Influence of Molecular Weight on the Surface Morphology of Aligned, Branched Side-Chain Polyfluorene

• Published in: Advanced functional materials 2005-09, Vol.15 (9), p.1517-1522
• Main Authors: Knaapila, M., Lyons, B. P., Hase, T. P. A., Pearson, C., Petty, M. C., Bouchenoire, L., Thompson, P., Serimaa, R., Torkkeli, M., Monkman, A. P.
• Format: Article
• Language: English
• Subjects: Polyfluorenes
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200500061

The surface structure of uniaxially aligned poly(9,9‐bis(ethylhexyl)‐fluorene‐2,7‐diyl) films on rubbed polyimide has been studied as a function of molecular weight (Mn = 3–150 kg mol–1, number‐average molecular weight) using polarized microscopy, atomic force microscopy (AFM), X‐ray reflectivity, and grazing‐incidence X‐ray diffraction. At the threshold Mn, Mn* = 104 g mol–1, there is a prominent transition in morphology from featureless (Mn  Mn*), corresponding to the nematic–hexagonal phase transition. The hexagonal phase reveals two coexistent crystallite types in the whole film and at least one crystallite type has been observed at the surface by AFM. The combined optimization of alignment and surface smoothness is achieved slightly below Mn* while the combined optimization of orientational and local order and moderately smooth surface is achieved slightly above Mn*. The relationship between macroscopic–submicrometer‐scale morphology and self‐organized intermolecular structure and uniaxial alignment is highlighted for a series of poly(9,9‐bis(ethylhexyl)‐fluorene‐2,7‐diyl) films. At a threshold molecular weight, there is a distinct transition in morphology (see Figure: left panel: nematic phase; right panel: hexagonal phase).