Effects of mesomorphic β nanograins on crystallization and photoexcited emission of poly(9,9-di-n-hexyl-2,7-fluorene)

Structural evolution processes of solution-cast and melt-quenched poly(9,9-di-n-hexyl- 2,7-fluorene) (PFH) were quantitatively examined using small/wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry. SAXS/WAXS profiles of solution-cast PFH indicated presence of nanograins (radius of gyration Rg ≈ 5 nm) of 2D-ordered β phase. Upon heating above Tg, perfected packing order and growth of β nanograins up to Rg ≈ 10 nm result in suppressed formation of α crystallites up to 190 °C, beyond which extensive melting accompanied by significant (albeit partial) transformation of β nanograins into α nanocrystals of Rg ≈ 13 nm can be observed in a narrow temperature range of 190-200 °C. Thus transformed α crystallites remained stable in size before melting above 235 °C. The nanograin structures observed are correlated to phase-dependent optical absorption and photoexcited emission. Results suggest sensitive influences of the size/spatial distribution of β nanograins (particularly in the layering direction of the 2D-ordered chain packing) on photoexcited emission of the polyfluorene film.