Engineers have developed a new class of rubber-like material that not only self-stretches upon cooling but also reverts back to its original shape when heated - all without physical manipulation. The material is like a shape-memory polymer because it can be switched between two different shapes, but unlike other shape-memory polymers, the material does not need to be programmed each cycle - it repeatedly switches shapes with no external forces, simply upon cooling and heating. Mitchell Anthamatten, associate professor of chemical engineering at the University of Rochester and his team built on the success of a recently developed polymer that can also stretch when cooled.
Other polymers need to have small weights attached in order to direct the shape to be taken. But the polymer developed at Rochester has been "tricked it into thinking" a load was attached. The researchers introduced permanent stress inside the material."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," Anthamatten noted. 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.
The team showed that internal crystallization forces are strong enough to stretch the material along one direction. Once cooled below about 50 degree Celcius, polymer chain segments pack into highly ordered micro-layers called lamellae. This reorganization occurs within a network of polymer chains, causing the material's length to increase by over 15%. The stress 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. After multiple cycles of cooling and heating, the team found that the material assumed its programmed shape and returned to its initial state with no noticeable deviation. The team 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. "The next step is to optimize the shape of the polymer material and the energy released during the process," said Anthamatten. "That will be done by adjusting the type and density of crosslinks that tie the individual chains together." The findings were recently published in the journal ACS Macro Letters.
Shape memory polymers are "smart" plastic materials that can be transformed into a temporary shape and then return to their original shape, triggered by an external stimulus such as heat or pressure. These polymers can be programmed to create highly complex shapes-coils, knots, even origami-like shapes- but the transformations have been conventionally irreversible, as per phys.org. In recent years, reversible shape memory has been enabled in certain polymers, but achieving shape switching across broad classes of polymers has been elusive. Usually, switching from one shape to another requires a persistent external mechanical force. Now, researchers have turned to elastomers - polymers with elasticity and viscosity - with carefully designed chemical structures.
A team of scientists from the University of North Carolina (UNC) at Chapel Hill, the University of Connecticut, and the US Department of Energy's Brookhaven National Laboratory, led by Sergei Sheiko (UNC), has devised a general method for enabling reversible shifting between programmable shapes. They used semicrystalline elastomers with uniform chemical compositions of various kinds and achieved shape transformations without applying a persistent external force. Their work was described in the journal Macromolecules. "We're uniquely able to achieve reversible transformations, not just one-way shape changes from one state to another, by taking advantage of interactions between crystalline polymer domains and chemical network of polymers," said Oleg Gang, group leader for Soft and Bio NanoMaterials at Brookhaven's Center for Functional Nanomaterials. Gang and his collaborators fabricated elastomers, twisted them into coiled shapes at 140 degree F and then cooled them to about 40 degree F. Once reheated, to 100 degree F, the straight elastomers gain took on the coiled shape spontaneously, without any applied force. The angles of the coil were repeatedly reproduced, even after many heating and cooling cycles. Similar reversible behavior was shown in a straight polymer bending to a predetermined angle and an origami-like star or gripper folding and unfolding on its own. Gang used small-angle x-ray scattering (SAXS) at Brookhaven's Center for Functional Nanomaterials and on beamline X9 at the National Synchrotron Light Source to determine the nanoscale structural changes in the polymers during shapeshifting and the molecular mechanism that causes the reversibility. "We are only beginning to understand the molecular, nano- and meso-scale effects in this material. Many structural changes are happening on various scales simultaneously, and it is not fully clear which phenomenon dominates," Gang said. "The complexity of these materials arises from the interplay of a scaffold of small crystallites and a chemically cross-linked polymer network, which makes the study more challenging but also more stimulating. We are excited about new possibilities that NSLS-II will provide, since it will address our currently limited ability to probe structure and dynamics at multiple scales in real time when macroscopic transformations occur or mechanical stress is applied."
He explained that shape memory materials are vital for minimally invasive surgery, hands-free packaging, the aerospace industry and microrobotics. Reversible shape memory polymers are particularly attractive for these fields, as they allow for highly complex shape transformations on broad length scales, and in response to a vast array of external stimuli – mainly heat, but also light, electro-magnetic fields, and even acoustic waves.