Responsive AM Feedstock Materials

Additive manufacturing (AM) based on polymeric feedstocks has accomplished complex three-dimensional shapes such as hierarchical metamaterials. However once printed, the properties of the materials remain fixed. Specifically, traditional stereolithography (SLA) processes use short molecular weight acrylate-based monomers that cross-link into rigid and brittle materials, which cannot be modified and are undesirable for most applications. Alternative AM techniques such as direct ink write (DIW) and volumetric additive manufacturing (VAM) provide means to use higher viscosity, higher molecular weight feedstocks, enabling more elastomeric and functional materials. Current research and development have focused on expanding LLNL’s leadership role in AM from static materials to dynamic materials.<br/>Sentient architected materials are being developed at LLNL with recent emphasis on DIW and VAM platforms, where 3D-to-3D shape change has been demonstrated with a change in temperature above the glass transition. Here, we present two different shape memory polymer feedstock materials: a novel dual-curing urethane-grafted acrylate polymer (UGAP) for DIW [1] and a highly tunable thiol-ene photopolymer for VAM [2]. While a broad range of complex printing motifs is made possible from the on-demand UV rheological control during rapid acrylate crosslinking, the predominately polyurethane elastomeric mechanical properties develop from the subsequent heat cure. Use of the low concentration of acrylate diluents allowed for a high solids loaded ink, contributing to the wide range of printable ink yield stress (between 11.5 and 8042 Pa) and tunable mechanical properties. With the integration of multiple printing motifs within the final multimaterial architected part, a cylinder representing a finger with designed softer and stiffer regions acting as the finger and joints respectively, was fabricated and the shape memory effect was demonstrated. The biocompatibility and tunability of this material enable its application in soft robotics. As for the radical thiol-ene photopolymers for VAM, our previous manuscript [2] of tuning the thiol-ene kinetics with a radical inhibitor to provide spatial and temporal curing control reports the highly tunable and tough material properties of this feedstock material. In following up on this work, we present the shape memory capabilities of these materials, reporting a 20% strain and thermal recovery cycling with minimal signs of plastic deformation. This was enabled by VAM’s ability to print higher viscosity resins without layering artifacts. To demonstrate the potential of this material in soft robotics, a printed gripper was shown to release an object in free space. This work represents the first VAM printed sentient materials work to date. Apart from these works, we will discuss some ongoing and future efforts in printing materials such as liquid crystal elastomers on multiple length scales in order to expand our portfolio of sentient materials for AM.<br/><br/>References<br/>1. Howell, B.; Cook, C; Grapes, M; Dubbin, K; Robertson, E; Sain, J; Sullivan, K; Duoss, E; Bukovsky, E; “Spatially Controlled 3D Printing of Dual-Curing Urethane Elastomers.” Advanced Materials Technologies. 2021, 2100700<br/>2. Cook, C.; Fong, E; Schwartz, J.; Porcincula, D.; Kaczmarek, A.; Oakdale, J.; Moran, B.; Champley, K.; Rackson, C.; Muralidharan, A.; McLeod, R.; Shusteff, M; “Highly Tunable Thiol-ene Photoresins for Volumetric Additive Manufacturing.” Advanced Materials, 2020, 32 (47) 20003376