Influence of Cross-Linker Functionality and Photoinitiator Loading on Network Connectivity and Actuation in 3D-Printed Model Thermosets

Vat photopolymerization additive manufacturing has gained traction as a platform to produce high-performance materials for a wide range of applications in areas such as tissue engineering and soft robotics. However, a gap in the current knowledge remains on how seemingly minute variations during resin formulation can impact the properties of these cross-linked, 3D-printed parts, especially with respect to how network inhomogeneity develops within the print process in (meth)acrylate resins. In this study, a series of acrylate-based model resins are printed with precise control over cross-linker functionality (difunctional or tetrafunctional) and photoinitiator concentration to influence the resulting structure of the cross-linked network. A significant impact on connectivity and inhomogeneity is observed as initiator content is increased, but this relationship is not consistent across cross-linkers with varied functionalities. The shifts in thermomechanical properties are probed by time-temperature superposition (TTS) studies which highlight that greater inhomogeneity results in more fragile & horbar;or temperature-sensitive & horbar;networks and that this is a result of the changes in effective cross-link density across the prints. These small changes in the nano- and microscale structuring of the 3D print critically influence its functionality when incorporated into a printable hygromorphic actuating bilayer. Moreover, the reported findings emphasize the need for a deep understanding of the polymerization pathways utilized in resin 3D printing, as it is the foundation for precisely predicting the thermomechanical and functional properties of 3D-printed cross-linked systems.