Architected Liquid Crystal Elastomers with Spatially Programmed Alignment, Shape Morphing, and Mechanics

Abstract

Liquid crystal elastomers (LCEs) are responsive soft materials that undergo reversible shape morphing when cycled above their nematic-to-isotropic transition temperature. This property, coupled with their programmable alignment and mechanics, makes LCEs ideal for advanced applications in adaptive structures, energy absorption, and artificial muscles. However, fabricating monolithic LCEs with spatially varying director alignment in arbitrary architected structures remains a significant challenge. To address this, my Ph.D. thesis focuses on developing a universal framework to correlate printing conditions with director alignment, laying the groundwork for fabricating architected LCE lattices with spatially programmed alignment, shape morphing, and mechanics. During extrusion-based 3D printing, LCE inks experience coupled shear and extensional flows, that enable spatial control of nematic director alignment along prescribed print paths. Combining experiments and computational modeling, we investigated the effects of ink composition, nozzle geometry, and printing parameters on flow-induced alignment. Rheological measurements revealed that the Weissenberg number (Wi) strongly predicts alignment, with uniform alignment achieved at Wi >> 1. COMSOL simulations and in-operando X-ray measurements confirm that hyperbolic nozzles produced printed LCE architectures with improved alignment compared to tapered nozzles, resulting in enhanced stiffness and actuation strain. By varying Wi during printing, LCE architectures with uniform composition yet locally encoded degree of alignment, and hence shape-morphing transitions were realized. Next, we fabricated architected LCE lattices with a high degree of flow-induced alignment via direct ink writing and systematically characterized their shape morphing, stiffness, and energy absorption across strain rates spanning six orders of magnitude. Compared to non-mesogenic elastomeric (silicone) counterparts, LCE lattices exhibit superior energy absorption, with energy absorption ratios up to 18-fold higher at the highest strain rates. A finite element model capturing their shape-morphing response shows excellent agreement with experimental data. In summary, this work demonstrates the potential of architected LCEs as programmable soft materials for myriad applications that require stimuli-responsive, tunable properties.