Self-Assembly of crystallinity controlled conjugated polymer based rod-coil copolymers

Abstract

Conjugated polymer based rod-coil copolymers are composed of covalently linked two distinctive class of polymers―one contains extended $\pi-conjugation$ along the polymer backbone, leading to optical and electrical properties, and the other block is classical Gaussian coil polymer which allows tuning of morphology, solubility and/or functionality of the copolymer. Self-assembly of the rod-coil copolymers has been offered great promise for the construction of desirable nanostructures of active layers in organic electronics. Strong chain rigidity and crystallinity of the conjugated polymers, however, introduce complex interactions between rod blocks that significantly complicates the phase behaviors, affording entirely different from that of conventional coil-coil copolymers. In this thesis, I studied self-assembly of rod-coil copolymers containing conjugated blocks in a variety of contexts including melt-phase assembly, interfacial assembly at polymer blends and solution state crystallization-driven assembly with systematical tuning of crystallization behaviors of the conjugated polymers that provides new levels of tailiorability of nanostructures form rod-coil copolymers. i) Melt-phase assembly of rod-coil copolymers―an architectural molecular design of conjugated polymer-based rod-coil copolymers was explored to control the crystallinity of the copolymers. A series of well-defined poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP) copolymers were synthesized via a microwave assisted click chemistry between azide-functionalized P3HT (P3HT-azide) and alkyne-terminated P2VP (P2VP-alkyne) that were prepared with two di？erent controlled polymerization methods. We observed that controlling the molecular weights ($M_n$) of the grafted P2VP chains allowed us to regulate the rod-rod interaction of the copolymers systematically. As the $M_n$ of the grafted P2VP chains increased, the crystallinity of the P3HT block in the copolymers gradually decreased so that the enthalpic interaction and chain entropy become more dominant than the crystallization of the P3HT moiety. Therefore, we could produce thermally-annealed, well-ordered non-fibril nanostructures including lamellae, hexagonally packed cylinders, and spheres with architectural engineering of P3HT-based rod-coil copolymers. ii) Interfacial assembly of rod-coil copolymers at polymer blends ―while conjugated polymer blends have been widely used in the organic electronic devices, thermodynamically unstable and sharp interfaces cause serious morphological instability against thermal and mechanical stress. We utilize architecturally engineered rod-coil graft copolymer (P3HT-g-P2VP) as a compatibilizer that effectively modifies the sharp interface of P3HT/fullerene based solar cell device, resulting in a dramatic enhancement of mechanical and thermal stability. It was evident from the results of the thermal and mechanical stabilities of the solar cell that the P3HT-g-P2VP copolymers had much better compatibilizing efficiency than linear-type P3HT-b-P2VP copolymers. Strong crystallization of the block copolymers drives to form fibril-like bundles that disturb the accumulation of the block copolymer at the interface whereas the graft architecture promotes preferential segregation at the interface, resulting in broader interfacial width and lower interfacial tension as demonstrated by coarse-grained molecular dynamics simulations. iii) Solution state crystallization-driven assembly ―solvent mediated self-assembly of conjugated polymers allows unique one-dimensional growth of conjugated polymer block into nanowires (NWs) with high aspect ratio and well-ordered structure, resulting in excellent optoelectronic properties and long-range charge transport pathways with nanometer-sized cross-sectional areas. Herein, we studied solution state crystallization-driven assembly of regioregularity (RR) controlled P3HTs and their copolymers. A series of P3HTs and P3HT-b-P2VP copolymers having a wide range of RR was synthesized. The crystalline behaviors of the polymers were systematically modulated as changing the RR from semi-crystalline to amorphous, and thus the solution state assembly of NWs was tailored to have different growth kinetics and structures.