Investigation of the electrical conductivity of the iodine‐polyphenylacetylene charge‐transfer complex

The electrical conductivity of the charge transfer complex between iodine and polyphenylacetylene (PPA) has been studied. The room temperature d‐c conductivity (σ) increased by seven orders of magnitude with iodine addition, reaching a maximum of 10−5 Ω−1 · cm−1 for a 1:1 ratio of iodine to repeating unit, and its concentration dependence could be described by: σ = σ0 exp [γc(1–c) – γ/4], where c is the mole fraction of iodine. Both crystalline and amorphous cis‐PPA followed this form, the crystalline polymer having a lower conductivity at low iodine levels, but attaining the same maximum value. The trans‐polymer iodinated much less readily than cis‐PPA. Molecular weight variations did not influence iodine uptake or conductivity in the range studied. It was possible to remove virtually all iodine from the charge‐transfer complex in dynamic vacuum, and an iodine diffusion coefficient of D ≈ 10−11 cm2.s−1 was determined from the rate of weight loss. Passage of current resulted in the evolution of iodine and iodine compounds at the anode. The experiment is consistent with a predominantly ionic conduction mechanism, in which the strong concentration dependence of σ is associated with a concentration‐dependent activation energy for ionic charge‐carrier formation.