Session J63: 2021 Dillon Medal Symposium

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Electronically-active polymers continue to impact emerging technology landscapes in myriad device applications. The oft-utilized design paradigm associated with conducting polymers is one rooted in molecules containing large degrees of conjugation along their backbones and the inclusion of chemical dopants that serve to change the oxidation state of the polymers. However, moving from this archetype and towards one with a single-component, charge-neutral macromolecule has significant fundamental and practical benefits, as this type of macromolecular conductor should result in materials that can be designed in a more straightforward fashion and should have longer stability when implemented in devices. As such, we will describe redox-active macromolecules known as radical polymers (i.e., macromolecules that are composed of non-conjugated backbones and have pendant groups that contain open-shell entities), and we will demonstrate how controlling their physical properties allows for control of their structure at the local (i.e., ~10 nm) scale. This ordering does not extend to longer scales, which results in radical polymer films that are completely amorphous. Despite lacking any type of conjugation or crystalline domains, these macromolecules demonstrate high electrical conductivity values. Specifically, we show that the solid-state electrical conductivity of a designer radical polymer exceeds 20 S/m, and this places these non-conjugated polymer conductors in the same regime as many grades of common commercially-available, chemically-doped conjugated conducting polymers. Additionally, we highlight how controlling the macromolecular physics of the redox-active materials allows for their utilization in advanced organic electrochemical transistors. Thus, this work showcases how polymer physics insights can lead to next-generation organic electronic materials and their utility in myriad device archetypes including in the realm of stretchable bioelectronics.   

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Block polymers have been used in the development of advanced ultrafiltration membranes. They offer incredible control over pore size and pore wall functionality in the resultant separation media. Numerous demonstrations have highlighted the advantages of such block polymer-based strategies, and the future prospects for translation of these examples to technological applications have motivated new practical approaches. To this end, we have emphasized the use of the disordered state of lamellar-forming block polymers as key components given that they adopt bicontinuous morphologies that allow for percolating domains of a mechanically robust material and a sacrificial domain used for pore formation. I will report on our efforts to integrate disordered block polymers into dual layer membranes through a co-casting strategy that combines the commonly used non-solvent induced phase separation method with solvent casting of a designer block polymer that traps the disordered state as a thin separation layer post pore formation. The design concept, the strategy used to prepare the dual-layer membranes, and the characterization of the membranes will be presented.   

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Radical-containing polymers are ascendant organic electronic materials with potentially advantageous and complementary properties to more established conjugated polymers. Moving from a conjugated chemistry to a radical-based chemistry presents several opportunities, including the creation of intrinsically conductive materials with highly tunable mechanical properties and attractive solution-processing profiles. Despite these advantages, the structure-function and processing-function relationships of radical-containing polymers are only now being investigated in earnest. Moreover, the scope of radical species that have been incorporated into radical polymers remains extremely limited, with extensive reliance on the TEMPO radical. In these respects, radical polymers are closer to where conjugated polymers were in the nineties, before structure-function rules had been established and when the range of conjugated polymer chemistries that had been characterized was extremely narrow. In this talk, I will present the work that my group has been doing in collaboration with this year’s Dillon Medal Recipient, Bryan Boudouris, to computationally characterize next-generation radical materials that disrupt the TEMPO paradigm.   

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Polydienes like polyisoprene (PI) and polyolefins like hydrogenated medium-vinyl polybutadiene (hPB) generally show limited compatibility (high interaction energy density, X). Both the regular mixing model and the copolymer equation suggest that styrene units (S) should boost inter-block miscibility when incorporated in minor amounts into hPB, up to 40 wt% S, via random copolymerization. Mixing thermodynamics in symmetric polydiene-polyolefin block-random copolymers composed of PI and a selectively saturated random copolymer of medium-vinyl polybutadiene and styrene (hSBR) were investigated via location of their order-disorder transition (ODT) temperatures. Block and “block-random” copolymers were prepared by anionic polymerization, followed by selective saturation of the butadiene units. At 100°C, X = 0.72 MPa at 0 wt% S, decreasing to below 0.28 MPa at 27 wt % and generally exhibiting a parabolic variation with wt% S, with a minimum near 40 wt% S. While both the regular mixing model and the copolymer equation qualitatively predict this parabolic behavior, the copolymer equation—with all three binary interaction energies determined independently—provides a better quantitative description of how S incorporation tunes X.   

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Recent discoveries of complex low-symmetry phases in soft matter have inspired advances in molecular and materials design. However, understanding the mechanisms underlying symmetry selection across soft matter remains a challenge. Block polymers represent attractive model materials that permit wide synthetic tunability and provide access to multiple length scales. To date the block polymer design space has been largely limited to variations in molecular weight, block volume fraction, and conformational asymmetry. Molecular architecture offers rich potential but remains relatively unexplored in experimental block polymers. This work bridges this gap, connecting molecular architecture, space-filling demands, and symmetry selection in block polymer self-assembly. Three series of block polymers were synthesized by living polymerization, tuning the architectural asymmetry across the linear-b-linear and linear-b-bottlebrush limits. The bottlebrush architecture amplifies two key ingredients for the formation of Frank-Kasper phases: high conformational asymmetry and high self-concentration. Analysis by small-angle X-ray scattering provides insight into the impact of architectural asymmetry on block polymer self-assembly.   

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Dynamic hydrogel platforms have recently emerged as a subject of intense interest with applications in bioprinting, injectable cell delivery vehicles, and viscoelastic tissue mimics or scaffolds. Dynamic covalent chemistries in particular present an opportunity to carefully control the rate of bond exchange kinetics, enabling decoupled control over material properties such as elastic modulus or stress relaxation lifetime. Within this context, we developed a poly(ethylene glycol) (PEG) hydrogel platform using a reversible thia-conjugate addition crosslinker. By controlling the aromatic substituents on the conjugate acceptor, we demonstrate that we can preferentially control the forward reaction kinetics, which alters the overall modulus of the hydrogel with minimal effect on the stress relaxation properties. Interestingly, our data demonstrate a role of bond exchange kinetics in the ease of injectability of these hydrogels through clinically-relevant syringes. Finally, our hydrogels exhibit good cytocompatibility with live mammalian cells, indicating promise for biomedical applications. Overall, this work suggests a strategy to decouple bulk hydrogel parameters using tunable bond exchange kinetics.   

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Clustering of stickers along an associative polymer chain often leads to enhanced toughness and creep resistance. Sticker clustering changes network topology, and it can also change sticker dynamics due to the proximity of the associating groups. Here, the effect of clustering is investigated by comparing associative polymer hydrogels with randomly-distributed metal coordinate groups to triblock copolymers with the metal-coordinate groups concentrated in the endblocks. Polymers were synthesized with histidine associative groups that coordinate Ni atoms in a 2:1 ligand:metal ratio. The combination of rheology, neutron scattering, and forced Rayleigh scattering (FRS) are used to study the structure, mechanics, and molecular-level dynamics of these systems. Although rheology shows slower stress relaxation for clustered polymers due to the need for cooperative dissociation of multiple bonds, faster diffusion is observed in clustered polymers by FRS due to the presence of topological defects. Brownian dynamics simulations of the clustered systems provide further insight into the molecular origin of these dynamic differences, demonstrating the key cooperative effect of proximate stickers in producing loop defects which alter the dynamic properties of these systems.   

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This talk will present a short overview of polymer-based electrochemical transistors (ECTs) in which an electrolyte is employed as a gate dielectric and a polymer semiconductor serves as the switchable channel material. Polymer ECTs have applications in biosensing and printed electronics, but they also are an important platform for examining electrical conductivity as a function of controlled electrochemical doping. Examples of both applications and fundamental measurements will be discussed.   

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As interest in decentralized water treatment technologies grows, the development of nanofiltration (NF) as a complementary process to reverse osmosis has emerged. However, NF membranes that overcome the trade-off between selectivity and permeability, that expand the range of molecules that can be separated selectively, and that resist the detrimental effects of fouling must be developed in order to realize the potential of NF-based processes. One approach for overcoming the permeability-selectivity trade-off is through the development of membranes with better controlled pore size distributions. In fact, the performance of NF membranes based on self-assembled materials is already pushing the limits of these size-selective separation mechanisms. Therefore, many researchers are considering approaches that move beyond improved control of size-selective separation mechanisms. In this regard, the functionality lining the pore walls of NF membranes offers another means through which to increase selectivity without sacrificing permeability. In this talk, we will discuss membrane platforms that allow for the post-assembly modification of the pore wall chemistry such that the membranes can be tailored to myriad separations for the treatment and conservation of water resources.   

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This study shows that it is possible to obtain homogeneous mixtures of two chemically distinct polymers with a lithium salt for electrolytic applications. This approach is motivated by the success of using mixtures of organic solvents in modern lithium-ion batteries. The properties of mixtures of a polyether, poly(ethylene oxide) (PEO), a poly(ether-acetal), poly(1,3,6-trioxocane) (P(2EO-MO)), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt were studied using small angle neutron scattering and electrochemical characterization in symmetric cells. The SANS data are used to determine the miscibility window and quantify the effect of added salt on the thermodynamic interactions between the polymers. In the absence of salt, PEO/P(2EO-MO) blends are homogeneous and characterized by attractive interactions, i.e., a negative Flory-Huggins interaction parameter. The addition of small amounts of salt results in a positive effective Flory-Huggins interaction parameter, and macrophase separation. Surprisingly, miscible blends and negative interaction parameters are obtained when salt concentration is increased beyond a critical value. A brief summary of electrochemical properties of the miscible polymer blend electrolytes will also be presented.   

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While polymer electrolytes hold the promise of improving safety and mechanical durability of electrochemical devices, most suffer from relatively low ionic conductivities especially at ambient temperature. This combination of challenges becomes even more pronounced in the conduction of the multivalent metal ions likely to be necessary for next generation, high energy density energy storage devices where the ions are likely to have complex, multi-functional interactions with the polyelectrolyte matrix. Metal-ligand coordinating polymers utilize labile bonds between polymer-bound ligands and free cations to delocalize and conduct mono- and multi-valent metal ions in the solid state. We have shown that the metal-ligand bonds also act as reversible cross-links, leading to delayed terminal relaxation in oscillatory rheology and universal behavior where conduction is related strongly to bond lifetime. Interestingly, these metal-ligand interactions appear to facilitate salt dissociation so the dielectric constant of the polymer backbone is less important than the ligand design.   

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Radical-containing polymers have promising application in fast-charging batteries, in which energy is stored through a combined ion-electron transfer process at the radical unit. In the battery environment the solid radical-containing polymer is swollen with the contacting liquid electrolyte. In this talk, experimental methods and theory that capture the nature of the redox mechanism are discussed. Specifically, electroanalytical techniques are applied to obtain relevant electron transfer rate constants and diffusion coefficients for comparison with Marcus-Hush theory. To examine the coupled ion-electron transfer process, electrochemical quartz crystal microbalance is utilized for thin films of radical-containing polymers during charge and discharge. With this approach, it is possible to distinguish between cation and anion transfer and to estimate the number of solvent molecules transferred. These results are discussed in the context of polymer-solvent interactions, solvent-salt interactions, and polymer segmental mobility.   

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The hierarchical assembly of conjugated polymers has gained much attention due to its critical role in determining the optical/electrical/mechanical properties. The complex morphology in the solid state is fully determined by the polymer assembly pathway in the solution state, which in turn is sensitively modulated by molecular structure and processing conditions. However, molecular pictures of polymer assembly pathways remain elusive, due to the lack of detailed structural characterizations in the solution state and lack of understanding on how various factors impact the assembly pathways. In this talk, we discuss pre-aggregation and liquid-crystal mediated assembly pathways and their relationship. We develop SAXS models for in-depth analysis of the complex solution state structure of conjugated polymers complemented by imaging and spectroscopy. Further we discover a chiral, helical liquid crystal mediated assembly pathways from achiral conjugated polymers. We elucidate a hierarchical helical structure spanning the molecular scale to micron scale. We further trace the molecular origin of chiral liquid crystal assembly pathways. These studies change the way we perceive structure of conjugated polymers and may bring forth exciting new optoelectronic properties not imagined before.