentration of metallic silver in solution, which is further considered t o depend upon the size of a (latent-image) silver speck. This equation can be rewritxen in the form E’ = Eo - RT (pAg - log C,)
212
S. E. SHEPPARD, R. H. LAMBERT, AND ,R. D. WALKER
The conception of a definite true solubility of metallic silver is not immediately acceptable t o all, including the present aut+ors, so that we prefer t o consider the log C, term in a more generalized fashion, as an ad hoc unknown function of the activity of metallic silver atoms adsorbed to silver halide. For either the “print-out” or the “latent” image, the potential can be represented as a difference term in the equation for a reversible silver electrode of the second kind (29). According to certain investigators (lo), a higher oxidation potential is needed to destroy (oxidize) the latent image than the “print-out” image. This would imply, in terms of dispersity or “particle size,” a higher dispersity for the “printout” image, contrary to many other indications, but again the question has not been adequately considered in respect of “external” and “internal” images, Nor is it certain whether the criteria here ( 5 , 17) differentiate the “activity” of photosilver on a purely topographic basis (occlusion which is simply mechanical), or whether there exists a true energy of adsorption of silver atoms to silver halide, as made likely by the investigations of Esterman and others (3). At present we can only distinguish speculatively between the measured, and so far, known potentials of massive silver I silver halide electrodes, and the moot but only indirectly measurable potentials of photohalides. The interesting data presented by Reinders and Beukers, e.g., in the latter’s thesis-monograph (6), do not need to be interpreted exclusively in terms of solubility [of silver] but remain, qua measured potential differences, values important per se and liable to interpretatton in terms of adsorption potentials. I n fact, the question as to the effective potentials of photographic reagents and substrates, whether redox potentials of photographic developers or of desensitizing or (optically) sensitizing dyes, cannot be answered definitively from the behavior of solutions a t inactive electrodes. The equilibria which we may be able to define, i.e., measure, at such non-attackable electrodes as platinum, are not necessarily identical with their effective redox potentials a t the photohalide surfaces. Not only is the photosilver a difficultly definable variable but solution potentials (pK’s of ionization as well as redox potentials) are likely to be more or less radically displaced on adsorption of the solutes to silver halide, particularly where this is of the irreversible, oriented type? Thus, for example, the pK values of a number of cyanine dyes (19) having been determinedP6it was observed that a considerably higher acidity-of the order of 0.10 e. v.-was required to decolorize such a dye when adsorbed to silver bromide than when frec! in solution. Very much to the point, as regards redox potentials, is an observation by A. Steigmann (28). It was to the effect that, although, according to the general effect of pH on such potentials, the oxidation potential should be greatly reduced -and thereby the desensitizing power, considered as an oxidation of nascent 3 The technique of comparison used by Luther (19), Sheppard (25), and Beukers (6) with graded redox mixtures is probably the only one feasible. 4 Formerly termed pseudo-adsorption. 6 With H. R. Brigham.
213
DESENSITIZING BY DYES
silver (latent image in statu nascendi)-in strongly alkaline solutions such as developing baths, yet practical desensitizers just do defy that condition. As an example he points out the case of methylene blue (which is an effective desensitizer under such conditions, though not a practical one because it causes “fog”).’ Citing values from W. M. Clarke shown in table 1, where EA is the redox potential of the half-reduced dye (1:1molar), Stejgmann says, ..... since desensitizing is also effective a t pH 9 and pH > 9, it can hardly be assumed that it depends upon an oxidation of the silver arising from the action of light. This is, in the case of still more negative Phenosafranine (v. infra fig. 1) at pH 9, virtually excluded.” Steigmann’s point is well taken, but it is questionable whether his conclusion is inevitable. We have to consider that the potential of the system adsorbed to the silver halide-more specifically, adlineated at the interface silver-silver halideis by no means identical with that measured in solution, but is displaced to a degree which is not a t present always measurable, or even calculable. It has been suggested (26) that the displacement of the pK of cyanine dyes may be a measure, since here an estimate can be obtained, of the adsorption energy, but even this case requires further examination and extension. The adsorption in question is usually irreversible-what has been sometimes termed pseudoTABLE 1 p H . .. . . . . . . . . . . . . . . . . Eh . . . . . . . . . . . . . . . . . . . .
5
+o. 101
7 f0.011
9 -0.050
adsorption and which does not seem immediately susceptible of treatment according to Gibbs’ theorem. With the proviso that no exact correlation is to be expected yet between redox potentials (of dyes) measured in solution (at conventional electrodes) and desensitizing action, it remains worth while to obtain comparative measurements inter se of typical classes and groups of dyes. These measurements can be further correlated with reference to silver electrode potentials a t the same temperatures, and a t various pAg values incident a t such stages of the photographic process as emulsion preparation, exposure (to light for image production), and development. MEASUREMENT O F REDOX POTENTIALS
The potentiometric determination of redox potentials (12) has been fruitfully supplemented and extended by the method of polarography. The “half-wave” potential as defined by the IlkoviE equation should ‘correspond rather closely t o the oxidation-reduction potential as determined potentiometrically. This was first shown for reversible organic redox systems by Muller and Baumberger (22). But not only does this in a well-buffered system represent the E; of that 6 The origins of fogging by (desensitizing) dyes, not resulting from impurities, requires independent study. It is possibly related to semiquinone formation.
214
S. E. SHEPPARD, R. H. LAMBERT, AND R. D, WALKER
“system” (23), but evidently significant “waves” are obtained in poorly buffered systems affected by semiquinone formation and other deviating factors. In this connection there may be considered particularly recent investigations of Mdler (21) and of Muller and his collaborators (22, 23), and also of Breyer el al. (8) (on acridine dyes). We have included in our observations polarograms of some (optically) sensitizing cyanines, which potentiometrically give no evidence of reversible reduction (see figure 2). EXPERIMENTAL
As the reaction vessel there was used a Pyrex-glass funnel, 40 mm. in diameter, having a sintered glass disk of medium porosity at base. For potentiometric titrations a rubber stopper was pierced to carry three bright platinum electrodes, a thermometer, the reference electrode, a glass electrode (for pH control), and a gas exit trap; and a hole for a buret was inserted in the top of the funnel. Purified nitrogen was introduced from the bottom to sweep out oxygen. The gas was commercial but was purified by passage through chromous sulfate solution and finally through a solution of leucoindigodisulfonate as indicator. The gas also served to stir the solutions. As reference electrode was used silver I silver chloride (Ag I AgCl 1 0.1 N KCI), using freshly anodically deposited silver. The junction or bridge was an ordinary ground-glass salt-bridge valve, but the glass plug was replaced by a 4 per cent agar jelly made 0.1 N to potassium chloride. The single potential of this half-cell plus bridge, compared t o a standardized normal hydrogen electrode at 25”C.,was found to be 0.292 volt, a value which compares reasonably with the value given by Kolthoff and Lingane for this 0.290 volt, there being no very minute preelectrode (without bridge) of cautions taken. The polarogruph was manually operated. A Leeds and Northrup portable potentiometer was modified by connecting two standard cells in series to read up to 2.2 volts, and the working battery voltage was boosted to at least 2.2 volts. This was enclosed in a metal case, with jacks and plugs leading to the galvanometer and the electrode, and switches for reversing and to control the galvanometer sensitivity. The galvanometer was a Leeds and Northrup Type P (sensitivity 5 X 1O-Io amperes per millimeter, period 14 sec.), critical damping resistance 20,000 ohms and coil resistance 1171 ohms. The shunt resistance was macle equal to the critical damping resistance, so that the sensitivity could be varied while the instrument remained critically damped. It was found necessary t o use 50 per cent alcoholic solutions of the various dyes, because many are salted out by the buffers.’ The latter were made t o give 0.1 N potassium chloride on dilution of the buffer by alcoholic-aqueous dye solutions. Potentiometric measurements were made as follows: 25 cc. of buffer, 5 cc. of a palladium sol, and 25 cc. of an ethyl alcohol solution of the dye were placed in the funnel; the gas inlet tube was connected t o the lower end of
+
+
7
This, of course, introduces a variation in the “activity” factor.
21 5
DESENSITIZING BY DYES
the funnel and the whole placed in the thermostat. The stopper (carrying electrodes, etc.) was inserted, and hydrogen bubbled through the solution until it was decolorized. Then nit,rogen was bubbled through to discharge all hydrogen. The buret containing an oxidant was inserted in its proper place, and a potentiometric titration carried out in the usual way. For polarographic operation, 25 cc. of buffer and 25 cc. of ethyl $coho1 solution of the dye mere introduced into the funnel-vessel, the gas inlet tube was connected, and the whole placed in the thermostat. The stopper, droppingmercury electrode, and gas inlet tube were inserted, and nitrogen was bubbled through until oxygen was discharged. This takes about 5 min. with a suitable siatered glass, giving in this time equivalence to a solution of sodium sulfite, in respect of deoxygenation. After this the nitrogen is led over the surface of the liquid, instead of being bubbled from below. The polarograms are run as usual in manual operations, changing the voltage, and measuring the corresponding current. RESULTS AND DISCUSSION
In figures 1 and 2 are shown polarograms of the reduction (at a droppingmercury electrode) of some desensitizing and sensitizing dyes. Pronounced “maxima” were observed in most instances, and any interpretation at present is quite rough and provisional. So far, no dye has been found which does not give a reduction “wave” with the polarograph, while many dyes show several such waves. Figure 1 includes polarograms of two desensitizing dyes, methylene blue and phenosafranin, and figure 2 of two phthaleins, one, erythrosin, an optical sensitizer, and the other, Rose Bengal, a semi-desensitizer. I n figure 3 are polarograms of two other semi-desensitizers,viz., two azacyanines. I n table 2 all half-wave potential data from the polarograms are collected,giving first the E.M.F. values at the “half-wave” potentials and, under E; and E: (W.M.C.), corresponding potentials and potentiometric potentials from Clark (12). The values are collected in figure 4,and attention may be directed to the potentials at a level of pH 8. Two scales of measurement are indicated for the abscissae, the upper E for the potentials at pH = 0, and the lower for the pAg corresponding to the silver electrode potential vertically above. The following points may be noted in relation to this chart: There is good agreement between the potentiometrically determined previous values of E: pH for methylene blue and phenosafranin, without, however, taking account of semiquinone formation. Silver in the pAg range 5 t o 0 should be oxidized by methylene blue pH 0 to 5 (the arithmetical quasi-identity is coincidental), but over the pH range of developers, say pH 8 to 12, methylene blue leucomethylene blue would be a reductant for silver ions, not an oxidizer, which returns us t o Steigmann’s contention. The conditions for phenosafranin are similar but less pronounced, since it is more inclined to reduction. The fact that such an effective desensitizer is also
-
216
S. E. SHEPPARD, R. H. LAMBERT, AND R. D. WALKER
qualitatively an optical sensitizer (it was first mentioned photographically in this connection by Eder (13)) may indicate that 6he mechanism proper of optical sensitizing is per se quite independent of desensitizing or, in general, of oxidationreduction processes. Such seems indicated by the exciton process suggested for optical sensitizing (27). But it appears that we may have t o take into account
2 90
-I5-
250
-
210
-
170
-
o
W
-J
$ 0
a w
g 110 U 3 90 U "E 7 0 E 50 I-
w ' 130
r
30
-
0.2
0
-0.2 -0.4 -0.6 VOLTS VS N . H . E .
-0.8
-1.0
FIG.1. Polarograins of t w o desensitizing dyes. Drop time = 2 sec.; T = 30°C.; pH = 8.0; 10-3 AI dyc. Half-wave potentials are indicated: X, methylene blue; 0,phenosafranin.
certain counter-indications in the shape of the relations of the above-noted pseudo-potentials of the sensitizing dyes (where no reversibility is found) to the potentials of (massive) silver at various pAg values; according t o the equation
EAg = 0.8 = 0.8
+ RT In [Ag] volts - 0.6 pAg volts
21 7
DESENSITIZING BY DYES
giving with sufficient accuracy for these comparisons the values in table 3. Indicating in reference to these potentials (table 3), those assigned to the phthalein and cyanine dyes (cf. figure 4),then if these corresponded, if only transiently, to reversible redox systems, they would indicate reductants capable of decomposing water (cj. chromous and titanous salts). Practically, of course, nonreversibility is a large factor in the e m e n c y of a sensitizing dye; the asserted effect of oxygen in connection with desensitizing (7) may well be connected
-0.5
-
-1.0 1.5 N.H.E. FIG.2. Polarograms of phthalein dyes. Drop time = 2 sec.; T = 30°C.; pH = 8.0; 10-3 M dye. Half-wave potentials are indicated: X , erythrosin; 0 , Rose Bengal.
0
VOLTS V S
therewith, that is, with momentary restoration of the oxidized-andoxidizingform of a dye. In regard t o the question of reduction of the cyanines, it may be noted that these are reversibly decolorized by metallic sodium in liquid ammonia, which may betoken a s o d i m compound equivalent t o the leuco form of the dye, Reverting to the cases of Rose Bengal and of the azacyanines, these appear t o be borderline cases, as indicated by the primary waves, while if the tentatively assigned secondary-wave characteristics are significant, these dyes should have latent optical sensitizing properties. Evidently, further and fuller data should
218
S. E. SHEPPARD, R. H. LAMBERT, AND R. D. WALKER
TABLE 2 DYE
4
E.Y.F.
Methylene blue. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phenosafranin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
........................... Dye (IV) ............................................... (2).. . . . . . . . . . . . . (3).. . . . . . . . . . . . .
--
5, (W.M.C.)
-0.30
-0.01
(-0.02)
-0.61 -1.10 -1.25 -1.05
-0.30 -0.82
(-0.30)
-1.45
-1.16
-0.97 -0.73
-1.65?
Azacyanines : a) 3,3‘-diethyl-8-azathiacarbocyanine iodide (pH 8) b) 3,3’-diethyl-9-aeathiacarbocyanine iodide
...............................................
~ 1 . .
b i . . .............................................
-0.49 -0.56
-0.78 -0.85
z
0 t-
o w
-1
LL
W
0
U W IW
5: 0
z a
3
a
c3
E E
0
-0.5
-1 J
2
20
E E
10
0.1
0
-0.1
-0.2
-0.3
- 0.4 -0.5
V O L T S VS N.H.E.
FIG. 5. Polarograms of a n ani1 dye (2-p-dimethylaminophenyliminomethylquinoline etho-p-toluenesulfonate) at different p H values. Half-wave potentials are indicated.
latent-image formation? The latter may be equivalent to destruction of latent image in statu nascendi. I n some respects the behavior of a (practical) desensitizer is antithetic to that of a developer-or, more correctly, perhaps t o that of a latensifying agent. SUMMARY
Measurements have been made of the redox potentials of a number of photographically desensitizing and sensitizing dyes. The desensitizing dyes form reversible redox systems, with a dependence upon pH such that at relatively
DESENSITIZING BY DYES
221
-
low pH values they should be capable of oxidizing metallic silver at low emulsion pAg values, e.g., pAg 4 to 5, but not in developing solutions having pH values of 10 and upwards. Powerful optical sensitizers, such as most cyanine dyes, and some of the fluoresceins, e.g., erythrosin, behave irreversibly in potentiometric determinations, but definite “half-wave” values of uncertain origin are obtained polarographically. It is pointed out that irreversibility is a valuable factor for the efficiency of an optical sensitizer-desensitizing dyes such as phenosafranin are optical sensitizers, but of lorn efficiency. However, while the “potentials” of the polarographically found “half-waves” imply very high reduction potentials, > to >> that of hydrogen, their significance for the mechanism of optical sensitizing cannot yet be definitely assigned. If desensitizing is to be ascribed to an oxidation of nascent silver germs, then it is suggested that the anomalous behavior in relatively alkaline (and reducing) solutions must be due to a process of adsorption displacement at the surface of the grain. Finally, several questions for future investigation are posed. REFERENCES (1) ABEGG,R.: Eder’s Jahrb. f . Phot. 18, 65 (1904). (2) ARENS, H.: 2. physik. Chem. 114, 379 (1925). R. M.: Diffusion in and through Solids, Chapter 8 , p. 337 et seq. The Mac(3) BARRER, millan Company,. New York (1941). (4) RAUR,E.: Helv. Chim. Acta 12, 793 (1929). A., A N D STEVENS, G. W. W.: J. Optical SOC.Am. 31,385 (1941). (5) BERG,W.F., MARRIAGE, (6) BEUKERS,W. C. F.: Fotografische Ontwicklaare (Thesis, Delft, 1934). H.: Z. wiss. Phot. 33, 191 (1934); 34, 253 (1935). (7) BLAU,M., A N D WAMBACHER, (8) BREYER, G., BUCHANAN, G. S., A N D DUEWELL, H.: J. Chem. SOC.1944,360. L. G. S.: I n C. E. K . Meeds Theoryofthe Photographic Process, p. 1040. The (9) BROOKER, Macmillan Company, New York (1944). (IO) BULLOCK, E. R.: “Chemi’cal Reactions of the Photographic Latent Tmage,” Vol. I, Monograph No. 6 in the Theory of Photography. D. Van Nostrand Company, New York (1927). (11) CARROLL, B. H.: J. Phys. Chem. 29, 693 (1925). (12) CLARK,W. M.: Chem. Rev. 2, 127 (1925). E.: Beitraje zur Photochemie und Spectralanal. Graph(13) EDER,J. M.,A N D VALENTA, ische Lehr-u. Versuchsanstalt, Wien (1904). (14) EVANS,R . M., AND HANSON, W. T., JR.:J. Phys. Chem. 41, 509 (1937) ; but cf. CAME R O N , A. E.: J. Phys. Chem. 42, 521, 629 (1938). (15) HAMER, F. M.: 7th Intern. Congr. of Photography (London, 1928), p. 92. (16) JAMES, T. H., AND KORNFELD, G.: Chem. Rev. 30, 1 (1942). G.: J. Optical SOC. Am. 31, 598 (1941). (17) KORNFELD, (18) LUPPO-CRAMER: “Die Grundlagen der photographischen Negativverfahren,” Vol. 11, Part 1, of Eder’s Ausfiihrliches Handbuch der Photographie, Chap. 16, p. 678. W. Knapp, Halle (1927). (19) LUTHER,R.: Z. physik. Chem. 30, 628 (1899). (20) MICHAELIS,L. : Die Wasserstojionkonzentration; Oxydations-Reduktions Potentiale, Part 11,p. 68. J. Springer, Berlin (1929). (21) M ~ ~ L L E0. RH.: , J. Am. Chem. SOC.62, 2434 (1940). .
4
I
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(22) M ~ ~ L L E 0.RH., , A N D BAUMBERGER, J. P.: Trans. Am. Electrochem. SOC.71, 169 (1937). (23) MULLER,0 .H . , AND BAUMBERGER, J. P . : Trans. Am. Electrochem. Soc. 71,181 (1937). (24) NIETZ, A. H . : “The Theory of Development,” Monograph N o . d in the Theorv of Photography. D. Van Nostrand Company, New York (1922). S. E . : J. Chem. SOC.87, 1311 (1905). (25) SHEPPARD, (26) SHEPPARU, S. E.: Rev. Modern Phys. 14, 324 (1942). , (27) SHEPFARD, s. E . , LAMBERT, R . H., AND WALKER, R . D.: J. Chem. Phys. 9, 96 (1911). (28) STEIQMANN, A , : Z.wiss. Phot. 30, 73 (1931). (29) THIEL, A , : Z.anorg. Chem. 24, 1 (1900).
SHRINKAGE PHENOMENA ON CELLULOSE FILMS R.PRINCE A N D J . SEIBERLICH Engineering Experiment Station, University of N e w Hampshire, Durham, N e w Hampshire Received March 27, 1946
Films prepared from colloidal solutions of high-molecular and highly polymerized compounds show birefringence under certain experimental conditions. As long as the films contain water or solvents, the birefringence is little pronounced. If the swelling agent is gradually removed by drying, tension is exerted in the film, owing to the loss of solvent; thus the birefringence phenomenon becomes more apparent. This phenomenon is caused by gradual shrinking of the swollen high-molecular units. According to Katz (2),all high-molecular compounds exhibit swelling phenomena before forming a colloidal solution. If such a solution is allowed t o dry and the larger part of .the solvent is removed, a highly swollen mass remains which gives up the solvent very slowly, generally under shrinking conditions. In many cases, films possess a certain amount of Orientation which is due to their method of preparation. The orientation is produced by casting the film in one direction; greater birefringence is exhibited in this direction, since greater stress occurs in this direction after the removal of the dispersing agent. Gray (1) has recently studied very extensively these phenomena on cellulose films and has proven that birefringence is always observed in cellulose films if stresses produced during the manufacture remain present in the end product. If films are allowed t o shrink, shrinkage patterns of a variety of designs are formed. Since little is known of the relationship between such shrinkage patterns and birefringence, it has been the object of this investigation to study the conditions under which shrinkage patterns are obtained and to ascertain how the shape of the patterns may be controlled by variation of the experimental conditions. At the same time, it could be proven that cellulose films prepared from viscose with any kind of a shrinkage pattern, made according to special methods, do not exhibit birefringence as a normal procedure. Only if great tension is applied to the pattern may a slight birefringence be detected. I n other words, the shrinkage of the film and the formation of the shrinkage pattern proceed without producing substantial stresses in the film which remain in the end product and may be detected by examination in polarized light.
