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

Hebner, Tayler S

Graduate Thesis Or Dissertation

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

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/6969z258b

Abstract

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.

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.

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.

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.

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