Available at: http://digitalcommons.calpoly.edu/theses/1768
Date of Award
MS in Polymers and Coatings
Chemistry & Biochemistry
Conjugated polymers represent a class of semi-conducting materials with numerous applications in optoelectronic devices, including organic light-emitting diodes, field-effect transistors, and photovoltaics. Because of the numerous advantages of macromolecular systems, including solution processing and mechanical flexibility, conjugated polymers have become a burgeoning field of research with the hopes of producing cost-effective solution-based electronics. Importantly, optoelectronic device performance is heavily influenced by conjugated polymer backbone orientation and overall thin film morphology. As such, the processing conditions of these systems are important to the construction of high- performance optoelectronics. Polythiophenes are model conjugated polymers that have been studied extensively in halogenated organic solvents. However, the self- assembly of these systems from dilute solution to the solid state remains ambiguous for solvents with high dielectric constants.
Here, two derivative compounds of poly(3-hexylthiophene) are correspondingly investigated in high dielectric solvents by way of ultraviolet-visible absorption and fluorescence spectroscopy, Fourier transform infrared spectroscopy, small-angle X-ray scattering, polarized optical microscopy, and four- point probe conductivity measurements. In dilute solutions, both systems are found to undergo self-assembly when exposed to various stimuli, including temperature, solvent composition, and side-chain characteristics. The kinetics of these transitions are investigated, and a model is put forth to explain contrasting self-assembly mechanisms. At higher concentrations, both systems form lyotropic liquid crystalline phases. Characteristics of the liquid crystalline phases are found to be heavily influenced by dilute solution self-assembly mechanisms and processing. Through the application of a mechanical shear force along still-wet liquid crystal films, alignment of the polythiophene long axis is attained. This morphological characteristic is found to carry over to the solid-state for both systems, and clear optoelectronic anisotropy of the thin films is observed. As such, these methodologies may provide a route to the production of environmentally friendly high-performance optoelectronic devices.