No abstract

The integration of soft, stimuli‐responsive materials in robotic systems is a promising approach to introduce dexterous and delicate manipulation of objects. Electrical control of mechanical response offers many benefits in robotic systems including the availability of this energy input, the associated response time, magnitude of actuation, and opportunity for self‐regulation. Here, a materials chemistry is detailed to prepare liquid crystal elastomers (LCEs) with a 14:1 modulus contrast and increase in dielectric constant to enhance electromechanical deformation. The inherent modulus contrast of these LCEs (when coated with compliant electrodes) directly convert an electric field to a directional expansion of 20%. The electromechanical response of LCE actuators is observed upon application of voltage ranging from 0.5 to 6 kV. The deformation of these materials is rapid, reaching strain rates of 18% s−1. Upon removal of the electric field, little hysteresis is observed. Patterning the spatial orientation of the nematic director of the LCEs results in a 2D–3D shape transformation to a cone 8 mm in height. Individual and sequential addressing of an array of LCE actuators is demonstrated as a haptic surface.

Liquid crystalline elastomers (LCEs) are functional materials whose stimuli response is strongly influenced by the composition and structure of the polymer network. LCEs are commonly fabricated by copolymerizing acrylate monomers with thiols. This work explores the formation of LCE polymer networks via photopolymerization reactions of diacrylate liquid crystalline monomers with dithiols. Detailed analysis of model systems based on monoacrylate–dithiol reactions indicates that the polymer network architecture of LCEs prepared by this reaction contains extensive unreacted thiol content as dangling ends. Further, kinetic analysis indicates that polymerization in a liquid crystalline phase strongly hinders the mobility of reactive species during the formation of the LCE polymer network, which is evident in the substantial difference in viscosity between liquid crystalline monomers (0.0195 Pa s) and non-liquid crystalline monomers (0.0025 Pa s). In copolymerization with the diacrylate liquid crystal monomers, the dithiol comonomer significantly decreases the elastic modulus and T g. The residual thiol in the LCE polymer network can be postfunctionalized with reactive additives.

Liquid crystal elastomers (LCEs) are soft materials that undergo large anisotropic shape change in response to stimuli. Rational organization of the local director field can impart spatial control of the strain profile, enabling stretch-based deformation capable of nearly 20 J kg −1 of output force. LCEs are increasingly being considered in end-use applications in robotics, therapeutics, and optics. Here, a new synthetic approach is introduced to prepare LCEs composed of main chain mesogens via the cationic photopolymerization of the epoxy liquid crystal monomer (LCM). This examination details the optical, mechanical, and thermal properties of epoxide-based LCEs as a function of spacer length (3, 6, or 11 carbons). The oxygen insensitivity of the cationic photopolymerization of these monomers makes this approach particularly attractive for implementation with emerging additive manufacturing techniques. This contribution focuses on microstructuring LCEs via 2-photon direct laser writing (2P-DLW). A custom heated cell facilitated 2P-DLW of the aligned LCE epoxy resin melts to fabricate diverse geometric arrays. Enabled by the orthogonality of the reaction chemistry, hybrid and microstructured material compositions are prepared via the encapsulation of LCE epoxy micropatterns with free-radical polymerization of acrylate-based LCEs. The distinct thermomechanical response of the hybridized and microstructured LCE composites enables local and spatially controlled actuation.

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress‐strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x‐ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol‐ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.