Conductive supramolecular liquid-crystalline polymers by ionic self-assembly
Final Report Abstract
Polyaniline is set aside from other well-known conductive polymers because of its unique acid-base doping chemistry. From its insulating state, it can be brought to a conductive state both electrochemically and by treating it with a sufficiently strong acid. If bulky organic acids are used for this purpose, they can additionally plasticise the material. However, polyaniline is difficult to process, and suffers from poor solubility, coiling, and a mixture of crystalline and amorphous regions. To gain better solubility and systems that are easier to understand, the well-known oligomer approach was used and only the minimal functional unit of polyaniline – tetraaniline – was considered. To achieve an additional level of control and to improve solublilty in polar solvents such as water, a tetraaniline moiety was equipped with a charged quarternary amine-headgroup and a long aliphatic chain was used to decouple the two moieties. Although the synthesis of this molecule proved to require more steps than originally planned, gramquantities of clean material could be obtained. The material was fully water-soluble despite its bulky organic part and the critical micelle concentration was determined to be 0.34 M by means of conductivity measurements. Interestingly, the observed micelles seem to undergo a dynamical change in their morphology: while spherical after preparation of the solution, they transform to worm-like and sheet-like structures in the course of 2-3 days. When doped with HBr, the TANI- amphiphile self-assembles predominantly into highly anisotropic sheet-like structures. It was speculated that this is caused by the dominance of the stacking of the tetraaniline unit over the repulsive interactions of the equally charged ionic head-groups. In order to gain simpler, onedimensional structures, a bola-form amphiphile with a charged headgroup on both sides, was synthesised and it indeed self-assembled into nano-wires when doped with HBr and into vesicles when doped with sulphuric acid. Water-soluble conductive molecules are good candidates for green processing of nano-electronic devices. For this purpose, the orientation of the molecules on surfaces as well as in water is important. Promisingly, the acid-doped as well as the undoped tetraaniline-surfactant showed well ordered lyotropic phases in water that are easily orientable by, for example, mechanical shear forces or surface effects. If drop-cast from water or methanol onto polar surfaces, well-ordered phases, with the molecular main axes parallel to the film’s normal were observed both by polarised optical microscopy and by grazing incidence X-ray spectroscopy. The latter method showed strong indication of π-π-stacking of the tetraaniline units for the undoped as well as for the doped surfactant. As the phase structure of such films is uniform over several square centimetres, the films are excellent candidates to further study the relation between molecular order and conductivity of the tetraaniline oligomers. For certain applications it might be interesting to further improve the processability by combining the conductivity of the tetraaniline amphiphile and the orientability of the liquid-crystalline phases they form with the good processability and film-forming properties of polymers. Therefore, the surfactants were assembled with a polyanion in water. This led to the precipitation of a selfassembled supramolecular polymer by ionic self assembly. Such polymers are soluble in dilute acids and they exclusively form layered lyotropic phases over a wide range of concentrations. When dry, they are orientable by mechanical shear, which, again, is promising for potential applications. With a relatively simple molecular design, conductive amphiphiles were synthesised that proved to be water-soluble and thus can potentially be used for green processing. Over large areas, they selfassemble into well-ordered phases on polar surfaces and they form liquid-crystalline phases in water that are easily orientable by external fields.
