Zhen-Gang Wang, Ph.D. - US grants

9531914 Wang Motivated by the high promise of block copolymers as materials for nanotechnology, and by their intrinsic scientific interest as model systems for self-assembling complex fluids, a systematic program of theoretical investigations will be undertaken on a number of topics involving self-assembled block copolymer nanostructures using statistical mechanical techniques. Some of these topics deal with phenomenologies that are expected to apply to a large class of amphiphilic systems, while others are unique to block copolymers. The thermo-mechanical properties of ordered nanostructures of block copolymers will be studied. By including mechanical stress or strain, a new dimension is added to the study of self-assembling complex fluids. Focus will be on various thermo-mechanical instabilities and phase transitions that occur when an ordered nanostructure is subjected to anisotropic stress or strain. Some of these instabilities and transitions are unique to ordered block copolymer nanostructures and are manifestations of length- scale dependent ordering,i.e., fluid-like behavior on the monomer scale and solid-like on the polymer length-scale. Also to be studied will be kinetic pathways of order- disorder and order-order transitions in diblock copolymers using a time-dependent Ginzburg-Landau approach. Focus will be on possible non-trivial intermediate states during the various transitions after a sudden change in the system parameters. Of special interest will be the existence of the so-called pseudo-stable states which correspond to saddle points on the free energy surfaces. These states will be explored both dynamically and in equilibrium. Finally, the phase behavior and nanostructures of ABC triblock copolymers will be studied using self-consistent field techniques. In contrast to the better studied AB diblocks, a fundamentally new feature here is that the morphological structures and phase behaviors depend not only on the fraction of ea ch block and the overall molecular weight, but also crucially on the sequence of the blocks in the chain and on the relative strengths of the interactions between the three unlike pairs of blocks. %%% This new award is to support theoretical research on nanostructure and phase transitions in block copolymers. The research particularly looks at the behavior of this important class of polymers when subjected to mechanical forces. In addition to providing insight into fundamental issues in polymer physics, the results will also be useful in applications of this class of polymers for practical use. ***

9970589
Wang
This grant supports theoretical research on the statistical mechanics of polymers. The use of flexible chain models for polymers, e.g., the freely-jointed chain model, the bead-string or its continuous version of the gaussian model, have contributed considerably to our current understanding of the dynamic and thermodynamic properties of polymers. Many polymer systems, especially biopolymers such as DNA and actin filaments, and main-chain liquid-crystalline polymers, require that the stiffness of the polymer chain be taken into account explicitly. A number of fundamental issues will be addressed in the static and dynamic properties of semiflexible polymers. These include: (1) understanding the single chain dynamics of a semiflexible polymer; (2) understanding the conformation and dynamics of a semiflexible chain in a nematic field, with a focus on multiple hairpins; and, (3) understanding the phase behavior, fluctuations and instabilities in nematic semiflexible polymer solutions.

While the main focus of this work is understanding of the fundamental physics of semiflexible polymers, the results will also be highly relevant in engineering applications. For example, the study of chain dynamics includes calculating the stress of a polymer under various steady flow conditions and the stress relaxation for a deformed chain. These results will enable us to predict the constitutive behavior of polymer melts and solutions in steady flows as well as their viscoelastic properties. Likewise, one of the problems to be addressed in this research, namely the banding instability in sheared multi-chain liquid crystals, has important consequences in the processing of these materials such as in fiber drawing and injection molding.
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This grant supports theoretical research on the statistical mechanics of polymers. The use of flexible chain models for polymers, e.g., the freely-jointed chain model, the bead-string or its continuous version of the gaussian model, have contributed considerably to our current understanding of the dynamic and thermodynamic properties of polymers. Many polymer systems, especially biopolymers such as DNA and actin filaments, and main-chain liquid-crystalline polymers, require that the stiffness of the polymer chain be taken into account explicitly. A number of fundamental issues will be addressed in the static and dynamic properties of semiflexible polymers.

While the main focus of this work is understanding of the fundamental physics of semiflexible polymers, the results will also be highly relevant in engineering applications. For example, the study of chain dynamics includes calculating the stress of a polymer under various steady flow conditions and the stress relaxation for a deformed chain. These results will enable us to predict the constitutive behavior of polymer melts and solutions in steady flows as well as their viscoelastic properties. Likewise, one of the problems to be addressed in this research, namely the banding instability in sheared multi-chain liquid crystals, has important consequences in the processing of these materials such as in fiber drawing and injection molding.
***

0965812
Wang

Intellectual Merit

This energy-and manufacturing related project is a collaborative experimental and theoretical study of the effect of morphology on electron and ion conduction in nanostructured polymer materials with clearly defined independent channels for electronic and ionic transport. Regio-regular poly(3-hexylthiophene)-block-polyethyleneoxide (PHT-PEO) will be synthesized by coupling aldehyde terminated PHT chains with living styryl-PEO anions, and doped with the appropriate salts to make the PHT domain electron-conducting and the PEO domain ion-conducting. The morphology of the mixtures and charge carrier distribution will be characterized by standard techniques such as electron microscopy and X-ray scattering, as well as element-specific techniques such as energy filtered EM and resonant soft X-ray scattering. A combination of DC- and AC-impedance spectroscopy will be used to measure the ionic and electronic conductance of the doped copolymer. Concurrently with the experimental efforts, theoretical and simulation studies will be performed to understand the underpinnings of the experimental observations regarding morphology and dopant distribution, and to provide insight for designing second generation systems with optimal properties. In particular, a ribbon-coil model will be developed to predict the morphology of PHT-PEO systems. Theories that incorporate both ion solvation and chain deformation will be used to predict dopant distribution. Computer simulations used to predict ion transport will be validated using experimental measurements.

This work will be the first study of the simultaneous electronic and ionic transport in nanostructured polymer materials. The combined experimental and theoretical efforts will yield rich insights into: how charge carries are distributed in nanostructured materials, how the motion of charge carriers couples to the segmental dynamics of the polymers, how the local nanostructure and large-scale grain structure influences charge transport, and how doping agents alter the morphology of the self-assembled polymeric structures. These insights may lead to entirely new design strategies for electrode architectures in rechargeable batteries and fuel cells.

Broader Impacts

The research is in sync with the nationwide efforts at creating and ultimately manufacturing clean and more efficient energy technologies. The systems studied have potential to directly translate into new battery technologies. Furthermore, in both PIs? home departments,there is an increasing need among the graduate students to work in energy-related research areas; the projects fulfill that need by providing them with the opportunity to do research in a technologically important area, while receiving a multidisciplinary training in theory, simulation, modeling, thermodynamics, synthesis and characterization of polymers, optics, scattering, and electrochemistry. Equally important, the proposed research serves as a platform for developing new educational packages for high school and undergraduate students. In this respect, the PIs will develop and execute lectures and demonstrations on electrochemistry and batteries as part of the Math and Science Summer Academy program at Berkeley

With this award from the Major Research Instrumentation (MRI) program and the Chemistry Division, Professor Thomas F. Miller and colleagues John F. Brady, William A. Goddard, Zhen-Gang Wang and Niles A. Pierce from the California Institute of Technology will acquire a computer cluster with graphical processing units. The proposal will enhance research in a variety of areas characterized as soft matter behavior/simulations. The projects include investigations aimed at the rational design of nucleic acid, protein and enzyme systems, conformational dynamics of proteins and molecular motors, enzyme-catalyzed electron-transfer and hydrogen-transfer dynamics, trans-membrane signaling and transport processes, the nucleation of membrane adhesion, protein secretion across a cellular membrane, the formation of gels, the dynamics of ring-polymer mixtures, and polymer-based tissue engineering.

A computer cluster is a group of linked processors that work in concert to achieve vastly more computational power that individual computers. These are employed to investigate complex problems using computational methods based on theoretical models and programs. Such calculations, often used in conjunction with experimental data, allow chemists and biochemists to better understand many types of complex chemical and biological phenomenon. This resource will be used by students and faculty to develop the use of computer clusters based on graphical processing units (GPUs) rather than CPUs. This approach can speed up calculations and simulations enabling larger, more complex systems to be investigated.

Thomas Miller and Zhen-Gang Wang of the California Institute of Technology, Nitash Balsara of the University of California - Berkeley, and Geoffrey Coates of Cornell University are supported by an award from the Chemistry Designing Materials to Revolutionize and Engineer our Future (DMREF) program in the Chemistry division for a collaborative research effort aimed at advancing the development and discovery of high-performance, non-flammable, solid polymer electrolytes for rechargeable lithium batteries. The project integrates expertise in theory and simulation of materials, materials synthesis, and the characterization of polymer electrolyte materials. The project involves a tightly coupled research approach in which state-of-the-art theoretical and coarse-graining methods drive the screening and design of new polymer electrolytes, as well as the detailed understanding of ion diffusion mechanisms; promising polymer electrolyte candidates are synthesized in the laboratory, and their ion transport characteristics are measured and tested in full cells.

This research effort is providing fundamental and essential knowledge to enable the development of battery technologies that are critical for transportation and other large-scale energy storage applications. The development and deployment of safe, low-cost, high-performance batteries is of great societal need for affordable and reliable energy storage. A key strength of the research effort is the tight integration and synergy between simulation, theory, synthesis, characterization, and cell assembly. Guidance from experimentally verified theoretical constructs allows the focused design of new polymer electrolytes. Synthesis of these materials, preselected using theory, allows for more rapid production of materials with superior properties. Measurement of material properties is then used to validate and further guide theory and modeling. Pursuit of this integrated research effort will lead to a significant progress towards the goal of rational materials design for polymer systems.