Influence of Molecular Weight on Self-Organization, Uniaxial Alignment, and Surface Morphology of Hairy-Rodlike Polyfluorene in Thin Films

We present investigations of the self-organized structure, overall uniaxial alignment, and larger-scale interface morphology in thin films of low-molecular-weight hairy-rodlike π-conjugated poly(9,9-bis(ethylhexyl)-fluorene-2,7-diyl), (LMW-PF2/6) ( M n = 7600 g/mol) and a comparison of high-molecular-weight PF2/6 (HMW-PF2/6) ( M n = 150 000 g/mol). Photoabsorption, grazing-incidence X-ray diffraction, small/wide-angle X-ray scattering, and X-ray reflectivity methods have been used. The experiments have been supported by molecular mechanics-calculated molecular structure and recursively simulated reflectivity curves and discussed in terms of the semiquantitative theoretical analysis of the self-organization of hairy-rodlike polymers. As with HMW-PF2/6, LMW-PF2/6 is found to be a thermotropic liquid crystal consisting of rodlike 5/2 helices. Blue photoluminescence with an absolute photoluminescence quantum yield of 32% in the solid films is observed. After thermotropic alignment on rubbed substrates, considerably higher dichroic ratios in absorption, >10, are found, indicating a far higher degree of axial alignment compared to the similarly processed HMW-PF2/6. The degree of spatial order has been found to be high along the rubbing direction, the z axis but, in contrast to HMW-PF2/6, the structure perpendicular to the z axis on the ( ab 0) plane is observed to be less ordered, and no multiple orientation (Knaapila, M.; Lyons, B. P.; Kisko, K.; Foreman, J. P.; Vainio, U.; Mihaylova, M.; Seeck, O. H.; Pålsson, L.-O.; Serimaa, R.; Torkkeli, M.; Monkman, A. P. J. Phys. Chem. B 2003, 107, 12425−12430) is seen. Both in bulk and in thin aligned films, LMW-PF2/6 is suggested to approach a nematic instead of a well-defined hexagonal structure. These findings are in agreement with the presented theoretical arguments. LMW-PF2/6 is also shown to form well-developed larger-scale morphology and surface roughness below 2 nm in films over the thickness range from 20 to 200 nm. There is also a difference in macroscopic texture in polarized micrographs between these materials, but the influence of the molecular weight cannot be rigorously established at present.