Self-assembly behaviors of polythiophene-based homopolymers and block copolymers by tuning regioregularity and molecular weight

Abstract

Self-assembly of semi-crystalline conjugated polymers has been widely considered as the facile and efficient strategy for offering highly ordered crystalline structures due to their strong $\pi$-$\pi$ interaction, which has great potential for application as optoelectrical devices. Study of physical properties to control crystallinity is important to achieve various nano-structures and understand self-assembly behaviors. In this thesis, I studied synthesis and self-assembly of conjugated polymer in a variety of routes including solution assembly with systematical tuning of regioregularity and molecular weight of the conjugated polymers that provides synthetic rules to regulate their crystallinity and morphology. i) Modulating Regioregularity of P3HT-based Amphiphilic Block Polymers to Control Solution Assembly from Nanowires to Micelles—In selective solvents, poly(3-hexylthiophene) (P3HT)-based block copolymers (BCPs) assemble into one-dimensional nanowires (NWs) due to strong $\pi$-$\pi$ stacking interactions of the P3HT block. Herein, we report the effect of P3HT regioregularity (RR) on the assembly of P3HT-based amphiphilic BCPs in solution. We synthesized a series of P3HT-block-poly(2-vinylpyridine) (P3HT-b-P2VP) copolymers with similar molecular weights and P3HT volume fractions but with different RRs, ranging from 55% to 95%, and studied their assembly in tetrahydrofuran/n-butanol mixtures. P3HT-b-P2VP copolymers with high RR (> 80%) crystallized into well-ordered NWs with core widths consistent with fully extended P3HT chains. In BCP nanostructures with decreasing RR, more flexible P3HT chains produced gradual increases in the width of the NWs, from 12 to 24 nm. Eventually, a morphological transition to spherical micelle structures was observed at 55% RR. The structural differences were visualized by incorporating Au nanoparticles, which locate at the interface of P3HT and P2VP blocks, onto the NWs and imaging the resulting hybrid nanostructures by transmission electron microscopy (TEM). In addition, the crystalline behaviors of the assembled nanostructures were determined using differential scanning calorimetry (DSC) and grazing incidence- X-ray scattering (GIXS). We elucidated important relationships between the solution assembly behaviors and the crystalline interactions of conjugated semi-crystalline–coil BCPs, which will guide design of future versatile BCP nanostructures. ii) Chain Length Dependent Self-Assembly Behaviors of Discrete Conjugated Oligo(3-hexylthiophene)— Discrete oligomers having highly monodisperse nature can provide a precise understanding of chain-length dependent properties of polymers and their self-assembly behaviors. Herein, we obtain discrete oligo(3-hexylthiophene)s (D-o3HTs) with a dispersity (Đ) of 1.0 and degree of polymerizations (DP) between 6 to 18 through simple synthetic procedure of 3-hexlythiophene trimer-based polymerizations and reversed phase auto-column chromatography purification. As the DP of D-o3HTs increases, longer conjugation lengths cause a red shift in their optical properties and yield enhanced crystalline properties. Interestingly, D-o3HTs with DP <10 have face-on dominated Form II structures and do not crystallize in either solution or thin films. In contrast, D-o3HTs with DP ≥ 10 assemble into edge-on dominated Form I structures. These D-o3HTs show highly ordered fiber morphologies and well-defined lattice patterns in thin films, indicating that the monodisperse nature of D-o3HTs allows for single molecule-like packing unlike conventional regioregular poly(3-hexylthiophene) with Đ = 1.1. Finally, we demonstrate the formation of 2-dimensional flower-like nanostructures in micrometer scales from D-o3HTs via solvent-mediated self-assembly. These results can offer the advanced understanding of relationship between DP and crystallization of conjugated polymers, leading to exquisite control over their morphology.