Synthesis and self-assembly of regioregularity-controlled polythiophene-based block copolymers

Abstract

Conjugated polymer based block copolymers are composed of covalently linked two distinctive class of polymers—one contains extended π-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 conjugated-amorphous 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 conjugated blocks that significantly complicates the phase behaviors, entirely different from that of conventional coil-coil copolymers. In this thesis, I studied synthesis and self-assembly of conjugated polymer based block copolymers in a variety of contexts including melt-phase assembly and crystallization kinetics with systematical tuning of regioregularity of the conjugated polymers that provides new levels of tailiorability of nanostructures form conjugated-amorphous copolymers.

i) Synthesis and Crystallization Behavior of Regioregular-block-Regiorandom Poly(3-hexylthiophene) Copolymers—Herein, we report the development of poly(3-hexylthiophene) regioblock copolymers (block-P3HT), which are composed entirely of identical 3-hexylthiophene repeating units, with one block being highly regioregular (rre) and the second block being regiorandom (rra). This block-P3HT was prepared via catalyst-transfer polycondensation by sequential addition of different monomer batches, with 3-hexylthiophene (3HT) monomers used for the rre block and a mixture of 3HT and 3,3ʹ-dihexyl-2,2ʹ-bithiophene (BT) monomers for the rra block. The average regioregularity ($RR^{avg}$) of the block-P3HTs were tuned from 58 to 79% by differentiating the weight ratio between the rre and rra blocks, and the crystalline behavior of block-P3HTs in comparison to polymers having similar RRavg but a random distribution of 3HT and BT units (co-P3HT) was investigated. This study allowed for the exploration of the effects of the distribution of regiodefects within the polymer chain (e.g. confined to a single block versus uniformly distributed throughout the polymer) on the crystalline behaviors of the polymer. For example, block-P3HT with 72% $RR^{avg}$ formed long nanowires by solution assembly, due to the strong crystallinity from a sufficient effective conjugation length in the rre block, whereas co-P3HT with same $RR^{avg}$ formed spherical micelles indicating amorphous characteristics. Overall, the block-P3HTs showed a strong tendency to crystallize, even with very low RRavg, driven by the rre block in the polymers, whereas the crystallinity of the co-P3HTs was substantially suppressed due to chain distortion by the distributed regiodefects.

ii) Regioregularity-Driven Morphological Transition of Poly(3-hexylthiophene)-Based Block Copolymers—Conjugated polymer-based block copolymers (BCPs) offer great potential to provide beneficial nanostructures for efficient organic opto-electronics. However, their complicated self-assembly behavior, which is attributed to the strong crystallization of conjugated blocks, is still not well understood due to the lack of a model BCP system. Herein, we develop a series of novel conjugated polymer-based BCPs, poly(3-hexylthiophene)-block-poly(2-vinylpyridine) (P3HT-b-P2VP), in which the regioregularity (RR) of the P3HT block was varied from 95 to 73%. The tunable RR content allows for precise regulation of P3HT crystallization with minimal influence on the microphase-separation force between the P3HT and P2VP blocks. When RR is high (i.e., 95 or 85%), structure formation is controlled by crystallization of P3HT, and the ultimate structure is characterized by nanoscale P3HT fibrils in an amorphous matrix. In contrast, as RR decreases to 78 and 73%, P3HT crystallization is suppressed. The self-assembly is controlled by the enthalpic interaction between P3HT and P2VP blocks, much like typical BCPs having two flexible blocks, and thermal annealing drives the formation of well-ordered lamellar or cylindrical phases. This morphological behavior is consistent with a Monte Carlo simulation based on a newly-developed coarse-grained model. Significantly, this novel class of RR-controlled P3HT-based BCPs provides a simple method to tune bulk and thin film morphology for a variety of applications in nanostructured organic electronics.

iii) Domain Structures of Poly(3-Dodecylthiophene)-based Block Copolymers Depend on Regioregularity — Microphase-separation behavior of conjugated–amorphous block copolymers (BCPs) is driven by a complex interplay between Flory–Huggins interaction (χ), liquid crystalline (LC) interaction and crystallization. Herein, in order to elucidate the influence of LC interaction on the morphology of the BCPs, we report the effects of regioregularity (RR) on the microphase separation and molecular packing structures of poly(3-dodecylthiophene)-block-poly(2-vinylpyridine) (P3DDT-b-P2VP). To decouple the effect of LC interactions from crystallization kinetics, we investigate the morphological behavior of the P3DDT-b-P2VP at above the melting temperature of P3DDT (~ 160 °C). Both electron microscopy and X-ray scattering show an abrupt reduction in the domain spacing of both lamellar and cylindrical phases as the RR of P3DDT block increases. Specifically, lower RR (i.e., 85, 79, 70%) BCPs have larger domain spacings than high RR (94%) by 50% (lamellar) or 80% (cylindrical), even though the overall molecular weights and P2VP volume fractions were similar for each morphology. We propose that the RR-driven transition in domain spacing is caused by a change in P3DDT conformations and interchain interactions. When RR is low, the system assembles into a typical bilayer structure like other semiflexible and flexible block copolymer systems. When RR is high, the less flexible P3DDT chains are extended, driving their assembly into an LC monolayer. Significantly, this study demonstrates that tunable RR provides a simple route to manipulate melt state self-assembly of conjugated-amorphous materials.

iv) Crystallization Modes of Poly(3-dodecylthiophene)-based Block Copolymers Depend on Regioregularity and Morphology—Conjugated block copolymers (BCPs) can self-assemble into highly ordered nanostructures in a melt state. However, when cooled below the melting temperature, crystal growth can disrupt the self-assembled structure and produce a poorly-ordered fibrillary texture. We demonstrate that crystallization modes of conjugated BCPs based onpoly(3-dodecylthiophene) (P3DDT) and poly(2-vinylpyridine) (P2VP) can be tuned through P3DDT regioregularity (RR), as this attribute controls the melting temperature and crystallization rates of P3DDT. When RR is low (70-80%), crystallization is observed at temperatures near or below the glass transition of P2VP, so crystal growth is largely confined by the glassy cylindrical or lamellar BCP structure. When RR is high (94%), crystallization occurs at 40 K above the glass transition of P2VP, so there is no longer a restriction of glassy domains. Importantly, crystal growth remains confined by the rubbery P2VP lamellae, but breaks through the rubbery P2VP cylinders. This morphology-dependent behavior is attributed to geometric compatibility of P3DDT crystal growth and the self-assembled symmetry: In a lamellar phase, the P3DDT chain orientations at the P3DDT-block-P2VP interface are compatible with crystal growth, and both the alkyl-stacking and π-π growth directions are unrestricted within a lamellar sheet. In a cylindrical phase, the radial orientation of P3DDT chains at the P3DDT-block-P2VP interface is not compatible with crystal growth, and the hexagonal close-packed symmetry only allows for one direction of unrestricted crystal growth. Significantly, these studies demonstrate that tuning RR of polyalkylthiophenes can open up multiple crystallization modes with the same monomer chemistries and block lengths, thereby decoupling the parameters that govern classical BCP self-assembly and crystal growth.

v) Impact of Regioregularity on the Crystallization Modes of Poly(3-Dodecylthiophene)-based Block Copolymers under Soft Confinement— Understanding crystallization and morphological behavior of conjugated–amorphous block copolymers (BCPs) is important for further applications of organic electro-active materials. Herein, we demonstrated crystallization modes of regioregularity (RR) using poly(3-dodecylthiophene)-block-poly(ethyl methacrylate) (P3DDT-b-PEMA) copolymer series. All copolymers are designed to have similar molecular weight, but different RR form 96 to 65%. Lamellar and hexagonal cylinder morphology at the melt state was achieved by differentiating volume fraction of PEMA (fPEMA) of the copolymers. Due to the low Tg of PEMA block, crystallization occurs with soft second block domain. As the temperature of polymer melt samples decreases through the TWAXS of P3DDT block, the crystallization proceeds and its effects on melt morphology was monitored by X-ray scattering. For HPC forming copolymers, only break-out and template crystallization modes are observed. This is attributed to the geometric frustration between the hexagonal morphology and linear growth of P3DDT crystal. The crystallization modes were determined by crystallization strength depending on RR. For example, break-out occurs by strong crystallization with higher RR (96, 81 and 76%) and template crystallization is observed in lower RR copolymers cases (70 and 65%). For LAM forming copolymer, template and confined crystallization are observed. Similar to HPC case, RR dependent crystallization mode transition was observed between 76 and 70%, showing template crystallization for higher RR (96, 81 and 76%) and confined crystallization for lower RR (70 and 65%). Significantly, our work on crystallization modes under soft confinement environment can pave a new method for modulating crystallization and self-assembly behavior of conjugated–amorphous BCPs.