High Temperature Electrical Conductivity of Solution-Cast Bbl Films
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ASHWINI K. AGRAWAL,# CHYI-SHAN WANG+, AND HYUN H. SONG+ #Systran Corporation, 4126 Linden avenue, Dayton, OH 45432 +University of Dayton Research Institute, 300 College Park Drive, Dayton, OH 45469
ABSTRACT Thin films (18 g. thickness) of ladder polymer poly[(7-oxo-7H,IOHbenz(d,e)imidazo(4',5':5,6)-benzimidazo(2,1-a)isoquinoline-3,4:10,11-tetrayl)- 10-carbonyl} (BBL), which were cast from an isotropic methanesulfonic acid solution of BBL, were characterized for electrical conductivity in the temperature range of 25-480 °C. The electrical conductivity was measured in situ while the films were heated at a rate of 2 °C/minute under vacuum. The in-plane conductivity of BBL films increased rapidly from below 10-10 S/cm at the room temperature to 5 x 10- 3 S/cm at 280 °C and then increased slowly to a peak value of 5 x 10-2 S/cm at about 460 oC. The out-of-plane conductivity of the films was comparable to their in plane conductivity at temperatures below 270 oC, however, was lower by a factor of 10 than inplane conductivity at temperatures above 270 oC. This anisotropy in conductivity of BBL films is discussed with respect to the film morphology determined by wide angle x-ray scattering studies. INTRODUCTION BBL (Figure 1) is an important conjugated polymer since it combines ladder structure, ease of processability,J and remarkable electronic, 2 optoelectronic, 3 and nonlinear optical properties. 4 Recently, high conductivity was obtained when BBL films or fibers were heat treated at a high temperature. 5 -7 In one of the studies,6 solution cast BBL films preheat-treated at 477 oC were shown to have an in-plane conductivity which increased monotonically from 10-10 S/cm at 27 °C to 10-5 S/cm at 427 oC. The out-of-plane conductivity of these films was about three-orders of magnitude lower than the in-plane conductivity for the same temperature range. BBL chains being rigid due to their ladder structure may induce structural anisotropy in the solution cast BBL films. The conductivity results together with possible structural anisotropy suggest that charge conduction in solution cast BBL films may be mainly in the in-plane direction. In another study, 7 the room temperature dc conductivity of a uniaxial BBL fiber was significantly enhanced by heat treatment at high temperatures. The highest room temperature conductivity of 3 x 10-4 S/cm was achieved for a fiber heat-treated at 350 oC for two minutes. The conductivity was found to be stable for many days under ambient conditions. The room temperature conductivity was significantly lower than 3 x 10-4 S/cm for samples heat-treated beyond 350 oC. These studies indicate that although high intrinsic conductivity can be induced in BBL samples by subjecting the samples to high temperatures, the magnitude of conductivity is strongly dependent on thermal history of the samples. Since the previous in situ studies 5 ,6 on BBL conductivity versus temperature were performed on preheat-treated samples, the conductivity behavior of pristine BBL film at high temperatures remains unknown. 2
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