Self-assembly of conjugated polymers at the air/water interface. Structure and properties of Langmuir and Langmuir-Blodgett films of amphiphilic regioregular polythiophenes

This paper provides the first direct structural evidence describing conjugated polymer self-assembly at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity measurements on a number of derivatives of amphiphilic regioregular polythiophenes (e.g., poly(3'-dodecyl-3-(2,5,8-trioxanonyl)-2',5-bithiophene), polymer 1) show that these conjugated polymers self-assemble as 2-dimensional polycrystalline monolayers at the air/water interface with the amphiphilic polymers preorganized into rigid boards standing edge-on on the water surface. The monolayer consists of highly ordered (similar to 70% crystalline) domains, with a centered rectangular unit cell having the polymer backbone along the a-axis and the thiophene, pi-stack along the b axis with a distance of 3.85-3.94 A depending on the applied surface pressure. These domains are connected by soft, more disordered boundaries. This is evidenced by the macroscopic compressibility of the entire LB film (C-macro approximate to 4-7 m/N) being one order of magnitude larger than the microscopic compressibility (C-micro approximate to 0.6 m/N) of the polycrystalline domains. The alkyl chains in the 3-position of the thiophene rings are in a crystallographically disordered state due to their cross-sectional mismatch with the packing of the thiophenes. The importance of having the side chains coupled in a regioregular fashion to the 3-position of the thiophene rings is evidenced by a dramatic increase in the coherence length of the crystalline domains for highly regioregular samples (>95% head-tail couplings) as compared to less regioregularly coupled polymers (similar to 80% head-tail couplings). Transfer to solid support by the Langmuir-Blodgett technique induces an overall orientation of the domains in the film, giving rise to a dichroic ratio of up to 4. Reflection-absorption infrared spectroscopy (RAIRS) shows that the alkyl chains of transferred films are in an all-trans conformation with a locally ordered environment, having only few gauche defects.