Thermal degradation of electrical conductivity of polyacrylic acid doped polyaniline: effect of molecular weight of the dopants

This paper describes the effect of molecular weight of dopants on thermal degradation behaviour of electrical conductivity of polyacrylic acid (PAA) doped polyaniline (PANI). Two PAA with weight-average molecular weight (M,) of 5000 and 250,000, respectively, were used as dopants to synthesise PANI-PAA complexes by in situ oxidative polymerisation. PANI doped with the low M-w PAA has lower electrical conductivity. which is attributed to its smaller size of crystal islands due to the end group effect. When annealed at 180 degreesC, within a period of 2 h the conductivity of PANI doped with the high M-w PAA decreases continuously with the annealing time following sigma = sigma(0) exp [-(t(a)/tau)(1,2)] law, while the one doped with the low M-w PAA obeys this law only within 1 h of annealing. This indicates that the decrease of conducting island size is responsible for the thermal degradation of the conductivity of the complexes in a certain period and the length of this period depends on M-w of the dopants. TGA study shows that at 180 degreesC weight loss rates of the low and high M-w PAA doped PANI are about the same, which implies that the faster reduction of the conductivity in the low M-w PAA doped PANI is mainly due to its smaller initial crystal size. FTIR study shows that annealing leads to de-doping, but it is less pronounced in the high M, PAA doped PANI. (C) 2002 Elsevier Science B.V. All rights reserved.