Researchers from the Technical University of Vienna in Austria (TU Wien) have developed a technique for producing tough, high resolution 3D printed polymers. The method, which could help to overcome existing limitations for light-cured 3D printing materials, involves tailoring the production of methacrylate-based photopolymers without affecting the curing process.
The research study, recently published in the journal Angewandte Chemie, explains how photopolymer materials have tended to be brittle once they are been cured with light, a reality that has significantly limited applications for the materials in 3D printing as well as microelectronics and biomedicine.
Now, however, thanks to the research project led by Robert Liska at TU Wien, there is now a way to produce “methacrylate-based, homogenously crosslinked, tailored, tough polymers” which are suitable for high resolution additive manufacturing.
In photopolymer light curing, the researchers explain, an initiator is typically split into radicals by light energy, causing radical chain polymerization. In this process, the radicals “attack” the monomer in the photopolymer (like the C=C double bond in a vinyl group, for instance) that results in the formation of a new radical which then “becomes the starting point of a growing polymer network by attacking more monomers and binding to them.”
Over the years, the radical photopolymerization process has been made more controllable by researchers, though these advances have generally resulted in materials that are slower to cure. This, of course, has presented a challenge for additive manufacturing, which necessitates fast photopolymer curing to ensure economic production times and good quality prints.
By adding an ester-activated vinyl sulfonate ester (EVS), however, the Austrian researchers have demonstrated the ability to produce methacrylate-based photopolymers that don’t affect the curing process. The addition of EVS in the photopolymer functions as a chain transfer agent, which “easily splits off one portion of itself.”
A release on the study elaborates further saying: “If the growing polymer network attacks EVS instead of the next monomer, an intermediate forms and quickly splits apart to form a terminated polymer chain in the network and a highly reactive radical (tosyl radical), which starts a new chain reaction. The more EVS is added, the shorter the average chain length in the polymer network.”
In practical terms, because the shorter polymer chains are more mobile, the risk of cracking or shrinkage are lessened during the curing process, which can increase applications for photopolymer materials in 3D printing, biomedicine, bioelectronics and more.
To demonstrate their novel method, the researchers created a scaffold-like structure from a methacrylate copolymer, in which individual layers had a thickness of 50 μm and were spatially well resolved. With the addition of varying amounts of EVS, the researchers were able to change the material properties of the structure, making it solid, elastic, impact resistant and with high tensile strength. The same structure without EVS was very brittle after curing.
Ultimately, the innovative advancement in photopolymer materials has opened the doors for the creation of tough photopolymers for a range of 3D printing applications, including the production of shape-memory polymers for tissue growth and dental fillings.