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
