Influence of solvent quality on the self-organization of archetypical hairy rods -: Branched and linear side chain polyfluorenes:: Rodlike chains versus beta-sheets in solution

We report on the nanoscale structure and solvent-induced phase behavior of two, nearly similar pi-conjugated hairy-rod polymers, branched side chain poly[9,9-bis(2-ethylhexyl) fluorene-2,7-diyl] (PF2/6) and linear side chain poly[9,9-dioctylfluorene-2,7-diyl] (PFO or PF8), in good and bad (or poor) solvents-deuterated toluene and deuterated methylcyclohexane (MCH) sat 20 degrees C. Small-angle neutron scattering (SANS) measurements exploiting contrast variation with side chain deuterated PFO polyfluorene have been employed and complemented by optical absorption measurements. In toluene both PF2/6 and PFO adopt an elongated (rodlike) conformation containing predominantly only a single polymer chain (diameter of the order of 1 nm), which indicates dissolution down to the molecular level. In contrast, in MCH, PF2/6 shows an elongated structure while PFO forms sheetlike structures (characteristic thickness of 2-3 nm), thus dissolving down to the colloidal level. The elongated structure of PFO consists of individual polymer chains adopting dominantly a conformational isomer CR. The thickness of sheetlike PFO particles corresponds to that of around two polymer layers and side chain contrast variation gives an evidence for an even distribution of the backbones within the sheets. These sheets are potentially an initial stage of PFO crystallization and also contain conformational isomer C-beta of those chains observed in the so-called beta-phase (or beta-sheets) in the solid state. The observed phenomena were not found to depend on concentration over the concentration range 5-10 mg/mL.