Self-Assembly of Conjugated Polymers at the Air/Water Interface. Structure and Properties of Langmuir and Langmuir−Blodgett Films of Amphiphilic Regioregular Polythiophenes

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-(2,5,8trioxanonyl)-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 π-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 (C macro ≈ 4-7 m/N) being one order of magnitude larger than the microscopic compressibility (C micro ≈ 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. Reflectionabsorption 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.

Molecular level studies of polymer behaviors at the water interface using sum frequency generation vibrational spectroscopy

“…82,83 By studying the microscopic ordering of amphiphilic neutral polymers at different interfaces, scientists may better understand the microscopic driving forces behind complicated macroscopic polymer aggregation. [84][85][86][87][88][89] SFG has been applied to study interfacial behavior of PEG, poly(propylene glycol) (PPG), and the copolymer of PEG and PPG. 88,[90][91][92] The Somorjai group studied the adsorption of PEG and PPG, common amphiphilic polymers, in 2003.…”

Functional Organized Molecular Assemblies Based on Polymer Nano-sheets

“…Therefore, several groups reported the construction of ultrathin PHT films by the Langmuir-Blodgett (LB) technique to prepare and ordered ultrathin film with well defined structure. [76][77][78][79][80] However, the reported work for fabricating PHT LB film has problem with stability and processability, or a special regioregular polythiophene have to be synthesized to fabricate PHT LB film. We have tried to fabricate PHT LB film using pDDA nanosheet.…”

Introduction of Curvature in Amphipathic Oligothiophenes for Defined Aggregate Formation

“…It was found that such thiophenes prefer to self-assemble at the air-water interface and can be used to construct Langmuir-Blodgett multilayers. [33] From these simple models it can be seen that the introduction of two neighboring hydrophilic or hydrophobic thiophenes, in an otherwise alternating sequence, would introduce a cis conformation in polar environments, which would result in a nonlinear or curved shape of the oligothiophene. Such oligothiophenes with a defined number of hydrophilic-hydrophilic (or hydrophobic-hydrophobic) thiophene neighbors can most conveniently be constructed for terthiophene building blocks, as depicted in Scheme 1 C. It is anticipated that such amphipathic oligothiophenes will be able to self-assemble in a similar fashion as surfactants.…”