Near-Field Scanning Optical Microscopy (NSOM) Studies of Nanoscale Polymer Ordering in Pristine Films of Poly(9,9-dialkylfluorene)

Abstract

Near-field scanning optical microscopy (NSOM) is used to characterize nanoscale topographic and fluorescence features of pristine films of the stiff-chain polymer polyfluorene. The pristine films of polyfluorene bearing two hexyl (1) or dodecyl (2) groups at the 9 position were studied. Pristine films appear isotropic using conventional polarized light microscopy. NSOM images show two distinct types of film morphology, the first of which is characterized by 50−150 nm clusters in the topography. These clusters seen in films of 1 and 2 correlate directly with regions of lower fluorescence. The cluster correlated fluorescence (CCF) is unpolarized and has a slightly greater percent contribution from low energy emission than the rest of the polymer film. The larger and more frequent regions of CCF in films of 1 versus films of 2 indicate they are formed due to poor solubility. NSOM images show a second type of morphology that is characterized by 50−500 nm polarized domains denoted as long range order (LRO). These domains are distributed uniformly in pristine films of 1 and 2. Unlike CCF, the topography and fluorescence of LRO do not correlate. A method of NSOM image math is introduced to quantify film anisotropy from simultaneously collected fluorescence images at orthogonal polarizations. The anisotropy of 2 is found to be significantly larger than 1. NSOM images collected at 440 & 600 nm show that intra- and interpolymer emitting species are distributed evenly throughout the film's LRO. Additional polarization images show that intra- and interpolymer fluorescence are polarized along the same axis. The dominance of LRO in 2 and the large interpolymer emission in 2 (as measured in previous studies) implies that the LRO, not the clusters, is responsible for most of the interpolymer emission in these polymer films.