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Domain Structures of Poly(3-dodecylthiophene)-Based Block Copolymers Depend on Regioregularity
Kim, Jin-Seong,Han, Junghun,Kim, Youngkwon,Park, Hyeonjung,Coote, Jonathan P.,Stein, Gila E.,Kim, Bumjoon J. American Chemical Society 2018 Macromolecules Vol.51 No.11
<P>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)-<I>block</I>-poly(2-vinylpyridine) (P3DDT-<I>b</I>-P2VP). To decouple the effect of LC interactions from crystallization kinetics, we investigate the morphological behavior of the P3DDT-<I>b</I>-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, and 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 RR. 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.</P> [FIG OMISSION]</BR>
Coote, Jonathan P.,Kim, Jin-Seong,Lee, Byeongdu,Han, Junghun,Kim, Bumjoon J.,Stein, Gila E. American Chemical Society 2018 Macromolecules Vol.51 No.22
<P>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 fibrillar texture. We demonstrate that crystallization modes of conjugated BCPs based on poly(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-<I>block</I>-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-<I>block</I>-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.</P> [FIG OMISSION]</BR>
Regioregularity-Driven Morphological Transition of Poly(3-hexylthiophene)-Based Block Copolymers
Kim, Jin-Seong,Kim, Yongjoo,Kim, Hyun-Jeong,Kim, Hyeong Jun,Yang, Hyunseung,Jung, Yeon Sik,Stein, Gila E.,Kim, Bumjoon J. American Chemical Society 2017 Macromolecules Vol.50 No.5
<P>Conjugated polymer-based block copolymers (BCPs) offer great potential to provide beneficial nanostructures for efficient organic optoelectronics. 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 8S%), 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.</P>
Symmetry Transitions of Polymer-Grafted Nanoparticles: Grafting Density Effect
Yun, Hongseok,Yu, Ji Woong,Lee, Young Jun,Kim, Jin-Seong,Park, Chan Ho,Nam, Chongyong,Han, Junghun,Heo, Tae-Young,Choi, Soo-Hyung,Lee, Doh C.,Lee, Won Bo,Stein, Gila E.,Kim, Bumjoon J. American Chemical Society 2019 Chemistry of materials Vol.31 No.14
<P>We examined the packing structure of polystyrene-coated gold nanoparticles (Au@PS) as a function of grafting density. A series of Au@PS nanoparticles with grafting densities in the range of 0.51-1.94 chains nm<SUP>-2</SUP> were prepared by a ligand exchange process using thiol-terminated PS and then self-assembled at a liquid-air interface. We observed a transition from disordered to body-centered cubic (bcc) to face-centered cubic (fcc) arrangements with increasing grafting density, even though the ligand length-to-core radius ratio (λ) was as high as 3.0, a condition that typically favors nonclose-packed bcc symmetry in the self-assembly of hard nanoparticles. To explain this phenomenon, we define λ<SUB>eff</SUB> to include the concentrated polymer brush regime as part of the “hard core”, which predicts that the softness of Au@PS nanoparticles is reduced from 1.53 to 0.14 in a theta solvent as the grafting density increases from 0.51 to 1.94 chains nm<SUP>-2</SUP>. This new definition of λ can also predict the effective radii of nanoparticles using the established optimal packing model. The experimental findings are supported by a combination of coarse-grained molecular dynamics simulation and adaptive common neighbor analysis, which show that changes in grafting density can drive the observed transitions in nanoparticle packing. These studies provide new insights for controlling the self-assembled symmetries of polymer-coated nanocrystals using a simple ligand exchange process to tune particle softness.</P> [FIG OMISSION]</BR>