Study of the different structural phases of the polymer poly(9,9′-dioctyl fluorene) using Raman spectroscopy

The polymer poly(9,9′-dioctyl fluorene) (F8) can be easily driven between a number of different structural phases by different thermal or solvent treatments. The phases which we have identified include (i) as-spin-coated, (ii) semi-crystalline, (iii) glassy, (iv) with an extended intrachain conformation. Atomic force microscopy has been used to investigate the surface structures associated with the different phases of the polymer and different solvents used to cast the film. Raman spectroscopy has been performed on these samples to understand how the polymer conformation changes with the phases. Variations in the relative intensity of the peaks and shifts in energy are observed. We have compared the F8 spectrum with a trimer of fluorene with butyl side chains and acrylate/hydroxy endgroups attached to the fluorene main chain via hexyl spacers. We identify a very small frequency dispersion in the vibrational modes connected to an increase of the conjugation length. Therefore, the assignment of the vibrational modes of the trimer is very important for understanding the F8 spectrum.