By tweaking the chemistry of a single polymer, researchers have created a family of synthetic materials that ranges in texture from ultra-soft to extremely rigid. The materials are 3D-printable, self-healing, recyclable, and naturally adhere to each other in air or underwater. These characteristics make them suited for more realistic prosthetics and soft robotics as well as broad military applications such as agile platforms for air vehicles and futuristic, self-healing aircraft wings.
Synthetic polymers are made up of long strings of repeating molecular motifs, like beads on a chain. In elastomeric polymers, or elastomers, these long chains are lightly crosslinked, giving the materials a rubbery quality; however, these crosslinks can also be used to make the elastomers more rigid by increasing the number of crosslinks. Although previous studies have manipulated the density of crosslinks to make elastomers stiffer, the resulting change in mechanical strength was generally permanent.
Crosslinks are like stitches in a piece of cloth — the more stitches, the stiffer the material gets and vice versa. But instead of having the stitches be permanent, the researchers sought to achieve dynamic and reversible crosslinking to create materials that are recyclable. The team focused their attention on the molecules involved in the crosslinking. First, they chose a parent polymer, called prepolymer, and then chemically studded these prepolymer chains with two types of small crosslinking molecules — furan and maleimide. By increasing the number of these molecules in the prepolymer, they found that they could create stiffer materials. In this way, the hardest material created was 1,000 times stronger than the softest.
Furan and maleimide participate in a type of reversible chemical bonding. In this reaction, furan and maleimide pairs can “click” and “unclick,” depending on temperature. When the temperature is high enough, these molecules come apart from the polymer chains and the materials soften. At room temperature, the materials harden since the molecules quickly click back together, once again forming crosslinks. Thus, if there is any tear in these materials at ambient temperatures, the researchers showed that furan and maleimide automatically re-click, healing the gap within a few seconds.
The temperatures at which the crosslinkers dissociate or unclick from the prepolymer chains are relatively the same for different stiffness levels. This property is useful for 3D printing — regardless of whether they are soft or hard, the materials can be melted at the same temperature and then used as printing ink. By modifying the hardware and processing parameters in a standard 3D printer, they were able to use the materials to print complex 3D objects layer by layer.
As the 3D part cools to room temperature, the different layers join seamlessly, precluding the need for curing or any other chemical processing. Consequently, the 3D-printed parts can easily be melted using high heat and then recycled as printing ink. The researchers also noted that their materials are reprogrammable; after being set into one shape, they can be made to change into a different shape using just heat. In the future, the researchers plan to increase the functionality of the new materials by amplifying the materials’ multifaceted properties.