Almasri, Moayad (2008) Liquid crystalline conducting polymers. (PhD thesis), Kingston University, .
Abstract
Side chain liquid crystal polypyrroles have been synthesised and investigated in order to study their liquid crystallinity and its possible effect on their electronic conductivity. The synthesis ofN-substituted dithienylpyrrole was studied using a number of methods. Firstly, there was the synthesis of a side chain which consisted of a mesogenic group and nine units of methylene as a spacer. This synthesis was followed by the preparation of the N-substituted monomer, its polymerisation and the characterisation of both the low molecular weight compounds and the polymer. The next stage was the hydrolysis of the cyanobiphenyl (the mesogenic group) and esterification with a chiral alcohol. The purpose was to synthesise N-substituted polypyrrole with a chiral liquid crystalline moiety in order to produce new liquid crystalline conductive polymers which could exhibit a chiral smectic Cmesophase. A larger monomer (dithienylpyrrole) was synthesised. The substitution of the monomer was carried out using several methods, and the production of the N-substituted monomer was attempted using the Gabriel Synthesis and alternative Suzuki Coupling methods. The polymers were studied electrochemically, and the conductivity of the chemically polymerised compounds was measured (2.8 x 10-8 S cm-1 for the nitrile polymer). The effect of heat treatment on a film of the electrochemically polymerised product was studied, and the energy gap before and after the annealing was measured (2.7 to 2.6 eV for the nitrile polymer and the acid polyer). The liquid crystalline properties were studied 'using differential scanning calorimetry and polarised hot-stage optical microscopy. The acid intermediate (product 3) showed two liquid crystal mesophases. Nematic and smectic C mesophases were observed to be stable over a wide range of temperature (144.8 to 241.4 [degrees]C). The nitrile polymer showed a highly organised phase which was probably a smectic B or crystal. B phase. The nitrile polymer and the acid polymer were studied electrochemically showing that doping and dedoping potentials for the acid polymer were higher than those of the nitrile polymer, and the energy gap of the acid polymer was lower. The energy gap of the nitrile polymer was higher than that of polymer with shorter spacer (6 units of methylene), and that was consistent with the results obtained from the conductivity measurement, as the longer spacer polymer is less conductive (higher energy gap and lower conductivity). Annealing the polymers decreases the energy gap due to self-organisation of the liquid crystal phase .: Modelling the monomer using Quantum Cache indicated molecular dimensions (the length of the nitrile monomer with an LC group attached is 27.8 AO) consistent with the repeat distances indicated by X-ray diffractometry (27.6 AO for the nitrile polymer). The result was proposed that there was inter-layer of the smectic LC phase indicated by hot¬stage polarised optical microscopy.
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