Nascent morphology of polyacetylene

Abstract

Acetylene was first polymerised to a linear polymer in 1958¹ but the resulting grey powdery material, insoluble, infusible, and devoid of any interesting electrical properties, remained a scientific curiosity until Ito et al.² showed that it was possible to polymerise acetylene into a continuous film on the wall of a reactor. This polyacetylene film, (CH)_x, a large band gap semiconductor, can be transformed into a highly conducting material with σRT∼ 1,000 (Ω cm)⁻¹ (refs 3–12) upon doping with either electron donors or acceptors^13,14. Although fibrous electron micrographs (EM) of unaligned² and partially aligned (stretch elongated) samples having enhanced conductivities have been reported³, it has been suggested that the observed morphology might be misleading, possibly resulting from a post-polymerisation handling of the film, or even an artefact such as the result of ultrasonication. The aim of this work is to determine the true nascent morphology of polyacetylene. We have polymerised acetylene directly on electron microscope gold grids and examined the polymer with scanning and transmission modes. The ultrathin specimen is entirely fibrous with diameters of ∼5–40 nm, with 20-nm fibrils the most abundant. Some films show surface fractures revealing partially or completely aligned fibrils bridging the gaps. Some globular features were also observed suggesting local fusion. Thick films of polyacetylene also have fibrous morphology throughout. The observed structure is consistent with surface area measurements. Therefore, acetylene polymerises directly into fibrils, possibly due to the rigidity of the polymer backbone which is comprised of conjugated double bonds.