Dual-vat photopolymerization 3D printing of vitrimers

Abstract

Herein, the processing of multi-material vitrimers with heterogenous properties along their x-,y- and z-axis is demonstrated using dual-vat digital light processing (DLP) 3D printing. The printer is based on a conventional vat exchange set-up containing two vats and a cleaning station, which are positioned on a linearly moving platform, and are exchanged automatically during the layer-by-layer build-up of the object. Two thiol-acrylate resins are selected bearing ample -OH and ester moieties, which undergo thermo-activated transesterification in the presence of an organic phosphate as catalyst. Based on the functionality of the acrylate monomers, printing of multi-material structures with flexible (σ = 1.1 MPa, ε = 14.0%) and rigid (σ = 33.3 MPa, ε = 4.4%) domains is feasible. A good interlayer adhesion between the soft and hard domains is evidenced by uniaxial tensile tests, whilst optical microscopy is used to study the interface. Dynamic mechanical analysis further shows that the stiffness of the photopolymers varies over several orders of magnitude (E’23 °C = 209 kPa - 507 MPa). Jacobs working curves of the two different resins reveal comparable printing parameters at higher exposure doses, which do not only facilitate the printing of multiple materials between individual layers (z-heterogeneity) but also within the same layer (x,y-heterogeneity). This is confirmed by the printing of a multi-material gripper as “proof of concept” demonstrator, which contains soft inner teeth and a stiff outer core. Gripping and releasing of objects is shown by exploiting the glass transition-based shape memory properties of the stiff domains. Moreover, due to the dynamic nature of the covalent bonds, damages inserted into the gripper are intrinsically repairable due to a thermo-activated macroscopic reflow. Shape memory experiments further confirmed that the multi-material gripper fully regains its function after the thermal mending process. Thus, the results clearly show that objects with a high degree of functionality can be realized by combining multi-material 3D printing with the chemistry of vitrimers. This is of particular interest for future soft robotic applications and for mimicking biological composite structures.