X-ray-diffraction studies of the three-dimensional structure within iodine-intercalated poly(3-octylthiophene)

Comprehensive x-ray-diffraction data and structure factor calculations are presented for oriented poly(3-octylthiophene) films intercalated by iodine vapor to ${\mathrm{I}}_3^{\mathrm{-}}$ concentrations of 0, 4, 14, 26, and 45 mol wt. % per octylthiophene monomer. These data reconfirm previous reports that this polymer-iodine complex exhibits a continuous and reversible structural evolution. [M. J. Winokur, P. Wamsley, J. Moulton, P. Smith, and A. J. Heeger, Macromolecules 24, 3812 (1991); T. J. Prosa, M. J. Winokur, J. Moulton, P. Smith, and A. J. Heeger, Synth. Met. 55, 370 (1993); J. Moulton and P. Smith, ibid. 40, 13 (1991)]. Structure factor modeling calculations are used to fully specify the overall changes in the three-dimensional host polymer structure. This molecular restructuring involves both translational and rotational rearrangements of the host polymer. The fundamental lamellar base structure of the pristine host polymer is found to remain fully intact throughout the entire structural transformation. The large-scale changes in the observed interlayer repeat distances result from continuous variations in the angular orientation of the polymer backbone and in the average position of the alkyl side chains. Translational displacements of the polymer chains parallel to the main chain axes are found to occur within individual lamellae and this motion creates essentially one-dimensional intercalant channels perpendicular to the main chain axis. This latter process facilitates iodine intercalation and is, in part, responsible for the ability to achieve the very high iodine concentrations which have been measured experimentally. Scattering features are seen at the highest iodine concentrations which are indicative of additional structural ordering by the intercalant within the newly formed one-dimensional columns.