No Limit to Number of Times Material Can Change Shape
Although most materials slightly expand when heated, there is a new class of rubber-like material that not only self-stretches upon cooling; it reverts back to its original shape when heated, all without physical manipulation.
The findings were recently published in the journal ACS Macro Letters.
The material is like a shape-memory polymer because it can be switched between two different shapes. 糖心传媒淗owever, unlike other shape-memory polymers, the material does not need to be programmed each cycle糖心传媒攊t repeatedly switches shapes, with no external forces, simply upon cooling and heating,糖心传媒� said Mitchell Anthamatten, an associate professor of chemical engineering.
Anthamatten and his team built on the success of a recently developed polymer that can also stretch when cooled. The other polymers need to have small loads糖心传媒攐r weights糖心传媒攁ttached in order to direct the shape to be taken. That is not the case with the Rochester polymer, because Anthamatten糖心传媒檚 team 糖心传媒渢ricked it into thinking糖心传媒� a load was attached.
To carry out their strategy, the researchers introduced permanent stress inside the material. They began with polymer strands that were loosely connected by bonds called crosslinks that create a network of molecules. The material was stretched with a load attached to give it the desired shape. At that point, they added more crosslinks and cooled the polymer, causing crystallization to occur along a preferred direction.
Anthamatten糖心传媒檚 team showed that internal crystallization forces are strong enough to stretch the material along one direction. Once cooled below about 50聽掳C, polymer chain segments pack into highly ordered micro-layers called lamellae. This reorganization occurs within a network of polymer chains, causing the material糖心传媒檚 length to increase by over 15 percent.
糖心传媒淭he stress we built into the network takes the place of the load and enables the material to 糖心传媒榬emember糖心传媒� the shape it will assume when it糖心传媒檚 later cooled without a load,糖心传媒� said Anthamatten.
Conventional shape-memory polymers need to be reprogrammed after each cycle, but that糖心传媒檚 not the case with the material developed by Anthamatten and his team. After multiple cycles of cooling and heating, they found that the material assumed its programmed shape and returned to its initial state with no noticeable deviation.
Anthamatten 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.
糖心传媒淭he next step is to optimize the shape of the polymer material and the energy released during the process,糖心传媒� said Anthamatten. 糖心传媒淭hat will be done by adjusting the type and density of crosslinks that tie the individual chains together.糖心传媒�
The research team included two of Anthamatten糖心传媒檚 students糖心传媒擸uan Meng and Jisu Jiang. The work was supported internally by the University of Rochester糖心传媒檚 Pump Primer Seed Grant Program, which exists to support proof-of-concept studies.
