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_w) 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°C, within a period of 2h the conductivity of PANI doped with the high M_w PAA decreases continuously with the annealing time following σ=σ₀exp[-(t_a/τ)^1/2] law, while the one doped with the low M_w PAA obeys this law only within 1h 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°C 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. FT-IR study shows that annealing leads to de-doping, but it is less pronounced in the high M_w PAA doped PANI.