Weak bonds, strong material!
Achieving a desirable combination of solid-like properties and fast self-healing is a great challenge due to slow diffusion dynamics. In this work, we describe a design concept that utilizes weak but abundant coordination bonds to achieve this objective. The designed PDMS polymer, crosslinked by abundant Zn(II)-carboxylate interactions, is very strong and rigid at room temperature. As the coordination equilibrium is sensitive to temperature, the mechanical strength of this polymer rapidly and reversibly changes upon heating or cooling. Based on these features, this polymer not only exhibits fast thermal-healing properties, but is also advantageous for applications in orthopedic immobilization.
The paper "A Rigid and Healable Polymer Cross-linked by Weak but Abundant Zn(II)-Carboxylate Interactions" in Nature Communications is here:
Self-healing abilities are an important survival feature in nature, as these properties allow living beings to repair damage when wounded. The self-healing abilities of living beings have inspired scientists to invent various methods for restoring the functionality of damaged materials. However, for most self-healing materials, there is often a trade-off between the mechanical properties and the dynamic healing; strong bonds result in mechanically robust but less dynamic systems, precluding autonomous healing, while weak bonds afford dynamic healing, but yield relatively soft materials. Therefore, it is quite challenging to achieve self-healing in strong and solid-like materials.
Nature has given us hints to solve this conundrum. Hydrogen bonds are considerably weaker than other interactions, but these weak interactions can form very strong materials in some situations. For example, chitin, which consists of polysaccharides (sugars) assembled through extensive hydrogen bonding, has amazing high mechanical strength and serves as a protective shell (such as lobsters claws, beetle carapaces, and tree bark) for living organism. As another example, at low temperature, water molecules aggregate into solid ice via ordered hydrogen bonding. The compressive strength of ice can be up to 5–25 MPa over the temperature range −10 °C to −20 °C.
Inspired by nature, we describe in this paper a new design concept that utilizes weak but abundant coordination bonds to achieve rigid and healable materials. The coordination bonds used in our study are weak but still significantly stronger than hydrogen bonds. Therefore, the resulting polymer is very strong (with flexural Young's modulus as high as 480 MPa) and rigid (with an elongation at break smaller than 4%) at room temperature. The coordination equilibrium is sensitive to temperature; thus, the mechanical strength of our polymer exhibits distinct (as high as almost 4 orders of magnitude in a narrow temperature range (∆T < 100 °C)), fast (within tens of seconds) and reversible change upon heating or cooling.
Our polymer is useful in various situations. For example, due to its rapid softening and hardening property, our polymer can be used in orthopedic immobilization to replace traditional plaster casting, and it also has the advantages of being lightweight, removable and recyclable. Our polymer can also be used for 3D printing since it turns into a viscous liquid upon heating to 120 °C and quickly forms a rigid solid upon cooling. With its thermal healing properties, objects made of our polymer using 3D printing can be healed when damaged. We can also obtain large or complex objects with only a small 3D printer by taking advantage of the healing processes of this material. Thus we can combine the advantages of modern 3D-printing processes and traditional brick-and-mortar operation using our materials. Moreover, our polymer can be used to prepare conductive composites/adhesives that are reshapable, healable, and 3D printable.
In 11-Apr-2018 to 15-Apr-2018, we exhibited our invention of “A Rapid Shaping Synthetic Polymer for Use in Orthopedic Immobilization” in the 46th International Exhibition of Inventions in Geneva. Our invention was highly appreciated by the visitors and Judges. Above is a video demonstrating our invention.