The National Institute for Computational Sciences

Polyelectrolytes and Sustainable Energy

The Darter Supercomputer Allows Investigation of the Physics of Versatile Macromolecules

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On her morning commute, a woman checks her gas gauge and notes that she'll have to fill up soon. As she drives down the road, she notices a coffee shop and decides a hot beverage would be a perfect start to the day. Soon a styrofoam cup full of steaming hot coffee is sitting in the cup holder. Already she's had to stop for at least three traffic lights, and she'll likely encounter one or two more before she gets to work. While everyone's day is a little different, we all rely on myriad resources along the way, and many of them are nonrenewable.

Even in the short window of time in this person's day, she used gasoline to power her car, drank from a cup made of oil-based plastic pellets that she'll probably use only once, and stopped at lights run by electricity, which is often produced by coal or oil. We are surrounded by nonrenewable resources and depend on them every day.

"One of the greatest challenges facing us in the 21st century is sustainable energy," says Bamin Khomami, a professor and researcher at the University of Tennessee, Knoxville. "To meet the quality-of-life and economic demands, we need to provide sustainable energy sources to both developed and developing nations. The need for addressing this energy challenge is greater than any recent time. To this end, a multifaceted approach to developing sustainable energy technologies is required. Polymer electrolytes [polyelectrolytes] play a significant role in many such technologies, including energy conversion and storage."

Khomami currently has a variety of projects running on the supercomputer Darter at the National Institute for Computational Sciences (NICS). He is modeling the behavior of materials composed of minuscule-based units to better understand their structural properties and interaction tendencies.

Although Khomami's projects are investigating a wide range of topics in macromolecular systems, "there is a central theme of using novel multiscale modeling and simulation strategies to elucidate important fundamental phenomena and predict system or process-level features that are integral to the discovery, design, development, and processing of a wide range of nano- and micro-structured materials."

The Importance of Polyelectrolytes

Polyelectrolytes perform indispensable roles in biological environments and emerging technologies, and so polyelectrolyte research, along with the investigation of other sustainable energy sources, is essential.

The four nonrenewable energy sources we use most often in the U.S. are oil and petroleum products (such as gasoline, propane, and diesel fuel), natural gas, coal, and uranium in nuclear power production. As the term nonrenewable implies, those materials can't be rapidly replenished and could run out, compromising the ability of future generations to meet their energy needs.

Beyond the energy sources, polyelectrolytes are found in DNA, protein, disposable diapers, and high-range water reducers (superplasticizers) found in cement, to name just some of the items.

With respect to the science behind polyelectrolytes, they are molecules that consist of a large number of covalently linked ionizable subunits. A covalent bond is a type of chemical bond that involves atoms sharing pairs of electrons. Ionizable indicates that polyelectrolytes have the capability of being broken down into the individual ions or atoms of which they are composed.

Polyelectrolyte chains should be studied, Khomami says, because "they have broad applications in many sustainable energy technologies, including batteries, fuel cells, super capacitors, and even photoelectrochemical and electrochromic devices." But since a comprehensive picture of chain behavior doesn't exist, he plans to shed light on those interactions.

A project Khomami is working on creates a simulation of the dilute solution of the screened flexible polyelectrolyte chains. These simulations are computationally intensive "since both the number of macromolecules in the solution and the number of statistical segments per macromolecule must be large in order to recover the micro-mechanical response of the flexible [polyelectrolyte] chain accurately."

Darter's Vital Contribution to the Investigation

Khomami says that the large size of the solution needed to capture the underlying physics calls for the use of high-performance computing and reliable extended run times, and he adds that communication between computer processor cores must be efficient for message-passing interface (MPI) and OpenMPI parallelism. Supercomputers such as Darter make complicated computationally dependent projects possible, Khomami adds.

He explains that most of the previous approaches of showing chain behavior are based on weak intermolecular forces and excluded volume. Excluded volume is defined as the volume that is inaccessible to other molecules in the system due to the presence of that first molecule—the idea being that one part of a long chain molecule cannot occupy space that is already taken up by another part of the same molecule. Khomami has extended this approach to use equations to describe and map the motion of the molecules.

In his research, Khomami is looking at an assortment of chain segments to understand and map the movement and interactions of polyelectrolytes. They are broken up into as small of workable pieces as possible, but to get accurate data responses they have to be long chains.

Because only long chains can accurately depict the micro-mechanical responses of chains with extended volume, Darter's high computational abilities are vital to this project, Khomami explains. Darter, he says, has provided the necessary simulation clarity and enabled the investigation to be successful in developing a new "entropic force law for the coarse-grained model [concerning large subcomponents] of polymeric chains."

Details of Khomami's project will be provided in a paper that is soon to be submitted to the journal Macromolecules.

Research such as this contributes to the body of knowledge that will inform the development of better ways to store and use energy. In the meantime, society can take advantage of strides that have been made to allow the use of fewer nonrenewable resources.

In the early light of the morning another woman heads to work. Her car is a hybrid, which allows her to travel more miles per gallon. The city she lives in is a Green Power Community, so the traffic lights she encounters on her commute are powered by a variety of renewable resources, such as wind and solar power. She stops at a local cafe for a steaming cup of coffee. But instead of using a disposable cup from the cafe, she has them fill her reusable travel mug.

Jennifer Bailey, science writer, NICS, JICS

Article posting date: 9 September 2014

About JICS and NICS: The Joint Institute for Computational Sciences (JICS) was established by the University of Tennessee and Oak Ridge National Laboratory (ORNL) to advance scientific discovery and state-of-the-art engineering, and to further knowledge of computational modeling and simulation. JICS realizes its vision by taking full advantage of petascale-and-beyond computers housed at ORNL and by educating a new generation of scientists and engineers well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS runs the National Institute for Computational Sciences (NICS), which had the distinction of deploying and managing the Kraken supercomputer. NICS is a leading academic supercomputing center and a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment, known as XSEDE. In November 2012, JICS sited the Beacon system, which set a record for power efficiency and captured the number one position on the Green500 list of the most energy-efficient computers.