The influence of the molecular weight on the thermotropic alignment and self-organized structure formation of branched side chain hairy-rod polyfluorene in thin films

We report on the influence of the molecular weight (M) on the alignment and structure of poly(9,9-bis(ethylhexyl)fluorene-2,7-diyl) (PF2/6) in thin films on rubbed polyimide in the equilibrium. The degree of alignment has been studied using optical spectroscopy and compared to theoretical arguments. The structure of PF2/6 has been studied using grazing-incidence X-ray diffraction. PF2/6 realizes a threshold molecular weight M-n* = 10(4) g/mol separating a low M-n (LMW, M-n < M-n*) and a high M-n (HMW, M-n > M-n*) region. LMW materials show only the nematic (Nem) phase while HMW compounds show hexagonal (Hex) and Nem phases at low and high temperatures, respectively. LMW samples align equally well at any temperature above the glass transition, and the dichroic ratio, R, increases with M-n. HMW material aligns well in the Nem regime, and R drops exponentially with M-n, which is in agreement with the theory. The orientational order is maximized near M-n*, and the orientational. order parameter is > 0.9. HMW samples, whether aligned or not, reveal two kinds of coexistent Hex crystallites distributed in the sample plane having one crystal axis a perpendicular or parallel to the surface. Local order within the triaxially aligned Hex phase surpasses that of the in-plane aligned Hex phase.