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Controlled Self Assembly of Conjugated Polymer Containing Block Copolymers

  • Author(s): McCulloch, Bryan
  • Advisor(s): Segalman, Rachel A
  • et al.
Abstract

The discovery and development of conjugated polymers has led to a large and vibrant research field due to their unique semiconducting properties and possibility of offering a completely new paradigm due to their abundant, lightweight, flexible and solution processable properties. In particular, the optoelectronic properties of these materials make them very well suited to applications such as organic light emitting diodes or organic photovoltaics and their relatively high charge mobility also make them useful in organic circuits. There are several reasons why the performance of these materials is presently limited compared to inorganic alternatives. Recently, significant work has been done trying to improve the performance of these materials by synthetically tuning the electronic properties such as the band gap and energy levels. There have been many other studies trying to improve intermolecular transport by enhancing crystallinity through annealing. In all of these studies the performance organic electronics still tends to be limited because the morphology of these materials is very complex and difficult to optimize. Many different length scales must be simultaneously optimized because the structure of a single polymer chain, their interactions with other polymer chains, the orientation of these chains and their degree of mixing with other components in the device all are extremely important to the performance of these materials.

In this work, block copolymers containing conjugated polymers are used to optimize the morphology of these materials through self assembly processes. Block copolymers can be used to produce a wide variety of thermodynamically stable morphologies with long range order and tunable domain sizes. The self assembly of conjugated polymers using block copolymers is complicated because the delocalization of electrons along the backbone, which produces their interesting semiconducting properties, also makes these polymers rod-like, liquid crystalline and drastically increases their intermolecular interactions. This thesis focuses on some of the basic issues that must be understood when trying to create and optimize techniques that can be used to produce self assembled structures containing conjugated polymers using block copolymers.

The chain shape of conjugated polymers is related to their intramolecular interactions and is a fundamental property of the polymer chemistry. This contributes to their specific properties that make every conjugated polymer behave slightly differently, making it difficult to build design rules that can be used for all conjugated polymers or even classes of polymers. By understanding the chain shape of these conjugated polymers, we can begin to understand how to tune their intermolecular interactions and block copolymer phase diagram. In this work we have examined the chain shape of polythiophenes, one of the most commonly used classes of conjugated polymers. We have also explored the use of polythiophenes in block copolymers to control the morphology in polymer photovoltaics. In order to optimize the properties of these materials it is also important develop techniques to align the block copolymer structure and the conjugated polymer backbone. This work examines the use of magnetic field alignment as a method to achieve these two goals. It has been shown that high degrees of alignment have been achieved with easily accessible field strengths. Additionally, work has been done examining methods to optimize the alignment procedure for magnetic field alignment of block copolymers by examining the effect of the field strength on the block copolymer phase behavior and through detailed work focusing on the dynamics of the alignment process. By building methods to optimize the morphology of block copolymer containing conjugated polymers we hope see these techniques applied to increase the performance of polymer optoelectronics.

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