All-conjugated block copolymers of the rod-rod type came into the focus of interest because of their unique and attractive combination of nanostructure formation and electronic activity. Potential applications in a next generation of organic polymer materials for photovoltaic devices ("bulk heterojunction"-type solar cells) or (bio)sensors have been proposed. Combining the fascinating self-assembly properties of block copolymers with the active electronic and/or optical function of conjugated polymers in all-conjugated block copolymers is, therefore, a very challenging goal of synthetic polymer chemistry. First examples of such all-conjugated block copolymers from a couple of research groups all over the world demonstrate possible synthetic approaches and the rich application potential in electronic devices. A crucial point in such a development of novel polymer materials is a rational control over their nanostructure formation. All-conjugated di- or triblock copolymers may allow for an organization of the copolymer materials into large-area ordered arrays with a length scale of nanostructure formation of the order of the exciton diffusion length of organic semiconductors (typically ca. 10 nm). Especially for amphiphilic, all-conjugated copolymers the formation of well-defined supramolecular structures (vesicles) has been observed. However, intense further research is necessary toward tailor-made, all-conjugated block copolymers for specific applications. The search for optimized block copolymer materials should consider the electronic as well as the morphological (self-assembly) properties.
The impact of thermal annealing on the growth of crystalline nanodomains of poly(3-alkylthiophene) (P3AT) in thin films (15−20 nm thick) was investigated as a function of length of alkyl side chain by combined low-dose highresolution transmission electron microscopy (HR-TEM) and grazing incidence X-ray diffraction (GIXD). Statistical analysis of the data yields the characteristic dimensions of the face-on oriented P3AT nanocrystals, i.e., average stem length l c along backbones and lateral dimension perpendicular to the stems l a along side chains. The following trends were identified: (i) in as-spin-coated films, the proportion of face-on oriented nanocrystals increases with the number of carbon atoms in the side chain, (ii) annealing favors the lateral in-plane growth of the nanocrystals along the side chain direction (a P3AT axis), (iii) for a given P3AT, the proportion of face-on oriented domains increases with annealing temperature, (iv) lateral growth along the a P3AT axis is most efficient for the longer octyl side chains, and (v) thermal annealing induces only modest lamellar thickening which is limited by the poor slide diffusion of π-stacked P3AT chains as opposed to lateral growth favored by weak van der Waals interactions between layers of n-alkyl side chains. The increase in the population of face-on oriented crystallites, observed when the length of the side chain increases, coincides with a corresponding decrease in the field effect mobility in annealed P3AT thin films.
Based on their rigid-rod structure all-conjugated, rod-rod block copolymers show a preferred tendency to self-assemble into low-curvature vesicular or lamellar nanostructures independent from their specific chemical structure and composition. This unique and attractive behaviour is clearly illustrated in a few examples of such all-conjugated block copolymers. The resulting nanostructured heteromaterials may find applications in electronic devices or artificial membranes.
Interfacial behavior and surface morphology of an amphiphilic, all-conjugated rod-rod diblock copolymer, poly [9,9-bis(2-ethylhexyl)fluorene]-b-poly [3-(6-diethylphosphonatohexyl)thiophene] or PF2/6-b-P3PHT, were investigated by a combination of Langmuir-Blodgett (LB) techniques, optical spectroscopy, and atomic force microscopy (AFM). For the PF2/6-b-P3PHT diblock copolymer aggregates, well-defined gas, liquid-expanded, liquid-condensed, and solid states were observed at the air-water interface. The backbones of the polar P3PHT blocks exhibited an edge-on arrangement which is driven by the pendant alkyl chains with the polar phosphate groups; i.e., the polymer main chain orients parallel to the air/water interface with the planes of the thiophene rings in vertical orientation (edge-on). For comparison of the optical properties, three different LB films (transferred at surface pressures of 5, 15, and 50 mN/m), spin-or drop-cast films, and solutions were investigated. Spectral shifts and intensity changes of UV-vis absorption and photoluminescence emission of the films were correlated to changes of the surface morphology. The emission properties after excitation into the higher-bandgap PF2/6 absorption band were governed by both Fo ¨rster resonance energy transfer (FRET) and conformational changes within the P3PHT block. The AFM images illustrate the formation of vesicular species and their transition into a monolayer lamellar phase upon increased surface pressure and a correlation of the optical properties and aggregation state at the air/water interface.
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