Molecular orientation of crystalline and glassy polyfluorene layers induced by photo-aligned polyimide films

We have investigated the alignment of poly (9,9-dioctylfluorenyl-2,7-diyl) (PFO) induced by a photo-aligned polyimide film, which is optically, thermally, and chemically stable. The similar to 30 nm-thick PFO layer was spin-coated onto a photo-aligned polyimide film. Slow cooling and quenching of the sample from the liquid crystalline phase of PFO formed uniaxially-aligned crystalline and glassy layers of PFO, respectively. For comparison, we also investigated the alignment of PFO induced by a rubbed polyimide film. For the crystalline PFO layer on the photo-aligned film, an extremely high polarization ratio was observed in photoluminescence (PL), which was similar to 30. It was much larger than that of the crystalline PFO layer formed on the rubbed film (similar to 20). Indeed, such a high PL polarization ratio has not previously been reported in crystalline PFO layers formed by any other alignment method. For the glassy PFO layer formed on the photo-aligned film, the PL polarization ratio was 10.8. It was slightly larger than that of the glassy PFO layer on the rubbed film (9.5). These results show that the photo-aligned polyimide film used in this study is an excellent alignment film for PFO layers, especially for crystalline layers. (c) 2005 Elsevier B.V. All rights reserved.