Electronic-structure evolution upon thermal treatment of polyacrylonitrile: A theoretical investigation

We present valence effective Hamiltonian nonempirical band-structure calculations on polyacrylonitrile. We first consider the compound in its pristine state and discuss the results of calculations on an helical conformation as well as on a linear conformation. We then investigate the electronic properties of various geometric structures that have been proposed in the literature as appearing upon thermal treatment up to 300°C. These structures namely correspond to a single conjugated structure (due to ring closure along the nitrile groups) and, at higher pyrolysis temperature, to a doubly conjugated ladder structure (polypyridinopyridine). In order to facilitate the discussion of the electronic-structure evolution in pyrolized polyacrylonitrile, we also present band-structure calculations on related compounds, such as polyethylene, polymethineimine, and polyacene. From the band stuctures, we derive the theoretical photoemission spectra of the studied polymers. The comparison with available spectra from photoelectron spectroscopy is very good and allows a detailed interpretation of the evolution of those spectra upon pyrolysis up to 300°C.