How to Make a Better Polymer
AMHERST, Mass. – New research, recently published in Science Advances, unveils a process for counting the number of strength-enabling entanglements in glassy polymers—which are used widely in an enormous range of applications, from the windows in hyperbaric chambers and airplanes to water filtration elements, the coatings of circuit boards and the membranes used in gas separation. Until now, however, scientists haven’t had a clear molecular-level view of exactly how polymers develop their amazing strength.
“Think of a polymer as a bowl of spaghetti, where lots of long molecules are tangled together,” says Alfred Crosby, professor of polymer science and engineering at the University of Massachusetts Amherst, co-director for the Center for Evolutionary Materials at UMass, and one of the paper’s co-authors. “Sometimes, when you stick your fork in to get a bite, all the spaghetti tangles together and comes out of the bowl at once.” It turns out that a polymer is like a piece of spaghetti, and the longer the molecule, the more it can entangle with other polymer strands. When individual molecules of polymer wrap around each other like pieces of spaghetti, they’re said to be ‘entangled,’ and it’s these entanglements that account for the ultimate strength of the polymer itself.
“But,” says Cynthia Bukowski, graduate student in polymer science and engineering at UMass Amherst, and, along with Tianren Zhang of the University of Pennsylvania, the paper’s co-lead author, there’s a tradeoff. “The stronger and more entangled the polymer, the more energy it requires to shape that polymer into whatever its final form will be.” It takes toxic, volatile solvents or very high heat to shape these stronger polymers, which is costly, both environmentally and economically.
The goal, then, is to figure out the minimum number of entanglements needed for each specific application and to then engineer an efficient polymer that is both strong, cost-effective and easier on the environment. But, until now, researchers have long been stumped in trying to figure out exactly how to count the entanglements that matter.
Bukowski, Crosby, Zhang and Robert Riggleman, professor of chemical and biomolecular engineering at the University of Pennsylvania and a co-author of the paper, devised an innovative pairing of computer-based simulations, which approximate how polymer molecules would interact with each other in real life, with an experimental process that involved creating a polymer film 100 nanometers thick (or 500 times smaller than the width of a human hair) and stretching it until it reached its breaking point. The team then developed a theory that allowed the simulation to accurately predict their experiment results—and when they had this, they knew that that model was close enough to real life that it could allow them to accurately count a polymer’s entanglements.
“This was our breakthrough,” says Crosby. “We could change the number of entanglements in the polymer, and the model would give us an accurate picture of how the polymer’s molecular structure controlled the material’s strength.”
The researchers, who were supported by funding from the National Science Foundation, discovered that not every entanglement actually contributes to the polymer’s final strength and that opportunities exist for precisely tuning the crucial load-bearing entanglements to maintain strength and decrease the energy needed for processing them for a range of applications.
“It’s exciting to finally understand what’s happening on the molecular level,” says Bukowski. “And we hope that our research contributes to the next generation of polymer materials.”