Synthesis and Self-Assembly of Poly(3-alkylthiophene)-containing Rod-Coil and Rod-Rod Block Copolymers

Abstract
Optoelectronic devices fabricated from organic or polymeric materials have received a great deal of attention due to their significant potential as low cost, flexible and lightweight large-area devices. Controlling ordered morphology of material in 10-nm length scale is expected as one of important issues for optimizing the performance of organic devices. Using intriguing self-assembled behaviors of block copolymers (BCPs) is an emerging and promising strategy for achieving such nanomorphology. Manipulating the non-lamellar and bicontinuous nanostructures through changing volume fraction is well-developed technique for conventional coil-coil block copolymers, but it is not always effective for π-conjugated polymer-based rod-coil block copolymers. In addition, using coil-coil BCPs as photoactive layer cannot improve the performance of devices efficiently due to their insulating properties. In this dissertation, the synthesis of poly(3-alkylthiophene)-b-poly(methyl methacrylate) (P3AT-b-PMMA) rod-coil block copolymers via click chemistry are explored. The fundamental self-assembly of P3AT-b-PMMA is studied and their versatile nanostructures are observed under rational molecular design. Besides, we also synthesize amphiphilic rod-coil block copolymer via click reaction, poly(3-(2-ethyl- hexyl)thiophene)-b-poly(ethylene oxide) (P3EHT-b-PEO), n-type ID4 amphiphile, and prepared the P3EHT-b-PEO(ID4)x complexes. The self-assembly and optoelectronic properties of complexes are explored. Finally, in order to further enhance the performance of devices, the novel all-conjugated donor-acceptor rod-rod block copolymers, poly(3-alkylthiophene)-b-poly(thiophene-alt-isoindigo) (P3AT-b-PTID), are designed, synthesized and characterized.
A series of well-defined P3AT-b-PMMA rod-coil block copolymers with different PMMA volume fractions (fPMMA) have been successfully synthesized via Grignard metathesis (GRIM) polymerization of P3AT and anionic polymerization of PMMA followed by click chemistry, where poly(3-hexythiophene) (P3HT), poly(3-dodecylthiophene) (P3DDT), and poly(3-(2-ethylhexyl)thiophene) (P3EHT) are used as P3AT blocks. While using P3AT-b-PMMA as a model, versatile self-assembly morphology of rod-coil copolymer can be achieved by simultaneously adjusting the rod-rod interaction, rod-coil interaction and conformational asymmetry. By altering the alkyl side chain of polythiophene from linear hexyl to longer dodecyl and to branch 2-ethyl hexyl, both rod-coil and rod-rod interaction are decreased with increasing spatial occupation of alkyl side chain which have been quantitatively determined for this type of rod-coil copolymer. With tunable conformational asymmetry, competition between rod-rod and rod-coil interactions, and crystallization-driven force, the presence of versatile morphology, i.e. lamellar, hexagonal structures, cylinder-to-gyroid phase transition and disordered phase, can be observed for long sought composition at approximately fPMMA = 0.5.
We also successfully use click chemistry to synthesize amphiphilic rod-coil block copolymer, P3EHT-b-PEO. For optoelectronics applications, lowering insulating coil segments in rod-coil block copolymers is crucial. We thus focus on the self-assembled behaviors at low PEO volume (fPEO) fractions. After thermal annealing, P3EHT-b-PEO block copolymers would self-assemble into hexagonal and lamellar structures at room temperature at fPEO = 0.18 and 0.30, respectively, and they both show gyroid morphology through order-order transition at elevated temperature. Besides, a new amphiphilic n-type acceptor, ID4, is successfully synthesized. After ID4 is blended with P3EHT-b-PEO with fPEO = 0.30, a series of P3EHT-b-PEO(ID4)x complexes have been successfully prepared through hydrogen bonding, where x is ID4 per repeating unit of PEO in molar ratio. As increasing binding fractions, the complexes not only change their morphology from lamelle to gyroid, but also shows reduced segregation strength compared to neat P3EHT-b-PEO block copolymer.
A series of fully conjugated donor-acceptor rod-rod block copolymers, P3AT-b-PTID, have been synthesized using Stille coupling reaction under microwave irradiation. While P3HT and P3EHT are used as donor block, PTID is used as acceptor block. These novel block copolymers can self-assemble into unique donor-acceptor morphology with different crystalline structures in bulk state. When P3AT-b-PTID block copolymers are used as photoactive layer to fabricate all-polymer solar cells, their Voc values are both higher than 0.9 volt. The P3HT-b-PTID device has the highest efficiency with a value of 0.79, which is better than devices fabricated with P3EHT-b-PTID and P3HT/PTID blend.
In summary, we can increase the conformational asymmetry of rod-coil block copolymer, and reduce its rod-rod interaction through introducing bulky side chain on the thiophene ring. Thus we can obtain the bicontinuous and interpenetrating gyroidal structure, which is one of ideal morphology for organic devices. We can also obtain gyroid morphology, as we further add n-type amphiphilic acceptor into rod-coil block copolymer. Based on the study of self-assembly of rod-coil block copolymer, we can synthesize novel all conjugated donor-acceptor P3AT-containing block copolymer with different side chain structure on the thiophene ring. Using this strategy, gyroid of donor-acceptor block copolymer might be achieved in the future, which could enhance the performance of organic photovoltaics more efficiency.