Abstract
Block copolymers made by covalently linking two or more conjugated polymers have significant potential for organic optoelectronic applications, particularly those requiring a p/n junction. Herein, we report the structure of all-conjugated diblock copolymers poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene) and poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene-co-benzothiadiazole) in thin films and in the bulk. The diblock copolymers are prepared using a combination of Grignard metathesis polymerization and Suzuki polycondensation and purified using solvent extraction and column chromatography. 1H NMR, SEC, and UV/Visible absorbance measurements are used to characterize the materials and quantify the amount of homopolymer impurities. Thin films and bulk structure are characterized using a combination of differential scanning calorimetry, x-ray diffraction, small-angle x-ray scattering, and atomic force microscopy. Atomic force microscopy images reveal nanoscale lamellar domains in solvent-annealed diblock copolymer thin films, and peaks in x-ray diffraction data correspond to poly(3-hexylthiophene) crystallites. On cooling from temperatures above the crystallization temperature to below the crystallization temperature, two peaks appear in temperature-dependent small-angle x-ray scattering traces - one associated with poly(3-hexylthiophene) crystallites and a second low-angle peak indicative of a self-assembled nanostructured. These measurements show all-conjugated diblock copolymers self-assemble into nanoscale crystalline domains present throughout the bulk samples which may be useful for improving the performance of organic photovoltaics and organic light-emitting diodes.