Synthesis and characterization of coil-rod-coil triblock copolymers comprising fluorene-based mesogenic monodisperse conjugated rod and poly(ethylene oxide) coil

A series of coil rod coil triblock copolymers (i.e., F3T8EO8, F3T8EO17, F3T8EO45, and F3T8EO125) with a mesogenic monodisperse conjugated oligomer comprising 3 fluorene, 8 thiophene, and 2 phenyl units as the rod and poly(ethylene oxide) (PEO) as the coil were synthesized. A reference compound, that is F3T8ME2, with the identical rod but without PEO was also prepared for comparison. The volume fraction of PEO (fPEO) was 0, 0.16, 0.28, 0.50, and 0.73 for F3T8ME2, F3T8EO8, F3T8EO17, F3T8EO45,and F3T8EO125, respectively. It was found that the introduction of PEO into the triblock copolymers encouraged the formation of H-type aggregation and fPEO-dependent highly ordered mesophases while fPEO 0.73. For F3T8ME2, only nematic mesophase was observed. In contrast, F3T8EO8 and F3T8EO17, with fPEO of 0.16 and 0.28, respectively, are smectic A (SA) mesomorphism. For F3T8EO45 with fPEO of 0.50, several highly ordered phases, such as hexagonal columnar crystalline phase and rectangular columnar, smectic B (SB) and SA mesophases, appear in succession upon heating. However, large fPEO suppresses the formation of mesophase. For F3T8EO125 with fPEO 0.73, only a crystalline phase was observed. Meanwhile, thin films of above triblock copolymers exhibit lamellar or cylindrical nanostructures consistent with the phase structures at room temperatures, which render the block copolymers different spectroscopic properties. These results indicate that it is possible to control the supramolecular nanostructures of conjugated materials and then to tune their optoelectronic properties by designing rod coil block copolymers with appropriate chemical structures.