Influence of the molecular weight on the thermotropic alignment of thin liquid crystalline polyfluorene copolymer films

Mechanisms that limit the thermotropic alignment of a liquid crystal semiconducting polymer on a rubbed polyimide film were studied using three different molecular weights of poly(9,9dioctylfluorene-co-benzothiadiazole) (F8BT), one of a large class of polyfluorene polymers currently being investigated for use in highly polarized light-emitting diodes and high-performance polymer-based transistors. The molecular weight was found to influence the melting temperature of the polymer, the speed at which the polymer aligns to a rubbed surface, and the ultimate molecular alignment that can be achieved. The alignment of F8BT with the highest molecular weight was severely limited by the viscosity of the polymer, while the films with low and intermediate molecular weights reached a maximum alignment in reasonable processing times. The maximum saturated dichroic ratio (D-max) was consistently higher, the largest D-max observed was over 29, in the lowest molecular weight films. Optical microscopy revealed that the lower dichroic ratio could be attributed to an inability of the domains in the higher molecular weight film to reorient to the templating direction; thus, the final film retains a multidomain structure. This has serious implications on the charge transport through these materials since any domain boundaries will adversely affect the charge carrier mobility. Therefore, despite the advantages that higher molecular weights could have on optoelectronic device performance, the long chain lengths will also limit macroscopic liquid crystalline alignment. For F8BT, the maximum molecular weight in which monodomain alignment is still achieved appears to be in the range 62 000-129 000.