Near-field scanning optical microscopy (NSOM) study of alkyl-substituted polyfluorene films: the affect of alkyl substituent length on nanoscale polymer ordering and cluster formation.

Abstract

Pristine and Annealed 170 nm films of the stiff-chain polyfluorene, bearing two hexyl (1) or dodecyl (2) groups at the 9 position were studied using Near-field Scanning Optical Microscopy (NSOM). NSOM images reveal two distinct types of nanoscale film morphology: clusters and long-range order (LRO). Dual polarization and dual wavelength fluorescence collection schemes are combined with the use of simple image math provide both qualitative and quantitative characterization. In pristine films, shorter alkyl substituents (1) reduce the solubility of the polyfluorene in the spinning solution used for spin-coating films which leads to the formation of 50-150 nm clusters which are not present in films made from 2. Clusters can be removed by annealing as a function of time. Polarization shows the clusters are amorphous with fluorescence from both intra and inter-polymer species. The second variety of nanoscale ordering, LRO, is characterized by regions of polarized emission in the polymer films. This polarization can be quantified with an anisotropy value and is barely present in pristine films of 1 but increases dramatically upon annealing to induce a liquid crystalline phase transition. Pristine films of 2 have larger anisotropy values than pristine films of 1, due to longer alkyl-substituent-induced ordering during the spin-coating procedure, but films of 2 have only a slight increase in their anisotropy value upon annealing. Both intra and inter-polymer emitting species are distributed homogeneously throughout both pristine and annealed films within the resolution of our microscope (40-70 nm).