A University of Rochester team has fabricated a material that self-stretches as it cools. The rubber-like polymer reverts back to its original shape when heated.
After multiple cycles of cooling and heating, the material, similar to a shape-memory polymer, switches between two different shapes. Unlike other shape-memory polymers, however, the material does not need to be programmed each cycle.
To carry out their strategy, the researchers introduced permanent stress inside the material. Polymer strands were loosely connected by bonds called crosslinks. To achieve the desired shape, the material was stretched with a load. The team then added more crosslinks and cooled the polymer, causing crystallization to occur along a preferred direction.
Mitchell Anthamatten, an associate professor of chemical engineering, and his team demonstrated that internal crystallization forces are strong enough to stretch the material along one direction.
Once cooled below about 50 °C, polymer chain segments pack into highly ordered micro-layers called lamellae. The reorganization occurs within a network of polymer chains, causing the material’s length to increase by over 15 percent.
“The stress we built into the network takes the place of the load and enables the material to ‘remember’ the shape it will assume when it’s later cooled without a load,” said Anthamatten.
The researchers envisions the material being applied to a number of areas in which reversible shape-changes are needed during operations, including biotechnology, artificial muscles, and robotics.
