Disorder in Conjugated Polymers
Abstract
This thesis explores the effect of disorder on the electronic and structural properties of conjugated polymers which are particularly sensitive, by virtue of their low dimensionality, to defects which disrupt translational order. The thesis is divided into two major parts. The first is devoted to the nonlinear optical properties. We calculate the effect of quenched random bond and site disorder on the third harmonic generation susceptibility of a finite chain, using the standard two band tight binding model. The large calculated changes in the magnitude of chi ^{(3)} as the amount of disorder grows over a reasonable range may explain the discrepancies between various experimentally reported values, or between experiment and pure chain theories. Site disorder leads to the appearance of a two photon peak absent from the long chain limit of the pure Su-Schrieffer-Heeger theory, and quantitatively consistent with experimental observations. The second part concentrates on thermal disorder and solution properties of conjugated polymers via Monte Carlo simulation, using a Hamiltonian which has been designed to describe the complex coupling of the electronic and conformational degrees of freedom within a simple two parameter model. The thermochromic changes in the optical absorption spectra as a function of temperature or the length of the side-group are quantitatively similar to those observed experimentally and can be understood in this framework as coming from the shift in the energy gap due to disruption in the pi electron conjugation. We note that this shift in the absorption peak is very sensitive to the parameters of the model, which suggests the importance of sigma - pi overlap and puts a bound on the magnitude of steric hindrance. Although the electronic states get increasingly localized with disorder, and the conjugation and persistence lengths decrease, the abruptness and order of magnitude change in the radius of gyration is not captured very well and may require inclusion of additional effects, such as screened coulomb or polymer -solvent interactions.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1993
- Bibcode:
- 1993PhDT.......245S
- Keywords:
-
- OPTICAL ABSORPTION;
- Physics: Condensed Matter; Chemistry: Polymer