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Characterization of Heterogeneity in Polymer Systems using Optical Spectroscopy and Fluorescence Microscopy
Conjugated organic materials have become industry standards for handheld devices due to their advantageous properties leading to thinner and ecofriendly optoelectronics. However, both small molecules and polymers are limited by their susceptibility to aggregation at concentrations required for device fabrication. Aggregation can lead to enhanced charge transport but will then quench the emission of the conjugated molecules. Therefore, it is important to be able to control the degree and type of intermolecular packing that occurs in these materials. Poly(3-hexylthiophene) (P3HT) is a well-studied benchmark system for semiconducting polymers used in optoelectronic devices. Within this work, we use P3HT to develop methods to more accurately characterize the heterogeneity of this polymer in various environments achieved through solvent poisoning techniques. We first explored the underlying assumptions of the classical weakly coupled H-aggregate (WCH) model. We find that, while this model sufficiently describes the degree of aggregation in homogenous systems, it does not for heterogenous systems. A modified model is then proposed that better accounts for multiple emissive species (MES). Within this work, we show the residual monomeric chains must be accounted for to properly assign either H- or J-type aggregation. We next attempted to determine the impact of the solvation environment on the polymer chain conformation. Specifically, we analyze the properties that classify a solvent as either “good” or “poor” for a given polymer. The results from solvent poisoning with both a higher dielectric constant and lower dielectric constant solvent show that while this property along with the polarizability of the environment do impact the photophysical properties, they do not fully describe the differences. We then show the emission heterogeneity is retained when pre-aggregated samples undergo a phase transition from solution to solid. Moreover, we find an enhancement of the emission intensity in the aggregated samples compared to that of the monomer intensity. Finally, we show the general utility of the FCS method combined with the MES model for understanding the spectra of a variety of other conjugated polymers.
- Doctor of Philosophy (PhD)