These images illustrate how the polymer hand changes shape depending on the glass transition temperature of the individual layers. Image: Autonomous Materials Systems Group.
These images illustrate how the polymer hand changes shape depending on the glass transition temperature of the individual layers. Image: Autonomous Materials Systems Group.

Researchers from the University of Illinois at Urbana-Champaign have improved the technique of frontal polymerization, where a small amount of heat triggers a moving reaction wave that produces a polymeric material. Their new method allows the creation of a wider range of materials with better control over their thermal and mechanical properties. The researchers report their new method in a paper in ACS Macro Letters.

"Most of the previous research looked at stiffer materials. This paper is the first time frontal polymerization has been used to synthesize a rubbery material," said Nancy Sottos, head of the Department of Materials Science and Engineering and leader of the Autonomous Materials Systems (AMS) Group at the Beckman Institute for Advanced Science and Technology. "The new technique allows us to have more control and makes materials that have good engineering properties in terms of strength and stiffness."

The researchers used a mixture of two monomers, 1,5-cyclooctadiene and dicyclopentadiene, to create materials tailored for a wide range of applications.

"These materials are chemically similar to what is used in tires," said Leon Dean, a graduate student at AMS. "Conventionally, the synthesis of rubbers requires an organic solvent, multiple steps and a lot of energy, which is not environmentally friendly. Our solvent-free manufacturing method speeds up the process and reduces energy consumption."

Using this new technique, the researchers were able to make materials for producing a shape-memory polymer hand. The shape-memory effect occurs when a pre-deformed polymer is heated beyond its glass transition temperature, which is the point at which the polymer changes from a hard, glassy material to a soft, rubbery material. The sequential change in shape was driven by the differences in glass transition temperature between each layer making up the polymer hand.

"We made a layered material in the shape of a hand, where each layer had different amounts of the two monomers and therefore different glass transition temperatures," explained Qiong Wu, a postdoctoral fellow at AMS. "When you heat the polymer above the highest glass transition temperature and then cool it, it forms a fist. As you raise the temperature again, the digits of the fist open sequentially."

The researchers hope to further develop this technique by improving their control over the polymer properties. "Although we have demonstrated the tunability of several properties over a wide range, it remains a challenge to adjust each property individually," Wu said.

"Scaling up the technique will also be a challenge," Dean added. "Most of our work has been done on a lab scale. However, in larger scale manufacturing, there is a competition between bulk polymerization and frontal polymerization."

"This study demonstrates the Beckman Institute at its best," said Jeff Moore, a professor of chemistry and the director of the Beckman Institute. "It brought together two groups that have different perspectives on a problem, but share a common goal."

This story is adapted from material from the University of Illinois at Urbana-Champaign, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.