Influence of the casting solvent on the thermotropic alignment of thin liquid crystalline polyfluorene copolymer films

The morphology of a conjugated polymer thin film determines how it will behave as an active layer in optoelectronic devices. The liquid crystalline phase present in some conjugated polymers offers a unique way of controlling the film morphology. For example, films in which the chains are aligned in a single direction can be fabricated by processing films on a templating layer and annealing them above the liquid crystalline melting point of the polymer. However, the ability to align to a templating layer will be influenced by the character of the as-cast film. This effect was examined by casting poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) films from a range of both aromatic and chlorinated solvents on top of rubbed polyimide layers and aligning via the thermotropic melt. Measurements of the dichroic ratio as a function of annealing time revealed that films cast from high boiling point aromatic solvents like isodurene (ISO) and o-xylene (OXY) aligned well with a maximum dichroic ratio over 10. Films cast from toluene (TOL), a lower boiling aromatic solvent, exhibited a lower D-max of only around 5, while the films cast from chloroform (CHL) and tetrachloroethane (TCE), both chlorinated solvents, exhibited very low dichroic ratios. The best alignment (D-max = 15) was achieved when a F8BT film was annealed while still wet, which proves that a plasticizer (in this case ISO molecules) can improve the mobility of the polymer chains as they initially align to the rubbed surface. However, it is unclear whether a plasticizing effect (from residual solvent) is responsible for the differences in the alignment observed in films that were dried before annealing, as D-max for these samples did not directly correlate to the volatility of the solvent. Ellipsometric measurements showed that all the films that aligned well exhibited a much more dramatic densification upon annealing than those that did not align well. Since all the well-aligned films were cast from aromatic solvents, we believe that aromatic interactions between polymer and solvent is critical in spinning a film that will align well. These results will guide future efforts into improving molecular orientation of liquid crystalline polyfluorene films.