Crystalline and Noncrystalline Forms of Poly(9,9-diheptylfluorene)

The formation of ordered morphologies in poly(9,9-diheptylfluorene) (PF7) was investigated using X-ray diffraction and grazing incidence X-ray diffraction. Two crystalline phases were found. The alpha-phase is orthorhombic with a = 2.60 nm, b = 2.25 nm, and c = 3.34 nm, and it is structurally very close to the alpha-phase in poly(9,9-dioctylfluorene) (PF8). The gamma-phase is monoclinic with a = 2.88 nm, b = 0.96 nm, and c = 1.68 nm, and the oblique angle is close to 90 degrees. The gamma-phase is the stable form in the bulk while the alpha-phase preferentially forms in thin films. Well-ordered and aligned crystalline films were produced from both good (toluene) and moderate (methylcyclohexane, MCH) solvent. Preparing films from MCH without annealing resulted in mesoscopic crystal with decreased order along the a-axis. This mesoscopic structure differs from the beta-phase found in PF8 and is more related to the crystalline gamma-phase. This difference may explain why mesoscopic PF8 has a phase transition into the alpha-phase, whereas the mesoscopic PF7 rather into the gamma-phase.