The secret of how young children can play with yellow rubber bands and make different shapes is their resilience. Shape memory polymers, which have similar properties, show promise for innovative applications in a variety of industries by utilizing their ability to return to their initial shape.
Two young children are playing with a yellow rubber band. Using their tiny fingers to make stars and shields, they are competing to see who can make the most shapes. How can they make so many different shapes with one rubber band? It’s because rubber bands like to return to their original shape. If you stretch it into a star shape and then release it, it returns to its original shape, so you can make a sharpie shape again. In this simple game, we can easily observe the unique elasticity and resilience of rubber bands.
The elasticity and resilience of rubber bands has many useful applications. For example, they’re used in sports equipment, clothing, and even building materials, and thanks to their properties, they have many uses in everyday life. Thanks to their elasticity and resilience, rubber bands are useful in a variety of situations that require strong tensile strength and flexibility. The properties of rubber bands are expanding their use in our daily lives.
One such material that exhibits these same properties is shape memory polymer. Shape memory polymers are polymers that have the ability to change the shape of an object, but when the environment is created under the same conditions as the environment in which the object was initially shaped, the object reverts to its original shape. This technology is opening up innovative possibilities in various fields such as medicine, aerospace, and robotics.
The principle of shape memory polymers can be explained through crosslinking points, which are points that chemically connect polymer chains. When the position of the crosslinks changes due to deformation, they are internally memorized and return to their original shape. This property has great potential, especially in the medical field. For example, stents using shape-memory polymers can be inserted into narrowed blood vessels and then revert to their original shape when heated by the body to widen the vessel.
Let’s dig deeper into the principle. A polymer with an initial shape is deformed and temporarily immobilized by increasing or decreasing heat. When heat is applied above a critical temperature, recovery from the temporary shape occurs and the original shape appears. This is the shape memory effect. The power to recover the deformation comes from the change in entropy, which comes from the elasticity of the polymer. Entropy is simply the degree of disorder. To use an analogy, students during school hours have low entropy because they are in an orderly state, while students during recess are very disorganized and have high entropy. According to the second law of thermodynamics, reactions occur in a way that increases the entropy of the entire universe. The initial polymer is in a state of high entropy because its molecular arrangement is disordered, and deforming it is an unstable reaction in the direction of decreasing entropy because it is ordering the molecular arrangement. Therefore, in this temporarily stationary situation, applying heat creates conditions for entropy to increase, so it will return to its initial shape. This is the principle of shape memory polymers.
The structure of a shape memory polymer is similar to that of a jungle gym or a net. This structure usually comes from the coexistence of fixed (hard) and reversible (soft) parts. The reversible phase is the main part of shape memory polymers and plays an elastic role in deformation and recovery. Above a critical temperature, the reversible phase becomes fluid and can move freely. When deformation is applied to the shape memory polymer, the polymer chains align and the entropy decreases. This unstable state can be maintained by rapidly cooling the polymer under strain. The reversible structural rearrangement of the phase by tugging strain is strictly limited at temperatures below a critical temperature, and recovery of the polymer chains does not occur.
Shape memory polymers are flexible due to their low density and high elasticity, and depending on the nature of the polymer, they can also have properties such as biocompatibility and biodegradability. These properties have led to their use as composites with other materials in many applications, including toys and medical devices. Currently, they are commonly used to restore the original state through temperature, but other conditions such as light and pH are not widely used yet, so they have a bright future.
Shape memory polymers also have the potential to provide innovative solutions in a variety of industries. For example, in robotics, shape memory polymers can be used to create flexible and adaptable artificial muscles. These artificial muscles can enable more natural movements than traditional robotic parts, expanding the range of applications for robots.
