Web Release Date: June 2,
Self-Assembly of Conjugated Polymers at the Air/Water Interface. Structure and Properties of Langmuir and Langmuir-Blodgett Films of Amphiphilic Regioregular Polythiophenes
and
rnholm*
CISMI, Laboratory for Materials Science, Department of Chemistry, University of Copenhagen, Fruebjergvej 3, DK-2100 Copenhagen, Denmark, Condensed Matter Physics and Chemistry Department, RISØ National Laboratory, DK-4000 Roskilde, Denmark, Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, and Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712
Received July 12, 1999
Abstract:
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 (~70% crystalline)
domains, with a centered rectangular unit cell having the polymer backbone along the a-axis and the thiophene
-stack along the b axis with a distance of 3.85-3.94 Å 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 (Cmacro 
4-7 m/N) being one order of magnitude larger than the microscopic
compressibility (Cmicro 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 (~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.
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