Interplay of vesicle and Lamellae formation in an amphiphilic polyfluorene-b-polythiophene all-conjugated diblock copolymer at the air-water interface

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 Förster 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.