X-ray scattering from oriented poly(3-alkylthiophenes)

Abstract

Solution-cast films of poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) were oriented by stretching to obtain more detailed information about their crystalline structure. For the first time reflections associated with the stretch direction l = 1–7 were revealed using a combination of X-ray film exposures and detailed intensity mapping by various detector scans. The reflections were located at finite angles to the stretch axis indicating a monoclinic (or triclinic) unit cell. The equatorial diffraction patterns ( l = 0) of P3HT and P3OT show that the two structures are equal only with one differing lattice dimension caused by the length of the alkyl side chains. The crystalline structure for the unstretched (isotropic) and the stretched sample is found to be the sa me. Existence of h00 and absence of hk0, h0 l and hkl reflections, all with finite integers h, k and l, indicate layer stacking disorder between bc-planes. Intensity calculations based upon this model corroborate well with the measured diffraction pattern on the assumption of disordered alkyl side chains.