Digital Light 3D Printing of Double Thermoplastics with Customizable Mechanical Properties and Versatile Reprocessability

Digital light processing (DLP) is a 3D printing technology offering high resolution and speed. Printable materials are usually based on multifunctional monomers, resulting in the formation of thermosets that cannot be reprocessed or recycled. Some efforts have been made in DLP 3D printing of thermoplastic materials. However, these materials exhibit limited and poor mechanical properties. Here, we present a new strategy for DLP 3D printing of thermoplastics using two polymers with contrasting mechanical properties, where stiff and flexible linear polymers are sequentially constructed. The inks consist of two vinyl monomers, which lead to the stiff linear polymer, and α-lipoic acid to form the flexible linear polymer via thermal ring-opening polymerization in a second step. By varying the ratio of stiff and flexible polymers, the mechanical properties can be tuned with Young's modulus ranging from 1.1 GPa to 0.7 MPa, while the strain at break increased from 4% to 574%. Furthermore, these 3D-printed thermoplastics allow for a variety of reprocessability pathways including self-healing, solvent casting, reprinting, and closed-loop recycling of the flexible polymer, contributing to the development of a sustainable materials economy. Last, we demonstrate the potential of the new material in applications ranging from soft robotics to electronics.