Heterogeneous modelling of polymer conductivities

Abstract

The following thesis shows that the electronic transport in conducting polymers can be accounted for by the heterogeneous model. This was done in two parts; the first examines the effects of annealing and gas absorption have on the DC conductivity of polyaniline. Following this the AC conductivity of polyaniline and polypyrrole are modelled in light of the DC heterogeneous model. Finally the origin of the DC to microwave Drude response is investigated using a classical oscillator analysis. The effects of annealing and gas absorption on the conductivity of PAN-CSA were successfully modelled using the DC heterogenous model with a variable range hopping resistivity term. This is the first time the effects of gas absorption have been modelled this way. It was found that as the annealing temperature increased the magnitude of the conductivity decreased, there was a shifting of the conductivity peak to higher temperatures and a strengthening in the temperature dependence. The latter two effects result in an increase in the hopping parameter T0. The results indicate that the removal of water from the sample results in more localised electronic wave functions. The exposure of the annealed sample to N2 and O2 had little effect on the conductivity. The exposure of the annealed samples to moist air almost entirely reversed the effects of annealing. The AC conductivity of PAN-CSA and PPY-PF6 are modelled by a frequency dependent generalisation of the heterogeneous model. The model allows an estimate of the extent of disorder relative to the sample as a whole to be made. It is expected that the dielectric constant of PAN-CSA and PPY-PF6 will be similar to that of other polymers such as polyethylene (εr = 2.3). This would result in the estimate of the extent of disorder to be between 1% and 10% of the sample length. The DC conductivities of the "good" metallic components of the model are found to be of the order of 400 - 1500S/cm. Such conductivities are of the same order of magnitude as the room temperature conductivities of intrinsic Silicon and Germanium. The heterogeneous model curve fits are compared to those found using the localisation modified Drude model for a series of PPY-PF6 conductivity spectra. It is found that the frequency dependent generalisation of heterogeneous model can describe the AC conductivity as successfully as the localisation modified Drude model. The heterogenous model offers a description of the AC conductivity that is related very intuitively to the ideas held about the morphology of the many polymers. Large negative values of the real component of the dielectric function (εr) have been reported for both PAN-CSA and PPY-PF6 by Kohlman et al. The reported conductivity for one of these samples was modelled using a classical oscillator analysis and the real component calculated from the conductivity model. The calculated εr reproduced the reported behaviour very well over the energy range (.005-1.2eV). There was however no evidence for the reported Drude response below .005eV in the calculated behaviour.