Meridional orientation in biaxially aligned thin films of hairy-rod polyfluorene

The uniaxial chain alignment and equatorial patterning of poly [9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (M(n) = 29 kg/mol, M(w) = 68 kg/mol) thin films atop rubbed polyimide substrates have been studied by grazing incidence X-ray diffraction. This specific molecular weight yields among the highest observed levels of chain alignment and optical anisotropy and, after thermal annealing, the sample undergoes transformation to a highly textured crystalline hexagonal phase. The two dominant equatorial orientations (i.e., types I and II crystallites) are found to have almost identical meridional orientation distributions. The X-ray deduced orientation is in quantitative agreement with that obtained by optical absorption measurements. It is also shown that the equatorial ordering is paracrystalline in nature, and both type I and type II crystallites are similar in this respect. This equatorial scattering is superimposed on a background of a hexagonal phase polymer with a cylindrically isotropic orientation (type III).