Bipolar conducting polymers: Blends of p-type polypyrrole and an n-type ladder polymer

Bipolar conducting polymers, in which both hole transport and electron transport contribute to electronic conductivity, have been explored by chemical template synthesis of p-type polypyrrole (PPy) in the matrix of an n-type conjugated ladder polymer, poly(benzimidazole-benzophenanthroline) (BBL). Transmission electron microscopy images of the conducting polymer blends show that 5-20 nm diameter x 100-180 nm long rodlike PPy particles are randomly and homogeneously distributed in the BBL matrix, with connectivity of the PPy phase occurring at a volume fraction of about 0.17. The volume fraction dependence of conductivity of the BBL/PPy blends did not exhibit a percolation threshold at volume fractions as low as 0.007 nor can it be described by percolation-type effective medium theory. Room temperature conductivities as high as 60-70 S/cm were observed in the blends compared to 2 S/cm in pure PPy. The enhanced conductivity and the nonpercolation nature of these blends originate from bipolar charge transport involving both conjugated polymer components of the blends. Existence of the oxidized (p-type) polypyrrole and reduced (n-type) BBL that facilitate bipolar charge transport in these blends was established by cyclic voltammetry.