Session E21: Polymer Physics Prize

Polymer Physics Prize Talk

Polymer electrolytes have been particularly difficult to describe theoretically given the large number of disparate length scales involved in determining their physical properties. The Debye length, the Bjerrum length, the ion size, the chain length, and the distance between the charges along their backbones determine their structure and their response to external fields. We have developed an approach that uses multi-scale calculations with the capability of demonstrating the phase behavior of polymer electrolytes and of providing a conceptual understanding of how charge dictates nano-scale structure formation. Moreover, our molecular dynamics simulations have provided an understanding of the coupling of their conformation to their dynamics, which is crucial to design self-assembling materials, as well as to explore the dynamics of complex electrolytes for energy storage and conversion applications.   

Emerging Insights into Directed Assembly: Taking Examples from Nature to Design Synthetic Processes

There is considerable interest in controlling the assembly of polymeric material in order to create highly ordered materials for applications. Such materials are often trapped in metastable, non-equilibrium states, and the processes through which they assemble become an important aspect of the materials design strategy. An example is provided by di-block copolymer directed self-assembly, where a decade of work has shown that, through careful choice of process variables, it is possible to create ordered structures whose degree of perfection meets the constraints of commercial semiconductor manufacturing. As impactful as that work has been, it has focused on relatively simple materials – neutral polymers, consisting of two or at most three blocks. Furthermore, the samples that have been produced have been limited to relatively thin films, and the assembly has been carried out on ideal, two-dimensional substrates. The question that arises now is whether one can translate those achievements to polymeric materials having a richer sequence, to monomers that include charges, to three-dimensional substrates, or to active systems that are in a permanent non-equilibrium state. Building on discoveries from the biophysics literature, this presentation will review recent work from our group and others that explains how nature has evolved to direct the assembly of nucleic acids into intricate, fully three-dimensional macroscopic functional materials that are not only active, but also responsive to external cues. We will discuss how principles from polymer physics serve to explain those assemblies, and how one might design a new generation of synthetic systems that incorporate some of those principles.   

Programming the Assembly of Unnatural Materials with Nucleic Acids.

Nature directs the assembly of enormously complex and highly functional materials through an encoded class of biomolecules, nucleic acids. The establishment of a similarly programmable code for the construction of synthetic, unnatural materials would allow researchers to impart functionality by precisely positioning all material components. Although it is exceedingly difficult to control the complex interactions between atomic and molecular species in such a manner, interactions between nanoscale components can be directed through the ligands attached to their surface. Our group has shown that nucleic acids can be used as highly programmable surface ligands to control the spacing and symmetry of nanoparticle building blocks in structurally sophisticated and functional materials. These nucleic acids function as programmable ``bonds'' between nanoparticle ``atoms,'' analogous to a nanoscale genetic code for assembling materials. The sequence and length tunability of nucleic acid bonds has allowed us to define a powerful set of design rules for the construction of nanoparticle superlattices with more than 30 unique lattice symmetries, tunable defect structures and interparticle spacings, and several well-defined crystal habits. Further, the nature of the nucleic acid bond enables an additional level of structural control: temporal regulation of dynamic material response to external biomolecular and chemical stimuli. This control allows for the reversible transformation between thermodynamic states with different crystal symmetries, particle stoichiometries, thermal stabilities, and interparticle spacings on demand. Notably, our unique genetic approach affords functional nanoparticle architectures that, among many other applications, can be used to systematically explore and manipulate optoelectronic material properties, such as tunable interparticle plasmonic interactions, microstructure-directed energy emission, and coupled plasmonic and photonic modes.   

Does Prescribed Randomness Hold the Key to Interface Synthetic and Natural Systems?

The bottlenecks to engineering biomimetic functional materials are not only to duplicate hierarchical structures, but also to manipulate the system dynamics. Bio-inspired responsive materials have been investigated extensively within the past few decades with much success. Yet, the level of control of these complex systems is still rather simplistic. More importantly, we have yet to uncover the design rules to synergize natural and synthetic building blocks that allows us to go beyond just a few specific families of natural building blocks. I am going to discuss our recent studies that demonstrated the feasibility to develop synthetic protein-like polymers that can interface with natural proteins and biomachinaries. Rational design of these protein-like polymers thus opens a viable approach toward functional materials based on natural components.   

Integration of Covalent and Supramolecular Polymers

Supramolecular polymers with their energy-tunable noncovalent bonds among structural units have inherent potential as materials that are reversibly responsive, dynamic, adaptable, or capable of integrating synergistic functions. The possibility of programming interactions in these systems also provides a good platform to design hierarchical structures. Covalent polymers, on the other hand, can provide robust mechanical properties but have only limited capacity to sustain long range order and short time scale dynamics. The integration of covalent and supramolecular polymers offers potential to design soft matter with novel functionality by virtue of the combination of order, rapid dynamics, and high mechanical properties. This lecture will provide examples of such hybrid polymers and some of their properties, including systems generated by simultaneous covalent and supramolecular polymerization, and others in which the covalent and supramolecular phases are formed sequentially.