We report on solution aggregates and backbone conformation of poly(9-undecyl-9-methyl-fluorene) (PF1-11) and poly(9-pentadecyl-9-methyl-fluorene) (PF1-15), having two different side chains compared with poly(9,9-dihexylfluorene) (PF6) and poly(9,9-dioctylfluorene) (PF8) with two identical side chains. In the poor solvent methylcyclohexane (MCH), X-ray scattering indicates that PF1-11 and PF1-15 appear as three-dimensional aggregates (5-10 nm wide and thick), forming ribbon-like agglomerates (correlation lengths of 100 nm). PF6 and PF8 appear as two-dimensional aggregates (>10 nm wide and 2-3 nm thick) involving ribbon-like agglomerates (correlation lengths much greater than 100 nm). Upon heating, all aggregates undergo a gel-sol transition which occurs at lower temperatures for PF1-11 and PF1-15 (<60 degrees C) than for PF6 and PF8 (>80 degrees C). In the good solvent toluene, PF1-11 and PF1-15 form networks of cylindrical particles. The mesh size and the cylinder radius are smaller in 24 degrees C toluene (60 nm, 0.5 nm) than in 60 degrees C MCH (300 nm, 1-2 nm). Nuclear magnetic resonance spectra in toluene-d(8) together with density functional theory calculations suggest higher torsion angles between polymer repeat units for PF6, PF8, and PF1-11 (less planar conformation) and a gauche arrangement of the dihedral angles between the bridge carbon atom and the side chain methylene groups in PF1-15. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 826-837