Formulating Liquid Crystal Elastomer Resins for Digital Light Projection Additive Manufacturing

Liquid crystal elastomers (LCEs) are a novel class of responsive polymer-based materials that change shape upon exposure to external stimuli such as heat, light, and pH. To formulate elastomers capable of exhibiting a maximum degree of shape change, resins must be comprised of functional monomer “reactive mesogens” linked to thiol or amine based “spacers”, that when oligomerized, form a viscous ink. While such an ink is suitable for facile extrusion based processing via Direct Ink Write (DIW) additive manufacturing(AM) methods, it is incompatible for processing using digital light projection (DLP) due its high viscosity. Because of this the additive manufacturing of LCEs via digital light processing (DLP) remains a nascent field, with many of the current uncertainties related to how low viscosity and suitable ink formulations can be made for DLP, while still maintaining the alignment capability, and thus, actuation of LCEs. Moreover,a more general understanding of how additives in oligomer inks can alter properties like viscosity, alignment, and transition temperature, without inhibiting actuation has yet to be systematically investigated. . Here, we screen various synthetic pathways to highlight property features that lead us to the ideal LCE formulation for DLP processing. This includes tuning the oligomer size by monomer type, spacer composition, use of additives, and evaluating their effect on resin viscosity and printed LCE actuation. This tuning process enables some of the first truly 3D printed LCE parts with 3D-to-3D shape change, enabling us with the knowledge to move towards printing parts with greater complexity, as seen with more conventional photopolymer resins but which are incapable of shape change. <br/><br/>This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.