(303g) Synthesis and Characterization of All-Conjugated Block Copolymers/Supramolecular Block Copolymers in Thin Films
All-conjugated block copolymers with p- and n-type blocks represent a promising approach to improving the performance of all-polymer organic photovoltaics (OPVs). Large scale phase separation can be avoided and block copolymer self-assembly may lead to ideal structures for charge dissociation and transport. In this work, we present both covalent and supramolecular conjugated block copolymers as potential routes to self-assembled polymer OPVs. Covalent block copolymers are made using a bromine-terminated poly(3-hexyl thiophene) macroreagent and palladium catalyzed poly-condensation reaction to attach a second poly(fluorene) (PFO) or poly(fluorene-co-benzothiadiazole) (PFBT) polymer block. A combination of nuclear magnetic resonance spectroscopy (NMR) and size-exclusion chromatography (SEC) provides the absolute molecular weight, the molecular weight ratio, and an estimate of homopolymer impurities. Supramolecular block copolymers can be made using 2-ureido- 4[1H]-pyrimidinone (UPy) quadruple hydrogen bonding interactions between distinct polymer blocks. A practical advantage of UPy interactions compared with all-conjugated block copolymers is a greatly simplified synthetic route and straightforward preparation of a series of supramolecular block copolymers through solution blending. UPy terminated polymers are synthesized by coupling hydroxyl or primary amine terminated polymers to a reactive isocyanate-UPy group in a one-step reaction. All polymer films are subsequently prepared by solution casting onto a surface and thermal and/or solvent annealing. Thin film microstructure and polymer crystallinity is analyzed by optical microscopy, atomic force microscopy (AFM), and grazing-incidence wide-angle x-ray scattering (GIXS). Optical microscopy images show that UPy terminated polymer blends do not undergo large-scale phase separation after extended thermal annealing at 160 oC, which are conditions typically used to prepare bulk heterojunction polymer OPVs. AFM together with GIXS reveal the self-assembled nanostructures and crystallinity of block copolymers and polymer blends. The crystallinity of the resulting film depends on the relative molecular weights of each block. In the case of a large PFO block, crystallization of PFO dominates the morphology, while a larger P3HT results in exclusively P3HT crystallization. In UPy polymer blends, only nano fibers and P3HT crystallinity are observed under AFM and GIWAXS, respectively. These results show that covalent block copolymers can be used to control the resulting nanostructures and crystallinity by changing the ratios of each block, and UPy-mediated interactions can suppress large scale phase-separation in bulk heterojunction polymer blends at temperatures and processing conditions typically used to prepare bulk-heterojunction OPVs.