The National Institute for Computational Sciences

Polymer Potential

Fundamental Research Could Lead to Product Innovations

Brad Olsen

Muzhou “Mitchell” Wang

Among the most ubiquitous types of substance in the modern world, polymers are in many things we take for granted every day. These chemical compounds are used to make plastic, rubber, and more, but beyond that, with further understanding of their molecular dynamics, modified versions of them could create innovations in electronics, clothing, auto parts, and other products.

"One of the exciting things about polymers is that their structure can be controlled at the nanoscale, which is necessary for making more efficient organic electronic materials, for harvesting light in photovoltaics, for example," says Muzhou "Mitchell" Wang, who investigated polymers as a graduate researcher at the Massachusetts Institute of Technology (MIT). "But before we can use these polymers as electronic materials, we need to understand how to effectively process them so that they have the correct structure and properties, and so we need to know about dynamic behavior such as the way their molecules move and how fast [diffusion], and their response to an external deformation [rheology], or how flexible they are."

Wang and Brad Olsen, project principal investigator and assistant professor in the Department of Chemical Engineering at MIT, in collaboration with Alexei Likhtman, professor of mathematical physics at the University of Reading in the U.K., performed fundamental research on what are called rod–coil block copolymers, polymers with characteristics that make them good study models because not only are they are a class of organic electronics, they also have molecules that are part rigid and part flexible. Polymers similar to these might be used to make electronics that are lighter and more compact, or so flexible they could be rolled up; clothes that have stronger fibers; plastic automotive parts that are lighter and yet very durable; and plastic materials mostly composed of enzymes that could catalyze chemical reactions, explains Olsen.

"A nice analogy for polymers at the molecular level is to think of them as long strands of spaghetti that entangle each other," says Wang. "This is particularly relevant to our study, which concerns the difficulty that rod–coil block copolymers have moving relative to each other when they are entangled."

Like spaghetti, entangled polymers move more easily along their own shape. "If they were to move perpendicular to themselves, they would run into the surrounding chains," says Wang. "One approximation for this motion is to think of the polymer being confined in a tube; this is a common approach to addressing the problem because it's difficult to consider everything moving simultaneously. One of the key conclusions of our work is that the rigid rod doesn't like to move into a curved section of the tube, so a polymer that contains a rod moves more slowly than one that does not contain a rod."

Confined to a tube to approximate the spaghetti-like motion of entangled polymers, a rod–coil block copolymer (rod in blue, coils in pink) moves through a sea of molecules. Flow is impeded when a molecule with a rod passes through a curved portion of the tube (denoted in red). [Image credit: Muzhou “Mitchell” Wang and Christopher N. Lam, Massachusetts Institute of Technology.]

Details of the project are contained in the paper "Tube Curvature Slows the Motion of Rod–Coil Block Copolymers through Activated Reptation," published on January 21, 2015, in ACS Macro Letters.

"The computing time was really critical to this study and several previous papers," says Olsen. "Without it, we would not have been able to complete any of the studies."

Compute allocations from the National Science Foundation's eXtreme Science and Engineering Discovery Environment (XSEDE), allowed the researchers to use the Kraken (now decommissioned) and Darter supercomputers at NICS, and Gordon at the San Diego Supercomputer Center (SDSC).

Olsen touts the benefits of XSEDE from his perspective in this way: "The opportunity for anyone with a good idea to write a proposal and get computation time is something that is absolutely critical for our scientific community. XSEDE is essential to helping the best ideas get out, even if they are not coming from groups that are dedicated to simulation and, therefore, have large computational resources in house."

Wang has an appreciation for the user interface and other aspects of XSEDE, as well. "In particular, it [the user interface] seemed easy to transfer computational time from resource to resource, or from project to project," he says. "The online Globus tool for transferring data was also especially useful. The policies on requesting short-term extensions were generous and flexible. Finally, I found that User Assistance, through, was very responsive and helpful."

The group has a follow-on manuscript to the rod–coil block copolymer diffusion project in progress and hopes to apply the knowledge gained from it to the design of new polymer systems with desired rheological properties, says Olsen.

Olsen reports another positive outcome, as well: "This project has launched a really wonderful collaboration with Alexei Likhtman, and we are looking forward to continuing the research together."

[Writer's note: Wang is now a postdoc at the National Institute of Standards and Technology, NIST.]

Scott Gibson, science writer, NICS, JICS

Article posting date: 3 August 2015

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 leading-edge 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 to be well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS operates the National Institute for Computational Sciences, NICS, one of the nation's leading advanced computing centers. NICS is co-located on the UT Knoxville campus and ORNL, home of the world's most powerful computing complex. The center's mission is to expand the boundaries of human understanding while ensuring the United States' continued leadership in science, technology, engineering, and mathematics. NICS is a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment (XSEDE).