Directional epitaxial crystallization and tentative crystal structure of poly (9,9'-di-n-octyl-2,7-fluorene)

Directional epitaxial crystallization is used to prepare highly oriented films of poly(9,9'-di-n-octyl-2,7-fluorene) (PFO). This orientation method implies the use of a crystallizable aromatic solvent (1,3,5-trichlorobenzene) playing successively the role of the solvent for PFO for T > 63 degrees C and the role of an orienting substrate below this temperature. Extended areas of the oriented PFO can be formed on a given substrate after simple evaporation of the crystallized solvent under vacuum. The effect of annealing on the thin film structure and morphology was followed by transmission electron microscopy (TEM) in bright field and dark field modes and atomic force microscopy. Upon annealing, oriented crystalline PFO lamellae and axialites are formed by coalescence of smaller nanocrystalline domains. As shown by electron diffraction, a single-crystal-like structure is formed after a short annealing of the as-oriented films at 210 degrees C and slow cooling (0.5 degrees C/min) to room temperature. The films consist of oriented lamellar crystals in edge-on orientation on the SiO2 substrate, and the PFO chains are oriented parallel to the TCB stacking direction (C-TCB). The reflection rules deduced from various selected area electron diffraction (SAED) patterns obtained by rotation-tilt indicate that PFO crystallizes in an orthorhombic structure with Pnb2(1) space group. A tentative crystal structure was obtained from the simulations of the SAED patterns and molecular modeling. The epitaxial relationship at the PFO/TCB interface suggests that ID epitaxy is involved: the PFO chains are oriented parallel to the stacking direction of TCB (c-axis); the 3.95 angstrom distance between TCB molecules matches closely the reticular distance of 4.1 angstrom corresponding to dense planes of n-octyl side groups of interdigitated PFO chains.