Stretchable, flexible and recyclable: 3D printing creates amazing plastics

Princeton engineers have developed an easily scalable 3D printing technology to produce soft plastics with programmed stretch and flexibility. The technology is also recyclable and inexpensive, with qualities not typically combined with commercially manufactured materials.

In an article in the journal Advanced Functional Materials, a team led by Emily Davidson reported that they used a type of widely available polymer called a thermoplastic elastomer to create soft 3D printed structures with tunable stiffness.

Engineers can design the printing paths used by 3D printers to program the physical properties of the plastic, allowing the device to repeatedly stretch and contract in one direction while remaining rigid in the other. Davidson, assistant professor of chemical and biological engineering, said this approach to engineering soft building materials has many applications, including soft robots, medical devices and prosthetics, strong and lightweight helmets, and custom high-performance shoe soles. He said there is a possibility.

The key to a material’s performance is its internal structure at the most minute level. The researchers used a type of block copolymer that forms rigid cylindrical structures 5 to 7 nanometers thick (for comparison, a human hair is about 90,000 nanometers) inside a stretchable polymer matrix.

The researchers used 3D printing to orient these nanoscale cylinders, resulting in a 3D-printed material that is stiff in one direction, but soft and stretchy in nearly every other direction. Designers can orient these cylinders in different directions throughout an object, allowing for soft architectures that exhibit stiffness and stretch in different areas of the object.

“The elastomers we use form nanostructures that we can control,” Davidson said. This gives designers a great deal of control over the finished product. “We can create materials with properties tailored in different directions.”

The first step in developing this process was selecting a suitable polymer. The researchers chose thermoplastic elastomers. It is a block copolymer that can be heated and processed as a polymer melt, but when cooled it solidifies into an elastic material.

At the molecular level, polymers are long chains of linked molecules. While traditional homopolymers are long chains of one repeating molecule, block copolymers are made up of different homopolymers connected together. These different regions of the block copolymer chain are like oil and water: they separate rather than mix. The researchers took advantage of this property to create a material with rigid cylinders within a stretchable matrix.

Researchers are using knowledge of how these block copolymer nanostructures form and how they respond to flow to effectively align these rigid nanostructures. We have developed a guided 3D printing technology. The researchers analyzed how printing speed and controlled underextrusion can be used to control the physical properties of printed materials.

Alice Fergerson, a graduate student at Princeton University and lead author of this article, spoke about the technique and the important role played by thermal annealing, the controlled heating and cooling of materials.

“I think one of the best things about this technology is the many roles that thermal annealing plays. This will allow the item to self-repair if it becomes damaged or damaged.

Davidson said one of the goals of this project is to create soft materials with locally tunable mechanical properties in a way that is affordable and scalable for industry. It is possible to create similar structures with locally controlled properties using materials such as liquid crystal elastomers.

However, Davidson said these materials are expensive (more than $2.50 per gram) and require multi-step processing, including carefully controlled extrusion followed by exposure to ultraviolet light. . The thermoplastic elastomers used in Davidson’s lab cost about 1 cent per gram and can be printed on commercially available 3D printers.

Researchers have demonstrated the ability of their technology to incorporate functional additives into thermoplastic elastomers without reducing their ability to control material properties. In one example, Professor Lin Lu’s group added an organic molecule developed by Professor Lin Lu that causes the plastic to glow red when exposed to ultraviolet light. They also demonstrated the printer’s ability to create complex, multilayered structures, such as small plastic vases and printed text that uses sharp turns to spell out Princeton.

Annealing plays an important role in the process by perfecting the order of the internal nanostructures. Davidson said annealing also enables the material’s self-healing properties. As part of their work, the researchers are able to cut flexible samples of printed plastic and re-glue them together by annealing the materials. The repaired material showed the same properties as the original sample. The researchers said they “found no significant differences” between the original and restored materials.

As a next step, the research team hopes to explore new 3D printable architectures that are compatible with applications such as wearable electronics and biomedical devices.