Abstract
In this thesis the developments of detailed atomistic molecular models for poly(phenylene), substituted and unsubstituted polypyrrole and their corresponding oligomers has been outlined. From these models and using simulation software structural, electronic and physical properties were calculated and compared with experimental data. Two approaches have been assessed: The quantum mechanical and semi-empirical approaches using quantum mechanics. The molecular mechanical and dynamical ones using classical Newtonian mechanics. In chapter 3 energies and structures of benzenoid and quinonoid polyphenylenes were calculated at the ab-initio level for oligomers containing up to 10 rings. By considering the series of oligophenyls, nPPP, and QnPPP, the picture of a distinct larger energy difference in the oligophenyls, as compared to the case of oligopyrroles. The energy difference between the benzenoid and the quinonoid structure of polyphenylenes was found to be around 88 KJ mol-1 for n = 3. For larger n, ΔΔE (QnPP) quickly approaches zero. Without explicitly performing the polymer calculations including periodic boundary conditions, a distinctly smaller value 67 kJ/mol in favour of the benzenoid structure had been found for polypyrrole, respectively, in that work on a similar methodological level. The energetic reason for this behaviour lies in the substantially larger energy difference between benzenoid and quinonoid valence isomers in the case of the PPP series. In chapter 4 ab-initio and semi-empirical calculations have been performed on pphenylene, pyrrole dimers and polymer chains as a function of the torsion angle between consecutive aromatic rings. A presentation of 3-21G, AM1 and PM3 calculations on polymeric materials have been made for poly(p-phenylene) and polypyrrole. The development of electronic properties such as ionization potential, the carboncarbon bond length between rings, the band gap and width of the highest occupied bands were studied. It was found that on going from a coplanar to a perpendicular conformation, the ionization potential and band gap values increase and the bandwidth of the highest occupied bands decreases. This indicates that in these conjugated systems, the electronic interactions between rings decrease in the same way as the overlap between the p atomic orbitals on adjacent rings, as far as the highest occupied and lowest unoccupied electronic levels are concerned. Moreover, by their incorporation of parameters derived from experimental data some of the approximate methods can calculate some properties more accurately than even the highest level of ab-initio methods, like using 6-31G** basis set. In both cases the coplanar conformation is considered the best for π-overlapping and maximum conductivity. In chapter 5 atomistic molecular models have been developed with the help of molecular mechanics and semi-empirical quantum mechanical calculations. Structural, volumetric, and mechanical properties, e.g. geometrical values and density, have been calculated using simulations on these models. The results from both methods have been compared with experimental data and conclusions have been drawn about the methodology and the approximations used. The values of physical, chemical and mechanical properties found in the literature for the conducting poly(p-phenylene) were close to those derived from modelling ones. The oligomer with six phenyl rings gave a value of 520 MPa for the tensile strength at room temperature. The model with six phenyl rings using the mechanical properties module of Cerius showed values <1000 MPa at room temperature. Theoretically, a model is expected to possess a higher value because of its perfect aromatic structure as well as lack of defects and impurities. In chapter 6 the vibrational spectra of isolated pyrrole monomers and oligomers from n = 1 and 2, where n is the number of structural repeat units used, have been computed using the ab initio 3-21G basis set. The results obtained are compared with data for the case of oligomers with n = 2 to 5 for both neutral benzenoid and quinonoid oligopyrroles, from semi-empirical predictions obtained by AM1 and PM3. The trends in the computed harmonic force fields, vibrational frequencies and intensities are monitored as a function of the chain length. Also, the heats of formation of these two degenerate forms have been examined with respect to increases in the number of rings and the change of methods from AM1 to PM3. The literature vibrational values for the conducting polypyrrole were very close to the modelling ones. In chapter 7 side chain polypyrrole polymers have the potential to be processable conducting materials. The minimised structure of polypyrrole and differently substituted polypyrroles, the minimisation of several monomers of the conductive polymers under study and minimised structures of several chains of each polymer are reported. Information on the angles between neighbouring rings in the backbone together with molecular dynamics simulations calculating physical properties (Tg, density) of our models were presented and compared with experimental values. Tg values for two new substituted polypyrroles were also presented, for which no experimental values are knovm.