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10 November 2004 Multiple orientation and nematic-hexagonal transition in uniaxially aligned polyfluorene thin films
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We present a structural study of poly(9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl) (PF2/6) in aligned thin films on a rubbed polyimide substrate. PF2/6 forms 5/2-helical hairy-rodlike molecular structure which is self-organized in the hexagonal structure. In thin films, the aligned rigid polymer chains are parallel to the substrate in the direction corresponding to the molecular backbone, c axis. The cells are flattened in the direction of the surface normal and -in particular- reveal a multiple orientation where the greater proportion of the crystallites have one crystal axis a perpendicular to the substrate surface but a small proportion are aligned with the crystal axis a parallel to surface. We find further that by reducing the polymer chain length to approximately twenty repeat units the degree of axial alignment of PF2/6 is considerably improved while the local order changes from hexagonal packing towards nematic phase with no sign of multiple orientation.
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Matti Knaapila, Benjamin P. Lyons, Kaisa Kisko, Oliver H. Seeck, Joel P. Foreman, Ulla Vainio, Ritva Serimaa, Mika Torkkeli, and Andrew P. Monkman "Multiple orientation and nematic-hexagonal transition in uniaxially aligned polyfluorene thin films", Proc. SPIE 5519, Organic Light-Emitting Materials and Devices VIII, (10 November 2004);


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