Synthesis and properties of liquid crystalline conjugated polymers
Novel conducting polythiophenes, which have mesogenic side-chains have been synthesised and investigated in order to study their liquid crystallinity and its possible effects on electronic conductivity. The backbone was based on substituted polythiophenes. Four main types of liquid crystalline conjugated polymers were investigated: l Polymers without liquid crystal groups attached 2 Polymers with laterally attached liquid crystal groups 3 Polymers with terminally attached liquid crystal groups 4 Polymers with terminally attached liquid crystal groups that have potential negative dielectric anisotropy A large variety of suitable thiophene-substituted precursors with potential liquid crystalline properties was synthesised and investigated by optical microscopy and differential scanning calorimetry. Williamson ether synthesis and boronic acid coupling chemistry were mainly used to synthesise the polymer precursors. Some of these monomers proved difficult to homopolymerise due to the large bulky . laterally attached moieties, therefore copolymerisations with 3-alkylthiophenes were carried out. Cyclic voltammetry of the liquid crystalline substituted thiophene precursor was investigated but even when redox peaks were observed. no polymer film was formed. Therefore these liquid crystalline conjugated polymers/copolymers were chemically synthesised by free radical polymerisation using FeCh and then synthetically modified. The polymers obtained were fully characterised by spectroscopic analysis and the physical properties of these potentially liquid crystalline conjugated polymers were investigated. Comparative studies of the laterally-attached liquid crystalline conjugated copolymers,poly[(I-(3-thiophenemethoxy)-6-(4-n-hexoxy-2-oxybenzoic acid)hexane-co-(3-hexylthiophene)] and of its terminally attached analogue poly[(I-(3-thiophenemethoxy )-6-( 4-oxybenzoic acid)hexane )-co-(3- hexylthiophenej], revealed that the conductivity of the former was much greater than the latter. Also observed was the noticeable increase in conductivity when the copolymers were converted from the ester form poly[(I-(3- thiophenemethoxy)-6-( methyl 4-n-hexoxy-2-oxybenzoate )hexane )-co-(3- hexylthiophene)] to the liquid crystal acid form poly[(I-(3-thiophenemethoxy)-6- (4-n-hexoxy-2-oxybenzoic acid)hexane )-co-(3-hexylthiophene)]. It became evident that an alkyl spacer was necessary to attach the lateral liquid crystal group. as the alkyl chains provided enough space for the bulky group to orientate into position. However copolymer ratio studies revealed that the ratio of alkyl chain to liquid crystal groups did not significantly affect the overall conductivity of the copolymers for this class of compounds. Also the electrical conductivities as a function of temperature for some undoped LC copolymers of poly[(I-(3- thiophenemethoxy)-6-( 4-n-hexoxy-2-oxybenzoic acid)hexane )-co-(3- hexylthiophene)] increase with increasing temperature. The temperature range in which these increases occur corresponds to LC transitions in the differential scanning calorimetry and by polarising hot-stage microscopy.