Biomimetic photonic thin films from polymer-polymer self-assembly

Polymer-polymer blends provide a cost-effective solution process fabrication method to generate a variety of self-assembled structures. Previously, we have demonstrated that blending poly(3-hexylthiophene-2,5-diyl) (P3HT) with polystyrene (PS) leads to the fabrication of nanoporous structures with variable pore diameters ranging from 150 to 600 nm.[1] In the present study, we demonstrate that by adding polymethylmetacrylate (PMMA) to the mixture, the pore dimensions increase while maintaining their honeycomb-like ordered structure. The formation of self-assembled polymer nanostructures can be theoretically predicted in binary blends but this becomes more complicated for ternary blends. Here, we use confocal microscopy to characterize our ternary blends by replacing the commercial (non-emissive) PS with a boron-dipyrromethene functionalized PS (PS-bodipy). In the ternary blends with high PMMA contents, we can distinctively locate the green-emitting PS phase, which acts as a compatibilizing agent at the interface between the red-emitting P3HT and PMMA. After selective removal of the PS and PMMA phases, we demonstrate that structured P3HT thin films exhibit angle dependent reflectivity (iridescence) in a similar manner to morpho-butterfly wings. This effect is further emphasized when covered with a thin metal layer and the metal covered nanostructured P3HT films display structural coloration properties with blue and red emission at low and high reflection angles, respectively. The angular dependence of the reflectivity peak is well reproduced by the Bragg formula that describes the iridescence of opal-like systems and is often used to characterize the parameters of colloidal crystals. The large inter-pore distance of the film and the presence of a long-wavelength reflection tail suggests that the observed visible iridescence originates from higher order Brillouin zone reflectance wavelengths, as in the case of the blue feathers coloration of some bird wings.