Brush Printing Creates Polarized Green Fluorescence: 3D Orientation Mapping and Stochastic Analysis of Conductive Polymer Films

Brush printing is a unique method used to obtain uniaxially oriented films, whereby a polymer solution is brushed onto a substrate. However, there have been only a few reports on the brush-printing method. Here, we report the preparation of a uniaxially oriented film of a green light-emitting conductive polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). The fluorescence polarization ratio of the oriented F8BT films was as high as 11.3, and the average orientation factor reached 0.74 +/- 0.06. The orientation factor and the torsion angle of F8BT were visualized by two mappings of fluorescence and Raman spectral measurements by confocal spectromicroscopy, respectively. These two x-y mapping data with many pixels (similar to 750 pixels) were evaluated by x-y-z mapping of the film thickness at a single position and were used to reveal the three-dimensional (3D) orientation mechanism from a stochastic approach. Polarized green fluorescence originates from polymer chains uniaxially oriented along the brush direction. The high orientation for a film thickness < 100 nm is established by shear stress, faster capillary flow, and flow-induced chain extension for a thin solution film on a substrate. The high orientation factor was also demonstrated by a high brushing speed, whereas an optimized brushing speed existed. We found that this optimization is attributed to the property of a non-Newtonian fluid. By applying this brush-printing method to the fabrication of an optoelectrical device, polarized green electroluminescence was preliminarily demonstrated by the OLED assembled from an oriented F8BT film.