Gabaston, Laurence Isabelle (1996) Synthesis and properties of liquid crystalline conducting polymers. (PhD thesis), Kingston University, .
Abstract
Novel conducting polyanilines which are thermoplastic and have mesogenic side-chains have been synthesised and investigated in order to study the use of liquid crystal alignment to produce higher conductivity. The backbone was based on ortho-substituted aniline polymers. A large variety of suitable ortho-substituted nitrobenzene precursors with liquid crystalline properties were synthesised and investigated by microscopy and differential scanning calorimetry. A study of the 4-methoxyphenyl 4-[x-(2-nitrophenoxy)alkoxy] benzoate series (x = 2-10) showed that only the molecules with even spacer lengths were liquid crystalline whereas the ones with odd spacer lengths were not. All the even molecules were nematic in character. Some substituted biphenyl nitrobenzenes were also synthesised and 4-cyano 4'-[6-(2-nitrophenoxy)hexoxy]biphenyl exhibited a monotropie nematic phase. A large variety of ortho-substituted nitrobenzenes with transition temperatures above 100°C were also synthesised. Depending on their structure, nematic as well as smectic phases were characterised. The ortho-substituted nitrobenzene molecules were subsequently reduced to the required anilines. Their liquid crystal properties were similar to their respective nitro-compounds.These liquid crystalline aniline monomers were chemically polymerised by use of an emulsion system. The polymers obtained were fully characterised by spectroscopic analysis, and were a head-to-tail emeraldine type substituted polyaniline. The physical properties of these potentially liquid crystalline conducting polymers were fully investigated. They were found to be soluble in DMF, DMSO, THF and CHCI3• UV/visible study revealed an average band gap of 3.4 eV. Gel permeation chromatography analysis showed the presence of oligomers and higher polymers in smaller proportion for each samples. These polymers were also structurally stable up to 300°C. Polarising hot-stage microscopy and differential scanning calorimetry showed the existence of liquid crystal properties for two of these polymers namely : poly[2-(hexoxy)-6-(4- biphenyloxy-4' carboxylic acid)]aniline and poly{2-[4-cyanophenyl-4-carboxylate 6-(4'-oxybiphenyl)hexoxy aniline. In fact, a nematic phase was exhibited for the carboxylic acid polymer between 225 and 261°C and between 120 and 175°C for the ester polymer. Study of their electrical conductivities showed some semi-conducting characteristics with values as high as 1.2xlO-3 S ern" for the H2S04 vapour doped carboxylic acid polymer and equal to 8xlO-6 S cm" for the HCI doped ester poÍymer. An increase in conductivity values was also observed as the temperature reaches the nematic liquid crystal transitions, demonstrating the effect of the mesogenic groups on the ordering of the polymer backbone. Laser alignment of these two liquid crystal semi-conducting polymers was also carried out. A conductivity anisotropy was observed along the laser written samples. This technique could also be of great potential interest as a method of localised conductivity enhancement.
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