Designing new materials depends upon understanding the
properties of today’s materials. One such material, Nafion, is a polymer that
efficiently conducts ions and water through its nanostructure, making it
important for many energy-related industrial applications, including in fuel
cells, organic batteries, and reverse-osmosis water purification. But since
Nafion was invented 50 years ago, scientists have only been able to speculate
about how to build new materials because they have not been able to see details
on how the molecules come together and work within Nafion.
Now, two Virginia Tech research groups have combined forces
to devise a way to measure Nafion’s internal structure and, in the process,
have discovered how to manipulate this structure to enhance the material’s
applications.
The research is published in Nature Materials in the Letters article,
“Linear coupling of alignment with transport in a polymer electrolyte
membrane,” by Jing Li, Jong Keun Park, Robert B. Moore, and Louis A.
Madsen, all with the chemistry department in the College of Science and the
Macromolecules and Interfaces Institute at Virginia Tech.
Nafion is made up of molecules that combine the non-stick
and tough nature of Teflon with the conductive properties of an acid, such as
battery acid. A network of tiny channels, nanometers in size, carries water or
ions quickly through the polymer. “But, due to the irregular structure of
Nafion, scientists have not been able to get reliable information about its
properties using most standard analysis tools, such as transmission electron
microscopy,” said Madsen, assistant professor of physical, polymer, and
materials chemistry.
Madsen and Moore, professor of physical and polymer
chemistry; Madsen’s post-doctoral associate Jing Li; and Moore’s PhD student
Jong Keun Park, of Korea, were able to use nuclear magnetic resonance (NMR)to
measure molecular motion, and a combination of NMR and x-ray scattering to
measure molecular alignment within Nafion. “We were looking at water
molecules inside Nafion as internal reporters of structure and efficiency of
conduction,” said Madsen. “The new feature we discovered is the
locally aligned aggregates of polymer molecules in the material. The molecules
align like strands of dry spaghetti lined up in a box. We can measure the speed
(diffusion) of the water molecules and the direction they travel within those
structures, which relates strongly to the alignment of the polymer molecule
strands.”
The researchers observed that the alignment of the channels
influenced the speed and preferential direction of water motion. And a
startlingly clear picture presented itself when the scientists stretched the
Nafion and measured its structure and water motion.
“Stretching drastically influences the degree of
alignment,” said Madsen. “So the molecules move faster along the
direction of the stretch, and in a very predictable way. These materials
actually share some properties with liquid crystals—molecules that line up with
each other and are used in every LCD television, projector, and screen.”
These relationships have not been previously recognized in a
polymer electrolyte, Madsen said.
The ability to observe motion and direction, and understand
what is happening within Nafion, has implications for using the material in new
ways, and for designing new materials, the researchers write in the Nature Materials article. Ion-based
applications could include actuator devices such as artificial muscles, organic
batteries, and more energy efficient fuel cells. A water-based application
would be improved reverse osmosis membranes for water purification.
Madsen and Moore started this collaborative project shortly
after they arrived at Virginia Tech (Madsen in 2006, Moore in 2007), and they
are furthering their work together by investigating new polymeric materials
using their unique combination of analysis techniques.
“Alignment provides for a better flow of the molecules
through the polymer,” Madsen said.
