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Communications to the Editor
00 90
well below those which cause complete sieve dehydration. Figure 2 shows Raman spectral3 of the same sieve lot calcined at 400 “C for 1h before (a) and after (b) washing with 0.2 N NaOH. The spectrum of the untreated sieve is identical with that in Figure 2a. Clearly the spectrum of the sieve washed with NaOH has considerably less background fluorescence and, therefore, more pronounced features. For example, the 1550-cm-l 1602band is prominent (S/N 3) in the caustic washed sieves whereas it is obscured by fluorescence (S/N 0.2) in the unwashed sieve. Sieves that are heated to much higher temperatures, e.g., 600 “C for 5 h, also show fluorescence reduction but the improvement is only two-thirds of that achieved by washing. Moreover, such high temperatures alter cation positions in 4A sieves12athus affecting subsequent adsorption. The reduction of fluorescence through washing with NaOH appears quite general14and, therefore, may be a useful technique for improving Raman spectra of 4A sieves and other alumina/silica adsorbents.
-
80
70
N
60 50
--ae
,$
40 30 20 IO I
1
I
I
1900
1700
1500
I300
I 1100
I
I
I
1
900
700
500
300
L 10
0
Frequency (cm-’)
Flgure 2. Raman spectra with associated fluorescence of 4A sieves (a) calcined at 400 O C and (b) washed with NaOH prior to calcining at 400 “ C : Cary 82, slit = 5 cm-’, excitation frequency = 514.5 nm, power = 200 mW.
height at 2324 cm-l to that at 2329 cm-l, for four different samples of sieves. The effect upon relative adsorptivity of calcining and caustic extractinglO the sieves is also included. The untreated sieves show a 3.5-fold range in their N2 capacities, whereas the treated sieves are more uniform in their adsorption capacities. The spread in N2 adsorption capacities of the untreated sieves probably reflects a poisoning effect by adsorbed species (both organic and inorganic) on the sieve surface. Dry sieves4 which are calcined at 400 “C show increased N2 adsorption due to the removal of the organic surface impurities.ll Sieves which are washed in 0.2 N NaOH and then calcined exhibit a somewhat diminished capacity for Nz sorption as a result of “steaming” effects which occur when wet sieves are dehydrated at elevated temperatures.8*12b Thus, Raman scattering can provide a unique in situ measurement of N2 adsorbed on sieves which might be useful in kinetic and other studies. 11. Reduction of Fluorescence. The typical Raman spectrum of 4A molecular sieve^,^ as well as other adsorbents,ll is often obfuscated by low-level fluorescent impurities. When these fluorescent impurities are caused by adsorbed species, e.g., hydrocarbons, rather than by structural impurities such as color centers, the fluorescence can be minimized by (1)removal of the impurity through pretreatment or burning with the laser, and (2) changing the excitation wavelength. The usual pretreatment method for A-type sieves involves heating at 350-500 “C for ca. 24 h, usually under vacuum, to desorb the impurit i e ~ . ” ~ JFor other closely related material, e.g., silica, porous vycor, heating at 500-600 “C under O2 for ca. 1 2 h is recommended for removing impurities.ll While these methods are satisfactory for fluorescence reduction, they are time consuming, and, moreover, if the latter method were used for the A sieve, complete dehydration of the sieve would occur.12a An interesting consequence of washing sieves with NaOHlO is the diminution of the associated fluorescence in a relatively short period of time, 2 h, at temperatures
Acknowledgment. We thank Dr. Seemon Pines for his aid in the preparation of this manuscript. References and Notes (1) (a) A. M.Ahharov, B. P. Bering, I. A. Kalinnikova, and V. V. Serpinskii, Izv. Akad. Nauk. SSSR, Ser. Khim., 6, 1434 (1972); (b) A. M. Arkarov, I. A. Kalinnikova, and V. V. Serpinskii, ibM., 3, 538 (1972). (2) E. I. Borzenko, Zh. PrM. Khim. (Leningrad), 42(4), 891 (1969). (3) M. Nakagaki and T. Fujie, Yakugaku Zasshi, 90(3), 384 (1970). (4) Linde 4A Molecular Sieves, powdered, 600 Mesh, ca. 2-4% hydration. (5) Cary 82 spectrophotometer, Spectra-Physics argon laser Model 165, power = 500 mW, A,, 514.5 nm, slit = 2.0 cm-’, pen period = 10 s, scan speed = 0.1 cm-l/s, sensitivity = 1000 counts/s full scab. (6) S. K. Freeman, “Applications of Laser Raman Spectroscopy”, Wiley-Interscience, New York, N.Y., 1974: (a) p 314; (b) p 30. (7) C. L. Angell, J. Phys. Chem., 77, 222 (1973). (8) N. T. Tam, R. P. Cooney, and G. Curtheys, J. Chem. SOC., Faraday Trans. 1, 72, 2577 (1976). (9) H. Forster and M. Schuldt, J. Chem. Phys., 86, 5237 (1977). (IO) 1 gram of sieves is boiled for 15 min in a 0.2 N NaOH solutlon, washed free of excess base, fikered, pulverized, and calcined at 400 OC for 1 h. (1 1) (a) E. Buechler and J. Turkevich, J. Phys. Chem., 76, 2325 (1972); (b) T. A. Egerton, A. H. Hardin, Y. Kozirovski, and N. Sheppard, Chem. Commun., 887 (1971); (c) R. 0. Kagei, J. Phys. Chem., 74, 4518 (1970). (12) 0.W. Breck, “Zeolite Molecular Sieves”, Wiley, New York, N.Y.: (a) see p 133 and 442 f f ; (b) p 490. (13) Power = 200 mW, A,, = 514.5 nm, slit = 5.0 cm-’, pen period = 5 s, scan speed = 1.0 cm-’/s, sensitivity = 5000 counts/s full scale. (14) Silica gel (J. T. Baker Co.) washed with 0.2 N NaOH’ shows a 50% higher SIN (lower fluorescence) throughout its Raman spectrum (20-2500 cm-I) than does either untreated sllica gel or silica gel calcined at 600 OC in air for 2 h. (15) R. C. Weast, Ed., “Handbook of Chemistry and Physics”, 56th ed, Chemical Rubber Co., Akron, Ohio, 1976. Merck Sharp and Dohme Research Laboratories Division of Merck and Company, Inc. Rahway, New Jersey 07065
Davld D. Sapersteln’ Alan J. Reln’
Received June 20, 1977
High Protonic Conduction of Polybenzlmldazole Films’ Publication costs assisted by the National Science Foundation
Sir: Pressed disks of polybenzimidazole2(PBI) are known to be insulators for electronic cond~ction.~We have confirmed this finding for polybenzimidazolefilm, but find that protonic conduction is high, the conductivity being in the semiconductor range. The Journal of Physical Chemistry, Vol. 8 I, No. 22, 1977
2136
Additions and Corrections
TABLE I : P r o t o n i c C o n d u c t i v i t y Results for PBI Film Conductivity Premeasurement
Resistances
humidity, %
Bulk
Surface
0 7 15 31 100
7 3 2 3 2
16 11 4 13
Bulk: fi-’ c m - ’
Surface,b
2 x 10-4 5X 8X 5 x 10-4 8X
4 x 10-3 6X 2 X lo-’
n-’
5X
u = c m - ’ ) = ( l / R ) ( L / A=) 1.58 x lO-’(l/R). ( l / R ) { ( r l - r,)/2r[(rl r,)/2]} where rl = i.d. of guard ring, 0.375 in. a n d r2 = radius of small disk, 0.25 in. a
u(n
+
1
’
s t r u c t u r a l unit of P B I
Bulk and surface resistances were measured simultaneously with guard ring electrodes made of brass, using a modified Wheatstone bridge.4 The electrodes were two disks, one 0.5411 in diameter, the other 0.25411. with a guard ring of 0.5-in. 0.d. and 0.375-in. i.d. They were set in a lucite holder and applied to the film with slight pressure. These electrodes measure electronic conduction. If they are covered with platinum black and exposed to hydrogen gas so that the hydrogen is absorbed onto the electrode surface they become “protodes” [Pt/H2 H” (in polymer film) e- (in wire)15and measure protonic conduction. We have found that the hydrogen remains absorbed on the Pt black even when the electrodes are exposed to air for several weeks. This convenient property let us measure protonic conductivity without exposing the polymer film to gaseous hydrogen. The PBI film was provided by Dr. H. J. Davis of Celanese Corp. and was fiber quality, approximately 0.01 mm thick. The film was boiled in distilled water for several hours to remove traces of lithium chloride, resulting in a salt-free film containing approximately 4 mol of tightly held water per structural unit.6 Conduction measurements were made at room temperature in the presence of air. Because of the abundant evidence that absorbed water affects electronic conduction in polymers, the PBI films were pretreated by exposing them to a controlled humidity for 24 h and then quickly transferred to the measuring
+
apparatus. Bulk and surface conductivity for electronic conduction were so low that they could not be measured with our equipment, but we could place an upper limit of Q-l cm-l on them. On switching to the protodes the results shown in Table I were obtained. The change from the conductivity of an insulator to that of a semiconductor proves a change in conduction mechanism, probably from electronic conduction to protonic conduction. The results show no significant trend in conductivity as a function of premeasurement humidity. The hypothesis that conduction involves a protonic mechanism has been put forward to explain the d.c. conductivity and low frequency dielectric loss of pressed disks of polyhexamethylene sebacamide, a linear polyamide.I Additional work8p9has led to the generalization that polyamide solids as a class exhibit proton conduction which is associated with the disordered parts of the solid. However, the conductivities of these materials were smaller by at least six orders of magnitude than our values for PBI film. In explaining the high conductivity of PBI film, it may be significant that the polymer chains are linear, that the aromatic rings have a marked tendency to be coplanar, and that the film contains tightly held water molecules which may assist bifunctional proton transfer. Our decision to search for polymers with a high protonic conductivity was prompted by the known high speed of intramolecular proton transfer of diamines in aqueous ~ o l u t i o n . ~ ~ - ~ ~
References and Notes (1) This work was supported by the National Science Foundation. (2) C. S. Marvel and H. A. Vogel, U S . Patent No. 3 174947;Chem. Abstr., 63, P7137h. (3) H. A. Pohl and R. P. Chartoff, J. Polym. Sci. Part A , 2,2787-2806 (1964). (4) A. P. Zielinger, M. Tapiera, and C. Noguet, J. Phys. E, 6, 579 (1973). (5) L. Glasser, Chem. Rev., 75,21-65 (1975). (6) Private communication from Dr. D. R. Wilson of Celanese Corp. and confirmed by us.
(7) W. 0. Baker and W. A. Yager, J. Am. Chem. Soc., 64,2171 (1942). (8) R. H. Boyd, J. Chem. Phys., 30, 1276 (1959). (9) D.W. McCall and E. W. Anderson, J. Chem. Phys., 32, 237 (1960). (10) D. W. Fong and E. Grunwald, J . Am. Chem. Soc., 94,7371 (1972). (11) E. Grunwald, K. C. Chang, P. L. Skipper, and V. K. Anderson, J. Phys. Chem., 80, 1425 (1976). (12) K. C. Chang and E. Grunwald, J. Am. Chem. Soc., 98,3737 (1976). (13) K. C. Chang, E. Grunwald and L. R. Robinson, J. Am. Chem. Soc., 99, 3794 (1977). Chemistry Department Brandeis University Waltham, Massachusetts 02 154 Received June 24, 1977
ADDITIONS AND CORRECTIONS 1976. Volume 80 Maurice L. Huggins: Thermodynamic Properties of Liquids, Including Solutions. 12. Dependence of Solution Properties on Properties of the Component Molecules. Page 1318. Equation 3 should read € A = 0lo(2f12 - €11 - f22)/2
Nothing else in the paper is affected by this correction. -Maurice L. Huggins
The Journal of Physical Chemistry, Vol. 8 1, No. 22, 1977
Donna Hod* Ernest Grunwald
