Stimulus Response in Liquid Crystalline Elastomers: Fundamental Characterization To Functional Design

Liquid crystalline elastomers (LCEs) are functional materials capable of undergoing large deformations. The distinctive deformation of these materials is founded upon the ordered packing of mesogenic moieties and subsequent order disruption by stimuli such as heat or light. Numerous approaches have been explored to locally dictate (program) the stimuli-response of LCEs to realize 3-D shape transformation. Motivated by potential for use in applications ranging from soft robotics to biology, this thesis details fundamental structure-property relationships relating to material composition, processing, and programming.&nbsp; The stimuli-response of LCEs was found to be strongly correlated to fundamental properties spanning the molecular to macromolecular level. First, LCEs were prepared with a liquid crystalline monomer exhibiting reduced intermolecular interactions. These LCEs had faster, higher magnitude photomechanical response as a function of reducing energy required for order disruption. Further, preparation of LCEs via radical-mediated thiol-acrylate photopolymerization was studied, elucidating network structures that retain unreacted pendant thiols. Next, thermomechanical actuation properties were characterized for LCEs prepared using two-step polymerizations. Differences were attributed to discrepancies during aza- or thiol-Michael oligomerization before photopolymerization of acrylate-capped oligomers. Additionally, theoretical predictions based on the state of order during crosslinking inspired study of stimuliresponse of LCEs prepared with common alignment methods. Results demonstrated the contribution of alignment method on the thermomechanical response of LCE.&nbsp; These fundamental studies inspired functionally-motivated examinations. In one instance, LCEs prepared with variation in modulus were laminated to create mechanical elements with a through-thickness modulus gradient and a +1 topological defect director pattern. The LCE element leapt from the surface when heated. In another demonstration, crosslink density was patterned spatially across an LCE with a uniform director. This LCE exhibited thermomechanical deformations with Gaussian curvature.&nbsp; LCEs were also prepared with dynamic covalent bonds to realize shape permanence and reprogrammability. One composition incorporated a thermally stable photochrome and thermally active dynamic bonds to allow photoinduced deformation and shape retention via thermal bond rearrangement. Further, photo-active dynamic bonds were incorporated in LCEs via a thiolMichael reaction amenable to surface enforced alignment. This LCE exhibited complex deformations upon heating by combining the directed self-assembly and material reprogrammability via dynamic bond exchange.</p

Lifting, Loading, and Buckling in Conical Shells

Liquid crystal elastomer films that morph into cones are strikingly capable lifters. Thus motivated, we combine theory, numerics, and experiments to reexamine the load-bearing capacity of conical shells. We show that a cone squashed between frictionless surfaces buckles at a smaller load, even in scaling, than the classical Seide/Koiter result. Such buckling begins in a region of greatly amplified azimuthal compression generated in an outer boundary layer with oscillatory bend. Experimentally and numerically, buckling then grows sub-critically over the full cone. We derive a new thin-limit formula for the critical load, $\propto t^{5/2}$, and validate it numerically. We also investigate deep post-buckling, finding further instabilities producing intricate states with multiple Pogorelov-type curved ridges arranged in concentric-circles or Archimedean spirals. Finally, we investigate the forces exerted by such states, which limit lifting performance in active cones.Comment: 7 pages, 4 figures. This version published in PRL, open acces

Surface-Enforced Alignment of Reprogrammable Liquid Crystalline Elastomers

Liquid crystalline elastomers (LCEs) are stimuli-responsive materials capable of undergoing large deformations. The thermomechanical response of LCEs is attributable to the coupling of polymer network properties and disruption of order between liquid crystalline mesogens. Complex deformations have been realized in LCEs by either programming the nematic director via surface-enforced alignment or localized mechanical deformation in materials incorporating dynamic covalent chemistries. Here, the preparation of LCEs via thiol-Michael addition reaction is reported that are amenable to surface-enforced alignment. Afforded by the thiol-Michael addition reaction, dynamic covalent bonds are uniquely incorporated in chemistries subject to surface-enforce alignment. Accordingly, LCEs prepared with complex director profiles are able to be programmed and reprogrammed by (re)activating the dynamic covalent chemistry to realize distinctive shape transformations. &nbsp;</p

Discontinuous metric programming in liquid crystalline elastomers

Liquid crystalline elastomers (LCEs) are shape-changing materials that exhibit large deformations in response to applied stimuli. Local control of the orientation of LCEs spatially directs the deformation of these materials to realize spontaneous shape change in response to stimuli. Prior approaches to shape programming in LCEs utilize patterning techniques that involve the detailed inscription of spatially varying nematic fields to produce sheets. These patterned sheets deform into elaborate geometries with complex Gaussian curvatures. Here, we present an alternative approach to realize shape-morphing in LCEs where spatial patterning of the crosslink density locally regulates the material deformation magnitude on either side of a prescribed interface curve. We also present a simple mathematical model describing the behavior of these materials. Further experiments coupled with the mathematical model demonstrate the control of the sign of Gaussian curvature, which is used in combination with heat transfer effects to design LCEs that self-clean as a result of temperature-dependent actuation properties