Poly[2,7-(9,9-dihexylfluorene)]-block-poly[3-(trimethoxysilyl)propyl methacrylate] (PF-b-PTMSPMA) rod-coil block copolymers: Synthesis, morphology and photophysical properties in mixed solvents

We report the synthesis, morphology, and photophysical properties of poly[2,7-(9,9-dihexylfluorene)]-block-poly[3-(trimethoxysilyl)propyl methacrylate] (PF-b-PTMSPMA) with two different coil lengths. Ethyl acetate (EA) and methanol (MeOH) are used as selective solvents for the PF rod and PTMSPMA coil blocks, respectively, while THF as the common solvent. Micelle morphologies of the PF-b-PTMSPMA in THF/MeOH or THF/EA mixed solvents are characterized by transmission electron microscopy (TEM) and scanning force microscopy (SFM). In the THF/MeOH mixed solvent, PF-b-PTMSPMA assembles into diverse morphologies of sphere, short cylinder, cylinder, and cylinder bundles. Besides the selective effects, the strong π-π interaction of PF contributes partially to the above morphologies. In the THF/EA mixed solvent, morphologies of the PF-b-PTMSPMA changes from large compound micelles (LCM) to hollow spherical micelles due to the strong core chain stretching. Stable micelles are obtained by crosslink Si(OR)3 groups of the PTMSPMA block by triethylamine (TEA). The micellar morphology significantly affects the photophysical properties. In the THF/MeOH mixed solvent, blue shifts on the UV-vis and fluorescence spectra are observed probably attributed to the formation of the H-aggregation in the PF core. However, the photophysical properties are insensitive to the different ratios of THF/EA, due to the insignificant aggregation of short PF corona. The present study reveals that the morphology and photophysical properties of fluorene-based rod-coil polymers could be significantly manipulated through solvent, rod/coil ratio, and π-π interaction.