Anisotropic Ionic Conductivity of Lithium-Doped Sulfonated PBI R. J. SPRY, 1 M. D. ALEXANDER, JR., 2 S. J. BAI, 2 T. D. DANG, 2 G. E. PRICE, 2 D. R. DEAN, 1 B. KUMAR, 2 J. S. SOLOMON, 2 F. E. ARNOLD 1 1 Polymer Branch, Materials Directorate, Wright Laboratory, Wright-Patterson AFB, Department of the Air Force, 2941 P. St., Ste 1, Ohio 45433 2 University of Dayton Research Institute, Dayton, Ohio 45469 Received 16 May 1997; accepted 11 August 1997 ABSTRACT: The conductivity study results of lithium-doped sulfonated PBI, a conju- gated rigid rod polymer, poly [ ( 1,7-dihydrobenzo [1,2-d:4,5-d  ] dimidazole-2,6-diyl ) -2- (2-sulfo)- p -phenylene ] , derivatized with pendants of propane sulfonate Li / ionomer are reported. The room-temperature DC four-probe conductivity parallel to the surface of cast films was as large as 8.3 1 10 03 S / cm. Similar measurements with an eight- probe configuration showed no difference between bulk and surface conductivity. The ionic nature of the conductivity was indicated by constant voltage depletion experiments and by secondary ion mass spectroscopy measurements of the residues near the elec- trodes. The DC two-probe conductivity measured transverse to the sample surface was three to four orders of magnitude smaller than longitudinal conductivity, while the AC two-probe conductivity was even less. Electron microscopy indicated that the films had a layered structure parallel to the surfaces. This structural anisotropy was confirmed by refractive index values obtained from wave-guide experiments and by wide angle X-ray scattering.  1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2925 – 2933, 1997 Keywords: ionic conductivity; DC conductivity; rigid-rod polymer ; depletion measure- ment; X-ray scattering; anisotropic; polymer electrolyte; polyelectrolyte; conducting polymers INTRODUCTION the utilization of comb polymers, 7 solvent-con- taining polymers, 3 and blend-based polymers. 4 The desire to maintain strength, stiffness, and Polymer electrolytes with high ionic conductivity are required for high energy density rechargeable thermal stability along with high ionic conductiv- ity has led to the study of the conductivity of rod- batteries. Many have room temperature conduc- tivities of about 10 06 S / cm but practical applica- like polymers, for example, lithium-doped sulfo- nated poly ( p -phenyleneterephtalamide ) ( sulfo- tions require conductivities in the range of 10 03 to 10 04 S/cm. 1–4 The ionic conduction mechanism nated Kevlar ), 8 sodium-doped sulfonated poly involves both the movement of ions in an electric ( benzobisthiazoles ) (PBTs), 9 and acid-doped field and the thermally induced assistance of poly- PBI. 10 In the present study, the conductivity of mer chain segments. 2,5 Recent methods to in- sulfonated PBI, derivatized with pendants of pro- crease the ionic conductivity of polymer electro- pane sulfonate Li / ionomer was investigated as lytes include the introduction of plasticizers, 6 and a function of excess lithium concentration. This is an extension of previous research on similar materials containing H / and Na / moieties in sto- Correspondence to: R. J. Spry ichiometeric quanitites. 11,12 The objectives of the Journal of Polymer Science: Part B: Polymer Physics, Vol. 35, 2925–2933 (1997)  1997 John Wiley & Sons, Inc. CCC 0887-6266/97 / 172925-09 present study were to enhance the water solubil- 2925 9713021 / 8Q46$$3021 10-24-97 13:48:28 polpal W: Poly Physics