Scanning force microscopy and optical spectroscopy of phase-segregated thin films of poly(9,9 '-dioctylfluorene-alt-benzothiadiazole) and poly(ethylene oxide)

We describe the formation of ordered phase-segregated domains at the hundred nm scale in thin films prepared from two molecular systems of interest as active materials in light-emitting electrochemical cells, i.e. the ion-transport polymer poly( ethylene oxide), PEO, and the well-known electron-transport macromolecule poly(9,9'-dioctylfluorene-alt-benzothiadiazole), F8BT. Scanning force microscopy investigations revealed signs indicative of self-organization processes taking place during film deposition, and characterized by the formation of PEO crystalline lamellae arranged either face-on or as intertwined fibres assembled in an edge-on fashion surrounding large and disordered F8BT grains. This self-segregated architecture, prepared by a single step co-deposition of the two components from a chloroform solution, provides unambiguous evidence for the poor miscibility of the two polymers. Fluorescence titration studies in solution and measurements of the photoluminescence quantum efficiency of thin solid state films showed little change of the optical properties upon addition of PEO to F8BT, thus confirming the modest interaction, at the molecular level, between the PEO and F8BT macromolecular strands. The results obtained are significant to the understanding of the parameters contributing to the interfacial and intermolecular interactions.