Poly [2,7-(9,9-dihexylfluorene)]-block-poly(2-vinylpyridine) Rod-Coil and Coil-Rod-Coil Block Copolymers: Synthesis, Morphology and Photophysical Properties in Methanol/THF Mixed Solvents

New rod-coil diblock and coil-rod-coil triblock copolymers containing conjugated poly[2,7-(9,9-dihexylfluorene)] (PF) and coil-like poly(2-vinylpyridine) (P2VP) were synthesized by combining coupling reaction and living anionic polymerization. Scanning force microscopy (SFM), transmission electron microscopy (TEM), cryo-transmission electron microscopy (cryoTEM), and dynamic light scattering (DLS) were used to obtain the morphologies of the aggregates in selective solvents of different methanol(MeOH)/tetrahydrofuran (THF) compositions. The effects of micellar morphologies on the photophysical properties were studied by optical absorption and photoluminescence (PL). The experimental results showed that the diblock PF-b-P2VP (di-PFPVP) maintained spherical micellar aggregates as the MeOH content increased. However, the triblock PF-b-P2VP(tri-PFPVP) were found to readily aggregate in elongated cylinders due to its symmetric structure. Consequently, tri-PFPVP polymer chains could stack together favorably and have stronger pi-pi interchain stacking compared with di-PFPVP, leading to the higher absorption wavelength maximum. The quantum efficiencies were gradually quenched with increasing the MeOH content for both copolymers. Moreover, for di-PFPVP, the increase of the MeOH content induced a blue shift in both absorption and PL spectra, suggesting an H-type aggregation. However, tri-PFPVP exhibited a blue shift in absorption but a red shift in PL by increasing the MeOH content, which reflected a different type of aggregation. The present study revealed the effects of polymer chain architecture and the aggregated structures of the rod-coil block copolymers on the photophysical properties.