Structure and Morphology in Highly Oriented Films of Poly(9,9-bis(n-octyl)fluorene-2,7-diyl) and Poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) Grown on Friction Transferred Poly(tetrafluoroethylene)

This study focuses on the epitaxial orientation of two polyfluorenes (PF) namely poly(9,9-bis(n-octyl)fluorene-2,7-diyl) (PFO) and poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) (PF2/6) on friction transferred poly(tetrafluoroethylene). PF2/6 films with a single-crystal like orientation are obtained on PTFE substrates after proper annealing whereas only marginal orientation is observed for PFO films. For both polymers, the crystalline lamellae grow essentially perpendicular to the PTFE chain direction and the epitaxial conditions were determined as [0 0 1]_PF//[0 0 1]_PTFE and (0 1 0)_PF//(1 0 0)_PTFE. For PF2/6 samples with different molecular weight distributions, the lamellar thickness tends to increase with increasing average molecular weight of the polymer. The close examination of the selected area electron diffraction (SAED) patterns corresponding to the [0 1 0]_PF26 zone supports a 21₄ helical conformation of the PF2/6 chains. A simple structural model based on a trigonal cell with P3 space group, a = b = 1.68 nm and c = 16.9 nm containing a triplet of 21₄ helices accounts well for the main features of the observed electron diffraction pattern. The three 21₄ helices of the triplet show periodic short contacts every fifth monomer along the helix axis, suggesting that the supramolecular assembly of the PF2/6 chains forming the triplet is stabilized by efficient π-stacking between fluorene monomers. The observed difference in orientation of PF2/6 and PFO on PTFE substrates supports an orientation mechanism involving the PTFE macrosteps.