The synthesis of dendritic & liquid crystal conducting polymer hybrids

• Main Author: Banfield, Sarah
• Published: Kingston University 2011
• Subjects: 548; Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.587364

The aim of this project was to synthesis N-substituted pyrrole monomers with liquid crystal and dendritic side groups, followed by their subsequent polymerisation and mesaurement of the polymer properties. The second objective beyond the synthesis of these novel polymer materials was to investigate how potential liquid crystals and dendritic side groups affected the conjugation and planarit of the polypyrrole backbone, for the purpose of conductivity applications. Four differnt types of molecules were synthesised: 1. Polypyrroles with terminally and laterally attached mesogenic side groups (monomers/polymers 1&2) 2. Polypyrroles with first and second generation dendritic side groups, terminated by alkyl chains (monomers/polymers 3&4) 3. Polyprroles functionalised by first and second generation dendritic moities with hydrophilically-terminated alkyl chains (monomers/polyers 5&6) 4. Polypyrrole hybrid materials: second generation dendrimer with terminal liquid crystal groups (monomers/polymers 7) Although seven polymers were intended to be synthesised, polymers (4) and (6) were unable to be characterised, due to very poor solubility. However polymers (1), (2), (5) and (7), successfully underwent hydrolysis of the terminal and lateral ester side groups to carboxylic acids, making a total of nine novel N-substituted polypyrroles synthesised, within this project. The synthesis began with the generation of the monomer compounds (monomers 1-7). This was done by nucleophilic substitution of the potential side chain liquid crystal or dendritic moieties on a benzyl ring using a Williamson ether synthesis. A flexible hexyl spacer group was sustituted onto the intermediate compounds, followed by the attachment of pyrrole at the heteroaton site. Thin layer chromatography was used to follow the progress of each reaction and the intermediate compounds were subjected to column chromatography and high vacuum distillation for purification. Characterisation techniques such as Fourier-transform infrared spectrophotometry, gas chromatography/mass spectrometry, [sup]1H NMR spectrometry and elemental analysis were used to determine the purity and structural indentity of each monomer compound. Electrochemical (EC) polymerisation of monomers (1) and (3) on conductive indium-tin oxide (ITO) glass were attempted first. The cyclic voltammograms indicated that the polymerisation was successful, and the monomer oxidation and polymer doping and dedoping potentials were determined. However this method of polymer synthesis was abandoned, as the yield of the polymer film was too low and the polymer product was very difficult to remove from the ITO substrate for further analysis. Chemical polymerisation with FeCl[sub]3 in chloroform was successful, as indicated by [sup]1H NMR, KBr IR, and UV-visible spectrometry, and the yields of the polymer products were significantly higher than those of the EC method. Physical properties measurements (electrical conductivity, scanning electron microscopy, hot-stage optical microscopy, solubility testing and differential scanning calorimetry) were carried out, in order to determine the general natures and to indicate any possible applications of the novel polypyrrole materials synthesised. From the spectroscopic data and physical measurements, it was concluded that polymers (1), (2), (3), (5) and (7) and their corresponding carboxylic acids (1a), (2a), (5a) and (7a) had been successfully synthesised, as proton NMR confirmed the disappearance of the 2,5-hydrogens of pyrrole, and in addition conductivity measurements using simple 2-probe and ven der Pauw 4-probe methods on undoped polymers (1a), (2a), and (7a) gave reproducible conductivity values which classified them as being potential semi-conductors (2.2x10[sup]-4, 6.7x10[sup]-5 and 7.8x10[sup]-5 Sm[sup]-1 respectively). UV-visible spectrophotometry indicated that hydrolysis of the laterally attached liquid crystal ester groups to carboxyliv acids significantly reduced the pp* energy gap of polymer (1). (from 3.9eV to 2.48eV). It was suggested that hydrogen bonding in the carboxylic acid might have improved the planarity and conjugation of the polymer backbone and transformed the insulating ester polymers to potential semi-conductors. Differential scanning calorimetry (DSC) and hot-stage optical microscopy (HSM) were used to determine the phase transitions of polymers (1a) and (2a). The DSC traces showed small peaks, indicating liquid crystal phase transitions (nematic phases observed from 105-248[degrees]C, 105-170[degrees]C respectively). These were later confirmed by the use of HSM, as nematic textures were observed in the expected temperature region for nematic liquid crystals. In addition the ethalpy changes for the phase transitions were estimated, and polymers (1a) and (2a) were found to have nematic-isotropic phase transitions within the expected range of values. Scanning electron microscopy was used to examine the morphology of the polymer materials. Generally the N-substituted polypyrroles with dendritic side groups were found to have more porous morphologies, while N-substituted polypyrroles with potentially mesogenic side groups appeared to have more continuous and smoother morphologies. However, polymer (7a) was found to have the most porous morphology and it also add an unexpectedly high conductivity value. Polymer (7a) was the only polypyrrole hybrid material synthesised in this project, and it appeared that combining all three polymeric units (dendrimers, LC and CP) into one polymer system improved the planarity if the polymer backbone, encouraged the formation of a porous structure which facilitated p-type doping of the polymer by iodine vapour.