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 ð-conjugated poly(9,9-bis(ethylhexyl)-fluorene-2,7-diyl), (LMW-PF2/6) (Mn) 7600 g/mol) and a comparison of high-molecular-weight PF2/6 (HMW-PF2/ 6) (Mn) 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, i