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Self-Assembly and Crystallization of Conjugated Block Copolymers

  • Author(s): Davidson, Emily Catherine
  • Advisor(s): Segalman, Rachel A
  • et al.
Abstract

Conjugated polymers are class of electrically conductive polymers used in applications including organic photovoltaics, organic light emitting diodes, and organic field effect transistors. Incorporating this class of materials into block copolymer architectures offers the opportunity to combine the optoelectronic properties of conjugated polymers with precise, long-range nanopatterning via block copolymer self-assembly. Importantly, conjugated polymer crystallinity is essential for determining the charge transport properties of the material. Achieving high performing materials in block copolymer geometries requires that conjugated polymer crystallinity be both confined and optimized. Furthermore, while crystallinity in conjugated polymers is essential for excellent performance, many aspects of conjugated polymer crystallinity are still poorly understood. Block copolymers offer an opportunity to study conjugated polymer crystallization in highly-defined geometries with controlled chain tethering at the interfaces, allowing a precise level of control over the crystallization conditions.

This dissertation demonstrates the utility of molecular design in conjugated polymers to create diblock copolymers that robustly self-assemble in the melt and confine crystallization upon cooling. This work leverages the model conjugated polymer poly(3-(2′-ethyl)hexylthiophene) (P3EHT), which features a branched side chain, resulting in a dramatically reduced melting temperature (Tm ~80°C) relative to the widely-studied poly(3-hexylthiophene) (P3HT) (Tm ~200°C). This reduced melting temperature permits an accessible melt phase, without requiring that the segregation strength (χN) be dramatically increased. Thus, diblock copolymers containing P3EHT demonstrate robust diblock copolymer self-assembly in the melt over a range of compositions and morphologies. Furthermore, confined crystallization in the case of both glassy (polystyrene (PS) matrix block) and soft (polymethylacrylate (PMA) matrix block) confinement is studied, with the finding that even in soft confinement, crystallization is constrained within the diblock microdomains. This success demonstrates the strategy of leveraging molecular design to decrease the driving force for crystallization as a means to achieving robust self-assembly and confined crystallization in conjugated block copolymers.

Importantly, despite the relatively flexible nature of P3EHT in the melt, the diblock copolymer phase behavior appears to be significantly impacted by the stiffness (persistence length of ~3 nm) of the P3EHT chain compared to the coupled amorphous blocks (persistence length ~0.7 nm). In particular, it is shown that the synthesized morphologies are dominated by a very large composition window for lamellar geometries (favored at high P3EHT volume fractions); cylindrical geometries are favored when P3EHT is the minority fraction. This asymmetry of the composition window is attributed to impact of conformational asymmetry (the difference in chain stiffness, as opposed to shape) between conjugated and amorphous blocks. These results emphasize that targeting curved morphologies with majority conjugated polymer – even when the conjugated polymer is fairly flexible, as is the case with P3EHT – will continue to be an important challenge.

The detailed balance between the unique properties of conjugated polymer crystallization and diblock copolymer self-assembly in these materials is illuminated by examining the crystallite orientation and the response of microdomains to crystallization. A critical parameter is found to be the P3EHT drive for extended-chain crystals. It is found that under all probed conditions in lamellar P3EHT-b-PMA, P3EHT chains crystallize with their chains perpendicular to the diblock interface. Further, in P3EHT-b-PMA with a deformable amorphous block, P3EHT drives domain expansion during crystallization despite increasing P3EHT density. This expansion corresponds to the formation of extended-chain crystallites. This resulting conformation is not necessarily expected to be favorable, given that it induces a stretching penalty in the coupled amorphous block. However, this expansion appears to be not only preferred but necessary: crystallization in lamellar confinement with a glassy PS matrix suppresses not only domain extension but also P3EHT crystallization. Interestingly, in cylindrical confinement, it is shown that this drive for extended chain crystals results in local deformation of the cylindrical domains themselves.

Finally, the relationship between the detailed crystallization process and the diblock structure is examined. The degree of crystalline perfection of P3EHT can be controlled in confinement by controlling the crystallization temperature (Tc) or, alternatively, via re-crystallization at temperatures below the melting temperature. Surprisingly, in P3EHT-b-PMA increasing the crystallization temperature both improves the crystalline perfection and results in less domain extension. By tracking the changes in domain structure during melting, three distinct melting regimes are identified. A structural model of the conjugated block melting process is proposed, consisting of (I) excluded-chain relaxation followed by (II) chain inter-digitation during melt-recrystallization, and finally (III) complete melting independent of the initial crystallization conditions. These detailed studies of the impact of processing conditions suggest that crystallization processes coupled to temperature-dependent diffusion and nucleation are critical for determining the final crystalline state in confinement. They also suggest that, surprisingly, improvement in conjugated polymer crystallinity may correspond to a more compact structure along the chain dimension.

Throughout this dissertation, we consider how key parameters governing structure formation impact self-assembly and crystallization. In particular, we consider the driving forces for crystallization versus microphase separation, the impact of conformational asymmetry, the drive for extended chain crystallites, and finally the role of detailed crystallization processes. The findings presented here demonstrate the biases introduced by conjugated polymers for the self-assembly and crystallization of such systems, and suggest design rules for the targeted creation of block copolymers containing high-performing conjugated polymers.

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